Advances in Corrosion Engineering

Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web CourseLecture 1 Corrosion: Introduction – Definitions and Types Keywords: Definition of Corrosion, Corrosion Types, Environment. Corrosion can be viewed as a universal phenomenon, omnipresent and omnipotent. It is there everywhere, air, water, soil and in every environment, we encounter. There is no single figure for loss to the nation due to corrosion. It can be a minimum of 3.5% of the nation‟s GDP. Losses due to corrosion could be around Rs. 2.0 lakh crores per annum in India. Corrosion costs manifest in the form of premature deterioration or failure necessitating maintenance, repairs and replacement of damaged parts. In the US, total direct cost of corrosion is estimated at about 300 billion dollars per year; which is about 3.2% of domestic product. Corrosion has a huge economic and environmental impact on all facets of national infrastructure; from highways, bridges, buildings, oil and gas, chemical processing, water and waste water treatment and virtually on all metallic objects in use. Other than material loss, corrosion interferes with human safety, disrupts industrial operations and poses danger to environment. Awareness to corrosion and adaptation of timely and appropriate control measures hold the key in the abatement of corrosion failures. Definitions: Corrosion is the deterioration or destruction of metals and alloys in the presence of an environment by chemical or electrochemical means. In simple terminology, corrosion processes involve reaction of metals with environmental species. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course As per IUPAC, “Corrosion is an irreversible interfacial reaction of a material (metal, ceramic, polymer) with its environment which results in its consumption or dissolution into the material of a component of the environment. Often, but not necessarily, corrosion results in effects detrimental to the usage of the material considered. Exclusively physical or mechanical processes such as melting and evaporation, abrasion or mechanical fracture are not included in the term corrosion” With the knowledge of the role of various microorganisms present in soil and water bodies, the definition for corrosion need be further widened to include microbially- influenced factors. Corrosion can be classified in different ways, such as Chemical and electrochemical High temperature and low temperature Wet corrosion and dry corrosion. Dry corrosion occurs in the absence of aqueous environment, usually in the presence of gases and vapours, mainly at high temperatures. Electrochemical nature of corrosion can be understood by examining zinc dissolution in dilute hydrochloric acid. Zn + 2HCl = ZnCl2 + H2 Anodic reaction is Zn = Zn++ + 2e with the reduction of 2H+ + 2e = H2 at cathodic areas on the surface of zinc metal. There are two half reactions constituting the net cell reaction. Environmental effects such as those of presence of oxygen and other oxidizers, changes in flow rates (velocity), temperature, reactant concentrations and pH would influence rates of anodic and cathodic reactions. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course Even though the fundamental mechanism of corrosion involves creation or existence of corrosion cells, there are several types or forms of corrosion that can occur. It should however be borne in mind that for corrosion to occur, there is no need for discrete (physically independent) anodes and cathodes. Innumerable micro level anodic and cathodic areas can be generated at the same (single) surface on which anodic (corrosion) and cathodic (reduction) reactions occur. Each form of corrosion has a specific arrangement of anodes and cathodes and specific patterns and locations depending on the type can exist. The most important types are Uniform corrosion. Galvanic corrosion, concentration cells, water line attack Pitting. Dezincification, Dealloying (selective leaching) Atmospheric corrosion. Erosion corrosion Fretting Crevice corrosion; cavitation Stress corrosion, intergranular and transgranular corrosion, hydrogen cracking and embrittlement Corrosion fatigue. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course Table 1.1 ASM classifications of corrosion types Metallurgically Mechanically Environmentally General Corrosion: Localized Corrosion: Influenced Assisted Induced Cracking: Corrosion: Degradation: Corrosive attack High rates of metal Affected by alloy Corrosion with a Cracking dominated by penetration at chemistry & heat mechanical produced by uniform thinning specific sites treatment component corrosion, in the Atmospheric Crevice Intergranular Erosion presence of stress. corrosion corrosion corrosion corrosion Stress – Galvanic Filiform Dealloying Fretting Corrosion corrosion corrosion corrosion corrosion Cracking Stray-current Pitting corrosion Cavitation (SCC) corrosion Localized and water Hydrogen General biological drop Damage biological corrosion impingement Liquid metal corrosion Corrosion embrittlement Molten salt fatigue Solid metal corrosion induced Corrosion in embrittlement liquid metals High – temperature corrosion (Ref: Sully J R, Taylor D. W, Electrochemical Methods of Corrosion Testing, Metals Hand Book. Vol 13, 1987.) 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course Crevice corrosion is a localized attack on a metal adjacent to the crevice between two joining surfaces (two metals or metal-nonmetal crevices). The corrosion is generally confined to one localized area to one metal. This type of corrosion can be initiated by concentration gradients (due to ions or oxygen). Accumulation of chlorides inside crevice will aggravate damage. Various factors influence crevice corrosion, such as. Materials: alloy composition, metallographic structure. Environmental conditions such as pH, oxygen concentration, halide concentrations, temperature. Geometrical features of crevices, surface roughness. Metal to metal or metal to nonmetal type. Filiform corrosion is a special type of crevice corrosion. Pitting corrosion is a localized phenomenon confined to smaller areas. Formation of micro-pits can be very damaging. Pitting factor (ratio of deepest pit to average penetration) can be used to evaluate severity of pitting corrosion which is usually observed in passive metals and alloys. Concentration cells involving oxygen gradients or ion gradients can initiate pitting through generation of anodic and cathodic areas. Chloride ions are damaging to the passive films and can make pit formation auto-catalytic. Pitting tendency can be predicted through measurement of pitting potentials. Similarly critical pitting temperature is also a useful parameter. Uniform corrosion is a very common form found in ferrous metals and alloys that are not protected by surface coating or inhibitors. A uniform layer of „rust‟ on the surface is formed when exposed to corrosive environments Atmospheric corrosion is a typical example of this type. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course Galvanic corrosion often referred to as dissimilar metal corrosion occurs in galvanic couples where the active one corrodes. EMF series (thermodynamic) and galvanic series (kinetic) could be used for prediction of this type of corrosion. Galvanic corrosion can occur in multiphase alloys. Eg: - Copper containing precipitates in aluminium alloys. Impurities such as iron and copper in metallic zinc. Differential aeration (oxygen concentration cell) and ion concentration (salt concentration) cells create dissimilar polarities (anodic and cathodic areas) Eg:-Pitting of metals. Rusting of iron (Fig. 1.1). Fig. 1.1 Differential oxygen cells in rusting of iron Selective leaching (Dealloying) refers to selective dissolution of active metal phase from an alloy in a corrosive environment. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course Examples: a) Brass containing copper and zinc. Since zinc is anodic to copper, selective dezincification occurs in a corrosive medium, enriching the cathodic copper in the matrix (colour of brass turns red from yellow). b) Graphitization of grey cast iron-graphite being cathodic enhances dissolution of iron in the matrix, leaving behind a graphite network. There are several other examples of dealloying besides the above. Tin Bronzes in hot brine or steam-Destannification. Precious metal alloys such as gold containing copper or silver – strong acids, sulfide environment - preferential dissolution of copper or silver. Cupro-nickel alloys in condenser tubes-denickelisation. Localised attack at or nearer to grain boundaries in a metal or alloy can be termed as intergranular corrosion. Generally the following factors contribute to intergranular corrosion. Impurities and precipitation at grain boundaries. Depletion of an alloying element (added to resist corrosion) in the grain- boundary area. A typical example is sensitized 18-8 stainless steels when chromium carbide is precipitated along grain boundaries. Lowered chromium content in the area adjacent to grain boundaries, leads to formation of anodic and cathodic areas. Such intergranular corrosion is common in stainless steel welded structures and is referred to as weld decay. Intergranular attack can occur in other alloys as well. For example, Duralumin-type alloys (Al – Cu) due to precipitation of CuAl2. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course Erosion corrosion is the deterioration of metals and alloys due to relative movement between surfaces and corrosive fluids. Depending on the rate of this movement, abrasion takes place. This type of corrosion is characterized by grooves and surface patterns having directionality. Typical examples are Stainless alloy pump impeller, Condenser tube walls. All equipment types exposed to moving fluids are prone to erosion corrosion. Many failures can be attributed to impingement (impingement attack). Erosion corrosion due to high velocity impingement occurs in steam condenser tubes, slide valves in petroleum refinery at high temperature, inlet pipes, cyclones and steam turbine blades. Cavitation damage can be classified as a special form of erosion corrosion. This is usually caused by formation and collapse of vapour bubbles in liquids closer to a metal surface. Typical examples include ship‟s propellers, pump impellers and hydraulic turbines. Surface damage similar to that of pitting can occur and both corrosion and mechanical factors are involved. Corrosion occurring at contact regions between materials under load subjected to slip and vibration can be termed Fretting. Such friction oxidation can occur in engine and automotive parts. Fretting is known to occur at bolted tie plates on rails. Parameters promoting fretting include: Relative motion between two surfaces. Interface under load. Both the above produce slip and deformation of surfaces. Wear-oxidation and oxidation-wear theories are proposed to explain fretting corrosion. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 1: Corrosion: Introduction – Definitions and Types NPTEL Web Course Stress corrosion cracking (SCC) refers to failure under simultaneous presence of a corrosive medium and tensile stress. Two classic examples of SCC are caustic embrittlement of steels occurring in riveted boilers of steam-driven locomotives and season cracking of brasses observed in brass cartridge cases due to ammonia in environment. Stress cracking of different alloys does occur depending on the type of corrosive environment. Stainless steels crack in chloride atmosphere. Major variables influencing SCC include solution composition, metal/alloy composition and structure, stress and temperature. Crack morphology for SCC failures consists of brittle fracture and inter - or trans-granular cracking could be observed. Higher stresses decrease time before crack initiation. Tensile stresses of sufficient threshold levels are involved (applied, residual or thermal stresses). Hydrogen embrittlement although many a time classified under stress corrosion, need be considered separately since the two types respond very differently to environmental factors. Fracture of metals and alloys under repeated cyclic stresses is termed fatigue and corrosion under such circumstances is corrosion fatigue (reduction of fatigue resistance). Electrochemical factors come into play in many of the above corrosion forms. Both thermodynamic and kinetic aspects of electrochemistry of corrosion are discussed in the following lectures with respect to both corrosion mechanisms and corrosion protection. 9 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 2: Electrochemical Cells – Definitions and Principles NPTEL Web Course Lecture 2 Electrochemical Cells – Definitions and Principles Keywords: Electrodes, Cells, Liquid Junction. Definitions and types of electrochemical cells. Electrode: An electric conductor, the electrode metal and an ionic conductor, electrolyte solution, form an interface at which electrode process occurs. An electrochemical cell contains two electrodes (anode and cathode); a liquid-liquid junction separates two electrodes. Anode is the electrode where oxidation occurs. Cathode is the electrode where reduction occurs. Types of electrodes: Metal – Metal ion Cu / Cu++ Ion – Ion (redox) Pt / Fe+++, Fe++ Gas Pt / H2, H+ Metal – insoluble salt Hg / Hg2Cl2 / KCl Liquid Junction: Serves as galvanic contact between electrodes (can be a salt bridge or porous membrane). Salt bridge, very commonly used – an intermediate compartment filled with saturated KCl solution and fitted with porous barrier at each end or agar solidified incorporating saturated KCl. Salt bridge minimises liquid junction potential (diffusion potential) that develops when any two phases such as two solutions are contacted each other. This potential (if not corrected) introduces 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 2: Electrochemical Cells – Definitions and Principles NPTEL Web Course errors and interferences in the measured cell potentials. With introduction of a salt bridge, two liquid junction potentials are created; but they tend to cancel each other (Fig.2.1). Fig.2.1 Electrochemical cell with salt bridge IUPAC sign convention for electrode potentials Sign of the electrode potential, E 0 is positive when the half – cell is spontaneous as cathode. is negative when the half – cell behaves as anode. is a measure of the driving force for the half – reaction. E0 is referenced to standard Hydrogen Electrode. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 2: Electrochemical Cells – Definitions and Principles NPTEL Web Course 2H+ + 2e = H2 E0 = 0.000V Fe+++ + e = Fe++ E0 = +0.771V E0 is independent of number of moles of reactant or product. Positive means reaction is s spontaneous with respect to Hydrogen electrode. In accordance with the recommendations of IUPAC, the present practice is to use reduction potentials. Ecell = Eright - Eleft Standard hydrogen electrode (SHE) is the reference point. Types of electrochemical cells Galvanic Electrochemical cells Electrolytic Table 2.1 Comparison of galvanic and electrolytic cells Galvanic Electrolytic Chemical energy to electrical energy Electrical to chemical energy Spontaneous/Reversible/Thermodynamic Non spontaneous/Kinetic cell / irreversible Cathode (+) Cathode (-) Anode (-) Anode (+) AG0 <O, E0cell >O AG0 > O, E0cell < O Eg: Dry cell Eg: Electroplating, Daniel cell Impressed current cathodic protection 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 2: Electrochemical Cells – Definitions and Principles NPTEL Web Course Definition of electrochemical cell Electrodes (anode, cathode) in the presence of electrolyte. Driving force for electrochemical reaction is the potential difference between the electrodes, Ecell. ∆G = -nFEcell n = moles of electrons in half reaction F = 96500c ΔG0 = - RTlnK Nernst relationship: E = E0 + ln Eh = E0 + ln Eh = E0 + log at 250c Where E0 is the standard electrode potential, is the ratio between activities of oxidized and reduced species. Daniel cell is an example of a galvanic cell - contains zinc immersed in zinc sulfate solution and copper in copper sulfate solution separated by a diaphragm or salt bridge (Fig. 2.2). 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 2: Electrochemical Cells – Definitions and Principles NPTEL Web Course Fig.2.2 Diagrammatic representation of Daniel cell (-)Zn | Zn++ (1M) || Cu++ (1M) | Cu(+) + Right Rule Ecell = (Right) - (Left) The sign of cell reaction potential (E cell) should be positive indicating spontaneous reaction. Single vertical line denotes electrode – solution interface. Double vertical lines in the middle indicates liquid junction is eliminated. Zn = Zn++ + 2e (Anodic) E0 = - 0.76 V Cu++ + 2e = Cu0 (Cathodic) E0 = + 0.34V Zn + Cu++ = Zn++ + Cu0 (Net reaction) Zinc ions enter the aqueous phase leaving two electrons behind. (oxidation, polarity of anode is negative): copper ions deposit by taking up two electrons (reduction, polarity of cathode is positive). at 1M concentration for Zn ++ and Cu++ Ecell = + 0.34 – (-0.76) = + 1.1V The cell is spontaneous 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 2: Electrochemical Cells – Definitions and Principles NPTEL Web Course For the reaction, Fe++ = Fe+++ + e Eh = 0.771 + 0.059 log (-)Pt | Fe++ , Fe+++ || H+, H2 | Pt (+) (a=1) For the reaction 2H+ + 2e = H2 Eh = 0 + log [H+]2 at pH2 = 1, Eh = 0 - 0.059 pH 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course Lecture 3 Potential Measurements - Galvanic Cells, Concentration Cells Keywords: Potentials, Reference Electrodes, Concentration Cells. Electrode potentials can be measured using a reference electrode in any desired electrolyte at room temperature. Potential of a corroding metal or alloy (E corr) can be measured in the corrosive media (Fig. 3.1). Fig. 3.1 Measurement of electrode potential with reference to standard electrode. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course Brief description of a few generally used reference electrodes given below: Often measured electrode potentials are referred to standard or Normal Hydrogen Electrode (SHE or NHE) whose standard potential is 0.0V at partial pressure of hydrogen and hydrogen ion activity both at unity 2H+ + 2e = H2 NHE consists of a platinum electrode immersed in a solution of hydrogen ion of activity = 1. Hydrogen gas at 1 atmos. pressure is bubbled around the platinum electrode. This is a universal reference standard (Fig 3.2). Fig. 3.2 Hydrogen reference electrode 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course Due to efforts and expenses involved in its construction and also due to difficulties in handling H2 gas, NHE is not generally preferred. Instead more convenient and less cumbersome standard reference electrodes such as Saturated Calomel Electrode, silver/silver chloride or Cu/CuSO4 electrodes can be used. Saturated Calomel Electrode (SCE). Hg|Hg2Cl2 (Sat’d), KCl (x M)|| - - - - - - - - - - - (see Fig. 3.3) Fig. 3.3 Saturated calomel electrode (SCE) Hg2Cl2 + 2e = 2Hg + 2Cl- Standard potential = + 0.244V 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course Silver / Silver chloride Electrode Silver wire coated with AgCl Solution of KCl saturated with AgCl Similar electrode construction as that of SCE Ag | AgCl (Sat’d), KCl (a = xM) || - - - - - - - - - (see Fig. 3.4) AgCl + e = Ag + Cl- Standard potential = + 0.222 V Fig. 3.4 Ag/AgCl reference electrode Copper / Copper sulfate electrode Cu++ + 2e = Cu (sat’d CuSO4) Standard potential + 0.316 V 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course Types of galvanic cells a. Dissimilar electrode cells b. Concentration cells Important concentration cells Salt concentration • Differential aeration (Oxygen Concentration) Differential temperature Metallographic / mechanical heterogeneities Differences in residual stress levels Dissolved oxygen concentration in water, soil etc frequently varies from region to region due to various reasons. Such a difference in aeration produces a different equilibrium potential. 1 O2 + 2H+ + 2e = H2O 2 1 2 2 00.059 [O ] H Eh = E + log 2 2 [ H 2 0] Higher oxygen concentration provides a more nobler potential. Oxygen-enriched areas in a corroding metal serve as cathodes, while oxygen depleted regions act as anodes (undergoing corrosion). Eg: Rusting of iron in atmosphere (O2, moisture) Fe = Fe++ + 2e (anodic reaction at O2 – starved regions) Net reaction: Fe++ + 2H2O = Fe (OH)2 + 2H+ 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course See the model for rusting of iron given in Fig. 1.1. A diagrammatic arrangement of an oxygen concentration cell is illustrated in Fig. 3.5. Fig.3.5 Oxygen Concentration cell Similarly, metal ion concentration gradients at metal-solution interfaces can create a potential difference and generation of anodic – cathodic sites (Fig 3.6). 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course Fig. 3.6 Salt concentration cell Differential concentration cells (-)M | M+(a1) || M+ (a2) | M(+) E1 = E0M + ln a1 E2 = E0M + ln a2 Ecell = E2 – E1 = ln Water – line corrosion and corrosion of electric poles at the base are typical examples. Electrical poles fixed to soil can buckle due to failure at the junction (pole – soil interface) due to prevalence of oxygen concentration cells. In water – line 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 3: Potential Measurements – Galvanic Cells, Concentration Cells NPTEL Web Course corrosion, a steel specimen submerged in a beaker of saline water can experience corrosion at the water line due to similar reasons. Dissimilar electrode cells or bimetallic couples are separately discussed in Lecture 4. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course Lecture 4 EMF and Galvanic Series - Bimetallic Couples Keywords: EMF Series, Galvanic Series, Galvanic Corrosion. EMF Series a) EMF series lists only metals (little engineering application). Alloys not included b) Electrode potentials listed calculated from thermodynamic principles (corrosion potentials are more relevant). c) Equilibrium potentials with concentrations at unit activity (Exact prediction of galvanic coupling not possible). d) Predicts only tendency to corrode (Role of passive films and oxidation kinetics not predicted). e) Effect of environment not predicted (Eg: Sn – Fe couple as in Tin cans) Galvanic series a) Instead of standard electrode potentials, actually measured rest potentials of metals and alloys in a given environment arranged with respect to nobility and activity. b) Practically measured potentials vs reference electrode. c) Effect of coupling of metals and alloys on corrosion rate can be predicted. Certain anomalies Eg: Stainless steels (active and passive) Galvanic series is generally good for stagnant conditions and not for turbulent conditions. EMF and galvanic series are illustrated in tables 4.1 and 4.2. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course Table 4.1 EMF Series Reaction E0,V(SHE) Au++++ 3e = Au +1.42 Noble Pt++ + 2e = Pt + 1.2 O2 + 4H+ + 4e = 2H2O +1.23 Pd++ + 2e = Pd +0.83 Ag+ + e = Ag +0.799 O2 + 2H2O + 4e = 4OH- +0.401 Cu++ + 2e = Cu +0.34 Sn+++ + 2e = Sn++ +0.154 2H+ + 2e = H2 0.00 Reference Pb++ + 2e = Pb -0.126 Sn++ + 2e = Sn -0.140 Ni++ + 2e = Ni -0.23 Co++ + 2e = Co -0.27 Cd++ + 2e = Cd -0.402 Fe++ + 2e = Fe -0.44 Cr++++ 3e = Cr -0.71 Zn++ + 2e = Zn -0.763 Al+++ + 3e = Al -1.66 ++ Mg + 2e = Mg -2.38 Na+ + e = Na -2.71 K+ + e = K -2.92 Active 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course Table 4.2 Galvanic Series in Seawater Platinum Gold Graphite Silver Noble Hastelloy C 18 – 8 stainless steel (passive) Chromium steel > 11% Cr (passive) Inconel (passive) Nickel (passive) Monel Bronzes Copper Brasses Inconel (active) Nickel (active) Tin Lead Lead-tin solder 18-8 Mo stainless steel (active) 18-8 stainless steel (active) Ni-resist Chromium steel<11% Cr (active) Cast iron Steel or iron Active 2024 aluminium Cadmium Commercially pure aluminium Zinc Magnesium and its alloys. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course The EMF series is an arrangement of various metals in the order of their electrochemical activities based on their standard oxidation-reduction potentials (E0). The most active metal in the series will be having a high negative standard potential while nobler metals possess relatively less negative (or more positive) standard potential (E0). If we consider a couple of two metals in the EMF series, the one with higher negative E0 will act as anode (and will corrode) compared to the other with a relatively less negative E0 value (cathode). There are several exceptions to the predicted activity of a metal (or couple) as arranged in the EMF series. Eg: Aluminium exhibits higher corrosion resistance due to Al2O3 layer present on surface. Chromium exhibits stable Cr2O3 layer and is used as alloying element for corrosion resistance in stainless steels. Many metals alter their potentials depending on the environment. Reversal in polarity can occur in some environments, leading to changes in anodic (and cathodic) behaviour. Tin (Sn) is nobler to iron (Fe) in the EMF series. Internally tinned (tin-coated) steel cans are used to preserve vegetable and fruit juices. Such a cathodic protection of iron by tin is however only limited since many food constituents such as organic acids can combine with Sn++ to form soluble tin complexes, resulting in lowering the activity of stannous ions. The polarity of Fe – Sn couple can reverse under these conditions. Fe++ + Sn = Sn++ + Fe The cell polarity reverses when Ecell = 0 Sn log can be calculated and works out to be -10.30 Fe 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course Sn Ratio of must be < 5 x 10-11 for tin to become more active than iron. Fe ratio within the can must be very small for the reversal of polarity to occur. Amenability of galvanic corrosion in bimetallic contacts can be predicted by the EMF and galvanic series. Bimetallic and Concentration cells are mainly responsible for galvanic corrosion. Typical example is rusting of iron in a moist environment where oxygen concentration gradients come into play. Galvanic corrosion rates are influenced by two factors, namely distance and area effects. Severity of corrosion is the highest near the junction of the bimetal contacts. Area effect refers to ratio of anodic to cathodic areas and a larger cathode in contact with a small anode is considered ‘unfavourable area ratio’. For a given current flow in a galvanic cell, the current density is higher for a smaller electrode than for a larger anode. Higher current density results in larger rates of anodic corrosion. Examples demonstrating the area effect: a. Copper plates (larger cathodes) connected by steel rivets (smaller anodes) exposed to sea water. b. Steel plates (larger anode) connected with copper rivets (smaller cathode) exposed to sea water. Case (a) represents unfavourable area effect leading to severe corrosion of steel rivets. Case (b) represents favourable area effect. Larger anode and smaller cathode results in negligible galvanic corrosion. A graphical representation of area effect with respect to anodic corrosion rate is illustrated in Fig 4.1. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course Fig 4.1 Graphical representation of area effect Galvanising of steels for corrosion protection is a classic example of corrosion protection through proper galvanic (sacrificial) method. Zinc is anodic to iron and hence corrodes away protecting the steel base metal surfaces. Consider a uniformly zinc coated steel surface exposed to a corrosive environment. Even if portions of zinc coating are abraded away, the base steel will still be protected! (Due to favourable area effect). See Fig 4.2. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course Fig 4.2 Pictorial representation of zinc coated steel surface in a corrosive environment All coatings have defects in the form of pinholes and mechanical damage. Corrosion of steels can be concentrated at coating defects (small anodes). For example, in a carbon steel (anode) structure having contact with stainless steel (cathode), surface coating of only the carbon steel could lead to disastrous corrosion due to unfavourable area effect. The best alternative would then be, if one of two dissimilar metals (alloys) in contact is to be coated, the more noble one should be coated (or painted). The following factors need be considered for prevention of galvanic corrosion. a) Select combinations as close together in the galvanic series. b) Avoid unfavourable area effect. c) Insulate dissimilar metal contacts. Corrosion currents can be generated due to several reasons in metals and alloys, namely a) Impurities b) Grain orientation and grain boundaries c) Differential thermal treatment d) Surface roughness. e) Alloying elements (Brass, Zn corrodes with respect to Cu) f) Metallographic defects g) Strain/stress 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 4: EMF and Galvanic Series and Bimetallic Couples NPTEL Web Course Fig 4.3 Electrode potentials of some metals and oxidising – reducing agents With respect to arrangement of the electrode potentials for metals on the one hand and those of various oxidising and reducing agents on the other, it becomes easy to predict the relative oxidising or reducing power of various reagents with respect to a desired metal / metal ion reaction (Fig 4.3). A wide selection of strongly oxidizing species is available for oxidation of most of the metals excluding perhaps the nobler metals such as gold and platinum. Similarly, reducing power of hydrogen with respect to precipitation of metal ions can also be predicted. However, it may be borne in mind that not all equilibria are always oxidizing or reducing. Effect of pH and gaseous partial pressures on oxidisability and reducibility need be taken into consideration. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 5: Eh-pH Diagrams - Fundamental Aspects NPTEL Web Course Lecture 5 Eh-pH Diagrams – Fundamental Aspects Keywords: Eh-pH Relationship, Water Stability, Oxidation, Reduction. Eh and pH as environmental parameters in the electrochemical equilibrium diagram. Eh-pH diagrams showing reactions and products at electrochemical equilibrium are often referred to as Pourbaix diagrams. As shown in Fig. 5.1 below, there are four regions in the diagram corresponding to oxidizing (acidic), oxidizing (alkaline), reducing (acidic) and reducing (alkaline) environments. The basic diagram for aqueous environment involves upper and lower, stability limits for water, represented by the oxygen (universal oxidizing agent) and hydrogen (universal reducing agent) reactions. Fig. 5.1 Basic regions in a Eh – pH diagram 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 5: Eh-pH Diagrams - Fundamental Aspects NPTEL Web Course Stability limits of water a) O2 + 4H+ + 4e = 2H2O E0 = +1.23V Eh = 1.23 – 0.059 pH (at po2 =1) b) 2H+ + 2e = H2 E0 = 0.00V Eh = 0-0.059 pH (at pH2 = 1) These equilibria are plotted in Fig. 5.2. Above the oxygen line, oxygen liberation occurs. Below the hydrogen line, hydrogen liberation occurs. Water is stable between the two lines. In neutral or alkaline solutions, the following reactions hold good. 2H2O + 2e = H2 + 2OH- O2 + 2H2O + 4e = 4OH- Electrochemical evolution of hydrogen represents water decomposition. At more positive potentials, oxygen reduction or water oxidation takes place. Slope of both lines correspond to 59 mV/pH. Fig 5.2. Stability limits of water 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 5: Eh-pH Diagrams - Fundamental Aspects NPTEL Web Course Three types of lines in the diagram a) Those depending only on Eh, but independent of pH (Horizontal to the X- axis). b) Those dependent only on pH, but independent of Eh (Vertical to the X- axis). c) Those dependent on Eh and pH (Slanted with definite slopes). The above types of reactions and general effects of Eh and pH on redox reactions (oxidation, reduction) are illustrated in Fig. 5.3. Fig 5.3. Effects of changes in Eh and pH 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 5: Eh-pH Diagrams - Fundamental Aspects NPTEL Web Course The following aspects are noteworthy: As the potential increases in the positive (noble) direction Loss of electrons is favoured (oxidation). Metal dissolution is favoured. The system becomes more oxidizing. Ox Ratio of increases. Re d When the potential decreases in the negative direction Gain of electrons favoured (reduction). The system is more reducing. Metal deposition (plating) favoured. Ox Ratio of decreases. Re d Increasing pH favours metal hydroxide precipitation. Still higher pH may lead to solubilisation again (eg: dihypoferrite and aluminate) Consider two redox reactions: OX1 + n1e= Red1 OX2 + n2e = Red2 OX1 + Red2 = OX2 + Red1 As shown in Fig. 5.4, through Eh – pH diagrams, one can predict oxidizability and reducibility of different reactants. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 5: Eh-pH Diagrams - Fundamental Aspects NPTEL Web Course Fig 5.4 Role of oxidising and reducing agents Oxidant OX1 can oxidise Red2 to OX2 while OX1 get reduced to Red1. Relative positions of various oxidation and reduction reactions in the diagram indicate possibilities of cell reactions as shown above. Oxidizability of various metals such as gold, silver, copper, nickel, cobalt and iron by oxidants such as oxygen, hydrogen peroxide, halides (chloride, bromide, iodide), permanganate and dichromate can be predicted. Similarly, possibility of using hydrogen (and other reducing agents) to reduce and precipitate metal ions such as Ag+, Cu++, Ni++, Zn++ and Fe++ can also be predicted based on relative positions of respective lines in the diagram. For example, all metal-metal ion redox lines which are placed above the hydrogen line can be thermodynamically reduced by hydrogen. Iron can be used to displace copper from acidic solutions (cementation). Cu++ + Fe = Cu + Fe++ E0 for Cu++ / Cu is + 0.34V, while E0 for Fe / Fe++ is -0.44V. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram) NPTEL Web Course Lecture 6 Construction of Eh – pH Diagrams (Fe- H2O – O2 Diagram) Keywords: Iron Diagram, Corrosion Regions, Advantages and Limitations. Construction of an Eh – pH diagram based on thermodynamic data is illustrated with respect to Fe-H2O – O2 system. Seven major reactions which are thermodynamically feasible are illustrated along with calculations leading to simplified Eh – pH relationships which are plotted on the electrochemical equilibrium diagram. The diagram can be drawn for a given concentration of the metal ion species. The lines will shift as a function of varying concentrations. Assume Fe++, Fe+++ ion concentrations (activity), aH2O = 1, pO2 = 1, pH2 = 1. Room temp 25oC. Eh – pH diagrams drawn for specific ion activities and partial pressures of gases at room temperature. 