Corrosion Tutorial

CORROSION TUTORIALDated: June 14, 2004 Hota GangaRao Eung H Cho Sucharitha Bachanna Srinivas Aluri Robert Creese Constructed Facilities Center West Virginia University, Morgantown, WV 26506 Corrosion Tutorial FOREWORD Corrosion is the result of chemical reaction between a metal and its environment. It is the tendency of the refined metal to return to its mineral state. Corrosion engineers study the corrosion mechanisms to determine the causes of corrosion and the methods to minimize the resulting damage. Also, corrosion engineers design and apply various methods to prevent corrosion by practical and economical means. The importance of corrosion is not only related with economy, but also with safety issues. The loss of material due to corrosion results in the failure of the machines, structures, bridges, etc. resulting in damages worth billions of dollars. The annual direct cost of corrosion and of protection against corrosion in the United States for Department of Transportation alone is estimated to be around 276 billion in the year 2001. Direct costs mean the costs of replacing corroded structures and machinery. Indirect costs resulting from actual or possible corrosion are more difficult to evaluate and maybe more than $276 billion. Some of the major industries affected by corrosion are 1) Defense, 2) Nuclear power plants, 3) Aircraft, 4) Pipeline, 5) Storage Tanks, 6) Highways and bridges, 7) Water systems, 8) Gas distribution, 9) Transportation, 10) Petroleum, 11) Oil and natural gas, 12) Chemical plants, etc. In this tutorial, some of the basics issues dealing with corrosion are explained. The essential elements of electrochemistry that are needed to understand the basics of corrosion reactions are presented in Chapters 1 and 2. Corrosion has been classified in many different ways. One way is to classify by the forms in which corrosion occurs. The forms of corrosion are discussed in Chapter 3. Corrosion of composites is typically called Aging. Composites have different mechanical properties compared to most metals. Hence aging of composites is discussed in Chapter 4. Use of composites is rapidly becoming prevalent in many applications. Some of the important applications are discussed in Chapter 5. Although corrosion cannot be prevented, its rate can be retarded using many different methods. Some of the important methods are discussed in Chapter 6. Chapter 7 consists of brief information on typical metals and composites that are used extensively in the industry. chemical kinetics of the corrosion reactions. Chapter 8 deals with advanced electro- i Corrosion Tutorial TABLE OF CONTENTS 1 1.1 1.2 1.3 1.4 1.5 2 2.1 2.2 2.3 FUNDAMENTALS OF CORROSION ELECTROCHEMICAL CELL STANDARD ELECTROCHEMICAL POTENTIAL NERNST EQUATION FREE ENERGY AND ELECTRODE POTENTIAL POTENTIAL MEASUREMENT OF HALF-CELL REACTION PASSIVITY AND ELECTROCHEMICAL CORROSION MEASUREMENTS PASSIVITY TAFEL EXTRAPOLATION LINEAR POLARIZATION RESISTANCE METHOD 1 1 3 5 6 7 9 9 12 13 13 14 15 15 2.3.1 DERIVATION OF THE POLARIZATION RESISTANCE 2.3.2 PRINCIPLE OF MEASUREMENT 2.3.3 ADVANTAGES OF POLARIZATION RESISTANCE MEASUREMENTS 2.3.4 ERRORS AND LIMITATIONS IN THE USE OF POLARIZATION RESISTANCE MEASUREMENTS 2.4 OTHER METHODS TO DETERMINE POLARIZATION RESISTANCE 16 16 16 2.4.1 ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY 2.4.2 ELECTROCHEMICAL NOISE 3 3.1 FORMS OF CORROSION UNIFORM CORROSION ATMOSPHERIC CORROSION 18 18 20 3.1.1 3.1.1.1 3.1.1.2 3.1.2 Mechanism Prevention of Atmospheric Corrosion GALVANIC CORROSION 20 21 21 3.1.2.1 3.1.2.2 CORRODED END (ANODIC OR LEAST NOBLE) PROTECTED END (CATHODIC OR MOST NOBLE) 26 28 ii Corrosion Tutorial 3.1.2.3 3.1.2.4 3.1.2.5 3.1.3 Factors Affecting Galvanic Corrosion Galvanic Series Prevention of Galvanic Corrosion STRAY CURRENT CORROSION 30 31 33 33 3.1.3.1 3.1.3.2 3.1.3.3 3.1.4 Direct stray current corrosion Alternating Stray Current Corrosion Telluric Effects GENERAL BIOLOGICAL CORROSION 33 34 34 36 3.1.4.1 3.1.4.2 3.1.5 3.1.6 Causes and Prevention of Biological Corrosion Prevention of biological corrosion MOLTEN SALT CORROSION CORROSION IN LIQUID METALS 36 37 38 39 3.2 3.2.1 LOCALIZED CORROSION PITTING CORROSION 39 39 3.2.1.1 3.2.1.2 3.2.1.3 3.2.2 Initiation of Pitting Corrosion Propagation of Pitting Corrosion Prevention of Pitting Corrosion CREVICE CORROSION 40 40 42 43 3.2.2.1 3.2.2.2 3.2.3 3.2.4 Initiation and Propagation of Crevice Corrosion Prevention of Crevice Corrosion PACK RUST FILIFORM CORROSION 43 45 45 46 3.2.4.1 3.2.5 Prevention of Filiform Corrosion LOCALIZED BIOLOGICAL CORROSION 48 49 3.3 3.3.1 ENVIRONMENTALLY INDUCED CRACKING STRESS CORROSION CRACKING 49 49 3.3.1.1 3.3.1.2 3.3.1.3 3.3.2 3.3.3 3.3.4 3.3.5 Metallurgical Effects Electrochemical Effects Prevention of Stress Corrosion Cracking SULFIDE STRESS CRACKING LIQUID METAL EMBRITTLEMENT SOLID METAL INDUCED EMBRITTLEMENT CORROSION FATIGUE CRACKING 50 50 51 52 52 53 54 iii Corrosion Tutorial 3.3.5.1 3.3.5.2 3.3.6 Comparison with Stress Corrosion Cracking Prevention of Corrosion Fatigue Cracking HYDROGEN INDUCED CRACKING 55 55 56 3.3.6.1 3.3.6.2 3.4 3.4.1 Comparison with Stress Corrosion Cracking Prevention of Hydrogen Induced Cracking 56 57 57 57 MECHANICALLY ASSISTED DEGRADATION EROSION CORROSION 3.4.1.1 3.4.2 Prevention of Erosion Corrosion IMPINGEMENT CORROSION 58 58 3.4.2.1 3.4.3 3.4.4 Prevention of Impingement Corrosion CAVITATION CORROSION FRETTING CORROSION 59 59 60 3.5 3.5.1 METALLURGICALLY INFLUENCED CORROSION INTERGRANULAR CORROSION 61 61 3.5.1.1 3.5.1.2 3.5.1.2.1 Exfoliation / Lamellar Corrosion Weld Decay Prevention of Weld Decay 61 62 63 3.5.1.3 3.5.2 3.5.3 Sensitization (Intergranular Corrosion of Austenitic Stainless Steels) DEALLOYING DEZINCIFICATION 63 65 65 3.5.3.1 3.6 3.6.1 3.6.2 3.6.3 Prevention of Dezincification 65 65 66 66 67 HIGH TEMPERATURE CORROSION OXIDATION SULFIDATION CARBURIZATION 4 4.1 4.2 AGING OF COMPOSITES INTRODUCTION COMPOSITION FIBER REINFORCEMENTS RESIN SYSTEMS FILLERS ADDITIVES 69 69 69 70 71 71 71 4.2.1 4.2.2 4.2.3 4.2.4 iv 3.3.2.3.3.2 5.1 APPLICATIONS OF COMPOSITES FOR CIVILIAN PURPOSES AUTOMOTIVE 161 161 v .2.2.3.2.2 5.2 FIBER REINFORCED POLYMERS CHARACTERISTICS OF FIBER REINFORCED COMPOSITES SHORT-TERM MECHANICAL AND HYGRO-THERMAL BEHAVIOR 72 75 78 4.4 5 5.2 4.3 5.1 5.1 4.3.2 4.4 5.3.1 4.3. Vinyl Ester.3 4.3.3 4.1.3.3.1.2 4.3.2.2.3.3. Polyestor and Phenolics Effect of Moisture on Fiber-Matrix System Effect of Temperature and Polymer Structural Variables on Sorption of Water 78 81 82 87 94 104 LONG TERM MECHANICAL AND HYGROTHERMAL BEHAVIOR (AGING) 4.3.1 Aging Due to Environmental Factors Environmental Factors Influencing the Durability of Composites 131 132 4.2 4.3 Fatigue and Fracture Fatigue Process Fatigue in Unidirectional Composites Fatigue in Multidirectional Composites 121 124 127 130 4.Corrosion Tutorial 4.3.3.2.3 Thermal Coefficient and Conductivity Moisture Diffusion/Plasticization Diffusion Through Unreinforced Epoxy.3.2.3.3 4.2.3.1.5 4.2.2 4.3.1.3.3.2.1 4.3 Creep Theory Effect of moisture and temperature on Creep Effect of Physical Aging on Creep Effect of Ultraviolet (UV) Radiation on Creep 104 115 119 120 4.3.4 4.3.1 4.1 4.3.3.5.1.3.3.1 4.1 Knockdown Factors Durability Models Analytical Methods to Predict the Effects of Environment on Composite Materials 144 147 147 4.1.3.2.1.1.1 5.5 SUMMARY AND CONCLUDING REMARKS APPLICATIONS OF COMPOSITES APPLICATIONS OF COMPOSITES FOR DEFENSE PURPOSES AIRCRAFT SYSTEMS GROUND SYSTEMS INDIVIDUAL AND CREW SERVED SYSTEMS ROCKET AND MISSILE SYSTEMS SHIPBOARD SYSTEMS 155 156 156 156 158 158 159 160 5.3.3.3.3.3. 2.2 6.2.2.2.5 6.3.3 Portland Cement Coatings Ceramics Chromate Filming Phosphate Coatings Nitriding ORGANIC 175 175 175 176 176 176 6.1.2.2 6.1 RETARDATION METHODS FOR CORROSION CATHODIC PROTECTION SACRIFICIAL ANODE SYSTEM 168 168 168 6.4 5.6 5.2.2.2 Advantages of Sacrificial Anode Systems Disadvantages of Sacrificial Anode Systems IMPRESSED CURRENT CATHODIC PROTECTION 170 170 170 6.5 5.5 6.1 Binders 176 vi .2.2 6.9 5.2.2.1.2.1 6.2.1.1.1.1.1.1 6.1 6.2.2.8 5.3 5.3 6.1.2.2.1.2.2 5.1 6.2 6.4 6.3 6.1.2.Corrosion Tutorial 5.2.2.1 6.1.2.1.2.2.2.2.2.2 Hot Dipping Chemical Vapor Deposition (CVD) Ion Vapor Deposition (IVD) Spraying Electroplating INORGANIC 172 173 173 173 174 175 6.10 INFRASTRUCTURE CONSTRUCTION TRANSPORTATION BIOMEDICAL COMPUTER PRODUCTS CORROSION RESISTANT PRODUCTS ELECTRICAL RECREATIONAL MARINE 162 162 163 164 164 165 165 166 166 6 6.2.2.1 Advantages of Impressed Current Cathodic Protection Disadvantages of Impressed Current Cathodic Protection 172 172 172 172 COATINGS METALLIC 6.4 6.1.2.7 5.2 6.2. 6 7.1.10 7.1.1.4 7.1.1.3.1 6.2.1.1.3.8 7.7 7.3 7.3.4 Plain Carbon Steels Low Alloy Steels Stainless Steels ALUMINUM AND ITS ALLOYS MAGNESIUM AND ITS ALLOYS 184 184 185 185 186 7.1.5 7.3.3 7.2.1.1 7.3.1 7.2.4 7 7.1.2.1.1 7.1.12 Lead and its Alloys COPPER AND ITS ALLOYS NICKEL AND ITS ALLOYS ZINC AND ITS ALLOYS CADMIUM TITANIUM AND ITS ALLOYS COATED ALLOYS MOLYBDENUM TANTALUM 186 186 187 187 187 188 188 188 188 vii .3 7.1.1.3 6.11 7.1.3 6.2.4 Pigments Solvents NONSTICK COATINGS 176 177 177 6.2.1.2 White Cast Iron Malleable Iron Ductile Iron Gray Cast Iron High Silicon Cast Iron STEELS 182 183 183 183 183 184 7.2 6.Corrosion Tutorial 6.9 7.1.1 ANODIC PROTECTION METALS AND COMPOSITES DICTIONARY METAL ALLOYS CAST IRON 179 182 182 182 7.1.1 7.1.4.1.1.1.3.1.1.5 7.2 7.2 6.3 6.1.4 INHIBITORS ANODIC INHIBITORS CATHODIC INHIBITORS MIXED INHIBITORS APPLICATIONS 177 178 179 179 179 6.2 7.1. 1.5.3 7.6 7.1.2.1 7.2.1 MANUFACTURING PROCESSES OF COMPOSITES OPEN-MOLD PROCESSES 197 197 7.4.8 7.5.2 7.5 7.4.4.2.2 PLASTICS THERMOSETS THERMOPLASTICS 190 190 191 7.4 7.5 7.12 7.5.4.4.5.1.17 TUNGSTEN ZIRCONIUM GOLD PLATINUM SILVER 189 189 189 189 190 7.4.Corrosion Tutorial 7.5.4.2.5.2 Hand Lay Up Tube Rolling CLOSED-MOLD PROCESSES 198 198 199 7.5.5.4.11 7.1.1 7.15 7.5 7.2.1.5.3 7.4.10 7.2.16 7.1.4.1.13 7.2 7.13 CERAMICS COMPOSITE MATERIALS ORGANIC MATRIX COMPOSITES (OMCS) METAL MATRIX COMPOSITES (MMCS) CERAMIC MATRIX COMPOSITES (CMCS) PARTICULATE REINFORCEMENTS WHISKER REINFORCEMENTS CONTINUOUS FIBER REINFORCED COMPOSITES BRAIDED FABRICS HYBRID FABRICS KNITTED OR STITCHED FABRICS WOVEN COMPOSITES CARBON FIBER REINFORCED PLASTICS GLASS FIBER REINFORCED PLASTICS ARAMID FIBER REINFORCED PLASTICS 191 191 192 193 193 193 194 194 194 195 196 196 197 197 197 7.4 7.7 7.1 7.2 7.2.4.9 7.3 7.6 7.7 Resin Transfer Molding (RTM) Vacuum Assisted Resin Transfer Molding (VARTM) Resin Injection Molding Process Compression Molding Pultrusion Extrusion Filament Winding Process 199 200 200 201 202 203 203 viii .4 7.1 7.4.5.2 7.5.4.14 7.2.2. 2 8.7 7.6 7.1.7.1 7.7.5 COMPOSITES TERMINOLOGY REAGENTS SULFURIC ACID HYDROCHLORIC ACID NITRIC ACID HYDROFLUORIC ACID PHOSPHORIC ACID 204 212 212 212 213 213 214 8 8.7.1.3 MIXED POTENTIAL THEORY EXPERIMENTAL POLARIZATION CURVES 222 224 LIST OF FIGURES Figure 1-1: Electrochemical Cell _________________________________________________ 2 Figure 2-1: Schematic Active-Passive Behavior of the Anodic Polarization of a Metal _______ 9 Figure 2-2: Schematic Representation of Corrosion of Stainless Steel with Two Oxidation Reagents ___________________________________________________________________ 10 Figure 2-3: Comparison of Galvanostatic and Potentiostatic Anodic Polarization Curves_ __ 11 Figure 2-4: Cathodic and Anodic Polarization Curves________________________________ 12 Figure 3-1: Uniform Corrosion Figure 3-2: Example of uniform corrosion damage on a rocket assisted artillery projectile Figure 3-3: Atmospheric corrosion of galvanized anti crash railing due to marine aerosol condensation on wooden post Figure 3-4: Galvanic Corrosion of Brass Coupled With Black Iron Figure 3-5: Mechanism of Galvanic Corrosion of a Two Metal Couple Figure 3-6: Galvanic cell showing corrosion process in its simplest form 19 20 21 22 17 18 ix .7.7.1 8.2 8.1 CORROSION KINETICS POLARIZATION ACTIVATION POLARIZATION CONCENTRATION POLARIZATION 215 215 215 220 8.3 7.Corrosion Tutorial 7.2 7.4 7. Figure 3-10: Effect of cathode to anode ratio in galvanic corrosion Figure 3-11: Occurrence of Stray current corrosion in pipelines. Figure 3-25: Filiform Corrosion Causing Bleed Through a Welded Tank Figure 3-26: Stress Corrosion Cracking Showing Branched Cracks in Aluminum Plates Figure 3-27: Schematic of Active-Passive Behavior of the Anodic Polarization of a Metal Figure3-28: Liquid Metal Embrittlement Figure 3-29: Solid Metal Induced Embrittlement of a cadmium plated B7 bolt 23 24 25 28 33 34 35 36 37 39 41 43 44 45 46 46 47 48 49 50 51 53 54 Figure 3-30: Brittle crack in a cadmium plated B7 bolt from solid metal induced embrittlement ____________________________________________________________________________55 Figure 3-31 Impingement corrosion in a bent tube Figure 3-32: Exfoliation of Aluminium Figure 3-33:Exfoliation of aircraft component Figure 3-34:Intergranular corrosion in stainless steel Figure 3-35: Intergranular Corrosion of 7075-T6 aluminum adjacent to steel fastener 59 62 63 64 65 x . The introduction of a less noble metal will decrease the corrosion rate of the more noble metal.Corrosion Tutorial Figure 3-7: Corrosion rate determination for a two electrode process system Figure 3-8: Corrosion rate determination for a three electrode system Figure 3-9. Figure 3-12: Ionic current flow onto the pipeline Figure 3-13: Current flow onto pipeline at coating discontinuities Figure 3-14: External stray current sources. Figure 3-15: Corroded surface of carbon steel in its natural condition Figure 3-16: Pitting in Aluminum Figure 3-17: Propagation of Pitting Corrosion Figure 3-18: Crevice Corrosion Figure 3-19: Mechanism of Crevice Corrosion Figure 3-20: crevice corrosion in rivets Figure 3-21: A crevice formed into an open atmosphere Figure 3-22: Example of Pack Rust Figure 3-23: Mechanism of Filiform Corrosion Figure 3-24: “worm like” filiform corrosion tunnels. Corrosion Tutorial Figure 4-1: Fibers as reinforcement a) Random Fibers________________________________70 Figure 4-1: Fibers as reinforcement b) Continuous Fibers (Long)______________________ _70 Figure 4-2: The combined effect on modulus of the addition of fibers to the resin matrix_____ 73 Figure 4-3: Typical Sorption Curve (Vijay et al. 2001)______________________________ 83 Figure 4-4: The Sorption Curves for Epoxy. Vinyl ester. Temperature (Stokes. _______________________ 102 Figure 4-11: Typical Creep Behavior of Plastics (GangaRao. 1999)_________________________________________84 Figure 4-5: Fickian Diffusion Curves for Epoxy in (a) Water. 1990) ___________________ 90 Figure 4-8: (B) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Across Ply Direction (Stokes. 1990)________________________________________________________93 Figure 4-9: Difference Between (a) Diamine and (b) Aniline Hardener __________________ 99 Figure 4-10: Bonds Between Glass Fiber and Coupling Agent. (b) Salt Solution. 1999)__________________________________________ 86 Figure 4-6: Thermal Expansion Measured by Stokes (Stokes. and (c) Concrete Pore Solution at 22 °C (Chin et al. and Isopolyester Resin When Exposed to the 3 Different Solutions (Chin et al... 1990)_________ 88 Figure 4-8: (A) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen as a function on the humidity of conditioning environment. 1990)________________________________________________________92 Figure 4-8: (D) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Wrap Direction (Stokes.1998) ________________________________________________ 116 Figure 4-18: Moisture Absorption Behavior_______________________________________ 118 Figure 4-19: Effect of Physical Aging on Creep_____________________________________120 xi .. Temperature. 2001) ___________________ 105 Figure 4-12: Maxwell's Model__________________________________________________ 108 Figure 4-13: Kelvin Model_____________________________________________________110 Figure 4-14: Four Element Model _______________________________________________111 Figure 4-15: Behavior of creep and stress relaxation in four element model ______________112 Figure 4-16: Behavior of creep when subjected to a series of stresses ___________________113 Figure 4-17: Schematic Representation of the Effects of Time. 1990)_____________________ 88 Figure 4-7: Moisture-Induced Thermal Expansion vs. 1990)_____________________________________________________91 Figure 4-8: (C) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in Ply Direction (Stokes. (Stokes. and Moisture on Creep Compliance (Liao. 1986] ______________________________ 129 Figure 4-24: Fatigue Life Diagram of Unidirectional Composites Under (a) Loading Parallel to Fibers. 1987]________________________________________________________125 Figure 4-21: Two-Stage Strength Degradation Model for Fatigue Reliability of Composites [Talreja. 1987] __ 130 Figure 4-25: Normalized S-N Curves for (0/±45) CFRP Laminates with Varying Percentage of 0o Fibers [Curtis and Dorey. (c) Interfacial Shear Failure [Talreja.Corrosion Tutorial Figure 4-20: Fatigue Damage Mechanism in Unidirectional Composites Under Loading Parallel to Fibers: (a) Fiber Breakage.GAU –19A b. 1987] _____________________________________________________________ 127 Figure 4-22: Comparison of S-N Curve for Three Different Unidirectional Composite Materials [Curtis and Dorey. CFC-WVU _________________________ 162 Figure 5-12: Energy Plant Towers ______________________________________________ 163 Figure 5-13: Third Rail Protection in Monorail System _____________________________ 163 Figure 5-14: MRI Units ______________________________________________________ 164 Figure 5-15: Composite Computer Chip _________________________________________ 164 Figure 5-16: Waste Water Plant________________________________________________ 165 Figure 5-17: Telecommunication Towers_________________________________________ 166 Figure 5-18: Recreational Products_____________________________________________ 166 xii . Laurel Lick. 1986] _____________________________________________________ 128 Figure 4-23: Comparison of S-N curve for Four Different Materials with Different Carbon Fibers in Same Epoxy Resin [Curtis and Dorey. Interfacial Debonding. (b) Matrix Cracking. F18C/D ____________________________________________ 158 Figure 5-4: Reactive Armor and XM-301 Gun ____________________________________ 158 Figure 5-5: Objective Crew Served Weapon ______________________________________ 159 Figure 5-6: Delta II _________________________________________________________ 160 Figure 5-7: Missiles from the Hydra 70 Family____________________________________ 160 Figure 5-8: Destroyers _______________________________________________________ 161 Figure 5-9 Goalkeeper: In-Ship Defense System ___________________________________ 161 Figure 5-10: Composite Fire Truck Panels _______________________________________ 162 Figure 5-11: All Composite Bridge. (b) Off-Axis Loading (Dotted line correspond to on-axis loading) [ Talreja. 1986] _____________________________________________ 131 Figure 5-1: F-22 Raptor Aircraft -Tactical Fighter Aircraft __________________________ 157 Figure 5-2: RAH-66 Comanche ________________________________________________ 157 Figure 5-3: a. Corrosion Tutorial Figure 5-19: Sheet Piling and Fender Applications_________________________________ 167 Figure 6-1: Steel Tank Protected by Sacrificial Anode System ________________________ 169 Figure 6-2: Mechanism of Anodic Protection System _______________________________ 170 Figure 6-3: Steel Tank Protected by Impressed Current System _______________________ 171 Figure 6-4: Mechanism of Impressed Current Systems Explained Using Anodic Polarization Curves ____________________________________________________________________ 171 Figure 6-5: Mechanism of Electroplating ________________________________________ 174 Figure 6-6: Effects of Applied Anodic Current on the Behavior of Active-Passive Alloy ____ 181 Figure 7-1: Particulate Reinforcement___________________________________________ 193 Figure 7-2: Whisker Reinforcement _____________________________________________ 194 Figure 7-3: Continuous Fiber Reinforcement _____________________________________ 194 Figure 7-4: Braided Fabrics___________________________________________________ 195 Figure 7-5: Hybrid Fabrics ___________________________________________________ 195 Figure 7-6: Knitted or Stitched Fabrics __________________________________________ 196 Figure 7-7: Woven Fabric ____________________________________________________ 196 Figure 7-8: Hand Lay Up _____________________________________________________ 198 Figure 7-9: Tube Rolling _____________________________________________________ 199 Figure 7-10: Resin Transfer Molding Machine (CFC-WVU) _________________________ 200 Figure 7-11: VARTM –Tabletop Model of VARTM and Schematic Process of Manufacture _ 200 Figure 7-12: Injection Molding Machine (CFC-WVU) ______________________________ 201 Figure 7-13: Compression Molding Machine (CFC-WVU)___________________________ 202 Figure 7-14: Schematic Representation of Pultrusion Process (Bedford Plastics) ____________ 202 Figure 7-15: Basic Extruder___________________________________________________ 203 Figure 7-16 a: Winding Machine Showing Carriage and Mandrel b: Filament Winding____ 204 Figure 8-1: Activation Polarization _____________________________________________ 216 Figure 8-2: Butler-Volmer Equation and Tafel Equation ____________________________ 220 Figure 8-3: Combined Polarization _____________________________________________ 222 Figure 8-4: Behavior of Metal M in Acid Solution__________________________________ 223 Figure 8-5: Behavior of Coupled Metals in Acid Solutions ___________________________ 224 Figure 8-6: Showing Cathodic and Anodic Polarization Curves_______________________ 226 xiii . 1998) ______________________________________ 146 xiv . Vinyl Ester. and Isopolyester Resins___________ 85 Table 4-2: Variation of Equilibrium Moisture Content with Degree of Cure ______________ 98 Table 4-3: Effect of Hardener on Equilibrium Moisture Content ______________________ 100 Table 4-4:Calculated Values of Gth _____________________________________________ 103 Table 4-5: Knockdown Factors (Vijay.Corrosion Tutorial LIST OF TABLES Table 1-1: E0 Values for Metals __________________________________________________ 4 Table 1-2: E0 Values for Common Oxidation Reagents ________________________________ 5 Table 3-1: Galvanic Series of Metals/Alloys in Seawater _____________________________ 32 Table 4-1: Diffusion Coefficients of Epoxy. electro-chemistry. sulfuric acid catholyte and an anionic membrane (allows only negatively charged ions to pass through the membrane). and metallurgy among others. an electrolyte. Physical chemistry and its various disciplines are very useful for studying the mechanisms of corrosion reactions. Figure 1. can predict the approximate rate of corrosion. Nernst equation. In the case of applications that require aesthetic appeal.Corrosion Tutorial 1 FUNDAMENTALS OF CORROSION In the design and fabrication of machines and structures. the final choice frequently depends on many other factors like economics. a platinum cathode. appearance is the most important consideration. The electrochemical cell. design. 1. This chapter introduces the fundamentals of the electro-chemistry needed to understand the basic corrosion mechanism. etc. a cathode. and other basic properties. standard electrochemical potential. Although corrosion resistance is an important factor. In order to determine how corrosion occurs. Corrosion resistance or chemical resistance of a material depends primarily on thermodynamics. and an anionic membrane. Metallurgical factors influence the corrosion resistance of the material.1 shows an electrochemical cell.1 Electrochemical Cell Corrosion is an electrochemical reaction between the metal and its environment. Electro-chemistry along with environmental factors.. availability. Thermodynamics can determine whether or not the material is prone to corrosion. The corrosion cell consists of an anode. which consists of a steel anode. and free energy and electrode potential are briefly discussed. we must understand the formation of a corrosion cell. including its corrosion behavior. 1 . etc. the surface conditions of metals. the choice of the material depends on many factors. 2) This cathodic reaction is also called half-cell reaction. The anionic membrane allows only the anions (negatively charged ions) of the sulfate ions to pass through. iron is oxidized and dissolved into the electrolyte. the dissolution of Fe introduces Fe++ ions into the anodic compartment. So.Corrosion Tutorial Figure 1-1: Electrochemical Cell At the anode. For example.= H2 (g = ‘gas’) reaction is a reduction of H+ to H2 (g). Fe = Fe2+ + 2e…(1. This reaction cannot occur alone. …(1.transferred from the cathodic compartment to maintain the electrical neutrality. one mole of Fe++ introduced needs one mole of SO42. it needs a partner cathodic reaction. This transfer of anions is needed to maintain the electrical neutrality of the solutions at both anodic and cathodic compartment. We can see that cathodic 2 .1) is an anodic reaction and it is also called half-cell reaction. The cathodic reaction takes place on the surface of the platinum electrode according to: 2H+ + 2e.1) Equation (1. as per the international convention.. The list of E0 values for various metals is provided in Table 1.Corrosion Tutorial 1.1. The E0 values for both tables are given for cathodic reaction. i.2 shows the E0 values for common oxidation reagents.2 Standard Electrochemical Potential Standard electrochemical potential is defined as the potential under standard conditions.e. 3 . Table 1. 25 °C and 1 atmosphere pressure and when the reactants of the reaction have unit activity. 799 +0.= Ag Hg22+ + 2e.= Cu Pb2+ + 2e.= Co Cd2+ + 2e.138 -0.277 -0.= 2Hg Cu2+ + 2e. SHE) +1.799 +0.744 -0.= Sn Ni2+ + 2e.=Na K+ + e.= Ni Co2+ + 2e.372 -2.= Au Ag+ + e.= Zn Al3+ + 3e.342 -0.662 -2.= Cd Fe2+ + 2e.Corrosion Tutorial Table 1-1: E0 Values for Metals Reaction Au3+ + 3e.447 -0.71 -2.250 -0.= Fe Cr3+ + 3e.= Mg Na+ + e.= Al Mg2+ + 2e.= Cr Zn2+ + 2e.126 -0.498 +0.931 Active Noble Note: SHE = Standard Hydrogen Electrode 4 .762 -1.= Pb Sn2+ + 2e.= K Standard Potential E0 (volts vs.403 -0. 30 kcal/mole. The E values are under the conditions that deviate from the standard condition as defined in section 1.229 +0.3) where F is the Faraday constant and is 23.(pH 7) 2H2O + 2e.= Sn2+ 2H+ + 2e.= 2ClO2 + 4H+ + 4e.771 +0.401 +0.= Fe The GFE change for reaction (1.(pH 14) Standard Potential E0 (volts vs.= 2H2O (pH 0) NO3.000 -0.= Fe O2 + 2H2O + 4e. For example.= 4OH.= NO + 2H2O O2 + 2H20 + 4e.= H2 + 2OH.413 -0.82 +0.= 4OHFe3+ + 3e. the reverse reaction of reaction (1.+4H+ + 3e.3 Nernst Equation Nernst equation is used to calculate E values from the E0 values.2.1) is: Fe2+ + 2e.957 +0. SHE) +1.(pH 14) Sn4+ + 2e. the E0 of reaction (1.44 eV. But.828 The E0 values can be calculated from ΔG0 (GFE = Gibbs Free Energy) value of the reaction.= H2 2H2O + 2e.nFE0 …(1.Corrosion Tutorial Table 1-2: E0 Values for Common Oxidation Reagents Reactions Cl2 + 2e.358 +1. Then. ΔG0 = .06 kcal/equivalent-volt and n is the number of electrons or equivalents/mole.4) …(1.= H2 + 2OH. 1.3) is 20.15 0.3) will be -0. The Nernst equation can be derived from: 5 . K is the equilibrium constant. these values should be converted to free energy. Then. F. and F is the Faraday constant (=23060 cal/equivalent-volt).7) is called the Nernst Equation. and T (=2980K) into Equation (1. Substitution of equation (1.6) Substitution of R.5) yields: E=E0–(RT/nF)*ln(K) …(1. so that the overall electrochemical reaction results in no electrons. T is the absolute temperature. Then the free energies of the half-cell reaction can be added to obtain the free energy of the overall reaction.Corrosion Tutorial ΔG=ΔG0+RTln(K) …(1.303 log( K ) n * 23060 0. n for each half-cell reaction should be determined to give no electrons in the overall reaction. The free energy of the overall reaction can be determined as follows: First E value of the half-cell reactions should be calculated using the Nernst equation. 6 .5) where R is the gas constant (=1. 1. Since potential is an intensive (having same potential value for every subdivision of a system) property. If the number of electrons is not identical in both half-cell reactions.4 Free Energy and Electrode Potential The overall electrochemical reaction is a combination reaction of cathodic and anodic reactions. ΔG. mathematical arrangement should be made to equate the number of electrons.7) Equation (1.987cal/mole-K). the potentials of the half-cell reactions cannot be added in order to determine the free energy of the overall reaction.987 * 298 * 2.4) and the similar term for ∆G into Equation (1. In this procedure.059 log( K ) n …(1.6) and converting ln (log to the base e) to log (log to the base 10) yields: E = E0 − E = E0 − 1. 059 log +2 2 = −0. To calculate the free energy of reaction (1.499 eV.44 − 0. which is -20. the overall reaction for Figure 1. is: potential of the anodic reaction. Similarly the E value for reaction (1.8) is the sum of those for the halfcell reactions.499) = -23.499 log 2 [ Fe ++ ] …(1.059 2 [H ] …(1.01 kcal/mole.8) when [Fe++] = 10-2 molar. H+ = 10-1 molar and PH 2 = 1 atmosphere The E value for cathodic reaction is: E = 0− PH 0. Also the reverse is true.2) and the anodic reaction is Equation (1. the reaction is at equilibrium. Then the free energy ΔG = -2F(0.059 1 = −0.1 can be written as: Fe + 2H+ = Fe++ + H2 …(1.1). the reaction is spontaneous because the energy level at the final state is lower than that of initial state.3) is: E = −0. Since the potential of reference electrode is known and we measure the 7 . its potential should be 0. If ΔG is negative. When ΔG is zero. The sign of this free energy indicates that the overall reaction is spontaneous.10) Since we need the This E value is the potential for the cathodic reaction.5 Potential Measurement of Half-Cell Reaction The reference electrode is needed to measure the potential of the half-cell reaction.8) The cathodic reaction is Equation (1. Now the free energy of the overall reaction (1.29 kcal.9) Then the ΔG = -2F(-0.Corrosion Tutorial For example.059) = 2. 1.72 kcal/mole. The next reference electrode is copper-copper sulfate electrode and it has the following half-cell reaction: CuSO4 + 2e. The measurement should be conducted in a circuit with a high impedance value.318 eV.14) …(1.= Cu + SO4.241 eV The potential at the saturation of chloride ion is 0.241 eV. The standard electrode is prepared with platinum wire immersed in a chamber.13) …(1. Otherwise.E0 = 0.318 eV The potential at the saturation of copper ion is 0. Thus.= Ag + Cl. The next reference electrode is silver-silver chloride electrode and it has the following half-cell reaction: AgCl + e. other reference electrodes are used. The potential of this reference electrode is given as: 2H+ + 2e.222 eV …(1. the standard hydrogen electrode is not practical because it involves flammable hydrogen gas.Corrosion Tutorial potential difference between the half-cell reaction and the reference electrode. which contains 1 molar H+ ion.= H2 E0 = 0 eV …(1. a significant current flows through the circuit and then the potential changes to give a wrong value of the half-cell reaction.11) Thus. we can measure the potential of the half-cell reaction. There are several types of reference electrodes. One of them is calomel electrode and it has the following half-cell reaction: Hg2Cl2 + 2e.= 2Hg + 2Cl.12) 8 .E0 = 0.E0 = 0. and pure hydrogen gas is bubbled through the solution. titanium) is immersed. However.1 is produced. a typical polarization curve as shown in Figure 2. the curve follows Tafel behavior. ipass (passive current) Etp transpassive passive Oxygen evolution E icc (critical current) active Ecorr (corrosion potential) Epp (passivation potential) Log i Figure 2-1: Schematic Active-Passive Behavior of the Anodic Polarization of a Metal In the active zone. iron. nickel. chromium.Corrosion Tutorial 2 PASSIVITY AND ELECTROCHEMICAL CORROSION MEASUREMENTS 2.. the current starts decreasing until it reaches ipass (passive current).1 Passivity When an oxidizer such as ferric ion is added to a solution in which a metal (e.g. This passive current is retained until the potential reaches Etp 9 . when the voltage increases (by adding more oxidizer). Titanium alloys are frequently used in an aggressive environment to induce passive film. the current starts increasing again like in the active zone. Stainless steels are generally immune to atmospheric corrosion where oxygen is involved but they are not immune to acid environment. applying a potential to form a passive film protects acid storage tanks. its potential. It can be seen from Figure 2. For example.2 that the higher potentials with oxygen can induce passivity while the lower potential with acid induces higher corrosion rate. Passive films are intentionally produced to control the corrosion rates. Above Etp. The mechanism is illustrated in Figure 2.2. The passive zone has the lowest current due to the formation of metal hydroxide/oxide film. At Etp the film becomes unstable making the film break down. This film is usually impermeable for the corrosion reagents such as oxygen and water thus reducing the metal corrosion drastically.Corrosion Tutorial (transpassive potential). Since titanium is an active metal. The passivity is one of the important aspects in corrosion. in anodic protection. it induces passive film easily. and the current starts increasing above 2H2O+O2+4eE 2H++2e-=H2 Log i Figure 2-2: Schematic Representation of Corrosion of Stainless Steel with Two Oxidation Reagents 10 . Figure 2.Corrosion Tutorial There are two methods to generate anodic polarization curves. the passive zone will be revealed. 11 . However. Tafel extrapolation 2. E Potentiostat Galvanostat Log i Figure 2-3: Comparison of Galvanostatic and Potentiostatic Anodic Polarization Curves The methods available for measurement of corrosion by electrochemical polarization are as follows: 1. continuous monitoring and non-destructive measurements. The dotted line is produced by galvanostat where potential is measured at each increment of current. since the potentiostat measures current at each increment of This potentiostat should be used to potential.3 compares both methods. speed. measure the passivity. Thus. Polarization resistance Polarization measurements are found to be useful in engineering and research applications due to its inherent advantages such as accuracy. the passivity cannot be revealed by this method. The accuracy of this technique is better than the conventional techniques. In Figure 2. 2H++2e-=H2(g) Experimental Ecorr E Fe=Fe2++2e- icorr Tafel equation Log i Figure 2-4: Cathodic and Anodic Polarization Curves 12 . This linear region is referred to as Tafel region.4.Corrosion Tutorial 2. the Tafel region is extrapolated to obtain the corrosion potential. the solid lines represent the experimentally measured cathodic polarization and anodic polarization data. The disadvantage of this system is that it cannot be applied for systems having more than one reduction reactions. To determine the corrosion rate from this measurement. As the current density increases. the region is found to be linear compared to less current density values.2 Tafel Extrapolation The Tafel extrapolation technique uses the data obtained from the cathodic polarization measurements. Consider a metal M in de-aerated acid solution. Ecorr )→0 = βa * βc 2..) and by neglecting higher terms when ΔE/β < 0. This method is limited to electrolytically conducting liquids.Corrosion Tutorial 2.g. This simplified relationship has the following form Rp = [ E iapp ](E .1 Derivation of the Polarization Resistance It is experimentally observed that iapp is almost linearly related to applied potential within a few millivolts of polarization from Ecorr. iapp = I corr [exp( 2.3 Linear Polarization Resistance Method Linear Polarization Resistance Method (LPR) is the only corrosion monitoring method that allows corrosion rates to be measured directly in real time.. Stern and Geary simplified the kinetic expression to provide an approximation to the charge transfer controlled reaction kinetics given by Equation (2.1. The response time and data quality makes this technique superior to other methods.(2.1) where βa = anodic Tafel slope βc = cathodic Tafel slope Icorr / icorr = corrosion rate/corrosion current density Ecorr = corrosion potential Equation (2.1) for the case of small overvoltage with respect to Ecorr.3.2) Rearranging icorr = βa * βc B = 2.3icorr ( β a + β c ) .3( E − Ecorr ) βa ) − exp( 2. ex = 1 + x + x2/2! + x3/3! +….3) Rp is the polarization resistance 13 .3R p ( β a + β c ) R p …(2.. 2.3( E − Ecorr ) βc )] …(2.1) can be approximated mathematically by taking its series expansion (e. and βc enables direct determination of the corrosion rate at any instant in time using equation (2.3). βc).3. cathodic mass transport control results in B = βa/2. the measured Rp value in ohms is halved. It can be infered that corrosion rate is inversely proportional to the Rp. if unequal. The B factor is dominated by the smaller of the two anodic and cathodic Tafel slopes (βa. After which. 14 .2 Principle of Measurement Before making any measurements. βa. The current (ΔI) required to sustain the 10 mV potential shifts is proportional to the corrosion rate of the test electrode. The flow of current between the auxiliary electrode and test electrode will increase until the test electrode potential is shifted 10 mV with respect to the reference electrode. Knowledge of Rp. Therefore.3 and similarly anodic mass transport control results in B = βc/2. Consequently the method is called linear polarization method (LPR).Corrosion Tutorial βa = anodic Tafel slope βc = cathodic Tafel slope B is the proportionality constant The units of Rp are ohms as obtained from E-Iapp data when the applied current is not normalized by electrode area (such data must be multiplied by electrode area to yield Rp (Ω-cm2)). and this intrinsic polarization resistance value remains the same. If the electrode area is doubled. current will flow from the auxiliary electrode (A) onto the test electrode. 2.3. However. The extent of approximately linear E – iapp region can vary considerably among corroding systems. This gives the result that corrosion rate per unit area is independent of electrode area. residual potential difference between the reference electrode (R) and the test electrode (T) should be nullified. the working electrode area must be known to calculate the corrosion rate. iapp is approximately linear with potential within ± 5 mV to 10 mV of Ecorr . 3. Accuracy of the measurement of even smaller corrosion values. • Resistances from the presence of films on the electrodes and the electrolyte resistance between the working and reference electrode in high resistivity media can produce an underestimation of corrosion rates due to IR (current times resistance) losses on ΔE and must be compensated to obtain accurate measurements. • • • This equation is valid only for activation controlled process This method is not applicable under the special non-Tafel conditions corresponding to passivity or cathodic diffusion limiting current densities. Ecorr can cause errors in the measurement of polarization resistance when linearization techniques are involved. or the latter rates will dominate the polarization resistance measurement.3. The corrosion rate icorr must be much larger than any other exchange currents of half-cell reactions.Corrosion Tutorial 2.3 Advantages of Polarization Resistance Measurements • • • Rapid Measurement.4 Errors and Limitations in the Use of Polarization Resistance Measurements • • The Stern-Geary relationship is only mathematically valid only when ΔE is equal to zero Curvature of polarization curves about the corrosion potential. 2. Can be used to monitor corrosion continuously which cannot be inspected visually or by other methods. 15 . 2. It has the same media conductivity limitations and requirement for a known electrode area as other electrochemical techniques. a reference electrode. which directly measures naturally occurring electrochemical potential and current disturbances due to ongoing corrosion activity. It is generally less quantitative than linear polarization resistance for corrosion rate calculations.4.4 Other Methods to Determine Polarization Resistance 2. AC impedance measurements can be used to predict corrosion rates and characterize systems under study and are commonly used for performance studies of chemical inhibitors and protective coatings to evaluate the resistance of alloys to specific environments etc.2 Electrochemical Noise Electrochemical noise is a monitoring technique. and instrumentation capable of measuring and recording electrical response of the test corrosion cell over a wide rate of AC excitation frequencies. Again.1 Electrochemical Impedance Spectroscopy Electrochemical impedance spectroscopy is a well-established laboratory technique used to determine the electrical impedance of the metal-electrolyte interface at various AC frequencies. 16 . some investigation of this method is now being made in the field. Laboratory and field interpretations are still under development. measurements.Corrosion Tutorial 2. AC impedance is capable of characterizing the corrosion interface more comprehensively and with good quality equipment specifications of achieving measurements in lower conductivity solution or high resistivity coatings. it is useful in detecting transient effects in marginally conductive situations. One advantage is that this technique can provide a large quantity of information. One disadvantage is that there is too much information to analyze. with the evolution of more rugged computers. it is necessary to have a corrosion cell of known geometry.4. which can be useful in determining what is actually happening in real time with corrosion activity in piping or equipment being monitored. Impedance measurements combine the effects of DC In order to make impedance resistance with capacitance and inductance. Corrosion Tutorial especially. when there is a contact but acceptable level of corrosion activity in the system or another source of potential electrical disturbance in the system. 17 . (Reference:http://corrosion.gov/html/unifcor.htm ) 18 .ksc.nasa.Corrosion Tutorial 3 FORMS OF CORROSION The various forms of corrosion that are prevalent in metals are discussed in terms of their characteristics. This is the most important form of corrosion on the basis of tonnage wasted. This leads disastrous failures relatively rare.1 Uniform Corrosion Uniform (or general) corrosion is a form of corrosion where there is uniform reduction of thickness over the surface of a corroding material. mechanisms. The breakdown of protective coating systems on structures often leads to this form of corrosion. 3. Figure 3-1: Uniform Corrosion. Uniform corrosion can be easily measured and predicted. and preventive measures in this chapter. Realtime corrosion monitoring systems can detect such transitions. NDT techniques for thickness measurements). Corrosion monitoring is therefore advisable.corrosion-doctors. this variability is not accounted for by single "textbook values". Much data on uniform corrosion has been published that can be used for design purposes and estimating a corrosion allowance". In most practical cases.corrosion-club. corrosive environments tend to differ from "textbook" cases (even small differences can be very significant). flow rate changes. This undesirable transition can occur if the passive surface film is disrupted by mechanical effects.org/Forms/projectile. a chemical change in the environment etc. generally the simplest methods suffice (coupons.htm) By selecting suitable materials and protective coatings. Furthermore.Corrosion Tutorial Figure 3-2: Example of uniform corrosion damage on a rocket assisted artillery projectile (Reference: http://www. “It is relatively easy to monitor uniform corrosion. Unexpected rapid uniform corrosion failures can occur if the material's surface changes from the passive (low corrosion rate) to the active (high corrosion rate) state. ER. actual uniform corrosion rates tend to vary with time. uniform corrosion can be controlled. Cathodic protection and corrosion inhibitors are other preventive methods.” (This excerpt is taken from www.com) 19 . The resultant increase in uniform corrosion rate is typically several orders of magnitude. The rate at which the corrosion takes place and the severity is primarily dependant upon the properties of the surface formed electrolyte. which takes place in the presence of an electrolyte.1.1 Mechanism of Atmospheric Corrosion Atmospheric corrosion is an electrochemical process. This has been identified as one of the oldest forms of corrosion and has been reported to account for more failures in terms of cost and tonnage than any other single environment.org/AtmCorros/AtmCorr.1. which in turn depends upon the factors such as relative humidity.Æ 4OH- Figure 3-3: Atmospheric corrosion of galvanized anti crash railing due to marine aerosol condensation on wooden post (Reference: http://www. climatic conditions.Corrosion Tutorial 3. atmospheric pollutants etc. 3. The following reactions take place. Anodic reaction 2M Æ 2M2+ + 4eCathodic reaction O2 + 2H2O + 4e. relative humidity and temperature. temperature.1.corrosion-doctors. Depending upon the climatic conditions. a certain humidity level is reached which tends to form a thin electrolyte on the metallic surfaces.1 Atmospheric corrosion Atmospheric corrosion is defined as the corrosion or degradation of the material exposed to the air and its pollutants.htm) 20 . The first is a temporary one which is used during storage and which consists of lowering of atmospheric humidity by using a desiccant.htm) 21 . the system will tend to adjust itself so as to nullify the effect of that change. The anodic reaction involves the dissolution of the metal in the electrolyte.4 shows galvanic corrosion between the pipe made of black iron and a brass valve. a galvanic cell is formed.eci-ndt. Therefore. 3. while the cathodic reaction involves reduction. 3. Corrosion of active metal is increased and of noble metal is decreased upon galvanic coupling. inorganic and metallic coatings effectively. In a galvanic cell.2 Prevention of Atmospheric Corrosion There are two approaches to prevent Atmospheric corrosion.Corrosion Tutorial It is an established principle that if a change occurs under which a system is in equilibrium. Oxygen from the atmosphere is readily supplied to the electrolyte in thin film conditions.1. heating devices.1. Figure 3. in the presence of an electrolyte.com/gallery_a. the more active metal is corroded while the noble metal is protected. atmospheric corrosion proceeds by balancing cathodic and anodic reaction. or by treating the surface with vapor phase or surface inhibitors.2 Galvanic Corrosion When two dissimilar metals are coupled and immersed in an electrolyte solution. Permanent solution is by applying organic. Figure 3-4: Galvanic Corrosion of Brass Coupled With Black Iron (Reference: http://www.1. Resistance to current flow in the conducting materials and in the connection between them. This cell includes the following essential components: a metal cathode. The Figure 3-6 illustrates a cell showing the corrosion process in its simplest form.5. metal anode. the corrosion mechanism may be depicted in a graphical representation as shown in Figure 3. The magnitude of the galvanic current flow is controlled by the potential difference and the total resistance to current flow as shown in the Figure 3-6. with N being more active. The electrons produced by the dissolution of N are consumed by the cathodic reaction that takes place on the noble metal surface. a metallic conductor between anode and the cathode and an electrolyte in 22 . 2H+ H2 M 2e- N N2+ Figure 3-5: Mechanism of Galvanic Corrosion of a Two Metal Couple The more noble metal M undergoes the cathodic reaction while the active metal N undergoes the anodic reaction. are connected and immersed in an acid solution.Corrosion Tutorial When two dissimilar metals M and N. corrosion of the noble metal retards while that of the active metal accelerates because of the galvanic effect generated on the metal surface. which can be broken down further into the following resistance components. Resistance to current flow of the electrolyte. Thus. Resistance associated with polarization behavior of the anode and cathode. When contact with a dissimilar metal is made. the rate of corrosion would be very rapid. the self-corrosion rates will change: corrosion of the anode will accelerate. The opposite would be true if there was a very large anode compared to the cathode. They are so arranged to form a closed electric path. this is where the corrosion occurs. the anode is very small compared to the cathode.com/Resources. The difference in charge provides potential voltage which is the driving force for the current to flow in the cell. The rate of which depends upon the relative sizes of the anode and cathode and also the potential difference between cathode and anode. however.edstrom.cfm?doc_id=131) The cathode is positively charged and anode is negatively charged. corrosion of the cathode will decelerate or even stop. for instance.Corrosion Tutorial contact with the anode and the cathode. Hydrogen gas is produced at the cathode and no destruction will occur while the anode gives off its ions in the form of rust. Figure 3-6: Galvanic cell showing corrosion process in its simplest form (Reference: http://www. If. 23 . ’ 24 . Figure 3-7 shows the corrosion rate for a single metal in solution.htm) Figures 3-8 and 3-9 shows the rate determination when a third electrode process is added at a potential between the first two electrode reactions.uri.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion. The rule that must be applied is that the ‘total oxidation rate must be equal to the total reduction rate. E (V) Cathodic Reaction 1 Ecorr Anodic reaction 2 log Current Density 2 μA/cm icorr Figure 3-7: Corrosion rate determination for a two electrode process system (Reference: http://www.Corrosion Tutorial This is shown in Figures 3-7 to 3-9.egr. (Reference: http://www.htm) From Figure 3-8. The second electrode dissolution rate increased significantly by the introduction of the third electrode processes.uri.Corrosion Tutorial E (V) Cathodic Reaction 1 Anode Reaction 3 Ecorr 1+2 Cathode Reaction 3 Total Cathode 1+3 Anodic Reaction 2 log Current Density 2 μA/cm icorr 1+2 Figure 3-8: Corrosion rate determination for a three electrode system. when the corrosion potential for three electrodes is above the two electrode potential and when the three electrode corrosion potential is below the two electrode potentials respectively. 25 .egr. but the third electrode corrodes at a lower rate than the second electrode. it is seen that the corrosion rate for electrode 2 has increased from icorr to icorr 1+2 as it is the only anodic reaction. Two cases are shown in Figures 3-8 and 3-9. thus contributing to the cathodic reaction and protecting the third electode from corrosion. the resulting corrosion potential from the three electrodes is more negative than the double electrode potential. In Figure 3-9. In this case both the second and third electrodes are corroding. In Figure 3-8 the resulting corrosion potential is more negative than the third electrode reverse potential.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion. uri.1. The seawater Galvanic Series. Metals with negative voltage charges (anodic–least noble) are listed first. The metals below are arranged according to their tendency to corrode galvanically.2. It can also be used for protection by galvanizing.htm) Both these Figures 3-8 and 3-9 show that introducing a more anodic metal will decrease the corrosion rate in a more noble metal. 3. The introduction of a less noble metal will decrease the corrosion rate of the more noble metal. 6053 26 .1 CORRODED END (ANODIC OR LEAST NOBLE) • • • • MAGNESIUM MAGNESIUM ALLOYS ZINC ALUMINUM 5052.edu/che/course/CHE534w/chapter3EnivronmentalCorrosion. 3004. (Reference: http://www. This is the process behind galvanic corrosion. 3003. followed by metals with positive charges (cathodic–more noble). 1100.egr.Corrosion Tutorial E (V) Cathodic Reaction 1 Anode Reaction 3 Ecorr 1+2 Cathode Reaction 3 Anodic Reaction 2 Total Anode 1+3 log Current Density 2 μA/cm icorr 1+2 Figure 3-9. shown in table 3-1 below can be used to predict which metal will become the anode and how rapidly it will corrode. 303. 321.RESIST 316. 2024 MILD STEEL (1018). ALUMINUM.NICKEL ALLOY 90-10 COPPER . STAINLESS STEEL (ACTIVE) CARPENTER 20 CB-3 STAINLESS (ACTIVE) ALUMINUM BRONZE (CA 687) HASTELLOY C (ACTIVE) INCONEL 625 (ACTIVE) TITANIUM (ACTIVE) LEAD . WROUGHT IRON CAST IRON.NICKEL ALLOY 80-20 430 STAINLESS STEEL NICKEL. K500 27 . 410. 304.Corrosion Tutorial • • • • • • • • • • • • • • • • • • • • • • • • • • • • • • CADMIUM ALUMINUM 2117. BRONZE (CA 630. 347. 632) MONEL 400. TIN BRONZE (CA903. 2017.416. 430 SERIES (ACTIVE) 302. 905) SILICON BRONZE NICKEL SILVER COPPER .TIN SOLDERS LEAD TIN INCONEL 600 (ACTIVE) NICKEL (ACTIVE) 60 NI-15 CR (ACTIVE) 80 NI-20 CR (ACTIVE) HASTELLOY B (ACTIVE) BRASSES COPPER (CA102) MANGANESE BRONZE (CA 675). LOW ALLOY HIGH STRENGTH STEEL CHROME IRON (ACTIVE) STAINLESS STEEL. STAINLESS STEEL (ACTIVE) NI . 317. This action raises the voltage of the lower-voltage metal above its natural potential.1. 321.MOLYBDEUM .2. the lowervoltage metal discharges a current in to the electrolyte.15 CR (PASSIVE) INCONEL 600 (PASSIVE) 80 NI. it is true that each metal has a different electrical potential when immersed in the same electrolyte (an electrically conductive fluid such as sea water). 303.) HASTELLOY C & C276 (PASSIVE). To establish the equilibrium. the current flows from higher voltage metal to the lower one. INCOLOY 825 NICKEL . 317. STAINLESS STEEL (PASSIVE) 316. The current passes through the electrolyte back to the higher-voltage metal and completes the electrical circuit between the two pieces.1.1.CHROMIUM . if two dissimilar metals are placed in the same electrolyte.2 PROTECTED END (CATHODIC OR MOST NOBLE) From the list 3. 28 .) GRAPHITE ZIRCONIUM GOLD PLATINUM 3. 304.2. their different electrical potentials will produce a voltage that can be measured on the two pieces of metal. The current flowing through the electrolyte is generated by an electrochemical reaction that steadily consumes the lower-voltage metal a process known as galvanic corrosion.20 CR (PASSIVE) CHROME IRON (PASSIVE) 302. 347. As a result.Corrosion Tutorial • • • • • • • • • • • • • • • • SILVER SOLDER NICKEL (PASSIVE) 60 NI.IRON ALLOY (PASSIVE) SILVER TITANIUM (PASS. INCONEL 625(PASS. STAINLESS STEEL (PASSIVE) CARPENTER 20 CB-3 STAINLESS (PASSIVE). According to the potential difference of these two metals. Corrosion of the anodic metal is both more rapid and more damaging as the voltage difference increases and as the cathode area increases relative to the anode area.udel. the situation is reversed: the stainless steel rivet is small. In both couples A and B. corrosion of the large aluminum plate in couple B will be much less. the situation is reversed: the stainless steel rivet is small. even though the potential difference is the same in each case. In couple A. the aluminum rivet is comparatively small. Corrosion of the aluminum rivet in couple A will be severe. 29 . the cathode-to anode area ratio. and the C/A ratio is also small.ocean. even though the potential difference is the same in each case. The two major factors affecting the severity of galvanic corrosion are (1) the voltage difference between the two metals on the Galvanic Series.html) The effect of the second factor. corrosion of the large aluminum plate in couple B will be much less. In couple B. However.edu/seagrant/publications/corrosion. is illustrated in Figure 3-10 for a rivet in a plate. and the C/A ratio is large. and stainless steel is the cathode. C/A. and the C/A ratio is large. aluminum is the anode. Figure 3-10: Effect of cathode to anode ratio in galvanic corrosion (Reference: http://www. However.Corrosion Tutorial In couple A. and the C/A ratio is also small. and (2) the size of the exposed area of cathodic metal relative to that of the anodic metal. Corrosion of the aluminum rivet in couple A will be severe. the aluminum rivet is comparatively small. In couple B. 3 Factors Affecting Galvanic Corrosion Area Effect The current flowing between anode and cathode will be the same independent of the surface area of each electrode. 30 . To minimize the galvanic corrosion.Corrosion Tutorial 3. It is important in several other forms of corrosion including pitting corrosion.2. the rate of corrosion depends on the current density in the anode. For the anodic and cathodic reactions. crevice corrosion. Distance Effect Distance effect is another important factor for galvanic corrosion. the anode area should be very large compared to the cathode area. The transportation of the ions becomes more difficult when the distance between anodic and cathodic reaction site increases thus decreasing the corrosion rate.1. if the anode is painted. then according to the rule as explained. then the damage to the paint causes large cathode to small anode ratio resulting in large corrosion rates in the anode and thus penetrating into the metal. Galvanic corrosion rates are the largest at the interface between the anode and cathode and decrease with distance away from the contact region. Since the corrosion occurs at anode. For example if corrosion appears at a constriction some distance from a galvanic contact. it is the current rather than current density which is equal. the cathode of the system should be painted. If a galvanic corrosion is suspected. Essentially the resistance of the electrolyte increases with distance. Conversely. The anode to cathode area effect is an important characteristic. stress corrosion cracking and corrosion fatigue. If the corrosion rate is far away from this area then different type of corrosion may be involved. Inorder to protect the system from galvanic corrosion. If the paint is damaged. This is an important factor in determination of the form of corrosion. then small cathode to large anode area ratio is formed which results in minimizing corrosion rates. then erosion corrosion was the cause and not the galvanic corrosion. the rate of corrosion adjacent to the galvanic contact region should be higher. The list is formed by polarization of two or more half-cell reactions to a common mixed potential. titanium is a noteworthy element. For every change in environmental conditions. the potential changes due to changes in electrolyte composition and temperature. Metal at the top of the system are highly cathodic while metals at the bottom highly anodic. This may be due to the fact that titanium surface is easily covered with passive film with normal potential oxidizing reagents such as oxygen.= Ti …(3.1) However. Titanium is a very active metal with E0 value being -1. Clearly.Corrosion Tutorial Distance Apart in the Galvanic Series Galvanic series is an empirical listing of the corrosion resistance of metals. In the galvanic series. it has a very noble corrosion potential. not the rate. 3. it is undesirable to connect metals widely spaced on the galvanic series. For selection purposes metals close together on the list are desirable as there is little driving force for corrosion to be accelerated. 31 .1. Ecorr on the corroding surface.1 lists some of the metals/alloys for the galvanic series in seawater.4 Galvanic Series Galvanic series is a list of metals/alloys according to their corrosion potentials. Table 3.1. Corrosion potentials in the galvanic series are measured in real or simulated service conditions. In Table 3.2. Its advantage over the Redox series is that it refers to alloys in a real environment. a new series needs to be established. The galvanic series gives only tendencies for galvanic corrosion.63 V for the reduction of: Ti2+ + 2e. 17 Cr. 7 Fe) Cast Iron Wrought Iron Copper Red Brass Cadmium Manganese Bronze Aluminum 52SH Tin Lead Zinc ⇑ ⇓ Active Magnesium Alloys Magnesium 32 .Corrosion Tutorial Table 3-1: Galvanic Series of Metals/Alloys in Seawater Noble Platinum Gold Titanium Silver Hastelloy C (62 Ni. 15 Mo) 18-8 Stainless steel (passive) Inconel (passive) (80 Ni. 13 Cr. cathode protection systems of nearby equipment or pipelines. The damage caused to the metal components due to this unwanted current refers to stray current corrosion.2. This current mostly originates due to bad earthen systems of electrical equipments and eventually leaks through the metal structures or other conductive systems. paint the cathodic area.5 Prevention of Galvanic Corrosion • • • Coupling of two dissimilar metals should be avoided when they are much apart in the galvanic series. the corrosion rate in the area will increase drastically. DC-driven elevators. 3. Thus local oxidation occurs and the metal is consumed rapidly.3 Stray Current Corrosion Stray current corrosion is the current flowing through unintended paths due to some kind of leakage from extraneous sources.3. Metals like aluminum under soil or water are affected due to this kind of corrosion. The mechanism involved in stray current corrosion is that an electrolysis cell is formed that forces the metal structure through which it passes to act as an anodic site.1. etc. DC welding equipment. Instead.1.1. The common sources of these stray current include electric railway systems. In the case where such coupling is necessary. 3.1 Direct Stray Current Corrosion This type of corrosion occurs due to direct current from sources like rail transit system. break the electrical contact between the two metals by using insulation such as gaskets and rubber. 33 .Corrosion Tutorial 3. Again this type of corrosion can be classified as dynamic current corrosion when the current is not steady or flows irregularly and as static current corrosion when the flow is steady. The effects of direct stray current are very severe compared with alternative currents. Do not paint anodic area because if the paint is ruptured in a small area. htm) Basic Theory A protective coating is used as the primary form of protection for buried pipelines. Figure 3-11: Occurrence of Stray current corrosion in pipelines. Thus a combination of a protective coating system and cathodic 34 .corrosion-club. Additionally.1.2 Alternating Stray Current Corrosion This type of corrosion is caused due to alternate currents from sources like overhead AC power lines.com/stpickup.3 Telluric Effects The disturbances in the earth’s magnetic field called geomagnetic activity may lead to the production of dynamic stray currents. (Reference:http://www.Corrosion Tutorial 3. These may flow onto a buried pipeline varying the magnitude of current flow and the position of current pick-up.3.1. 3. These currents induced naturally due to the geomagnetic activity are called telluric effects. The discharge areas will also fluctuate with time. cathodic protection is designed to provide protection at coating discontinuities.3. Under these idealized conditions. Figure 3-12 schematically illustrate current flow in an impressed current cathodic protection system (the principle is similar for a sacrificial anode system). current flows through the electrolyte (soil) onto the pipeline.corrosion-club. Such external stray current sources interfere with the normal operation of the cathodic protection system leading to corrosion problems.htm) The schematic diagram in Figure 3-13 details the current flow onto the pipeline at coating discontinuities. (Reference:http://www. Figure 3-12: Ionic current flow onto the pipeline. (Reference:http://www.com/sttheory. in the form of ionic current. 35 .Corrosion Tutorial protection system is used to reduce the corrosion risk factor.corrosion-club.htm) Current flow in the electrolyte that does not originate from the cathodic protection system designed to protect the pipeline is referred to as stray current. under the protective influence of the cathodic protection system.com/sttheory. Figure 3-13: Current flow onto pipeline at coating discontinuities. Sulfate Reducing Bacteria (SRB) and Heterotrophic Aerobic Bacteria (HAB) grow on iron and steel pipes that lead to the corrosion of these metals. (Reference:http://www. can survive only when free oxygen is not present. hydrogen.corrosion-club. 36 .4 General Biological Corrosion Microorganisms such as Iron Related Bacteria (IRB). aerobic bacteria sustains only when oxygen is present.1.4. On the other hand.Corrosion Tutorial Figure 3-14: External stray current sources. Metals are subject to corrosion when the bacteria deplete the oxygen supply or releases metabolic products.htm) 3.com/sttheory. such as iron. anaerobic bacteria.1. The metabolism of these bacteria requires them to obtain part of their sustenance by oxidizing inorganic compounds. The resultant chemical reactions cause corrosion. sulfur. This type of corrosion can occur through aerobic and anaerobic corrosion. Sulfur is oxidized to produce sulfuric acid and thus corrosion is accelerated. 3.1 Causes of Biological Corrosion Bacteria Bacteria may be either aerobic or anaerobic. The other name for this corrosion is Biofouling. and carbon monoxide. htm) 37 .1. yet effective method for monitoring the population size and/or activity of specific groups of bacteria is Biological Activity Reaction Test.4. Bacteria collect inside the holes and produce FeS.it/images/bcorr2_i. (Reference: http://www.Corrosion Tutorial Fungi Microorganism feed on organic materials and this leads to the growth of fungi. A simple.esemir. Bacteria produce tubercles and. In the analysis chamber of the ESEM XL30 TMP Philips Electron Microscope. Figure 3-15: Corroded surface of carbon steel in its natural condition. it slows their growth and may prevent damage due to corrosion. Low humidity does not kill microbes. 3. Below is an image of a carbon steel plate section that is bio-corroded. Biological corrosion also affects copper and aluminum alloys. Microbial growth may also be removed with steam at 100 psi. Microbial growth should be removed by hand with a firm non-metallic bristle brush and water. The corrosion product is brown. Temperatures between 68 F and 108 F and relative humidity between 85 and 100 percent are ideal conditions for the growth of most of the microorganisms. the image was obtained in high vacuum. Removal of microbial growth is easier if the growth is kept wet with water. Protective coating is used when using the steam for removing microbial growth.2 Prevention of Biological Corrosion Microbial growth must be removed completely to avoid corrosion. under the tubercles a small hole. 1.5. Many principles that apply to aqueous corrosion also apply to molten salt corrosion. Molten salt corrosion can be identified as the intermediate form of corrosion between molten metal and aqueous corrosion.2 Types of Molten salts ƒ ƒ ƒ ƒ ƒ ƒ Molten Fluorides Chloride Salts Molten Nitrates Molten Sulfates Hydroxide Melts Carbonate Melts In nuclear reactor cooling systems.Corrosion Tutorial 3.5 Molten Salt Corrosion The corrosion of metal containers by molten or fused salts is known as molten salt corrosion. Molten salts are partially electronic conductors as well as ionic conductors. fluoride melts are used. Corrosion in fluoride molten-salt melts is enhanced because protective surface films are not formed. The corrosion process is mainly electrochemical in nature. such as anodic reactions leading to metal dissolution and cathodic reduction of an oxidant.1 Mechanism of Molten Salt Corrosion Metal dissolution and Metal oxidation are the two common mechanisms involved in this corrosion.1. Cathodic reactions increase causing a substantial increase in corrosion growth. 3. Molten salt systems operate at higher temperatures than aqueous systems.5. This phenomenon mostly occurs at high temperatures. due to the ionic conductivity of most molten salts. Nickel- 38 .1. This fact allows for reduction reactions to take place in the melt. 3. 3.1 Pitting Corrosion When passive metal surface is exposed to a chloride solution. The following are different types of localized corrosion. a material should be selected that will form a passive non-soluble film in the melt.6 Corrosion in Liquid Metals Liquid metals are widely used in nuclear reactors and nuclear power systems. In order to prevent molten salt corrosion. erosion and other forms of chemical attack. Chloride salts attack steels rapidly with preferential attack of carbides. corrosion is initiated by forming pits on the metal surface. fatigue.1. Pitting is a form of extremely localized attack that results in holes in the metal. 3. It usually takes place when there is intense attack at the localized sites than at the rest of the original surface.2 Localized corrosion Localized corrosion is defined as the selective removal of material by corrosion at small areas and zones in contact with corrosive environment. used in the above said applications. 3. Minimizing the entry of oxidizing elements such as oxygen and water into the melt is important. Liquid metal corrosion may be formed due to the dissolution of the solid metal.2. Pits are difficult to determine because they are 39 . potassium and cesium are some of the examples of liquid metals.Corrosion Tutorial base alloys show better corrosion resistance than iron-base alloys. Liquid lithium. to form an alloy or due to compound reduction. It is mostly accompanied with other destructive processes such as stress. The metal surface may react with the liquid metal due to the presence of some impurities. mercury. These hydrogen ions further attack the substrate of the metal matrix.Corrosion Tutorial smaller in size and they will be covered with the corrosion products. Most of the metal oxide/hydroxide films are soluble by chloride ions.clihouston.htm) 3. respectively. Figure 3-16: Pitting in Aluminum (Reference: http://www. is most susceptible and nickel alloys are least susceptible to pitting corrosion.com/pitting. Further.1 Initiation of Pitting Corrosion Pitting corrosion is initiated when the passive film is attacked by chloride ions in solution. hydrogen ions are produced by the hydration of metal chloride. 3. These results show that stainless steel.1 and 3.2. Jones and B. which contains chromium. Stainless steel is well known to be susceptible to pitting corrosion.1. the metal ions produced by corrosion attract more chloride ions by Coulomb forces into the pits.2 Propagation of Pitting Corrosion Once the pits are initiated in the presence of chloride ions. The reason is that upon hydration of CrCl3. A. D. 40 . E. Figure 3-16 shows pitting corrosion on aluminum surface. very low pH is produced.2. Wilde observed that upon hydration of 1 N CrCl3 (N=normality) produced a pH of 1. because there would not be passive films formed in the presence of chloride ions.0.1.1 while 1 N of FeCl2 and NiCl2 produced pH of 2. 2) We can see that the hydration produces an acid. One is by acid produced upon the hydration of metal ions. which further corrodes the pit. The growth of the pit is by two distinct mechanisms. The other is formation of an electrical circuit between an anodic reaction of metal dissolution which takes place at the pit and the cathodic reaction of oxygen reduction reaction which takes place outside the pit. Since the solubility of oxygen is virtually zero in concentrated solutions.17 in the case of steel. The metal ions are hydrated and the aqueous metal hydroxide is transported to outside of the pit where it is further oxidized in the case of steel.4) 41 .Corrosion Tutorial making the pit solution more concentrated. The hydration of ferrous ion is given by Fe2+ + 2H2O = Fe(OH)2(aq) + 2H+ …(3. no oxygen reduction occurs within a pit. These mechanisms can be seen graphically in Figure 3.3) The other mechanism where iron is dissolved with oxygen can be written as: 2Fe + 2H2O + O2 = 2Fe2+ + 4OH…(3. The reaction is: Figure 3-17: Propagation of Pitting Corrosion Fe + 2H+ = Fe2+ + H2 …(3. When the metal corrodes in the pit. 42 . thereby causing corrosion failure in a short time. sulfides.g. Initiation sites may be non-metallic inclusions. pitting attacks often penetrate at a high rate.2. This will accelerate localized attack and pits will develop at the anodic spots.1. but may have larger cross-section areas deeper inside the metal.g. Equipment should be designed for complete drainage avoiding areas that retain standing solution. dissolved metal ions generate an environment with low pH and chloride ions migrate into the pit to balance the positive charge of the metal ions. The electrolyte inside the growing pit may become very aggressive (acidification) which will further accelerate corrosion.5) Pitting corrosion occurs when a passive film or another protective surface layer breaks down locally. As a result. e. a pitting attack often remains undiscovered until it causes perforation and leakage. Since the attack is small at the surface and may be covered by corrosion products. etc. Elimination of stagnant areas because this area can be a cathodic site. The pits often appear to be rather small at the surface.3 Prevention of Pitting Corrosion Selection of an alloy. Chloride ions facilitate a local breakdown of the passive layer.Corrosion Tutorial Finally the oxidation of Fe(OH)2 to Fe(OH)3 can be written also: 4Fe(OH)2(aq) + O2 + 2H2O = 4Fe(OH)3(s) …(3. After this initiation (local breakdown of the film) an anode forms where the film has broken. or deposits formed by slag. Nickel Alloy). micro crevices caused by coarse grinding. which is resistant to pitting corrosion in the chloride medium (e. suspended solids. especially if there are imperfections in the metal surface. Pitting occurs mainly in the presence of neutral or acidic solutions containing chlorides or other halides. 3. while the unbroken film (or protective layer) acts as a cathode. Thus the environment inside a growing pit gradually becomes more aggressive and repassivation becomes less likely. 3.2. Wood.2. Contact between metallic and non-metallic surfaces can cause crevice corrosion as in the case of a gasket. asbestos. plastics.htm) Environmental factors affecting crevice corrosion are sand. dirt. But these two are differentiated.ksc. This form of corrosion is called crevice corrosion or deposit or gasket corrosion. The deposit could also be a permeable corrosion product.2 Crevice Corrosion Crevice corrosion is similar to pitting corrosion in the initiation and propagation mechanisms. Pit corrosion occurs on the open surface of metal while crevice corrosion occurs on hidden areas of the metal surface. rubber. concrete.Corrosion Tutorial 3. These deposits act as a shield and create a stagnant condition.1 Initiation and Propagation of Crevice Corrosion 43 .gov/html/corr_forms. and surface deposits under bolt and rivet heads (Figure 3-18).2. glass. The crevice corrosion is usually associated with small volumes of stagnant solution retained on gasket surfaces.nasa. corrosion products and other solids. lap joints. Figure 3-18: Crevice Corrosion (Reference: http://corrosion. wax and fabrics are examples of materials that can cause this type of corrosion. Oxygen is dissolved into water and reduced to consume electrons supplied by the anodic reaction of steel corrosion at the hidden crevice site. O2 OH- e - Fe2++2H2O=Fe(OH)2+2H+ Fe+2H+=Fe2++H2 Fe=Fe2++2e- Crevice Figure 3-19: Mechanism of Crevice Corrosion Figure 3-19 shows a connection of two plates by a bolt.Corrosion Tutorial The basic mechanism of initiation and propagation of crevice corrosion are basically the same as those of pitting corrosion. The mechanism of crevice corrosion is illustrated in Figure 3-19. The difference is that the cathodic site of the crevice corrosion is a small amount of water retained by gasket or similar materials. which can serve as a cathode site. more chloride ions are attracted and help propagate the crevice corrosion. 44 . The upper left hand side corner retains a small amount of water. the anodic half-cell reactions cannot take place because of the formation of passive film. This anodic site has high concentration of iron and is unsuitable for cathodic site because oxygen cannot be dissolved in the high concentrated solution. After initiation of crevice corrosion. Otherwise. Corrosion Tutorial Figure 3-20: crevice corrosion in rivets (Reference:http://www. of steel bridges which are already showing signs of rust between steel plates.htm) 3.2 Prevention of Crevice Corrosion Crevice corrosion should be prevented at the design stage itself. This particular form of corrosion is often used in relation to bridge inspection to describe built-up members.2. Figure 3-21: A crevice formed into an open atmosphere 45 .2.3 Pack Rust Pack rust is a form of corrosion typical in steel components that develop crevice in to an open atmospheric environment.corrosion-doctors.2. 3.org/Localized/Crevice. Inspection of equipment and removing deposits frequently. • • Design vessels for complete drainage to avoid sharp corners and stagnant areas. gold. silver.org/Bridges/Pack-rust. Filiform corrosion has been observed on steel.Corrosion Tutorial Figure 3-22: Example of Pack Rust (Reference:http://www. The mechanism of filiform corrosion is depicted in Figure 3-23. magnesium and aluminum surfaces covered by tin. surface appearance. O2+H2O OH- O2+H2O Fe2++2H2OÆ Fe(OH)2+2H + eOHFe(OH)3 O2+H2O Fe2+ H+ Fe2+ Fe(OH)2 Steel e- e- Figure 3-23: Mechanism of Filiform Corrosion 46 .4 Filiform Corrosion Filiform Corrosion is a special type of crevice corrosion. The filament has two ends: one is head and the other is tail. phosphate.corrosion-doctors. enamel and lacquer coatings. since it does not weaken or destroy metallic components but only affects the It appears under thin coatings. defects of the coatings and propagates in the form of thread like filaments. Filiform corrosion is an unusual type of It initiates on the scratched attack.htm) 3. The head is anodic site where metal is dissolved while the tail is cathodic where oxygen is reduced.2. Corrosion Tutorial A special form of oxygen concentration cell is formed on metal surfaces having an organic coating leading to filiform corrosion. and for relative humidity higher than 90 percent. It has a distinguished pattern and is known by its characteristic worm-like trace. Polyurethane finishes are especially susceptible to filiform corrosion. Filiform corrosion can be removed using glass bead blasting material with portable abrasive blasting equipment or sanding. . using coating systems having a low rate of diffusion 47 . If the relative humidity is lower than 65 percent. the metal is unaffected. The effected area should be treated properly and a good protective finish should be applied. microscopic defects in the coating and then grows out into a network. Lacquers and "quick-dry" paints are most susceptible to this problem. Filiform corrosion normally begins as small. but they will cross under one another on aluminum which makes the damage deeper and more severe for aluminum. This corrosion usually attacks steel and aluminum surfaces. This could be seen especially around fasteners and at seams. The traces never cross on steel. If the corrosion is not removed and allowed to grow. This corrosion starts under the painted surface when moisture permeates the coating on the metal. Their use should be avoided unless absence of an adverse effect has been proven by field experience. Zinc-rich coatings should be considered for coating carbon steel because of their cathodic protection quality. The paint or plating should have certain properties that could sustain or avoid any causes that lead to this type of corrosion. The most important environmental variable in filiform corrosion is the relative humidity of the atmosphere. Filiform corrosion can be prevented by storing aircraft in an environment with a relative humidity below 70 percent. Low water vapor transmission and excellent adhesion are some of the characteristics that are to be exhibited by the coatings that are to be used. Zinc-rich coatings should also be considered for coating carbon steel because of their cathodic protection quality. the corrosion can lead to intergranular corrosion. Filiform corrosion occurs when the relative humidity of the air is between 78 and 90 percent and the surface is slightly acidic. corrosion primarily appears as blistering. g.nasa.1 Prevention of Filiform Corrosion • • Controlling the humidity to lower values (e. titanium or copper do not exhibit filiform corrosion.pdf) Figure 3-25: Filiform Corrosion Causing Bleed Through a Welded Tank (Reference: http://corrosion.faa.Corrosion Tutorial for oxygen and water vapors.gov/avr/afs/300/pdf/2g-CH6_2.gov/html/filicor. Corrosion resistant substrates of stainless steel. Figure 3-24: “worm like” filiform corrosion tunnels. and by washing the aircraft to remove acidic contaminants from the surface. 48 .4. (Reference:http://www.ksc.2.htm) 3. less than 65%). 3.2. biological organisms are the sole cause or accelerating factor in the localized corrosion. The reason behind this is that organisms do not form continuous film on the metal surface. corrosion fatigue cracking (CFC) and hydrogen embrittlement or hydrogen induced cracking (HIC). 49 . EIC includes stress corrosion cracking (SCC).5 Localized Biological Corrosion In this case. The large fouling organisms in marine environments settle as individuals. 3.1 Stress Corrosion Cracking Stress corrosion cracking (SCC) is due to the low static tensile stress of an alloy exposed to a corrosive environment.Corrosion Tutorial • Coatings and inhibitors eventually retard corrosion to some extent. Figure 3. a susceptible alloy and some component of tensile stress. but not completely. Three conditions must be present together to produce SCC: a critical environment.3 Environmentally Induced Cracking Environmentally Induced Cracking (EIC) is due to brittle mechanical failures that result in presence of tensile stress and a corrosive environment. and it is often a period of months or even years before a complete cover is built up.3. 3.9 shows SCC of aluminum plate with branched cracks. Microscopic organisms also tend to settle on the metal surfaces in the form of discrete colonies or spots rather than continuous films. probably because chromium in the alloy may react with chloride ions to form CrCl3.gov/html/corr_forms. Transgranular failures are less common than intergranular ones. Sometimes.3. The cracks are usually produced by SCC by normal to the tensile component of stress.Corrosion Tutorial Figure 3-26: Stress Corrosion Cracking Showing Branched Cracks in Aluminum Plates (Reference: http://corrosion.ksc. SCC usually occurs where the passive films are less stable.3. but both may exist in the same system. Chloride ions cause SCC to austenite stainless steels.27 50 .1 Metallurgical Effects Pure metals are more resistant to SCC than alloys of the same base metal.1. For example. 3.nasa.htm) 3. ammonia reacts with copper in brass to cause SCC because ammonia can form various complex ions with cupric ion. or even in the same failed part.1. Intergranular failure suggests that some inhomogeneity exists at the grain boundaries. Higher strength alloys are more susceptible to SCC. The regions near the two ends of the passive zone as shown in Figure 3. specific chemicals react with alloy components to make the alloy more brittle which develops cracks.2 Electrochemical Effects Electrochemical potential has a critical effect on stress corrosion cracking (SCC). depending on the conditions. but they are not completely immune. Hydrofluoric acid may react with nickel and copper in the nickel-copper alloy (Monel alloy 400) because fluoride forms compounds of NiF2 and CuF2. Corrosion Tutorial ipass (passive current) Etp transpassive passive Oxygen evolution E icc (critical current) active Ecorr (corrosion potential) Epp (passivation potential) Log i Figure 3-27: Schematic of Active-Passive Behavior of the Anodic Polarization of a Metal In the upper zone. environment or susceptible alloy. In lower zone.3 Prevention of Stress Corrosion Cracking Prevention of SCC requires elimination of one of the three factors: tensile stress. SCC and pitting are associated in adjacent or overlapping potential ranges. SCC occurs where passive film is relatively weak. 3.3. the magnitude will be reduced by the shotpeening. Shotpeening is a process in which small. • When a tensile stress is applied. 51 .1 to 1. hard particles of 0.1.0 mm are projected at high velocities onto the surface to increase the compressive stress. which is resistant to the particular environment. which may fail by HIC when cathodic protection is applied. sulfide cracking and sulfide corrosion cracking. there will be decrease in its ductility. LME requires an incubation period for the liquid metal to penetrate the oxide or passive layers. but will accelerate HIC. Like local de-passivation prior to stress corrosion cracking. Annealing may be impractical for some stainless steels. sulfidic environments undergoes corrosion damage.Corrosion Tutorial • Removal of residual stress may be accomplished by stress-relief annealing. 3. • • • • • Reducing the oxidizing agents. This causes brittle failures in steels and other high strength alloys. Small amounts of liquid metal are sufficient to result in embrittlement. The decrease in ductility can result in brittle failure of a normally ductile material.2 Sulfide Stress Cracking (SSC) When metals and metal alloys come into contact with moist hydrogen sulfide. Cathodic protection will usually stop SCC. commonly known as sulfide stress cracking. The acidic effect of the sulfides combined with stress in the environment leads to this type of corrosion. Inhibitors are also effective in limited situations.3 Liquid Metal Embrittlement (LME) When liquid metal comes into contact with a metal. which sensitize and become susceptible to intergranular attack. Choosing an alloy. LME shows many characteristics of both stress corrosion cracking and hydrogen embrittlement corrosion. Caution must be exercised with higher strength alloys.3. Tensile strength can also be reduced along with the ductility of the metal. This phenomenon is called liquid metal embrittlement. 52 . SSC is also called hydrogen sulfide cracking.3. 3. Changing the composition of the alloying elements for lower strength. hgtech. However reduction in mechanical properties due to de-cohesion can occur. welding.4 Solid Metal Induced Embrittlement The embrittlement caused above the melting point of the embrittler is Liquid metal embrittlement and the same phenomena below the melting point are pertained as Solid metal embrittlement.htm) Intergranular or transgranular cleavage fracture is the common fracture modes associated with liquid metal embrittlement. LME can be considered as a prerequisite for the occurrence of SMIE. Some events that may permit liquid metal embrittlement under the appropriate circumstances are brazing. The susceptibility to SMIE is stress and temperature sensitive and does not occur below a 53 .3. In addition to an event. crack initiation at the interface is the necessary conditions for SMIE to occur. Figure3-28: Liquid Metal Embrittlement (Reference:http://www. the presence of tensile stress.com/Corrosion/Hg%20LME. Hence a ductile fracture mode results occurring at reduced tensile strength. Intimate contact between the solid and the embrittler. soldering. Notched tensile specimens of various steels are embrittled by solid cadmium. heat treatment and hot working. it is also required to have the component in contact with a liquid metal that will embrittle the component.Corrosion Tutorial Liquid metal embrittlement can occur at loads below yield stress and thus failure can occur without significant deformation or obvious deterioration of the component. 3. html) 3.5 Corrosion Fatigue Cracking Corrosion fatigue cracking (CFC) is a brittle failure of an alloy caused by fluctuating stress in corrosive environment. (Reference:http://www. But SMIE requires multiple cracks to propagate. a single crack usually propagates to failure.hghouston.html) Figure 3-30: Brittle crack in a cadmium plated B7 bolt from solid metal induced embrittlement (Reference:http://www. In LME. This type of corrosion occurs because the cyclic stresses do not give enough time for the metal to recover its structural integrity.com/x/57.Corrosion Tutorial specific threshold value. The crack propagation rate depends on the frequency and cyclic amplitude of the 54 .hghouston.3. Figure 3-29: Solid Metal Induced Embrittlement of a cadmium plated B7 bolt.com/x/56. Corrosion products are usually absent in case of SCC whereas it is present in CFC.2 Prevention of Corrosion Fatigue Cracking Corrosion fatigue cracking (CFC) can be minimized using inhibitors. cathodic protection. The susceptibility to corrosion fatigue cracking increases due to stress raisers such as notches. 3. The frequency of cyclic stress is important in CFC.1 Comparison with Stress Corrosion Cracking • • • • • CFC is similar to SCC in that a corrosive solution induces brittle fracture in an alloy that is normally ductile in non-corrosive environment. CFC cracks propagate perpendicular to the principal tensile stress.5. but at high temperatures when creep is possible.3. CFC cracks propagate perpendicular to the principal tensile stress whereas SCC cracks propagate normal to the tensile stress. CFC cracks are usually transgranular and blunt whereas SCC cracks are usually transgranular or intergranular and sharp. R has its effects. Lower frequency leads to greater crack propagation per cycle. 55 . At ambient temperature the stress ratio R has no effects. and reduction of oxidizers or increase in pH. Removal of cyclic stress by designing will prevent CFC. of the minimum to the maximum stress in the cycle generally decreases the resistance to corrosion fatigue cracking. CFC occurs due to cyclic stress whereas SCC occurs due to static stress. Increasing the ratio R. 3.Corrosion Tutorial stress. Shotpeening increases the resistance to CFC.5. Very high frequencies eliminate the effects of the corrosive environment.3. 56 . the cracks will be intergranular for HIC. niobium and tantalum. SCC cracks are mostly intergranular whereas the HIC cracks are usually transgranular.3. vanadium. SCC cracks are usually branched.1 Comparison with Stress Corrosion Cracking HIC is similar to stress corrosion cracking in that brittle fracture occurs in a corrosive environment under constant tensile stress. The presence of atomic hydrogen results in decrease in the toughness or ductility. HIC has been recognized classically as being of two types.Corrosion Tutorial 3. Failures by HIC are usually maximized at/or near room temperature. aluminum and nickel are more resistant to HIC because of their inherent high ductility and low diffusivity for hydrogen. susceptible to HIC. which are embrittled by insoluble hydrides. But reactive alloys of titanium. the face centered cubic (FCC) stainless steels and FCC alloys of copper. Cathodic protection aggravates HIC but suppresses or stops SCC. hydrogen in the ferrite is supersaturated and thus leads to embrittlement of the metal. The second type of HIC results from hydrogen being absorbed by solid metals.3. whereas CFC cracks are unbranched. The first is known as blistering. forming microscopic voids (blisters). The solubility of hydrogen in austenite is higher than that in ferrite.6.6 Hydrogen Induced Cracking HIC usually occurs in hardened or high stressed steels. are 3. zirconium. However. nascent hydrogen (H atom) which will recombine to form molecular hydrogen gas. If the alloys are cold worked. Accumulations of this gaseous molecular hydrogen develop high pressures sufficient to rupture inter-atomic bonds. Upon forming ferrite from austenite below their phase transformation temperature of 723 °C. which causes HIC. When hydrogen builds up at metallurgical inhomogeneities (traps). This type is especially prevalent in iron alloys because of the solubility difference of hydrogen in ferrite (BCC) and austenite (FCC). Metals that are soft such as copper and lead are quite susceptible to erosion corrosion. Sometimes passive films are developed on the metal surface. aqueous additions. All types of equipment exposed to moving fluids are subject to erosion corrosion. Some of these are piping systems. which are more resistant to HIC such as FCC stainless steel and copper.2 Prevention of Hydrogen Induced Cracking • • Selection of alloys. which has high corrosion resistance such as aluminum. centrifugals. impellers. In this velocity. hydrodynamics. rounded holes.1 Erosion Corrosion Erosion corrosion is due to the relative movement between a corrosive fluid and the metal surface. 57 . are gases. and usually exhibits a directional pattern. lead and stainless steels.6. 3. elbows and tees. The corrosive media. and valleys. Stop cathodic protection.3. and liquid metals. etc. agitators etc and equipments subjected to spray. Erosion corrosion results when these protective surfaces are damaged. play a major role and has a significant effect in the corrosion behavior. valves. abrasion. such as bends. Erosion corrosion is characterized in appearance by grooves.4 Mechanically Assisted Degradation Mechanically assisted degradation of metals is defined as the degradation which involves both corrosion mechanism and a wear or fatigue mechanism. 3. waves. which cause erosion corrosion. blowers. organic systems. propellers.Corrosion Tutorial 3. gullies.4. pumps. It is not the result of mechanical erosion of the metal itself but is the result of removal of the film of corrosion products by erosion which is ordinarily protective at lower velocities.htm) 58 .Corrosion Tutorial 3. 3.1. Avoid sharp elbows and angles in the design of equipment. It may be accelerated by entrained gas bubbles.1 Prevention of Erosion Corrosion • • Choose materials with better resistance to erosion corrosion such as aluminum and stainless steel. Impingement corrosion is a form of erosion corrosion associated with impingement action of liquids.dk/Html/alulib/modul/A00115.alu-info.4. More specifically it is caused by the impingement action of water carrying entrained gas bubbles and striking the metal surface at an angle. Figure 3-31 Impingement corrosion in a bent tube (Reference:http://www.4.2 Impingement Corrosion Impingement is a process resulting in a continuing succession of impacts between particles and a solid surface. This type of corrosion can be seen at the suction of a pump. Parts like pump casings. It is desirable to separate out solids. 3.1 Prevention of Impingement Corrosion Suitable resistant material (harder materials) is to be chosen. at pipe elbows and expansions. This form of corrosion will eat out the volutes and impellers of centrifugal pumps. Cavitation should be designed out by reducing hydrodynamic pressure gradients and designing to avoid pressure drops 59 .Corrosion Tutorial Impingement corrosion is usually seen at or near the entrance of the tubes and in bends. This depassivation results in an accelerated corrosion and causes gas pockets and bubbles to form and collapse. These bubbles may result from boiling phenomena or may arise because of the release of dissolved gases from the fluid as a result of pressure drops. Design. nozzles and valve seats.4. High flow velocities and the entrainment of solid particles are the main causes of erosion corrosion. water or gas early in a flow system in order to avoid two phase flow.2. tubes are some of the examples where impingement corrosion can be seen.4. at the discharge of a valve or regulator. 3. shape and geometry are some of the aspects that are to be considered. pump shafts and impellers. Bubble implosions are responsible for the breakthrough of the passive film in cavitation corrosion.3 Cavitation Corrosion Cavitation corrosion occurs under conditions of severe turbulent flow and rapid pressure changes. The cavitation process is responsible for the breakdown of the protective surface film on the metal. It takes the form of pitting or grooving and eventual perforation of the wall at that location while the remainder of the tube shows no sign of corrosion. Inhibitors and coatings are to be applied. As a result of surface initiated fatigue. Bolted assemblies and ball bearings are most vulnerable cases of fretting corrosion. This rubbing occurs at small amplitudes. wear particles break out of the material. fatigue cracks are initiated by high shear stresses. 3. Parameters that need to be controlled in fretting corrosion evaluations include corrosive environment. Increased surface hardness and use of lubricants are some of the preventive methods to be used for fretting corrosion. Contact surfaces exposed to vibration during transportation undergo the effects of fretting corrosion.4. The metal surface gets exposed to the atmosphere. 60 . while the oxide film is removed due to this effect. which in severe cases may lead to immediate bearing failure. Resilient coatings and cathodic protection can be considered as control methods.Corrosion Tutorial below the vapor pressure of the liquid and air ingress.4 Fretting corrosion The rapid corrosion that occurs at the interface between contacting highly loaded metal surfaces which are enhanced through slight vibratory motions. become trapped between the surfaces and oxidize. contact load. amplitude and frequency of load fluctuations. these oxidized particles prevent free axial displacement and introduce increased load in the bearing. While the rubbing motion continues. Large pressure changes should be avoided and surface layers should be hard to prevent from cavitation corrosion. temperature and availability of moisture. It is caused by the combination of corrosion and the abrasive effects of corrosion product debris often seen in component with moving or vibrating parts. Apart from causing dismounting problems. Pits or grooves and oxide debris characterize this damage. Bearing loads has to be reduced on mating surfaces. 1 Exfoliation / Lamellar Corrosion Exfoliation corrosion is a special form of intergranular corrosion. Alloys that have been extruded or worked heavily undergo this type of damage. 61 . chromium precipitates as chrome carbide in the grain boundaries. The metallurgical influences considered are the relative stability of the component alloys.5. It is usually seen in high strength aluminum alloys and carbon steels. the metallic surface grains get lifted up and results in this form of corrosion. It may progress through an entire section. metalloid phases and local variations in composition in a single phase. An example is stainless steel upon extended exposure to temperatures between 400 and 510 °C. 3. This is because chromium is the element that can reduce the corrosion by forming the passive film. metallic phases. which depletes the element and thus causes intergranular corrosion along the grain boundaries.1 Intergranular Corrosion Intergranular corrosion (IGC) can be caused by depletion of one of the alloying elements in the grain boundary areas.1. holes and grooves.5. Corrosion products building up along these grain boundaries exert pressure between the grains. In this case.5 Metallurgically Influenced Corrosion This is classified as a result of the significant role that metallurgical factors affect corrosion. The damage often begins at end grains seen in machined edges. 3. typically during welding. By the force of expanding corrosion products at the grain boundaries.Corrosion Tutorial 3. Thus a leafing effect can be observed eventually giving a layered appearance to the metal. htm) Figure 3-33:Exfoliation of aircraft component (Reference:http://www.2 Weld Decay Sensitization of austenitic stainless steels during welding is known as weld decay.corrosion-doctors. at the interface of base metal and weld the temperature was too high to sensitize the alloy.5.1. However.ksc.Corrosion Tutorial Figure 3-32: Exfoliation of Aluminium (Reference:http://corrosion. areas next to the 62 .nasa. During welding of austenitic stainless steel.htm) 3.gov/html/intercor.org/Forms/exfol-examp. 1 • Prevention of Weld Decay Solution annealing is heat treatment of an alloy to high temperature just below its melting point to dissolve the alloying elements. followed by quenching to prevent the precipitation of chromium carbides.5. 3.2. This happens in environments where the alloy should exhibit excellent corrosion resistance.1. However. in the temperature range between these two temperatures.1. the diffusion rate of carbon is too low to permit the formation of carbides. conditions are optimum to precipitate chromium with carbon. they become sensitized or susceptible to intergranular corrosion. precipitates as chromium carbide leaving the grain boundary areas with less chromium.5. When these steels are heated in approximately the Here sensitizing means that chromium temperature range of 425 °C to 815 °C (800 to 1500 °F). Above 815 °C. • • Low carbon alloy modifications allow a minimum reduction of chromium by precipitation along the grain boundaries.3 Sensitization (Intergranular Corrosion of Austenitic Stainless Steels) Numerous failures of 18-8 stainless steels have occurred because of intergranular corrosion. Stabilized alloys containing niobium and titanium in austenitic stainless steels will not reduce chromium content because these elements will react with carbon in the sensitization temperature range. 63 .Corrosion Tutorial base metal/weld are sensitized since it reaches the critical temperature range of 425 °C to 815 °C. Thus IGC occurs along this weld decay zone. the chromium carbides are soluble and below 425 °C. 3. and 7000 series alloys) can also have this problem. Figure 3-35: Intergranular Corrosion of 7075-T6 aluminum adjacent to steel fastener (Reference:http://www. Heat-treatable aluminum alloys (2000. This corrosion can be eliminated by using stabilized stainless steels (321 or 347) or by using low-carbon stainless grades (304L or 3I6L).faa.gov/avr/afs/300/pdf/2g-CH6_2. 6000.Corrosion Tutorial Figure 3-34: intergranular corrosion in stainless steel Figure 3-34 shows a stainless steel which corroded in the heat affected zone a short distance from the weld. This is typical of intergranular corrosion in austenitic stainless steels.pdf) 64 . Dealloying can be controlled by the use of more resistant alloys such as inhibited brasses and malleable or nodular cast iron. Usually when the zinc content is less than 15 weight percent. gray cast iron.5. For example.6 High Temperature Corrosion Corrosion can occur when metal is exposed to an oxidizing gas at high temperatures.3.2 Dealloying Dealloying or selective leaching is the removal of one or more active element(s) from the alloy. Dealloying is a form of corrosion found in copper alloys.Corrosion Tutorial 3.3 Dezincification Dezincification occurs when the more active element of zinc in an alloy is preferentially leached in a corrosive environment leaving the copper behind. and some other alloys. but in most cases these methods are not economical. 3. Common yellow brass consists of 30% zinc and 70% copper.5. red brass (15% Zn) is almost immune. One is uniform or layer type and the other is localized or plug-type. Dezincification is readily observed with the naked eye because the alloy assumes a red or copper color contrasting with the original yellow. There are two types of dezincification and both are readily recognizable. 3. This form of corrosion that does not require the presence of a liquid 65 .1 Prevention of Dezincification Dezincification can be minimized by reducing the aggressiveness of the environment or by cathodic protection. 3. the brass is immune due to dezincification. Further improvement can be obtained by adding small amounts of arsenic. antimony or phosphorus as inhibitors. A preferable method is to select a resistant brass. Another method is the development of better brasses with the addition of 1% tin to a 70-30% brass.5. This behaves as a solid electrolyte. 3. or a combination of these. When sulfur activity of the gaseous environment is sufficiently high. oxidation participates in the high temperature corrosion reactions. Regardless of the predominant mode of corrosion.2 Sulfidation Sulfidation is defined as the reaction of a metal or alloy with a sulfur-containing species to produce a sulfur compound that forms on or beneath the surface on the metal or alloy. which may eventually serve to initiate or propagate cracks under thermal cycling conditions. It is also called as dry corrosion.Corrosion Tutorial electrolyte is known as high temperature corrosion. aerospace and gas turbine. high temperature oxidation and scaling. power generation (nuclear and fossil fuel) etc. These protective oxide scales are formed at all the surface discontinuities. Alloys intended for high-temperature applications are designed to have the capability of forming protective oxide layers. High temperature corrosion is a widespread problem in various industries such as heat treating. 3. rate of attack also increases. Hence there is a possibility that notches of oxides are formed at occluded angles in the surface. alloying requirements for the production of specific oxide scales have been translated into minimum levels of scale-forming elements. possibly carbides. but also be sulfides. As temperature increases. tarnishing. The surface film thickens as a result of reaction at the scale-gas or metal-scale interface due to the ion transfer through the scale. wherever the alloy surface is exposed to the ambient environment.6. 66 . Depending on the base alloy composition and the intended service temperature.1 Oxidation Oxidation is the most important high temperature corrosion reaction. High temperature scales are usually thought of as oxides.6. Corrosion Tutorial sulfide phases. nickel and the rare earths have been noted as promoting resistance to carburization. Carburization is based on carbon transport across the metal/gas interface which is very slow. molybdenum. In particular silicon (1. Sulfidation attack mainly occurs at sites where the protective oxide has broken down. 3. titanium. Carbon has large influence on the mechanical properties of the steel like hardness. cobalt. whereas lead. depleting the metal matrix locally of chromium and making it more sensitive to corrosion. These base metal sulfides are responsible for the accelerated attack as they grow much faster than the chromium or aluminum oxides or sulfides and have low melting points. Once sulfides have formed alloys they have a tendency to get oxidized to form new sulfides in grain boundaries or at the sites of chromium or aluminum. Carbon forms carbides (like Cr23C6. zirconium and borium are considered detrimental.5 to 2.3 Carburization Carburization is defined as the increase of the carbon content of steel due to interactions at elevated temperatures with the environment. there is little danger of accelerated attack. As long as the sulfur is present in small amounts as sulfides. Cr3C 2 or Cr7C3).6. carbon monoxide and hydrocarbons. niobium. Carburization therefore results in the formation of a hard top layer that is more brittle than the core material. It occurs kinetically in many carbon-containing environments like carbon. and alloy pretreatments likely to promote such scales forms smooth 67 . If sufficient sulfur enters the alloy so that the available chromium and aluminum is converted to sulfides.0%). instead of oxide phases can be formed. tungsten. Minor alloying elements can exert an influence on the susceptibility to carburization of various alloys. then the less stable sulfides of the base may form due to different morphological and kinetic reasons. strength etc. which act to localized stress or reduce the load-bearing section. Corrosion Tutorial surfaces or pre-oxidation. This oxide film will decrease the carburization attack. Because of the high solubility of carbon in austenite, austenitic steels carburize more readily than ferritic steels. 68 Corrosion Tutorial 4 AGING OF COMPOSITES 4.1 Introduction A composite material is a complex solid material, composing of atleast two materials, that, when combined, remain discrete but function interactively to form a useful material. The composite is designed to exhibit the best properties that cannot be predicted by simply summing the properties of its constituents. This combination of materials is done by physical means unlike the chemical bonding that takes place in the alloys of monolithic materials etc. Composites are made of fiber reinforcements, resin, fillers, and additives. The fibers provide stiffness and strength. The resin offers high compressive strength and binds the fibers together into a matrix. The fillers serve to reduce cost and shrinkage. The additives help to improve not only the mechanical and physical properties of the composites but also workability. A true composite might be considered to have a matrix material completely surrounding its reinforcing material. The matrix holds the reinforcement to form the desired shape while the reinforcement improves the overall mechanical properties of the matrix. After designed properly, the combined material produces characteristics not attainable by either constituent acting alone. 4.2 Composition Composites are composed of resins, reinforcements, fillers, and additives. Each of the above mentioned constituents play a vital role in the processing of the final product. Resin holds the composite together thus influencing the physical properties of the final product, while the reinforcement provides the mechanical strength. The fillers and additives are used to impart special properties to the final product. 69 Corrosion Tutorial 4.2.1 Fiber Reinforcements The fiber is an important constituent in composites. The fiber generally occupies 30% - 70% of the matrix volume in the composites. The primary function of fibers is to carry load along the length of the fiber to provide strength and stiffness in one direction. The fibers can be chopped, woven, stitched, and/or braided. They are usually treated with sizing such as starch, gelatin, oil or wax to improve the bond as well as binders to improve the handling. The most common types of fibers used in advanced composites for structural applications are the fiberglass, aramid, and carbon. The fiberglass is the least expensive and carbon being the most expensive. The cost of aramid fibers is about the same as the lower grades of the carbon fiber. a. Random fiber (short fiber) reinforced composites b. Continuous fiber (long fiber) reinforced composites Figure 4-1: Fibers as the reinforcement (Fibrous Composites) (Ref: http://www.efunda.com/formulae/solid_mechanics/composites/comp_intro.cfm) 70 Corrosion Tutorial 4.2.2 Resin Systems The resin is another important constituent in composites. The primary function of resin is to transfer stress between the reinforced fibers and protect them from environmental damage. A thermoplastic remains solid at room temperature and becomes soft when heated. It may be shaped or molded while in a heated semi-fluid state and become rigid when cool. On the other hand, thermoset resins are liquids or low melting point solids in their initial form. Once cured, solid thermoset resins cannot be converted back to their original liquid form. Thermosetting resin will cure permanently at elevated temperatures. This characteristic makes the thermoset resin composites very desirable for structural applications. The most common resins used in composites are the unsaturated polyesters, epoxies, vinyl esters polyurethanes and phenolics. 4.2.3 Fillers It is cost effective to fill the voids in a composite matrix purely with resin as the resins are expensive. Hence fillers are added to the reisn matrix to control the material thus improving its mechanical and chemical properties. The three major types of fillers used in the composite industry are the calcium carbonate, kaolin, and alumina trihydrate. Some other common fillers are mica, feldspar, wollastonite, silica, talc, and glasses. 4.2.4 Additives Different varieties of additives are used to improve the material properties, aesthetics, manufacturing process and performance. The additives are divided into different groups – catalysts, promoters, and inhibitors; coloring dies; and releasing agents. The most common man-made composites can be divided into three main groups: 71 Corrosion Tutorial Polymer Matrix Composites (PMC’s) – These are also known as Fiber Reinforced Polymers (or Plastics). They use a polymer based resin as the matrix, and variety of fibers such as glass, carbon, and aramid as the reinforcement. These ate the most common used composites. Metal Matrix Composites (MMC’s) – These materials use a metal such as aluminum as the matrix and fibers such as silicon carbide is used as the reinforcement. This is mainly used for automotive industry. Ceramic Matrix Composites (CMC’s) - Used in very high temperature environments, these materials use a ceramic as the matrix and reinforce it with short fibers, or whiskers such as those made from silicon carbide and boron nitride. 4.3 Fiber Reinforced Polymers The mechanical properties of resins such as epoxies, polyesters are not very high when compared to metals and hence they have limited use for the manufacture of their own structure. But their ability of ease formation into complex shapes makes them more desirable. Glass, aramid and boron have high tensile and compressive strengths but in solid form, these properties are not apparent because when stressed, surface flaws will cause the material to crack and fail before its breaking point. In order to overcome this problem, the material is produced in the fiber form, thus restricting the flaws to a small number of fibers while the remaining exhibiting the material’s strength. Therefore a bundle of fibers reflect the optimum performance more accurately. However, fibers alone can only exhibit tensile properties along the fiber’s length. Inorder to obtain exceptional properties, resin systems are combined with reinforced fibers such as glass, aramid, carbon etc. The resin matrix protects the fibers from damage caused by abrasion and impact and also distributes the load applied to the composite between each of the individual fibers. Since the Fiber Reinforced Composites combine a resin system and reinforcing fibers, the properties of the resulting composite will combine some properties of the resin on its own with that of the fibers, which are summarized as shown in the Figure 4-2. 72 90o. Important considerations for the design of composite products are as follows: • • • • • • • • Type of fiber reinforcement Fiber volume fraction Orientation of fiber (0o. In addition to the manufacturing process to fabricate the product.Corrosion Tutorial Fiber Tensile Stress FRP Composite Resin Strain Figure 4-2: The combined effect on Modulus of the addition of fibers to a resin matrix. the type and quantity of materials will also affect the mechanical properties and performance.45 o or a combination of these) Type of resin Cost of product Manufacturing process Volume Production Service conditions Composites are manufactured using two major processes. +/. They are: open mold process and closed mold process. The mechanical properties and composition of FRP composites can be tailored for their intended use. Types of open mold processes are hand lay up and 73 . They increase chemical wear and corrosion resistance. They increase secondary uses and recyclability.g. compression molding. They increase toughness (impact strength). Closed mold processes include. They maintain strength/stiffness at high temperatures while under strain conditions in a corrosive environment. They decrease water absorption. They increase some electrical properties (e. They increase mechanical damping. They are used in defense equipments. They decrease thermal expansion. • • • • • • • • • • • • • • • • • • • • They have high stiffness. and to reduce any negative impact on the environment. increase electrical resistivity). infrastructure. Composites when compared to conventional materials are expensive. This is because of the advantages offered by composites in comparison to conventional materials. Potential for real-time monitoring. vacuum assisted resin transfer molding (VARTM). They are non-magnetic and have high dielectric strength (insulator). They are tailored to loading conditions to optimize structural performance. They have low density. or dimensional stability. strength. They improve design flexibility. resin transfer molding (RTM). They are easy to handle. They increase heat-deflection temperature. pultrusion.Corrosion Tutorial tube rolling. 74 . but are still widely used in various applications. medical equipment etc. They have good fatigue response and damage tolerance. extrusion and filament winding process. Radar Transparency. Tailored surface finish.. resin injection molding. They reduce permeability to gases and liquids. acid or aqueous solutions. cyclic loads and environmental conditions. and moisture diffusion/plasticization. thermal behavior (thermal coefficient and conductivity). 75 . fiber degradation. shear and combinations). Some of the causes of corrosion/aging of composites are absorption of solvents. over traditional structural materials. or fiber-matrix interface degradation. Aging of composites may depend on environmental factors on the composite material when exposed for a period of time and type of liquids. etc.3. fatigue. changes with exposure time take place in the mechanical properties such as strength. These service conditions render environmental effects. creep. which are to be considered in design. composite laminates also have inherent weaknesses. fatigue and creep or when it is exposed to chemical solutions like alkaline. such as susceptibility to impact damage and long-term aging. bending. creep.1 Characteristics of Fiber Reinforced Composites Fiber-reinforced polymer (FRP) composites offer many advantages. Short term behavior of FRP composites are determined to establish parameters such as mechanical resistance (axial. The response of a composite has been classified in terms of its short-term behavior and long-term behavior. This may occur as a result of the matrix degradation. and weathering effects on FRPs. The physical or chemical weathering conditions occur when a composite material is subjected to mechanical loadings such as static load. stiffness. such as enhanced corrosion resistance and high specific mechanical properties. UV radiation. and thermal degradation. An FRP component may sustain constant or repeated loading under environmental exposure for a prolonged period of time. oxidation. and fatigue life and chemical properties such as glass transition temperature. However. As a result of the above mentioned environmental factors. Long-term responses of composite materials are determined to establish the properties of aging in composites due to sustained loads.Corrosion Tutorial 4. the difference in curing and operating temperatures of the composite material may be as high as 2000F. These and other mechanical properties that are functions of temperature will be evaluated. Mechanical properties of fiber reinforced composites change when the material is exposed to elevated temperatures (370C to 1900C). The molecules dissolved in the surface layer of the polymer migrate into the bulk of the material under a concentration gradient. 3) fatigue strength and creep resistance. Water penetration into cracks or other flaws occurs by capillary flow. which is dependent upon polymer type and temperature. In cold regions. Similarly. 2) tensile and flexural strength. Matrix tensile strength reductions up to 50% may be possible because of residual stress build-up under low temperature effects. Temperature affects the rate of moisture absorption as well as the mechanical properties of a composite. decrease in temperature can lead to possible reduction in: 1) elongation and deflection. Moisture Diffusion / Plasticization: Water penetrates FRPs through two processes: diffusion through the resin. 3) compressive strength.Corrosion Tutorial Short term Mechanical and Hygro-Thermal Behavior Thermal Coefficient and Conductivity: A lower coefficient of thermal expansion of glass fibers in relation to resin produces residual stresses within the material microstructure during temperature drop. and 4) thermal coefficient. Increase in temperature may accelerate time-dependent effects such as creep and stress relaxation. Water also penetrates at the fiber-matrix interface. It is reported that the primary mechanism of moisture through the cracks is an after effect. The decrease in temperature can lead to possible increase in: 1) modulus. thus resulting in residual stresses that are high enough to cause microcracking within the matrix and the matrix-fiber interfaces. but consists of molecules or groups of molecules that are linked together by hydrogen bonds to the polymer. and 4) adhesive strength. and flow through cracks or other material flaws. Also. The increase in stiffness at low temperature is attributed to crystallization and instantaneous thermal stiffening. During diffusion absorbed water is not in the liquid form. Moisture 76 . 2) fracture toughness and impact strength. evaluation of composite systems at low temperatures is essential since high strength and stiffness degradation rate under thermal cycling is observed in cold region structures. maximum sustained stress for GFRP reinforcement embedded in concrete is limited to 20% of the failure stress. However. The relationship between survival probability (creep-rupture probability) and logarithm of creep-rupture time will be included in addition to the effects of initial sustained strains with respect to moisture absorption and fracture. Long Term Mechanical and Hygro-Thermal Behavior (Aging) Creep: Creep of composites is significant as compared to fibers particularly when sustained stress levels exceed particular threshold levels. The equilibrium content of water determines the magnitude of swelling stresses. In this report. 77 .Corrosion Tutorial pickup leads to loss of chemical energy. Different creep models are also discussed. For example. increased hydrostatic pressure reduces water uptake due to closing of micro cracks. Fatigue and Fracture: This section mainly focuses on the models that predict the fatigue life of composites and hybrids. fabric configuration and temperature. and fracture initiation. rate of hydrolysis and property degradation are reviewed as a function of different parameters such as moisture concentration. The review consists of empirical fatigue strength theories. Diffusion of water into the resin causes swelling stresses. diffusion coefficients. humidity level. The effects of moisture on long-term strength and stiffness of the FRP composites and hybrids are discussed. strength and stiffness based degradations under fatigue. The chemical composition of resin influences the solubility of water in the resin and its susceptibility to hydrolysis. which is attributed to hydrolytic scission of ester groups. Exposure of composites to moisture for longer duration results in resin (matrix) plasticization and interface bond strength reduction. composite thickness. over its 75-year service life.3. aging is a complex phenomenon. Review on durability and aging in this report includes effects of different pH conditions.. thus resulting in residual stresses that are high enough to cause microcracking within the matrix and matrix-fiber interfaces. thus compromising structural integrity and safety. i. Change in properties of polymers in the absence of load is referred to as ‘aging’.1 Thermal Coefficient and Conductivity A lower coefficient of thermal expansion of glass fibers in relation to resin produces residual stresses within the material microstructure during temperature drop. Mechanical properties of fiber reinforced composites.2. For example. Temperature affects the rate of moisture absorption as well as the mechanical properties of a composite.2 Short-Term Mechanical and Hygro-Thermal Behavior 4. Since different factors influence mechanical properties of polymer composites.e.3. In cold regions. temperature and sustained stress level on structural composites. Increase in temperature may accelerate time-dependent effects such as creep and stress relaxation. change when the material is exposed to elevated temperatures (37°C to 190°C). The increase in 78 . the difference in curing and operating temperatures of the composite material may be as high as 200°F. leading to serious malfunction of the pads and consequent failure of the structure. Matrix tensile strength reductions up to 50% may be possible because of residual stress buildup under low temperature effects. Similarly. under thermal cycling is observed in cold region structures. 4. and bond. Aging phenomenon can be very significant during the service life of strengthened concrete-frame or masonry wall. stiffness.Corrosion Tutorial Aging due to Environmental Effects: In-service FRP composites and hybrids exposed to harsh environments may lose strength. evaluation of composite systems at low temperatures is essential since high strength and stiffness degradation rate. high stiffening (50 to 100 times over ambient temperature of 78°F) of resins at low temperatures reduces the desirable movement of elastomeric bearing pads under bridge seats. 2) where vf. The resins used have thermal expansion coefficients that are positive (~10 to 20x10-6/C). Composites have two coefficients of thermal expansion.Corrosion Tutorial stiffness at low temperature is attributed to crystallization and instantaneous thermal stiffening. The decrease in temperature can lead to possible increase in: 1) modulus. Thermal Coefficient Temperature effects can induce the dimensional change of a composite. to account for creep. in addition to its mechanical deformations.1) and in the direction perpendicular to the fibers α 2 = (1 + Vf )α f Vf + (1 + Vm )α m Vm − α1V12 ……(4. Thermal mismatch between fiber and matrix may cause matrix cracks in composites under severe temperature fluctuations. vm. α1 refers to expansion in the direction of the fibers.(4. decrease in temperature can lead to possible reduction in: 1) elongation and deflection. α1 is computed according to Schapery. as is the case with carbon fibers. 3) fatigue strength and creep resistance. 3) compressive strength. Also. Such stresses can be predicted by complex FEA of the microstructure or by complex micro-mechanical models of the same 79 . and 4) thermal coefficient. and 4) adhesive strength. and the fibers can have either a low value of α or even a negative value. The thermal expansion in the direction of the fibers. α1 = 1 [α f E f (T )Vf + αmEm (T )Vm ] E1 (T ) ….. T. and v12 are the values of the respective Poisson’s ratio. 2) tensile and flexural strength. which is dependent upon polymer type and temperature. 2) fracture toughness and impact strength. It is noteworthy to point out that the moduli of elasticity of the constituent parts are shown as function of time. while α2 is the expansion in the direction perpendicular to the fibers. 1968. Halpin.) are grouped together and termed the transport properties. and km are the transport properties of the composite in the longitudinal direction. 1991). the property k2 is computed by the Halpin-Tsai equation: k 2 1 + ξηVf = km 1 − ηVf where ⎛ Kf ⎞ ⎜ ⎜ K − 1⎟ ⎟ m ⎝ ⎠ η= ⎛ Kf ⎞ ⎜ ⎜ K + ξ⎟ ⎟ ⎝ m ⎠ ….(4.5) ⎛a⎞ log(ξ) = 3 log⎜ ⎟ ⎝b⎠ ……(4. 1984. 1981] for both the longitudinal and transverse directions in an FRP composite. etc.3) where k1.Corrosion Tutorial (Bowles. Equations are suggested by [Hashin. These equations accurately predict the thermal and electrical conductivities of carbon-epoxy composites for Vf up to 80 ..4) ……(4.6) and a and b are the dimensions of the fiber along and perpendicular to the direction of measurement of the transport coefficient. respectively. 1968. in the fiber. electrical conduction. 60%. and in the matrix. and Springer. The longitudinal properties are computed through the rule of mixtures as follows: k 1 = Vf k f + Vmk m …(4. because they deal with diffusion through the composite. For the transverse coefficient. Thermal and Electrical Conductivity Transport Properties The properties of materials (such as heat conduction. kf. Propagation of such mode of damage has been observed only for very high or very low temperature applications. permeation. which can be restored by drying. and delamination caused by swelling or internal stresses. which is attributed to hydrolytic scission of ester groups. externally applied stress. Temperature increases the rate of absorption. Moisture pickup leads to loss of chemical energy. temperature.2. and the chemical structure of the polymer and fiber/matrix interface. It could also result in irreversible damage such as matrix cracking. During diffusion. The sorption rate depends on the properties of the constituents.2 Moisture Diffusion/Plasticization Water penetrates FRPs through two processes: diffusion through the resin. The molecules dissolved in the surface layer of the polymer migrate into the bulk of the material under a concentration gradient. Improved bonding between fiber and matrix. The rate of degradation of the polymer composite exposed to fluid environment is related to the rate of sorption of the fluid. and transfer of moisture through the cracks is an after effect. Water also penetrates at the fiber-matrix interface. particularly the chemical bonding tends to delay the corrosion process. Tg. volume fraction and orientation of the fibers. the state of the material.Corrosion Tutorial 4. but consists of molecules or groups of molecules that are linked together by hydrogen bonds to the polymer. hydrostatic pressure. However. Diffusion coefficients have been found to increase with temperature. The damage done in FRP due to the diffusion of fluids depending on the exposure time may be plasticization and swelling of the matrix. fluid concentration. due to Plasticization of the matrix as a result of the disturbance of the Van der Walls bonds between the polymer chains. It is reported that the primary mechanism of moisture pickup is diffusion through resin. of the polymer matrix. absorbed water is not in the liquid form. The sorption behavior depends on the type of fluid. and flow through cracks or other material flaws. increased hydrostatic pressure reduces water uptake due to closing of micro cracks. debonding of the fiber/matrix interface region. and process variables. Water penetration into cracks or other flaws occurs by capillary flow. the degree and type of cross-linking and the presence of voids.3. Moisture decreases the glass transition temperature. damage to fibers. 81 . It is governed in the most part by the chemical structure of the resin. this is often known as Fick’s first law. such as a solid polymer. [1999] studied the sorption and diffusion of water. The mass flux is generally proportional to the local concentration gradient. When a concentration gradient exists in a material. impact strength and interlaminar strength are reduced due to exposure to moisture. salt water and concrete pore solution in epoxy (EPON 828 RS).3. short beam shear strength and modulus. They observed Fickian diffusion in all the resins and solutions. 4.Corrosion Tutorial The tensile strength and modulus. transverse tensile strength and modulus. Polyestor and Phenolics Chin et al. Vinyl Ester. vinyl ester (Derakane 411-350PA) and polyester (Aropol 7240-T15) matrices. there is a natural tendency for the concentration difference to be reduced and eliminated by the process of mass transfer.1 Diffusion Through Unreinforced Epoxy.2. 82 . Typical dimensions of the polymer film used in the experiments were 25mm X 25mm and the thickness of the vinyl ester and polyester film ranged from 230 to 260 microns whereas the thickness of epoxy film was 300 microns.2. compressive strength and modulus. and the constant of proportionality is called the diffusivity or the diffusion coefficient. flexural strength and modulus. Fracture energy or fracture toughness seems to increase in some cases and reduce in some cases. vinyl ester and isopolyester resins. and then slowed between 10 and 100 hours as equilibrium was attained. 2001) The Figure 4-3 shows sorption curves for epoxy. The curves for moisture content ratio show an initial linear region up to about Mt/M∞ = 0.Corrosion Tutorial Figure 4-3: Typical Sorption Curve (Vijay et al. 83 . The epoxy resin has a higher concentration of hydrophilic hydroxyl groups located along the backbone thus exhibiting higher moisture uptake than other resins.6 followed by a region concave to the abscissa. Uptake was rapid for the first 10 hours.. Corrosion Tutorial Figure 4-4: The Sorption Curves for Epoxy.. and Isopolyester Resin When Exposed to the 3 Different Solutions (Chin et al. Vinyl ester. 1999) 84 . vinyl ester.9 ----(x 10-9 cm2 s-1) 60 °C 13. D Matrix Sorbent 22 °C Distilled water Epoxy Salt solution Pore solution Distilled water Vinyl Ester Salt solution Pore solution Distilled water Isopolyester Salt solution Pore solution 8. Vinyl Ester.3 The sorption curves for epoxy.75 8.89 0.6 8.0 24. and Isopolyester Resins Diffusion Coefficient.53 1.5 24.88 8.72 41.82 19.Corrosion Tutorial Table 4-1: Diffusion Coefficients of Epoxy. and isopolyester resin when exposed to the 3 different solutions are shown in the Figure 4-4.54 9.04 0.67 6. 85 . (b) Salt Solution. 1999) 86 .Corrosion Tutorial Figure 4-5: Fickian Diffusion Curves for Epoxy in (a) Water.. and (c) Concrete Pore Solution at 22 °C (Chin et al. The specimens were 0. Both swelling and plasticization are reversible and the thickness and properties of the dried sample are restored. Moisture sorption into polymeric material lowers the glass transition temperature (Tg). Ghorbel and Valentin [1993] studied the changes in structure with time of aging and found that the above reactions were contributing to hydrolysis of the resin. it is concluded that the permeability of vinyl ester is greater than epoxy. strain in the axial direction was measured as 87 .. it had higher diffusion coefficients than epoxy. which was heated uniformly at a constant rate of 5.635 cm in diameter and 2. Wong and Broutman [1985] found the epoxy EPON 828 samples to regain their initial thickness and Tg upon drying them after saturation.54 cm in length and were fabricated such that the direction transverse to the fabric plane was aligned with the axial direction of the specimen. in his work made cylindrical specimens of FM5055 carbon phenolic. The results indicate a decrease in Tg from 568 K to 477 K as the weight of absorbed water increased from 0% to 4. where the chemical structure was not affected. The reduction in Tg is due to plasticization of the resin. 1998]. Since diffusion coefficient is a function of both permeability and solubility. Water causes the ester linkages to break into acid and alcohol groups. As the specimens were heated.Corrosion Tutorial Although vinyl ester resin exhibited the lowest equilibrium moisture uptake.25%. This established a clear correlation between Tg and amount of water absorbed. A reduction in Tg is also accompanied by swelling of the sample. hydrolysis of the matrix is observed for long periods (>4000 h) [Buck et al. 4. Especially in vinyl ester resins.50C/sec.2 Effect of Moisture on Fiber-Matrix System Stokes [1990].2. Hiltz and Keough [1992] studied the influence of absorbed water on the Tg of a poly amideimide using dynamic mechanical analysis (DMA) and DSC. Decrease in Tg is also attributed to increase in the chain mobility and voids.2.3. Hence reduces the maximum working temperature of the material. Corrosion Tutorial a function of temperature. 8% initial moisture. Figure 4-6: Thermal Expansion Measured by Stokes (Stokes. 1990) Figure 4-7: Moisture-Induced Thermal Expansion vs. The measured results from Stokes are shown in Figure 4-6. 4%. The oven chamber in which the specimens were heated was maintained at zero percent relative humidity. 1990) 88 . Temperature (Stokes. Stokes measured the transverse thermal expansion strain for specimens containing three different initial moisture contents: 0%. and across ply) as a function of equilibrium moisture content of the material. is evident. Using regression analysis. Stokes [1990] measured the swelling response of a typical carbon phenolic composite in the three primary material directions. The larger swelling in the across-ply direction. (C) and (D) show the equilibrium linear swelling of each specimen of FM 5055 in the three primary orthogonal directions (warp. water is moving largely into the carbonized fibers. (C) and (D).Corrosion Tutorial Strain response due to water is obtained by subtracting the strain response for the 0% moisture specimen from the response for the 4% and 8% moisture specimens. as opposed to the two fiber directions. the incremental increase in moisture absorption can be attributed primarily to the resin whereas at intermediate relative humidities. Separated strain profiles for 4% and 8% moisture levels obtained through subtraction technique are shown in Figure 4-7 referred to as the moisture-induced thermal expansion. 89 . a fourth-degree polynomial was fit to each of the equilibrated data sets as shown in Figures 4-8 (B). Figures 4-8 (B). fill. Figure 4-8 (A) shows the mean equilibrium moisture content of each set of specimens from each conditioning chamber as a function of the relative humidity of the conditioning environment. A sigmoidal (shape of the alphabet S) relationship was found between the two variables. The data obtained suggest that at low and high relative humidities. Corrosion Tutorial Figure 4-8: (A) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen as a function on the humidity of conditioning environment (Stokes. 1990) 90 . Corrosion Tutorial Figure 4-8: (B) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in the Across Ply Direction (Stokes. 1990) 91 . Corrosion Tutorial Figure 4-8: (C) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in the Fill Direction (Stokes. 1990) 92 . 93 . 1990) From the work as shown in figures 4-8 (A) to 4-8 (D).Corrosion Tutorial Figure 4-8: D) Moisture Absorption/Swelling Response of Carbon Phenolic Specimen in the Wrap Direction (Stokes. which indicates a large free volume within the composite. whereas the fibers absorb most of the intermediate moisture content. These findings appear to indicate that there are multiple sites for the absorption of water in the cured resin and that these sites vary in their affinity for water. The unaccounted volume of water absorbed by the fully saturated composite indicates that dry FM 5055 has a free volume of approximately 11. less than 20% of the water absorbed by the composite can be accounted for by the swelling of the material. Moreover.61 g/cm3. It was proposed that the phenolic network polymer is responsible for the majority of the absorption of water at low and high moisture levels. Stokes concluded that large increases in moisture content at intermediate moisture levels resulted in negligible increases in volume of the composite.5% and an apparent density of approximately 1. respectively. Verghese et al. The two values seem to be consistent. debonding and degradation of the glass fibers. Ishai [1975] found significant damage to the resin (shown by Scanning Electron Microscopic (SEM) studies of cut samples) and glass fibers at temperatures close to boiling. damage at lower temperatures was negligible.(4. At higher temperatures (close to Tg). (1999) too found that the diffusion coefficient follows an Arrhenius relation with E/R= 4650 K.2. The diffusion coefficient has been found to increase with temperature according to the Arrhenius relationship [1981] D = D0 exp(− E / RT ) …. An important conclusion of their study was that accelerated aging could not be simulated by subjecting the resin to higher temperatures because under these conditions other phenomena (such as permanent damage to 94 .3. Significant damage was found to reduce fiber regions. the sorption approaches that of the Fickian response. In addition.e. Marsh et al.Corrosion Tutorial 4.7) where E is the activation energy for diffusion. However. the value of activation energy does not seem to vary too much between neat resins and epoxies glass fiber composites. which was attributed to imbalance of stress distribution.3 Effect of Temperature and Polymer Structural Variables on Sorption of Water Temperature Temperature increases the rate of sorption. Significant damages included extensive cracking.2. temperatures close to boiling) irreversible damage to the resin and composites was observed. Samples immersed in distilled water at room temperature for over 3 years showed no damage to their structure. Bonniau & Bunsell (1981) calculated E to be of the order of 11000 cal/mol for (Diglycidyl ether of bisphenol A) DGEBA epoxy cured with different hardeners. (1984) in their experiments calculated E for neat resins and composites to be 9500 cal/mol and 9940 cal/mol. However. Bunsell and Dewimille [1983] studied sorption in DGEBA composites and found similar results. under extreme conditions (i. Corrosion Tutorial resin) take place. Woo and Piggott [1987] observed that the effective dielectric constant of water is only 55-77% that of free water. which was attributed to a stretching vibration of the carbon monoxide and the bonding of the primary alcohol OH bond. It was believed that water might cluster within a polymer. Verghese et al. However. and/or diamino-diphenyl sulfone. contrary theories were postulated [Netravalli et al. and water molecules from hydrogen bonds may become bound with polymer groups. demonstrated the influence of polar OH groups [1999]. Hence.. Presence of --OH group in Polymer Interactions of water with polar groups within the polymer chain of vinyl ester resins have been confirmed through FTIR (Fourier Transform Infrared Spectroscopy) studies on saturated samples. 1984]. Nature of Water Water may exist as either “bound” or “unbound” within the system. while the saturation level of the resin was lower as compared to the Derakane 441-400 vinyl ester resin. i.. This study also led to the conclusion that at higher temperatures. other factors controlled the sorption process. The model resin did not exhibit the thermal spiking phenomenon. The existence was confirmed by 2H – NMR studies by Klotz et al.e. [1996] on diglycidyl ether bisphenol A resin cured with dicyandiamide. Specific interactions through hydrogen bonding were observed in Derakane 441-400 vinyl ester resin composites (which had an OH group in its structure) as against their model resin in which the OH groups was substituted for a CH3 group. the mobility of water within the polymer is between that of ice and free water. FTIR studies made on vinyl ester resins before and after aging showed changes in the shape of the peak at 1450 cm-1. Changes in the Resin Structure 95 . cracking/crazing of the resin and degradation of the interfacial layer. [1985] showed that slow rearrangement of the polymer chains takes place due to the ingress of the water molecule. Wong and Broutman [Part I and II. However. In some cases.Corrosion Tutorial Changes taking place within the polymer due to the sorption of water may be reversible or irreversible. They concluded that the absorbed water can act as a plasticizer. this rearrangement was not permanent and the polymer collapsed to its original form when annealed or heated to temperatures greater than its Tg. which clearly show extensive cracking attributed mainly due to imbalance in stress and degradation of the interface. cracks developed in the fiber rich zones. Due to additional cure. and Dewimille and Bunsell [1983]. Hence the history of the polymer has a definite bearing on subsequent sorption. which caused increased water absorption.2 x 10-9 cm2/sec. the resin had more fractional free volume and more–OH Groups. [1984] used (Differential Scanning Calorimetry) DSC technique to obtain the glass transition temperatures and curing energies of wet and dry samples. Netravalli et al. [1984] reported that the resins retained some moisture on drying. Weitsmen [1988] and Wong et al. McMaster and Soane [1989] also found the second and third sorption to be faster than the first. The diffusion coefficient for one such result was first sorption. The irreversible changes that may take place are cracking/crazing. the behavior was found to be the same as the original sample. The researchers also observed that the post-cured sample did absorb more water. They proposed that the water molecules caused polymer chains to rearrange and did not cause any damage to the epoxy. They took SEM photographs. 1985] studied the sorption of water into EPON 828 resin (DGEBA cured with m-phenyldiamine/aniline) and concluded that Fickian sorption was taking place although the diffusion coefficient was concentration dependent.5 x 10-9 cm2/sec and second sorption 4. 3. This rearrangement of polymer chains causes changes in the free volume and hence the second and subsequent sorption is comparatively faster. and degradation of the matrix/fiber interface are shown by Ishai [1975]. When this sample was subjected to the sorption experiment after annealing. but the effect was 96 . This is because the water plasticized the resin and rendered the resin rich zones more resistant to damage. Marsh et al. More cured resins exhibit higher sorption because the highly cross-linked specimen has a lower density and consequently a greater free volume. 31%. The water was found to cause the unreacted epoxide groups to react. The Tg of a sample increased as the time of an isothermal cure was increased and hence the extent of cure increased. 215 °C) of the fully cured sample. Similarly. the amount of water sorbed has been found to increase with the curing.3% and 93%. Six hundred minutes was not enough to fully cure the sample. Cure The effect of curing temperature and time of cure has been reported by a number of researchers. The extent of cure of a resin can be found using Torsional Braid Analysis (TBA). The samples were then subjected to a humid environment at 25 °C. 79. Torsion Pendulum and FTIR (Fourier Transform Infrared Spectroscopy).Corrosion Tutorial reversible. a greater cure leads to the formation of more –OH groups. The result of the sorption experiments was that the sample with the highest percent of cure absorbed more water at all the conditions. A typical result is shown in Table 4-2. 97 . 51%. Enns and Gillham [1983] cured DGEBA with stoichiometric amount of DDS at 175°C for four different times: 50. Sahlin and Peppas [1991] found the post cured TGDDM/DDS resin to consistently sorb more water than the one without the post cure. 180 and 600 minutes. thereby causing some permanent changes in the resin structure. Four different humidities were used. They also found that the curing energies progressively decreased as the amount of water taken in increased. In all cases. 100. Wong and Boutman [1985] found this in their study of DGEBA/mphenyldiamine/aniline epoxy resin. as the Tg was below the Tg∞ (approx. 2334 M∞ 1. it is desirable to have a fully cured polymer.58885 1. it is found (Gupta.7707 1. The results have shown that the degree of cure does not accurately reflect the mechanical property development during cure.69 1..737 D x 109 cm2/s 1. This is because a partially cured resin has a lower modulus compared to a fully cured resin.074 1. dα n = k1 + k 2 αm (1 − α ) dt ( ) …. In comparison to the thermal degree of cure. The most common method of determining the extent of cure is with the help of a differential scanning calorimeter (DSC).Corrosion Tutorial Table 4-2: Variation of Equilibrium Moisture Content with Degree of Cure Cure time (minutes) 50 100 180 600 Tg °C 135 160 185 195 Density (g/ml) 1. α .8) in which the ks are temperature-dependent rate constants while m and n are temperature independent constants. White and Mather [1991] used an ultrasonic cure monitor technique to assess the simultaneous extent of cure and mechanical property development during the cure on an epoxy resin EPON 815/V 140 and compared the results with DSC monitoring.7170 From the viewpoint of mechanical properties. The extent of reaction. heat is liberated. When dα dt is plotted as a function of time. then is simply the ratio of the total heat evolved to the heat of reaction. the modulus extent shows that 98 .2357 1. During the course of a curing reaction.65 1. t. and the instantaneous rate of energy evolution can be measured using a DSC.237 1. and it displays more creep.(4.2339 1.136 1. 2000) that data can be described by the following equation. The modulus extent was derived and presented as a characterization parameter similar to the degree of cure in thermal cure characterization. [1981] attempted to keep the polarity of the resin a constant by replacing the diamine hardener with aniline in DGEBA (Figure 4-9). thus reducing the density of cross-links in the resin without affecting the polarity and increased chain mobility. 25. The commonly used hardeners are Diamino Diphenyl Sulphone (DDS).Corrosion Tutorial significant mechanical property development is still occurring in the later stages of cure when the degree of cure is fully developed. the ultrasonic method is likely to provide the needed information while the thermal method may not provide such information. The polarity of the hardener also had an effect. hardeners influence the sorption of water. anhydride and Lewis acid hardeners. Hardener A hardener is used in the cure of an epoxy resin to cross-link the epoxy chains. The former method is a non-destructive method. Sahlin and Peppas [1991] cured TGDDM resin with different amounts (5. In other words. thus giving structural rigidity. Diamant et al. Within a reasonable deviation. By keeping the polarity a constant. dicyandiamine (DICY). 45 wt %) of DDS. if the goal is to determine how close a sample is to being fully cured. Due to their polarity. 35. the increase was linear with the amount of water. diphenyl diamine. morphology of the polymer has changed as the length of the matrix between cross-linking increased. 99 . 15. The sorption increased as the amount of the hardener was increased. for example. A residual hardener. DICY means greater affinity for water.92 3. the inherent polarity of diamine and aniline was not taken into account. the resin cured with only diamine hardener was observed to have the maximum sorption.7 100 .3 M∞ % (20 °C) 1. Table 4-3: Effect of Hardener on Equilibrium Moisture Content %TETA in DGEBA 5 15 Tg (dry) °C 109 142 Tg (Wet) °C 105 109 M∞ % (70 °C) 1. Higher cross-linking density caused hindrance to the movement of the water molecules and effectively reduced the sorption. which surrounded areas of lower density. However. Diamine is more polar than aniline and hence could have accounted for the increased uptake. In addition to polarity of individual hardeners. which is attributed to (and confirmed by etching experiments) regions that had a greater cross-linking density.Corrosion Tutorial Figure 4-9: Difference Between (a) Diamine and (b) Aniline Hardener Contrary to expectations. the presence of excess hardener increases the affinity of the resin towards water. 15 and 25 phr (parts per hundred parts of resin) of triethylene tetramine (TETA) at 100 °C.5 2. The results are tabulated in Table 4-3 [1982]. DGEBA was cured with 5. Corrosion Tutorial 25 95 52 8.6 10.8 It is clear that the amount of water absorbed increased with the amount of the hardener and temperature. However there was a discrepancy from expected Tg. Thus, important parameters that influence sorption of water are: • • • • • • Extent of cure, like curing temperature and time of cure. Type of hardener used to cure the resin as well as the amount used. Amount of free volume present in the resin. Sizing agent (whether it forms a good bond or not between the matrix and fiber). Environment like pH, temperature, etc. Effect of Glass Reinforcement Influence of glass fibers in polymers on the sorption of water can be determined by sorption experiments on neat samples as well as composites. The polymer resin used should be cured with the same hardener/catalyst as well as under the same conditions. Contradictory results have been obtained on the above subject. Marsh et al. [1984] studied the sorption of water in bisphenol A and cresol novolac epoxy cured with dicyandiamide. Neat resins and composites with 40% E-type glass were studied at 75°C /100% RH. The sorption of water in the glass composite was the same as that of the neat resin. Both the neat resin as well as the glass composite showed an intermediate saturation before the onset of residual moisture. These similarities led the authors (Marsh et al., 1984) to the conclusion that water did not enter the interface between the matrix and the fiber; hence, there was no difference in the sorption of neat resin and composites. On the other hand, Ishai [1975] showed that the behavior of neat resins was quite different from composites. In the case of diffusion of moisture into Epon 828 resin with E-glass fibers, significant damage was found not only to the 101 Corrosion Tutorial interface but also to the glass fiber (confirmed by SEM pictures). The degradation was signaled by a drastic change in the sorption curve. Plotting the Relative Weight Change (RWC) Vs. Relative Length Changes (RLC) also showed the onset of degradation. The extent of degradation was more for samples exposed to extreme environments, i.e., temperatures of 80°C. Also the degradation of the interface was considerably less at lower temperatures i.e. 20°C. Similar results were obtained by Dewimille and Bunsell [1983], who found that composites (DGEB/Anhydride with E-Glass) degrade when exposed to water at high temperatures (80°C and above). Similar sorption studies were also made on vinyl ester glass fiber reinforced composites. Pai et al. studied the effect of glass fiber lay-up sequencing in various acidic environments [Parts I and II, 1997], using six different types of resins including vinyl esters. They found that the composite with the Chopped Strand Matrix exhibited least resistance to all liquids (water, 15%, 25% and 35% Sulfuric acid). Although the diffusion process became sluggish as the concentration of sulfuric acid increased, the saturation levels were much higher. They also studied the extent of degradation on the composite. They assessed the fiber/matrix interface by performing an interlaminar shear strength Test. The loss of the shear strength increased as the concentration of sulfuric acid increased, showing an increased rate of degradation with increased acidic pH. In order for a composite to function properly, there must be a chemical bond between the matrix and the reinforcing fibers so that the applied load (applied to the matrix) can be transferred to the fibers. In 'fiber glass' the fiber is inorganic while the matrix is organic, and these two do not bond readily unless the fibers are treated to modify their surface. Silica (SiO2) is hygroscopic, i.e., however slow it absorbs water onto its surface where the water breaks down into hydroxyl (-OH) groups. The coupling agent takes the form of a silane (R-SiX3), where R is an organic radical that is compatible with the polymer matrix and X is a hydrolisable organic group such as an alcohol. The most common silane couplant is tri-ethoxy-silane. Heat will force the elimination of water between the -OH pairs at the hydrated silica surface and the silane 102 Corrosion Tutorial as well as between the adjacent silane molecules forming a strong bond between the matrix and the fibers as shown in Figure 4-10. Figure 4-10: Bonds Between Glass Fiber and Coupling Agent. If the bonds are chemical, then the presence of the glass will have a negligible effect. If, however, the bonds are weak and can be displaced by the hydrogen bonding due to water, debonding and degradation of the glass fiber can be pronounced. This was studied by Ritter et al. [1998]. They studied the propagation of a crack in monolithic glass (soda-lime and fused silica) untreated glass-epoxy interface and glassepoxy interface sized with 2-amino propyl triethoxy silane (3-AMPS) using a double cleavage drilled compression test. The increase in resistance of the silane bonded epoxy interface is attributed to chemical bonding of epoxy to the glass via the coupling agent. The rupture or debonding will take place only by breaking of the Si-O-Si bonds formed between the glass and the silane-coupling agent. However, the highest threshold energy release rate, Gth for the soda-lime glass is lowered as the alkali molecules can bond with water and hence prevent the silane molecules from properly bonding to the glass surface. The Gth calculated is given in the Table 4-4. Table 4-4: Calculated Values of Gth 103 Corrosion Tutorial Specimen Soda-Lime glass (SLG) Fused Silica (FS) Untreated SLG-Epoxy Untreated FS-Epoxy Silane treated SLG-Epoxy Silane treated FS-Epoxy Gth Jm-2 1.69 2.76 0.25 0.25 1.32 3.31 4.3.3 Long term Mechanical and Hygrothermal Behavior (Aging) Polymers and composites used for the renewal of civil infrastructure will be exposed to complex infrastructure service environment conditions like range of combinations of stress, time, temperature, moisture, radiation, chemical, and gaseous environments and are expected to perform more than forty years. These materials should be required to go through a series of specifications based on inherent, and residual mechanical, physical, and thermal properties after accelerated service environment exposure conditions. The lack of understanding of the fundamental parameters controlling long-term materials performance necessarily leads to overdesign and in-service prototype evaluations and, furthermore, inhibits greater utilization. The determination of long term mechanical and hygro-thermal behavior of these materials and the understanding of the phenomena in relation to civil infrastructure renewal are critical to the further use of FRP composites in civil infrastructure. 4.3.3.1 Creep Theory Creep is defined as the continuous deformation (strain increase) over a prolonged period under constant load. i.e., when a polymeric material is subjected to a constant, sustained load, it deforms instantly, but then the deformation continues over a 104 Corrosion Tutorial long period. This phenomenon of increasing strain is known as creep. It is a very slow plastic deformation process that occurs at stress levels below the yield point. Conversely, if a constant strain is imposed on a polymeric material, it exhibits instantaneous stress, but then the stress increases with increase in time; this is known as stress relaxation. It is a universal phenomenon and is exhibited by most materials, including metals. Polymers behave viscoelastically, and exhibit creep and stress relaxation to a high extent. Viscoelasticity arises because polymers are long-chain molecules, and under stress, parts of a molecule or even entire molecules can rearrange and slide past each other. This is especially easy above the polymer glass transition temperature Tg. Furthermore, creep and stress relaxation are more pronounced in thermoplastics than in thermosets; cross linking in thermosets restrict polymer chain mobility. The presence of fillers and reinforcements can further restrict creep. If the deformations are large enough, chain rupture may occur, particularly in thermosets where the chains are crosslinked into a network. Environmental factors such as temperature, moisture, and irradiation can all exercise their effects on molecular activity in the polymer, thus altering the macroscopic (creep) behavior. At high stress levels, this leads to relative slip between fiber and matrix. Rupture of fibers may also occur, resulting in higher fiber stresses in surrounding intact fibers, thus increasing elongation and rate of creep over time. Constant Constant Strain dε/dt Primar y Secondar y II Time, t Teritiray III tf 105 In general this effect (dashed line for constant stress) only really manifests itself in the region of interest in the secondary region. particularly along the direction of fibers. the creep deformation should be linked to an applied stress. 3. strain gages are generally employed. as compared to the substantial creep in the transverse direction and under shear stress. which is beyond the Creep and Relaxation FRP with glass fibers is expected to have very limited creep in the longitudinal direction. stage II). The load in FRP composite materials is carried primarily by the fibers. compression. For FRP. rapid initial elongation (ε0) of the specimen. and 4. In practice. or flexure) at constant temperature and the corresponding deformation is measured as a function of time. Thus creep and stress relaxation are not as significant in composites as in the bulk polymer matrix itself. a four-stage response is generally observed: 1. which behave elastically. stage III). it simpler to maintain a constant load. instantaneous test results cannot be obtained to define material response under sustained stress or deformation. When reporting creep test results the initial applied stress is used. a sustained load is applied to a specimen in one of several standard configurations (such as tension. and rate of loading. temperature. This is because stress is a unique function of strain. A rapid increase in response and fracture (tertiary creep. In principle. 2. Consequently. Steady state (secondary creep. The effect of constant load and constant stress is shown in Figure 4-11. tertiary region. as the specimen elongates the cross sectional area decreases and the load need to be decreased to maintain a constant stress. the stress-strain behavior is not dependent on time. even if it exhibits a nonlinear stress-strain relationship. stage I). which is typically designed to be the primary load path. Since the deformations are small. Rapid reduction in response rate (primary creep. For a purely elastic material. Thus.Corrosion Tutorial Figure 4-11: Typical creep behavior of plastics The properties of viscoelasatic materials are dependent on time. 106 . 11) When a sinusoidal strain is applied. This is a parallel combination of a spring and dashpot attributed to Kelvin 107 . the compliance is found by integrating the above equation as J (t ) = 1 t + E0 η ………………………. such as a Newtonian fluid. i..(4.. When a sinusoidal strain is applied.e.. the stress is proportional to the rate of strain.Corrosion Tutorial σ = Е0ε ε = J0σ ……. From equations 4. the stress is maximum. the applied stress is the sum of the first part of Equations 4.13) and creep increases passively with time. For a purely viscous material.(4. E0 is the elastic modulus and J0 is the elastic compliance. This resembles the behavior of dashpot. the stress is zero.. (4. the stress and strain are out-of-phase. and when the strain goes through zero.. If the elastic and viscous parts are subjected to the same strain.9) …… (4.12) For a creep test.10) This resembles the behavior of a spring.10 and 4. where a constant stress is applied at t=0.11 εt = σ (t ) E0 + η∫ 0 1 t σ (r )dr ….10 and 4. as well as most real materials exhibit a combination of elastic and viscous responses. meaning that they follow each other and related only by proportionality constant. when the strain is maximum. They can therefore be represented as combination of springs and dashpots (Maxwell’s model). the strain and stress are in phase. σ(t) = η dε dt ……(4.11. Polymers. we get the compliance as: − 1 J (t ) = [1 − e η ] E0 E 0t ….15) where. Clearly the creep behavior shown in the Figure 4-11 is more complex than what can be predicted based on the above equations. ε0 = initial strain in a creep test ε(t) = creep strain after time t σ0 = intial stress measured at the beginning of a stress-relaxation test σ = stress after time t Maxwell’s Model Maxwell’s model consists of a spring (Hookean) and a dashpot (Newtonian) in series as shown in Figure 4-12. 108 . Therefore. (4. creep and stress-relaxation tests are essentially the inverse of one another. Overview of Creep Models Maxwell and Kelvin Models are used for representing the creep of thermoplastic and thermoset resins. ⎛ ε (t ) ⎞ ⎛ σ0 ⎞ ⎜ ⎟relax ⎜ σ (t ) ⎟ ⎜ ε ⎟ ⎟creep = ⎜ ⎠ ⎝ ⎝ 0 ⎠ … (4. If the deformations and stresses are small and the time dependence is weak.Corrosion Tutorial model.. For a creep test under constant stress. integrating for the strain.14) and creep is predicted to attain a constant value with increasing time. to a first approximation stress relaxation data can be converted into creep by the following equation. 18) 109 . Most of the relaxation takes place when t is close to .(4.html) The modulus of the spring is E.matter. Since a dashpot deforms instantly.uk/matscicdrom/manual/pm. all the initial deformation takes place in the spring. the model is given a fixed strain ε while the stress σ is measured as a function of time as shown in equation 4.org.17) where ‫ = ح‬η/E is known as the relaxation time.16) Since. In a stressrelaxation experiment. later the dashpot starts to relax and allows the spring to contract. the solution of the equation is ⎜ ⎟ ⎟ −⎜ ⎜ σ = e ⎝ η ⎠ = e⎝ τ σ0 ⎛E⎞ ⎛ −t ⎞ ⎟ ⎠ …(4.(4.16. and the viscosity of the dash pot is η. ε = σ/E for the spring and σ/η=dε/dt for the dashpot. the stress relaxation can be written as σ (t ) σ ⎜ Er = = e⎝ τ ε σ0 ⎛ −t ⎞ ⎟ ⎠ …. dε 1 dσ σ = + =0 dt E dt η …..‫ح‬ Mathematically.Corrosion Tutorial Figure 4-12: Maxwell’s Model (Ref: http://www. Corrosion Tutorial Kelvin Model Kelvin Model consists of a spring and dashpot connected in parallel as shown. in the four element described later. Four Element Model 110 . Therefore. According to Maxwell model. Whereas. Maxwell’s model does not describe the creep behavior completely.19) Neither the Maxwell nor the Kelvin model can describe the creep of thermosets. which indicates that all creep and corresponding creep stress are recoverable. which models the actual behavior of polymers.15) and (4.16). fraction if creep is not recoverable for the flow that occurred in the dashpot with viscosity η3. and hence the applied stress is the sum of the first part of (4. In thiis the elastic and viscous parts are subjected to same strain. as t goes to infinity the stress ratio becomes zero. A more realistic model is the four-element model. the Four Element Model accounts for the actual creep behavior. For a creep test under constant stress Figure 4-13: Kelvin’s Model − 1 E(t ) = [1 − e η ] E0 E 0t …… (4. 20) If at time t1. the load is removed.22 is shown in the Figure 4-15: 111 . Later. When a constant load is applied. The total elongation is the sum of the individual elongation of the three parts. the spring with modulus E1 deforms first. (4.22) As tÆ ∞.. it is found that the creep of dashpot with viscosity η1 cannot be recovered. The equation for subsequent creep recovery is: ε = ε 2 e ( − ( t −t ) / τ + 1 σ 0 t1 η3 ……… … (4. the dashpot with the viscosity η1 deforms. ε= σ0 E2 [1 − e − E 2t η2 ]+ σ0 E1 + σ0 t η1 ….Corrosion Tutorial Figure 4-14: Four Element Model A four-element is shown in Figure 4-14. the spring with modulus E1 retracts instantly.21) where ε= σ0 E2 (1 − e (−t1 / τ ) ) ……. it is the spring E2 and dashpot η2 that deform. Finally. Equation 4. (4. (4. ti.21.. the total creep is ε (τ ) = J (t )σ 0 + J (t − t1 )(σ 1 − σ 2 ) + . σ3. t2. the response of a material to a given load is linear and additive. t3. experimental data covering about 10-15 decades of time are needed. the data can be obtained in short intervals but over a series of constant temperatures.…. According to the Boltzman superposition principle....……… σi applied at times 0. σ1. This is a very fine consuming proposition. which depends on temperature. Boltzman superposition principle and the principle of timetemperature superposition are employed. if there are several stresses σ0. For the case of creep. ε (τ ) = Kσt n …. Figure 4-16 illustrates the behavior for a polymer that obeys the following equation 4. + J (t − t i )(σ i − σ i −1 ) ….23) where the creep ε(t) at time t depends on the compliance function J(t). σ2.24) 112 . In practice.Corrosion Tutorial Figure 4-15: Behavior of Creep and Stress Relaxation in Four Element Model Master Curves In order to compute the spectrum of relaxation or retardation times.t1. σ4. In this task. and then they can be superimposed to extend the time scale of measurement.(4. t4. Corrosion Tutorial Figure 4-16: Behavior of Creep when subjected to series of stresses where K and n are temperature dependent constants. The extent of shifting is given numerically by an equation called the WLF equation. which holds between Tg or Tg+10K and about 100K above Tg. For these reasons the vertical shift factors are largely empirical. 113 . dynamic mechanical data. Aging and heat treatments may also affect the shift factors. Note that master curves can be made from stress relaxation data. and the result is called a master curve. Landel and Ferry (WLF). Sometimes a vertical shift may be needed in addition to the horizontal shift and the vertical shift may be needed in addition to the horizontal shift and the vertical shift may depend on temperature. or creep data. It has also been found that the creep curves made at different temperatures can be superposed by horizontal shifts along a logarithmic time scale to give a single creep curve. It describes a system when subjected to a series of stresses σi and ti. creep is given by the sum of identical responses. one may use an Arrhenius equation with a low activation energy. Above the upper limit of applicability of the WLF equation. This procedure was originally suggested by William. (4. it reads J(s) s =1/ ( E(s) s) …(4.26) Taking the inverse Laplace transform. not on stress.. Unidirectional Composites To predict the creep behavior of unidirectional lamina micromechanics is used. multiplying by s2 and inverting we get the Laplace transform of the relaxation E(s) = E/ ((E/η) + s) …(4. Taking the Laplaces of the Maxwell model equation. the relaxation time domain is E (t) = E0e-(E0/η) t …. the correspondence principle is used. The concept and methods of analysis remain the same.e. The correspondence principle is valid for a linearly viscoelastic material. Increasingly complex equations can be used in order to fit the experimental data better. they 114 . any viscoelastic problem in the time domain can be solved as an elastic problem in the Laplace domain.. proportional way. but the number of parameters and the complexity increases accordingly. Stress only magnifies the deformations in a linear.27) If the material follows a Maxwell model. The Kelvin model does not have a simple relaxation equation because the stiffness at t=0 is infinity. By transforming all the equations to the Laplace domain. non-linearity is in time. According to the correspondence principle. Since all the micromechanics models are formulated for elastic fiber and matrix. i. this equation should model well the data of relaxation test.25) where s is the Laplace variable. and because the stress is applied suddenly to the viscous component.Corrosion Tutorial Creep and Relaxation in Composite Laminates Correspondence Principle: To find the relaxation corresponding to the compliance of the Maxwell model. 1 Effect of moisture and temperature on Creep Moisture and time.3. 4. G12. E2. All that is needed are the elastic properties of the fibers.1. Instead. they obtained all the creep compliances and relaxation functions in all directions for the composite. complicated expressions for the creep and relaxation tensors of unidirectional composites. temperature and moisture on the creep compliance are illustrated 115 . as well as many others. Since the properties of the matrix often differ from the bulk properties. Although the concept is simple. like temperature and time. The effects of time. Harris and Barbero. Harris and Barbero [1998] used such a method to predict composite behavior of various laminates. Simple micro-mechanics formulas such Halpin -Tsai cannot be used because they contain adjustable parameters. a simple program or spreadsheet can be used. This has not been done to present. Luciano and Barbero [1995] used micromechanics of periodic microstructure to find analytical. Both of these effects can accelerate the time-dependent behavior of material. the equations are too complicated for hand calculations. the fiber volume fraction.3.Corrosion Tutorial must be used in Laplace domain. Models that include empirical adjustable parameters cannot be used because the time dependence of the adjustable parameters is not known. G23 and all the Poisson components. such as E1. so it is not possible to take the Laplace transform of them. struggled to perform creep tests on bulk matrix. Harris and Barbero [1998] suggested that creep tests be performed on the composite to back-calculate the creep behavior of the matrix. Laminated Composites The approach for prediction of laminate creep is to use classical lamination theory in the Laplace domain. often have an interchangeable effect on the creep behavior of polymers. Moisture absorption in polymer-matrix composites will result in the development of residual stresses and will plasticize the resin. On the contrary. From these. and representation of the creep behavior of the matrix. This was attributed to the redistribution of 116 . at a given stress level. data on viscoelastic behavior of the materials under the influence of fluid absorption are scarce. temperature. If the resin swells. Figure 4-17: Schematic representation of effects of time. creep ceased after sometime when the applied stress was low. (Liao. Furthermore. the rate of creep increased with immersion in water. Usually. In the dry-condition. the moisture absorption level is history-dependent. and moisture on creep compliance. stress will be generated. creep compliance is increased with an increase in moisture content and temperature over time.1998) When moisture is absorbed in a polymer-matrix composite the resin is plasticized. However. [1987] investigated the effect of water immersion on compressive creep of uni-directional glass-reinforced composites. The first of these effects will soften the polymer and increase creep. Although there are some studies on creep-rupture for pultruded FRP in fluids. a steady state creep rate (dε/dt=constant) was reached. Moreover. when the applied stress exceeded 85% of the ultimate strength. In liquid media. similar behavior was observed but considerably lower stress levels compared to tests conducted in air. and therefore sorption behavior under temperature and/or humidity cycling is not the same as under a constant humidity and temperature level.Corrosion Tutorial schematically in the Figure 4-17. Parasyuk et al. but exhibited a significant increase in creep deformation when the temperature was increased to 1020C. These general conclusions remained unchanged when data were obtained at higher temperatures. Wang and Wang [1980] have investigated the influence of moisture. 00 laminates exhibit minimal creep compared to the 900 and 450 laminates. Creep of the 450 and 900 laminates was also strongly influenced by moisture and temperature. Figure 4-18: Moisture Absorption Behavior (adapted from Weitsman [116]) 117 .2 MPa and moisture content (mass fractions from 0.Corrosion Tutorial stresses between glass fibers and the polymer matrix. even at low stress levels. At from temperature with low stress 6. 450. and between damaged and undamaged reinforcing elements. and 900 with respect to the fiber direction. Unidirectional laminates were loaded at angles 00.5% to 0. temperature and stress on the tensile creep behavior of Scotchply 1002 glass/epoxy composites.94%). For instance. This is due to the fact that there are some periodic changes in the aforementioned environments. the moisture absorption behavior depends on temperature. and presence of voids. applied load. or relative humidity. Curve A represents pseudo-Fickian behavior where the moisture weight gain never reaches equilibrium after the initial take-off. and high solvent concentration will often result in behavior described by curves C or D. which are often irreversible. For instance. For curve D. time. type of media. Curve B is two-stage diffusion with an abrupt jump in moisture weight gain after initial take-off. as shown in Figure 4-18. S-glass/epoxy followed curve C in water at above 80°C. As a consequence. 118 . the diffusion process can be actively controlled by using a matrix with lower uptake or lower permeability. glass/polyester at 40°C and under an applied load of 50% ultimate tensile stress displayed type D behavior. the degree of cross-linking. The rate of fluid sorption and the quantity absorbed are governed for the most part by the chemical structure of the resin. Curves C and D represents the most adverse situation pertained to the material performance. The cause for the jump is attributed to a change of environment such as temperature. and is inseparable from other performance aspects concerning durability.Corrosion Tutorial Moisture absorption behavior of composite materials can be categorized into several types. the type of crosslinking. for example. E-glass/vinyl ester with acryl-silane or epoxy silane surface treatment follows linear Fickian behavior for water absorption up to 80°C. also an irreversible process as a result of leaching out of the material from the bulk following chemical or physical break-down. In general. and material system. a hybrid matrix composite. Glass fiber reinforced plastics (GFRP) exhibit such behavior under specified conditions. Curve LF represents linear Fickian behavior. applied load. moisture weight gain follows a decreasing trend after the initial take-off. fiber/matrix debonding and matrix cracking. external load. Sorption process involving severe circumstances such as elevated temperatures. where after a rapid initial take-off the moisture weight gain gradually attains equilibrium. The rapid moisture weight gain depicted by curve C results from large deformation or damage of the material. The effects of physical aging continue until the material reaches volume equilibrium. The time required to reach volume equilibrium depends on the aging temperature. Aging is a characteristic of the glassy state and is found in all polymer glasses.2 Effect of Physical Aging on Creep Physical aging is a process. or delayed increases in creep compliance.3. The gradual approach towards equilibrium affects the mechanical properties of the polymer. it can be visualized that as the free volume decreases towards its equilibrium values. where the macromolecules gradually change their packing in order to approach the equilibrium free volume state. giving rise to a stiffer response. In some cases this may be “desirable” if the changes are understood and predictable. typical for infrastructure.1. they become stiffer and more brittle. During this time the mechanical properties may change significantly. especially when the material is subjected to an aging time as long as 50 or more years. In terms of free volume theory. the mobility of chain segments is hindered. as polymeric materials physically age. 119 . so that the compliance is decreased (or the modulus increased) than expected in a viscoelastic material without aging.3. Effects of physical aging on long-term performance of FRP could be substantial. In general.Corrosion Tutorial 4. often resulting in a material that is stiffer and more brittle. occurring below the glass transition temperature Tg. show less stress relaxation. show less stress relaxation. Physical aging can lead to changes in the short-term as well as long-term mechanical properties.3. or delayed increases in creep compliance. especially on aging the material for 50 years or more. Radiation below approximately 290 nm is effectively eliminated by stratospheric ozone. This is because the materials become glassier. In addition. In general. 4. they become stiffer and more brittle. In some cases this may be “desirable” if the changes are understood and predictable.3 Effect of Ultraviolet (UV) Radiation on Creep Ultraviolet radiation that reaches the earth’s surface comprises about 6% of the total solar radiant flux and has wavelengths between 290 nm and 400 nm. This phenomenon has been investigated extensively in the recent past. Since most polymers have bond dissociation energies on the order of the 290 120 . as polymeric materials physically age. and then the long-term compliance decreases relative to simple extrapolation of the short-term compliance. The remainder of the solar radiation is composed of visible (52%) and infrared (42%) radiation.1. the process of physical aging is found to be both temperature and load dependent. It is found that the short-term creep compliance curves shift to longer times with increasing initial aging time.3. Effects of physical aging on long-term performance of FRP can be substantial. and it is well understood.Corrosion Tutorial Figure 4-19: Effect of physical again on creep behavior. 2 Fatigue and Fracture Fatigue is defined as the failure or rupture of a plastic article under repeated cyclic stresses. they found that during the time that the UV radiation was turned on. freeze-thaw and other environmental components. (1967) suggested that the increase in creep of exposure to UV radiation was the result of increased stress relaxation or easier relative movement of polymer chains. carboxyl (COOH). the creep rate returned to the original value. as flaws that result from surface photo degradation can serve as stress concentrators and initiate fracture at stress levels much lower than those for unexposed specimens. or peroxide (O-O). or photo degradation. windborne abrasives. as it is exhibited by a large number of polymers. showing that the creep-rate change was reversible.3.0 nm. The effect of UV radiation on the creep behavior of different polymeric materials was investigated by Regel et al. subsequent reactions with oxygen result in the formation of functional groups such as carbonyl (C=O). Bond dissociation is initiated by the absorption of UV radiation. By irradiating a loaded specimen in the wavelength interval from 248. This increase occurs at each and all steps of creep and is fairly general. moisture. The effect of ultraviolet radiation is also compounded by the action of temperature.Corrosion Tutorial nm to 400 nm wavelengths in the ultraviolet region. resulting in chain scission and/or cross linking. at a point below the normal static breaking strength.3. Regel et al. On turning off UV radiation. degradation at the surface of a polymeric component has been shown to affect mechanical properties disproportionately. in some cases. creep strain increased sharply. Chemical changes induced by UV exposure are the result of a complex set of processes involving the combined effect of UV and oxygen. they are greatly affected by exposure to this portion of the solar spectrum. Fatigue failure occurs when a specimen has completely fractured into two parts.3 to 300. However. [1967]. are usually confined to the top few microns of the surface. The effects of UV exposure. has softened or has 121 . 4. ) • Laminate Stacking Sequence • Environmental conditions (mainly temperature and moisture absorption) • Loading conditions (stress ratio R. No significant reduction of stiffness is observed during the fatigue process. braiding. In metals. fiber buckling. matrix cracking. and the fatigue process can be simulate d in most of the cases by a linear analysis and linear fracture mechanics. The main reasons for this are the different types of damage that can occur (eg. the stage of gradual and invisible deterioration spans nearly the complete lifetime. fiber matrix interface failure. The final stage of the process starts with the formation of small cracks. Fiber Reinforced composites have a rather good rating regards to life time in fatigue. and their behavior is more complicated than that of homogeneous and isotropic materials such as metals.Corrosion Tutorial otherwise significantly reduced in stiffness by thermal heating or cracking. mat. Gradual growth and coalescence of these cracks quickly produce a large crack and final failure of the structural component. cycling frequency …) and boundary conditions There are a number of differences between the fatigue behavior of metals and fiber reinforced composites. The following are the parameters that influence the fatigue performance of composites • Fiber type • Matrix type • Type of reinforcement structure (unidirectional. Fiber fracture.…). delaminations. their interactions and their different growth rates.. It can also be arbitrarily defined as having occurred when the specimen can no longer support the applied load within the deflection limits of the apparatus. As the stiffness of a metal remains quasi unaffected. which are the only form of macroscopically observable damage. The same does not apply to the number of cycles to initial damage nor to the evolution of damage. Composite materials are inhomogeneous and anisotropic. 122 . matrix crazing. the linear relation between stress and strain remains valid. fabric. may result in an increase in the long life fatigue strain in the resin. humidity. 123 . (1983) noted that the fatigue degradation rate of GFRP preconditioned in boiling water is slower compared to dry specimens. It is a quantity defined by the intersection of two linear parts on the stress strain curves. Studies comparing glass and carbon fibers suggest carbon fiber composites are superior in fatigue performance in terms of fatigue life and rate of damage development.. As a consequence an appraisal of the actual state or a prediction of the final state (when and where final failure is to be expected) requires the simulation of the complete path of successive damage stress. The authors attribute this phenomenon to several factors. stress transfer capacity of the resin-fiber interface will be reduced by boiling. Third. compared to those fatigued without imposed environment. The gradual deterioration of a fiber reinforced composite-with a loss of stiffness in the damaged zones – leads to a continuous redistribution of stress and reduction of stress concentrations inside a structural component. and other corrosive fluids (such as acids) on FRP are to shorten their fatigue life. small matrix cracks leading to large size delaminations). The effects of elevated temperatures. Fatigue behavior of composite laminates is both frequency and temperature dependent. Plasticization of the resin Second. preconditioning in boiling water permitted relaxation of the thermal strains introduced during processing. Jones et al. which in turn will reduce the stress concentration in the vicinity of the broken fibers or resin micro-cracks. The matrix also influences fatigue performance at a low number of cycles. while the damage type in these zones can change (e. Low deflection fatigue is matrix and interface dominated while high deflection levels include matrix cracking.g. Higher frequencies and higher temperatures tend to reduce the fatigue life of the composite material. It was found that the influence of the matrix on the quasi-static properties is on the position of the knee point in the stress-strain curve. Fatigue loading above the knee point results in degradation of stiffness followed by sudden drop in stiffness leading to the death of the composite material and loading below knee point is the no-failure situation.Corrosion Tutorial In fiber-reinforced composite damage starts very early and the extent of the damage zones grows steadily. glass fiber fracture at the tensile surface. fiber matrix debonding. first. and plane fiber buckling and delamination on the compressive side. The damage accumulation in these Fatigue damage mechanism in loading mode (e. (1971) have shown that the susceptibility of different epoxies to water is different. tensile. implying direct impact of water on the interphase region.. 1987] namely: Fiber breakage Matrix cracking 124 .3. fatigue damage is initiated by debonding between fiber and matrix. parallel or inclined to the fiber direction. In unidirectional fiber reinforced composites.g. The rate of reduction on the off-axis specimens in water is higher than the unidirectional specimens. In uni-directional composites. bending. FRP composites accumulate damage in general rather than developing localized damage. the damage mechanism in tensile fatigue is of three stages as shown in Figure 4-20 [Talreja. The fiber matrix interface region has a controlling effect on the environmental fatigue of composites. It has been indicated that fluid absorption during fatigue is faster than under static condition. torsion or combinations) and on the loading direction i. Specimens exposed to water have reduced fatigue life and is attributed to the combined effects of mechanical fatigue and localized pressure exerted by water trapped in the micro cracks.2..Corrosion Tutorial Romans et al. delamination and fiber fracture [Mathews. leading to the failure of the composite. But matrix micro cracks transverse to the loading axis develops and propagates. matrix cracking. unidirectional composites primarily depends on compressive. and there is a dynamic interaction of fatigue behavior and the environment. which includes fiber/matrix debonding. thus breaking fibers or causing interfacial failure.3. 2000].1 Fatigue Process Unlike homogeneous materials. and fracture does not always occur by propagation of a single macroscopic crack. 4. which suggests that the effect is matrix and/or interface controlled and there is a close relationship between chemical structure and environmental susceptibility. materials is micro structural.e. Typically. fractures in fibers occur but the accumulation is slower and the life of the composite is not dependent on fatigue fractures in fibers. For every new material with a new lay-up. Under low strains. which are the last to fail. a matrix crack stops at the interface. approximately 50% of ultimate tensile strain of matrix. the stresses at crack tips exceed the fracture stress leading to fiber pullout or breakage of adjoining fibers due to higher stresses. which is useful information in materials selection for improvement in service properties. (b) Matrix Cracking. The fatigue behavior of composite materials is conventionally characterized by a Wöhler or S-N curve. 125 .Corrosion Tutorial Interfacial shear failure Figure 4-20: Fatigue Damage Mechanism in Unidirectional Composites Under Loading Parallel to Fibers: (a) Fiber Breakage. However. (c) Interfacial Shear Failure [Talreja. altered constituents or different processing procedure.e. A study of the fatigue damage mechanisms would also give indications of the weakest microstructural element. at least qualitatively. a whole new set of fatigue life tests has to be repeated for such a characterization. In tensile fatigue of a multidirectional laminate. which causes shear-stress concentration at the interface i. The debonded area leads to matrix cracking when the stresses exceed the fatigue limit. to predict the macroscopic fatigue behavior. the critical elements are the longitudinal plies. close to the tip of the broken fiber. it would be possible. 1987] Mechanical fatigue is the most common type of failure of structures in service.. at high strains. If the active fatigue damage micromechanisms and the influence of the constituent properties and interface were known. leading to debonding of the fiber from surrounding matrix. Interfacial Debonding. Strength degradation is assumed also to take place in these two stages reflecting the development of the underlying damage process. Fiber breakage is due to the failure of the weakest fiber in the laminate due to excess stress. residual stresses and number of cycles at the CDS. In the second stage.29) Nc = …(4.30) where Rc. The evolution of damage is expressed by the rate of growth of the crack dimension.maximum applied stress. The complete two-stage strength degradation model for fatigue reliability 126 . a general weakening of the material is assumed and is considered dependent on damage parameters representing the stage of damage.28) …(4. 1987]. taking it as a function of an effective stress. respectively. m. Nc . A power law is assumed for the rate of increase of the damage parameter [Talreja. and S. k . strength degradation is assumed to result from the localized zones of damage which are conceptually replaced by a single crack capable of releasing the same amount of elastic energy as that released collectively by the various crack growth mechanisms.Corrosion Tutorial In the first stage. Assuming a power law for the crack growth. Residual strength (R) is related to a characteristic dimension of the "equivalent” crack C through a fracture mechanics type relationship R = αC −1 / 2 …(4. respectively. A relationship between the residual strength (R) and the initial strength (R0) is then given as: ⎛ N R = R c + (R 0 − R c )⎜ ⎜1 − N c ⎝ m' = 1 (1 + m) 1 k (1 + m )S m ⎞ ⎟ ⎟ ⎠ m' …(4. m’.31) where α is the material constant characterizing material toughness [Talreja. a relationship between residual strength and the applied maximum stress is derived. This relationship forms the basis for determining the probability distribution of the residual strength and the probability distribution of the number of cycles to attain the Characteristic Damage State. 1987].material constants. which is assumed to depend on the current state of damage given by the current crack dimension. The use of stiff fibers such as carbon fibers results in low strains (1.2.8%) to failure and less stiff fibers like glass lead to relatively higher strains (2.3.3. The S-N curve for carbon fibers with different stiffness in the same standard epoxy resin is shown in Figure 4-21. Figure 4-21: Two-Stage Strength Degradation Model for Fatigue Reliability of Composites [Talreja. 127 .1. Rc and Nc are the residual strength and the number of cycles corresponding to CDS. 1987] 4.Corrosion Tutorial analysis of composites is illustrated in Figure 4-21 where R is the initial strength.2 Fatigue in Unidirectional Composites The S-N curve for carbon fiber. S is the maximum applied stress and Nf is the number of cycles to failure.5 – 3. glass fiber and aramid fiber in the same standard epoxy matrix is shown in Figure 4-22.5 %) to failure. the curve is steep for glass fibers while it is shallow for carbon fibers in Figure 4-22.0 . The slope of the curve (Figure 4-21) is a function of the strain in the matrix [Curtis and Dorey 1986]. Hence. plots of mean strain rather than stress versus log cycles to failure are commonly used for composite materials (Figures 4-22 and 4-23). This is because the fatigue behavior of composites is dependent on the strain in the matrix and interfacial characteristics rather than fiber strength. Due to this reason. 128 . 1986] It can be seen that there is little improvement in the fatigue behavior with change in fiber stiffness.Corrosion Tutorial Figure 4-22: Comparison of S-N Curve for Three Different Unidirectional Composite Materials [Curtis and Dorey. It can be seen that as the fiber stiffness reduces. 129 . 1986] A typical fatigue life diagram (Figure 4-24a) for a unidirectional composite under loading parallel to fibers is shown. distinct progressive damage band (matrix cracking) is observed before fiber breakage.Corrosion Tutorial Figure 4-23: Comparison of S-N curve for Four Different Materials with Different Carbon Fibers in Same Epoxy Resin [Curtis and Dorey. fatigue limit of the matrix is defined as the maximum strain below which no cracks or only non-propagating cracks maybe initiated in the matrix material. This matrix material property is taken as the lower limit of the progressive matrix damage. In Figure 4-24a. This leads to a mixed mode crack growth parallel to the fibers. the increase in the number of non-axial plies in a laminate reduces the strength and stiffness since only fewer fibers 130 . The limiting values of crack tip displacement will depend on the off-axis angle.2.3 Fatigue in Multidirectional Composites The damage mechanism in multidirectional composites is similar to off-axis loading in unidirectional composites except that delamintion is found to occur in these laminates [Talreja.3. In multidirectional composites. 1987]. delamination and hence overstressing of the 0o ply. i. thus. an opening normal to the fibers and a sliding parallel to the fibers. [Talreja. It was found that for off-axis angles more than a few degrees.. as matrix and/or interfacial cracking will become the predominant damage mechanism for strain up to fracture strain. 1987] For off-axis loading angles between 0o and 90o. the first event of failure is debonding of transverse fibers.e.Corrosion Tutorial Figure 4-24: Fatigue Life Diagram of Unidirectional Composites Under (a) Loading Parallel to Fibers. the fiber breakage bond would be lost. the tip of crack initiated in the matrix will be subjected to two displacement components. with crack tip displacement increasing with an increase in off -axis angle. The fatigue life diagram for off-axis loading is shown in Figure 4-24 (b). (b) Off-Axis Loading (Dotted line correspond to on-axis loading) [ Talreja. 1987] 4.3. The debonded crack then grows towards the ply This leads to interface causing stress concentration in the interfacial layers. which causes a small reduction in strength and stiffness [Curtis and Dorey. 1986]. an extensive review of the 131 . Figure 4-25 gives the comparison of S-N curve for varying percentage of 0o plies Angled ply layers with fibers typically at ±450 can also develop intraply damage. often giving rise to delamination between layers.Corrosion Tutorial are available to support the mean applied stress in the axial direction [Mathews. when they are manufactured with different constituent materials. In the following sections.3 Aging Due to Environmental Factors Several researchers have investigated the changes in properties and performance of composites under various environments. 2000]. 1986] 4.3. Figure 4-25: Normalized S-N Curves for (0/±45) CFRP Laminates with Varying Percentage of 0o Fibers [Curtis and Dorey.3. Also stress concentration is developed at the ends of intraply cracks. For a multidirectional laminate. Multidirectional laminates also develop edge-induced stresses because of different elastic properties of the layers. which causes delamination between the layers. 4. and interfacial bond strength of the composite material change with exposure time.Corrosion Tutorial literature dealing with durability/aging of composites is carried out. temperature (free-thaw cycling.3. fatigue life. humid air or liquid). 3) additives. Special attention is paid to understanding the existing experimental and analytical methods to qualify and predict the effects of constituent materials and environmental conditions on composites.3. 5) moisture and pressure. fire) and other weathering conditions (physical. The chemical weathering condition occurs when either the neat resin or composite materials is exposed to chemical solutions such as alkaline. water or any liquid. When composite materials are subjected to these environmental factors. Composite material property changes over a long service life (» 50 years) can be predicted by experimental methods simulating the environmental conditions in the laboratory. The physical weathering condition occurs when either the neat resin or a composite material is subjected to mechanical loadings such as static load. elevated temperature. acid or aqueous.3.3. Degradation of mechanical properties depends mainly on: 1) chemical and physical structure of the polymer. chemical.3. commonly known as “moisture problem.” can be predicted by analytical methods such as Fick’s diffusion law [Springer. 4) time and temperature. 2) physical state of the material.1 Effect of Moisture The effect of moisture on composite materials occurs when the composite materials are exposed to humid air. mechanical and chemical properties such as strength. Depending on the environmental conditions and the condition of the material. glass transition temperature. stiffness. 1981].1. fatigue and creep.3. and 6) nature of stress [Kilen. the material either absorbs 132 . and UV rays exposure) affect the performance of both neat resin and composite materials.1 Environmental Factors Influencing the Durability of Composites Environmental factors such as moisture (water. creep. 4. 1983]. The moisture and temperature distribution inside the composite materials. nanomers. vinyl ester and isopolyester) followed the Fickian process [Chin. Neat resins show higher moisture absorption than the composites show. and even at the interface of fiber/matrix level. Further. The swelling of the matrix causes stress within the material. Chin [1999] observed the moisture uptake for vinyl ester and isopolyester resin exposed to distilled water. First. many resin systems tend to recover their properties upon drying. Moisture affects the composite materials at the matrix or the fiber level. In general. Diffusion in all three liquids (water. and a non-Fickian behavior occurs when the resins or composites undergo damages such as cracks. Among epoxy. which eventually tends to decrease the strength and stiffness. ester groups are distributed along the main chain. additives. mass loss occurred after a certain period in alkaline solution and salt water. alkaline) and in three resins (epoxy. salt. The structure and morphology of a resin affect the moisture uptake. 1982]. salt water and concrete pore solution at 22oC and 60oC.. salt-water uptake was higher than pure aqueous or alkali solution uptake. moisture penetrates through the resin and later moisture is transferred through the cracks [Springer. type of liquid to which the resin is exposed to. For vinyl ester (at both ambient temperature and 60oC). 1996]. Most of the resins or composites follow the Fickian process. Moisture absorption and desorption in a composite material eventually leads to the degradation in its properties. etc. 1981]. making them more available to hydrolysis reactions at higher temperatures.Corrosion Tutorial or loses moisture as manifested by weight gain or weight loss [Springer. and isopolyester resins. but at 60oC. This is attributed to the fact that in a polyester resin. the sorption was greater for epoxy compared to the other two resins because more hydroxyl groups are present in epoxy matrix. However. because in the resins the matrix swells when exposed to moisture. vinlyester. the percentage of moisture absorption varies from resin to resin based on their chemical structures. temperature. fillers. a high concentration of polar functional groups can promote increased sorption of polar penetrants [Apicella et al. 1981]. Effect of moisture in resins Moisture content in resins depends on various factors such as type of resins. Similar results were observed in the case of isopolyester at ambient temperature. 133 . Stress corrosion in turn is dependent on the type of attacking fluid. wherein more concentration of corrosive fluid leads to greater detriment to glass fibers. the room temperature flexural strength of the laminates decreased by 40% over the dry state. the moisture content depends on the relative humidity in the air. however aramid fibers alone absorb significant amount of moisture when exposed to high humidity [MIL-HDBK-17. the strength drop was found to be in the range of 60 – 70%. or debonding and 2) reduction in energy required to break the Si-O bonds. 1997]. The maximum 134 . The moisture absorption of most fibers used in practice is negligible. When composites with polymer matrix are placed in a wet environment. Effect of moisture in composites Moisture content in composites depends on the type of composites and environmental conditions that the composite is exposed to over a certain period and range of temperature.Corrosion Tutorial Effect of moisture in glass fibers Studies conducted by Ehrestein and Spaude [1984] showed that both glass fiber and glass fiber-resin bond are susceptible to degradation through moisture content. Similarly. Acceleration is thought to be the result of two factors: 1) reduction in surface energy of glass fibers after exposure to moisture that reduces the energy required for interfacial crack formation. at an elevated temperature. The effect of temperature and moisture on Kevlar/epoxy laminates was studied by Allred [1984]. glass fibers do not recover their properties but tend to corrode. Glass fibers can lose up to 10% of their bending strength when exposed to moisture. the moisture content depends on the type of the attacking liquid. When composites are exposed to acidic or alkaline liquids. [1998] show that water accelerates the rate of crack growth in glass with the degradation being more severe and following different mechanisms with higher temperature exposures. the matrix begins to absorb moisture. While resins recover their lost properties while drying. Results from a companion study of Karbhari et al. who showed that in the saturated state. eventually leading to loss of effective cross-section. The strength of E-Glass is time dependent in the presence of moisture and is susceptible to stress corrosion. When composites are exposed to humid air. but the 135 . consequently. 1998]. These internal stresses can be calculated as in MIL-HDBK-17 [1997]. This hypothesizes that the degradation of the material has started at the interface of the fiber/matrix level. The maximum moisture content was nearly twice in the alkaline solution compared to tap or salt water.3.22 ~ 0. 4.Corrosion Tutorial moisture content in the Graphite/epoxy composite was found to be lower in salt solution (1. the interface tends to become more hydrophilic when exposed to moisture and the following is noted: 1) The fibers weaken due to crack growth that is accelerated by water in the resin. tensile and flexural strength. The mechanical and visoelastic behavior of composites could degrade dramatically at elevated temperatures and under freeze-thaw Elevated temperature When the fiber reinforced composite materials are exposed to elevated temperature.3.1.33% in the tap water [Vijay. The magnitude of these temperature effects depends on the type of liquid to which the composite is exposed. fatigue strength and creep resistance and adhesive strength may decrease [Kelen. mechanical properties such as modulus. Free swelling of layers does not take place and. In the case of E-glass /polyester composites.3. Temperature affects the rate of moisture absorption and chemical and mechanical properties of the composites. 3) plasticization of resin by water results in increase in viscoelasticity. and leads to fiber debonding and consequent weakening of composite. When a composite absorbs moisture.2 Effect of Temperature Temperature plays a vital role in the durability of composites.25%) than in distilled water [Springer. cycles. A similar trend was noted in the Eglass/vinyl ester composite with maximum moisture content of 0. Composite materials generally degrade when exposed to moisture. 1981]. 2) The resin swelling produces radial stresses at interface that is reinforced by water pressure. internal stresses are developed.29% in the salt solution compared to the moisture content of 0.18 ~ 0. which is discussed below. 1983]. the swelling coefficient of fiber is lower than the matrix. Vinyl ester composites aged at high temperatures in an aqueous environment showed an overall decrease in strength with an increase in the duration of conditioning at a given condition. differential scanning calorimetry 136 .4oC to –17. The alkali affects the fiber/matrix debonding as well as the fibers. As the temperature increases. particularly at higher temperatures [Buck. alkaline solution will be the most damaging solution to glass composites and also to the neat resins in terms of tensile strength. At 99oC the alkaline solution was found to be more corrosive and weight losses found to be about two to five Freeze-thaw cycles The excess moisture in composites generally expands upon freezing and causes internal stresses.Corrosion Tutorial degradation is further enhanced at elevated temperatures mainly due to matrix degradation. 300 and 500 cycles. The durability of pultruded vinyl ester composites was investigated by means of freeze-thaw tests. At an elevated temperature. 100. the mechanical properties of a composite degrade largely under freeze-thaw cycling. there was significant strength and stiffness loss of GFRP composites in alkaline solution under freeze-thaw cycling (0 – 70 oF) [Vijay and GangaRao. The strength and strain to failure were found to be approximately 50% lower for E-Glass/vinyl ester and E-Glass/epoxy samples when subjected to 4. Samples were subjected to three levels of exposure. For example. which initiate cracks or delaminations. 1994]. 2000].8oC freeze-thaw cycling temperature [Lesko. An increase in temperatures may also cause time-dependent effects such as creep and stress relaxation [Janas and McCullough. 1987]. Similarly. 2000]. et al. the ultimate tensile strength of Kevlar laminates was found to decrease by 23% after 360 cycles and by 63% after 1170 cycles when subjected to two hour temperature cycles from –20oF to 125oF [Allred. 1995]. times greater at temperatures higher than at 66oC. [1999]. An average strength reduction of about 10% was observed for glass reinforced isopolyeser and vinyl ester structural plates when treated in 4% salt solution and exposed at both room temperature and cyclic temperature of 0 – 70oF [Ajjarapu. whereas low temperatures may result in brittle failures. the pH value of an alkaline system also increases. Due to the formation of cracks. 1998]. thereby increasing the corrosiveness inhibited by the alkaline solution. In the work done by Vergheese. freezing process. Low temperature thermal cycling has shown that both bending and shear moduli degrade in case of plain weave glass composites. concluded that low temperatures stiffen the polymer composites. In one other study. In fact.1. accumulation of damage due to the growth of cracks.. Heat flow measurements during thawing taken for a single cycle (-150oC to +50oC) on saturated.e. [1994]. Strength gain (less than 10%) and stiffness loss (less than10%) were observed for a glass/vinyl ester composite when conditioned in salt 137 . Dutta. which further impede the However. Thus. 4. The flexural behavior of composites is matrix-dominated and the increase of bending and shear modulus at lower temperature controls the composite’s flexural properties. The slow diffusion rate prevents early damage of fiber/matrix interface and the glass fibers. hence the diffusion rate is slower. Strength gain is mainly due to post curing which improves the properties of matrices in the composites. This was attributed to the fact that water resides in the free volume of the resin.3.3. unreinforced vinyl ester resin samples indicated no melt endotherm and thus the absence of freezable water. This is because salt molecules are larger than other liquid molecules. i. the crack dimensions in composite systems are large The freezability of water leads to enough to facilitate the freezability of water.3. This is due in part to geometric space constraints in addition to hydrogen bonding.3 Effect of Solutions with Different pH Levels Effect of salt solution The effect of salt solution on a composite is very low compared to the effect of other environmental aging conditions.20 A. matrix and interface. polymer matrix and its type play a crucial role in the composite’s behavior at subfreezing temperatures. Since this free volume size is in the order of about 4. several researchers have observed a strength gain in composites when exposed to salt solution. these voids are thermodynamically too small for water to freeze.Corrosion Tutorial (DSC) was used to identify the nature and presence of freezable water for each constituent material within an E-glass/vinyl ester composite. It is highly unlikely to freeze water in highly cross-linked amorphous polymers. The ester linkages in the outer layers of the continuous strand mat bind the free water through hydrogen bonds in the surface layers. boron free glass fiber shows improved performance over traditional E-glass fiber because the former has improved corrosion 138 . The tensile strength loss of about 19% was observed in a glass/epoxy composite conditioned in salt solution at 60oC. cohesive failure [Kajorncheappunngam. the acid diffuses through the matrix and subsequently reaches the surface of glass fibers. 1999]. From the SEM micrographs.. Durability of composites in an aqueous solution greatly depends on the type of fiber that reinforces the composite. When composites are exposed to acids. For example. Effect of aqueous solution\acid solution Acid affects on the composite in terms of reduction of strength and stiffness at room temperature. 1998].e. The degradation in tensile strength (when exposed to salt solution) can be reduced to some extent by adding chopped strand mats in the composites. thereby increasing the rate of diffusion and leading to debonding of fibers. the effect of acid on strength reduction is low because the ion exchange reaction reduces at elevated temperature. it was concluded that the failure of composites was due to matrix degradation. and hence the damage to glass fibers is low compared to room temperature damage. Once the acid contacts the glass fibers. In general. eventually leading to surface shrinkage. the ion exchange takes place between glass fibers and acid. i. The surface shrinkage causes internal stress within the fibers and initiates cracks in the fibers. The reduction in tensile strength of a glass/epoxy composite At elevated exposed to acid solution over a 5-month period was 73% at room temperature. temperatures. Sometimes. 1999]. hence reducing its effect through the thickness. the degradation in strength and stiffness of composites exposed to salt solution is insignificant compared to other aging liquids. the ions in the matrix cause fracturing of matrix.Corrosion Tutorial water at room temperature [Vijay. The reduction in strength and stiffness is greater when composites in salt solution are exposed at elevated temperature because the rate of diffusion increases at elevated temperatures. but it was only 48% at elevated temperature [Kajorncheappunngam. The effect of alkali can be anywhere in the composites.. 1996]. thus reducing the strength and stiffness of the composite. For example. matrix or at the interface of fiber/matrix. At an elevated The temperature alkali has a greater effect on the strength and stiffness of the composites. The rate of strength and stiffness loss was about twice in alkaline environment for E-glass/vinyl ester composite over that for glass composite exposed to salt environment [Vijay.e. 1998]. This is attributed to better matrix cross-linking reaction and becomes brittle. jackstraw.. This loss is attributed to the fact that the rate of moisture absorption in composites is more when exposed to alkaline solution than other liquids. matrix degradation and fiber attack [Altizer et al. The efficacy of application of gel coats and protective coating has been shown by the marine industry to prevent blistering. The aqueous solutions cause dramatic increase in hydrolysis of Kevlar 49 yarn. fibers. The degradation in such glass fibers can be protected to some extent by choosing the appropriate resin system. especially in the combination of temperature and stress. The hydroxide ions in an alkaline environment attack the primary component of glass (silica) and cause the breaking of Si-O-Si bonds in glass This results in fiber corrosion and reduction in strength. A reduction of about 70% in tensile strength and ultimate strain to failure was observed in a glass/polyester composite (Vijay and GangaRao. brittleness leads to matrix cracking. Composite with Advantex glass fibers (boron free glass) had 139 . corrosive resistant glass fibers and alkali resistant resins can be used to make composites. Hence. application of gel coats and providing appropriate protective coatings. 1999). durability of composites in alkaline solution can be improved by selecting proper fiber and/or resin. Effect of alkaline solution Significant loss in strength and stiffness occurs when the composites or neat resins are exposed to alkaline conditions. fiber. The alkaline solution mainly attacks at the interface of fiber/matrix debonding. i.Corrosion Tutorial resistance. Boron free glass fibers (ECR) show improved performance over traditional E-glass fibers because of their improved corrosion resistance. ECRGLAS (boron-free glass) laminates were found to have 30% higher flexural strength than E-glass laminates. vinyl ester has better alkaline resistance compared to other resins and exhibits excellent strength and stiffness properties. while those with E-glass had only about 85% retention in the tensile strength Devalpura [1998]. Cementitious extract with pH 10 buffer and water had the greatest degradation in composite strength. which acts as a catalyst in degrading the glass fibers. The penetration of alkali into such fibers is mostly time dependent. Performance characteristics of vinyl ester changes with cure time. For GFRP bars. Glass fibers are more sensitive to alkali environment when compared to aramid or carbon fibers. 140 . which reduces strength of the bars.. The glass fibers deteriorate in the area where alkali has penetrated. 1996]. Higher alkali concentration increases the degradation on composites. The ultimate tensile strength of vinyl ester samples after 1300 hours of immersion in alkaline solution was about 70 MPa while that of isopolyester was only about 50 MPa. The increased distance between cross-linkages in vinyl ester implies that it does not completely polymerize. 1998].Corrosion Tutorial about 95% tensile strength retention. The strength of GFRP bars decreased with time when immersed in alkali solution. whereas those of AFRP and CFRP bars were not decreased [Katsuki. the penetration of alkali increases with time. With respect to performance of resin in the alkaline solution. The effect of alkali solution on composite materials can be potentially reduced by fiber sizings (to promote fiber/resin debonding) and by selecting alkali resistant resins and corrosive resistant fibers [Altizer et al. 1995]. This is hypothesized due to greater concentration of Ca ions available for formation of calcium hydroxide crystals at the surface of glass fibers. The pH level of an alkaline solution is another important factor. GFRP bars immersed in 5gm/L of sodium hydroxide (over a 4 month period) had about 20% reduction in strength while those in 20 gm/L of sodium hydroxide had about 30% reduction in strength [Alsayed. The performance of composite material in terms of Tg varies with temperature.3. The Tg varies depending on the type of aging environment and the exposure temperature of composites. 1995]. In contrast to neat resins. The aging solutions (depending on moisture absorption) lower the glass transition temperature (Tg) and enhance the apparent phase separation in composites due to the effect of polymer plasticization.3. polymer molecules have sufficient additional cross-linking reactions take place and there is continued resin curing process.. At temperatures above glass This is because. As the cure temperature increases. no significant changes were observed in Tg for vinyl ester and isopolyester resins following 1300 hour immersion in water.Corrosion Tutorial 4. once the hydrogen bond formation reaches its maximum state due to the finite number of accessible active sites available for H+ ions. and concrete pore solution [Chin. and then remains constant. Tg increases rapidly due to the hydrogen bonding. at elevated temperature. 1985]. salt solution. 1999]. the reinforcing fibers in a composite and the resulting interface regions actually enhance resin plasticization and hydrolysis.3. 141 . instantaneous mobility to get back towards equilibrium during temperature changes. When the composites are exposed to acidic environment. the lack of instantaneous mobility results in free volume in the system. At ambient temperature. the glass transition temperature also increases. The effect of plasticization was greater in glass/epoxy composites than it was in epoxy neat resin samples. When a polymer is quenched from above-to-below glass transition temperature. This change in free volume during the movement towards equilibrium results in altering mechanical properties of the bulk polymer [GangaRao et al.1. transition.4 Effect of Environmental Aging on Glass Transition Temperature (Tg) The glass transition temperature (Tg) generally indicates the thermal stability of a composite material. This was probably due to exposed edges that allowed solution to diffuse easily into the composite than the neat resin [Kajorncheappunngam. slowing down Tg kinetics. physical and chemical exposure. 4) fiber orientations.1. vinyl ester. because alkali solution breaks the glass bonds in the fiber leading to fiber breakage. However. Generally.1. it is clear that the resins and fibers made of polymers have significant property changes.5 Effect of Ultraviolet (UV) Rays The UV radiation affects polymeric composites. In addition. 1989].3. boron free glass fibers are corrosion resistant and perform well when compared to the traditional E-glass fibers.3. Although carbon fibers do not absorb moisture and are resistant to many 142 . Based on the above data. accelerated UV exposure tests under wet/dry cycles on marine fabrics with polyesters or nylon fibers were found to have significant reductions in tensile strength and elongation [Moore and Epps. 1992].3. 5) fiber volume fractions. 4. but also to some extent on structural factors.6 Structural and Manufacturing Factors Influencing Durability of Composites Durability of composites not only depends on environmental factors such as moisture.). no significant damage in terms of composite material properties has been found [Ashbee. 8) manufacturing techniques 9) others. Performance of a composite as function of some of the structural factors is addressed. temperature. vinyl ester/glass etc). On the other hand.3. Fibers The performance of composites under aging conditions depends on the type of fiber reinforcement. 6) thickness of the composites. The fiber should be selected in such a way that they are alkali resistance. FRP composites primarily driven by glass or carbon fibers do not vary significantly. Since the fibers are the main load resisting constituents of composites. 2) type of resins (epoxy. 3) type of composites (epoxy/glass. The factors that influence structural properties are: 1) type of fibers (glass. As mentioned earlier. 1989]. polyester etc. aramid. 7) interfacial bond.Corrosion Tutorial 4. especially at and near the surface [Ashbee.). the early degradation of the fiber should be avoided.3. carbon etc.3. herein. the penetration of alkali occurs more in glass fibers than in aramid or carbon fibers. Resins The durability of composites varies with the type of resin used in the composites. ECR-glass has an enhanced chemical resistance. The fiber volume content is one of the factors responsible for the durability of composites. Most of the aging solutions penetrate the resin and reach the core. Similarly. because the former was more corrosion resistant. A chopped strand mat is used for a smooth finish of a composite. carbon composites do absorb moisture. especially in acid environment [EUROCODE.Corrosion Tutorial chemical solutions. This is mainly attributed to the chemical structure of vinyl ester resins. The fiber orientation plays a vital role in the moisture absorption. Polyesters have double bonds at about 250g/mol level while vinyl esters 143 . Interlaminar shear strength and impact strength of the laminates were measured and found to have the properties of the composites with higher fiber content degraded faster than those with lower fiber content [Singh. when formulated with polymer matrices. Although the degradation of carbon fibers may not occur by itself. The extra moisture absorption effects in specimens with triaxial fabrics were hypothesized due to absorption along the fiber-resin interfaces with increased directionality resulting in increased crossover or contact points. The chopped strand mats are actually resin rich surfaces and increase the moisture absorption and diffusion coefficient. [1998] found higher moisture absorption but lower strength degradation in triaxial fabrics compared to the uniaxial fabrics. 1996]. Failure in the composites primarily initiates at the resin level. Hence. 1991]. In one study. they are easily attacked by the aging solutions. one should be careful in selecting the type of resins. bisphenol polyster was found to perform much better as compared to the isophthalic polyester. thus degrading the fibers or the interface of fiber/matrix. Since resins have more void contents. Karbhari et al. Diffusion coefficient for laminates with chopped strand mats at the surface was found to be considerably higher than with continuous fabrics. Also several studies have proven that vinyl ester has good stability against harsh environments compared to the polyester. In one study. the oxidation reaction at the carbon cathode degrades the matrix material in that location causing degradation of the composites. can be obtained by conducting experiments at coupon.33 are equations used to calculate nominal strength and stiffness. Manufacturing Techniques Manufacturing techniques play a partial role in the durability of composites. the base values obtained should be multiplied with modification factors for actual field and environmental conditions. Possibility of high void content exists in a composite during manufacturing.4 Knockdown Factors The mechanical properties of a composite material.4. and 12 ipm) were aged. Based on the microscopy results. (AASHTO LFRD Bridge Design Specifications) F=F0CfCmCcCaCst …(4.3.Corrosion Tutorial have reactive double bonds at about every 500-1000 g/mol. i.32 and 4. no difference was observed in the rate of moisture absorption. which eventually leads to degradation in strength and stiffness on a longterm basis. 8. These modification factors are called knockdown factors. Controlling the line speeds can reduce the void fractions during pultrusion. shear etc. then different conditions and mechanical properties were evaluated [Garland. bending. Equations 4.3. the void contents in the composites should be kept as low as possible during manufacturing process. Pull speeds at higher line speeds may provide some differences in void fractions. performance characteristics of composites with vinyl ester change with cure time. In addition. such as strength in tension. Pultruded samples made at different line speeds (4. Hence. fiber wet-out and degree of cure. which may then show the rate of degradation in the strength and stiffness when exposed to environmental conditions. In order to calculate the nominal strength and stiffness values for design.5% if possible and certainly no more than 1%. no difference was observed in the void fraction due to the effect of line speeds. 4.e.1. less than 0. The presence of voids in the components increases the moisture absorption and diffusion coefficient.32) 144 . 2000]. component or system level on non-aged specimens or by using the analytical formulae given in section 1. hence. 33) F= Nominal resistance in bending (b).c. 7 and 10 years and tested in tension for this research. Vijay. (1998) has given a table to account for various knock down factors by conducting tests on 3 GFRP bars each. 145 .t. Cm = Moisture content factor and/or humidity factor with pH variation Cc = Environmental factor.t. or v E = Nominal modulus for b.(4. or shear (s) F0 =Base resistance of b. depth. if the tensile strength of a bigger diameter bar is required and the data are not available then the size effect factor can be used to establish these values. or torsion (t).or v Cf = Size effect factor for dimensions of width. For example.c.Corrosion Tutorial E=EoCm where ….t.. or compression (c).or v Eo = Base modulus for b. span etc.c. These factors can be used when no test data are available. weathered under natural atmospheric exposure for 3. which varies with the FRP material exposure to different temperature levels Ca = Physical aging factor that varies with number of years of service Cst = Sustained load factor These knockdown factors or reduction factors are established through different tests and field evaluations. 93.2 Parameter Knockdown Factor (Reduction Coeff.5 0 F ) (T − 52.5 ≤ T ≤ 92.50F then a minimum reduction of 0.9-0.1 is applied for every 100F increase in the mean annual temperature.50F and hence knock-down factor of 1 is chosen at that temperature.70 0.90 Physical Aging Factor Ca Notes: size effect factor can be used only for interpolating bigger bar diameter strengths when smaller diameter bar (in this table #4 is chosen as reference) is tested.75 0.85-#5 Size effect Factor Cf Diameter 0. This is an empirical approach purely based on strength reductions in GFRP bars under accelerated aging at 69.60-#8 Moisture Content Factor Sustained Load Factor (20%-40% on GFRP bar) Cm Salt (Ph ≈ 7) Alkaline (Ph ≈ 13) Salt/Water Cst Alkaline Mean Annual Temperature (T0F) Cc (In combination with alkalinity and stress) 0.Corrosion Tutorial Table 4-5: Knockdown Factors (Vijay.) 1.00-#4 0.65 0.5 0 F ) 0.65-#7 0.5) 1− for 100 (52.1998) Factor Notation Eqn.680F and 1500F in addition to natural weathering 146 . values in this investigation were correlated for a mean annual temperature of 52. If mean annual temperature is more than 52.85-0.70-#6 0.8-0.70-0.1.1 & 1.80F.40 Temperature Factor to be used with (Cm and Cst) 1(T ≤ 52. 5 Durability Models Various analytical models used to predict the effect of environmental factors on the durability of the composite materials is collected and cited in the paragraphs. 4. physical.3. 1981.3.Corrosion Tutorial results of Litherland et al.1 Analytical Methods to Predict the Effects of Environment on Composite Materials 4. 1981].3..3. 4.5.5. t) The moisture concentration inside the material as a function of position and time c (x. This factor is given considering that all the environmental factors that cause aging do not act at the same time and place. which follow.4 is provided as the reduction factor for combined effects (Vimala.1 Effect of Moisture Following parameters are necessary to describe the behavior of composite material exposed to an environment with temperature ‘Ta’ and moisture content ‘ca’ as a function of time ‘t’ [Springer.t) The total amount (mass) of moisture inside the material as function of time m(t) Changes in “performance (e. 2002).3.g. The temperature distribution inside the material as a function of position and time T (x.1. A limit of 0.3. Mean annual temperatures above 900F are not expected in any part of the globe. chemical or mechanical property)” of the material as a function of time P(t) 147 .. thermal conductivity K. Analytical predictions based on Fickian diffusion are a function of geometry. Fourier’s Equation of Heat Transfer Fourier’s equation of heat transfer is given by: ρC ∂T ∂ ∂T = Kx ∂t ∂x ∂x …(4. Heat transfer through the material is by conduction only and can be described by Fourier’s law The moisture diffusion can be expressed by a concentration-dependent form of Fick’s law Energy (Fourier) and mass transfer (Fick) equations are decoupled Thermal conductivity and mass diffusivity depend only on temperature and are independent of moisture concentration or the stress levels in the core. specific heat C. and K x = thermal conductivity. maximum moisture content Mm and a relationship between the maximum moisture content and the ambient conditions [Shen and Springer. analytical methods can be employed if the diffusion process is “Ficikian” Fickian Diffusion The diffusion process is said to be “Ficikian” if the following conditions are met [Springer. 1981]. C = specific heat. 148 . 1981].34) where ρ = density.Corrosion Tutorial When changes in temperature and moisture inside the material are to be determined and not the performance. initial conditions and material properties such as density (ρ). t= time. T= temperature. boundary conditions. x = distance. the percent of moisture content M of a composite is defined on weight gain M= Weight of the Moist FRP ( w ) − Weight of the dry FRP ( w d ) x100 …(4. if: Cracks develop in the material or delamination occurs leading to an altered structural representation Moisture propagation is dominated by fiber-matrix interface Many composites under ambient conditions follow Fickian diffusion and this process is extensively modeled.35) where c = moisture concentration. leading to lack of experimental data on specimens under service. Dx= diffusion coefficient.36) Weight of the dry FRP ( w d ) Mm is the maximum moisture content that can be attained under given environment and is found to be insensitive to the temperature but dependent upon the moisture content in the environment. whereas the non-Fickian diffusion models are sparsely used because excessively cracked specimens are removed from service well before the non-Fickian phenomenon sets in.Corrosion Tutorial Fick’s Equation of Mass Transfer Fick’s equation of mass transfer is given by: ∂c ∂ ∂c = Dx ∂t ∂x ∂x …(4. Mm depends upon the relative humidity (φ) . Moisture Content (M) In practice. and is given by: 149 . For a composite material exposed to humid air. t = time Diffusion is said to be non-Fickian [Shen and Springer. x = distance. 1981]. Diffusion Coefficient for Alkaline or Salt Solution (k) For FRP circular sections such as bars immersed in an alkaline or salt solution.. the diffusion coefficient is a function of the depth of liquid penetration.Corrosion Tutorial Mm=aφb (for humid air exposure) …(4.39) where Do = diffusion coefficient with respect to reference temperature. Ed = activation energy for diffusion. Dc = D0 exp −Ed / RT …(4. 1981] 2 2 ⎡⎛ ⎛ M2 − M1 ⎞ ⎤ ⎞ h ⎟ ⎥ ⎟ ⎜ D c = π ⎢⎜ ⎜ t − t ⎟ ⎥ 4Mm ⎟ ⎢⎜ ⎝ ⎠ 1 ⎠ ⎝ 2 ⎣ ⎦ …(4.37) where a and b are constants. Mm is a constant with time after the material reaches saturation level. 1981]. Mm= maximum moisture content. Diffusion coefficients (Dc) can be calculated by using [Rao et. t1 and t2 = time taken to reach the moisture contents M1 and M2.40) where h = thickness of the composite. T = absolute temperature. R = universal gas constant.al. Mm=constant (for liquid immersion) …(4. For a composite material immersed in a liquid.38) Diffusion Coefficient (Dc) The diffusion coefficient characterizes the speed at which moisture is transported through the material. The temperature dependence of diffusion coefficient for a rectangular composite exposed to moisture is characterized by Arrhenius relationship [Rao et al. concentration of 150 . . 1984]. PmD ⎡ TGW − T ⎤ = ⎥ Pmo ⎢ ⎣ TGD − T0 ⎦ where PmD = degraded mechanical property. TGW = glass transition temperature of wet resin. x = s ⋅k ⋅C⋅ t where x = depth from surface of rod C = alkali or salt concentration (mol/l) t = curing time (hrs) k = diffusion coefficient of alkali or salt in rod. Pmo = mechanical property at reference 0.5 …(4.(4. whereas TGW is given by empirical relation: TGW = (0.1M + 1) TGD (M≤ 10%) ….Corrosion Tutorial the liquid. exposure duration and the bar diameter [Katsuki.42) condition.005M2 – 0. 1995] as given in the following eqn. TGD = glass transition temperature of dry resin.43) where M = weight % moisture in the resin It is clear that the matrix with the higher TGD will yield a composite with better resistance to hygrothermal degradation.41) Effect of Temperature Empirical relations for predicting hygrothermal degradation effects have been studied at Lewis Research Center of NASA and given by [Chamis. …(4. To = reference temperature at which PmD is determined The manufacturer supplies TGD. 151 . σ t = tensile strength at time t.1. 1976]: where σmL = Vf E f Em [(α f − αm )(T − T0 ) + (β f c f − βm − c m )] VmEm + Vf E f Vm σmL Vf …(4.1. σ o = initial tensile strength. Subscript m and f correspond to matrix and fiber respectively.2 Prediction of Residual Stresses Due to Moisture and Temperature Moisture absorption at low temperatures results in matrix swelling and reduction in strength and stiffness. β = coefficient of moisture swelling.3.45) where E= elastic modulus.5.. residual stresses in the matrix and fiber along the longitudinal direction due to matrix swelling and thermal shrinkage is given by [Hahn. σ t > 450 days =σ o /2 ….47) 152 . V= Volume ratio. If uniform strain in longitudinal direction and uniform stress in transverse direction are assumed. 4.3.5. T = temperature σ L = longitudinal stress.3.3 Prediction of Rate of Degradation in Composite Strength under Harsh Environment Ajjarapu (1994) suggested the rate of degradation of the composite materials under harsh environmental conditions using the following relationship: σ t = σ 0 e − λt where …(4. To = stress-free temperature (usually cure temperature).44) σ fL = …(4. c = specific moisture content.3.(4.0015.Corrosion Tutorial 4.46) t < 450 days. λ = 0. α = coefficient of thermal expansion. (Di is zero for undamaged material. R. are constant or have negligible effects (Beaumont). Therefore: …(4. D ) dD …(4. for the design purposes. 4. then: dD = f (Δσ. and on Above eqn. (the number of cycles to increase D from Di to Df) is therefore: Nf = Df Di ∫ f (Δσ . the strength of the material due to long-term aging should to be taken a minimum of 50% of the initial ultimate strength.1. the load ratio. which is a function of the number of cycles applied on a composite.3. D is given by: E = E0 g(D ) where Eo is the initial or undamaged modulus. etc. Cyclic loading causes the damage to increase from Di to Df after N cycles at which point catastrophic failure of the composite laminate occurs. However. frequency.5. it was concluded that the maximum reduction was 50% in 450 days. the strength did not change considerably.3. beyond 450 days.R.50) 153 . Δσ. D) dN …(4. E. However. Nf.48) Assuming that the damage accumulation rate depends on the cyclic rate amplitude. of the composite laminate and the accumulated damage. Fatigue Damage Model The fatigue damage can be measured by the variable D. R. is valid when temperature..) the current level of D. The lifetime.4 Fatigue Damage Models This section deals with damage models and fracture energy absorption in composites.Corrosion Tutorial From the experiments.49) Relation between the axial Young’s modulus. 49).R. The aging of composites with relation to various environmental factors and analytical models used to predict the effects of environment on composite materials have also been discussed. Data of (E/Eo) is obtained as a function of N and. The function f is determined from the plot of these results.52) where g-1 is the inverse of g: ⎛E⎞ D = g' ⎜ ⎜E ⎟ ⎟ ⎝ 0⎠ …(4. g ⎜ ⎥ ⎜ ⎟ ⎜ ⎟ E0 dN ⎝ E0 ⎠ ⎦ ⎣ ⎝ E0 ⎠ ⎦ ⎣ …(4. D) = 1 ⎛ dE ⎞ ⎜ ⎟ ⎡ −1 E ⎤ E0 ⎝ dN ⎠ g' ⎢g ⎥ ⎣ E0 ⎦ 1 …(4.54) The right hand side of the equation is evaluated for different values of Δσ at a constant (E/Eo) and R. for different values of R at constant Δσ and R.R. was introduced. D. Following this subdivision we will discuss the applications of composite materials in various fields. the function of f (Δσ.Corrosion Tutorial 1 dE = g' (D ) E0 dN …(4. we get: ⎡ ⎛ E ⎞⎤ ⎡ ⎤ 1 dE −1 ⎛ E ⎞ ⎟ ⎟ = g' ⎢g−1 ⎜ ⎥ f ⎢ Δσ.51) where g’ stands for the derivative of g with respect to D. and not on how the damage. The reader can thus appreciate the variability of the use of composites in various fields and application of composites especially in the field of defense.53) The function g(D) has to be established either experimentally or theoretically before determining function f. Differentiating and substituting into Eq (4. 154 . knowing g(D). R. Function g(D) depends on the properties and the lay-up of the composite laminate. D) is determined experimentally using : f (Δσ. The mechanical and hygrothermal properties of a composite material have been elaborated upon in the earlier subheadings. creep. and aging due to environmental effects. fluid sorption and damage. The aging of the composite material due to physical and chemical factors are elaborated upon. Most of these results obtained from the tests conducted. At the present systematic studies on the size effect is not available.4 Summary and Concluding Remarks Technical issues concerning the long-term and short-term response of composite materials are dealt with in this chapter. thermal coefficient. and there is no clear conclusion on whether or not a size effect exists. fatigue.Corrosion Tutorial 4. The size of FRP structural elements for an actual application is much larger. This chapter brings to light to the reader that composite material also age under corrosive environments. Issues include mechanical properties. An important point worth mentioning is the effect of size on performance. are from coupon level specimens. The mechanical and hygrothermal properties of the composite are established first in terms of short-term response. But the rate of aging is slower than in the case of metals. A proper correlation between these two parameters in size should be done to implement these results in the field. 155 . Corrosion Tutorial 5 APPLICATIONS OF COMPOSITES Special This chapter deals with the applications of metal and composites. It is designed to penetrate enemy airspace and achieve a first-look. The F/A-22 raptor construction is 39% titanium.1. advanced integrated avionics. automotive. first-kill capability against multiple targets. stealth and range. The need for improved materials to withstand varied environments is leading to the demand of The F-22 Raptor aircraft (Figure 5-1) is the next-generation air superiority fighter for the Air Force to counter emerging worldwide threats. highly maneuverable airframe. and aerodynamic performance allowing supersonic cruise without afterburner. emphasis is given to the application of these materials in the manufacture of defense Composites are vastly used in defense applications and also in other The cost factor of industries such as construction. including some of the bulkheads.1 Aircraft Systems Composites are widely used in the manufacture of aircrafts. 5. The Raptor was built with a requirement for a fighter to replace the earlier fighters. 24% composite. and also for its heat-resistant qualities in the hot sections of an 156 . The F-22 is characterized by a low-observable. etc. Titanium is used for its high strength-to-weight ratio in critical stress areas. 5. composites in various fields of defense. with emphasis on agility. 16% aluminum and 1% thermoplastic by weight. equipment.1 Applications of Composites for Defense Purposes The use of composites in the field of defense is multipurpose. composite material makes it suitable for manufacture of weapons in the field of defense. recreational. The application of composites for defense equipment is elaborated in the following paragraphs. Thus the applications of composites can be broadly classified as those used in defense industry and in various other civilian purposes. com/projects/f22/ The RAH-66 Comanche is a helicopter designed for armed reconnaissance. and for the honeycomb sandwich construction skin panels. Lightweight components include a fiber reinforced plastic inner drum and rotating scoop disc assemblies.gdatp.airforce-technology. the doors. The F18C/D system (Figure 5-3b) is lightweight and the compact design is mounted on a common pallet structure. attack and air combat missions.com/Products/2002/arm_systems/rh66_comanche/RAH66. 157 . Figure 5-2: RAH-66 Comanche Reference: http://www.Corrosion Tutorial aircraft. The GAU-19/A gun is especially effective in high clutter environments and for engagements that are inside missile envelopes. intermediate spars on the wings. Figure 5-1: F-22 Raptor Aircraft -Tactical Fighter Aircraft Reference: http://www.html Externally mountable aircraft guns are also manufactured using composite materials. The GAU-19A (Figure 5-3a) is an externally mounting 12mm gatling gun and the F18C/D is a 20mm gatling gun system. Carbon fiber composites have been used for the fuselage frame. 1.gdatp.2 Ground Systems The use of composites in armor systems enables the manufactures to decrease the weight of the armor thus enabling capability of further increasing the protection without increasing self-weight.gdatp. Composites are resistant to the harsh environments and the impact loads.GAU –19A b. This promotes for carrying a larger quantity of Figure 5-4: Reactive Armor and XM-301 Gun Reference: http://www.3 Individual and Crew Served Systems The Objective Crew Served Weapon (OCSW) is the next generation replacement for current heavy and grenade machine guns.htm 5.Corrosion Tutorial Figure 5-3: a.com/Products/2002/aircraft. It is truly a lightweight. The ammunition the armor utilizes is a very insensitive high energy explosive with reduced weight.htm 5. F18C/D Reference: http://www.1.com/Products/2002/ground. ammunition for attack purposes. two-man portable 158 . or multiple payloads on the same mission. Some factors affecting the fairing are aerodynamic loads. dust. Figure 5-5: Objective Crew Served Weapon Reference: http://www. The use of lightweight composite material made it feasible for the manufacturer to develop a crew system to be handled by two men. and micrometeorites that may be encountered as the satellite is launched and accelerates through the atmosphere into space. noise. An aluminum 2. The fairing being made of composite reduces the weight of the entire system. dirt. The company also builds several fairing types to enclose and protect payloads on the launch pad and during ascent. one of the companies designed a variety of payload attach fittings. vibration.9-m (9. dual.generaldynamics. Fairing is the front end of the rocket that serves to protect the satellite being launched from the external environmental factors.Corrosion Tutorial system that incorporates the most modern technology advancements in fire control.com/ 5. 159 . A composite fairing is offered in a 3-m diameter (10-ft) size. rain. as is a 3-m diameter stretch composite fairing for certain payloads.5-ft) diameter fairing is available. snow.4 Rocket and Missile Systems One of the rockets made of composite component parts is the DELTA II rocket (Figure 6-4). Delta II can launch single. To accommodate these varying requirements. temperature extremes.1. materials and munitions. It makes use of the lightness and durability of composites to bring about the added features required for the increased serviceability and service life of the rocket. This brings about the use of composites that have better 160 .5 Shipboard Systems Ships such as the carriers. which is highly corrosive.com/defense-space/space/delta/delta2/delta2.org/man/dod-101/sys/missile/hydra-70. Ships are also prone to high rate of corrosion due to the environment.fas.htm Figure 5-7: Missiles from the Hydra 70 Family Reference: http://www. The need to design and manufacture better battleships and cruisers for combat is the high priority in the defense. battleships that are used in warfare need to carry heavy loads and need to resist high wear and tear.Corrosion Tutorial Figure 5-6: Delta II Reference: http://www. in which they operate.htm 5.1.boeing. 2 Applications of Composites for Civilian Purposes 5.mil/navpalib/factfile/ships/ship-dd. are subjected to severe corrosion under high 161 . Figure 5-8: Destroyers Reference: http://www.gdatp.navy. The weapon systems mounted on ships for battle are also built using composites making the weapons lighter and increasing their service life.html Figure 5-9 Goalkeeper: In-Ship Defense System Reference: http://www.Corrosion Tutorial corrosion resistance and higher fatigue lives.chinfo.1 Automotive Automotive body parts.com/Products/2002/arm_systems/goalkeeper/goalkeeper. body panels. and under hood parts.2. especially made of steel.html 5. structural components. wraps and jacketing systems are used. weapons. allows replacement of steel with composites in automotive parts. rehabilitation and retrofit of these infrastructures have set a stage for composites.2. support systems for bridge decks. Laurel Lick. In order to bring about this rehabilitation. The design and manufacturing versatility of composites along with high endurance to corrosion Figure 5-10: Composite Fire Truck Panels Reference: http://www. buildings. Figure 5-11: All Composite Bridge. Repair.2.com/automotive. The retrofitting of columns using FRP wraps is a fast developing technique to increase the service life of a bridge. corrosion resistant walkways.pultrude. in infrastructure are subjected to corrosion and high internal stresses. machinery etc.Corrosion Tutorial temperature. and railings. They are also 162 .html 5. They are used in a number of construction purposes such as bridge decks.2 Infrastructure Aging bridges. CFC-WVU 5. FRP laminates.3 Construction Pultruded profiles play a dominant role in composite construction applications. highways. Steel is prone to high corrosion under harsh environments. 4 Transportation Composites are used in various transportation vehicles such as aircrafts.2. Figure 5-13: Third Rail Protection in Monorail System Reference: http://www.pultrude.html 5.e. They have replaced aluminum.pultrude. also in railways in order to wrap the existing wooden crossties.html 163 . energy plant towers. A number of panels made of FRP material are used together in infrastructure development i.Corrosion Tutorial used as reinforcing material in concrete as steel bars corrode at a rapid rate and FRP bars have higher corrosion resistance. steel and other traditional materials due to their high resistance to corrosion and their ability to withstand high temperatures. They are used. Figure 5-12: Energy Plant Towers Reference: http://www. pedestrian bridges and many other structures.com/construct. rockets and heavy-duty vehicles. In transportation systems such as pedestrian walkways they are used as handrails etc.com/construct.. cooling towers. stm 164 . outer frame and even the microprocessors and chips. Figure 5-14: MRI Units Reference: http://www.2. computer boards.bbc.html 5. the computer The high factor of corrosion inhibition of composites enhances its use in the building of component parts of Figure 5-15: Composite Computer Chip Reference: http://news.uk/1/hi/sci/tech/2053539. Magnetic resonance imaging units and electromagnetic shielding application units are built using composites.6 Computer products Composites are used to manufacture computer-housing products.2.gemedicalsystemseurope.com/euen/rad/mri/products/vhi/vhi.5 Biomedical Composites are being applied today to medical devices ranging from artificial limbs to light weight tubing used in invasive surgery.co.Corrosion Tutorial 5. 8 Electrical Electrical cables. They also boast of minimum maintenance. telecommunication towers. They are used to manufacture pipes in sewer systems. Figure 5-16: Waste Water Plant Reference: http://www.html 5. smoke and toxic safe and have high ranges of electrical and thermal insulation.Corrosion Tutorial 5. fire. which lasts longer then the traditional pipes.com/ww.pultrude. are manufactured using composites. minimum initial cost and ease of installation. power transformers.7 Corrosion Resistant products Composites are used to manufacture various corrosion resistant products.2. power transmission lines etc. They are corrosion resistant. Equipment used in wastewater treatment plants must withstand sustained exposure to highly corrosive chemicals and composites are proving the best materials to be used in such environments. 165 . scrubbers and pressure vessels have expanded further into the industrial sector. They have various advantages over conventional material. Corrosion resistant composite tanks. optic fibers.2. cable supports. 2.pultrude.com/elec.pultrude.10 Marine Corrosion in marine environments has led to opportunities for composites in waterfront applications such as marine fenders and pilings.html 5. Composites are found in most of the outdoor sports and recreational activity equipments. golf club. Figure 5-18: Recreational Products Reference: http://www. Paddles.html 5.9 Recreational Composites have successfully replaced wood and conventional materials such as steel in various consumer recreational products such as fishing rods.Corrosion Tutorial Figure 5-17: Telecommunication Towers Reference: http://www. kites. windsurfing masts. tennis racquets. bicycle handlebars and various other fastenings. Some of the composite 166 .com/consumer.2. Corrosion Tutorial pilings used are pilings made with vacuum-assisted technology.pultrude. and extruded thermoplastic pilings reinforced with extruded thermoset composite rebar.html 167 . Figure 5-19: Sheet Piling and Fender Applications Reference: http://www. filament wound composite piling jackets filled with cement.com/mar. 1. The metal to be protected and the sacrificial anode is a coupling for galvanic corrosion. This effect results in metal dissolution and oxygen reduction reactions when the oxidizing reagent is oxygen.1 Sacrificial Anode System The sacrificial anode is a more active metal than the metal structure to be protected (Figure 6-1). The dissolution of sacrificial anode “takes over” for the dissolution reaction of the metal to be protected. The sacrificial anode makes the metal to be protected as cathode. This sacrificial anode method cannot be used on acid because the consumption of sacrificial anode is 168 . They are: ƒ ƒ ƒ ƒ Cathodic Protection Coatings Inhibitors Anodic Protection These methods can be employed individually or in combination with each other. They are: • • Sacrificial Anode System Impressed Current Cathodic Protection 6. 6. techniques employed in cathodic protection.Corrosion Tutorial 6 RETARDATION METHODS FOR CORROSION There are four main methods used for retardation of corrosion process. This mechanism is shown in Figure 62.1 Cathodic Protection A metal in contact with corrosive environment starts to corrode due to electrochemical reactions. Cathodic Protection reduces the There are two main corrosion rate by supplying electrons to the metal structure. . Thus. So.g. this method is rather suitable to protect the metal structure exposed to oxygen environment in neutral solution.Corrosion Tutorial very high. This system is economical and suitable to a pipeline that runs through short distance. Sacrificial anodes are installed at every 10 feet to protect the structure (e. Figure 6-1: Steel Tank Protected by Sacrificial Anode System 169 .5 miles. this system cannot be implemented for very long structures (pipelines). such as 0. pipeline). 170 .1 Advantages of Sacrificial Anode Systems • • Simple and easy to design.2 Impressed Current Cathodic Protection In this method current is provided to the structure by some means such as AC supply. to the ground beds of usually graphite.1. The current drains from there and reaches the structure to be protected.1. this method can be applied to pipelines. This method is only suitable to oxygen corrosion in neutral environments like the sacrificial anode method. electrons are supplied to the metal structure.1. reducing the corrosion rate. Soil resistance should be low. 6. To complete the circuit the negative terminal of the DC power supply is connected to the structure to be protected and the positive terminal is connected to the ground bed system. Thus.2 Disadvantages of Sacrificial Anode Systems • • Used only where small current requirements are needed. The system is shown in Figure 6-3. silicon cast iron or precious material surrounded with carbonaceous backfill. Low maintenance requirements. which is converted to DC by means of a rectifier and supplied.1. In this method.Corrosion Tutorial H2O O2 eFe OHeZn2+ Zn Figure 6-2: Mechanism of Anodic Protection System 6. 6. which run long distances. This method is not suitable to acid environment because current requirement will be too high. the corrosion rate is reduced from icorr to i’corr. The applied voltage is Eapp as shown in Figure 6-4.1. The mechanism is seen in Figure 6-4. Unlike the sacrificial anode method. Corrosion Tutorial Figure 6-3: Steel Tank Protected by Impressed Current System MÆ M++eE Eap O2+2H2O+2e- i’corr icorr Log i Figure 6-4: Mechanism of Impressed Current Systems Explained Using Anodic Polarization Curves 171 . The coating of 55% zinc and 45% aluminum is used because the addition of 172 .Corrosion Tutorial 6. Single installation can protect a large structure. 6. 6.1.1 Hot Dipping The process of hot dipping is carried out by immersing a pre-treated substrate in a bath of molten metal or alloy.2 Disadvantages of Impressed Current Cathodic Protection • • Greater maintenance requirements.2.1 Advantages of Impressed Current Cathodic Protection • • High output capacity.1. They are • • • Metallic Organic Inorganic 6. The steels coated with such metals are called galvanized steels. There are mainly three types of coatings. Metallic coatings are further classified as follows: 6.1 Metallic Metallic coatings are with more active metals than the substrate metal. Hot dipping is used to increase corrosion and wear resistance.1. Coatings of all low melting-point metals and alloys such as zinc and aluminum are deposited. Depends on availability of power supply.2 Coatings Coatings are used to cover or spread the material to be protected against corrosion with a finishing.2.2.2. protecting or enclosing surface. carbon steels and stainless steels.3 Ion Vapor Deposition (IVD) Ion Vapor Deposition is a process by which aluminum is vaporized at high temperatures and adheres to metal substrate. thus reinforcing the physical property of the coating. loss of hardness and significant dimensional changes and geometric distortions.2. electric arc spraying. IVD coatings can be applied to a wide variety of metallic substrates. Gaseous compounds of materials to be deposited are transported to a CVD offers many advantages over other processes. high velocity oxy-fuel etc. they form a lamellar structure to form the desired coating. melted and sprayed on the surface. Some common spraying process include flame spraying. 1000 °C.4 Spraying Spraying is the deposition of the metallic coating by dispersing it on the metallic substrate to be protected.1.1. As they cool. The advantages of IVD coatings are uniformity of thickness. All spraying processes feed the material into the heating device where it is heated. 173 . plasma arc spraying. 6.2. The molten particles strike the surface and form a thin coating on the substrate. alloy or polymer is deposited on the surface of the substrate in the form of thin film or powder. The disadvantages are unable to coat deep holes and simpler methods are not available to repair the coatings. The process requires a vacuum chamber and a vaporizing source. aluminum alloys. It can deposit any substrate surface where a thermal reaction/deposition occurs at temperatures around element or compound and it can also produce high purity and high density coatings economically.Corrosion Tutorial aluminum suppresses the transport of iron into the coating. 6. applicability to complex shapes and prevention of hydrogen embrittlement.1.2 Chemical Vapor Deposition (CVD) Chemical vapor deposition is a process in which gaseous molecules of a metal. The disadvantage of high temperature CVD process can be overcome by using plasma enhanced CVD process. which usually requires a temperature range of 200-300 °C. The disadvantages are high process temperature. 6.2. = Zn …(6. A schematic is shown in Figure 6-5 for electroplating zinc ions on steel. At the anode.1.1) eH+ O2 e- clclZn2+ Cathode (Steel) Anode (Inert alloy) Anionic membrane Figure 6-5: Mechanism of Electroplating Chloride ions are transported through the anionic membrane to the anode compartment in order to maintain the electrolytic neutrality. Zinc ions are attracted to the cathode where they are reduced to elemental zinc and are deposited on the cathode surface.2) At the anode. water is dissociated by the reaction: 2H2O = 4H+ + O2 + 4e…(6.5 Electroplating Electroplating is a deposition of metal ions onto the metallic substrate by passing current.2. the electrolytic neutrality is maintained by producing hydrogen ions against the influx of chloride ions from the cathode compartment. The cathodic reaction is: Zn2+ + 2e. Zinc chloride is added to the cathode compartment. 6. Some of the 174 .Corrosion Tutorial The thermal sprayings are capable of competing with paintings and plating for atmospheric corrosion. cheap. Portland cement. ceramics.2.2.2.2 Inorganic Inorganic coatings perform conversion coatings. The advantages The disadvantage is that coatings can be easily damaged by mechanical or thermal shock and also easily attacked by sulfate-rich water. 6. The coatings can be applied by centrifugal casting for interior surfaces of pipes and spraying for exterior surfaces.2. 6.2. However the disadvantage is that the electrode position requires higher energy. 175 . rocket nozzles and exhaust passages of hot gases.Corrosion Tutorial advantages are plating samples having complex shapes. The most suitable method for the formation of film will be the treatment with chromic acid. 6. 6.2. rapid technique and no sophisticated instrument is needed for the process. These films are about 5mm thick and form a color depending on the base alloy. and glass on surface of the materials. are low cost and easy to repair. They are further described below.3 Chromate Filming Chromate conversion coatings or chromate filming is a process by which the material is protected by applying chromate using immersion. uniformity and grain size can be improved by adding various additives. Another advantage is that the coating properties of strength.2 Ceramics Ceramic coatings are used to improve the wear and corrosion resistance to withstand high temperature applications such as internal combustion engines.1 Portland Cement Coatings Portland cement coatings are used to protect the pipes made of cast iron or steel on both water and soil environments. These coatings can also be applied to steel to provide acid and heat resistance.2. Corrosion Tutorial 6. liquid or gaseous nitrogen The It is typically carried out in the temperature ranges of 500-575ºC.2.3 Organic Coatings consisting of organic materials like esters. etc.5 Nitriding Nitriding procedures are thermo-chemical processes. Nitriding can be done using solid. The nature and amount of binder determines the performance properties such as toughness. completely non-toxic. advantages of Nitriding are increased fatigue strength. environmentally clean. It also can retain oil such as petroleum products. 6. The distinction between powders that 176 .2 Pigments Pigments are finely ground organic or inorganic powders that provide color. and color retention. Manganese and zinc phosphate coatings are most commonly selected for their wear-resistant properties. 6. adhesion. but can be inorganic compounds such as soluble silicates. where the surface of the material is enriched with nitrogen. which can avoid rusting.2. In this process.1 Binders Binders are usually resins or oils.3.2. The binder is the film-forming component in the paint. These coatings are produced by holding the metal in a hot bath of phosphoric acid and zinc. film cohesion.2.2. excellent dimensional stability without distortion. 6. and corrosion inhibition. and low energy consumption. media. opacity.3.4 Phosphate Coatings Phosphate coating is a type of chemical conversion process that transforms the surface of a base metal with a non-metallic crystalline coating.2.2. They are composed of hydrocarbon and are further classified into: 6. are called organic coatings. nitrogen is diffused into the bulk material from outside. Dispersing the inhibitor in the corrosive liquid 2. These inhibitors will reduce the rate of anodic. Adding the inhibitor to paints 3. The non-stick quality of the surface is due to the silicone.Corrosion Tutorial are pigments or dyes is based on their solubility. and extends the life of steel. After the inhibitor molecules are adsorbed.3 Inhibitors Corrosion inhibitors are used to reduce the corrosion rate of metallic surfaces in a corrosive environment by adding chemical compounds. It offers superior surface protection in both fresh and salt-water environments. and a wide range of other substrates.4 Nonstick Coatings These coatings provide a polished and smooth surface. 6. Emitting vapors of volatile inhibitors onto the metal surface 177 .2. aluminum. Some of them are: 1. Inhibitors can be transported to the metal surface by various methods. which is the non-stick. and chlorinated solvents. Solvents are usually organic liquids or water. 6. Solvent is often used to describe terpenes. they form a protective film on the metal surface. Chemisorption occurs due to the formation of equivalent chemical bonds between the metal surface and inhibitor molecules. cathodic or both reactions.3 Solvents Solvents are used to dissolve the binder and to facilitate application of the paint. A true solvent is a single liquid that can dissolve the coating. iron. concrete.3.2. Physisorption occurs due to inter-molecular forces between the metallic surface and inhibitor molecules. Inhibitors are adsorbed on the metal surface by physisorption or chemisorption. 6. hydrophobic and lubricious component. Pigments are usually insoluble and dispersed in the material whereas dyes are readily soluble in a solution. hydrocarbons. Inorganic phosphates are compounds of phosphorus and oxygen that exists in many forms and is usually combined with other elements (metals such as sodium. However. the anhydrous form is better suited for areas where high humidity could cause caking and handling problems.1 Anodic Inhibitors These inhibitors protect the metal by forming a passive film on the metal surface.3. Molybdate is a salt of molybdenum containing the group of MoO4 or Mo2O7. Mixed type inhibitors 6. In this case. calcium. the inhibitors are not effective. giving high corrosion rate. It is an environmentally benign inhibitor. This film retards the rate the anodic reaction. Molybdate inhibitors are effective in both conventional salt-spray tests and the newer cyclic salt-spray/UV exposure tests (e. where alkalinity is required. Anodic inhibitors 2. They are: 1. potassium.Corrosion Tutorial Inhibitors are classified into three types. pigment formulation. Sodium chromate is used in certain types of corrosion control applications as well as in the formulation of pigments. Salt or ester of a phosphoric acid is used as nontoxic inhibitors. and aluminum). This is because cathodic polarization curve will intersect the active region of the anodic polarization curve. if the concentration of the oxidizer is too low. and oil well drilling. the anodic inhibitors are not suitable for usage.g. Cathodic inhibitors 3. Some examples of anodic inhibitors are sodium chromate. Molybdate inhibitors have been proven effective as demonstrated in numerous outdoor exposure studies and used for commercial applications for more than 30 years. 178 .. ASTM D 5894). Since sodium chromate is hygroscopic (having the characteristic of drawing moisture from the atmosphere). phosphate and molybdate. Sodium chromate (Na2CrO4) is used for inhibiting metal corrosion. A device called potentiostat is needed to increase the potential. organic esters. cycloaliphatic. 6. ZnSO4). which is in the middle of the passive zone.2 Cathodic Inhibitors These inhibitors form a protective coating on the metal surface like anodic inhibitors but retard the rate of cathodic reactions on the metal surface. textile dyeing.Corrosion Tutorial 6.3. and in closed containers. Phosphonates are extensively used in scale/corrosion inhibition. Some common examples of cathodic inhibitors are zinc salts (e. ore recovery. in primer of the anti-rust paint. Consider Figure 6-6 in which the anodic polarization curve intersects with the cathodic polarization curve at the active region having the corrosion rate icorr under normal conditions..4 Applications They are used in open water-cooling systems.g. industrial cleansing. Sodium sulfonates are used as additives to oils and lubricants used in functional objects to extend the maintenance period of the object in oily. Some examples are aliphatic. Phosphonates are derivatives of phosphates. pulp. As the potential increases from Ecorr to E’corr the applied current density decreases from icorr 179 . metal finishing. sodium sulfonates (NaRSO3) and phosphonates (H2PO3). etc.3 Mixed Inhibitors These inhibitors influence both the anodic and cathodic reactions. Many organic compounds are of this type. closed cooling systems of motor vehicles. The idea is to increase the potential. waxy or greasy coatings. paper. aromatic and heterocyclic amines. 6. 6.3.3. oil drilling. The tank metal should exhibit active-passive behavior in order to be applied by this technique. In this situation corrosion rate will be much reduced.4 Anodic Protection This technique is used for tanks in which strong acids are stored. and crop production. The installation costs are high due to complex instruments including potentiostats and electrodes. another to the auxiliary electrode which is a cathode and third to the reference electrode. Thus.Corrosion Tutorial to i’corr. Anodic protection can be implemented using a potentiostat. The current requirements are very low in anodic protection whereas they are very high in cathodic protection and increases as the corrosivity of the environment increases. It has three terminals. The optimum potential can be determined by the anodic polarization curve. 180 . The optimum range for this applied current density would be in the middle of the passive region for the system to be more effective. The primary advantage of anodic protection is its applicability in extremely corrosive environment whereas cathodic protection can be utilized only in weak to moderate corrosive environments. The potentiostat maintains constant potential between the structure and the reference electrode. Many reference electrodes are used in the anodic protection systems along with the conventional reference electrodes such as glass and calomel electrodes. The limitation of anodic protection systems is that it is only applicable to activepassive alloys. the operating costs in anodic protection systems are lower than those of cathodic protection systems. One is connected to the structure to be protected which is anode. Corrosion Tutorial Transpassive iapp E’corr Passive E Active Ecorr i’corr Log i icorr Figure 6-6: Effects of Applied Anodic Current on the Behavior of Active-Passive Alloy 181 . Thus it is used for applications that necessitate a very hard and wear resistant surface such as rollers. The melting points of cast iron ranges from 1150 to 1300 °C. These irons are extremely hard and brittle because of the cementite inclusion. they are easily melted and amenable to 7. and grinding mills.1.1 Metal Alloys 7. malleable cast iron and ductile/nodular cast iron. most carbon exists as cementite instead of graphite. They can be alloyed for improvement of corrosion resistance and strength. These are the least expensive of the engineering metals.1 White Cast Iron When silicon content is less than 1% and the cooling rate is rapid. and cementite. content. Thus.1.Corrosion Tutorial 7 METALS AND COMPOSITES DICTIONARY 7. Most cast irons contain carbon content between 3. The cementite (Fe3C) is a metastable phase and is dissociated into ferrite (almost pure iron) and graphite under some conditions. This is called white cast iron. These materials are brittle and exhibit practically no ductility. the carbon exists as graphite and both microstructure and mechanical properties depend on composition and heat treatment.0 to 4. The common alloys include gray cast iron. casting.5 wt%. The important structural constituents of cast iron are graphite. crushers.1. which is Furthermore most cast irons are very brittle because of the high carbon considerably lower than that of steels. For most cast irons. white cast iron. ferrite. 182 .1 Cast Iron Cast iron is a common term that applies to high carbon iron alloys containing silicon. However its strength and ductility are much higher under compressive loads. 7.1.Corrosion Tutorial 7. A straight high silicon iron such as Duriron contains about 14. They are very effective in damping vibrational energy.5 to 4 wt% and 1. It is used as base structures for machines and heavy equipment that 7. respectively.1.1. main shafts and rotors for machinery drive etc. It is used in applications such as automobile crankshafts. The mechanical properties of ductile irons can be improved by heat treatments.1. The graphite exists in the form of flakes. It is usually corrosion resistant in many environments. Because of the high silicon content. valves. 7.5 High Silicon Cast Iron High silicon cast iron is produced by increasing the silicon content in gray cast iron to about 14%. Gray cast iron is weak and brittle in tension.2 Malleable Iron Heating white cast iron between 800 to 900 °C decomposes the cementite forming graphite into clusters.3 Ductile Iron Ductile iron is also called nodular cast iron.0 to 3.0 wt%.1. locomotives.5% silicon and 0. Adding magnesium to the gray cast iron before casting produces a distinctly different microstructure and mechanical properties.95% carbon. Their inherent hardness makes them corrosion resistant to erosion corrosion. The notable exception is hydrofluoric acid.1. road machinery. etc. cementite decomposes into ferrite and graphite. are exposed to vibrations.4 Gray Cast Iron The carbon and silicon contents of gray cast irons lie between 2. This composition is suited to provide the best combination of 183 .1. pipefittings. It is used in numerous applications in automobiles (such as connecting rods and transmission gears).1. It has high strength and appreciable ductility or malleability. 6%) and high-carbon (0. but the most important advantage is better corrosion resistance to atmospheric corrosion. axles.2 Low Alloy Steels Low alloy steels contain alloy content less than 8%. Low carbon steels are used in automobile body panels.1. 184 .0%). medium-carbon (0. The high silicon cast iron has a major application in pipelines and fittings.3 to 0. 7.1. 7.3%). and vanadium to produce low alloy steels. Strengths are higher than ordinary carbon steel.2 Steels Steels are basically iron-carbon alloys in which the carbon content is usually less than 1. The excellent corrosion resistance of high silicon irons is due to the formation of passive SiO2 surface layer. nickel.0 wt%.2. forgings and boiler plates. low alloy steels and high alloy steels. tin plate. couplings.6 to 1. crankshafts. and forgings. The plain carbon steels are further classified as lowcarbon (0. Better mechanical properties can be obtained by increasing the alloy content. phosphorus. molybdenum. These materials can be used for stampings. Carbon steel is usually alloyed individually or in combination with small quantities of chromium.1.2.1 Plain Carbon Steels Plain carbon steel contains carbon as the only alloying element. 7. They are plain carbon steels. brittleness increases. which forms during exposure to the environment. and wire products. High carbon steels are used in spring materials and high strength wires. along with the addition of other alloying elements. Medium carbon steels are used in shafts. The mechanical properties of steels depend on its carbon content. The mechanical properties of this steel are controlled by the carbon content. As the carbon content increases. Steels are classified into different types depending on their carbon content. gears.Corrosion Tutorial corrosion resistance and mechanical strength. copper. its strength and corrosion resistance. Aluminum alloys have a strong resistance to corrosion because of an oxide skin that forms as a result of reactions with the atmosphere.5 to 18%. (e. Martensitic stainless steels are also known as hardened alloy steels. The applications include chemical processing equipment.2.1. Ferritic stainless steels contain 11. Aluminum has a specific gravity of 2. composition. which can be metallic or non-metallic (such as silicon). Chromium content will be usually in the range of 11. However alkaline substances are known to penetrate 185 . hardening steels.3 Stainless Steels The most important advantage of stainless steels is their resistance to corrosion and good mechanical properties at atmospheric and elevated temperatures. They are austenitic stainless steels. Chromium tends to form passive film and exhibit excellent resistance to many environments. These steels find major applications in oil and gas industry. and characteristics.7. The stainless steels are classified into three groups based on their structure. and even many acids. Chromium is the main alloying element and the steel should contain at least 11%.Corrosion Tutorial 7. The mechanical properties can be improved by quenching. They are produced in the largest quantities. The most predominant alloying elements are chromium and nickel.5 to 27% chromium and the carbon content is usually low. and Austenitic stainless steels are also called as work martensitic stainless steels.1. This corrosive skin protects aluminum from most chemicals.3 Aluminum and its Alloys Aluminum alloys are a mixture of base metal aluminum with one or more alloying elements. which is usually in the range of 16 to 26% and 6 to 22%.g. 7. ferritic stainless steels. Stainless steels form pits when exposed to chloride environments. Austenitic stainless steels are the most corrosion resistant because of the high chromium contents and the nickel addition. The strength and hardness of this steel can be improved by cold working. The aim of alloying is to enhance the properties of the base metal. weathering conditions. respectively. These steels find applications in automotive industry. These stainless steels are also called non-hardening steels because they cannot be hardened by heat treatment. etc). solders. 7. lead. Magnesium is a silvery-white metal that is used as an alloy element for aluminum. 7. cable sheath. Lead has excellent atmospheric and sea-water corrosion resistance. and other chemicals. storage batteries. lead is alloyed with 0.74. sulfates. zinc.1 Lead and its Alloys Lead is one of the oldest metals used. Similarly. it forms bronze. It is used for water piping. However. and ammunition. easily formed and has a low melting point.4. The mechanical properties can be improved by cold working.4 Magnesium and its Alloys Magnesium is the lightest metal having specific gravity of 1.Corrosion Tutorial the protective skin and corrode the metal.1. In the process industry.1. radiation shields. Lead is soft. and other nonferrous alloys. Magnesium is extensively used in printing and textile industries. Magnesium finds various applications in the aerospace parts such as fuselage. Magnesium forms a film to protect against corrosion. Lead and its alloys are used in piping.06% copper to improve corrosion resistance. It is rapidly attacked by acetic acid and generally not used in nitric. When copper is alloyed with tin. Magnesium is ductile and suitable for all metal working processes. Hence magnesium is usually anodized to improve its corrosion resistance. etc. Aluminum has high conductivity for heat and electricity. hydrochloric. to enhance its strength. Magnesium is non-toxic and non-magnetic.5 Copper and its Alloys Copper is a base material alloyed with tin. bearings. It finds applications in aerospace and automotive industries. nickel. it is easily corroded by chlorides.1. The strength of the lead can be improved by addition of antimony or calcium. 7. The 186 . roofing. sheet linings. Lead also possesses excellent corrosion resistance against sulfuric acid. copper alloyed with zinc forms brass. Magnesium is easily susceptible to corrosion in marine environments. zinc. engine parts and other accessories. and organic acids. 8 Cadmium Cadmium is used exclusively as an electroplated coating. piping. and ductility.6 Nickel and its Alloys Nickel has low corrosion rates in acid solutions in the active state. electronics.1. Cadmium is more expensive than zinc and its salts are toxic. electric cords. 7. 187 . thermal conductivity. Its use is in galvanized steel for piping. Hastelloy D. pipelines. silver. The major applications of copper alloys are power utilities. 7. fencing. Many automobile components such as grilles and door handles are die cast but are usually plated with corrosion resistant metals. Iconel. Some of the important nickel alloys are Monel. Copper possesses higher electrical conductivity. etc. It is also utilized in the form of bars or slabs as anodes to protect ship hulls. etc. sulfuric acid and nitric acid. Constantan. corrosion resistance. etc. Nickel alloys possess high strength. and tungsten lowers the corrosion rate further.Corrosion Tutorial most popular brasses or bronzes are cold heading brass. pumps.1. and in the production of most minerals and organic acids.1. toughness. German The nickel alloys are used extensively for vessels. and high temperature steels. Nickel is one of the major alloying elements in the stainless steels. molybdenum. nails. Cadmium can be used to protect steel. 7. and other structures. Zinc alloy parts are made by die casting because of their low melting points. lighting and wiring devices. Alloying additions of copper. corrosion resistant steels.7 Zinc and its Alloys Zinc is not a corrosion resistant metal but it is utilized as a sacrificial anode for cathodic protection of steel. and leaded brass. naval bronze. appliances. and ductility. Cadmium is soluble in hydrochloric acid. but it is not as effective as zinc. Molybdenum is lighter in weight. and abrasion resistance. protective.1. are called coated alloys. It is used as an alloying element for steel to increase its hardness. and possesses high ductility. Tantalum is used for surgical 188 . easier for fabrication. toughness. Molybdenum finds applications in electronic tubes and in pressure applications where galling is a problem.11 Molybdenum Molybdenum shows good resistance to hydrofluoric. They have more resistance to corrosion. 7. Titanium alloys are often unsuitable for hot. but oxidizing agents such as nitric acid cause rapid attack. strength. Tantalum possesses high thermal conductivity and corrosion resistance. concentrated. It can be soldered and hence it finds 7. and adherent oxide film on the surface in presence of oxygen and moisture.9 Titanium and its Alloys Titanium is highly reactive forming a continuous. applications in radio and television industries. But tantalum is easily attacked by alkalies.1. and sulfuric acids. reducing acids in which the oxide film cannot form.12 Tantalum Tantalum is widely used in handling chemically pure solutions such as hydrochloric acid. hydrochloric.1. stable. implants. which have a coating material on it. Titanium finds wide spread application in the aircraft industry. 7.Corrosion Tutorial Cadmium plating is utilized on high strength steels in aircraft because of improved resistance to corrosion fatigue. 7.1.10 Coated Alloys Alloy materials. The coatings used should be compatible with the underlying alloy materials. electrical contacts. hydroiodic. stability and luster made this metal useful in jewelry and coinage. A common example is filament in bulbs.15 Gold Gold is one of the oldest metals used. Its chief use involves strength at high temperatures.13 Tungsten Tungsten has the highest melting point temperature.Corrosion Tutorial 7. hot concentrated hydrochloric. shows good resistance to acids and alkalies except nitric acid. The color. Gold and its alloys are used in jewelry.1. Zirconium has excellent corrosion resistance due to its protective oxide film.16 Platinum Platinum exhibits good mechanical properties such as high temperatures and wear resistance. Gold is not attacked by dilute nitric acid and sulfuric acid. chlorine and bromine.1. Platinum is attacked by aqua regia. plating. iron. burner nozzles. mercury and alkaline cyanides. Its corrosion resistance is affected by impurities in the metal such as nitrogen. electrical resistors. and bromine. Platinum has excellent corrosion resistance and oxidation resistance in hot oxidizing gases. aluminum. hydrobromic acids. tableware and decorative purposes. It is very expensive and hence it is used for limited applications. The inertness of platinum is attested by its extensive service as a catalyst. and carbon. and sulfuric acids. 189 . ferric chloride. 7. 7. Zirconium has found some applications in hydrochloric acid service.14 Zirconium Zirconium finds extensive application in the ceramic industry.1. Tungsten 7. It is used in spark plugs.1. and as anodes or cathodes in electrolytic cells. chlorine. dental inlays. This metal exhibits good corrosion resistance to alkalies and acids except for hydrofluoric acid. Moreover. zirconium is an important structural metal in atomic energy plants. But gold is attacked by concentrated nitric acid. and luster. ultraviolet light. Silver tarnishes in the presence of hydrogen sulfide and sulfur bearing environments. to become a relatively insoluble substance.Corrosion Tutorial 7. stability. and more resistant to chloride ions and hydrochloric acid. epoxy or polyester). catalyst. phenolics and polyesters. Upon the application of heat or catalysis the liquid resin becomes rigid due to curing process.. or brazed linings. Some common examples of thermosets are epoxies. 7. It is highly resistant to organic acids. Silver is attacked by nitric acid. The chemical bonds are formed through crosslinking of molecules and this provides thermal stability without any flow of resin upon 190 .2 Plastics A plastic is a material that contains organic substance of larger molecular weight.1 Thermosets A thermoset is a resin in liquid form before curing (eg. Silver possesses high electrical and thermal conductivity. jewelry and tableware. etc. which undergoes a chemical reaction by the action of heat. Plastics are classified into two types thermoplastics and thermosets. On the other hand thermosets harden when heated and retains the hardness after cooling.17 Silver Silver is best known for its use as coinage. mercury.1. Silver is also widely used in the chemical industry as solid silver and also as loose. less resistant to solvents. acrylic and cellulose. and alkaline cyanides and may be corroded by reducing acids if oxidizing agents are present. Plastics are generally much weaker. softer. is solid in its finished state. clad. 7.2. less resistant to concentrated sulfuric and oxidizing acids such as nitric. Some common examples of thermoplastic materials are nylon. Silver finds extensive applications in the photography industry. The resultant product is less sensitive to temperature and it is nonrecyclable. hot hydrochloric acid. Thermoplastic materials soften when heated and return to the original hardness when cooled. glass. abrasives etc. acetals.4 Composite Materials A composite material is made by combining two or more materials to give a unique combination of properties. These are linear or branched structures and they are recyclable. Ceramics include brick. Ceramic materials find extensive applications in jet engines and atomic reactors. glass or aramide fibers are called thermoplastic composites.Corrosion Tutorial heating. clay tile. Most ceramic materials exhibit good resistance to chemicals with exception to hydrofluoric acid and alkalies. fused silica. extrusion or slip casting.2 Thermoplastics Thermoplastics are those materials whose change upon heating is substantially physical rather than chemical. stone. composition and surface conditions. Ceramic parts are usually formed by pressing.2. 7. Thermoplastic matrices reinforced with carbon. Composites 191 . Individual polymer molecules are held together by weak forces. stoneware. A thermoset composite is a material containing a thermosetting polymeric matrix. such as Van der Waal forces. and polysulfones. 7. The polymer melts and flows upon heating and has a number of heat sensitive properties. hydrogen bonds and dipole. porcelain. The strength of the material depends on cross-sectional area. Ceramic materials have low tensile strength and high compressive strength. concrete. 7. polycarbonates.3 Ceramics A ceramic material consists of compounds of metallic and non-metallic elements. Composites maintain an interface between components and act in concert to provide improved specific or synergistic characteristics not obtainable by any of the components acting alone.dipole interactions. They are largely one-dimensional or two-dimensional molecular structures such as nylons. Carbon Matrix Composites (CMCs) Extra steps of carbonizing and densifying the original polymer matrix typically form carbon matrix composites. OMCs are further subdivided as: Polymer Matrix Composites (PMCs) A polymer is an organic compound. These long molecular chains consist of repeating chemical units held together by covalent bonds formed by a polymerization reaction. PMCs consist of polymer resin as the matrix. 7. These composites in which fibers are embedded in an organic matrix are called OMCs. composites.Corrosion Tutorial include: (1) fibrous (composed of fibers usually in a matrix). usually in a matrix). which is considered inorganic. and boron. The fibers can be arranged in almost any fashion. and (4) hybrid (combinations of any of the above). automobiles. ranging from totally random to highly structured and organized. whose structure can be represented by a repeated small unit (mer). When the matrix material is such a polymeric chain. and airframe structures. PMCs are typically used in lowtemperature structural applications such as in civil-structures. graphite. kevlar. The fibers typically provide the stiffness and strength to the composite and can be made from a wide variety of materials including glass. natural or synthetic. biomedical implants. (2) laminar (layers of materials). (3) particulate (composed of particles or flakes.4. They are primarily used in aerospace structures as they have high resistance to corrosion and fatigue damage.1 Organic Matrix Composites (OMCs) The matrix in the composite is a chemical compound containing carbon molecules with the exception of CO2. They are commonly refered to as carbon-carbon 192 . then it is called a PMC. wires including tungsten. discontinuous reinforcements. beryllium. The reinforcements are chosen to increase stiffness. Ceramic matrix composite focuses on achieving useful structural and environmental properties at high operating temperatures.4. whiskers and particulates. primarily silicon carbide. titanium and molybdenum and discontinuous materials such as fibers.asm-intl. These can be classified as Figure 7-1: Particulate Reinforcement Reference: http://www. rods.org/pdf/spotlights/IntroToComposites.pdf 193 .3 Ceramic Matrix Composites (CMCs) Materials consisting of a ceramic or carbon fiber surrounded by a ceramic matrix.4. 7. and wear resistance. The principal motive of using metal matrix composites was to dramatically extend the structural efficiency of the metallic materials while retaining their advantages including high chemical inertness.2 Metal Matrix Composites (MMCs) MMCs include metallic matrix materials reinforced with continuous fibers such as boron. 7.4 Particulate Reinforcements Particulate reinforced composites include those reinforced by spheres. Ceramic materials are inherently resilient to oxidation and deterioration at elevated temperatures. as well as heat. strength. graphite or alumina. flakes and many other shapes of roughly equal axes.Corrosion Tutorial 7.4. silicon carbide. high shear strength and good property retention at high temperatures. asm-intl.4. then the composite is considered to be continuous fiber reinforced. Figure 7-3: Continuous Fiber Reinforcement Reference: http://www. discontinuous fibers of polygonal crosssections.org/pdf/spotlights/IntroToComposites. made of a large number of materials such as graphite. silicon oxide. boron carbide and beryllium oxide.pdf 7.6 Continuous Fiber Reinforced Composites Continuous fiber reinforced composites contain reinforcements whose lengths are much greater than their cross sectional dimensions.pdf 7.5 Whisker Reinforcements Whisker reinforcements are short. Triaxial braids provide reinforcement in the bias direction with fiber angles ranging from ± 10o to ± 80o and axial (0o) direction. silicon carbide.4. Braided fabrics are engineered with a system of two or more yarns intertwined in such a way that all of the yarns are interlocked for optimum load distribution. aluminum oxide.4.org/pdf/spotlights/IntroToComposites.asm-intl. If any increase in length of the fiber does not provide further increase in elastic modulus or strength of the composite.7 Braided Fabrics Braided fabrics are fabrics in which two sets of continuous fibers are interwoven symmetrically about an axis. Figure 7-2: Whisker Reinforcement Reference: http://www. Biaxial braids provide reinforcement in the bias direction only with fiber angles ranging from ± 15o to ± 90o.Corrosion Tutorial 7. 194 . netcomposites. 90o direction (weft or fill).pdf 7. Figure 7-5: Hybrid Fabrics Reference: http://www. pultruded wires. (4) intimately mixed hybrids. in which one material is sandwiched between two layers of another. but it is more common to find alternating threads of each fiber in each warp direction. (3) interply or laminated. such as those reinforced with ribs. also known as core-shell. (2) sandwich hybrids. a hybrid fabric will allow the two fibers to be presented in just one layer of fabric.org/pdf/spotlights/IntroToComposites.asp 195 . where the constituent fibers are made to mix as randomly as possible so that no overconcentration of any one type is present in the material.4. where alternate layers of the two (or more) materials are stacked in a regular manner. in which tows of the two or more constituent types of fiber are mixed in a regular or random manner.Corrosion Tutorial Figure 7-4: Braided Fabrics Reference: http://www. (5) other kinds. It would be possible in a woven hybrid to have one fiber running in the weft direction and the second fiber running in the warp direction. thin veils of fiber or combinations of the above. There are several types of hybrid composites characterized as: (1) interply or tow-by-tow. the available fiber orientations include the 0o direction (warp).asm-intl.8 Hybrid Fabrics The term hybrid refers to a fabric that has more than one type of structural fiber in its construction. If low weight or extremely thin laminates are required.com/education. Typically. pdf 7.uk/ccm/projects/mpfs/departabs2.10 Woven Composites A planar woven fabric composite is a fabric produced by interlacing two or more sets of yarns. The assembly of each layer is then sewn This allows for together. This type of construction allows for load sharing between fibers so that a higher modulus.enae.9 Knitted or Stitched Fabrics Stitched fabrics are produced by assembling successive layers of aligned fibers. is typically observed. maximum resin flow when composites are manufactured. fibers.Corrosion Tutorial 7. and +45o direction (bias).eng. Figure 7-7: Woven Fabric Reference: http://www-mech. both tensile and flexural.4. Typically.edu/ASC/abstracts/abs102. rovings.cam.pdf 196 . or filaments where the elements pass each other essentially at right angles and one set of elements is parallel to the fabric axis.umd. Figure 7-6: Knitted or Stitched Fabrics Reference: http://www. 90o direction (weft or fill).ac. the available fiber orientations include the 0o direction (warp).4. and inertness. The open mold technique produces emission of volatile organic compounds. The composite is then cured. catalyzed resin is sprayed along with fibers. After the gel coat cures.4. The most important feature of it is that it has excellent mechanical properties though it has low density. cooled and removed from the reusable mold. which make them suitable for impact and ballistic protection.12 Glass Fiber Reinforced Plastics In this type of FRP the reinforcement is provided by glass fibers.4.4. 7.5 Manufacturing Processes of Composites 7. The fibers are chopped directly into the resin stream.5.11 Carbon Fiber Reinforced Plastics A composite material composed of carbon fiber as filler reinforcement and plastics as matrix.13 Aramid Fiber Reinforced Plastics The trade name of this type of fiber is Kevlar. They have high stiffness to weight ratios and provide better fatigue characteristics to the composite by reducing the amount of strain in the polymer matrix. they creep absorb moisture and are sensitive to UV radiation. 7. Tensile strength of glass fibers reduces with increases in temperature and with chemical corrosion. hand lay up and tube rolling. The mold is waxed and sprayed with gel coat. Types of open mold spray up are 197 . corrosion resistance. 7. The laminate is then compacted by hand with rollers.1 Open-Mold Processes Open mold processes make use of a single cavity mould and requires little or no external pressure. Aramid fibers have high-energy absorption during failure. They have low compressive strength. Glass fibers exhibit the typical glass properties of hardness.Corrosion Tutorial 7. Curing is normal at ambient temperature but if heating is done the curing process is accelerated.com/education. with an increasing use of nip-roller type impregnators for forcing resin into the fabrics by means of rotating rollers and a bath of resin. http://www. Figure 7-8: Hand Lay Up Reference. This technique is more environmentally friendly.2 Tube Rolling Tube rolling is the manufacturing process used to produce finite length tube and rod.1. knitted. 198 .5. The material is precut in particular patterns to achieve the required ply schedule and the fiber architecture.1.5.Corrosion Tutorial 7. Fibers must be continuously realigned to impart bending strength.netcomposites. The pieces are laid and the mandrel is rolled over under pressure. This is usually accomplished by using rollers or brushes. Resins are impregnated by hand into fibers.asp?sequence=55 7. stitched or bonded fabrics. which are in the form of woven. Laminates are left to cure under standard atmospheric conditions.1 Hand Lay Up An open mold process in which the reinforcements are applied to the mold and the composite is built by hand. Closed mold processes include RTM.co. cloth. In RTM. woven roving. compression molding.htm 7.harrisonrods. pultrusion.2 Closed-Mold Processes A technique used in composite fabrication that utilizes a two-piece mold (male and female). and filament winding process. The processes are usually largely automated.2. resin Injection molding. 7.1 Resin Transfer Molding (RTM) A closed-mold pressure injection system that allows for faster cure times as compared to contact molded parts. extrusion.uk/production. long fiber and chopped strand.5. VARTM. resins and catalyst are metered and mixed in the dispenser equipment before they are injected/infused into the mold. The process uses polyester matrix material systems in association with cold-molding and reinforcement material types such as continuous strand.5. Various methods are used to transfer liquid resin from an external source into the dry preform that will be placed in a two-sided matched closed mold. 199 .Corrosion Tutorial Figure 7-9: Tube Rolling Reference: http://www. Corrosion Tutorial Figure 7-10: Resin Transfer Molding Machine (CFC-WVU) 7.ncat. The material is forced from an external heated chamber through a runner or gate into a cavity of a closed mold by means of a pressure gradient. 200 .2.2.5.3 Resin Injection Molding Process Resin injection molding is similar to the resin transfer molding process except that it injects a resin/catalyst into the mold in two streams. independent of the mold's clamping force.5.2 Vacuum Assisted Resin Transfer Molding (VARTM) Figure 7-11: VARTM –Tabletop Model of VARTM and Schematic Process of Manufacture Reference: http://www.edu/~sasmith/C2. so that mixing and the resultant chemical reaction occur in the mold instead in the dispensing head.pdf 7. high-strength fiberglass reinforcements.4 Compression Molding Compression molding is a method of molding in which the molding material.Corrosion Tutorial Figure 7-12: Injection Molding Machine (CFC-WVU) 7. is first placed in an open. generally preheated. 201 .5. woven fabrics. material has cured. and pressure is applied to force the material into contact with all mold areas. high-pressure method suitable for molding complex. Heat and pressure are maintained until the molding Compression molding is a high-volume. The advantage of compression molding is its ability to mold intricate parts of large dimensions. heated mold cavity.2. Advanced composite thermoplastics can also be compression molded with unidirectional tapes. The mold is closed with a top force or plug member. randomly orientated fiber mat or chopped strand. Corrosion Tutorial Figure 7-13: Compression Molding Machine (CFC-WVU) 7. Figure 7-14: Schematic Representation of Pultrusion Process (Bedford Plastics) 202 . This process relies on relies on reciprocating or puller/clamping systems to pull the fiber and resin continuously from a resin impregnating bath through a heated steel die. Excess resin is squeezed out by the shaped bushings.5. The compacted part then enters the die where it cures.5 Pultrusion Pultrusion is a continuous automated closed molding process that is cost effective for high volume production of constant cross section parts.2. and is removed later. It is a thermoplastic process whereby pellets.reliance. or mandrel. Once the composite material is applied. etc. Fiberglass.pdf 7. granules.) is pulled from a large spool. etc. The mandrel is then removed by placing the mandrel 203 . A release agent is applied to the mandrel before winding which enables the composite part to be removed later.Corrosion Tutorial 7. and ensures that the composite material will be applied accurately.com/prodserv/standriv/appnotes/d7741. or powder are melted and forced through a die under pressure to form a given. a special non-stick plastic film is wrapped under tension around the part. pulled through a bath of resinous polymeric material (epoxy.).2. The mandrel is then placed in a computer-controlled oven.5. Figure 7-15: Basic Extruder Reference: http://www. continuous shape.7 Filament Winding Process "Filament Winding" is a highly automated process in which fiber yarn (Carbon Fiber. This film is applied to provide compaction to the composite.5. The mandrel is then placed under tension in the winding machine. and special heating profiles harden the polymeric resin. Kevlar.2. and "wound" upon a tool. A computer controls these motions.6 Extrusion Extrusion is a process used for forming composite preformed materials from mixtures of a matrix powder and short fibers suitable for MMCs. which rotates the mandrel while moving a carriage that applies the composite material. solidifying the composite material. com/education.Any set of conditions designed to produce in a short time the results obtained under normal conditions of aging. Figure 7-16 a: Winding Machine Showing Carriage and Mandrel b: Filament Winding Reference a: http://www.asp?sequence=57 7. finished. Adherent – A member of a bonded joint. Additives – This material modifies and enhances the final FRP product.htm Reference b: http://www. The part can then be machined.6 Terms Related to Composites Abraded – Make the surface rough by mechanical means such as wirebrushing.netcomposites.com/winding. Adhesive Failure – Failure caused due to the rupture of the adhesive bond leading to the separation of the adhesive .advancedcomposites. Aging . like moisture. 204 . and painted into a final form.adherend interface. temperature. etc. Adhesive – A substance capable of holding two materials together by surface treatment. on the composite material when exposed for a period of time.The effect of environmental factors.Corrosion Tutorial / part into an extraction machine which pulls the mandrel out of the part. Accelerated Aging . Bond – The adhesion of one surface to another. of the applied stress on a test specimen in bending. to the corresponding strain in the outermost fibers of the specimen. Aramid Fibers .Stiffness due to bending forces. Block Shear Failure . Bending Rigidity . Bending Strength . with the use of an adhesive or bonding agent. It may be tensile or compressive strength. at the instant of failure. 205 . Bending Modulus .Corrosion Tutorial Aging Factor – Aging Effects due to physical and chemical exposure are accounted for using aging factors in design methodology.The strength of a material in bending. within the elastic limit. Bi-directional Fibers/Fabrics (2-D) – Continuous rovings placed in a plane in any two directions. Anisotropic Material – Elastic properties are different in all the directions.Combination of tension failure along one plane and shear failure along another plane of the same component.A manufactured fiber in which the fiber forming substance consists of a long chain synthetic aromatic polyamide in which at least 85% of the amide linkages are attached directly to two aromatic rings. Bolt Shear Failure – Is caused by high shear stress in the fastener. Axial Strength – The strength of the material in tension or compression when the load is applied length wise along the axis. expressed as the stress on the outermost fibers of a bent test specimen.The ratio. Coefficient of Thermal Expansion . Composite .The coefficient of thermal expansion (linear) is the change in length per unit length of material. 206 .Moment of inertia due to joint action between the composite deck and girder. PAN or pitch in an inert atmosphere. Delams – Delamination in the FRP deck. and the fibers are chemically bonded to the resin matrix. Creep – Time dependent deformation of a material (or structure) under constant load.Composites are a combination of a reinforcement fiber in a polymer resin matrix. for a one degree Centigrade change in temperature. Delamination – The separation of the layers of material in a laminate. Debonding – Separation of fiber from the matrix. Cleavage Failure – Characterized by a single-plane cleavage failure where the apparent laminate transverse tensile strength is less than the corresponding in plane shear strength. Chemical degradation – The degradation or the deterioration of a material due to chemical attack. where the reinforcement has an aspect ratio that enables the transfer of loads between fibers.Corrosion Tutorial Carbon Fibers – Fibers produced by the pyrolysis of organic precursor fibers such as rayon. Composite Moment of Inertia . Crazing – Fine cracks at or near the surface of a plastic material. Cohesive Failure – Is caused due to the failure of the adhesive/adherend when subjected to loads exceeding the adhesive/adherend strength. this problem is accounted for assuming only a portion of the flange to the effective.In an FRP composite deck system. 207 . Extensional Bending Coupling Stiffness – Stiffness due to coupling of in-plane and bending forces. Failure strain – The ultimate strain at which the FRP material fails. Effective Flange width . Fatigue . In engineering practice. ECR Glass – ECR glass fibers are boron free. Durability – The ability of a structure to maintain strength and stiffness throughout the service life of the structure. Factored Resistance – Product of nominal resistance and the resistance factor. which may be of widely different structures but which are characterized by the reaction of the epoxy group to form a cross-linked hard resin. Ductility Factor – The ratio between the difference of total impact energy and energy to peak impact force to energy to peak impact force. which is provided to account for variability in material properties of FRP. very resistant to chemical attack and have similar properties to E glass.The failure or decay of mechanical properties after repeated applications of stress. the shear deformations tend to reduce the longitudinal stresses as they progress transversely from the web of the girder (shear lag effect).Corrosion Tutorial Ductility – Ability of material to undergo large deformations before fracturing. This reduced flange width is referred to as the effective flange width.Nonpermanent deformation. after which body returns to original shape or volume when deforming force is removed. Elastic Deformation . Epoxy Resin – Resins. or made into a fabric by interlacing in a variety of methods. Glass Transition Temperature – Temperature range over. 208 . Fiber Reinforced Polymer – Fiber reinforced polymer materials consist of fibers of high strength and modulus embedded in or bonded to a matrix with distinct interfaces (boundary) between them. to the corresponding strain in the outermost fibers of the specimen. of the applied stress on a test specimen in flexure. rather than a sharp point such as a freezing point or boiling point. Flexural Modulus . Gel Coat – A quick setting resin used in molding process to provide an improved surface for the composite. within the elastic limit. Fire Retardant Resins – Resins that are inflammable.The strength of a material in bending.The ratio. Fiber Direction . at the instant of failure.The orientation of the major axes of the fiber weave as related to the designated zero direction. which can be spun into a yarn or roving.Corrosion Tutorial Fiber – A single homogeneous strand of material having a length of at least 5 mm. Glass Transition – The reversible change in an amorphous polymer or in amorphous regions in partially crystalline polymer from a viscous or rubbery condition to a hard and relatively brittle one. resulting in significantly degraded mechanical and electrical properties. expressed as the stress on the outermost fibers of a bent test specimen. Flexural Strength . which a plastic changes from a rigid state to softened (rubber-like) state. This occurs in a narrow temperature range. Corrosion Tutorial Global Deflection – The maximum global deflection of the girder in a typical composite structure. Impact Resistance – Resistance offered by the structure against impact loads. In-Plane Shear Loading – Adherend shear loads that produce shear stresses in the bond line in lap and strap joints. In-Plane Stiffness - Stiffness in the in-plane direction due to in-plane forces. Isotropic – Elastic properties are same in all the directions; hence, the material contains an infinite number of planes of material property symmetry passing through the same point. Lamina – A single ply or layer in a laminate of layers. Limit State - A condition beyond which the structure ceases to satisfy the provisions for which it was designed. Limit Stresses – A value of stress beyond which the structure ceases to satisfy for provision for which it was designed. Local Deflection – Local deflection in the top flange of FRP deck. Matrix - Polymeric material used as binder for reinforcing fibers in FRP. Net-tension Failure – Occurs when the specimen is narrow compared to the bolt diameter and the crack propagates transverse to the loading. Nominal Resistance – The resistance of a component or connection to force effects, as indicated by the dimensions specified in the contract documents and by maximum permissible stresses, deformations, or specified strength of materials. Nonductile Material – A material, which is brittle, or a material that can undergo very little plastic deformation. 209 Corrosion Tutorial Orthotropic Material – FRP materials containing three orthogonal planes of material property symmetry. Overlay – The laying of FRP material or asphalt over a composite deck in order to prevent further corrosion or for decorative purposes. Overloads – Loads that are heavier than the design loads of the structure. Protective Coatings – A protective and finishing enclosure surface for the FRP material. Resin - Polymeric material used as binder for reinforcing fibers in FRP. Resistance Factor – Factor, which depends on, event of rupture of the section due to tension, flexure, shear, torsion or combination. Rovings – A number of ends, tows, or strands collected into a parallel bundle with little or no twist. Rule of Mixtures - Formed sub laminates undergo the same axial strain but not necessarily the same transverse strain, based on the linear volume fraction relationship between the composite and its corresponding constituent properties. Service Limit State – Limit states relating to stress, deformation and cracking under service loads. Shear - An action or stress resulting from applied forces that tend to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their plane of contact. Shear Rigidity – Stiffness due to shear forces. 210 Corrosion Tutorial Shear Modulus - The ratio of shearing stress τ to shearing strain γ within the proportional limit of a material. Shearlag – The non-uniform shear distribution within any given material. Shear-Out Failure - Occurs as the section of the material parallel to loading pushes past the remaining specimen. It is caused by shear stress and occurs along the shear- out planes. Single Lap Joint – A joint formed by overlapping one laminate over the other. Stitched Fabrics (3-D) – Multidirectional rovings in a plane with various layers in the direction perpendicular to the plane and stitched together (yarn, glass fiber, etc) either in-plane or out of plane. Temperature Gradient - Temperature gradient is the rate of change of temperature with distance in any given direction at any point i.e, the temperature gradient from the top to bottom of composite deck. Tensile Shear loading – Adherend shear loads that produce shear stresses to the bond line in lap and strap joints. Tensile Strength – Strength of a material in tension. Thermally Induced Loads – Loads induced due to thermal gradient in a FRP composite Deck. Thermoplastic – A plastic that can be repeatedly softened by heating and hardened by cooling through a temperature range characteristic of the plastic, and when in the softened stage can be shaped by flow into shapes by molding or extrusion. Thermoset – A plastic that is substantially infusible and insoluble after being cured by heat or other means. 211 Corrosion Tutorial Unidirectional Fibers (1-D) – Continuous long fibers in one direction for which strength and stiffness is maximum along the direction of fibers. Void Content - Amount of voids existing in a given material Wearing Surface - Overlay material applied on the surface of the FRP Deck. Young’s Modulus – The ratio stress/strain within elastic limit. 7.7 Reagents 7.7.1 Sulfuric Acid Sulfuric acid is a very strong acid. It is a dense, colorless, and corrosive liquid. It is used in manufacturing fertilizers, pigments, dyes, drugs, and explosives. It is also used in petroleum refining and metallurgical processes. The commercial industrial concentrations for sulfuric acid are 78%, 93% and oleum. Steel is the most common material used for storage and transportation of sulfuric acid. Dilute acids attack steel more rapidly than the strong acids. Moreover, the corrosion rate of steel increases as the temperature of the acid increases when the concentration of the acid is up to 70%. Above 70% concentration, there is no reliable data available to predict the corrosion rate. Aeration of sulfuric acid in the storage tanks will lead to drastic efforts. Cast iron shows similar effects like steel. High silicon cast iron containing 14.5% silicon has better performance against corrosion. Usage of high silicon cast iron should be avoided in case of fuming acids. applications. Copper alloys are not generally used for sulfuric acid 7.7.2 Hydrochloric Acid Hydrochloric acid is used in the manufacture of fertilizers, dyes, artificial silk, and pigment for paints. It is used in removing scales in boilers, clean membranes in desalination plants, and to clean other metals for coatings. Hydrochloric acid is one of the most corrosive of the non-oxidizing acids in contact with copper alloys, and is 212 Corrosion Tutorial handled in dilute solutions. It is soluble in benzene, alcohol, and ether. It is insoluble in hydrocarbons. It is incompatible/reactive with metals, hydroxides, amines, and alkalis. Hydrochloric acid’s fumes have an acid, penetrating odor. platinum, tantalum, and certain alloys. chlorine, and organic matter. Aqueous solutions of hydrochloric acid attack and corrode nearly all metals except mercury, silver, gold, It may be colored yellow by traces of iron, 7.7.3 Nitric Acid Nitric acid is a corrosive, non-volatile and inorganic acid. It is used in the manufacture of fertilizers, dyes, explosives, and other organic chemicals. It is a strong, monobasic acid and an oxidizing agent. In the presence of traces of oxides, it attacks all base metals except aluminum and special chromium steels. It is soluble in water (cold and hot), and ether. It increases the flammability of combustible organic and oxidizable materials and can also cause ignition of some of these materials. 7.7.4 Hydrofluoric Acid Hydrofluoric acid is a weak acid. It finds extensive applications in various industries such as glass industry, leather industry, textile industry, fertilizers, etc. Magnesium shows excellent corrosion resistance to hydrofluoric acid. Steel is used in handling the hydrofluoric acid when the concentration is between 60 and 100%. Monel shows excellent corrosion resistance at all concentrations and temperatures. aeration and presence of oxidizing salts leads to increase in corrosion rate. temperature. But Lead shows good resistance to hydrofluoric acid in concentrations below 60% at room 213 or a thick. extracting penicillin. and pickling. fertilizers. colorless. 214 . tile cleaning. and soft drinks. Phosphoric acid is incompatible with strong caustics and most metals. dibasic phosphates.5 Phosphoric Acid Phosphoric acid usually exists as a crystal or clear liquid. electro-polishing. It is a chelating agent. photoengraving operations. It can form three series of salts: primary phosphates. It is used as an acid catalyst. ceramic binding. It is either an oily. and odorless liquid. It is also used in the rust proofing and polishing of metals. hot stripping for aluminum and zinc substrates.Corrosion Tutorial 7. process engraving. It is corrosive to ferrous metals and alloys. unstable crystalline solid. It readily reacts with metals to form flammable hydrogen gas.7. and tribasic phosphates. thick. The liquid can solidify at temperatures below 21oC. cotton dyeing. operating lithography. soil stabilizer. and gasoline additive. It is used in the manufacturing of phosphates. bonding agent for refractory bricks. coagulating of rubber latex. antioxidant in food. colorless. electric lights. It has a low vapor pressure at room temperature. It is deliquescent and hygroscopic. water treatment. It is soluble in alcohol and hot water. Corrosion Tutorial 8 CORROSION KINETICS The corrosion reaction rates can be predicted using the thermodynamic principles only when the reactions are in equilibrium. When the corrosion reactions are not in equilibrium, the corrosion kinetics needs to be considered. Polarization, mixed potential theory, and experimental polarization curves are discussed in this chapter. 8.1 Polarization The polarization is a state where current flows in a circuit, where the cathodic and anodic reactions take place. Polarization can be divided into two types. They are activation polarization and concentration polarization. Activation polarization is related to chemical reaction control while concentration polarization is related to diffusion control. 8.1.1 Activation Polarization The activation polarization can be derived using Figure 8.1. The solid line and the dotted line represent the status of non-polarization and polarization, respectively. The directions of these reactions are conveniently chosen as H2 ⎯⎯ ⎯ ⎯→ ←⎯ ⎯ ⎯ ⎯ Cathodic Anodic 2 H + + 2e − …(8.1) The activation energies of the forward and reverse reactions are ΔG f * and ΔGr * . When the forward reaction is polarized by a potential E, the activation energy for the forward reaction becomes: ΔG f * − nFE + (1 − α )nFE = ΔG f * − α nFE and the activation energy for the reverse reaction also becomes: ΔGr * + (1 − α )nFE 215 Corrosion Tutorial ∆G*F –ά nFE ∆G F * ∆G*r ∆G*r + (1-ά)nFE nFE nFE (1-ά)nFE ά (1-ά) Figure 8-1: Activation Polarization Then the rates of the anodic and cathodic reactions become: Ra = PH 2 K a = PH 2υ e + + −( ΔEa ) RT …(8.2) ΔEc ) RT Rc = [ H ]K c = [ H ]υ e −( …(8.3) where ΔEa and ΔEc are the activation energies of the forward and reverse reactions, respectively. But generally i = nFR where i is the current density (amp/cm2) or (Coulomb/cm2-sec), F is the Faraday constant 96500 (Coulomb/equivalent) and R is the reaction rate (moles/cm2-sec). 216 Corrosion Tutorial Substitution of Equation (8.2) into (8.4) yields: r −( ΔG f * − α )nFE ) ) i = nFRa = PH 2 nFυ exp( RT …(8.4) ΔG f * = PH 2 nFυ exp( − RT ) exp( α nFE RT ) r α nFE ) i = PH 2 ka exp( RT where ka = nf υ exp( −ΔG f * …(8.5) RT ) r s r But when E = Erev i = i0 . Erev is equilibrium potential where i and i are same as i0 . Here i0 is called exchange current density. Then, the equilibrium rate can be written as: i0 = PH 2 ka exp( α nFErev RT ) …(8.6) Dividing Equation (8.5) by (8.6) yields: r i = i0 PH 2 ka exp( α nFErev RT ) PH 2 ka exp( α nFE RT ) r α nF ( E − Erev ) i = i0 exp RT …(8.7) But E - Erev = η . η is called overvoltage and it is a potential for driving force. Then Equation (8.7) becomes: r α nFη i = i0 exp RT …(8.8) The rate of reverse reaction can be derived similarly as: s −( ΔGr * + (1 − α )nFE ) i = nF R e = [ H + ]nFυ exp( ) RT 217 Corrosion Tutorial = [ H + ]nFυ exp( = [ H + ]kc exp( − −ΔGr * −(1 − α )nFE ) exp( ) RT RT (1 − α )nFE ) RT −ΔGr * ) RT …(8.9) where kc = nFυ exp( s But i = i0 when E = Erev i0 = [ H + ]kc exp( −(1 − α )nFErev ) RT …(8.10) Dividing Equation (8.9) by (8.10) yields: s i = i0 −(1 − α )nFE [ H + ]kc exp( ) −(1 − α )nFErev RT [ H + ]kc exp( ) RT s − (1 − α )nF ( E − E rev ) ) i = i0 exp( RT But η = E - Erev s −(1 − α )nFη i = i0 exp( ) RT The net anodic current is given by: r s ia = i − i …(8.11) ia = i0 exp( α nFηa RT ) − i0 exp( −(1 − α )nFηa ) RT …(8.12) Equation (8.12) is called Butler-Volmer equation. We can see that as the r s s overvoltage increases i increases but i decreases. Thus when ηa >> 0, i becomes negligible. By convention ηa takes positive values. Then Equation (8.12) becomes: ia = i0 exp( α nFηa RT ) …(8.13) 218 Corrosion Tutorial ηa = 2.303RT i log α nF i0 …(8.14) We can see that Tafel Equation (8.13) or (8.14) is called Tafel equation. equation does not consider the reverse reaction rate, where β a = the anodic Tafel slope. 2.303RT is known as α nF In the case of cathodic reactions, the net cathodic current is given by: s r ic = i − i ic = i0 exp( −(1 − α )nFηc α nFηc ) − i0 exp( ) RT RT …(8.15) Equation (8.15) is also called Butler-Volmer equation. But when ηc << 0, the reverse reaction rate becomes negligibly small. values. Then Equation (8.15) becomes: By convention ηc takes negative ic = i0 exp( −(1 − α )nFηc ) RT …(8.16) ηc = −2.303RT i log c (1 − α )nF i0 ηc = β c log ic i0 …(8.17) This is again called Tafel equation, more exactly the Tafel equation for cathodic reaction. The difference between Butler-Volmer equation and Tafel equation can be seen from Figure 8-2. One can see that when the potential is much away from Erev, the two equations are overlapping because the reverse reaction rates are negligible as mentioned previously. However, when the potential approaches Erev, the two equations differ much because the reverse reactions become significant. 219 2 Concentration Polarization Concentration polarization is a diffusion-controlled process. Erev = E 0 − P 2. Then. It is usually observed for cathodic reactions because it usually involves diffusion of a reactant. This means that the diffusion of hydrogen ion from the bulk solution to the metal surface is slower than the consumption rate of hydrogen ion by the chemical reaction. But the increase of the chemical reaction rate cannot continue above a certain overvoltage because the supply of the hydrogen ions to the surface becomes a rate controlling step.18) 220 . This state is called concentration polarization.303RT log + H22 2F [ H ] Bulk …(8. [H+]Surface is equal to [H+]Bulk.Corrosion Tutorial Butler-Volmer Anodic Erev E Cathodic log i i0 Tafel Figure 8-2: Butler-Volmer Equation and Tafel Equation 8. The concentration polarization equation can be derived as follows: When there is no current.1. The cathodic rate increases as the overvoltage increases in the activation polarization. This is called limiting current density iL. iconc will be at maximum because it will give the maximum concentration gradient. iconc = − FDH + [ H + ]Surface − [ H + ]Bulk δ …(8.303RT i + log(1 − conc ) i0 F iL …(8.20) …(8.total = β c log …(8.25) ηc .22) When [ H + ]Surface is zero.22) by (8. iL = FDH + [ H + ]Bulk δ …(8.303RT log + H 2F [ H ]2 Surface …(8. However.24) Substituting Equation (8.23) Dividing Equation (8.19) The overvoltage is: ηconc = E * − Erev Substitution of Equations (8.303RT η conc = − log F [ H + ] Bulk …(8.19) into Equation (8. general equation for any metal will take the form. there would be current generated by the diffusion of [H+] ion.Corrosion Tutorial When [H+]Surface is smaller than [H+]Bulk.21) From the Fick’s first law.20) yields [ H + ] Surface 2.24) into (8.21) yields ηconc = 2.303RT i log(1 − conc ) F iL ic 2.18) and (8.23) gives [ H ]Surface iconc = 1− iL [ H + ]Bulk + …(8.26) This is the equation for hydrogen electrode. 221 . E* = E 0 − P2 2. 2 Mixed Potential Theory Mixed potential theory explains the two simple hypothesis: one is that any electrochemical reaction can be divided into two or more oxidation and reduction reactions.29) …(8. The other is that at the corrosion potential the sum of cathodic current should be same as the sum of the anodic current.27) is plotted to show the combined polarization in Figure 8-3.= H2 and the anodic reaction is: M = M2+ + 2e…(8.Corrosion Tutorial ηc .28) 222 . Erev i0 Activation Polarization E Concentration Polarization Log i Figure 8-3: Combined Polarization iL 8.303RT i + log(1 − conc ) i0 nF iL …(8. For example when a metal is immersed in an acid solution.27) Equation (8. i0 and iL are known.total = β c log ic 2. This theory can be applied to determine corrosion rate if βa. The cathodic reaction is: 2H+ + 2e. the following reactions will take place. βc. Also. H2Æ 2H++2e- i0 2H++2e-ÆH2 Potential (V vs. βc and i0 are known (Figure 8-4). if M is copper. N could be iron. zinc. If M is iron. zinc and aluminum. which has E0 potential less than zero. respectively. nickel. when two metals are coupled. the anodic and 223 . magnesium or aluminum.M and icorr. βa and βc are the slopes of the anodic and cathodic polarization lines. N is more active metal than M. SHE) Icorr Ecorr i0 MÆM2++2eM2++2e-ÆM Log i Figure 8-4: Behavior of Metal M in Acid Solution The next example is the case where two metals M and N are immersed in an acid solution. provided that βa. However.Corrosion Tutorial M should be an active metal. The polarization lines of these reactions can be plotted. i0 is the exchange current density. icorr. Figure 8-5 shows two sets of polarization curves: one is for the anodic dissolution of metal M coupled with cathodic reduction of acid and the other is for the anodic dissolution of N coupled with cathodic reduction of acid. magnesium or aluminum. N will be zinc. such as iron. icorr can be determined by the intersection point between the cathodic polarization line for acid reduction and the anodic polarization line for the metal dissolution.N are the corrosion rates when two metals are not coupled. M icorr.total.M* while the corrosion rate of N increases from icorr.total M icorr.Corrosion Tutorial cathodic rates of M and N are summed and also.N to icorr. H2(N) i0. For the generation of anodic polarization curves. N Icorr. the anode of the electrochemical cell is connected to the working electrode while the cathode is connected to the auxiliary electrode or counter electrode of the 224 . i0. We can see that the corrosion rate of M decreases from icorr. these lines are generated experimentally using an instrument called potentiostat/galvanostat.M to icorr.N*. 8. This is particularly the basis of a cathodic protection method with metal N as a sacrificial anode. M* icorr. icorr. However.3 Experimental Polarization Curves Mixed potential theory is used to generate theoretical polarization lines according to Tafel equation.N* Total Oxidation Potential N Log i Figure 8-5: Behavior of Coupled Metals in Acid Solutions The intersection point of the dotted line represents the imaginary total corrosion rate. the cathodic rates of the acid on M and N are summed. This corrosion value is horizontally extrapolated into anodic polarization lines of the two metals. H2(M) Total reduction rate E icorr. Anodic polarization curve is generated when potential with respect to the reference electrode potential increases stepwise and current is measured at every step. However. There are two methods producing polarization curves. the two curves are almost identical. Let us take the cathodic polarization curve in Figure 8-6 when the potential is much away from Ecorr and the two curves are overlapping as mentioned previously. decreasing the cathodic rate. 225 . in the vicinity of Ecorr. However.Corrosion Tutorial instrument. one is potentiostatic. the cathodic reaction becomes less polarized. But at the same time. the electrons are produced by the anodic reaction and its rate increases. This is a negative effect to the cathodic reaction. A reference electrode is placed closely to the anode. as the potential increases. the two curves are significantly different because of the reverse reaction as mentioned previously with Butler-Volmer equation and Tafel equation. when the potential increases. in which potential is measured by varying current. This rate retardation is due to the negative effect by its opposite half-cell reaction near Ecorr. This means the consumption of electrons by the cathodic reaction retards. When potentials are farther away from Ecorr. This rate retardation is unavailable because we cannot polarize only cathode or anode. in which current is measured by varying potential and the other is galvanostatic. Cathodic polarization curve is similarly generated. The polarization curve produced experimentally is not exactly the same as the polarization curve as used in the mixed potential theory. and thus the cathodic current decreases. Similar phenomenon happens on the anodic polarization. However.Corrosion Tutorial 2H++2e-=H2(g) Experimental Ecorr E Fe=Fe2++2e- icorr Tafel equation Log i Figure 8-6: Showing Cathodic and Anodic Polarization Curves In order to determine the corrosion rate. if there is no linear range on the anodic polarization curve which is frequently observed. the linear portion of the cathodic and anodic polarization curves are extended. 226 . then the determination of icorr will be more difficult and its method is available in the literature. and its intersection point will be icorr. V. K. H. GangaRao.V. • Altizer. • • Ashbee.. R. B. 1969. J. H. L.Corrosion Tutorial REFERENCES • • Agarwal. • Apicella. 2nd Ed. S. J. L. • Allred. pp. S. V. “Thermoset Polymer Performance under Harsh Environments to Evaluate Glass Composite Rebars for Infrastructure Applications”. P. and Hu. S. • Alsayed. pp.E. “Analysis and Performance of Fiber Composites”. 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