1. Fe = Fe++ + 2e E0 = - 0.44V (Reaction dependent only on Eh, independent of pH) Eh = - 0.44 + log [Fe++] For aFe++ = 1, Eh= - 0.44V 2. Fe++ + 2H2O = Fe(OH)2 + 2H+ (Reaction dependent only on pH, independent of Eh) reaction = f(FeOH) 2 +2 F0 f(H+) - F0f(Fe++) - 2 -115.57 + 2(0) – (-20.30) – 2(-56.69) = 18.11 (k.cal) = - RT lnK = - 1.364 log K 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram) NPTEL Web Course log K = = - 13.28 2 H log = - 13.28 Fe 2pH + log [Fe++] = 13.28 Log [Fe++] = 13.28 -2pH pH = 6.64 3. Fe++ = Fe+++ + e E0 = + 0.771V (Eh dependent, but independent of pH) Eh = 0.771 + 0.059 log Eh = 0.771 (at unit activity) 4. Fe +++ + 3H2O = Fe (OH)3 + 3H+ (reaction dependent only on pH) Log [Fe+++] = 4.81 – 3pH pH = 1.6 5. Fe + 2H2O = Fe(OH)2 + 2H+ + 2e (dependent both on Eh and pH) First, estimate E0 for this reaction from reaction free energy values. E0 = - = -0.05V Eh = - 0.05 + log [H+]2 Eh = - 0.05 – 0.059 pH 6. Fe(OH)2 + H2O = Fe (OH)3 + H+ + e Eh = 0.27 – 0.059 pH 7. Fe++ + 3H2O = Fe (OH)3 + 3H+ + e Eh = 1.057 – 0.177pH – 0.059 log [Fe++] 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram) NPTEL Web Course The above Eh – pH relationships as above are plotted on a graph one by one as indicated below to yield the final Eh – pH diagram depicting all the seven reactions with respect to stability regions for the various cations and precipitated hydroxide products (Fig. 6.1). Fig. 6.1 Reaction – wise plotting of Eh – pH relationships Domains of immunity, corrosion and passivation From the Eh – pH diagram, it can be seen that when Fe++ , and Fe+++ are stable, the metal (Fe) is in the dissolved state, corresponding to regions of metallic corrosion. At more active potentials, iron is thermodynamically stable and immune to corrosion. In the regions where corrosion products such as Fe (OH)2 and Fe(OH)3 (or Fe3O4 and Fe2O3) are stable, the metal is oxidized with a surface film which can protect it from further corrosion (passivity). Corrosion behaviour of the metal (Fe) in an aqueous aerated solution can then be represented as a corrosion diagram. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram) NPTEL Web Course Corrosion diagram for the Fe-H2O – O2 system is given below (Fig. 6.2). Fig. 6.2 Corrosion diagram for iron Regions of corrosion for iron (steels) are not only confined to the acidic region, but also exist in the high alkaline region (beyond a pH of about 12) where dissolution of iron as HFeO2- species can occur. Regions of immunity (where metallic iron is stable thermodynamically) and passivation (stability phases for iron oxides which form a protective passive layer) are shown. Advantages and limitations of Eh – pH diagrams. Electrochemical equilibrium diagram shows conditions of solution oxidizing power (potential) and acidity or alkalinity (pH) at room temperature for a known activity of dissolved metal species. Stability regions for various solid (elemental form and precipitated compounds) and ionized forms are demarcated. This diagram has applications in several disciplines such as: Corrosion prediction and protection Extractive metallurgy Geology and geochemistry Geomicrobiology Fuel cells 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 6: Construction of Eh – pH Diagrams (Fe – H2O – O2 Diagram) NPTEL Web Course However, there are some limitations, such as: a) The diagrams are thermodynamically (theoretically) determined only for 25oC. Caution should be exercised to predict corrosion behaviour at higher temperatures using this diagram. b) Only thermodynamic amenability to corrosion and protection predicted. Kinetic factors are not considered. Corrosion rates cannot be predicted. c) No consideration for extraneous ions and effect of complexation. d) Only pure metals are generally considered. Effects of impurities, alloying and metallographic phases and heterogeneties are not considered. Unstable phases not represented. Modified Eh – pH diagrams for various metals can be prepared taking into consideration the effect of alloying additions and presence of complexing agents. Combined diagrams which take into consideration two or more metals simultaneously present would be more useful. For example, effect of chromium addition to steel to increase corrosion resistance can be represented in a modified Fe – Cr diagram. Anodic and cathodic protection limits with reference to potentials and pH can be predicted using the Eh – pH diagrams. Computer programmes and soft-ware kits are now available to construct Eh- pH diagrams for various metal systems. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course Lecture 7 Copper, Aluminium and General Corrosion Diagrams Keywords: Copper Diagram, Aluminium Diagram, Eh – pH Diagram for Metals. Similarly, Eh – pH diagrams for other metal systems can be drawn. Various reactions pertaining to the Cu – H2O – O2 system along with the constructed Eh – pH diagram are illustrated below (Fig.7.1). Fig. 7.1 Eh – pH diagram for copper 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course Construction of Al – H2O – O2 diagram 1. Al = Al+++ + 3e Eh = - 1.66 + log [Al+++] 2. 2Al + 3 H2O = Al2O3 + 6H+ + 6e Eh = -1.55 – 0.059 pH 3. 2Al+++ + 3H2O = Al2O3 + 6H+ Log K = 6 log [H+] – 2 log Al+++ For [Al+++] = 10-6M, = - 6 pH – 2 log Al+++ pH = - log = 3.9 4. Al + 2H2O = AlO2 - + 4H+ + 3e Eh = - 1.26 + 0.02 log AlO2 - – 0.079 pH 5. Al2O3 + H2O = 2AlO2- + 2H+ pH = 14.6 + log [AlO2-] There are two solid species (Al and Al2O3 H2O) and two ionic species (Al+++ and AlO2-). Constructed Eh – pH diagrams for the Al – H2O – O2 system are illustrated in Fig 7.2 and 7.3. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course Fig. 7.2 Stability phases in the aluminium diagram Immunity: Totally immune from corrosion attack and safe to use in the region. Cathodic protection may be used to bring the potential of a metal closer to that region by imposing cathodic shift. Passivation: Metal coated with an oxide or hydroxide film preventing all direct contact between the metal itself and the environment. Corrosion: Ionised form of aluminium is stable in this region and therefore susceptible to corrosion attack. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course Fig. 7.3 Corrosion diagram for aluminium 3 regions: corrosion, passivation, immunity. Al+++ / AlO2 - is stable - corrosion Aluminium oxide is stable - passivity Al is stable - thermodynamically immune to corrosion. Passivity? Caused by thin oxide or hydroxide layer forming on metal surface, protecting the metal from anodic oxidation. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course However, oxide will itself corrode under certain conditions. Aluminium is an amphoteric metal (acid and alkali reactions) if pH < 4 - Al+++ stable if pH > 8.3 - AlO2 - stable if 4 < pH < 8.3 - Al2O3 stable and thus protects the metal If the potential is sufficiently low – aluminium itself is immune to corrosion. Eh – pH diagrams as corrosion diagrams for several important metals such as aluminium, copper, magnesium, nickel, titanium and zinc are illustrated in Fig.7.4. A relative comparison of corrosion behaviour of the above metals in terms of Eh and pH brings about very useful comparisons: Nobler metals (copper, silver, gold and platinum) exhibit very large immunity regions. Active - passive metals such as titanium, and chromium exhibit a larger, stable passive region. Active metals such as magnesium exhibit a large corrosion region. Behaviour of copper, nickel, zinc and aluminium which are widely – used in several environmental conditions with respect to corrosion and passivity as a function of pH and redox potentials is very important in the development of corrosion resistant alloys and application of protection techniques. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course Fig. 7.4 Corrosion diagrams for various metals 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course Exercise 1. Through computer search and reference books construct and study Eh – pH diagrams for various metallic systems such as: Noble metals (gold, silver, platinum and palladium). Active – passive metals (chromium, titanium) High temperature metals (molybdenum, Tungsten) Active metals. 2. a) Comment on combination Eh – pH diagrams to represent alloy behaviour in corrosion. b) Role of inhibitors in corrosion prevention through modified Eh – pH diagrams. c) Modified Eh – pH diagrams to predict passive behaviour, cathodic and anodic protection parameters. 3. Study computer generation and software programmes for Eh – pH diagrams. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 7: Copper, Aluminium and General Corrosion Diagrams NPTEL Web Course References 1. M. Pourbaix, Atlas of electrochemical equilibria in aqueous solutions, NACE, Houston (1974). 2. D.C. Silverman and A. L. Silverman, Potential-pH diagrams as aids for screening corrosion inhibitors and sequestering agents, corrosion, 66 No. 5, (2010). 3. P. B. Linkson, B.D.Philips, C. D. Rowles, The method of maximum constriction. An imporved algorithm for the computer generation of potential – pH diagrams, Corrosion science, 19 (1979) p 613 -620. 4. Web information on software for corrosion prediction and generating of Eh – pH diagrams. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 8: Electrode – Solution Interface – Definition and Types of Polarization NPTEL Web Course Lecture 8 Electrode-Solution Interface – Definition and Types of Polarization Keywords: Electrochemical Interface, Polarization, Anodic Control, Cathodic Control Proportionality between current density (i) and reacted mass (m) in an electrochemical reaction is stated by Faraday’s law. m= Corrosion rate r= = Where n = number of electrons involved a = atomic weight t = time F = Faraday constant (96500 coulombs / equiv) Corrosion rate r= = Where i = current density A = surface area Corrosion rate in mpy [mils (0.001 in) per year] is given as r = 0.129 Where i = µA/cm2 D = density, g/cm3 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 8: Electrode – Solution Interface – Definition and Types of Polarization NPTEL Web Course Through above relationship, anodic current density in corrosion can be converted to metal loss in mpy. When electrode reactions take place, the potential will no longer be at equilibrium due to current flow through an electrochemical cell-causing a change in the electrode potential. This electrochemical phenomenon is termed POLARIZATION. Polarization of anode – anodic polarization Polarization of cathode – cathodic polarization Corroding systems are not in equilibrium-deviation from thermodynamics. Deviation of the potential from its reversible value is referred to as polarization. A cell or electrode is said to be polarised when there is little or no change in current with larger changes in potential. An electrode is not in equilibrium when a net current flow from or to its surface. Polarization can result from either a slow step in an electrode process or discharge of ions at an electrode surface. Distribution of anions and cations at a metal – solution interface is shown as electrical double layer (Fig. 8.1). A compact layer (Helmholtz) closest to metal surface in which charge distribution and hence potential varies linearly with distance (Fig. 8.2). A more diffuse innter layer (G. C. layer) where potential changes exponentially. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 8: Electrode – Solution Interface – Definition and Types of Polarization NPTEL Web Course Fig.8.1 Double layer at electrode – solution interface Fig. 8.2 Potential distribution across distance Processes at an electrochemical interface are schematically represented below : Fig. 8.3 Transport processes at electrochemical interface 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 8: Electrode – Solution Interface – Definition and Types of Polarization NPTEL Web Course Mass transport to metal surface involves forces such as convection, diffusion and migration, depending on presence or absence of electric field and flow conditions. Concentration polarization is controlled by diffusion gradients (Fick’s law). Activation polarization involves kinetic factors related to charge transfer, such as activation energy barrier and equilibrium current density. Transport processes at an electrode – electrolyte interface are illustrated in Fig. 8.3. Consider a solid-solution interface. Diffusion of electro-active ions from bulk to the interface and interaction at electrode resulting from charge transfer. diffusion Charge transfer ++ ++ M M M0 Slow step Slow step (bulk) (surface) (reduced) (concentration (activation polarization) polarization) ηTotal = ηconc + ηact (Total (concentration (activation polarization) polarization) polarization) Polarization resulting from concentration gradients is termed concentration polarization; while activation polarization is caused by a slow step in the electrode reaction (electrode reaction requires activation energy in order to reach the final state) 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 8: Electrode – Solution Interface – Definition and Types of Polarization NPTEL Web Course Examples of activation polarization include: Hydrogen overvoltage (overpotential) H+ + e = Hads 2Hads = H2 2H+ + 2e = H2 E0 = 0.00 V For this cathodic reaction, there will be deviation from the reversible value to more negative values and that deviation is the overvoltage. AE = Eapp - Erev (Overpotential) Similarly, we come across oxygen overvoltage, chlorine overvoltage etc. 2OH- = O2 + H2O + 2e The contribution to polarization due to IR drops is referred to as Resistance polarization So Total polarization ηT = ηAct + ηConc + ηresist When polarization occurs mostly at anodes, corrosion reaction is Anodically Controlled - Anodic process at electrode is accelerated by moving the potential in the positive direction. When polarization occurs mostly at cathodes, it is Cathodically controlled - Cathodic process accelerated by moving the potential in the negative direction. Resistance control when electrolyte / electrode resistance is so high that the current is insufficient to polarise either of the electrodes. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 8: Electrode – Solution Interface – Definition and Types of Polarization NPTEL Web Course Fig. 8.4 Anodic, Cathodic, Mixed and Resistance control Mixed control refers to the condition where both anode and cathode are polarized. Schematic illustrations of anodic, cathodic, mixed and resistance controls are given in Fig 8.4. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 9: Exchange Current Density – Polarization Relationships NPTEL Web Course Lecture 9 Exchange Current Density – Polarization Relationships Keywords: Exchange Current, Tafel Relation, Concentration Profile. A conceptual potential-current diagram depicting corrosion of zinc in an acid solution is given below (Fig. 9.1). Note that at the corrosion potential, E corr, there is a mixed equilibrium. Fig. 9.1 Potential – current diagram depicting acid corrosion of zinc Considering a reversible hydrogen electrode, under equilibrium conditions, rate of oxidation (H2  2H+ + 2e) should equal the rate of reduction (2H+ + 2e = H2). Activation polarization (slowing down of the reaction) is a result of a slow step in the reaction. For example, for hydrogen evolution at cathode, the reactions involve several steps. (i) H+ + e = Hads (ii) Hads + Hads = H2 (iii)Then evolution of hydrogen molecules. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 9: Exchange Current Density – Polarization Relationships NPTEL Web Course Any of the above steps can control the rate of the reaction and result in activation polarization. Concept of exchange current density When a metal in solution is at equilibrium, it implies rates of dissolution and deposition reactions are equal. When the above two reactions (anodic and cathodic) are in equilibrium, the rates (equal and opposite) of each of the two reactions are referred to as exchange current density. Expressing reaction rates in terms of current density, roxid = rred = based on Faraday’s law. where roxid and rred are equilibrium oxidation and reduction rates. io is termed exchange current density which is the rate of oxidation and reduction at equilibrium. There is no net current under the above conditions even through the concept is a useful method of representing rates at equilibrium. ic = ia = io inet = ia – ic = 0 A Kinetic expression for io is given as io = nFAKS ( ( Where KS is rate constant for the redox reaction = transfer coefficient A = Area 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 9: Exchange Current Density – Polarization Relationships NPTEL Web Course Exchange current density (io) is dependent on a) Nature of the redox reaction b) Electrode composition / surface c) Concentration ratio of oxidized and reduced species d) Temperature. Table 9.1 Approximate exchange current densities (io) for hydrogen reduction on various metals. Metal Io(A/cm2) Pb, Hg 10-13 Zn 10-11 Sn, Al 10-10 Ni, Ag, Cu 10-7 Fe, Au 10-6 Pd, Rh 10-4 Pt 10-2 For example, io for H+ / H2 reaction on platinum is about 10-2A/cm2 and for mercury it is about 10-13 A/cm2, which means it is easier to reduce hydrogen ions from acidic electrolyte on a platinum electrode unlike on mercury, which possesses a high Hydrogen-overvoltage. Exchange current densities are determined experimentally. The relationship between activation polarization (overpotential) and the rate of the reaction (ia and ic) is given below as Tafel relationship. ηact = log For Anodic polarization ηact = For Cathodic polarization ηact = and are termed Tafel constants. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 9: Exchange Current Density – Polarization Relationships NPTEL Web Course Magnitude of io will indicate as to whether the redox reaction is reversible or irreversible. Lower io denotes higher overpotential, while higher io indicates lower overpotential (which means the reaction tends towards reversibility). Concentration polarization Concentration profile at a solid-solution interface is given in Fig. 9.2: Fig. 9.2 Concentration – distance profile at electorde – solution interface C0 = surface concentration of electroactive species. Cb = Bulk concentration. = diffusion layer thickness (thickness of the concentration gradient) E1 = E0 + ln Cb 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 9: Exchange Current Density – Polarization Relationships NPTEL Web Course E2 = E0 + ln C0 = E| - Erev = ln Fick’s law of diffusion states = - AD Where = amount of species diffusing in unit time. D = Diffusion coefficient = concentration gradient = = m (Cb – C0) =-A (Cb – C0) i = nF iL (or id) = nFA (Cb – C0) (see Fig 9.3) iL = nFA Cb (C0  0 when current reaches steady state) Fig. 9.3 Depiction of limiting (diffusion) current Eapp = E0 + lan Cb 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 9: Exchange Current Density – Polarization Relationships NPTEL Web Course Where m = mass transfer rate constant. = mcb C0 = Cb conc = = ln = ln Total = act + conc red =- log + 2.3 log Fig. 9.4 Combined cathodic polarization Total cathodic polarization is the sum of activation and concentration polarization as shown in Fig. 9.4. Concentration polarization is generally absent in case of anodic polarization of metal dissolution. a = a log 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course Lecture 10 Polarization Techniques – Corrosion Rate Determination Keywords: Corrosion Rates, Polarization Diagrams, Linear Polarization Resistance. A weighed sample coupon of metal introduced into the corrosion process and after desired exposure period removed, cleaned of all corrosion products and reweighed. Weight loss can be converted to average corrosion rate (mpy) using Faraday’s law. There are ASTM standards G1, G4 and G31 for preparing, cleaning and evaluating corrosion test specimens, conducting corrosion coupon tests in plant equipment and laboratory immersion corrosion testing. Using corrosion coupons for weight loss (corrosion rate) measurements has advantages such as cheap and simple, permits analysis of corrosion products and can easily be done in a laboratory or on a service equipment. However, it requires long term exposures to be more accurate as short-term tests can yield misleading information. Different shapes of corrosion coupons, such as flat, ring type or cylindrical can be used. Coupons can be placed in industrial equipment using holders (electrically isolated) . Polarization techniques to determine corrosion rate Tafel extrapolation and polarization resistance are two methods to measure corrosion rates. Polarization methods are faster experimental techniques compared to classical weight loss estimation. Tafel relationship with respect to activation controlled anodic and cathodic processes has been discussed earlier. For an electrochemical reaction under activation control, polarization curves exhibit linear behavior in the E Vs log ( i ) plots called Tafel behavior. Typical polarization behavior of metals in acid solution in the presence and absence of oxygen are illustrated below. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course Typical cathodic polarization curves with respect to Tafel behaviour are also given. Extrapolation of cathodic and anodic Tafel slopes back to the corrosion potential (Ecorr) are shown. Intersection point corresponds to corrosion current density (icorr) or corrosion rate (Fig. 10.1). ia = ic = icorr (mixed potential theory) At least one decade of linearity in Tafel extrapolation is desirable to ensure good accuracy. When concentration polarization and ohmic resistance come into the picture, accuracy in Tafel extrapolation becomes more and more difficult. Steady state polarization curves need be obtained to be more representative of corrosion reactions. Potentiostatic and galvanostatic methods need be compared to ascertain the choice of a better technique to determine corrosion rates. There are some demerits in Tafel extrapolation. Since polarization curves are not reversible and are influenced by experimental and environmental conditions, Tafel constants can vary from system to system. Often anodic curves may not exhibit linear behavior near Ecorr. Polarization of a metal in deaerated acid solution To determine values of Ecorr and icorr, extrapolated linear sections from the anodic and cathodic curves are used as shown in Fig. 10.1. Anodic reaction M = M+++ 2e Cathodic reaction 2H+ + 2e = H2 Ecorr and icorr values can be directly determined from the cross-over point (Fig. 10.2). 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course Fig 10.1 Polarization behaviour of a metal (M) in deaerated acid solution At the corrosion potential, Ecorr, rate of cathodic reduction is equal to rate of anodic reaction (metal corrosion). Tafel constants ( βa and βc ) are calculated from the anodic and cathodic slopes 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course Fig 10.2 Tafel plot to estimate Tafel constants A plot of overpotential against log i showing exchange current density is illustrated in Fig 10.3. Fig 10.3 Overpotential vs log i Tafel plot 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course Current against overpotential for a reversible reaction such as the ferric – ferrous couple on a noble electrode surface is shown in Fig. 10.4. Anodic and cathodic curves along with the resultant current behavior for reversible reaction is illustrated. Fe+++ + e = Fe++ Fig 10.4 Polarization plot for a reversible reaction Polarization behaviour of a metal (M) in a stagnated aerated electrolyte at near neutral pH is illustrated in Fig 10.5. Total cathodic current corresponds to the sum of the currents for both hydrogen and oxygen reduction reactions and has to be balanced by the single anodic reaction current. Depending on the level of electrolyte agitation the magnitude of the limiting current for the oxygen reduction will vary. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course Fig 10.5 Polarization behaviour of metal M in unstirred aerated near neutral solution Another graphical example of electrochemical measurement of corrosion rate through Tafel extrapolation is illustrated in Fig. 10.6. At the corrosion potential (Ecorr), the rate of hydrogen reduction is equal to rate of metal dissolution. Corrosion rate (icorr) in terms of current density can be estimated. Tafel constants (βa and βc) can be calculated from anodic and cathodic portions of the Tafel plots. Fig 10.6 Tafel extrapolation method 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course Linear polarization resistance Change potential by about 10-20 mV from Ecorr and measure corresponding current (i). Plot a linear graph for (Eapp – Ecorr) Vs i. iapp (anodic) is positive while iapp (cathodic) is negative. The slope of the potential – current density plot near Ecorr is defined as POLARIZATION RESISTANCE (Rp). Rp = ∆E / ∆i (∆E  O) For reactions under activation, Rp can be related to icorr as: -icorr = B / Rp Log Rp = log B – log icorr When B = βa βc / 2.3(βa + βc) See Fig. 10.7 Fig 10.7 Linear polarization curve The extent of linearity of the potential – current plot depends on βa and βc values. Tafel slopes are necessary to calculate B. Since B varies within a factor of 2 around a value of 0.065 V for long range Tafel constants, corrosion rate (icorr) can be estimated within a factor of 2 (even of Tafel constants are not available). 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 10: Polarization Techniques – Corrosion Rate Determination NPTEL Web Course A Three-electrode cell is used for measurement of polarization resistance in a laboratory. Linear polarization corrosion probes are used in chemical process and water treatment industries for online monitoring, The probes are either three- electrode or two-electrode types. These techniques permit accurate measurement of even very low corrosion rates (< 0.1 mpy). 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 11: Mixed Potentials – Concepts and Basics NPTEL Web Course Lecture 11 Mixed Potentials – Concepts and Basics Keywords: Mixed Potential, Charge Conservation, Corrosion Potential Principle of charge conservation: Total rate of oxidation must be equal to total rate of reduction. i.e. Sum of anodic oxidation currents must be equal to sum of cathodic reduction currents. General Anodic reaction M = M++ + 2e Cathodic reactions can be different depending on environmental conditions. a) Evolution of hydrogen from acid or neutral solution 2H+ + 2e = H2 (acid) 2H2O + 2e = H2 + 2OH- (neutral or alkaline) b) Reduction of dissolved oxygen in acid or neutral solution. O2 + 4H+ + 4e = 2H2O (acid) O2 + 2H2O + 4e = 4OH- (neutral) e) Reduction of dissolved oxidizers such as ferric ions Eg: Fe+++ + e = Fe++ To understand the combined effect of various reducible species on the corrosion of a metal (M), it is essential to use the zero current criterion, = iaM = icH(M) + icO2(M) + icFe+++(M) 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 11: Mixed Potentials – Concepts and Basics NPTEL Web Course Take for example, the case of an active divalent metal, M corroding in an acid electrolyte. M = M++ + 2e (anodic reaction) 2H+ + 2e = H2 (cathodic reaction) M + 2H+ = M++ + H2 (net reaction) The metal corrodes with the evolution of hydrogen. There are two half-reactions as shown above and they cannot coexist as separate entities on the same metal surface. Each half reaction has its own electrode potential and exchange current density (see Fig. 11.1). Fig 11.1 Anodic and cathodic half – cell reactions occurring simultaneously on a corroding metal, M. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 11: Mixed Potentials – Concepts and Basics NPTEL Web Course Each electrode polarizes (shifts in potentials in anodic and cathodic directions) to an intermediate value (between the two half-cell potentials). Since such a polarized potential is a mixture of the two half – cell potentials, it is referred to as MIXED POTENTIAL (See Fig. 11.2). Fig 11.2 Polarization of anodic and cathodic reactions to yield a mixed potential Ecorr is corrosion potential which is a mixed potential. At Ecorr, rates of anodic and cathodic reactions are equal. ic= ia = icorr, at Ecorr icorr is the corrosion rate of the metal in the acid, and also represents the rate of hydrogen liberation at the metal surface. With a knowledge of and io for the system, corrosion rate of the metal in the acid solution can be estimated. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 11: Mixed Potentials – Concepts and Basics NPTEL Web Course From actual practice, corrosion of zinc or iron in hydrochloric acid can be represented as detailed above. A mixed potential due to anodic oxidation of zinc (or iron) and reduction of hydrogen ions from the acid (with liberation of hydrogen gas on the metal surface) could be realized. However, kinetic parameters including the exchange current density for the redox reaction at the given metal surface need be considered in assessing the corrosion behavior (and rate) of the metal in the corrosive medium. Comparing the corrosion rates for zinc and iron (when present separately) in dilute hydrochloric acid solutions, zinc dissolution is expected to be higher than that of iron from a thermodynamic view point (E0zn/zn++ = -0.76V compared to E0 for Fe | Fe++ = - 0.44V). The corrosion rate of iron will however be higher than that of pure zinc, when immersed in similar concentrations of hydrochloric acid due to differences in their exchange current densities for hydrogen liberation reaction. Exchange current density for hydrogen reduction on zinc is lower than that on iron (see Table 9.1). Engineering systems are heterogeneous and complex. The zero current criterion (Σ ia= Σic) in such multi-electrode systems in a corrosive environment becomes all the more relevant. Consider two electrodes X and Y with one reduction reaction in an acid solution. iaX + iaY = icH(X) + icH(Y) Relative areas of the anode and cathode are important in the prediction of anodic corrosion rates and current density (current / unit area) need be considered. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 11: Mixed Potentials – Concepts and Basics NPTEL Web Course Corrosion rates need to be estimated based on kinetic parameters such as overpotential and exchange current density. Driving force for corrosion in fact depends on the overpotential and not essentially on their electrode potential differences. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 12: Mixed Potential Theory – Bimetallic Couples NPTEL Web Course Lecture 12 Mixed Potential Theory – Bimetallic Couples Keywords: Bimetallic Couples, Area Ratio, Exchange Current Density. Application of the mixed potential theory to some bimetallic systems can now be considered. Iron – Platinum couple A very noble metal coupled to a very active metal, when significantly enhanced corrosion of iron in a corrosive medium can be expected. iaFe= icH(Pt) + i H(Fe) - Pt c ia Where ia and ic represent anodic and cathodic current densities and A is relative surface area. Since corrosion rate of platinum in the couple can be neglected due to its very high nobility, iaFe= icH(Pt) + i H(Fe) c Pictorial representation of half – cell reaction with respect to corrosion of iron when present alone and when present in contact with platinum (assuming equal areas) is illustrated in Fig 12.1. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 12: Mixed Potential Theory – Bimetallic Couples NPTEL Web Course Fig 12.1 Corrosion of iron in the absence and presence of platinum (as bimetallic couple) in acid medium. Presence of platinum in the couple is very significant due to the following factors: a) Provides additional cathodic surfaces for hydrogen reduction. b) Exchange current density for hydrogen reduction on platinum is very much higher than that on iron ( for Pt is 10-2 A/cm2 compared to 10-6 A/cm2 on Fe) c) If the area ratio of platinum to iron (larger cathode in contact with smaller anode), is increased, the effect of coupling with platinum on the corrosion rate of iron will be magnified. The corrosion rate of an active metal such as iron or zinc thus depends on a) What cathodic metal they are coupled with in relation to the exchange current density for the reduction reaction. b) pH and nature of the cathodic reactant For example, if the Fe-Pt couple (having higher cathode to anode surface area ratio) is exposed to neutral pH solution where oxygen reduction is the cathodic reaction (instead of H2), the expected effect of noble metal (Pt) would be not so significant since the exchange current densities for oxygen 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 12: Mixed Potential Theory – Bimetallic Couples NPTEL Web Course reduction on both the surfaces are nearly same. Also, if lead metal is coupled with iron instead of platinum in acid solution, the effect of coupling on anodic oxidation of iron would be rather negligible, since the exchange current density for hydrogen reduction on lead is very much lower than that on iron. Iron – Zinc couple Understanding the behavior of the iron-zinc couple in a corrosive environment is important since galvanizing is universally used to protect steel structures from corrosion. The role of zinc as a sacrificial anode is of industrial significance. To analyze the corrosive behavior of iron in presence of zinc, iaFe = icH(Zn) icH(Fe)– iaZn It is known that iaZn>>icH(Zn) Enhanced Zinc oxidation rate would lower the rate of corrosion for iron. The effect zinc depends on the ratio of icH(Zn) / iaZn and iron will be protected as long as this ratio is smaller. The relative areas of the two metals in a couple influence its galvanic behavior. Increasing cathodic surface areas for a fixed anode area will increase the anodic corrosion rate proportionately. Similarly, if the anodic surface areas are increased for a constant cathode area, anodic oxidation rate can be lessened. Effect of exchange current density for the cathodic reaction on two noble metals (gold and platinum) on the corrosion behavior of an active metal such zinc can now be examined. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 12: Mixed Potential Theory – Bimetallic Couples NPTEL Web Course From the EMF series, E0Au/Au+++ = + 1.42V E0Pt /Pt++ = + 1.2V Gold is nobler than platinum, However, in the galvanic series, the above position is seen to be reversed (platinum nobler than gold). Behavior of two types of noble metal-active metal couples on the anodic corrosion behavior in an acid solution can be understood taking into consideration Au – Zn and Pt – Zn couples. Exchange current density for hydrogen reduction on platinum and gold are 10 -2 A/cm2 and 10-6 A/cm2 respectively. Under the circumstances, higher corrosion rate (icorr) for zinc can be expected when coupled to platinum than when contacted with gold (cathodic surface area remaining similar in both cares). 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 12: Mixed Potential Theory – Bimetallic Couples NPTEL Web Course Effect of cathodic surface area on the corrosion rate of an anodic metal is illustrated in the figure given below. An active metal M is coupled to a noble metal N in an acid solution. Two surface areas for the noble metal N is considered, namely, 1cm2 and 10cm2. Fig 12.2 Effect of change in cathodic surface area on the corrosion rate of an active metal Corrosion rate for M is the highest when coupled to 10 cm2 of N compared to 1 cm2 of N (see Fig. 12.2). Unlike the case with coupling of an active metal with a very noble metal (such as platinum or gold) effect of coupling of two corroding metals in a corrosive acid medium can also be analyzed in the following lines (assuming equal areas). a) The combined potential of the couple (eg:- Fe – Zn) will lie in between the uncoupled corrosion potentials of the two metals. b) The corrosion rate of the most active one among the couple (Zn in case of coupling with Fe) will always be enhanced, with a simultaneous decrease in 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 12: Mixed Potential Theory – Bimetallic Couples NPTEL Web Course the corrosion rate of the relatively noble metal (iron for example in contact with zinc). c) It is generally true in case of all galvanic couples that their corrosion behavior is determined by their electrode potentials, exchange current densities, relative surface areas and magnitude of overpotentials. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 13: Mixed Potential Theory – Activation and Diffusion Controlled Processes NPTEL Web Course Lecture 13 Mixed Potential Theory – Activation and Diffusion Controlled Processes Keywords: Bimetallic Couples, Area Ratio, Exchange Current Density. Effect of added oxidant The driving force for corrosion is generally increased by the addition of a strong oxidizer such as ferric ions (ferric – ferrous redox couple). Ecorr (corrosion potential) of the active metal is shifted in a nobler direction. Corrosion rate is enhanced. Hydrogen evolution rate decreased in the presence of oxidizer (eg: in acid solution). In the presence of an oxidizer such as the Fe+++/Fe++ couple in an acid medium two possible cathodic reactions, namely, 2H+ + 2e = H2 and Fe+++ + e = Fe++ need be considered. The respective exchange current densities for the above cathodic reductions on the corroding metal will determine the corrosion rate. If for example, the exchange current density for ferric ion reduction is lower than that for hydrogen evolution, the addition of ferric ion oxidizer will not have any significant effect on the corrosion rate of the anodic metal in an acid medium. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 13: Mixed Potential Theory – Activation and Diffusion Controlled Processes NPTEL Web Course Effect of multiple reducible species on anodic corrosion is shown in Fig. 13.1. Fig 13.1 Effect of multiple reducible species on the corrosion rate of an anodic metal. Anodic and cathodic currents are proportional to concentrations of the redox species. Rate of cathodic reduction can be thus related to concentration of cathodic reactant such as hydrogen, oxygen, ferric ions etc. For example, ic is proportional to [H+]n With increase in pH, hydrogen ion concentration decreases in solution (and vice – versa). 7 = 10 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 13: Mixed Potential Theory – Activation and Diffusion Controlled Processes NPTEL Web Course For oxygen, from a solution concentration of about 10 mg/L if through deaeration, it is reduced to 0.01 mg/L, the cathodic reaction rate could be altered by a factor of 103. Effect of changes in cathodic reactant concentrations on the corrosion rate of an anodic metal is shown in Fig. 13.2. Fig 13.2 Effect of changes in cathodic reactant concentrations on the corrosion rate of a metal. Diffusion controlled cathodic processes If one considers the effect of concentration polarization also along with activation polarization for the cathodic process, effects on anodic oxidation rates need to be seen in a different light. There are instances when the cathodic process can be diffusion controlled (due to concentration gradients at the solid-solution interface). An example is metallic corrosion in sea water where oxygen diffusion controls the cathodic process. O2 + 2H2O + 4e = 4OH- 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 13: Mixed Potential Theory – Activation and Diffusion Controlled Processes NPTEL Web Course Solution velocity influences the corrosion rate of metals when the cathodic process is diffusion controlled (unlike in activation controlled systems) since limiting (diffusion) current is affected (see Fig. 13.3). Fig 13.3 Effect of concentration and velocity on diffusion controlled polarization However at higher solution velocity, the cathodic reduction process becomes activation controlled. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 13: Mixed Potential Theory – Activation and Diffusion Controlled Processes NPTEL Web Course The influence of solution velocity on the corrosion rate of a normal metal under a diffusion controlled cathodic process in depicted in Fig. 13.4. Fig 13.4 Change of corrosion rate with solution velocity for a normal metal. Corrosion rate increases with solution velocity as long as the cathodic process is under diffusion control and it becomes independent of velocity at higher velocities when the cathodic reaction is under activation control. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course Lecture 14 Prevention Strategies – Design and Coatings Keywords: Corrosion Prevention, Designs, Protective Coatings. There are a number of methods to control corrosion. The choice of any one control technique depends on economics, safety aspects and other technical considerations. Design Materials selection Protective coatings Inhibitors and environmental alterations Corrosion allowances Engineering design with a view to corrosion abatement is important. For example, a simple aspect such as providing drainage, as for an automobile side panel. Choice of appropriate materials keeping in mind the probability of corrosion in the existing environmental conditions is very critical. Among the materials available for selection; titanium, copper – alloys, stainless steels, carbon steels and aluminium and its alloys are often chosen. Proper design of equipment In the design of equipment, fittings such as baffles, valves and pumps to be considered Elimination of crevices Complete drainage of liquids Easy to clean Facilitate easy access to inspection and maintenance Avoid bimetal contacts – Insulation of Joints. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course General choice of metals and alloys for corrosive applications is given in Table 14.1. Table 14.1 Choice of materials for corrosive environments Material Environment Nickel and alloys Caustic solutions Monel Hydrofluoric acid Hastelloys Hot hydrochloric acid Stainless steels Nitric acid Lead Dil. sulfuric acid Tin Water Titanium Hot strong oxidizing acids/liquids Carbon steels are readily available cheaply and can easily be formed and worked into different shapes. Carbon steels can undergo different types of corrosion, such as rusting, hydrogen embrittlement and galvanic corrosion. Galvanization is commonly used to protect structural steels. Protective coatings, cathodic protection and inhibitor are extensively used to improve the structural life of carbon steels. Stainless steels are generally immune to corrosion in mild environments. However, they may experience pitting, crevice and stress corrosion cracking in aggressive environments such as sea water, chemical processing etc. Ferritic and austenitic stainless steels are used in thin wall tubing in heat exchangers and also in many industrial and marine applications. Type 304 stainless steel is used in valve parts, pump shafts and fasteners. Duplex stainless steels (Cr – Mo alloys of iron) are used in chloride and high temperature environments. Martensitic stainless steels possess good mechanical strength. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course Nickel and alloys are used in chemical process industries. Nickel – copper alloys as monel possess resistance to nonoxidizing acids. Nickel-chromium-iron alloys passivate in presence of oxidizers. Addition of molybdenum increases chloride resistance. Copper and its alloys are quite resistant to non-oxidizing aqueous and many atmospheric environments. Brass undergoes dezincification. Aluminium and naval brasses are more resistant. Bronzes and aluminium bronzes are resistant to impingement. Copper-nickel alloys exhibit good resistance to impingement and stress corrosion. Corrosion resistance of aluminium alloys vary widely depending on type of alloy addition and environments. Titanium and alloys show stable, protective oxide film (passivation). Very good corrosion resistance in hot acids and many other corrosive environments. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course Some general approaches for corrosion prevention are detailed in Tables – 14.2 and 14.3. Table 14.2 Corrosion protection methods and processes Approach Process Removal of oxidizers Boiler water Corrosion inhibition Inhibitors & pH control General corrosion prevention Anodic and Cathodic protection Coatings: Metallic Electroplating, galvanizing, metal spray or immersion. Organic Claddings and paints. Nonmetallic Anodizing, Conversion coatings. Metal modification Alloying Change in surface / Removal of corrosives (maintenance) environment conditions Proper designs • Avoid crevices Provide drainage Avoid bimetallic joints Since general corrosion is predictable, design considerations can include preventive measures whenever and wherever possible. Some examples: Wall thickness control Control of process stream composition (Elimination of chlorides) Prevention of acid contacts - neutralization. Minimization of vapor condensations and collection. Prevention of leakage of corrosives. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course For carbon steels: Cathodic protection combined with coatings. Channel and angle sections positioned to collect and drain water, liquids and debris. Table 14.3 Corrosion types with prevention strategies Type of corrosion Prevention Strategies Stress corrosion cracking More resistant alloys. Remove tensile stress, control of environment (elimination of chlorides) Corrosion fatigue Eliminate cyclic stress and corrosive environment. More rigid design to reduce stresses due to vibrations. Avoid stress concentration in design. Hydrogen embrittlement Choice of less – susceptible alloy / coatings. Avoid cathodic protection (steels in acid Environments) Galvanic Corrosion Selection of metals / alloys closer in galvanic series. Favorable cathode to anode ratio. Coating taking care not to create smaller anodes with larger cathodes, insulation of dissimilar joints. Crevice corrosion Proper design of junctions and joints to minimize crevices. Welded joints preferable to rivets and bolts. Pitting and crevice corrosion are enhanced in stagnant / slow flowing solution. Provide drainages. Erosion corrosion and cavitation Design to reduce velocity and turbulence, avoid abrupt changes in flow directions. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course Coatings Barrier between corrosive environment and metal. Coatings may serve as sacrificial anodes (zinc on steels ) or release substances that resist corrosion. Metal coatings - Noble coat - Silver, copper, nickel, chromium, tin, lead on steels (ensure pore - free, uniform, adherent coating; favorable anode / cathode ratio to minimise galvanic attacks). Sacrificial coatings – Zinc, aluminium, cadmium on steels. (steel is cathodic to plated metal). Coatings can be applied through hot dipping, hot spraying, electroplating, electro- less plating, vapour deposition and metal cladding. Aluminium, stainless steel, titanium, platinum etc can be cladded on various metallic substrate for enhanced corrosion protection (physical or chemical). Other types of surface treatments. Modification of substrates through ion implantation and laser processing. Inorganic coatings: glass, cement, ceramic and chemical conversion coatings. Chemical conversion: Anodizing, oxide, chromate, phosphatizing. Organic coatings: Paints, lacquers, varnishes (Resin, solvent + pigment in the coating liquid). High performance organic coatings used in petroleum industries. Development of corrosion – resistant synthetic resins. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 14: Prevention Strategies – Design and Coatings NPTEL Web Course Types of paint coatings Good adhesion, flexibility, impact resistance and protection from chemicals, moisture, and atmospheric conditions. Lacquer – synthetic resins (vinyl chloride, acrylic, rubber). Latex (Acrylics and Vinyls) Oil-based and Epoxy coatings (good bending, hard and flexible) Coal – tar – epoxy. Poly – urethanes, polyester and vinyl ester (hard, brittle or elastomeric). Organic zinc rich coatings (organic barrier + galvanic Zn protection) Co-polymeric protective coatings.(thermoplastic – copolymer - aromatic coatings). Anti - corrosion paints – various types additives to improve corrosion resistance, durability and impermeability. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course Lecture 15 Prevention Strategies - Inhibitors and Surface Engineering Keyword: Inhibitors, Passivators, Surface Engineering. Inhibitors are chemicals which adsorb on metal surfaces. A corrosion inhibitor can act in several ways: Arrest or slow down anodic or cathodic reactions by blocking active sites on metal surfaces. Eg: Amines, thiourea, benzoate, antimony trichloride. Promote surface passivation (active-passive metals and alloys). Eg: Chromate, nitrite, red lead, calcium plumbate. Formation of a surface layer blocking exposure of the bare metal to corrosive medium. Eg: Phosphate, silicate, bicarbonate, hexametaphosphate. Hexylamines or sodium benzoate – radiator fluids in cooling circuits of engines. Antimony trichloride – De-scaling of steels in sulfuric acid. Volatile (vapour phase) inhibitors (Amines) -Metal (steel) articles or equipment during transport. A classification of inhibitors based on their functionality is given below: Passivating inhibitors Cathodic inhibitors Organic inhibitors Precipitation inhibitors Volatile corrosion inhibitors. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course Two types of passivating inhibitors. Oxidizing anions – Chromate, nitrite and nitrate that can passivate steel in absence of oxygen. Nonoxidizing ions – phosphate, tungstate and molybdate that require oxygen to passivate steel. Inhibitors to be used in just the required concentration. Higher concentration – Over protection? or corrosion? Lower concentration do not protect! Inhibitors generally used in quantities less than 0.1% by weight. Cathodic inhibitors: • Slow down cathodic reaction or selectively precipitate on cathodic areas. Act as poisons, precipitates or as oxygen scavenger. Compounds of As and Sb make combination of and discharge of hydrogen difficult. Ions of Ca, Zn or Mg precipitate as oxides to form protective layers. Oxygen scavengers prevent cathodic depolarization due to O2 (Na2 SO3). Organic inhibitors – Both anodic and cathodic effects. Adsorption depending on charge of inhibitor. Precipitation inhibitors: Film forming compounds – block anodic and cathodic sites. (E.g.: calcium, magnesium precipitation of silicates and phosphates). Vapour phase inhibitors – used during transport in closed environment. Morpholine, Hydrazine. Vapor condenses and hydrolyzed by moisture to liberate protective ions. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course Corrosion inhibitors used in: Chemicals processing Petroleum refining Cement and concrete Pulp and paper Oil and gas production Metals Utilities Effect of addition of cathodic, anodic and mixed inhibitors on the corrosion rate of a metal is illustrated in Fig. 15.1, 15.2 and 15.3. Influence of the inhibitors on the anodic and cathode reactions, respectively could be seen. As can be seen, cathodic inhibitors selectively influence the cathodic polarization, bringing down corrosion rates. Similarly, anodic inhibitors specifically interfere with the anodic oxidation reactions, decreasing icorr values. On the otherhand, mixed inhibitors influence both anodic and cathodic reaction rates. Fig 15.1 Role of cathodic inhibitor on corrosion rate of a metal. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course Fig 15.2 Role of anodic inhibitors on the corrosion rate of a metal Mixed inhibitors (amines, selenides) Fig 15.3 Role of mixed inhibitors on the corrosion rate of a metal 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course General inhibitors used in some industrial operations are listed in Table 15.1 Table 15.1 Industrial uses of inhibitors Recirculation cooling water - Silicates, chromate, nitrate, polyphosphates Automotives coolants - Benzoate, borax, phosphate, nitrite Mercaptobenzothiazole. Steam condensates - Ammonia, amines (benzylcyclohexamine). Octadecylamine (long chain aliphatic) Sea Water and brines - Chromates, nitrite etc. Pickling acids - Phenylthiourea, mercaptans, quinoline, Pyridine, various long chain amines. Oil refining and production - Primary, amido-, quaternary amines Imidazoline. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course Surface modification approaches for corrosion protection of steels. Modification of surface region of engineering alloys through diffusion of different elements and formation of a layer having desirable chemical composition, microstructure and properties. Thermo-chemical treatments – Physical and chemical Vapour deposition. Coatings by plasma spraying Electrospark deposition Ion implantation Sputter deposition of selected elements and compounds. Surface layers developed by such materials, can be classified as: Overlay coatings Diffusion coatings Recast layers Thermo-chemical treatment for surface modification of steels – nonmetals or metals introduced into metal surfaces by thermo - diffusion after chemical reaction and adsorption. Caburizing, nitriding, carbonitriding, boronizing, chromising and aluminizing are some popular methods. Other examples include surface modification by Electrical Discharge Machining to remove surface material-Melted zones are transformed to recast layers with specific structures. Surface modification by electrical discharge treatment in electrolyte where a high energy thermal process is involved at surfaces leading to melting, vaporization, activation and alloying in an electrolyte. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 15: Prevention Strategies – Inhibitors and Surface Engineering NPTEL Web Course Laser surface engineering for corrosion protection a) Microstructure modification Laser surface melting Laser shock peening. b) Chemical composition and microstructure modification. Laser cladding Laser surface alloying Pulsed laser deposition Laser – based thermal spray Types of surface engineering Coatings – sputtering, CVD, spin coat, Passivation Chemical treatment Plasma treatment Surface derivitization Laser treatment Plasma deposition Polymerized coatings Fluropolymers and siloxanes Scratch - resistant coats Paint adhesion Electropolishing – 316 stainless steel, Nitinol (oxide enrichment). Conversion coatings Oxidation, passivation Chromate, phosphate, black oxide Pore surface engineering. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 16: Cathodic Protection – Principles and Classification NPTEL Web Course Lecture 16 Cathodic Protection – Principles and Classification Keywords: Cathodic Protection, Equipotential Surface, Impressed Current, Sacrificial Anode. Sri Humphrey Davy ‘s pioneering work (1824) on protecting the copper sheathing on wooden hulls in the British Navy by sacrificial zinc and iron anodes is considered to be the earliest example of application of cathodic protection. Copper-sheathed ship hulls protected by sacrificial blocks of iron. Zinc alloy as sacrificial anode. Galvanising – Typical example of sacrificial anode to protect steels. Various definitions Reducing or eliminating altogether corrosion by making the metal a cathode by application of either an impressed DC current or attaching the metal to a sacrificial anode. Corrosion occurs at anodic areas – if all anodic areas can be converted to cathodic areas, the entire structure will become cathode and corrosion is stopped. Corrosion occurs at the regions where current discharges from metal to environment (soil, water) (anodic areas). There is no corrosion at regions where current enters from the environment to metal (cathodic areas). Objective should then be to force the entire structure to collect current from the environment (making it cathodic entirely). 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 16: Cathodic Protection – Principles and Classification NPTEL Web Course Current flow depends on factors such as: a. Resistivity of environment and b. Degree of polarization of anode and cathodic areas. Cathodic protection is achieved by supplying electrons to the structure being protected. Driving force for corrosion is the potential difference. Equipotential surface - No driving force (no current flows). In Fig. 16.1, the above principles underlying cathodic protection are illustrated diagrammatically. Fig 16.1 Basic concept of cathodic protection. Reactions M = M++ + 2e (anodic, corrosion) 2H+ + 2e = H2 (cathodic – Acid Solutions) O2 + 2H2O + 4e = 4OH- (cathodic-neutral to mild alkaline) 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 16: Cathodic Protection – Principles and Classification NPTEL Web Course Principles governing cathodic protection are illustrated in Fig. 16.2 below. As per mixed potential theory, the zero current criterion is shown. An equilibrium is established on metal (M) in which anodic oxidation rate is equal to cathodic reduction rate [E corr and icorr(A)]. By cathodic polarization of the metal with an applied DC current (iapp), initial corrosion potential is seen shifted to a lower value [icorr(B)]. Complete stoppage of corrosion, requires polarization of the metal to the reversible potential of the metal (E oM). Fig 16.2 Electrochemical principles governing cathodic protection Principles of cathodic protection of a metal (steel, for example) in neutral aerated water or sea water are shown in Fig. 16.3. Diffusion controlled cathodic oxygen reduction is the cathodic reaction marked by a limiting current. Applied current and corrosion rate are limited by the limiting (diffusion) current density. Current requirements can be further reduced by surface coatings. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 16: Cathodic Protection – Principles and Classification NPTEL Web Course Fig 16.3 Electrochemical aspects of cathodic protection in neutral sea water. Two methods of cathodic protection a) Use of sacrificial anodes. b) Impressed current method. Fig. 16.4 and Fig. 16.5 illustrate the two types of cathodic protection, namely, sacrificial anode and impressed current methods. Fig 16.4 Sacrificial anode method 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 16: Cathodic Protection – Principles and Classification NPTEL Web Course Fig 16.5 Impressed current method Factors to be considered in the design and execution of cathodic protection installations. Impressed current system a) How much current necessary for complete protection? b) Source of DC Current. c) Installation, Design, erection and maintenance. d) Auxiliary anodes – choice, size, number, installation. e) How to assess elimination of corrosion through entire structure? There are a few limitations based on current flow reaching all through protected conducting structure. For example, in a pile-up of pipes, current may not efficiently reach pipe surfaces placed in between. Internal pipe surface may not receive protection. Similarly, portions of pipe lines above ground, valves etc, cannot receive complete protection. The above conditions are generally referred to as ‘electrical shielding’. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 16: Cathodic Protection – Principles and Classification NPTEL Web Course Current necessary for protection need be just sufficient; neither less nor excess. Excess current may do harm! Lower current do not protect! Requirements of galvanic sacrificial anodes a. Potential between the anode and the corroding metal structure should be large enough to overcome the anode-cathode cells. b. Sacrificial anode to have sufficient Electrical Energy Content (EEC) which predicts its life. c. Good current efficiency relevant to anodic corrosion. EEC can be estimated and expressed as ampere hours/weight (kg or lb) Eg: Pure Zinc that possesses high EEC of 372 ampere hour / pound. This means if the zinc sacrificial anode has to discharge continuously one ampere, on pound of its weight would be consumed in 372 hours. Lower current discharge will prolong its life further. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 17: Cathodic Protection – Influencing Factors and Monitoring NPTEL Web Course Lecture 17 Cathodic Protection – Influencing Factors and Monitoring Keywords: Coated Surfaces, Protection Criterion, Anode Materials, Pipeline Protection. For large structures such as underground pipe lines, impressed current cathodic protection is used, while for smaller structures such as house-hold water tanks, ship’s hull etc, sacrificial anodes can be effectively used. Painting of steel pipe lines and tubes can significantly reduce protection current requirements and thus save cost. Approximate current requirements for cathodic protection of steel pipes are given below: Uncoated in flowing sea water 10-15 mA/ft2 Well-coated in water 0.01-0.003 mA/ft2 Excellently coated and exposed to water or under soil 0.0003 or less mA/ft 2 As can be seen above, good surface coating significantly reduces protection current requirements. Electrochemical basis for protection criterion can be assessed: Protection of steel is taken as example: Fe = Fe++ + 2e E0 = - 0.44 V When polarized to half –cell potential of above reaction, corrosion rate reduces to 0. Rate of forward and reverse reaction are same when net reaction rate is zero. Eh = - 0.44 + log [Fe++] 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 17: Cathodic Protection – Influencing Factors and Monitoring NPTEL Web Course Fe++ + 2OH- = Fe (OH)2 Calculated potential (based on solubility product) is -0.59V (SHE) which corresponds to about -0.90V (vs Cu/CuSO4). Accepted criterion for protection of steel in water is -0.85V (vs Cu/CuSO4). Potential of structure to environment is generally measured using Cu/CuSO 4 reference electrode. Test coupons made of same metal and previously weighed can be electrically connected to protected structures. These coupons are also exposed to same cathodic current in the corrosive environment. Estimation of weight losses of such coupons is a better proof of cathodic protection. Table 17.1 Potentials for Cathodic protection (Cu/CuSO 4 electrode) Iron and Steel -0.85 to -0.95 V Lead -0.6 V Copper and alloys -0.5 to -0.66 V Aluminium -0.95 to -1.2 V 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 17: Cathodic Protection – Influencing Factors and Monitoring NPTEL Web Course Anode materials that can be used as ground-beds in impressed current cathodic protection are given Table 17.2 Table 17.2 Anode materials for impressed current cathodic protection Material Average consumption rate kg/A-year Cast Iron 5–7 Steel scrap 5-8 Aluminium 4–5 Graphite 0.6 – 1.0 Lead ----- Platinum ----- Magnesium, zinc and aluminium and their alloys can be used as sacrificial anodes. Design considerations for both impressed current and sacrificial anode systems have some common steps. a) Area to be protected – Exposed areas of the structure – in coated system, exposed area at breaks and deteriorated coatings. b) Polarised potential – Current density based on area need be estimated. c) Current demand – Current – density demands depend on the environment and nature of surface coating. d) Anode consumption – Required number and weights of anode materials determined from known consumption rates for the desired current demand. Anode number and distribution for the protected structure can be thus estimated. Anode resistance and design output current can then be estimated. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 17: Cathodic Protection – Influencing Factors and Monitoring NPTEL Web Course Monitoring of effectiveness of pipeline protection Most widespread method is based on potential measurements of a cathodically polarized structure with reference to a standard electrode. A potential of -0.85V (Cu / CuSO4) is sufficient for protection of steel in soil and natural water environments. It may however be borne in mind that the above criterion is not optimum and situations may arise when more negative (upto – 1.0V) may be required or even lower (-0.7V) potential may suffice for protection. Interference from IR components can introduce errors in pipeline potential measurements. Elimination of IR drop can be achieved using ‘switch – off’ method. Potential measurements in chosen control points in a pipeline are frequently insufficient to ensure effective protection. Close Interval Potential Survey (CIPS) is an intensive monitoring technique based on connecting a thin cable to a pipeline to monitor frequent potential readings all the way. Special computer software together with appropriate instrumentation can be used for gathering and processing the data. Another technique called Direct Current Voltage Gradient (DCVG) method enables protection evaluation and also detection of defects in insulation. Potential gradient is monitored in the soil with a sensitive potential measurement meter using two reference electrodes kept at both sides of the pipeline at shorter distances. Corrosion coupons (probes) are generally used for monitoring of cathodic protection. A schematic representation of a coupon probe connected to a cathodically protected pipeline is illustrated in Fig. 17.1 . The arrangement allows measurement of switch- off potential without any interruption of pipeline protection. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 17: Cathodic Protection – Influencing Factors and Monitoring NPTEL Web Course Fig 17.1 Circuit for monitoring cathodic protection. Different types of simulation probes are available for determination of : a) Level of protection in sections in casing pipes. b) Polarization resistance and depolarization rate. c) Insulation coating resistance. d) Any interference on neighbouring underground installations. e) Corrosion rate of protected structures. Such probes need be located in various geological locations through a running pipeline. Recently kinetic cathodic protection criterion has been proposed to allow maintenance of metal corrosion rate at a desired level. There are several pipeline corrosion rate control methods including both physical and electrochemical techniques, which allow determination of effective protection in chosen regions of structures. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 17: Cathodic Protection – Influencing Factors and Monitoring NPTEL Web Course Table 17.3 Corrosion rate control in pipelines. Electrochemical Physical Impedance Polarization curves Electrical resistance spectroscopy Polarization Radiography Electrochemical noise resistance Ultrasonic Harmonic synthesis Weight loss determination There are several developments in cathodic protection instrumentation. Use of thyristor – controlled rectifiers will enable automatic control of current output depending on corrosive environment requirements. There is also a possibility of controlled potential cathodic protection to suit specific structures. For example, in sea-going vessels, the hull is subjected to variations in flow velocities leading to alteration in limiting current density (with respect to oxygen reduction). Such limiting current fluctuations significantly influence cathodic protection current requirements from time to time. In such environments, controlling the potential (rather than current) would be more beneficial. Controlled potential protection is extensively used for ship hulls incorporating anode – reference electrode attachment along with automatically – controlled power supply unit. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 18: Design Aspects of Cathodic Protection NPTEL Web Course Lecture 18 Design Aspects of Cathodic Protection Keywords: Cathodic Protection Design, Choice of Protection, Engineering Aspects. Advantages and uses of cathodic protection: Compared to alternative protection methods, cathodic protection is applied by simply maintaining a DC power circuit and its effectiveness can be continuously monitored. Generally applied to coated structures to protect areas where coatings are damaged-enable longer life span for existing structures. Can avoid other design considerations for corrosion resistance (such as corrosion allowance) if cathodic protection is pre-specified. Can be applied to all metallic structures / including concrete). Application for protection of exterior surfaces of Ship hulls Pipelines Storage tank bases Seashore structures Off shore platforms and internal surfaces of • Large diameter pipelines. Storage tanks (water and oil) Water circulation systems • Can be applied to copper – base alloys (water systems), lead – sheathed cables, aluminium alloys and reinforced concrete structures (buildings, bridges, sea shore, and marine structures). 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 18: Design Aspects of Cathodic Protection NPTEL Web Course Basic requirements: For galvanic protection (sacrificial anode) Sacrificial anodes Direct connection to the structure. Minimum resistance between anodes – connection. For impressed current protection Inert anodes (backfill – ground-bed) DC power supply. Well insulated, minimum resistance and secure conducting connections Background information for choice of cathodic protection type and design considerations: Structure’s physical dimensions (surface area). Size, shape, material – type and locations. Electrical isolation and elimination of short circuits. Corrosion history in the area with respect to environment. Resistivity survey information. Information on pH, potential between structure and environment, current requirements per unit area. For ensuring reliable and cost-effective protection, the following aspects need be ascertained. Electrical continuity – minimize iR drop. Coatings to minimize current requirements. Structure isolation – introduction of isolation joints (insulating flanges). Availability of test stations with facilities for monitoring and data aquisition. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 18: Design Aspects of Cathodic Protection NPTEL Web Course Current requirements for complete protection can be assessed through. Actual tests on existing structure using a temporarily – organized cathodic protection setup. Based on prior experience and theoretical calculations based on coating efficiency. Suggested formula Total protective current = (Area in ft 2) (required current density) x (1.0 – coating efficiency) Table 18.1 Current requirements for cathodic protection of uncoated steels Approximate current requirements (mA/ft 2) for uncoated steel Soil at natural pH 0.4 – 1.5 Highly acidic soil 3 – 15 Fresh water (static) 1–6 Flowing water with oxygen 5 – 15 Seawater 5 - 10 Total current requirements can be estimated by multiplying current density requirements with surface area Choice between the two methods of cathodic protection depends on Conditions at site Current density requirements Soil resistivity If the soil resistivity is lower and current requirements are less than about 1mA/ft2, galvanic anodes can be used. For larger resistivity and current requirements, impressed current protection may be opted for. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 18: Design Aspects of Cathodic Protection NPTEL Web Course Design aspects for galvanic anode cathodic protection Soil resistivity assessment – Site of lowest resistivity to be chosen for location of anode. Choice of anode material – Data from commercially available anodes to be carefully assessed. Table 18.2 Properties of some sacrificial anodes. Metal Potential (Cu / CuSO4) Density, g/cm3 EEC (amp – h / Kg) Aluminium - 1.15 V 2.7 2700 Magnesium - 1.55 V 1.7 1230 Zinc - 1.10 V 7.1 780 Aluminium and magnesium – alloy anodes can also be chosen: Open circuit potentials for various anodes to be known to facilitate selection. Similarly, for protection of steel, its potential in soil or water need be known. Net driving potential between the metal to be protected and the sacrificial anode in the environment to be the criterion. This will involve the polarized potential of the steel (protected) when contacted with the anode such as magnesium. Estimate number of anodes required for desired protection and to compensate resistance limitations (anode to electrolyte and lead – wire resistance as well as structure to electrolyte resistance). Based on the knowledge of ground-bed resistance and life expectancy of anodes, requirement of number of anodes is calculated. Design aspects for impressed current cathodic protection Soil resistivity Estimation of required current density. Actual current requirements can be assessed using a provisional test setup, where battery-power supply can be used. Effectiveness of insulating joints (as in a pipeline) can be tested. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 18: Design Aspects of Cathodic Protection NPTEL Web Course Selection of appropriate ground-bed anode (high silicon, chromium bearing cast iron commonly used). Backfill materials such as coal-coke breeze, calcined petroleum coke or graphite can be chosen for ground-bed anodes for protection of subsoil steel structures such as pipelines. Number of anodes to meet current density and design requirement. Selection of anode sites and calculation of total circuit resistance. Selection of suitable DC power system. Table 18.3 Comparison between the two cathodic protection systems. Galvanic Impressed current No external power External power supply required Driving potential fixed Adjustable applied potential current Used in low resistivity environment Can be used even in high resistivity environment Lower maintenance High maintenance Cannot originate stray currents Can cause stray current problems Used for small and well - coated Suitable for larger structures (coated or structures uncoated) REFERENCES 1. Cathodic protection – Guide. www.npl.co.uk (from web) 2. J. P. Guyer, Introduction to cathodic protection, 2009, CED enginerring.com (from web) 3. J. B. Bushman, Impressed current cathodic protection system design, Bushman and Associates. Ohio (from web) 4. NACE literature on cathodic protection criteria: NACE, Houston (1989). 5. J. H. Morgan, cathodic protection, NACE, Houston, 1987. 6. D. A. Jones, Principles and prevention of corrosion, Prentice – Hall, N. J. (1996). 7. A.W.Peabody, Principles of Cathodic Protection, Chapter 5, NACE Basic Corrosion Course, NACE, Houston (1970) 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 19: Stray Current Corrosion NPTEL Web Course Lecture 19 Stray Current Corrosion Keywords: Stray Current, Electrical Bonding, Insulated Couplings Stray currents are currents flowing from external sources. Any metallic structure, such as a buried pipeline represents a low resistant current path and is thus vulnerable to the effect of stray currents. Stray-current effects are encountered in several impressed current cathodic protection systems. This is very common in industrial protected systems, such as oil production industries having innumerable buried pipe lines. Current leakage from auxiliary anodes associated with cathodic protection systems can enter unintentionally to a near-by unprotected structure and leave from the surfaces creating severe corrosion (see Fig. 19.1). Fig 19.1 Stray current leakage from a cathodic protection system to a nearby pipeline. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 19: Stray Current Corrosion NPTEL Web Course Other sources of stray currents include DC electric power traction, welders, electroplating units and ground electric DC power. If there is a current path due to a low resistance metallic object (for example, a pipe line or another metallic structure), current leakage from an impressed current protected system will enter such unprotected structure before returning to the protected object. Regions from where current leaves are susceptible to stray- current corrosion. A solution to such a problem is through electrical bonding of the near-by structure. Simultaneously additional anodes and increasing DC power capacity can accord full protection to all structures in the vicinity. Properly insulated couplings can help reduce the problem (see Fig. 19.2). Fig 19.2 Proper design through additional anodes to prevent stray current corrosion. When impressed current protection systems are installed, anode ground beds should be so located that stray current from them cannot make entry into other near-by structures. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 19: Stray Current Corrosion NPTEL Web Course Direct stray currents can cause anodic, cathodic or a combined interference. Anodic interference is generally found in close proximity to a buried anode. The pipeline will pick up current and will be discharged at a distance farther away from the anode. In the current pickup site, the potential of the pipe will shift in negative direction and is thus beneficial as cathodic protection. Sometimes, overprotection could be created by such potential shifts. On the other hand, cathodic interference is produced in close proximity to a polarized cathode; the potential shifting in a positive direction where current leaves the structure (causing corrosion damage). In combined interference, current pickup occurs close to anode and discharge occurs closer to cathodically polarized areas. The damage could be higher in this case since current pickup (overprotection) and discharge (corrosion) are both detrimental. Stray current corrosion control in DC rail transit systems. Fig 19.3 Stray current corrosion of a pipeline from a DC rail transit system. Consequences of transit stray current from DC traction are illustrated in Fig. 19.3. Stray current from the rails enters part of a water pipeline through soil and after traversing through the pipeline path, leaves at another end. Regions in the 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 19: Stray Current Corrosion NPTEL Web Course pipeline where current enters are protected, while those from where current leaves to reenter the rail line suffer unintentional corrosion. The question arises? How much corrosive stray current is harmful? It is estimated that for one ampere of stray current discharged from the transit system to earth, complete perforation of one square inch (0.25 inch wall thickness) steel pipe can occur within about a week. Besides unintentional severe corrosion of nearby structures such as pipelines. DC stray currents can result in ‘Free’ cathodic protection to regions where stray current enters a structure. Reduction in effectiveness and life of cathodic protection systems. Breakdown of reinforced concrete structures. Electrical shocks and loss of electrical grounding. As a control measure, track-to-earth potentials under multi-loads can be monitored through computer simulations and predictive modeling. On the other hand, controlling transit stray current at the source itself will be preferable. The following suggestion in this regard is noteworthy. Reasonable spacing of traction substations. Continuously welded rails. Stray current collector – Track slab rebar - epoxy coated. Use of high resistivity concrete for track slab. High track to earth resistance. Insulated track designs are available. Apart from this, pipeline designers and engineers can also keep in mind the potential for stray current and monitor pipeline currents and potentials frequently. Routine surveillance is required. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 20: Passivity – Definitions and Influencing Parameters NPTEL Web Course Lecture 20 Passivity – Definitions and Influencing Parameters Keywords: Definition of Passivity, Flade Potential, Anodic Polarization, Critical Anodic Current Density. In the Eh – pH diagrams, resistance to metallic corrosion is indicated at stability regions where either the metal remains thermodynamically stable (immunity) or the metal surface is covered with an oxide / hydroxide layer (passivity). Passivity is due to the formation of thin, impermeable and adherent surface films under oxidizing conditions often associated with anodic polarization. Only certain metals and alloys exhibit active-passive behavior, which is essentially an acquired property. Faraday in the 1840’s showed that iron reacted rapidly in dilute nitric acid, but was visibly unattacked in concentrated (fuming) HNO3. An invisible surface oxide film formed in concentrated acid was found to be unstable in dilute acid and through scratching, the surface oxide could be removed. Definitions of passivity as proposed by Uhlig are given below: 1. A metal active in the EMF series or an alloy composed of such metals is considered passive when its electrochemical behavior becomes that of an appreciably less active or noble metal. 2. A metal or alloy is passive if it substantially resists corrosion in an environment where thermodynamically there is a large free energy change associated with its passage from the metallic state to appropriate corrosion products. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 20: Passivity – Definitions and Influencing Parameters NPTEL Web Course Examples for definition 1 are Cr, Ni, Ti, Zr and stainless steels. Examples for definition 2 are lead in sulfuric acid, magnesium in water and iron in inhibited pickling acid. Two types of passivity thus exist. a) A metal is passive if it resists corrosion under anodic polarization (noble potential, low corrosion rate). b) A metal is passive if it resists corrosion in spite of thermodynamic amenability to react (active potential, low corrosion rate). The Eh – pH diagram for the Fe – H2O – O2 system can be superimposed on that for chromium to understand the role of chromium as an alloying addition in steel for enhanced corrosion resistance (Fig. 20.1). Chromium forms very stable, thin and resistant surface films in less oxidizing conditions. Chromium addition is the basis for stainless steels and other corrosion resistant alloys. Fig 20.1 Eh – pH diagram for iron superimposed on the chromium diagram (enhanced passivity range due to stable Cr2O3) 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 20: Passivity – Definitions and Influencing Parameters NPTEL Web Course Since chromium is capable of forming a very stable oxide at much lower potentials, alloying with chromium (minimum 12%) leads to development of corrosion resistant stainless steels and cast irons. Other metals that can form passive surface films include aluminium, silicon, titanium, tantalum and niobium. Electrochemical basis of active-passive behavior is illustrated in Fig. 20.2 Fig 20.2 Potentiostatic Anodic polarization curve Epp – Primary passive potential, above which passive film becomes stable. icrit = Critical passivating anodic current density, at which passivity is induced. ipass – Passive current density. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 20: Passivity – Definitions and Influencing Parameters NPTEL Web Course On increasing the potential beyond the passive region, the passive film breaks down and anodic corrosion current further increases in the transpassive state. Oxygen evolution at the anode occurs at higher potentials. Based on the above, it is possible to establish a) Passive potential region. b) Passive corrosion rate and c) Necessary conditions to achieve and maintain passivity. Decay of passivity on interruption of anodic current is characterized by Flade potential. If the potential as a function of time is monitored after interrupting the applied current, the potential value first changes to a value more noble on the hydrogen scale, then slowly changes and finally rapidly decays towards the normal active value. The noble potential reached just before rapid decay was found by Flade to be more noble, the more acid the solution in which passivity decayed (Fig. 20.3). Fig 20.3 Decay of passivity showing Flade potential 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 20: Passivity – Definitions and Influencing Parameters NPTEL Web Course EF = E0F – 0.059 pH (for Fe, Ni, Cr and alloys of Fe). Stability of passivity is related to E F. The lower the E0F, the easier it becomes for passivation and higher film stability. For Cr – Fe alloys, the value ranges from 0.63 V to -0.10V with 25% chromium addition. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 21: Passivity – Application of Mixed Potential Theory NPTEL Web Course Lecture 21 Passivity-Application of Mixed Potential Theory Keywords: Activation Controlled Reduction Process, Diffusion Control, Spontaneous Passivation Increasing temperature and hydrogen ion concentration (high acidity) tend to increase the critical current density for passivation. Similarly, chlorides are detrimental to passivity. To understand, mixed potential behavior for active – passive metals and alloys, it is essential to introduce cathodic reduction processes superimposed on the anodic polarization curve. Three different activation controlled reduction processes with different exchange current densities are superimposed on the passivity curve as shown in Fig. 21.1. Fig 21.1 Effect of activation – controlled cathodic processes on stability of passivity. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 21: Passivity – Application of Mixed Potential Theory NPTEL Web Course Three different cases are apparent: 1) Only one stable potential at M where the mixed potential theory is satisfied. High Corrosion rate at M. Eg:- Fe in dil H2SO4, Ti in dil H2SO4/ HCl. 2) Three points of intersection R, P and N where rate of oxidation is equal to rate of reduction. Point P is not in stable state. Only N and R are stable. N in active region (high corrosion rate) and R in passive state (lowest corrosion rate). This system may exist in either active or passive state. Eg:- Cr in dil HCl or H2SO4. Stainless steel in H2SO4 (containing oxidizers). 3) The most desirable condition-spontaneous passivation - Only stable potential S in the passive region. Eg:- Cr – noble metal alloys in H2SO4 or HCl. Ti – noble metal alloys in dil H2SO4. 18 – 8 stainless steel in acid (containing Fe+++, O2) Achievement of condition (3) is essential for the development of corrosion resistant alloys. The position of the current maximum or ‘nose’ of the anodic curve is important. Spontaneous passivation occurs only if the cathodic process clears the tip of the ‘nose’ of the anodic curve. For a stated reduction-curve, values of Epp and icrit will then decide whether a metal or alloy will spontaneously passivate or not. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 21: Passivity – Application of Mixed Potential Theory NPTEL Web Course Total cathodic current density at Epp should be equal to or greater than icrit to achieve spontaneous passivation. Such a criterion can be stated in terms of a passivity index (PI) defined as ic ( atE pp ) PI = icrit For PI ≥ 1, Spontaneous passivation occurs and for PI < 1, no spontaneous passivation occurs, even though as in condition (2), a stable passive region may exist. A comparison of the behavior of two active-passive alloys under an activation controlled cathodic system is depicted in Fig. 21.2. Fig 21.2 Active – passive alloys under activation controlled cathodic process. Alloy A corrodes readily at potential X, while alloy B spontaneously passivates at Y. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 21: Passivity – Application of Mixed Potential Theory NPTEL Web Course The above two alloys are exposed to a cathodic process under complete diffusion control as shown in Fig. 21.3. Fig. 21.3 Active – passive alloy behavior under diffusion controlled cathodic reaction Alloy A spontaneously passivate at potential X, while alloy B exhibits two stable states, namely, active at Q and passive at Y. Two significant factors emerge out of the above observations. a) To achieve passive behavior where cathodic reduction is activation controlled, a metal or alloy with an active E pp is superior. b) If the reduction process is diffusion controlled, a metal or alloy having a small icrit will passivate faster. (Ref: N. D. Greene, Predicting behavior of corrosion resistant alloys by potentiostatic polarization methods, Corrosion (NACE), 18, pp 136 – 1432 (1962). 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 22:Passivity – Design of Corrosions Resistant Alloys NPTEL Web Course Lecture 22 Passivity – Design of Corrosion Resistant Alloys Keywords: Alloy Design, Pitting Potential, Oxidizers. For the development of corrosion-resistant alloys through passivity criterion, two approaches then become possible. a) Increase ease of passivation by reducing icrit or making Epp more active. Anodic dissolution behavior can be changed by alloying (to decrease icrit) Examples are titanium, chromium – alloying additions, molybdenum, nickel tantalum and columbium. b) Increase cathodic reduction rates. Alloying with noble metals having high exchange currents for the reduction reaction. Metals with active Epp such as titanium and chromium and alloys containing these metals which possess high exchange current densities for hydrogen reduction can undergo spontaneous passivation. Effect of alloy additions on the corrosion resistance of titanium is given in Table 22.1. Table 22.1 Average corrosion rate of titanium after alloying addition. Corrosion rate (mpy) in 15% Alloying addition boiling HCl Ti (not alloyed) 4400 Addition of 0.5% Au 135 Addition of 0.5% Pt 110 Addition of 0.6% Ir 85 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 22:Passivity – Design of Corrosions Resistant Alloys NPTEL Web Course Effect of oxidizer concentration and solution velocity on the corrosion rate of a normal metal has already been discussed (lecture 13). It will be interesting to understand the role of oxidizer and solution velocity on the behavior of an active- passive metal or alloy. For an active-passive metal exposed to a diffusion controlled cathodic reaction, the corrosion rate will increase upto certain velocity levels, beyond which the corrosion rate decreases rapidly to a very low value on the onset of passivity and would remain at passive state for still higher velocities. Effect of oxidizer concentrations (ferric, chromate etc) on the electrochemical behavior of active-passive alloys can also be compared with those of normal metals under similar conditions. Corrosion rate of an active-passive alloy initially increases with oxidizer concentration (while in its active state). As soon as passive state is reached, the corrosion rate steeply decreases to a very low value and remains at this low corrosion passive level. With still further increase in oxidizer concentration, corrosion rate further increases due to transpassive behavior. It is however, interesting to note that, once the passive film has been formed, it is retained at oxidizer concentrations even lower than that needed for passive film formation. It may however be kept in mind that to maintain passivity, oxidizer concentration should be same or higher than the required minimum to induce spontaneous passivation. There is also a region of ‘borderline passivity’ in which any surface disturbance (scratching) will destabilize passivity, leading to increase in corrosion rate. The following conditions need to be kept in mind to judge passive behavior of an alloy. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 22:Passivity – Design of Corrosions Resistant Alloys NPTEL Web Course Corrosion rate is proportional to anodic current density in the active state irrespective of whether the alloy is passive type or not. Rate of cathodic reduction must exceed icrit to ensure lower corrosion rates. Border line passivity to be avoided. Avoid breakdown of passive films in oxidizing environments due to transpassivity. Stable passive state in oxidizing conditions is essential. Detrimental role of chloride concentrations and temperature on the passive region and critical anodic current density is illustrated in Fig. 22.1. Fig. 22.1 Effect of increasing chloride and temperature on passive behavior. Chloride ions breakdown passivity or even at times prevent passivation of Fe, Cr, Ni, Co and stainless steels. They can penetrate oxide films through pores and influence exchange current density (overvoltage). Breakdown of passivity by chloride ions is local and leads to pitting corrosion. However, chloride ions have no significant effect on the polarization curve of titanium, unlike that of stainless steels. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 22:Passivity – Design of Corrosions Resistant Alloys NPTEL Web Course Anodic polarization of active-passive metals and alloys can be established either potentiostatically or galvanostatically. The differences in the nature of the polarization curves in either case are illustrated in Fig. 22.2. Only potentiostatic approach allows a detailed study of the important parameters influencing passivity. Galvanostatic methods are not adequate for establishing the active-passive behavior. Above icrit, the curve no longer follows the anodic curve in the passive region; suddenly jumping into the transpassive region with oxygen evolution. Feg 22.2 comparison of potentiostatic and galvanostatic anodic polarization curves. Theories of passivation Major theories that have been proposed are the Oxide film theory and Adsorption theory The oxide theory attributes corrosion resistance of passive metals and alloys to the formation of a protective film on the metal surface; the film can be as a monolayer. There are different opinions expressed about the potential at which the oxide film is 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 22:Passivity – Design of Corrosions Resistant Alloys NPTEL Web Course formed, mechanisms of formation, causes of passivity and film thickness. Early theories proposed formation of a primary layer of lower conductivity and high porosity. As the current increases in the pores, passive layer is formed at a potential closer to the Flade potential. A stable passive film is free from porosity and presents a protective barrier between the metal and the corrosive environment. There are similar hypotheses regarding monolayer oxide formation. The adsorption theory is based on chemisorbed films. Oxygen adsorption on surfaces can reduce corrosion activity. Uhlig proposed in 1946 that an adsorbed oxygen film is the primary source of passivity. The observed Flade potential of passive iron is too noble by about 0.6V to be explained by any known oxides of iron at equilibrium. It is consistent with a chemisorbed film of oxygen, which is formed preferentially on transition metals due to interaction of oxygen with uncoupled electrons to form a stable bond. Adsorbed oxygen atoms significantly decrease the exchange current density, thus increasing anodic polarization, favorable for passivation. An alternative passivity mechanism could be direct film formation, dissolution and precipitation and anodic deposition. Several models also have been proposed to explain growth kinetics of surface oxide films. Anodic polarization curve is time-dependent in both active and passive regions. Passive current density (ipass) should be proportional to the rate of passive film formation and rate of its growth in thickness. ipass = K Logarithmic law of passive film formation has been derived taking into consideration continuous adsorption. Film thickness ( X) = A + B log t Inverse logarithmic rate law, = A – B log t 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 23: Anodic Protection NPTEL Web Course Lecture 23 Anodic Protection Keywords: Anodic Protection Range, Protection Design, Aggressive Environment. Anodic protection refers to prevention of corrosion through impressed anodic current. This method of protection tested and demonstrated by Edeleanu in 1954 however can be applied only to metals and alloys that exhibit active-passive behavior. The interface potential of the structure is increased to passive domain. If an active-passive alloy such as stainless steel is maintained in the passive region through an applied potential (or current) from a potentiostat, its initial corrosion rate (icorr) can be shifted to a low value at ipass as shown in Fig. 23.1. Fig 23.1 Polarization curves depicting principles of anodic protection As per mixed-potential theory, Applied anodic current density = oxidation current density – reduction current density. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 23: Anodic Protection NPTEL Web Course Anodic protection is more effective in acid solutions than cathodic protection. Current requirements for cathodic protection in acid solutions are several orders of magnitude higher than that necessary for complete anodic protection. Cathodic protection currents in acid solution can also lead to hydrogen liberation and embrittlement of steels. Anodic protection unlike cathodic protection is ideally suited for protection of active-passive alloys in aggressive environments such as high acidity and corrosive chemicals. Hence anodic protection is the most preferred choice for protection of chemical process equipment. Anodic protection parameters include. a) Protection range – range of potentials in which the metal/alloy exhibits stable passivity. b) Critical anodic current density. c) Flade potential. Potential corresponding to middle of the passive region can be taken as optimum for anodic protection. While choosing the desirable protection potential, an assessment of the aggressiveness of the environment need be made. Since chloride ions are detrimental to passivity, higher chloride concentrations can decrease the protection range. Metals and alloys having relatively larger pitting and protection potentials can only be chosen for very aggressive chemical environments. Higher temperatures can deleteriously influence the protection potential. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 23: Anodic Protection NPTEL Web Course Anodic protection of inner surface of a steel acid storage tank is shown in Fig. 23.2. Fig. 23.2 Anodic protection of inner surface of a steel acid storage tank A. Auxiliary cathode B. Reference electrode C. Anode connection to the tank Inert cathode materials having large surface area preferred-Recommended cathode materials for acid and corrosive industrial liquids include platinum-clad brass, chromium-nickel steel, silicon cast iron, copper, Hastelloy C and nickel-plated steel. Various types of reference electrodes such as Calomel, Ag/AgCl, Hg/HgSO 4 and platinum are used depending on the chemical environment. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 23: Anodic Protection NPTEL Web Course The DC power supply used in anodic protection is more or less similar to the one used in cathodic protection. There should be provisions for varying applied currents and also to reduce the minimum current output. Electronic controls to maintain and adjust current (or potential) in continuous (uninterrupted) mode could be very advantageous. Anodic protection can substantially reduce corrosion rate of active-passive alloys in very aggressive environments. For example, anodic protection of 304 stainless steels exposed to aerated sulfuric acid (5M) containing about 0.1 M chlorides could reduce corrosion rate from an unprotected value of about 2000 µm/year, to about 5 µm/year. It has been widely applied to protect chemical storage tanks, reactors, heat exchangers and even transportation vessels. A comparison between anodic and cathodic protection is given in Table 23.1: Table. 23.1 Comparison of cathodic and anodic protection methods Factors Cathodic protection Anodic protection To all metals in general. Only to those exhibiting active- Suitability passive behavior Only for moderate corrosion Even aggressive chemical Environment environment. corrosives. Low investment, but higher Higher investment, but low Cost benefit operative costs.. operative costs. Protective currents to be More precise electrochemical Operation established through initial estimation of protection range design and field trials possible. It has been mentioned in earlier discussions on passivity that the magnitude of anodic current density required for maintaining passivity is much lower than that required to 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 23: Anodic Protection NPTEL Web Course passivate the metal or alloy. Such a beneficial aspect can be used with advantage of low cost in anodic protection systems. Approximate current density requirements for anodic protection in some aggressive environments are given in Table 23.2: Table. 23.2 Current density for passivation and maintenance in different corrosive environments (Alloy S30400, room temperature) Average current density Environment for passivation mA/cm2 for maintaining µA/cm2 30-40% H2SO4 0.5 22 70% H2SO4 4.9 4.2 -5 Strong H3PO4 at high temp 2 x 10 1.2 x 10-4 20-25% NaOH 4.3 9 Ref: Anodic protection – (Web-PowerPoint and PDF). 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Lecture 24 Microbially – Influenced Corrosion (MIC) – Definitions, Environments and Microbiology Keywords: Microbial Corrosion, Microorganisms, Biofouling. Introduction Microbially-influenced corrosion (MIC) occurs in environments such as soil, fresh water and sea water and accounts for more than 30 percent of all corrosion damage of metals, alloys and several building materials. Microorganisms of interest in MIC belong to many types such as sulfur-sulfide oxidising, sulfate-reducing, iron oxidising, acid producing, manganese fixing and ammonia and acetate producing bacteria and fungi. The role of Sulphate Reducing Bacteria (SRB) in MIC has been extensively studied. Microbial activities under natural conditions influence many electrochemical reactions directly or indirectly. Microbe-metal interactions involve initial adhesion, biofilm formation and colonisation, generation of polymeric substances and inorganic precipitates and subsequent corrosion. Microbiological as well as physico-chemical and electrochemical aspects of microbially-influenced corrosion are analysed critically. Monitoring, diagnosis and prevention of MIC is illustrated along with suggested remedial strategies. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Seawater, fresh water and soil as corrosive media Sea water is an aggressive corrosive medium for biofouling and microbially- influenced corrosion (MIC). It contains about 3.4% salt and is a good electrolyte that can lead to galvanic and crevice corrosion. The rate of corrosion in seawater is influenced by oxygen content, temperature, velocity and microorganisms. Galvanic series for metals and alloys in flowing seawater could be used to predict potential corrosion involving metallic couples. Similarly, fresh water and sub-soil environments are conducive for microbial life leading to biofouling and MIC. With reference to biofouling, copper and copper-base alloys are more resistant compared to other ferrous alloys. Definition and practical significance The role of microorganisms in the deterioration and failure of materials can be classified into Biofouling, Biodeterioration and Biocorrosion or Microbiologically- influenced corrosion(MIC). The above terms could be complementary in their ultimate consequences. Biofouling refers to adhesion of micro- and macro-organisms onto material surfaces in marine, fresh water and soil environments leading to formation of fouled layers. Deterioration of nonmetallic materials like glass, concrete, cement, rubber, wood and plastics in the presence of microbes is termed biodeterioration. Corrosion of metals and alloys induced by the activities of microorganisms is defined as Microbially-influenced corrosion (MIC). The general definition for corrosion can be invoked in this case also by adding the superimposed microbiological forces. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Microorganisms are omnipresent and grow and reproduce at amazingly rapid rates in soil, water and air. The organisms exhibit extreme tolerance to hostile environments such as acidic and alkaline pH, low and higher temperatures as well as pressure gradients. Aggressive environments are generated by microorganisms, promoting direct or indirect corrosion. As early as in 1891, corrosion of lead sheathed cables was suspected to be caused by bacterial metabolites. Sulphur and iron sulphide accumulation at the interior and exterior portions of water pipes were attributed to the action of iron-sulphur bacteria during early 1900s. Anaerobic corrosion of bacteria was first reported in 1931. Tubercle formation due to microbial growth and reaction products has been reported almost forty years ago. However, a better understanding of MIC processes based on microbiological and electrochemical mechanisms, became available only since the last three decades. The practical significance of microbial corrosion can be seen from Table 24.1, where some industrial situations susceptible to microbial corrosion are listed. The extent of microbial corrosion processes is evident from the fact that many of the commercially used metals and alloys such as stainless steels, nickel and aluminium-based alloys and materials such as concrete, asphalt and polymers are readily attacked by microorganisms. Protective coatings, inhibitors, oils and emulsions can be biodegraded. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Table 24.1 MIC in industrial environments Nuclear and thermal power plants Cooling water tubes and pipes, sub-sea pipe lines, stainless steel and carbon steel, copper- alloys, aluminium-alloys Subsoil pipe lines Steels On-shore, off-shore oil and gas processing. Steels, Aluminium alloys Chemical industries Pipelines, Tanks, Condensers, Joints, heat exchangers. Civil engineering Concrete in marine, fresh water and sub-soil conditions, bridges, buildings. Water treatment and metal working Heat exchangers and pipes, Breakdown of oils, emulsions and lubricants Aviation (Defence and Civil) Aluminium fuel tanks Mining and metallurgical operations Underground machinery and engineering materials. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course A few cases of microbially-influenced corrosion reported more specifically in systems or components in power plants are listed in Table 24.2. Table 24.2 MIC in power plant materials Heat exchanger tubing Aluminium brass, 70:30 Copper-Nickel, Pitting 90:10 Copper-Nickel Rust, weld Water storage tank 316 stainless steel corrosion Water pipes 316 stainless steel weld Pitting Cooling towers Galvanised steel General corrosion Pumps Stainless steel Crevice, pitting 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Relevant Microorganisms Microorganisms that are known to cause corrosion can be grouped as shown in Table 24.3. Table 24.3 Microorganisms involved in MIC 1. Bacteria Sulphate Reducing Bacteria (SRB) Desulfovibrio Sulphur Oxidising and acid producing bacteria. Acidithiobacillus Iron Oxidising Bacteria (IOB) and metal depositing bacteria Gallionella, Crenothrix, Leptothrix Metal reducing bacteria Pseudomonas, Shewanella.. 2. Fungi Cladosporium resinae Aspergillus niger Aspergillus fumigatus Penicillium cyclospium Paecilomyces varioti 3. Algae Blue green algae 4. Microbial Symbiotic activity among different groups of consortia microorganisms 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course The sulphur cycle in nature is important to MIC. Sulphur and sulfide oxidising and sulphate reducing bacteria (SRB) are involved in a number of biogenic redox reactions leading to products such as H2S, metal sulphides and sulfoxy compounds. All these microbially - intermediated processes participate in corrosion processes in soils and aqueous environments. For example, sulphate reducing bacteria like Desulfovibrio reduce sulphate to sulphide and hydrogen sulphide, under reducing conditions. SO=4 + 4H2 S= + 4H2O 2H+ + S- - = H2S Sulphur (sulphide) oxidizing and sulphate reducing bacteria (SRB) involved in the biological sulphur cycle in natural environments are shown in Fig. 24.1. Fig. 24.1 Biological sulphur cycle in nature 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Sulphur and ferrous iron-oxidising bacteria such as Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans are acidophilic and aerobic promoting oxidation of sulfur and sulfides. 2H2S + 2O2 = H2S2O3 + H2O 5Na2S2O3 + 8O2 + H2O = 5Na2SO4 + H2SO4 + 4S 4S + 6O2 + 4H2O = 4H2SO4 Fe++ = Fe+++ + e Acidithiobacillus bacteria can exist over a range of pH from acidic, to alkaline conditions. For example, Thiobacillus thioparus could oxidise sulphur, sulphide and thiosulphate at a pH of 6-10. Microbiological features of some thio-bacteria involved in MIC are illustrated in Table 24.4. Morphological features of some bacteria implicated in MIC along with typical growth curves are illustrated in Fig 24.2 to 24.11. All these bacteria are implicated in microbial corrosion processes and their growth characteristics and metabolic reactions are important in understanding corrosion mechanisims. 9 10 No.of Cells / mL 8 10 0 10 20 30 40 50 60 70 80 Time (hours) Fig 24.2 Bacillus Fig. 24.3 Cell number as a function of time subtilis during growth of Bacillus subtilis 8 6x10 1.8 50 8 5x10 1.6 sulphate concentration(g/L) 0 Number of cells/ml 8 4x10 cell count 1.4 -50 EESE 8 EESE in mv 8 3x10 1.2 Course Title: Advances in Corrosion Engineering Sulphate concentration -100 Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore 2x10 8 1.0 -150 8 1x10 0.8 -200 0 0.6 -250 0 20 40 60 80 100 120 140 160 Time (min) Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Fig 24.5 Cell number, SO4 conc and ESCE as a Fig 24.4 Sulphate reducing bacteria function of time during growth of Sulphate reducing bacteria 10 2.0x10 8 2.5 550 and Fe conc (g / L) 8 2.4 500 8 1.6x10 6 No.of cells / mL 2.3 450 ESCE in mV Fe3+ 3+ 8 4 Fe2+ 1.2x10 2.2 pH 400 2 8.0x10 7 2.1 2+ Cell count 350 Fe pH 0 2.0 4.0x10 7 ESCE 300 0 10 20 30 40 50 60 70 1.9 Time (hours) 250 0 10 20 30 40 50 60 70 Time (hours) Fig. 24.6 Fig 24.7 Cell number, pH, ESCE as a Fig 24.8 Ferrous and ferric concentration Acidithiobacillus Sp function of time during growth of as a function of time during growth of Acidithiobacllus sp At.ferrooxidans 9 1.2x10 2.1 Cell count 28 Number of cells / mL Sulphate concentration (g / L) 9 1.0x10 1.8 24 8 8.0x10 1.5 20 pH 16 pH 8 6.0x10 1.2 Sulphate conc. 8 12 4.0x10 0.9 8 8 2.0x10 0.6 4 0.0 0 0.3 0 50 100 150 200 250 300 0 50 100 150 200 250 Time (Hours) Time (Hours) Fig. 24.9 Acidithiobacillus Fig. 24.10 Cell number as a function of Fig. 24.11 pH & SO4 conc. as a thiooxidans time during growth of At. thiooxidans function of time during growth of At. thiooxidans 9 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Fig. 24.12 to Fig. 24.14, illustrate typical morphological features of fungi such as Cladosporium and Aspergillus besides those of an iron and manganese oxidizing bacteria. Fig. 24. 12 Cladosporium Fig. 24.13 Fig 24.14 resinae Aspergillus spp Gallionella spp 10 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Morphological features of Aspergillus,SRB and Acidithiobacllus are more revealingly illustrated in Fig. 24.15. Fig. 24.15 Morphological features of Aspergillus fungal network, SRB with flagellum, Acidithiobacillus and SRB colonizing a steel surface. 11 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course Table 24.4 Microbiological features of some thio-bacteria Organism Environment Activity Desulfovibrio desulfuricans Mud, sewage oil wells, Anerobic, sulphate (Sulphate reducing) subsoil reduction, pH 6-7.5, Temp. 25-300C (some moderate thermophiles) Acidithiobacillus thiooxidans Sulphur and iron Anerobic, pH2 – 4, Acidithiobacillus ferrooxidans bearing minerals, soils 28 – 35oC, oxidizes and water sulphur, sulphides producing sulphuric acid, Ferrous to ferric oxidation. Thiobacillus Thioparus Water, mud, sludge, Aerobic pH 6-8, sulphidic soils 30-350C, oxidises thiosulphate and sulphur to sp. From the sulfur-bacteria cycle, bacterial oxidation and reduction cycles involving sulfur species are evident. Both these redox concepts are important in MIC mechanisms. 12 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 25: MIC – Electrochemical Aspects and General Mechanisms NPTEL Web Course Lecture 25 MIC – Electrochemical Aspects and General Mechanisms Keywords: Electrochemical Aspects, Direct Mechanism, Indirect Mechanism. Eh and pH are the important environmental parameters controlling the growth and activity of various aerobic and anaerobic organisms. The stability regions of various types of microorganisms corresponding to optimum activity can be defined through Eh – pH diagrams. Eh-pH diagram for sulphur – water – oxygen system wherein the stability and growth regions of various types of microorganisms are represented will be useful in the understanding of MIC. Iron and sulphur-oxidising acidophilic bacteria such as Acidithiobacillus ferrooxidans and Acidithiobacillus thiooxidans grow under higher oxidising potentials and acid pH levels. Sulphur and thiosulphate oxidising autotrophs such as Thiobacillus thioparus have optimum activity at near neutral pH ranges and relatively higher oxidising potentials. Sulphate reducing bacteria (SRB) grow under reducing and neutral pH environments. Iron oxidising heterotrophs are stable and active at neutral pH and higher oxidising conditions. Stability regions for some acidophilic chemolithotrophs and anaerobic heterotrophs such as SRB are shown in a S-H2O – O2 diagram in Fig. 25.1. Ferric-ferrous ratios at high acidic pH levels determine the potential limits for Acidithiobacllus ferrooxidans where as sulfate formation from sulphide oxidation at acidic pH dictates the stability limits for Acidithiobacillus thiooxidans. Sulfate reducing bacteria are anaerobes having optimum growth at neutral pH ranges. Stability region for SRB corresponds to reducing potentials at neutral to mildly alkaline pH. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 25: MIC – Electrochemical Aspects and General Mechanisms NPTEL Web Course Fig.25.1 Stability regions for Acidithiobacllus and SRB in a S-H2O-O2 diagram. Eh-pH corrosion diagrams can be readily constructed for various metal-water- oxygen systems in the presence of micro-organisms to predict the regions of MIC, immunity and passivation. Common Eh-pH diagrams cannot represent the corrosion behavior of metals and alloys in the presence of micro-organisms. Superimposition of bacterial stability regions on these diagrams may bring about significant changes in the regions of corrosion, immunity and passivation. There are instances where electrochemical prediction of corrosion went astray when microbial activities at the respective Eh and pH conditions were also considered. Due to microbial growth and biofilm formation, corrosion and protection regions in such diagrams can shift. Principal slime forming bacteria such as Bacillus subtilis, Bacillus cereus and species of Flavobacterium, Aerobacters and Pseudomonas are present in soil environments. Pseudomonas can grow in systems containing hydrocarbon sources such as oils and emulsions using hydrocarbons as energy source. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 25: MIC – Electrochemical Aspects and General Mechanisms NPTEL Web Course Algae range from single cell plants to multicellular species of diverse forms and shapes. They contain coloured pigments, the most important of which is the chlorophyll. Algae generally grow on moist surfaces such as cooling towers, screens and distribution systems. Some common algae groups are blue-green algae, the green algae and the diatoms. Owing to their ability to produce corrosive organic acids, oxygen and metabolites corrosion can be promoted. Fungi are similar to algae but do not contain chlorophyll. Mould fungi are filamentous in form but most of yeast fungi are unicellular. Some corrosion-causing fungi are Aspergillus niger, Aspergillus fumigatus, Penicilium cyclospium and Cladosporium resinae. Production of various types of organic acids such as oxalic acid, citric acid and gluconic acid by fungal metabolism create corrosive environment. Direct and indirect mechanisms Reactions involved in MIC are based on electrochemical reactions similar to general corrosion principles. Anodic: M = M++ + 2e Cathodic: O2 +4H+ + 4e = 2H2O (aerated, acidic) O2 + 2H2O + 4e = 4OH- (aerated, neutral and alkaline) 2H+ + 2e = H2 (in the absence of oxygen in acid solutions) Microorganisms, very often contribute towards corrosion without being solely and directly responsible for the failure. Both direct and indirect mechanisms are involved. Microorganisms can play both direct and indirect roles. In direct attack mechanisms, the organisms interlinks an electrode reaction (anodic or cathodic) 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 25: MIC – Electrochemical Aspects and General Mechanisms NPTEL Web Course through metabolism, while indirect mechanisms involves indirect microbial contribution to corrosion through creation of corrosive environments, such as differential aeration cells, acidic reaction products and other metal solubilising bioreagents . General mechanisms can therefore be seen in different perspectives: Changes in dissolved oxygen levels through microbial growth leading to formation of concentration cells. Biodegradation of additive reagents present in lubricants and emulsions. Biogeneration of corrosive products and hydrogen consumption. Microbiological breakdown or disruption of organic paint coatings, plastic fittings and linings, protective films and inhibitors. Typical examples of some of the corrosive metabolic products are illustrated below: Both organic and inorganic acids can be produced by microbial metabolism. Oxidation of inorganic sulphur compounds by Acidithiobacillus group of bacteria to produce sulfuric acid. Oxidation of iron sulphides by Acidithiobacillus ferrooxidans to produce acidic ferric sulfate. In the presence of organic carbon such as sucrose, fungi such as Aspergillus generate oxalic, citric and gluconic acids. Exopolysaccharides and bioproteins secreted by Bacillus species. Several bacterial enzymes are electrocatalysts. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 25: MIC – Electrochemical Aspects and General Mechanisms NPTEL Web Course At environmental pH, the following bacterial mechanisms are relevant. Corrosion by cathodic depolarization attributable to Sulfate Reducing Bacteria (SRB) which contain the enzyme, hydrogenase. Corrosion by differential aeration cells due to deposits and biofilms formed by iron bacteria and other slime bacteria. Corrosive products such as organic sulphides, mercaptides, amines, ammonia, phosphorous compounds and surfactants. Organic corrosion inhibitors such as diamines and aliphatics are used as nutrients by bacteria. For example, Nitrosomonas and Nitrobacter oxidise ammonia and amines to nitrite and nitrate, destroying the inhibition properties of several inhibitors. Ferric oxide coatings are degraded by Pseudomonas, exposing the base metal for corrosion. Iron sulphide films are broken down by Sulphate Reducing Bacteria. Protective aluminium oxide layers (passive film) on aluminium and its alloys could be destroyed by the fungus, C. resinae. Bacterial attachment and Biofilms. Under environmental conditions, submicroscopic bacterial cells can be considered as living colloids. Bacterial suspensions as in water and soil exhibit colloidal behavior. At natural pH, bacterial surfaces are negatively charged. Bacterial cell walls contain many types of cationic, anionic and nonionic polymeric substances such as polysaccharides, phospholipids and proteins. Cell surface hydrophobicity and hydrophilicity depends on cell wall architecture. Surface – chemical characteristics of microorganisms are important since they govern their adhesion behavior to solid substrates. Bacterial adhesion and biofilm formation on metals and alloys are initial events in ultimate metallic corrosion. Forces of bacterial adhesion (attachment) need to be understood to get an insight into biofilm formation mechanisms. A fully – developed microbial biofilm may consist of both micro – and macro- organisms 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 25: MIC – Electrochemical Aspects and General Mechanisms NPTEL Web Course along with their metabolic products and chemical reaction products. It should be understood that initial stages in biofilm formation invariably involves only bacterial attachment. Attached and colorized bacterial cells prepare the foundation on which macroorganisms subsequently attach and grow. Under the circumstances, it becomes imperative to understand forces and mechanisms of bacterial attachment to metals and alloys in different environments (water, air and soil). Attachment of Acidithiobacillus organisms on (A) aluminium (B) mild steel and (C) stainless steel are illustrated in Fig. 25.2. Fig. 25.2. Scanning electron micrographs illustrating attachment of Acidithiobacllus sp on (A) aluminium, (B) mild steel and (C) stainless steel surfaces. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 25: MIC – Electrochemical Aspects and General Mechanisms NPTEL Web Course Attachment of sulfate reducing bacteria such as Desulfovibrio and Desulfotomaculum on titanium surfaces is illustrated in Fig 25.3 Fig. 25.3 Scanning electron micrographs showing SRB attachment and biofilm formation on titanium surfaces 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course Lecture 26 MIC-Bacterial Transport, Attachment and Affected Materials Keywords: Bacterial Adhesion, Biofilms, Structural Materials. Bacterial transport to metal surface involve: Fluid dynamic forces (currents in water bodies and eddy diffusion in turbulent flow systems). Flocculation or sedimentation. Chemotactic response due to energy gradients. Brownian motion (colloids). Surface properties such as charge, free energy and roughness influence bacterial adhesion. There can be reversible and irreversible adhesion. Many forces such as electrostatic, chemical and hydrophobic forces may be involved in bacterial adhesion mechanisms. The following stages can be visualised to understand a fully developed biofilm on a metal surface. Transport of organics from bulk. Attachment and colonisation by bacteria. Incorporation of higher organisms (fungi, algae, protozoa). Build up of biofilms in thickness. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course Besides its contribution towards MIC, biofilms can pose several other engineering problems such as: Reduction in heat transfer leading to energy loss (condenser tubes). Reduction in mass and fluid transfer (water, oil, gas pipelines). Structural failures (buildings, bridges, platforms and construction materials) Increased fuel and operating costs (ships and engines). Several mechanisms and models have been proposed to understand biofilm formation. Aerobe-Anaerobe mutualism: Growth of aerobic bacteria such as Acidithiobacillus and iron oxidizers utilizing oxygen and nutrients at the metal-solution interface creating an anaerobic environment in the vicinity. Sulfate and other oxidised metabolic products formed in the biofilm due to activity of such aerobes serve as nutrients and energy source for anaerobic bacteria such as SRB which subsequently proliferate in the anaerobic environment. Bacterial mutualism leads to the formation and growth of a heterogeneous biofilm (often patchy and incoherent). Oxygen concentration cells would be formed under such conditions as illustrated below in Fig 26.1. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course Fig. 26.1 Model for bacterial film formation on metals involving aerobic and anaerobic bacteria Schematic representation of biofilm formation and consequent development of differential aeration cells are shown in Fig. 26.2. Fig. 26.2 Formation of differential aeration cells on metal surfaces due to biofilm growth. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course Tubercles (massive bio – chemical deposits) can result with time. Extensive pitting and cracks become visible under the biofilm. Anodic and cathodic reactions pertaining to MIC of steels in marine or soil environments are illustrated in Fig. 26.3 and Fig. 26.4. Fig. 26.3 Model for biocorrosion of ferrous alloys due to biofilm formation. Fig. 26.4 Anodic and cathodic reactions in differential aeration cells formed on metal surfaces. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course MIC of important structural materials There are no known metals or alloys which can completely resist biofilm formation and subsequent microbially-influenced corrosion. Behavior of various commonly used metals and alloys in relation to microbially-influenced corrosion is outlined below: Copper and copper alloys. Commonly used in heat exchangers, pumps, valves and condensers. 90-10 and 70-30 copper-nickel, brasses, aluminium bronzes and admiralty brasses are used in marine environments. SRBs present in marine environments contribute to localised corrosion of the above alloys. They are susceptible to microbially-influenced corrosion of different kinds. Extra-cellular polymers secreted by microorganisms can induce corrosion of copper-base alloys through differential aeration, selective dissolution and cathodic depolarization. Pitting, plug / dealloying and ammonia cracking of brasses and bronzes can occur. Sulphate-reducing bacteria generate tubercles through formation of sulphide-rich scales on copper alloys. In spite of copper toxicity, copper and copper alloys are not free from biological corrosion. Acidithiobacillus group of bacteria develop higher tolerance to copper ions and dissolve the metal. Slime forming bacteria together with iron were isolated from the corrosion products of copper-nickel alloy and monel tubes used in a nuclear power plant. Sulphate reducing bacteria can corrode underground copper tubes and pipes. Biologically generated ammonia is responsible for stress corrosion cracking of several copper alloys. Corrosion of brass in heat exchanger tubes by ammonia produced by bacteria is reported. Steels. Tubercle formation with pitting underneath is encountered in steel pipes and tubes, resulting in hampered flow and plugging problems. Carbon steels are used for water, oil and gas transport under sub-soil and marine environments. Aerobic 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course bacteria such as Gallionella, Leptothrix and Acidithiobacillus contribute to MIC resulting from differential aeration cells. These organisms oxidize ferrous to ferric resulting in the deposition of ferric oxyhydroxides. Anaerobic bacteria such as Sulfate Reducing Bacteria inhabit the tubercles. Aerobic bacteria can bring about MIC through formation of slimes, oxidation of iron and sulphides and generation of acidic metabolites. Hydrated slimes coat the metal surfaces, creating differential aeration cells. Iron oxidising bacteria listed in Table 26.1, oxidise ferrous ions to less soluble ferric ions, leading to the formation of insoluble tubercles, which consist of hydrated ferric oxides and biological slimes. Steel water pipes are prone to such attack. Massive tubercle formation inside steel pipes, hinders fluid flow, and creates severe corrosion problems, such as extensive pitting, fissures and crevices. Table 26.1 Role of slime – forming bacteria in metallic corrosion Organism Action Gallionella Sp Aerobic, Iron & Steels, Tubercle formation Sphaerotilus Sp Aerobic, Iron & Steels, Ferrous oxidation and tubercle. Pseudomonas Sp Aerobic, Iron & Steels, (Some iron reducing) P.aeroginosa Aerobic, Aluminium alloys (pitting) 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course Stainless steels. Stainless steels are used in nuclear power plants in sea water environments. Iron oxidising and depositing bacteria induce MIC of stainless steels characterized by pitting, usually adjacent to weldments. SRB can attack stainless steels, super stainless steels such as duplex steels and molybdenum steels. Slimes formed by bacteria can create sites for initiation of pits in stainless steels in sea water or fresh water. Destruction of passive films in stainless steels is observed through reducing environments created by SRBs. Nickel-based alloys. Monels and inconels are susceptible to MIC. Nickel-based alloys used in nuclear power plants corrode due to microbial attack under marine environments. Aluminium and its alloys. Protective oxide (passive) films present on aluminium and its alloys could be disrupted and destroyed through biological attack. Aluminium and 2024, 7075 alloys used in aircraft and fuel storage tanks are susceptible to MIC in the presence of hydrocarbons (fuels). The generation of water- soluble organic/inorganic acids by bacteria and fungi lead to corrosion of aluminium and alloys (pitting and intergranular corrosion). Aluminium-magnesium (5000 series) alloys used in marine applications are susceptible to pitting, intergranular corrosion, exfoliation and stress corrosion through microbial interaction. Aircraft fuel tanks and sea water components of aluminium and its alloys are corroded by organisms such as Pseudomonas, Leptothrix, Sulphate Reducing Bacteria and fungi. The fungus, Cladosporium resinae can proliferate on kerosene or paraffins as sole carbon sources, developing pinkish brown colonies. Fuel tanks of especially ground aircrafts are affected by fungal growth. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course The following microorganisms had been observed in an aircraft tank sludge. Pseudomonas aeruginosa. Aerobacter aerogenes. Clostridium, Bacillus, Desulfovibrio, Fusarium, Aspergillus, Cladosporium and Penicillium. Titanium. Titanium is susceptible to biofouling. SRBs and acid-producing bacteria may generate differential aeration cells leading to destruction of passive films. Titanium and its alloys used in marine environments are susceptible to biofilm formation involving manganese and iron oxidising bacteria as well as sulfate reducing halophiles. Surface passive films on titanium could be disrupted in the presence of anaerobes, leading to ennoblement and pitting. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 27:MIC – Role of Aerobic and Anaerobic Microorganisms NPTEL Web Course Lecture 27 MIC – Role of Aerobic and Anaerobic Microorganisms Keywords: Aerobic, Anaerobic, Sulfate-Reducing-Bacteria Corrosion initiated and accelerated by microorganisms (metals and alloys) MIC is also referred to as Biocorrosion Microbial corrosion Microbially-induced corrosion Biofouling – all types of biological attachment and growth on metal and nonmetal surfaces in contact with natural waters (fresh or sea water). Micro and macro-fouling refer to deposits through the growth of microbes and other higher organisms. Biodeterioration generally refers to deterioration of nonmetallic materials or degradation brought about by microbes. Microorganisms associated with MIC are generally characterized by a number of features such as: Small size (few micrometers) Ubiquitous and omnipotent Sessile or motile (active or sedentary) Ability to attach to substrates and grow colonies. Extremophiles (tolerant to wide range of metal concentrations, acidity, temperature, pressure, oxygen and lack of oxygen) Existence of consortia and mutualism Rapid reproduction. Generate organic and inorganic acids, alkalis, chelating agents and extracellular polymeric substances such as proteins and polysaccharides. Can oxidize or reduce metals and ions. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 27:MIC – Role of Aerobic and Anaerobic Microorganisms NPTEL Web Course Most of the microorganisms involved in MIC are chemolithotrophs and can be aerobic – anaerobic, mesophilic-thermophilic, autotrophs- heterotrophs, acidophilic- neutrophilic and many are slime formers. Chemotrophs get energy from chemical sources unlike photosynthetic organisms. Microorganisms involved in MIC can be generally classified as a) Sulfur-sulfide oxidizing bacteria. b) Manganese oxidizers / reducers c) Iron-oxidizing /reducing bacteria. d) Sulfate-reducing bacteria (SRB). e) Bacteria secreting exopolymers / slime and organic acids. Sulfate-reducing bacteria (SRB) are a group of diverse anaerobes which bring about dissimilatory sulphate reduction to sulfides. Although they are considered as strict anaerobes, some genera can tolerate oxygen, hydrogen serving as electron donor. Oil, gas and shipping industries are seriously affected by SRB activities (soil and water) due to H2S generation. Common SRB include Desulfovibrio, Desulfobacter and Desulfotomaculum. SRB are capable of growing in soil, fresh water and sea- water environments and also in stagnant areas. They oxidize organic substances to organic acids or CO 2, by reduction of sulfate to sulfide through anaerobic respiration. Tolerate pH ranges 5-9.5. Black deposits of precipitated sulfides and odour of H2S emanation are characteristic of SRB growth Environmental growth conditions and metabolic features of some corrosion – causing bacteria are illustrated in Table 27.1. Growth conditions and corrosion aspects of some heterotrophs are illustrated in Table 27.2. Characteristics of some sulfate reducing bacteria relevant to MIC are given in Table 27.3. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 27:MIC – Role of Aerobic and Anaerobic Microorganisms NPTEL Web Course Table 27.1 Environmental and metabolic aspects of MIC – causing bacteria Bacteria pH Temp Oxygen Corrosion Characteristics (0C) Desulfovibrio 4-8 10-40 Anaerobic Iron and Utilize hydrogen in D. desulfuricans steel, reducing SO4-- to S-- stainless and H2S promote steels, formation of sulfide aluminum films. zinc, copper alloys Desulfotomaculum D .nignificans 6-8 10-40 Anaerobic Iron and Reduce SO4-- to S-- steel and H2S (spore stainless formers). steels Desulfomonas ….. 10-40 Anaerobic Iron and Reduce SO4-- to S-- steel and H2S. Acidithiobacillus thiooxidans 0.5-8 25-40 Aerobic Iron and Oxidizes sulfur and steel copper sulfides to form alloys, H2SO4, damages concrete protective coatings. Acidithiobacillus ferrooxidans 1-7 25-40 Aerobic Iron and Oxidizes ferrous to steel ferric 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 27:MIC – Role of Aerobic and Anaerobic Microorganisms NPTEL Web Course Table 27.2 MIC – causing heterotrophs Organism pH Temp 0C Oxygen Affected Action metals Gallionella 7-10 20-40 Aerobic Iron and Oxidizes ferrous steel and manganous- tubercle formation Sphaerotilus. 7-10 20-40 Aerobic Iron and Oxidizes ferrous steel and manganous - tubercle formation S.natans ….. ….. ….. Aluminium alloys Pseudomonas. 4-9 20-40 Aerobic Iron and Some strains can steel reduce Fe+++ to Fe++ P.aeruginosa 4-3 20-40 Aerobic Aluminium alloys Cladosparium resinae 3-7 10-45 Aerobic Aluminium Produces organic (fungi) alloys acids. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 27:MIC – Role of Aerobic and Anaerobic Microorganisms NPTEL Web Course Table27.3 Important characteristics of some sulfate – reducing bacteria Desulfovibrio Single flagellum. Do not form spores. Hydrogenase present. Dv. desulfuricans Curved rods (vibrios); sometimes spirilloid, occasionally straight. Dv. vulgaris Typical size 3-5 µm / 0.5-1 µm single flagellum. Dv. Salexigens Dv. Gigas Large curved rods or spirilla, 5-10 µm / 1.2 – 1.5 µm. Desulfotomaculum Peritrichous flagella Dt .nigrificans Hydrogenase activity variable. Growth on pyruvate in sulphate- free media. Dt. Orientis Fat curved rods, 5 µm x 1.5 µm. Hydrogenase absent. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 28: Mechanisms and Models for SRB Corrosion NPTEL Web Course Lecture 28 Mechanisms and Models for SRB Corrosion Keywords: Anaerobic Corrosion, Hydrogenase, Biogenic Sulfides, Depolarization Microbiological characteristics of several sulfate reducing bacteria were illustrated in lecture 27. Role of SRB in metallic corrosion can be understood by a) H2S generation b) Creation of oxygen concentration cells c) Formation of insoluble metal sulfides d) Cathodic depolarization A well known mechanism involving SRB involves generation of several corrosion cells by their attachment and subsequent interaction and metabolism. Many hydrocarbons encountered in oil and gas exploration and mining sustain SRB through nutrient supply. Cathodic hydrogen (as a common corrosion reactant) generated at metal surfaces (as in pipes) promote SRB growth. Hydrogenase-positive SRB are implicated in hydrogen utilization which is used for bacterial reduction of sulfate. The bacteria thus scavenge away cathodically-generated hydrogen leading to cathodic depolarisation promoting anodic corrosion. A model for SRB attachment on steel surfaces resulting in subsequent hydrogen depolarization is shown in Fig. 28.1. Fig. 28.1 Model showing surface anchoring of SRB cells and cathodic depolarization. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 28: Mechanisms and Models for SRB Corrosion NPTEL Web Course Proposed reactions include Anode Fe = Fe++ + 2e Cathode 2H+ + 2e = H2 Depolarization SO4-- + 8H = S-- + 4 H2O Interaction with iron Fe++ + S-- = FeS Fe++ + 2OH- = Fe (OH)2 Overall reaction can be stated as 4 Fe + SO4-- + 4H2O = 3 Fe (OH)2 + FeS + 2OH- The following aspects need be stressed to explain promotion of metallic corrosion in the presence of SRB. a) Necessity of hydrogenase enzyme as a catalyst in hydrogen utilization. b) Bacterial generation of H2S (HS-) and formation of FeS (MeS) as a reaction product. c) Depolarization through cathodic hydrogen removal. d) Role of iron (and or metallic) sulfide. Possibility of galvanic cell formation between steel (Fe) and the iron sulfide which serves as cathode indeed promotes corrosion. The presence of bacteria can be seen as a biocatalyst promoting the electrochemical reactions as illustrated in Fig. 28.2. Fig. 28.2 Model showing galvanic interaction between Fe and FeS in the presence of SRB cells. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 28: Mechanisms and Models for SRB Corrosion NPTEL Web Course Metal corrosion rate is proportional to iron sulfide content in the environment. Many other factors can also influence metallic corrosion in the presence of SRB. a) Anodic depolarization. b) Formation of volatile phosphorus compound. c) Presence of iron-binding biopolymers. d) Sulfide-induced stress corrosion cracking. e) H2 induced stress corrosion and cracking. The basic idea of cathodic depolarization theory is the removal of hydrogen from the cathodic regions by the bacterial hydrogenase, coupled with sulfate reduction to sulfide. Severe corrosion of steels in water-logged clay soils can be attributed to this type of MIC. Ferrous sulfide and hydroxides are corrosion products. In fact, SRB uses the adsorbed hydrogen in sulfate reduction, increasing the anodic corrosion rate by allowing the cathodic reaction to proceed faster. Such a microbial reaction bypasses the recombination of adsorbed hydrogen atoms, which requires higher activation energy. Another suggestion regarding cathodic polarization in the presence of SRB is attributed to bacterially generated H2S. Cathodic reaction such as H2S + e = HS- + H2 can occur. Effect of precipitated iron sulfide need also be stressed. The depolarizing effect of ferrous sulfide on hydrogen evolution was also confirmed. In metals and alloys exhibiting active-passive behavior, breakdown of passivity by the metabolic products of SRB can occur promoting pitting corrosion. (Eg:- Stainless steels). Properties of biogenic sulfides generated by SRB and chemical sulfides need be compared. SRB metabolites containing thiosulfates and polythionates are corrosive to steel. Added presence of chlorides as in sea water would further aggravate the corrosion process. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 28: Mechanisms and Models for SRB Corrosion NPTEL Web Course MIC in anoxic environments in the presence of SRB need thus be understood in the light of various combinations of parameters such as: a) Biogenic sulfides – nature of protective film on metal surface. b) Depending on pH, poorly protective sulfide films could form. c) Breaking down of passivity by SRB metabolites. d) Cathodic depolarization – hydrogenase activity or iron sulfide precipitation. SRB – influenced corrosion of steel is seen to be significantly influenced by the nature and structure of the sulfide films. Metal-biofilm-solution interface and parameters such as pH, ionic concentration, oxygen levels and exopolymeric substances control the surface corrosion products. Corrosion or protective nature of such product films may vary depending on the environment. Presence of aggressive anions such as chlorides could impact the corrosion rate. SRB has been most troublesome among all organisms involved in MIC, especially in environments related to oil, gas and shipping industries. Considerable work has been focussed on the influence of SRB on mild steel and other iron-based alloys. Serious problems in petroleum industries include increased refining costs due to bacterial H2S generation (oil souring). SRB growth in sea water injection systems leads to equipment damage besides contamination of oil and gas with viable cells and their reaction products. They are termed obligate anaerobes obtaining energy from oxidation of organics, using sulfate as external electron acceptor. Corrosion of steels is generally realized as a localized attack as pitting. Many strains are capable of respiring oxygen with hydrogen serving as electron donor. General pH range of activity is from 5 to 10 within temperatures 50C to 500C. In industrial environments, SRB exhibit strong affinity for adhesion to available surfaces to develop often patchy biofilms. Such sessile SRB biofilm initiates localized pitting corrosion. Extracellular polymeric substances (EPS), entrapped particles, precipitates, adsorbed ions and organic molecules are present in SRB biofilms which are heterogeneous with thickness ranging from microns to centimeters. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 28: Mechanisms and Models for SRB Corrosion NPTEL Web Course Biofilm formation involves steps such as: Transport of organic matter to metal surface. Transport of planktonic cells to surface to become sessile cells. Attachment and growth of cells within biofilm. A pictorial representation showing stages in the attachment of SRB cells on metal surfaces is given in Fig. 28.3. Fig. 28.3 Stages in metal surface attachment of SRB cells 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 28: Mechanisms and Models for SRB Corrosion NPTEL Web Course Hydrogenase and depolarization activity of some SRB are given below: Fresh water isolates Desulfovibrio vulgaris ++ Depolarization Desulfotomaculum orientis - No depolarization Thermophiles Desulfotomaculum nigrificans + Depolarization Halophiles Desulfovibrio desulfuricans + Depolarization Desulfovibrio spp - No depolarization Activity of SRB in soils and waters should be reflected in the measured rest potentials. For example. Eh Corrosivity < 100 mV Severe 100 – 200 mV Moderate 200 – 400 mV Low >400 mV Insignificant Aggressive and nonaggressive soil sites can be classified in relation to soil resistivity, redox potentials, pH, water content and SRB cell counts. Similarly, soil types such as clayey, chalky, sandy and loamy can have a bearing on metallic corrosion. Water samples from fresh water and salty water environments can similarly be analyzed for the various parameters for corrosivity. Recording of redox potentials of liquid samples can be useful to establish the presence and growth conditions for SRB. Any contamination of the system by SRB can be monitored through physicochemical and biochemical tests. Accumulation of 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 28: Mechanisms and Models for SRB Corrosion NPTEL Web Course black iron sulfides and the emanation of H2S smell will invariably suggest SRB presence. SRB can be detected in solid and liquid samples using Starkey medium (in water or in presence of NaCl) Dipotassium hydrogen phosphate 0.5 g/L Ammonium chloride 1.0 g/L Sodium sulfate 1.0 g/L Calcium Chloride 0.1 g/L Magnesium sulfate 2 g/L Sodium lactate (70%) 5 g/L Addition of iron sulfate into the inoculated culture would result in black precipitate and evolution of H2S. SRB can usually be found in a) Stagnant regions in flow lines. b) Under sludge or in mud at bottom of pits. c) Under the scales in low velocity flow lines, crude oil storages and water tanks. d) Sand and gravel filters- sewerage lines. e) Injection wells, oil-fields, oil-water interface f) Buried pipe lines. Hydrocarbon degrading SRB proliferates in oil and gas reservoirs. Anaerobic bacteria such as SRB can be found even in oxygenated environment and the existence of oxygen-resistant proteins in some species has been observed. Possible use of oxygen as terminal electron acceptor by SRB is also mentioned. Oxygen gradients in biofilm include anoxic zones rich in SRB. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 29: MIC and Biofilms NPTEL Web Course Lecture 29 MIC and Biofilms Keywords: Biofilms, Potential Ennoblement, Indirect Role. Microbially-influenced corrosion (MIC) in natural marine environments involves two biomediated consequences: a) Corrosion potential ennoblement. b) Sulfate reduction-biogenic sulfide formation. High stainless alloys (Ni-Cr-Mo), titanium alloys, platinum and gold (noble metals) are capable of sustaining ennobled corrosion potential without enhanced susceptibility to crevice corrosion. Common stainless steels also exhibit corrosion potential ennoblement – But highly amenable to localised crevice corrosion. Irrespective of cuprous ion toxicity, copper alloys are not immune to biofouling. Microorganisms having high copper tolerance can attach and colonise on copper base alloys. Some copper alloys exhibit potential ennoblement. Invariably, MIC of copper alloys is caused by biogenic sulfides due to presence of SRB. For selection of alloys for marine applications, susceptibility for crevice corrosion is often the limiting factor. For most of the passive alloys, formation of biofilms results in corrosion potential ennoblement, in sea water and brackish waters. Microbially-deposited manganic and ferric oxides could well be the reason behind such ennoblement followed by localised corrosion. Proposed mechanisms for ennoblement in marine environments can be based on three aspects, namely, thermodynamic, kinetic and changes in the nature of the 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 29: MIC and Biofilms NPTEL Web Course reduction reaction. pH changes in the metal-film interface or an increase in partial pressure of oxygen can influence the potential of the oxygen reaction leading to ennoblement in corrosion potential. Shifts in the exchange current density for the oxygen reduction also would result in shift in the potential. Pitting potentials would also be affected under the conditions of ennoblement. Kinetic interpretation of ennoblement implies changes in oxygen reduction kinetics due to increase in exchange current densities. Cathodic oxygen reduction rates could be enhanced due to increase in exchange currents and biopolymers in the biofilm could further catalyze the cathodic reaction. Bio-enzymes are known to be excellent catalysts for several electrochemical reactions. Another viewpoint is the role of bacterial siderophores that possess excellent corrosion inhibition properties. The model for predicting SRB-mediated corrosion is based on the possibility of the metal interacting with biogenic sulfides. Reducing environment generated by bacterial sulfate reduction in the biofilms can destabilize the surface oxide (passive) layer, leading to metal dissolution. Thermodynamic feasibility for the conversion of metal oxide to sulfide need be ascertained. Corrosion and Ennoblement through biogenic manganese and iron oxide deposition Microbial colonization of metals and alloys can influence the mechanisms and rate of electrochemical reactions. For example, ennoblement of passive metals such as stainless steels and titanium can occur due to positive shifts in their open circuit potentials in the presence of microbial colonies. An increase in cathodic current density during cathodic polarization is often associated with enhancement of open circuit potentials. Biofilm formation on stainless steels changed their electrochemical behavior through significant positive shifts in open circuit potentials and increase in cathodic currents. Similar behavior could be observed on titanium as 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 29: MIC and Biofilms NPTEL Web Course well. Microbial colonization responsible for such ennoblement results in biogeneration of extracellular substances, organo – metallic complexes and metal – specific enzymes. Microbially-modified acidification at metal-solution interfaces and generation of oxidants such as H2O2 can also cause ennoblement. Microbial production of passivating siderophores is yet another possibility. Microbial deposition of iron and manganese oxides / hydroxides can lead to ennoblement of passive metals. Iron and manganese oxidizing bacteria inhabiting fresh and sea waters as well as soils can attach and colonise on metal surfaces and precipitate manganic and iron oxyhydroxides. Mn+2 + 2H2O = MnO2 + 4H+ + 2e MnOOH + OH- = MnO2 + H2O + e Depending on the pH of water, the above reactions can influence the measured open circuit potentials to deviate in a nobler direction. Electrochemical oxidation/reduction of manganese and iron is part of the natural manganese and iron cycles in natural environments. Both manganese and iron oxidizing and reducing microorganisms take part in the cycle involving biochemical and electrochemical pathways. Microbially produced MnO2 can also corrode active metals such as mild steel Fe + MnO2 + 4H+ = Fe++ + Mn++ + 2H2O Manganese – oxidizing organisms can corrode stainless steel welds. Mn-oxidizing microbes have been implicated in the microbial corrosion of stainless steel welds. Formation of MnO2 ennobles its potential in natural environments. Due to ennoblement, potentials shift above repassivation potential to the pitting potential. Mn++ + O2 MnO2 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 29: MIC and Biofilms NPTEL Web Course Indirect role of Microorganisms in MIC Role of SRB in cathodic depolarization through consumption of hydrogen iron has been discussed as a direct attack mechanism. Bacterial and fungal metabolites such as organic acids may decrease anodic polarization. Many biogenic organic acids such as citric and oxalic acids can form soluble metal ion complexes, promoting anodic oxidation. Similarly mineral acids such as sulfuric acid and nitric acids can be produced by microbes promoting metal dissolution. Microbially-generated phosphorus compounds and H2O2 produced in biofilms are aggressive corrosive agents. In many industrial water systems bacterial colonization of metal surfaces lead to slime formation containing extracellular polysaccharides. Slime-coated organisms provide conducive environment of further microbial growth. A slime (or capsuble) surrounding attached bacterium protects them from biocides and corrosion inhibitors. Developments of concentration cells under the biofilms (oxygen and ion concentration gradients) trigger formation of anodic and cathodic areas, promoting corrosion. Identification and enumeration of slime-forming microorganisms are important tools to predict MIC in cooling water systems. Many iron and manganese oxidizing bacteria contribute to formation of oxide scales on metal surfaces. Presences of chloride ions aggravate corrosion since they destroy oxides and passivity. Disruption of passive films and oxide layers from metal surfaces through biofilm formation and SRB metabolites has been discussed. Incorporation of extracellular polymeric substances into the copper oxide film is suggested to be one of the reasons for microbial corrosion of copper. 316 stainless steel interacted with and colonized with of Citrobacter freundii was found to be subjected to local chromium depletion under the passive layer. Similarly, depletion of iron with enrichment of sulfides could be observed in steels subjected to SRB interaction. Many industrial failures brought about by localised pitting in stainless steel condensers were identified as due 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 29: MIC and Biofilms NPTEL Web Course to MIC. Sulfoxy anions such as thiosulfates, and tetrathionates (SRB metabolites) present in circulating water could reduce the pitting potential of stainless steels. Continued activity of SRB would inhibit repassivation, when chloride ions are also present. Stress corrosion cracking of metals and alloys can be accelerated by microbial processes. For example, Stresses can be induced by biologically generated sulfide and hydrogen at neutral pH. Similarly, corrosion fatigue can be biocatalyzed. Microbial activity can influence cathodic hydrogen generation and its entry into the metal matrix. Hydrogen embrittlement of steels can thus be contributed by microbial activity. Further, biogenic sulfides such as H2S and thiosulfates can act as ‘poisons’ for the hydrogen recombination reaction and promote diffusion of atomic hydrogen into steel matrix. Under cathodic protection conditions, especially in the presence of SRB, hydrogen-induced stress cracking of steels could occur. Other stress-inducing biogenic products include CO2. Biological degradation of protective coatings can also occur. Souring of petroleum products due to H2S presence (due to SRB activity) is a serious problem. Degradation of Corrosion inhibitors Coatings and paints Lubricants and emulsions and Cathodic protection systems. are influenced by microbial activity. Microorganisms can grow in inhibitor-added electrolytes utilizing the carbon and hydrogen present in the organic chemical. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 29: MIC and Biofilms NPTEL Web Course Microorganisms are involved in corrosion in metal machining and processing operation. Destruction of Anticorrosive agents in coolants. Emulsions used in hydraulic fluids. Oil emulsions at alkaline pH favours bacterial growth. The microbes grow in aqueous phase and metabolize on nutrients available (carbon, nitrogen, phosphorous). Due to microbial activity, degradation of emulsifiers takes place and such an emulsion turns corrosive. Degradation of anticorrosive agents occurs in coolants used in several engineering operations. Recirulated lubricants are prone to bacterial infection. Protective coatings are provided for buried structures and interior surfaces of tanks and pipings. Microbial attacks can occur in chemical and polymer coatings. Biodegradation of adhesives is yet another problem. Biodegradation of coatings on pipe lines subjected to cathodic protection is of serious concern since it could enhance current requirements for protection ‘all of a sudden’. Experimental observations have recorded increasing protection current requirements for pipe lines under sub-soil conditions infested with SRB. [Ref – B.J. Little, J. S. Lee and R. I. Ray, The influence of marine biofilms on corrosion, A concise Review, Electrochim. Acta; 54 (2008), 2-7]. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course Lecture 30 MIC – Case Studies and Mechanisms Keywords: Aircraft Fuel Tanks, Steel Corrosion, Tubercles. To illustrate typical MIC – case studies, specific engineering systems prone to microbial attack are discussed. Aircraft fuel systems Microbial corrosion in aircraft systems was unknown till the1950s. Fungi and bacteria were identified as responsible for corrosion of structural alloys used in fuel tanks. Aviation fuel contains several types of hydrocarbons and can be contaminated with several types of heterotrophs. Spores of fungi and bacteria can survive for longer periods in fuel. Aliphatic hydrocarbons present in aviation fuels can be biodegraded. The ability of the fuel to serve as a carbon source for microbial metabolism is a key factor in fuel microbiology. Pseudomonads are frequently isolated from aqueous phases associated with aircraft fuel systems. The predominant organism present in aircraft fuel tanks has been observed to be a fungus, Cladosporium resinae which have been isolated from corrosion pits in integral tanks. Growth of Sulfate Reducing Bacteria (SRB) in ground storage fuel tanks has been reported. Aviation kerosenes may contain upto 100mg/L of water, and can promote active growth of the above heterotrophs. Water presence in integral tanks can occur through condensation. Cladosporium resinae are capable of spore formation in the kerosene layer and rapid build-up of high spore concentrations can occur. Ambient temperature determines their growth in ground aircraft and installations. Periods spent on ground therefore may be a factor that decides the amenability and severity of fuel tank corrosion. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course Fungi and bacteria produce a variety of water-soluble organic acids. Drained water from contaminated tanks record a pH of 3-4. Acid corrosion of structural metals and alloys can thus occur. Aluminium alloys are extensively used in aircraft parts and structures. Oxygen concentration gradients can also occur due to microbial colonies which can induce formation of corrosion cells on aluminium alloy surfaces. Fuel cleanliness and use of fuel biocides could mitigate the problem to certain extent. Microbe – related problems in fuel systems are illustrated in Table 30.1. Table 30.1 Microbial Problems in fuel systems. Type of Problem Organisms Clogging of pipes, valves and filters Fungi, polymer-generating bacteria Sludge accumulation Various microorganisms Reagent generation and oil / water emulsification. Fungi and aerobic bacteria Biocorrosion of storage tanks Fungi and anaerobes Suspended solids in the oil Various microorganisms Hydrocarbon biodegradation Fungi and aerobic bacteria Injectors biofouling Aerobic bacteria and fungi Sulfur accumulation SRB Engine parts breakage Bacterial corrosion 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course Aerobic bacteria and steel corrosion From a corrosion view-point, aerobic bacteria can bring about several consequences such as: Slime formation Sulfur and sulfide oxidation Iron oxidation Acid containing metabolites. Slime formation is a major problem in paper mills. There are several examples of slime formers such as filamentous fungi, algae, protozoa, diatoms and bacteria. Hydrated biofouling slimes coat the metal surface creating favorable conditions for initiation of concentration cells and also for the subsequent growth of anaerobic organisms. Sulfuric acid and reduced sulfur constituents generated by chemolithotrophs are corrosive. Iron oxidizers such as Gallionella, Sphaerotilus and Pseudomonas, oxidize ferrous to ferric ions. Insoluble tubercles consisting of hydrated ferric oxides and excreted slimes grow on the corroded steel surfaces. Fig. 30.1 shows advanced tuberculation inside water pipelines due to bacterial iron oxidation. Fig. 30.1 Tubercle formation inside water pipeline 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course Pipelines carrying water, oil and gas are susceptible to such tuberculation. Massive tubercles can hamper mass and heat flow. Differential aeration cells can be formed beneath such deposits. Oxygen-rich regions serve as cathodes O2 + 2H2O + 4e = 4OH- Fig. 30.2 Formation of differential oxygen cells in steel pipes due to tubercle formation. and oxygen-depleted areas act as anodes, promoting steel corrosion. Fe = Fe++ + 2e Tubercles can harbor different organisms including anaerobes such as SRB. Stages in tubercle build – up and generation of differential oxygen cells in steel pipes are illustrated in Fig. 30.2. Bio-slimes can also initiate pitting corrosion of stainless steels in sea water, fresh water or sub-soil environments. Aerobically generated biofilms catalyse cathodic oxygen reduction, enhancing corrosion potentials. Pitting and crevice corrosion (especially in presence of chlorides) can thus be propagated. Stainless steel weldments are susceptible to MIC. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course Microorganisms influence the corrosion of active-passive metals and alloys such as stainless steels and titanium through biofilm formation. a) Ennoblement of corrosion potential and enhancement of cathodic reaction rates. b) Initiation of pitting. The above electrochemical effects are due to biological interactions through formation of microbial biofilms. Differences in the behavior of bare and biofilm- formed metals confirm the above observation. The phenomenon of potential ennoblement due to biofilms is often variable and variations from sample to samples under different conditions could be observed. Percent surface coverage is shown to play a role in the above behavior (Fig. 30.3). Mechanisms involved in ennoblements are still inconclusive. Mixed potential theory can be applied to explain shift in potentials in the noble direction. Fig. 30.3 Potential ennoblement due to surface coverage. For example, for oxygen reduction as a cathodic process for stainless steel, ennoblement could be caused by noble shift in reversible potential for the oxygen reduction or enhancement in its exchange current density. Tafel slopes for the reaction would also be affected. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course Proposed mechanisms include: a) Biocatalysis of oxygen reduction. b) Change in pH under biofilm influencing the cathodic process or c) Emergence of new cathodic reactions. The role of biofilms in enhancing corrosion in the presence of microorganisms can be quite complex involving several bio-and physico-chemical parameters. Biofilms can act as a diffusion barrier for reactants such as oxygen and aggressive anions and cations. Detachment of biofilms from metal surfaces can lead to removal of protective films. Inducement of corrosion cells such as differential oxygen and differential concentration of reactive species can be a consequence of heterogeneous, patchy biofilms. All the above factors can lead to generation of potential differences at localized regions where corrosion current flow would be facilitated. Biofilms can also influence the nature of oxidation – reduction conditions at metal – solution interfaces. Redox conditions could thus differ in different regions of the biofilm. Microbial consortia within the biofilm can influence redox conditions. Consequences of biofilm formation can be summarized as follows: a) Localised corrosion initiation, especially in active-passive alloys. In marine environments, crevice corrosion can be initiated and propagated in presence of biofilms. Increase in probability for both crevice and pitting corrosion. b) Galvanic corrosion as a consequence of enhancement of cathode kinetics. For example, both the open – circuit potential and cathodic kinetics for stainless steel were seen to be increased in the presence of biofilms. Similarly, in a galvanic couple involving an active-passive alloys (stainless steel), corrosion currents for the couple with a biofilm was found to be higher than that without a film. c) Role of discrete bio - deposits on stainless alloys need also to be understood. Examples are biocorrosion of stainless steels where, (i) Pitting occurs under a bacterial colony or tubercle. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course (ii) Corrosion in the vicinity of the weld and (iii) Crevice attack In saline waters containing chlorides, biofilms involving manganese and iron oxidizing bacteria can initiate several complex reactions under discrete bio-deposits. Presence of anaerobic bacteria such as SRB and an iron-manganese oxidizer such as Gallionella can trigger formation of metallic sulfides and chlorides under discrete bio-deposits. Models to illustrate initiation of pitting corrosion due to biofilm formation on metal surfaces and welded joints are given in Fig. 30.5. Role of various bacterial species present in sea water in biofilm formation and subsequent metal corrosion in the presence of different reaction products such as iron sulphides, chlorides and reduced sulfur species is illustrated in Fig. 30.6. Fig. 30.5 Biofilm and pitting on a metal and welded joint. Fig. 30.6 Biofilm consisting of various bacteria and reaction products and generation of differential oxygen cells. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 30: MIC – Case Studies and Mechanisms NPTEL Web Course Generally, biofilms have been implicated in increasing corrosion rates of metals. This is especially true for patchy, heterogeneous bio-deposits which can initiate formation of concentration cells as indicated earlier. There are instances, where aerobic bacteria such as Bacillus spp can decrease biocorrosion. Ten-fold decrease in corrosion of SIS 1146 steel has been observed with cultures of Pseudomonas. Sp. Aerobic biofilms can decrease corrosion compared to sterile controls However, it is possible that such aerobic biofilms may ultimately enhance corrosion due to colonization of anaerobic SRB. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course Lecture 31 Biofouling of Titanium – Biofilm Studies Keywords: Biofouling, Sea Water, Titanium. Titanium has many attractive properties such as light weight, good mechanical strength and above all excellent corrosion resistance even in sea water. It will then be very useful to assess biofouling tendency of titanium in sea water environments in order to understand its utility in several nuclear power generation plants. Biofouling refers to undesirable accumulation of a biofilm deposit on an engineered surface. An organic film composed of microorganisms embedded in a polymeric matrix forms the basis for the attachment of various macro-organisms, inorganic and corrosion products. Such formation of biofilms on material surfaces poses a threat to the successful utilization of materials in industrial environments. Energy losses – reduced heat transfer efficiency resulting from insulating biofilm Increased capital costs. Enhanced maintenance costs and replacements. Shutdowns . Stages in marine biofouling: a) Successive coverage of a surface by growth of primary colonizing bacteria and other organisms. Aerobic followed by anaerobic organisms colonise in succession. b) A transient stage where multilayers of cells become embedded in their own polymer matrix. c) The final stage of development of a mature biofilm with high density of macrobial population (diatoms, protozoa etc). 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course Biofilms initially formed can be easily removed by physical or chemical means. However, those formed for extensive periods of time are difficult to be removed due to incorporation of inorganic and organic products formed by the metabolic activities of attached microorganisms. Biomineralization such as oxidized manganic and iron oxides occur within biofilms. Two fundamentally different forms of biomineralization are recognized. Biologically induced mineralization in which organisms generate conditions suitable for precipitation of extracellular mineral phases. The second one called boundary organized biomineralization involves growth of inorganic particles within organic biomatrix. Titanium is used as the tube material for seawater-cooled condensers in nuclear reactors and also, in plate-type heat exchangers of the auxiliary cooling water system. Titanium is known to be prone to microbial attachment and biofouling. Biofouling of heat exchanger surfaces may result in the deposition of biogenic MnO2 on the surfaces severely compromising efficiency. Investigations were carried out to study mineralization of manganese on titanium surfaces immersed in seawater due to the activities of marine bacteria. Microbial fouling and formation of inorganic deposits on titanium grade-2, commonly used for condenser tubes was established. Bacterial isolation, culturing, growth and attachment on titanium surfaces were studied. Total viable counts of culturable marine bacteria on titanium surface were monitored as shown in Fig. 31.1. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course Marine organisms Manganese oxidizers 5 10 2 Viable counts / cm 4 10 3 10 2 10 2 4 6 8 10 12 14 16 18 20 Exposure (Months) Fig. 31.1 Viable counts of marine bacteria and manganese oxidizers on titanium exposed to sea water. The percentage of Manganese oxidizing bacteria (MOB) in the total bacterial counts showed an increase from an initial value of about 30–40% to almost 100% at the end of about 20 months. Epifluorescence studies showed increased biofilm formation on titanium surface exposed to seawater with exposure time. Throughout the study the biofilm was always patchy and non-uniform. Epifluorescence and confocal images indicated highly irregular biofilms (Fig. 31.2). 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course Fig. 31.2 Epifluorescence micrographs of biofilm development on titanium. Surface coverage of the biofilm considerably increased with time. The EPS content, which is the other major component of the biofilm besides the microbial constituents, was determined as a function of exposure time. The Protein and carbohydrate constituents of the EPS were analyzed. Contribution of carbohydrate was more than that of protein in the biofilm. Confocal scanning laser microscope image of an year old biofilm showing mature biofilm is illustrated in Fig. 31.3. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course Fig. 31.3 confocal image of an year old biofilm on titanium. Protein levels in the biofilm however increased rapidly after a year of exposure and almost equaled the carbohydrate levels. The ability of the isolated bacteria to oxidize manganese was determined both in liquid as well as solid media. The formation of visible brown colonies on a solid agar plate containing 50 ppm of Mn (II) was confirmation of bacterial manganese oxidation. Solid agar plates inoculated with MOB and manganous ions are shown in Fig. 31.4. Bacterial manganese oxidation is evident as growth of coloured colonies. Presence of brownish colonies on filter membranes is a confirmatory test for the manganese oxidation ability of marine bacteria isolated from biofilms formed on titanium. Biomineralization of manganese by the isolated bacteria was further confirmed by studying manganese oxidation in liquid media. Various manganese oxidizing bacteria were categorized with respect to their ability to oxidize manganous ions at different concentration levels. The isolates exhibiting highest manganese oxidation rates were shortlisted for further detailed investigations. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course Fig. 31.4 Agar plates containing manganous ions and manganese oxidizing bacteria (MOB) showing growth of coloured colonies. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course Optical micrographs of some manganese (II) oxidizing bacteria isolated from the biofilms grown on titanium surfaces are illustrated in Fig. 31.5. Pseudomonas Vibrio Bacillus Micrococcus Bacillus Fig. 31.5 Predominant manganese oxidizing bacteria isolated from marine biofilms on titanium. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 31: Biofouling of Titanium – Biofilm Studies NPTEL Web Course 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 32: Biofouling of Titanium – Biomineralization and Corrosion Aspects NPTEL Web Course Lecture 32 Biofouling of Titanium – Biomineralization and Corrosion Aspects Keywords: Biomineralization, Biofilms, Titanium Corrosion. Pseudomonas spp. and Bacillus spp. dominated the consortia of manganese oxidizers, isolated from marine biofilms on titanium. 21 strains belonging to the genus Bacillus and 16 strains of Pseudomonas were isolated. Manganese oxidizing bacteria colonizing the titanium surfaces exposed to seawater were characterized by periodically retrieving the coupons over a period of two years. About 20% of the initial colonizers were capable of oxidizing manganese, and the dominant manganese oxidizing bacteria were those belonging to Gram - negative Pseudomonas spp. Gram positive, Bacillus spp, were also observed, though in smaller population. Almost 40-60% of the biofilm formers observed during 1 – 5 months were capable of oxidizing manganese. There was a gradual shift in biofilm colonisers from Gram - negative to Gram - positive bacteria, with Gram positive members increasing in numbers and diversity with time. Sulphate reducing bacteria (SRB) capable of growth under anaerobic conditions were also isolated from the biofilms even as early as two days of exposure to seawater. Different species of marine SRBs belonging to the genus Desulfovibrio and Desulfotomaculum were isolated. The frequency of occurrence of various isolated bacterial species on titanium biofilm is shown in Fig. 32.1. A percentage frequency of 100 indicates the presence of at least one species from this genus of bacteria in all the titanium surfaces studied during the entire period of study. Bacterial species belonging to the genera Bacillus and Pseudomonas showed 100% frequency. Genus Vibrio, Arthrobacter, 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 32: Biofouling of Titanium – Biomineralization and Corrosion Aspects NPTEL Web Course Micrococcus, Flavobacterium etc. showed frequencies of 25-35%. Members of the genera Staphylococcus, Citrobacter and E.coli, showed only a frequency less than 10%. The dominant bacterial species in the biofilms on titanium surfaces, capable of oxidizing manganese (II) were Pseudomonas and Bacillus spp. Fig 32.1 Frequency of occurrence of different bacteria in the biofilm on titanium Manganese and iron contents in the biofilm on titanium surfaces were analyzed. The maximum manganese content on a titanium specimen of approximately 220 cm2 surface area was about 250μg. This is a significant increase when compared to <6µg/L dissolved Mn (II) present in the coastal waters. Biofilm characterization over a prolonged exposure period substantiated the fact that titanium surfaces were susceptible to fouling. Within 24h of exposure, bacterial attachment and subsequent growth on titanium surfaces could be seen. Titanium is prone to biofouling due to its inert and non-toxic nature. The slime or the EPS is the 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 32: Biofouling of Titanium – Biomineralization and Corrosion Aspects NPTEL Web Course structural framework of the biofilm. EPS content of the biofilm increased with increasing exposure time. Titanium surfaces with the nutritionally rich organic biofilm are an attractive substrate for attachment and growth of marine bacteria. Predominant marine bacteria like Pseudomonas, Vibrio, Micrococcus, Flavobacterium and Bacillus were present in the biofilms. Formation of biogenic manganese oxide on the condenser tube surfaces and resulting decrease in heat transfer have been reported in power plants. Analysis of the fouled tubes revealed that MnO2 was formed on the inner surface due to the activities of Leptothrix spp. and Sphaerotilus spp. present in the condenser cooling water. Similar biofouling tests were also performed on titanium heat exchanger plates. Reddish brown colonies of manganese oxidizing bacteria could be seen deposited on titanium plates. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 32: Biofouling of Titanium – Biomineralization and Corrosion Aspects NPTEL Web Course Scanning electron micrographs depicting biofilms of Sulfate Reducing Bacteria(SRB) of the genus Desulfovibrio and Desulfotomaculum on titanium surfaces are illustrated in Fig. 32.2. Fig 32.2 Scanning electron micrographs depicting SRB biofilms on titanium surfaces. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 32: Biofouling of Titanium – Biomineralization and Corrosion Aspects NPTEL Web Course Open circuit potentials (SCE) with titanium in contact with MnO 2 and in the presence of biofilms of MOB and SRB measured in sea water are shown in Fig. 32.3. Ennoblement of potentials in the presence of MnO2 and MOB biofilms could be seen. In the presence of SRB biofilm, titanium potentials deviate in the negative (more reducing) direction. Fig 32.3 Variation of open circuit potentials of MnO 2 - and biofilm – coated titanium in sea water. The following observations with respect to the electrochemical (corrosion) behavior of titanium in sea water are noteworthy: An ennoblement of open circuit potential of titanium occurs in a culture of manganese oxidizing bacteria in artificial seawater nutrient broth, where as titanium surface shows active potential in an SRB culture. Cyclic anodic polarization curves in MOB culture shows good passivity and no breakdown up to a potential of 2V 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 32: Biofouling of Titanium – Biomineralization and Corrosion Aspects NPTEL Web Course Cyclic anodic polarization curves in SRB culture shows active dissolution prior to the passive region showing that titanium is almost film free under open circuit conditions in SRB culture. A very low value of charge transfer resistance in electrochemical impedance studies of titanium at open circuit potential in SRB culture confirms that passive film is not stable under this condition The implications of the above results are that while manganese oxidizing bacteria are harmless to titanium from a corrosion point of view, sulphate reducing bacteria and associated reducing environment makes titanium prone to local dissolution REFERENCES 1. Judy Gopal, P. Muraleedharan, H. Sarvamangala, R. P. George, R. K. Dayal, B.V.R. Tata, H.S. Khatak and K. A. Natarajan, 2008, Biomineralisation of manganese on titanium surfaces exposed to seawater, Biofouling, 24 (2008) pp 275 – 282. 2. H. Saravamangala, J. Gopal, P. Muraleedharan, R. P. George, R. K. Dayal and K. A. Natarajan, 2008, Biomineralization of manganese by Bacillus spp isolated from a marine biofilm. Minerals and Metallurgical Processing, 25, pp 149 – 155. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 33: MIC – Failure Analysis NPTEL Web Course Lecture 33 MIC – Failure Analysis Keywords: MIC Failures, Monitoring Programs, Biological Tools It is essential to assess susceptibility of an engineering system for MIC. Generally, any system exposed to water and nutrients can induce microbial growth. Microbial corrosion failures often manifest as Loss of efficiency in heat exchangers. Pressure drop and / or impaired flow through piping. Certain regions in piping such as spots where collection of water occur, dead-ends as well as tank bottoms where water-hydrocarbon interface exists are more susceptible to microbial fouling. MIC is prevalent in refineries, cooling water installations, oil- field water handling, sprinklers, oil, water and gas pipelines and pulp and paper mills. Power generation installations (both thermal and nuclear) in coastal belts are prone to different types of MIC. Analyses of MIC failures need to take into consideration the environment to which metals and alloys are exposed. For example, in soils, corrosion rates can be correlated with Eh, pH, moisture content, type of soil and resistivity. Aerobic and anaerobic soil environments attract growth of aerobic and anaerobic microorganisms. Factors correlating with SRB-induced corrosion in soils can be identified for buried steel pipes. Bacterial density - 103 – 108 cells / g Total organic carbon in ground water - upto 1.2% Soil resistivity - 500 – 30,000 ohm.cm Eh range - -300 to -390 mV Sulfate content - upto 200 mg / g Based on corrosion severity and its localised nature, potential MIC sites can be identified. Metallurgical damage, corrosion products, environmental and microbiological factors need be analysed to confirm MIC. Morphology, 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 33: MIC – Failure Analysis NPTEL Web Course directionalism and dimensions and distributions of pits need be metallurgically assessed. Biological analyses enumerate viable cells of different aerobic, anaerobic and acid producing bacteria. Chemical estimation of corrosion products also need to be undertaken at the corroded and neighbouring surfaces. For examples, corroded steel surfaces would yield yellowish brown or black ferric oxides and other iron oxides such as goethite, magnetite etc. in the presence of aerobic organisms. Corroded steel in the presence of SRB would reveal presence of blackish, amorphous and finely divided iron sulfides. Standard methods are available for identification of MIC corrosion products in different environmental and industrial conditions. (eg: oil and gas, cooling water, buried pipelines etc). For example, in cooling water systems, factors such as Microorganisms – byproducts Microbially – unique morphology Corrosion products, and Environmental conditions need be looked into. With reference to steels, reported MIC trends based on failure analyses are: Often associated with untreated, stagnant water in pipes – low or intermittent flow most damaging. Weld damage in stainless steel pipes. Pitting more prevalent in the bottom third of pipe. Low pH / High chlorides – enhance corrosion. Design of monitoring programs in industrial systems is documented. Probes, sampling coupons and other corrosion monitoring gadgets are available. For example, biofouling can be assessed by monitoring pressure drops across pipelines. Heat exchanger efficiency can be assessed before and after fouling. Electrochemical measurements to monitor biofilms development, bioporducts and corrosion are 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 33: MIC – Failure Analysis NPTEL Web Course available. Bioactivity can be detected through electrochemical probes. Measurements of corrosion potential, redox potential, linear polarization resistance, impedance and resistance through electrochemical instrumentation have become possible. As indicated above, detection, diagnosis and monitoring are critical to minimize MIC failures. Only through proper diagnosis, a corrective action plan for corrosion mitigation can be undertaken. Assessment should take the background history into consideration. Prior events, corrosion monitoring records, repairs undertaken, water chemistry and treatment etc need be evaluated. Data with particular emphasis on materials of construction, water and deposit analysis, microbial and metallurgical analysis and treatment are essential to assess MIC failures. Similarly, failure analysis constitutes of several principal stages such as: Collection of background information. Visual, non-destructive, destructive and chemical examination. Mechanical tests and analysis of failure causes. Various instruments and kits for the above are readily available. For microbiological analyses, standard procedures are available. Biological assays can be done on liquid and solid samples in order to identify involved microbes, their metabolic activities and concentrations of enzymes and other byproducts. Modern biological tools and techniques for characterizing microbial populations and their metabolic activities can be used. For example, separation of DNA/RNA, PCR (Polymerase Chain Reaction) and surface analytical tools such as mass spectrometry, AFM, SEM, TEM, etc. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 33: MIC – Failure Analysis NPTEL Web Course Some common analytical methods for biological evaluation are given below: Cell numbers Counting chamber Specific organisms Microscopy Viable cell count Serial dilution Aerobe, anaerobe, others MPN Identity Biochemical tests DNA/RNA Probes Reverse sample genome probing. Biomass Proteins, polysaccharides, nucleic acids Metabolism ATP Enzyme activity Hydrogenase assay, Radio-active labelling. Sulfate reduction 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 34: MIC – Prevention and Control NPTEL Web Course Lecture 34 MIC – Prevention and Control Keywords: Control of MIC, Design Aspects, Biocides. Prevention of MIC (Microbially- influenced corrosion) would be cheaper than its control after manifestation. Basic approach should be to keep microbial growth under control. How to maintain sterile environment? There are several factors to be considered. Appropriate design: Use correct materials Avoid improper design Environmental control For example, for designing water-containing systems, many parameters such as pH, source of water, salinity, hardness, oxygen levels, nature of impurities, rates of flow and temperature need be considered. For buried pipe lines, soil corrosivity need be assessed in terms of moisture content, pH, oxygen, availability of cations and anions, presence or absence of stray currents and resistivity. Pipelines are invariably coated for corrosion resistance and also to reduce cathodic protection current requirements. Barrier coatings on pipelines can at times disbond during service. In practice, cathodic protection is always used along with protective coatings. Cathodic protection can be applied to external surfaces of buried underground structures or installations submerged in water and also at times to inside of tanks, heat exchangers and clarifiers. Interrelationship between cathodic 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 34: MIC – Prevention and Control NPTEL Web Course protection and MIC in marine and sub-soil environments needs to be understood. Effect of applied current (or maintained potential) in attachment and growth behavior of microorganisms, such as SRB needs to be ascertained. In the presence of anaerobic bacteria, such as SRB, applied potentials for protection must be more negative than that usually considered (in the absence of microorganisms). Besides, effect of biofilms and bio-products on applied potential or current needs to be assessed. System design and operation are key factors in prevention and control of MIC. Avoidance of water seepage, collection and stagnation need be avoided. Tanks and fuel-storage vessels should be designed to drain of water at the bottom. For larger engines, such as seagoing vessels, breaking emulsions and separating water from oils can be done through recirculation. Prevention and timely removal of scales, debris and deposits will be beneficial. Pipeline cleaning devices can be effectively used frequently to remove deposits and tubercles. Chemical treatment using biocides is also recommended to deter bacterial attachment and growth. Chemical agents are designed as bio-stats and biocides and could also scavenge oxygen, inhibit corrosion, control pH and deposit formation. Biocides are generally added to chemical treatment packages to control biofouling and subsequent corrosion. For different industrial environments, chosen biocides need be used, such as: Chlorine / ozone Reactive aldehydes Quaternary ammonium salts etc. Screening tests and site-trials are essential in the choice of the most appropriate biocide. Biocides are classified into two families as oxidizing and non-oxidizing. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 34: MIC – Prevention and Control NPTEL Web Course Chlorine and bromine are oxidizers. Cl2 + H2O = HOCl + HCl HOCl = H+ + ClO- Strict regulations exist concerning chlorine gas and its residuals in chlorinated water. Chlorination may be continuously performed, preferably, in the range of 0.1 – 0.2 mg/L or intermittently as shock treatment in the range of 0.5 – 1.0 mg/L. Other oxygenating biocides for bacterial growth control include: Hypochlorites Chlorine dioxide Ozone Chlorinated or brominated donor molecules Biocides that inhibit bacterial metabolism include: Methylene bisthiocyanate Gluteraldehydes Amines Quaternary ammonium salts All the above can be effectively used in cooling water treatment systems. Thiocyanate compounds can be used to control algae, fungi and bacteria such as SRB. Today many types of newer instrumentation and analytical tools are available for assessment and evaluation of MIC. Electrochemical, microbiological, optical and other chemical methods could be effectively used not only in research laboratories, but also in industrial situations. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 34: MIC – Prevention and Control NPTEL Web Course The following developments are noteworthy: Chemical analysis within biofilms using micro-sensors to evaluate the chemistry at the surface and within the biofilm. Study of structure of biofilm using fiber-optic microprobe. Electric field mapping using scanning vibrating microprobe. Advanced microbial techniques such as DNA probes to investigate biofouling and MIC-Molecular techniques involving bacterial DNA and RNA. Environmental scanning electron microscope, confocal laser scanning microscopy and Atomic force microscopy to observe biofilm development in real time. Microelectrodes for in-situ oxygen measurements. Development of corrosion sensors and electrochemical measurement technology has attracted particular attention. An electrochemical sensor for monitoring biofilms has been recently developed, which provides an immediate indication of the biological activity and can be used to optimise and control biocide treatment. MIC affects the operation and maintenance costs of pipelines. About 40% of all internal pipeline corrosion in the petroleum industry can be attributed to MIC. In oil pipelines, hydrocarbon and water stratify at the bottom and cannot easily be drained off. Both aerobic and anaerobic bacteria besides various types of fungi may participate in the corrosion process. Presence of iron-and manganese-oxidizing bacteria, acid producing bacteria and methanogens may also be involved in oil and gas-pipelines. There are case studies of bacteria and fungi isolation from gas, oil and water pipelines. Sessile organisms are the dominant forms in the biofilms. Planktonic cells are not that numerous. Many such organisms isolated from pipelines may not grow in culture and thus there is a chance one may underestimate the true population of a system. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 34: MIC – Prevention and Control NPTEL Web Course Bacterial enumeration and identification studies in diesel and naphtha pipelines showed the occurrence of several bacterial species such as: Bacillus subtilis Bacillus cereus Pseudomonas aeruginosa Bacillus pumilus Bacillus megaterium Characterization of microbial populations in gas pipelines indicated the presence of colonies consisting of: Desulfovibrio vulgaris Desulfovibrio desulfuricans Citrobacter freundii Clostridium sporogenes It becomes clear that the microorganisms inhabiting pipeline systems significantly defer from place to place and system to system. Corrosion management strategies may thus change from system to system. Various management strategies need be adopted for a dedicated pipeline corrosion abatement programme, which should include. Corrosion modeling Corrosion risk assessment. Specific identification of corrosion types. Review of performance indicators for abatement processes. Biofouling is a severe problem in power plants. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 34: MIC – Prevention and Control NPTEL Web Course Microbial growth and its effects in fuel systems are listed in Table 30.1. By proper corrosion management, about 50% of the damages of condenser tubes could be prevented. Biofouling and MIC are serious problems in sea-going vessels. Biofouling is a serious problem in ship’s hull, propeller and rudder, and bottom parts. By appropriate surface coating (painting) technology which incorporates pertinent biocides, fouling of marine going vessels can be minimized. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course Lecture 35 Implant Materials And Corrosivity Of Human Body Keywords: Human Body, Implants, Biocompatibility. Recent innovations in the areas of biomedical engineering and biomaterials have resulted in the development of different implants for implantation in the human body. Biomaterials engineering deals with scientific study of the structure and properties of biomaterials in relation to their interaction with biological systems. Biomaterials have to interface with human biological systems so as to evaluate, augment or replace any organ, tissue or function of the body. Use of artificial metallic materials to repair fractured and diseased tissues and organs has been practiced since centuries. However, several developments have taken place in the use of advanced technology for manufacture of corrosion – resistant metallic and nonmetallic materials for human body implants since the last two decades. In lectures 35 and 36, developments in human implants with respect to corrosion resistance are illustrated. A cross section of a human body with a few implant sites such as dental cavity, hip joints and knee joints is shown in Fig. 35.1 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course Fig. 35.1 Some human body implants Human body environment presents a corrosive medium to various implant materials - a highly oxygenated saline electrolyte at a pH of around 7.4 and temperature of 370C, containing water, dissolved oxygen, sodium, chloride, bicarbonate, calcium, potassium, magnesium, phosphate, amino acids, proteins, plasma, lymph, saliva and various organic compounds. Various biological molecules influence the electrochemical behavior of implant metals and alloys. Anodic and cathodic reactions are influenced by adsorption and interaction of such organic polymers. For example, protein adsorption on implant materials can interfere with oxygen and hydrogen redox reactions. Besides, bacteria present in the human body can also influence the corrosion behavior of implant materials. Infections can create imbalance in the electrochemical equilibrium. Shifts in the physiological pH towards 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course acidic and alkaline directions can occur. Release of metal ions from the implant can change interfacial equilibrium and influence electrochemical kinetics. Tolerable corrosion rate for metallic implants is expected to be about 0.01 mils/year. Both thermodynamic and kinetic factors influence the corrosion behavior of many implant materials. From a corrosion angle, chloride and oxygen contents as well as pH of body fluids are very important. Biological macromolecules interfere with anodic and cathodic reactions at the implant – biotissue interface. Proteins can bind to metal ions and transport them across body upsetting double layer equilibrium, inducing enhanced metal solubilisation. Stability regions for body fluids such as saliva, bile, urine, gastric juice and intra - and intercellular fluids in terms of redox potentials and pH can be represented in Eh – pH diagrams. In the human body, saliva, interstitial and intracellular fluids occupy regions closer to oxygen line ( upper stability limit for water) since they are oxygen saturated. On the other hand, stability limits for urine, bile and gastric fluids are closer to the hydrogen line (reducing environment). In vivo corrosion of implants need be analyzed in relation to effect on life span of the device and levels of metal ion dissolution and transport. Since common methods of corrosion control such as inhibition and changing environmental conditions will not be suitable to human implants, the best solution would only be proper material selection and design. Corrosion forms such as galvanic corrosion, pitting, intergranular attack, uniform corrosion, stress cracking, crevice and fatigue corrosion are prevalent in different locations of the human body depending on the structural configuration and types of metallic implants. Both in vivo and in vitro tests are carried out to assess corrosion resistance of biomedical materials. In vivo tests are performed using animal 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course candidates, while in vitro measurements are done using simulated body fluids. There are ASTM standards for different tests and conditions, such as corrosion performance of biomaterials, galvanic corrosion in electrolytes, crevice corrosion of metallic surgical implant materials and for cyclic potentiodynamic polarization. Effects of fretting and rupture on surface oxide films need also to be evaluated. Surface oxide films formed on metals and alloys minimize release of metallic ions. Tissue compatibility and regeneration of surface oxides is also important. Regeneration periods of passive alloys differ depending on alloy composition (stainless steels, titanium alloys, Co-Cr alloys etc.). Regeneration time is observed to be longer for stainless steels, while shorter for Ti-6 Al-4V, an alloy widely used for orthopedic implants. Surface coatings on implant materials reduce the corrosion rate. Metal dissolution from implants contaminates human body systems and results in undesirable consequences. Important properties of (biomaterials) implants Mechanical properties – Enhanced risk of crack propagation and fatigue fracture. Biocompatibility A biocompatible (bio) material should not disrupt normal body function to any significant level. Many factors such as size of implant, material properties and surface-chemical characteristics influence biocompatibility. Long-term stability of metal implants critical for patient health and survival. - Stents - Arthroplasty - Fracture fixation - Pacemakers 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course Ideal implant materials should be Inert Nontoxic to body Corrosion resistant Inexpensive Strong Fatigue resistant However, most synthetic materials once implanted in the body can induce foreign- body reaction. Presence of metal ions triggers enhanced foreign body response. - Osteolysis, implant loosening. - Blood clotting (thrombosis). Corrosion and wear of implants produce changes in blood chemistry, such as elevation in metal concentrations. In case of stainless steel and chromium – containing alloy implants in Joints, dissolved chromium and nickel levels in serum could occur. Metal dissolution can damage cells. Approximate composition of some orthopedic implants is given in Table 35.1 Table. 35.1 Orthopaedic metallic implants Alloy Fe C Cr Ni Co Ti Al V Ti 6Al 4V 0.3 0.1 …. …. …. Bal 5-6 3 – 4.5 Co-cast 0.75 0.35 27-30 1.0 Bal …. ….. …. Co-forged 0.75 0.35 27-30 1.0 Bal …. ….. …. Co-wrought 3.0 0.05-0.15 19-21 9-11 Bal …. ….. …. Ti 0.5 0.1 …. …. …. Bal ….. …. 316L Bal 0.03 17-19 13-16 …. …. …. …. Synthetic biomaterials 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course Metals – Co-Cr alloys, Stainless steels, Gold, Titanium alloys, Vitallium, Nitinol (shape memory alloys) Uses – orthopedics, fracture fixation, dental and facial reconstruction, stents. Ceramics – Alumina, Zirconia, Calcium phosphate, Pyrolytic carbon. Uses – orthopedics, heart valves, dental reconstruction. Coatings – Bioglasses, Hydroxyapatite, Diamond like Carbon, Polymers. Uses – orthopedics, contact lenses, catheters, in-growth. Corrosion and erosion failures of implants can be categorized into: Mechanical (overload, fatigue, wear, abrasive motion) Electrochemical (different types of corrosion). Biological (inflammation and infection, calcification, biodegradation). There are also combined factors influencing corrosion, such as stress corrosion failure, fretting and fatigue. Biodegradation and bacterial interaction processes can lead to disastrous consequences such as bone loss, structural deformity, and osteolysis. Metal accumulation in body fluids (nickel, cobalt and chromium) impart toxicity (metal sensitization). Retrieval of wear particle remnants from the body fluids can help with failure analysis and development of newer implant materials. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course The following are the most relevant corrosion types for different implant materials: Uniform corrosion: For a successful implant material, long term uniform corrosion rate is expected to be less than one micrometer per year. Galvanic corrosion: Dissimilar metal corrosion is common in orthopedic, dental and other biomedical uses. Examples: Hip prosthesis with ball made of 316 stainless steel and a socket of Ti – 6 Al – 4V alloy. Co – Cr – Mo femoral head in combination with Ti – 6Al-4V femoral stem. Gold crown in contact with amalgam in oral cavity. Pitting and crevice corrosion: Pitting is a common form of corrosion in 304 stainless steel implants. Addition of molybdenum reduces pitting tendency. Other treatments for enhancing pitting resistance include nitrogen alloying and controlled cold working. Titanium is resistant to pitting. Crevice corrosion a) Micromotion between components results in fretting corrosion-lead to initiation of crevice attack. b) Passive oxide film for corrosion protection. c) Repetitive motion-continuous breakdown and repassivation. d) Crevice corrosion can be caused by localized oxygen depletion and metal ion concentration gradients. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 35: Implant Materials and Corrosivity of Human Body NPTEL Web Course Crevice corrosion is a problem in implants made of: Mo – containing stainless steels. 316 Stainless steels Co – Cr – Mo alloys Stress corrosion cracking is reported in cases of orthopedic implants made of stainless steel. Corrosion fatigue: Very uncommon if recommended specifications and processes are adhered to. Fretting corrosion is reported in orthopedic implants. Corrosion occurs at joints due to motion. Prosthesis implant materials such as stainless steels, Co-alloys and Ti – alloys are amenable to fretting corrosion. Oxidized corrosion products aggravate this type of corrosion. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 36:Medical Implants – Status and Developments NPTEL Web Course Lecture 36 Medical Implants – Status and Developments Keywords: Surgical Implants, Dental Implants, Surface Modification. Corrosion behavior of the most commonly used surgical implants Austenitic stainless steel (eg: 316L) used in cranial plates, orthopedic, dental implants, spinal rods, stents. Cobalt-chromium alloys - dental implants, orthopedic, heart valves, joint replacement. Titanium and its alloys - Cranial plates, dental implants, orthopedic fracture plates. Joint replacement, stents, catheters. a) Austenitic stainless steel (eg: 316 and 316L). Retrieved implants show that higher percent failure of 316L implants is due to pitting and crevice corrosion. Fatigue wear leads to disruption of passive layer. Fretting between implant and bone accelerate fatigue wear. Stainless steel implants in the thigh region are susceptible to crevice corrosion, pitting, crack initiation and propagation and stress corrosion. b) Cobalt-alloys: used in orthopedic implants. Co-Cr-Mo alloy used as a femoral head of joint prostheses. Release of metal ions cause toxicity problems. Serum proteins can interact with these alloys to release toxic metal ions. c) Titanium-alloys: very wide spread use due to combination of mechanical and corrosion resistance. Titanium is considered inert and immune to body fluids 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 36:Medical Implants – Status and Developments NPTEL Web Course and tissues and is therefore biocompatible. They are now used for joint replacements, bone fixation, dental implants, heart pacemakers, stents and artificial heart valves. In hip joints and bone plates, damage due to fatigue wear can occur. Comparison between Ti – Ta and Ti – 6Al – 4V alloys indicate that addition of tantalum reduces metal release from surface oxides. Ni-Ti alloys (Nitinol) used for dental, orthopedic and cardiovascular implants have attractive properties and corrosion resistance. Shape memory effect, damping capacity and superelasticity are attractive properties. The alloy can revert to its original shape following deformation. Within a given temperature range, Nitinol can be strained many a time more than other materials without plastic deformation. Widespread applications for Nitinol as stent materials are being made since the mid 1990’s. TiO2, with smaller amounts of nickel oxides form a surface film. Through surface treatments, such as coating with titanium nitride, its corrosion resistance can be further improved. Its corrosion behavior as dental arch wires has been studied. Dental implants: Very aggressive environment in the oral cavity. pH of saliva 5.2 – 7.8. Major causes of corrosion are pH, saliva, temperature, proteins, food materials etc. Galvanic corrosion very common in dental implants, especially when a pair of implants such as Co-Cr alloys, Ni – Cr, Ag – Pd, gold and Ti – implants are used. Pitting of Co, Ni and Cr containing alloys results in release of toxic metal ions into the body. Gold, palladium and platinum are chemically stable and are corrosion resistant. Titanium and its alloys such in Ti – 6Al – 4V and Ti – 15 V are good materials. Dental amalgams are among the oldest of materials used in oral health care. Formed through mixing liquid mercury and a powder of Ag, Sn and Cu, the biomaterial is 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 36:Medical Implants – Status and Developments NPTEL Web Course easy to manipulate for filling dental cavities. Since these amalgams are multiphased, galvanic and intergranular corrosion are possible. Dental alloys with precious metals are used for crowns and amalgam inlays. Gold and its alloys are used as electrodes in medical instruments and also in dentistry. Surface modification for corrosion protection: Surface chemical and corrosion resistance properties can be improved through surface engineering of biomaterials. Techniques such as plasma ion implantation, laser melting, laser alloying, ion implantation, vapour deposition and chemical and surface texturing can be used. Carbon-based coatings such as a DLC (Diamond like Carbon) improve corrosion resistance of Nitinol. Titanium dental implants are usually surface modified to decrease corrosion. Methods such as surface machining, electro - polishing, anodic oxidation, plasma-spraying and biocompatible coatings can be used. Nitrogen-ion implantation and nitrogen-alloying improve corrosion resistance of orthopedic implants. Laser surface engineering has advantages with respect to shorter periods, suitability to complex geometry and ability to process functionally structured and integrated materials. Biomimetics can be incorporated. Nanoceramic materials are yet another welcome introduction to orthopedic and dental implants. Examples include: Nanocrystalline diamond films coated onto titanium alloys and Co - Cr alloys. Nano Al2O3 – TiO2 coatings Nanoceramic hydroxy - apatite coatings. Future challenges 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 36:Medical Implants – Status and Developments NPTEL Web Course Electrochemical characterization in vitro of medical implants in the absence of real biological species (as done under simulated conditions) may not lead to conclusive results pertaining to biocorrosion in the human body. An alternative would then be to carry out electrochemical investigations on implant metals and alloys in media containing living cells and proteins. Complexity in biocompatibility testing comes into play when one uses cell cultures, animal models and clinical trials. Primary cell cultures or organ cultures derived from human body will be more acceptable to investigate biocompatibility. Implants of the future have been characterized as bioactive, corrosion resistant and antibacterial. The interaction of implanted materials with biological tissues and inhabitant organisms pose a challenge. Bacterial attachment and biofilm formation on implant surfaces can create not only corrosion related problems, but also tell on its biocompatibility. New developments in nanobiotechnology are expected to bring improvements with respect to biocompatibility. Development of antibacterial surfaces and use of targeted drug delivery with the help of nanobiotechnology are possible future outlooks. Another area of development in orthopedic implants is to have a bioactive surface to promote cellular adhesion and bone-in-growth. Widely used methods involve application of either hydroxyapatite or porous titanium coatings to implant surfaces. Newer orthopedic implants having a titanium-rich core and a hydroxyapatite-rich casing with controlled levels of micro - and macro-porosity are in the process of development. Use of graphene coatings on metal-based medical implants to improve corrosion resistance is being investigated. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course Lecture 37 Microbially Induced Concrete Corrosion Keywords: Concrete, Biocorrosion, Models, Prevention. Corrosion of reinforcing steel in concrete structures is one of the most expensive corrosion problems. Structural integrity of many bridges, buildings, terrestrial and ocean structures has been affected by microbial processes. Cement and concrete can be biodegraded and deteriorated through the action of various microorganisms. Compounds such as calcium carbonate, silicates and aluminates slowly react with water and bacterial metabolic products. Internal biocorrosion of reinforcing steel reduces the life of concrete structures. Biocorrosion products having larger space volume than the initial metal content initiate significant internal stresses leading to fracture. Microbial weathering of cement and concrete increases porosity enhancing water and corrodant penetration. Calcite precipitation and dissolution can be brought about by indigenous microorganisms. A beneficial aspect in this regard would be formation of calcium oxide (hydroxide) scales which can confer passive layer and also selectively plug up the pores. Biogenic cement and concrete can thus confer corrosion protection as well! This observation is of practical significance in curing of cements and concretes. Microbial precipitation of calcium carbonate and calcium hydroxide can lead to closure of pores, further increasing the corrosion resistance of reinforcing steel. Microbially induced concrete corrosion (MICC) has been known since 1900s. Concrete sewer pipes were observed to be degenerated into a pasty mass. Sulfur oxidizing bacteria could be identified and isolated from sewer pipes and channels. Bacterial sulfur cycle involving both sulfur-oxidizing and sulfate-reducing bacteria is relevant to MICC. SRB reduces sulfate present in sewage and soil materials to 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course sulfides leading to generation of gaseous H2S, dissolved H2S as well as reduced sulfur compounds such as hydrosulfides and polythionates. All the above sulfur- based reagents are corrosives to steel in cement and concrete structures. Bacterial oxidation of sulfur and sulfides lead to biogeneration of sulfuric acid which can corrode reinforcement steels and also dissolve cementaceous materials. Besides the above sulfur-oxidizing and sulfate-reducing bacteria, other neutrophilic heterotrophs of the genus, Bacillus and Pseudomonas present in sewer and soil environments generate organic acids, polysaccharides and proteins that can also corrode many metals and alloys. MICC can occur in different types of concrete systems such as Waste water treatment. Swimming pools and tanks. Cooling towers. Hydraulic structures, bridges and buildings. Rapid deterioration occurs in areas with elevated H2S and acid concentrations. A pictorial representation of microbial corrosion in sewage collection systems is illustrated in Fig. 37.1. 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course Fig. 37.1 Microbial corrosion of concrete pipes Reaction sequences: a) H2S generated by SRB in biofilms and sewerage-converted to various reduced-valence sulfur compounds. b) In presence of O2, water and S-bacteria, sulfuric acid is produced. c) Acids and S-compounds react with binder in concrete-disintegration, porosity, structural damage and reinforced steel corrosion. Bio-and physico-chemical variables such as biomass (cell number), pH, Eh and concrete ingredients change with time during the above mentioned process sequences. The initial pH being neutral is rendered acidic through bacterial acidification. Biological succession and pH reduction lead to deterioration of concrete and corrosion of reinforced steel. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course Colonization of concrete is an excellent example of bacterial succession. Concrete is generally alkaline with a pH of 11-13. Environmental reagents such as CO2, H2S, HS- etc can reduce the concrete pH to 9. Due to the availability of nutrients, bacterial attachment, growth and colonization on concrete surfaces take place. Neutrophilic Acidithiobacllus spp, such as T.thioparus can proliferate and the pH can drop to about 4-5 when other acidophiles take over. In concrete corrosion, the following Acidithiobacllus species were found to take part (Table 37.1). Table 37.1 Acidithiobacllus bacteria involved in concrete corrosion Bacteria pH range Substrate T. thioparus 4-10 S2O3, H2S T.novellus 5-9 S2O3 T. neapolitanus 4-9 H2S, S, S2O3 T. intermedius 1.5-9 S2O3 At. Thiooxidans 0.5-9 S2O3, S Besides bacteria, acidophilic and neutrophilic fungi can also grow on concrete surfaces, reducing pH levels to highly acidic values (pH 2 and lower). Reaction of biogenic sulfuric acid with the cementatious materials in concrete lead to structural failure. Corroding layers consist of gypsum and moisture. Ettringite (3CaO . Al2O3 . CaSO4 . 12H2O) produced due to bacterial action can induce internal stresses leading to cracking, pits and fissures, which facilitates further acid penetration. Corrosion rates up to 4-5 mm/year can be realized under the above conditions. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course A model for concrete biocorrosion is shown in Fig. 37.2 Fig. 37.2 Biocorrosion of concrete Corrosion potential of steel in reinforced concrete is measured relative to a reference electrode (Cu – CuSO4) in contact with concrete surfaces using high impedance voltmeters as shown in Fig. 37.3. Fig. 37.3 Potential measurements in reinforced concrete 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course Models have been proposed to predict microbial corrosion of concrete taking into consideration the following parameters. a) Sulfide generation. b) Sulfide flux to the pipe wall. c) Rate of acid reaction with concrete. Prevention strategies Corrosion effects of H2S and H2SO4 can be reduced by creating conditions that prevent or minimize their generation. This may not be that easy always. Proper treatment of sewer or modification of concrete could be useful. Concrete protection methods include modification of concrete mix and design, coatings or paintings on concrete surfaces or use of appropriate impermeable liners. Increasing and maintaining alkalinity could prevent acid corrosion. In sanitary constructions, appropriate specifications of cement need be used. Control of concrete sewer corrosion by the ‘crown spray process’ has been reported. A high pH mixture is sprayed into the crown area of sewer. Deactivation and sterilization of acid generating bacteria along with acid neutralization is aimed at. Activity of sulfate reducing and sulfide-oxidizing bacteria in the environment and formation of biofilms need be curbed. Magnesium hydroxide slurries of pH about 11 can be used to neutralize acid. Control of dissolved oxygen levels, sulfate concentrations and organic levels in the sewer environment could deactivate the corrosion-causing microbial load. Modification of concrete materials used in structures is another strategy. Closure of leakages and pores in the pipe lines through socket-seals would prove effective. ‘Microbial concrete’ is a novel strategy to restore damaged structures through biomineralization of calcium carbonate using microbes such as Bacillus sp. 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course Microbiologically induced calcite precipitation can generate highly impermeable calcite layer over the surface of already existing concrete layers. ‘Bacterial concrete’ can be made by embedding bacteria that are capable of calcite precipitation in the concrete. Bacteria such as Bacillus subtilis and Bacillus pasteurii can induce calcite deposition. The use of microbial concrete enhances durability of cement materials, sealing of decayed concrete structures and improves corrosion resistance of buildings and structures. Reduction in permeability through bioprecipitation of calcite in situ can prevent penetration of corrosion reactants. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course References and recommended reading (Lectures 24 – 37) 1. J.D.A Miller, Microbial Aspects of Metallurgy, Medical & Tech. Pub. Co, Lancaster (1971). 2. S.W.Borenstein, Microbiologically influenced corrosion handbook, Wood head Pub. Ltd., Cambridge (1994). 3. C.C. Gaylarde and H.A. Videla, (Eds), Bioextraction and Biodeteriortion of metals, Cambridge Univ. Press, Cambridge (1995). 4. M.G. Fontana, Corrosion engineering, Mc Graw Hall, New York (1987). 5. D.A Jones, Principles and Prevention of Corrosion Prentice –Hall Inc, New York (1996). 6. H. A. Videla and L. K. Herrera, Microbiologically influenced corrosion: looking to the future. Research Review. International Microbiology Vol.8, pp 169-180 (2005). 7. T.R. Jack, Biological corrosion failures. 8. I.B Beech, C.C. Gaylarde, Recent advances in the study of biocorrosion an overview, Rev. Microbiol 30, pp 1-22 (1999). 9. K.A. Natarajan, Microbially – influenced corrosion, chapter 3, in CORROSION SCIENCE AND TECHNOLOGY, (Eds) U. K. Mudali and Baldev Raj. Narosa, New Delhi (2008). 10. K.A. Natarajan, Microbes, Minerals and Environment, Geological survey of India, Bangalore (1998). 11. G.Manivasagam, D.Dhimasekharan and A.Rajamanickam, Biomedical Implants: Corrosion and its prevention. A Review, Recent patents on corrosion science, 2, 40-54 (2010). 12. D.C. Hansen, Metal Corrosion in the human body. The ultimate biocorrosion scenario. The Electrochemical society interface, pp 31 -34 ( 2008). 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 37: Microbially Induced Concrete Corrosion NPTEL Web Course 13. D.J Roberts, D. Nica, G. Zuo, J. L. Davis, Quantifying microbially induced deterioration of concrete: initial studies. International Biodeterioration and Biodegradation, 49, 227 – 234 (2002). 14. A.K. Parande, P.L. Ramasamy, S.Ethirajan, C.R.K Rao and N. Palanisamy, Deterioration of reinforced concrete in sewer environments, Proc. Inst of civil engg. Mucipal Engineer, 159, pp 11-20, (2006). 15. V.Achal, A.Mukherjee and M.S.Reddy, Microbial concrete: A way to enhance durability of building structures, Int. conf. sustainable construction materials and Technologies,. Ancona, Italy June (2010). 16. N.Eliaz, Biomaterials and corrosion, chapter 12, in CORROSION SCIENCE AND TECHNOLOGY. (Eds, U. K. Mudali and Baldev Raj, Narosa, New Delhi (2008). 17. U. K. Mudali, T. M. Sridhar and Baldev Raj, Corrosion of bio implants, Sadhana, 28, pp 601 – 637 (2003). 9 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course Lecture 38 Metallurgical Properties Influencing Corrosion Keywords: Metallurgical Properties, Intergranular Corrosion, Hydrogen Embrittlement, Iron Pillar Metallurgical properties and structure influence corrosion in various ways. Sulfide inclusion causes initiation of pitting in stainless steels. Grain boundary segregation of phosphorous, carbon and nitrogen can cause intergranular stress corrosion cracking of carbon steels. Susceptibility to hydrogen cracking of steels. Certain metallographic structures are unfavourable - for example, martensite reduces resistance to hydrogen cracking of steels. Crystallographic grain orientations can influence corrosion behavior. Grain boundaries susceptible - Segregation of alloying elements and precipitates to grain boundaries creating anodic and cathodic areas. Different metallographic phases in an alloy may exhibit varying resistance to corrosion. Cold working can influence corrosion rates. Intergranular corrosion is a localized attack at and adjacent to grain boundaries, with little corrosion in the grains. The alloy loses its strength and often disintegrates when exposed to a corrosive environment. The major causes are: Segregation of impurities at grain boundary (Iron in aluminium alloys). Depletion of an alloying element in the area of grain boundary. In stainless steels and nickel based alloys, intergranular corrosion is prevalent. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course When exposed to a temperature in the range of 425 – 8150C (sensitization), Chromium Carbide (Cr23 C6) precipitates at grain boundary leading to depletion of this corrosion-resistant element in the alloy. The role of chromium addition (upto 18%) to protect steels by inducing passivity is discussed already. Microphotographs for unsensitized and sensitized stainless steels are shown in Fig. 38.1. Fig. 38.1 Metallographic structures of stainless steel (unsensitized and sensitized) Chromium depletion as above induces corrosion in the grain boundaries, compared to surrounding grains. Decrease in chromium content in stainless steels increases critical passivating current densities and substantially restricting passive region stability. Below 12% Cr, the passive potential region is severely constricted. Percent carbon in the stainless steel influences sensitization time (higher the carbon, lower the time period to get sensitized). Nickel increases the activity of carbon in solid solution, facilitating carbide precipitation and enhancing sensitization. Titanium or niobium carbides can prevent sensitization. Sensitization of austenitic stainless steels during welding is termed WELD DECAY (see Fig. 38.2). 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course Knifeline attack is a localized form of intergranular corrosion occurring only a few grain diameters nearer to the weld zone in austenitic stainless steels. WELD ZONE Intergranular Corrosion Fig. 38.2 Weld decay in stainless steel. The following treatments are recommended for abatement of intergranular attack of stainless steels. Solution annealing – Heating above 815-0C to dissolve the precipitated chromium, followed by rapid cooling. Lower the carbon content (preferably below 0.03%). Addition of niobium or titanium for stabilization. Some recommended tests for intergranular corrosion of stainless steels are given in Table 38.1. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course Table 38.1 Some recommended tests for intergranular corrosion Strauss 16% Sulfuric acid, 72 h exposure Chromium 6% copper sulfate depleted area (Boiling) affected. Huey 65% boiling HNO3 Five 48h Determine weight exposures loss per unit area. Electrochemical a) Effect of degree of sensitization on anodic polarization in sulfuric acid. b) Electrochemical Potentiokinetic repassivation to establish sensitization to intergranular attack. Intergranular corrosion can occur in other alloys as well. Ni – Cr alloys: Containing Cr, Mo and Fe - precipitation of moly – carbides at grain boundaries. Aluminium alloys. Al – Mg - Mg2 Al8 is anodic to matrix and initiates corrosion selectively at grain boundary. Weldment corrosion is a serious problem. Drastic heating, melting, cooling cycles affects metallurgical and mechanical stability of the alloy. Residual stresses and stress concentrations promote susceptibility to corrosion. Weld metal composition need be so adjusted that the corrosion potential is shifted to nobler values. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course When an alloying element is more active than the other in an alloy, selective dissolution leading to dealloying would occur in a corrosive environment. Examples of dezincification of brasses and graphitic corrosion in grey cast-iron have been outlined already. Other examples are illustrated below: Aluminium bronzes in acid chloride, HF - dealuminification. Tin bronzes in hot brine or steam – destannification. Cupronickels in refinery condenser tubes - Denickelification Low-Zinc red brass (<15% Zn) is immune to dealloying. Tin additions to α + β castings accord good resistance. In single phase α admiralty brasses, small additions of As or Sb will be beneficial. Effect of potentials in chloride electrolytes for copper and zinc separation from α – brass has been reported. Below about 0 – 0.1V, slow zinc oridation begins and at potentials below -0.8 – 0.9V, selective zinc dissolution occur. Above 0.0V, copper dissolves with formation of chlorides along with dezinfication. Between 0.0 to + 0.2V, redeposition of Cu++ can occur. At more noble potentials, dissolution of copper and zinc increases. Mg, Cu, Zn and Si are the most generally used alloying elements for aluminium. Aluminium is active and readily oxidized to form protective oxide films on the surface. These surface films are stable in aqueous solutions of pH 4.5 – 8.5, but not in strong alkali and acid solutions. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course Coating of oxide film through conversion coating or anodising. Galvanic corrosion of aluminium and alloys depends on the nature of the oxide film which influences its polarization and resistance. Aluminium solid solution, microconstituents such as silicon and compounds such as Al2Cu Mg and Mg2 Si exhibit varying electrode potentials and can form concentiation cells within the alloy matrix. Al – Mg alloys are relatively resistant to corrosion. Al – Zn alloys used as galvanic anodes for protection of steel in marine environments. Constituents in the matrix induce nonuniform attack. Localised corrosion such as pitting and intergranular attack is common. Eg: Fe, Si, Al6Mn, Mg2Si, and Al3 Fe impurities promote electrochemical corrosion. Ageing and tempering heat treatments influence their corrosion behavior. Residual stresses can be introduced through thermomechanical treatments inducing stress corrosion cracking. Mechanical treatments including welding introduce corrosion, and susceptibility in heat-affected zones. Stress corrosion cracking develops in the simultaneous presence of tensile stress in a corrosive environment. Metallurgical factors influencing stress corrosion are: Grain boundaries anodic to grain – localised strains Matrix – precipitate composition Nature of dispersion of precipitates and solute concentration Hydrogen embrittlement – diffusion of hydrogen Adsorption of ions at crack interface. Cyclic stressing in a corrosion medium results in corrosion fatigue. Corrosion pits act as centres for corrosion propagation. Creep fracture at high temperatures 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course normally gets started at grain boundaries. Enhanced creep resistance can be provided by reinforcement with short fibres and introducing very minute intermetallics such as Al9Fe Ni or particles. Hydrogen in steels reduces ductility resulting in premature failure. Only a small quantity of ppm of hydrogen required to embrittle steel. Basic mechanism involves stress at a crack tip, where hydrogen can adsorb. Adsorbed atomic hydrogen then can migrate towards regions of localised stress and weaken the metal bond. H2  2H  H2 Sources of hydrogen : Steel making operations Processing Welding Storage or Containment Exposed environment Adsorption of hydrogen lowers surface free energy leading to crack propagation. Hydrogen diffusion into the metal-alloy can occur in different ways: Cathodic treatment (plating, pickling, cathodic protection) Hydrogen during casting / solidification Hydrogen occluded can create micro-voids and develop high pressure sites (H2 – blistering) Cracks can be developed through both intergranular and transgranular paths. Generally, hydrogen cracking is more prevalent at room temperature and the tendency deceases with higher temperatures. Best remedy for hydrogen embrittlement is minimization (or prevention, of possible) of hydrogen availability in the environment. 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course Iron pillar of Delhi – A metallurgical marvel for corrosion resistance. The iron pillar at Delhi (Fig. 38.3) is called a rustless wonder since it has withstood corrosion for the last 1600 years. It has naturally attracted the attention of corrosion engineers who are still without any definite scientific clue to the extraordinary corrosion resistance exhibited by such an ancient metallurgical structure exposed to atmospheric conditions. Even modern metallurgists are unable to reproduce such a corrosion-resistant metallic object, considering the advanced technologies available for iron and steel production, as at present. It becomes pertinent to analyze the secret of corrosion resistance of the Delhi iron pillar in the light of available research publications. Fig. 38.3 Iron Pillar Reported chemical analysis (percent) of the iron varies from report to report: C 0.08 -0.90 Si 0.004 – 0.05 S Trace to 0.008 P 0.114 – 0.48 Mn Nil 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course Fe 99.7 Specific gravity 7.5 – 7.8 Microprobe analysis in the near-surface regions revealed presence of 0.05% Cu, 0.05% Ni, and 0.07% Mn with no chromium. With respect to micro - structural features, the iron pillar material possesses a non- uniform grain structure, showing irregularly distributed slag inclusions in the unetched condition. Etched microstructure revealed medium to coarse polyhedral ferrite grains with slip bands in some grains. Pearlite not present at surface regions was observed to increase towards the interior. Absence of uniform pearlite distribution is attributed to segregation of phosphorous. Relatively high proportions of slag inclusions was characteristic of the iron pillar samples. Nouniform distribution of slag particles is indicative of the processing technique used in ancient periods of history, namely, solid state reduction of high grade iron ore with charcoal followed by hammering to remove part of liquid slag. Majority of phosphorous is in solid solution. The ancient process of iron extraction resulted in the presence of fine slag particles along with unreduced ore in the microstructure of the iron pillar samples. The microstructure of the iron in the Delhi pillar is typical of wrought iron. The composition of the iron, presence of phosphorous and absence of sulfur, its slag – enveloped metal – grain structure and passivity enhancement in presence of slag particles are attributed as possible reasons for its corrosion resistance. The nature of the protective passive layer on the iron pillar has been ascertained and found to be composed of iron hydrogen phosphate hydrate in the crystalline form in addition to - , γ – and - FeOOH and magnetite, all in amorphous form. 9 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 38: Metallurgical Properties Influencing Corrosion NPTEL Web Course REFERENCES 1. R. Balasubramaniam, On the corrosion resistance of the Delhi iron pillar, Corrosion Science, 42, pp 2103 – 2129, (2000). 2. T. R. Anantharaman, The Rustless Wonder – A study of the Delhi iron pillar, Vigyan Prasar, New Delhi, (1997). 10 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course Lecture 39 Laboratory Experiments In Corrosion Engineering - I Keywords: Corrosion Tests, Electrochemical Tests. Physicochemical, electrochemical and biochemical principles and mechanisms behind various types of corrosion and protection methods have already been discussed. Experimental protocols to understand and demonstrate various principles as above are also important. The last two lectures (39 and 40) are therefore devoted to illustrate laboratory experimental procedures and research techniques pertaining to some important corrosion types, electrochemical, biological and design aspects. These lectures, will introduce students to experimental techniques and corrosion evaluation methods. These lectures are so prepared as to enable the students to undertake design of corrosion experiments based on electrochemical and bio- chemical concepts. An understanding of both theoretical as well as experimental aspects of corrosion principles and processes would provide the synergy for a holistic knowledge of the subject matter. Only basic experimental procedures dealing with fundamental aspects are covered. For a detailed account of corrosion testing procedures, reference to recommended text books and other literature be made. Students are encouraged to prepare laboratory experimental plans and carry out as many experiments as possible and feasible. 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course Specific laboratory corrosion tests Atmospheric corrosion tests Salt spray tests (Fog testing) Neutral Acetic Acid Cyclic, acidified Cyclic, seawater acidified Cyclic SO2 Dilute electrolyte, Cyclic Fog. Dry Copper-Accelerated Acetic Acid Immersion tests Total immersion Partial immersion Intermittent immersion Tests for a given type of corrosion Stress corrosion (MgCl2, NaCl etc) Intergranular corrosion (Huey, Strauss etc) Pitting corrosion Selective leaching (Dezincification etc) Exfoliation Galvanic corrosion etc. Electrochemical tests Polarization resistance Impedance Cyclic potentiodynamic polarization 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course Galvanostatic polarization Scratch – repassivation Pitting and repassivation potential A. EMF and Galvanic series: Study EMF and galvanic series. a) Differentiate between EMF and Galvanic Series with respect to their application in corrosion prediction. b) With respect to Galvanic series, predict corrosion behavior (galvanic corrosion) in different bimetallic contacts? (i) For metals and alloys which are close together. (ii) Metals and alloys farther apart. c) Establish Galvanic Series in 3.5% wt. % NaCl and pH 2 sulfuric acid solution for the following metals and alloys. Mild Steel Zinc Copper Aluminum Platinum Tin Brass Bronze Aluminium – Mg alloy 18 – 8 Stainless steel Nickel Lead Cast-iron Titanium 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course Electrodes are made of the above metallic specimens. Saturated Calomel Electrode (SCE) is used as reference. Take the electrolytes in a beaker and immerse the electrodes. Measure steady state potentials using a pH meter. Arrange the various metals and alloys in increasing or decreasing order of measured potentials. B. In another series of tests, establish the potential difference for pair of electrodes. Measured potential differences between the half-cell potentials need be checked. Potential difference = Ecathode - Eanode The following combinations may be tried. Copper – mild steel Platinum – copper Zinc – mild steel Titanium – brass Stainless steel – copper For the above couples, discuss in electrochemical terms, corrosion behavior Which one is anode? Which is cathode? 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course C.Influence of surface area ratios in galvanic corrosion Copper and mild steel sheet specimens of different areas can be prepared. In an acid or chloride solution, different area combinations can be connected to a milli (micro) ammeter device and measure current. D. Demonstration of concentration cells Salt concentration cells – Two beakers containing pH 2 sulfuric acid solutions, separated by an agar bridge. Dip similar copper plates (same area) in the two beakers and measure potential difference. Now add 1M CuSO4 in one beaker and 0.1M CuSO4 in another. Measure potential difference between the copper electrodes. E.Differential aeration cell In the system as above containing 3% NaCl in both beakers separated by an agar bridge, dip the two copper plates. In one beaker bubble nitrogen gas continuously, while bubbling oxygen (or air) in the other. Measure the potential differences as function of time. Write your observation clearly. Which copper electrode is anode? Which is cathode? Why? Write down probable anodic and cathodic reactions? F.EMF Series From standard thermodynamic data, calculate E 0 (Standard Electrode Potentials) for various metals in the EMF series and arrange them in a series. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course To illustrate redox reactions involving displacement of ions in solution by a more active metal, the following tests can be performed. Immerse the metallic strip in the suggested solution and observe surface reactions. Acidic copper sulfate solution - Strip of iron (carbon steel) 10% hydrochloric acid - Strip of zinc 5% silver nitrate solution - Strip of copper In all the above cases, discuss the electrochemistry of the reactions based on EMF series. Write down anodic, cathodic and net cell reactions. G.Eh and pH Measurements and Standardisation Use a pH meter and learn how to standardize and calibrate the meter for both pH and Eh. pH standardization is done using standard buffer solutions of pH in the range 2, 4, 9 and 11. For redox potential measurements, ZoBell’s solution is used as a standard. 0.0033 M each of potassium ferro – and ferricyanide in 0.1 M of KCl (potassium chloride). Standard potential is +229 ± 5mV (Ag / AgCl) 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course H.Measurement of redox potentials The following couple can be used Fe+++ + e = Fe++ E0 = +0.771V Eh = 0.771 + 0.059 log Prepare different concentrations of standard ferrous sulfate and ferric sulfate at acidic pH of about 1.0. Using a platinum – SCE couple, measure the potentials for the following after deaeration of solution using nitrogen 1M ferrous sulfate alone 1M ferric sulfate alone Different ratios of ferric and ferrous sulfate solution mixed. Plot a graph depicting measured potentials as a function of log at a constant pH. What is the slope of the line? I. Laboratory Construction of Eh – pH diagrams Can you experimentally construct an Eh – pH diagram for the iron-water-oxygen system? Try measurements of Eh and pH corresponding to different concentrations of ferrous and ferric ions in the presence and absence of oxygen. The following points may be considered. a) Take distilled water in an electrolytic cell. Measure Eh and pH for different values of pH under oxygenated and deoxygenated conditions using platinum 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course – SCE couple. Plot Eh as a function of pH. Compare this experimental line with the theoretically established line for the reaction. O2 + 4H+ + 2e = 2H2O E0 = + 1.23V b) Establish experimental Eh – pH curves for the following reactions. 1. Fe = Fe++ + 2e 2. Fe++ + 2H2O = Fe (OH)2 + 2H+ 3. Fe+++ + e = Fe++ 4. Fe+++ + 3H2O = Fe (OH)3 + 3H+ 5. Fe + 2H2O = Fe(OH)2 + 2H+ + 2e 6. Fe (OH)2 + H2O = Fe (OH)3 + H+ + e 7. Fe++ + 3H2O = Fe(OH)3 + 3H+ + e Comment on your experimental observations with respect to theoretical Eh – pH diagram for iron based on the above reactions. c) Prepare 1M solution of acidic ferrous sulfate at a pH of about 1.0. Under changing pH in the presence of aeration, measure Eh as a function of pH. Report only steady-state values. Plot a curve depicting ferrous and ferric ion concentrations at different pH and time. J.Use of zinc in corrosion protection of steels Electrochemical principles governing corrosion of iron (mild steel) and zinc when present alone and when in galvanic combination can be demonstrated. Establish corrosion rate of zinc and iron (when present individually without contact) in dil. Hydrochloric acid. This can be done through weight loss measurements as a function of time. Then electrically join a sheet specimen of zinc and steel and immerse in the same medium for different periods of time. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 39: Laboratory Experiments in Corrosion Engineering - I NPTEL Web Course Tabulate your readings and comment? Conditions Initial weight Final weight Change in weight Steel Zinc Steel + Zinc Zinc can be coated into steel through either electroplating or hot dipping. Electroplating of zinc on a steel specimen can be experimentally demonstrated from an acid zinc bath under specified experimental conditions. Scratch away some of the zinc coating from the coated steel surface and study rusting of iron in salt water. Will steel corrode? 9 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course Lecture - 40 Laboratory Experiments in Corrosion Engineering – II Keywords: Polarization Experiments, Pitting Potentials, Microbial Corrosion. A. Electrochemical tests in a given environment Polarization curves and Tafel plots for generalized corrosion. Polarization resistance measurements. Corrosion potential, pitting and repassivation potential. Galvanic coupling – effect on polarization curves. Electrochemical impedance (to study behavior of coating, passivation). Anodic polarization for establishing active-passive behavior of metals and alloys in different environments – Anodic protection. Impressed current cathodic protection. In order to establish Tafel constants, corrosion potential, corrosion current and exchange currents, extrapolated regions of anodic and cathodic curves have to be used. Two types of approach a. Wide range of overpotentials with reference to rest potential (for example, -150mV to +150mV), to facilitate determination of Tafel slopes through extrapolation to the corrosion potential. b. Narrow range of overpotentials (+20 mV to -20mV), facilitating determination of linear polarization resistance (slow scan rate). 1 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course Description of cell and experimental arrangements are given in Fig 40.1 and 40.2. Fig. 40.1 Typical electrolytic cell with various electrodes for polarization measurements Fig. 40.2 Automatic polarization measurement 2 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course B. Measurement of pitting potentials: Cyclic polarization techniques can be used to evaluate pitting tendency of an active-passive metal or alloy (Fig. 40.3). A potential scan starting from Ecorr in the anodic direction is applied till significant current increase occurs. The final potential is negative with reference to repassivation potential. The potential where the loop closes on the reverse scan is the protection potential, which can also be estimated by extrapolating the reverse scan to zero current. Pitting potential (Epit) corresponds to the potential at which current increases sharply. The larger the loop, the higher the tendency for pitting. Pitting shows up as an increasing anodic current before transpassive corrosion or evolution of oxygen. Fig. 40.3 Cyclic polarization to determine pitting and protection potentials. New pits can initiate only above pitting potential, and not between E pit and Eprot. No hysteresis is exhibited by an alloy which is resistant to pitting. There will be potential and current distributions around pits. 3 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course C. Experiments for evaluation of sensitization in stainless steels. Study chromium depletion and precipitation as carbides at the grain boundaries. Oxalic acid test: Polished specimen is anodically etched at 1A/cm2 for a minute in 10% oxalic acid at room temperature. Examine the specimen under the microscope to reveal step, dual or ditch structures. D. Determination of effect of alloy chemistry on passivation parameters: For development of corrosion – resistant alloys with reference to active – passive behavior, the following key parameters need to be optimized. Epp – Primary passive potential icrit – Critical anodic passivating current density. 1. Establish Anodic polarization curves for iron , nickel and chromium in 1N H 2SO4. Comment on the passivity curves with respect to passivity potential range, E pp and icrit. 2. Establish the effect of chromium (0 – 30%) in stainless steels on Epp and icrit in 1N H2SO4. Plot your results with respect to (a) Epp Vs percent chromium (b) icrit Vs percent chromium 3. Determine pitting potentials for 18-8 stainless steel in different chloride concentrations. E. Corrosion testing for metals and alloys The following systems for corrosion testing are available, each covering different method of corrosion evaluation Humidity test chamber – Salt spray – Temperature and humidity control. Test set-up for alternate immersion testing of metals and alloys in 3.5% NaCl solution for stress corrosion testing. 4 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course Bimetallic corrosion testing in specific liquids under humidity control – current and potential difference recorded. F. Laboratory techniques for studying amenability to MIC due to biofilms: Evaluation of biofilms from deposit samples collected from various locations –organic and inorganic content of biofilms. Carbohydrate and protein analysis (spectrophotometer) Presence of aerobes and anaerobes in the deposits. Redox potential measurements in liquid samples. Corrosion potential measurements – Biofilm growth on metal surface influences anodic and cathodic reactions – Shifting of corrosion potentials in positive or negative directions to be monitored. Examples: Stainless steels in aerated seawater Mild steel in anaerobic seawater. Distinguish between aerobic and anaerobic corrosion. Polarization experiments in the presence and absence of biofilms on metals in the presence and absence of microorganisms. G. Monitoring and characterization procedures for different bacteria involved in MIC are listed in Table 40.1. Microbiological aspects of MIC – microbes are illustrated in lectures 24 – 27. Various strains of different bacterial species can be procured from culture banks and characterized as per recommended procedures. 5 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course Table 40.1 Testing and Analyses of different bacteria relevant to MIC, Type of Bacteria Monitoring and characterization 1. Acid producing Production of inorganic and organic acids – attack on various metals and alloys. 2. Denitrifying Ammonia production – attack on copper alloys. 3. Iron-related Ferrous oxidation and ferric-reduction- plugging of water and oil pipelines- tubercle formation. 4. Slime-forming Slime-sludge characterization. 5. Sulfate-reducing Sulfide production (H2S) – FeS production - corrosion of metal surfaces. Common media used for routine isolation of bacteria and fungi Filamentous fungi - Potato Dextrose Agar Aerobic and anaerobic bacteria - Nutrient Agar Pseudomonas Sp - Select media from literature Sulfate Reducing Bacteria (SRB) - Postgate media 6 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course H. Microbially influenced corrosion of aluminium alloys Choose desired aluminium alloys and make suitable specimens. Use naturally collected sea water and fresh water samples from identified locations. A biofilm growth chamber (under conditions of both stagnant and flowing liquid) can be constructed to expose the metallic specimens for different periods of time. Monitor biofilm growth by removing specimen frequently and characterize the biofilm with respect to thickness, microbial assay, chemical and metallurgical analysis, surface roughness and morphology. Isolate important bacterial species from the biofilm and carry out steady-state potential and polarization measurements in the presence and absence of isolated bacteria. I. Biofouling and MIC of stainless steels in sea water. Experiments similar to the previous one for aluminium alloys. J. Microbial diversity of pipelines and establishment of MIC Locate a pipeline transporting water and petroleum products. From the pipeline, collect aseptically, samples of water, oil and corrosion products (debris). a) Visual, physical and chemical characterization of the water, oil and debris samples for colour, chemical composition, pH. b) Isolation and enumeration of different types of microorganisms through standard microbiological procedures – characterization of isolated organisms with respect to Autotrophs, Heterotrophs. Bacteria, fungi Aerobe, anaerobe Iron oxidising, Manganese oxidising. c) Based on the microbial assay and characterization and failure analysis of the pipeline samples, predict nature of MIC (Microbially – influenced corrosion) 7 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course K. Examination of biocorrosion of concrete in the laboratory simulating sewer conditions Samples of sewer pipes collected from sewerage processing stations can also be used. Corrosion testing coupons Fresh coupons from new sewer pipes. Coupons prepared from corroded concrete slabs from sewer treatment plants. Corrosion chambers for exposing the coupons to bacterial activity under simulated conditions can be used. Monitor conditions with respect to pH changes, H2S generation, temperature and humidity. Growth of Anaerobic Sulfate Reducing Bactria (SRB), aerobic sulfur oxidizers such as Acidithiobaullus can be monitored and their role on concrete corrosion assessed. L. Bacterial kinetics of sulfur oxidation of Acidithiobacillus thiooxidans and its influence on concrete corrosion. Experimental strategy: Bacterial growth in recommended media. Growth curve with respect to cell number, pH and sulfate concentration as a function of time. Establish bacterial growth kinetics. Concrete corrosion tests in aqueous media at bacterial acidic pH under different conditions of temperature, metal-ion concentrations, and types of reinforcement steels. 8 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore Lecture 40: Laboratory Experiments in Corrosion Engineering – II NPTEL Web Course M. Corrosion testing for medical implants Compatibility Tissue response Dissolution rates Toxicity In vivo corrosion – How susceptible is the implant metal to corrosion? Effect of corrosion on body response Rest potential : Measurements over extended periods of time to predict metal dissolution. Cyclic potentiodynamic polarization: Corrosion susceptibility of small implant devices. Galvanic corrosion: Coupled and uncoupled leach rates. Fretting: Fretting corrosion in moving body parts. Various metal and alloy samples representative of implant materials can be shaped into electrodes and tested in body fluids and simulated electrolytes. 9 Course Title: Advances in Corrosion Engineering Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore