CORROSION SOLUTIONS WORLDWIDEMembers of the Sterling Fluid Systems Group www.sterlingfluidsystems.com ST-CORR-1 CORROSION SOLUTIONS WORLDWIDE STERLING FLUID SYSTEMS GROUP . Polypropylene. The material options include metals as standard as stainless steel and as exotic as titanium and hastelloy as well as rubber lining. Sterling has the facility for total engineering responsibility in the manufacture of the full pump range with its own specialist in-house foundries. a guaranteed material quality is assured. The economical construction and optimum reliable performance of centrifugal pumps . PVDF and PTFE. tested and supported to the highest standards attainable within the industry. Experienced in high alloy. Utilising laboratory data. high integrity castings and employing the latest analytical spectrometers. The materials are tailored to the particular industrial environment for which they are specifically designed. Sterling does not warrant its accuracy. while admittedly produced under stringently controlled conditions which quite often do not represent real life situations in industry.CORROSION AND EROSION IN CHEMICAL PUMPS SERVICE A WORLD LEADER IN MATERIAL TECHNOLOGY Sterling Fluid Systems specialised pumps are available in a diverse range of materials to combat the arduous nature of the pumped liquids. A QUALITY COMPANY Total quality procedures throughout the Sterling organisation ensure that every pump package installed in the field has been manufactured. nonetheless provides a useful guide toward alloy selection.or for that matter any other chemical equipment depends on selecting the right alloy. While every effort has been made to ensure that the information given in this booklet is technically correct. IN-HOUSE FOUNDRIES Uniquely. Thus the final selection also takes into consideration other factors such as presence of contaminants. galvanic attack. non-uniformity of solutions and differences in operating conditions. resistance of welded joints. Cost b. Suction and operation conditions b. Additionally. Continuous/intermittent service c. exhaust systems and water heaters. it is necessary to remember that the choice of material depends on several factors: a. and preventative maintenance. Corrosion contributes to the depletion of our natural resources and the recent concern over this is becoming increasingly influential in inducing people to be ”corrosion-cost“ conscious. and in particular to centrifugal pumps. Appearance 1 In dealing with pump systems. are to be imagined. Even though corrosion is here to stay. To view corrosion engineering in its proper perspective. its cost can be considerably reduced in industry through proper selection of materials. Temperature change The above factors influence the corrosion rates of any given material. etc. and each is dealt with in detail when considering a pump’s hydraulics. Are there several pumps involved. Type of seal e. or parallel? d. In an effort to apply corrosion principles (from fluid flow to underground soil and atmospheric) to contained chemical systems. The diagrams depict an impeller in a casing. Availability d. but the choice of a proper material depends on how accurately these factors have been calculated. the following schematic diagrams have been used (Page 2). in series. any changes subsequently made in operation or processing are critical and may make re-evaluation of the sizing and choice of material necessary. . Corrosion resistance c. Fabrication f. some other factors need to be considered: a. These indicate the different forms of corrosion that may take place in a pump. A plant may spend considerable amounts of money each year in painting steel to prevent rusting. The seals. bearing brackets. The world’s economy would be entirely different if it were not for corrosion. Strength e. correct design of products.INTRODUCTION The annual cost of corrosion and of protection against corrosion in the world is staggering. Flushing fluid f. Corrosion in radiators. increases this cost further. 1B CRACKING PHENOMENA CRACK STATIC STRESS DYNAMIC STRESS Stress Corrosion Cracking Corrosion Fatigue 2 . .. FIG..EIGHT FORMS OF CORROSION AND TWO FORMS OF CRACKING PHENOMENA .. 1A MECHANICS OF CORROSION General Corrosion Erosion FLOW BUBBLE Localised Corrosion Cavitation Pitting Crevice/Deposit Corrosion HEAT AFFECTED ZONE ALUMINIUM BASE WELD WELD HEAT AFFECTED ZONE ALLOY R53 NOBLE Galvanic Intergranular (Corrosion) Fig. . .... General corrosion proceeds by many different means: A. close examination will always reveal the fact that corrosion has left an etched appearance on the surface. the use of inhibitors. The corrosion reaction product may be protective. Description: Notice the attack is all over the surface without an exaggerated effect on the outer periphery. C. 3 CASE HISTORY ILLUSTRATING SEVERE GENERAL ATTACK Alloy: 316 S. the recognition of general corrosion is compounded by velocity and pressure variations. such as zirconium. In this case. the impeller face of the casting) and proceeding gradually and uniformly to the outer wall. For round bars and wires. B.S. . Most corrosion-resistant austenitic materials. it may form a passivating barrier that stifles further corrosion. Temperature 66 o C. This is illustrated very well when steel is used in oxygenated water. These solids may continually scour away the protective film which would otherwise form. which is resistant up to 37% HCI to 70 ºC. This type of protection is very sensitive to solids in the pumped fluids. show this type of passivating behaviour with the help of a surface film of oxides. Remedy: Review cost and evaluate possibility of using higher grade alloy. Castings suffer corrosion starting at the wall exposed to the fluid (for example.S. but continues to corrode at a low rate.GENERAL CORROSION CASE HISTORY ILLUSTRATING GENERAL ATTACK Alloy: Sterling R52 Environment: 30% Hydrochloric Acid with minor impurities. In pumps. or cathodic protection. the material is not inert. Environment: 70% Sulphuric Acid. Temperature 60 oC. the selection of a more corrosion-resistant material (a general rule is to select an alloy with a higher chrome and/or nickel content). These variations may appear to be caused by solids or erosive products in the fluid. The surface casing may show whorls and pockets where velocity variations have influenced the rate of corrosion. thus leading to erosion corrosion. impeller was substituted for an R-55 impeller. Methods of reducing or eliminating general corrosion are the use of coatings. Description: A 316 S. The corrosion product may be soluble in the pumped fluid at a rate determined by the electrode potential of the metal. A special case of an artificially controlled uniform dissolution process may be attempted by controlling the pH or current density of a given solution. General corrosion leads to relatively uniform thinning. Remedy: Do not interchange castings without checking suitability of application. This principle is utilised in chemical machining and electropolishing of stainless steels to improve either the corrosion resistance or frictional characteristics. and to continually repair the passive film. such as stainless steels. However. This pump was in service for two years. corrosion proceeds radially inward at an essentially uniform rate around the entire circumference. hardness and angularity of the particles that affect this type of erosion. The particles are now capable of destroying the protective oxide film continually by fluid shear. Erosion marks indicate direction of liquid flow.S. It is the angle of incidence of the abrasive particle that is of prime importance.EROSION . or flashing in the volute of a casing.24. thereby increasing the rate of damage. impeller. Mechanical-chemical or erosion-corrosion In the erosion-corrosion mode. Here the surface undergoes deformation and eventually extrusion. making the damage look erosive. By-pass throat area shows severe damage. Outer periphery shows damage increasing due to velocity and pressure. The surface discontinuity may be a weld bead. CASE HISTORY ILLUSTRATING EROSION FOR (A) AND (B) Alloy: 316 S. etc. the velocity of the liquid may cause flow aberrations and turbulence due to surface discontinuities. There is a defined ”breakaway velocity“ at which erosioncorrosion begins and is characteristic of a given alloy/pumped-fluid system. by causing mechanical damage. This will often correct the situation. the removal of the discontinuity may produce the same result. high pressure fluid in the constricted scratch zone causes the development of a channel. Remedy: a) Check rotation on impeller and b) Substitute with Sterling R48 or other more abrasionresistant material. In contrast. A number of closely knit channels causes the material to “wire-draw”. the flowing liquid may be free of abrasive particles. It is the angle of impingement. 4 .CORROSION FLOW Erosion consists of two types of damage modes: (A) A. Impingement of abrasive particles carried by a fluid can affect the surface of the casing. 1. Filtration of these particles wherever possible may be the best solution. GR. However. then reducing the flow rate will help reduce the erosion-corrosion. This usually reveals itself in a characteristic ripple pattern. This loosely adhered particle is removed by the liquid velocity. reducing the flow rate will not help. In this case. The impingement of hard particles causes multiple cratering. velocity. such as a scratch or a small pit. In this case. If this mode is identified and there is no particle impingement. Environment: Sodium Nitrate with other solids SP. It is extremely sensitive to the flow paths and thus may appear to be localised. A good example of discontinuity. causing damage is often seen in mechanical seals. Description: Very little damage around shaft opening. Purely mechanical or particle erosion In the case of mechanical erosion. Formation of bubbles: At the eye of the impeller the pressure on the liquid is sufficiently reduced to cause the liquid to vapourise or form bubbles. B. The central portion around the shaft opening is generally untouched. Improve design of system to reduce turbulence. It is here that the velocity and pressure of the liquid are the greatest. Collapse of bubbles: As the liquid is now pumped to the outer periphery of the impeller. causing the bubble to implode. Use coatings. if the coating has the required corrosion resistance. (B) CAVITATION This form of erosion is attributed to the following: A . alteration of the environment such as filtering to remove solids or reducing the temperature. Cathodic Protection has been found to reduce Erosion-Corrosion in some applications. Use materials with improved corrosion resistance to provide a stronger protective oxide film. as hardness. the pressure is increased. Repetition of this process at high speed causes the bubbles to form and collapse rapidly. such as hard facing. as well. 5 . the action is limited to the outer periphery of the casing. Methods of preventing or reducing Erosion-Corrosion can be accomplished by use of one or more of the following methods.B. b) Use a more cavitationresistant material. The closely spaced pits are usually seen on the lagging side of the blades. In certain violent instances. C.S. Cavitation damage is located anywhere between the inlet eye of the impeller and the tip of the blades. B. An imploding bubble causes the metal to be roughened. CASE HISTORY ILLUSTRATING CAVITATION TYPE DAMAGE Alloy: Sterling R55. CASE HISTORY ILLUSTRATING CAVITATION TYPE GAS CONCENTRATION CORROSION D. Remedy: a) Check the conditions under which the pump is operating. which is indicated by the presence of slip lines on the impeller and casing. Remedy: a) Review design of suction. Description: A restricted suction condition caused an insufficient amount of liquid to fill the discharge throat. Some measures which can be taken to alleviate this problem are: A. E. 6 . This caused a vacuum on the low velocity side as shown. Use of a rubber or plastic coating that inherently possesses a strong metal-coating interface. Description: The imploding of bubbles during re-absorption on the pressure side of the blade causes damage in the form of closely spaced pits. the temperature. These rapidly imploding bubbles may produce shock waves with pressure as high as 4400 bar. yet a large amount of gas was present. b) Use a more cavitationresistant material. Use of a more corrosion-resistant material.CASE HISTORY ILLUSTRATING CAVITATION TYPE DAMAGE Alloy: Sterling R55. partial pressures and the degree of recirculation flow inherent in the design. Environment: Water with 50 ppm chloride at 44 o C. Description: The imploding of bubbles during re-absorption on the pressure side of the blade causes damage in the form of closely spaced pits. Such forces cause plastic deformation in metals. Smoothing of the finish on the impeller to reduce nucleating sites for the bubbles. damage is noticed on the leading side of the blades. This is well beyond the yield strength of a number of materials. thereby cushioning the shock waves produced by the collapsing bubbles and thus preventing damage to the metal surface. The extent and location of the damage is dependent on the fluid being handled. The collapsing bubbles appear to cause closely-spaced pitted areas and considerable roughening of the surface. Cathodic protection can reduce cavitation by forming bubbles on the metal surface. This roughened area in turn acts as a nucleating site for a new bubble to form. Environment: Water with 50 ppm chloride at 44 o C. b) Determine if gas in liquid can be reduced. Environment: Unknown. Alloy: 316 S. Change of the design to minimise the hydrodynamic pressure differences in the process fluid. Remedy: a) Check the conditions under which the pump is operating. the depth increases. and hypochlorites. This resulted in the pits shown. Fluorides and iodides have comparatively lesser pitting tendencies.PITTING CASE HISTORY ILLUSTRATING PITTING CORROSION Alloy: 304L S. a type 304 stainless steel pump would give good service handling sea water if the pump ran continuously. Description: Several damage marks (caused during material handling) served as nucleating sites for an autocatalytic reaction to occur. This is substantiated by the fact that they require a dense concentrated solution for continuing activity. This form of attack is extremely localised. Non-oxidizing metal ions such as sodium chlorides and calcium chlorides are much less aggressive. depending on both the specific metal and the corrosive liquid. C. Pits have the following characteristics: A. Pitting is autocatalytic. They are difficult to detect because they are often covered with corrosion products. The addition of Molybdenum of 2% or greater in stainless steels contribute greatly in increasing resistance to pitting. For example. D. It usually results in a cavity that has approximately the same dimensions in breadth and in depth. Environment: Hydrochloric and nitric acid mixtures. This type of corrosion differs from crevice corrosion in that it creates its own crevice. such as chlorides.S. That is. E. the corrosion processes within a pit produce conditions which are both stimulating and necessary for the continuing activity of the pit. Methods for combating Crevice Corrosion generally apply for pitting. ferric. Pits usually grow in the direction of gravity. B. Materials that are susceptible to crevice corrosion do not necessarily become susceptible to pitting corrosion. but would pit if shut down for extended periods of time. Oxidizing metal ions. Pitting is usually associated with stagnant conditions. Remedy: Use alloy with molybdenum such as CF3M. and mercuric in combination with chlorides are considered to be the most aggressive. 7 . whereas the reverse may be considered to be true. Pitting usually requires an extended initiation period before visible pits appear. such as cupric. Most pitting is associated with halide ions. F. bromides. As the breadth increases. causing a hole through the wall of the casing. Temperature and concentration unknown. This period ranges from months to years. (threaded drain plug). and carbonaceous material that shield and create a stagnant condition.CREVICE/DEPOSIT CORROSION This type of corrosion occurs in restricted areas. Note the pitting that is usually associated with this type of corrosion. Use of gaskets that are non-absorbent. b) Consider welding the joint between impeller and shaft 8 . This kind of attack occurs in many media. such as teflon. it is very common in chloride-containing environments.There are a number of actions that can be taken to prevent crevice corrosion: A. however. but grows at an ever-increasing rate. either metal to metal. The external surfaces are protected cathodically. Use of flushing in seal areas to avoid stagnant conditions in the bore of the stuffing box cover. Thus. an autocatalytic reaction fosters the growth of crevice corrosion. or metal to non-metal. Temperature and concentration unknown. Use of welded joints instead of threaded joints. C. CASE HISTORY ILLUSTRATING CREVICE CORROSION Alloy: Sterling R55. where free access to the pumped fluid is restricted. As with pitting corrosion. Description: The gasketed shielded area has limited diffusion of oxidizing ions. the initial driving force is often an oxygen or metal ion concentration cell. but continued growth by accumulation of acidic hydrolyzed salts within the crevice. wherever possible. (gasketed joints). D. E. Environment: Dilute sulphuric acid with small amounts of hydrochloric acid and sodium chloride. B. thus avoiding penetration from either side. It is slow to start. Weld on both sides of a flange to pipe joint. thus creating an imbalance and the initiation of corrosion. Ensuring that the pump is completely drained. dirt. In certain cases corrosion products will deposit and form a crevice. Remedy: a) Review gasket material. It is aided by the presence of deposits such as sand. an electron flow is produced. There are several ways in which to combat galvanic corrosion: A.GALVANIC SERIES IN PURE SEA WATER Corroded End (anodic. Care should be taken to avoid wide separation in the relevant galvanic series. E. 2024 in this order) ↓ Mild Steel Wrought Iron Cast Iron ↓ NI-RESIST ↓ Type 410. Thus the series should be used only for predicting galvanic relationships. while the other metal is cathodic. Design changes involving the avoidance of the unfavourable area ratios.GALVANIC ALUMINIUM BASE TABLE 1 . Stainless Steel (passive) INCOLOY alloy 825 ↓ INCONEL alloy 625 HASTELLOY alloy C Titanium Protected End (cathodic. A similar series is needed for all of the various situations. Material selection is extremely important. 1100. The metal which is corroding at a faster rate becomes anodic. Stainless Steel (passive) Type 316. Cathodic protection by way of using sacrificial metals may be introduced. or most noble) STERLING R53 NOBLE A potential difference exists between two dissimilar metals when they are immersed in a corrosive and conductive solution. The separation between the two metals or alloys in the series is an indication of the probable magnitude of corrosive effects. B. The number of tests required would be almost infinite. avoiding dissimilar metal crevices. One of the two metals will corrode faster than the other. distance and geometry play a definite role in galvanic corrosion. If these metals are now connected electrically conductively on the outside. C. Barrier coatings and electrical isolation by means of insulators to break the electrical continuity are sometimes employed. The pumped fluid may be controlled by the use of a corrosion inhibitor. The most commonly used series. Stainless Steel (active) Type 316. 9 . based on electrical potential measurements and galvanic corrosion tests in unpolluted sea water is shown alongside. Substitution of impellers of different alloys in an existing system must be done carefully. 3003. and using replaceable sections with large corrosion allowances of the more active member. 2017. 3004. (See Table 1). D. M-Bronze ↓ Nickel 200 (passive) INCONEL alloy 600 (passive) ↓ MONEL alloy 400 ↓ Type 304. Effects such as polarisation (potential shifts as the alloys tend to approach each other). (as at threaded connections). G-Bronze Comp. Stainless Steel (active) ↓ 50-50 Lead Tin Solder ↓ Type 304. area. Stainless Steel (active) ↓ Lead Tin ↓ Muntz Metal Manganese Bronze Naval Brass ↓ Nickel 200 (active) INCONEL alloy 600 (active) ↓ Yellow Brass Admiralty Brass Aluminium Bronze Red Brass Copper Silicon Bronze 70-30 Copper Nickel Comp. using bolts and other fasteners of a more noble metal than the material to be fastened. 6053 in this order) ↓ Cadmium ↓ Aluminium (2117. or least noble) Magnesium Magnesium Alloys ↓ Galvanised Steel or Galvanised Wrought Iron ↓ Aluminium (5052. this condition would not be possible. The sensitisation of an austenitic alloy permits corrosive attack to start at the grain boundary. These crystal structures can be manipulated by varying chemistry and heat treatment. The chromium content varies in the manner shown in the lower portion of Figure 3. In translating this phenomenon to the macro-scale. Each type of atomic arrangement has specific physical and mechanical properties of its own. GRAIN BOUNDARY % CARBON % CHROMIUM CARBIDE PARTICLE Metals and alloys consist of individually oriented crystals which form from the molten state (castings or weld metal). The structure of low carbon austenitic stainless steels consists of three crystallographic phases: Ferrite.HEAT AFFECTED ZONE WELD INTERGRANULAR CORROSION WELD HEAT AFFECTED ZONE In a survey conducted by the Material Technology Institute of the Chemical Process Industries. GRAIN AVERAGE CONTENT 0. The figure depicts a transient state only. on the surface of the casting. Where the crystals come in contact with each other. through the grain boundary. it permeates the complete casting wall and results in leakage and possible grain dropping. corrosion follows the boundaries. which is typical of intergranular corrosion. The resulting energy states at the boundaries can promote the concentration of alloying elements. These boundaries have a much greater degree of structural imperfection than within the grains. and is mainly due to stress corrosion. Figure 2 shows the variation in carbon content in passing from one grain. however. their facets form a boundary that takes the form of a lattice. It should be noted that cracking. the carbide will precipitate in the grain boundaries. The effect of sensitisation on the chromium and carbon concentration is shown in Figure 3. If there was no carbon present in the alloy. to another grain. and of greatest importance .08% SATURATION VALUE 0. and of metallic and non-metallic impurities. but if they are heated to around 800oC for any appreciable length of time. does occur in austenitic alloys. hexagonal etc. the attack is first recognised as ditching along the grain boundaries. cubic. (lowest energy level). As the attack progresses. austenite and carbide under equilibrium condition. the development of facets indicative of their crystal structure is prevented when the growing crystals impinge on each other. Two important arrangements in stainless steels are called Ferrite and Austenite. These crystals develop into specific atomic arrangements known as crystal structures (e. 10 GROWING CRYSTAL FACETS FORMING A GRAIN BOUNDARY Fig 2 . During the solidification process. are more resistant than the boundaries (low energy level). This is below the 12% chromium minimum required for corrosion-resistance.EFFECT OF SENSITISATION ON CARBON AND CHROMIUM CONCENTRATIONS . Rapid cooling of these steels will ensure the retention of austenite (a high temperature phase).precipitates. There is a narrow region at the grain boundary which contains less than 12% chromium.02% 18% LEVEL FOR RESISTANCE12% BOUNDARY Fig 3 . where there is a deficiency of free chromium. it was shown that sensitisation and resulting intergranular corrosion were the cause of over half of the reported incidents of unsatisfactory performance.LATTICE MISMATCH EFFECT OF SENSITISATION ON THE CARBON & CHROMIUM CONCENTRATION CARBIDE PARTICLE Sensitisation may be referred to as carbide precipitation in the grain boundaries.g. Since the grains (high energy level).). Temperature: Ambient. a) The impeller on the left was solution annealed and then put in service. In practice at the foundry level. Environment: 25% Hydrochloric acid with 100 ppm Chlorine. However. Close-up of the impeller on the right in (a) above. Quench cracks have been noted in stress concentration areas also. it must be mentioned that sensitisation will occur during slow cooling in the mould. CASE HISTORY ILLUSTRATING INTERGRANULAR CORROSION (b) Alloy: Sterling R53. Sensitisation is prevented by providing a solution anneal above the sensitising range temperature. Temperature: Ambient. b) The impeller on the right is in the sensitised condition.CASE HISTORY ILLUSTRATING INTERGRANULAR CORROSION (a) Alloy: Sterling R53. Environment: 25% Hydrochloric acid with 100 ppm Chlorine. Immediate cooling of castings from the mould has been tried. This impeller has undergone severe corrosion with grain dropping occurring at the tip of the blades. 11 . the hostility of this production step has limited its applicability. however. Temperature 66OC. Compatibility of various materials in contact with respect to polarisation of potentials along with geometrics that increase salt ion concentrations (like crevices) should be considered in detail. liquid metal cracking. The surface net stress in contact with the pumped fluid will be the controlling parameter. and corrosion fatigue. but as the stress is increased above the threshold. Two characteristics are necessary for environment cracking to occur: a tensile stress and a corrosion reaction. Metallurgical: The list of specific environments that aid in stress corrosion cracking is different for each major alloy classification. Generally. These cracks may be single (as in corrosion fatigue) or multiple (as in stress corrosion cracking). The stresses that exist in a given situation are usually very complex. stresses below the threshold will not cause cracking. caustics being handled by a carbon steel. The most common method is to utilise another alloy that is not susceptible to this attack.STRESS CORROSION CRACKING STATIC STRESS CASE HISTORY ILLUSTRATING STRESS CORROSION CRACKING Alloy: 304L S. The cracks produced are perpendicular to the stress vector. There are several forms of environmental cracking such as stress corrosion cracking. and copper alloys in an ammonia environment. The latter generates compressive stresses in the material which often offset the tensile stress necessary for cracking to occur. For a given alloy/fluid system. hydrogen induced cracking. In such instances.S. They may be intergranular or transgranular. For example. cracking is immediately evident. chlorides pumped by stainless steel pumps. Remedy: Use another alloy type. Description: A drastic brittle fracture in a ductile material in a period of four months. intergranular corrosion does not require a tensile stress. Stress. a threshold stress for cracking exists. Lowering the residual and thermal stresses by heattreatment and shot-peening is carried out to decrease the stress levels below the threshold. For example. They are often wrongly interpreted. the morphology of the cracks may be very similar to stress corrosion cracking. B. Welding sometimes produces hot-short cracks which may be identified as stress corrosion cracking. 12 . Environment: 50% Caustic (Sodium Hydroxide) with trace amounts of Sodium Chlorides. Several of the parameters and the controlling methods used are discussed below: A. cracks produced by this method are unexpected and sometimes are dangerous. the temperature is high enough to remove the critical chemicals. There is some mystery in this type of attack due to the fact that failure in this mode may occur in the absence of corrosive action. This will not induce cracks. Pump shafts have often failed due to mechanical fatigue with no contribution by the corrosive. On the other hand.S. a sensitised 316 material in a nitric acid solution. C. a solution-annealed 316 material in the same solution. A review of the entire system is usually necessary if this attack has been identified. CORROSION FATIGUE DYNAMIC STRESS This attack results from the cyclic tensile and corrosive fluid in contact. Elimination of the critical chemicals from the liquid is probably the most desirable.The importance of microstructure may be illustrated by placing: 1. The phenomenon then covers a broad spectrum and is difficult to define clearly. E. CASE HISTORY ILLUSTRATING FATIGUE CORROSION Alloy: 316 S. D. 13 . An increase in temperature generally has a detrimental effect. 2. Stress corrosion cracking is noticed. stress corrosion cracking under static tensile stress has also been known to occur. The coatings normally act as a barrier between the metal and pumped fluid. The corrosive liquid in certain cases can be made less effective in causing stress corrosion cracking by the use of an inhibitor such as chromates in a caustic solution. however. that is. Most of the discussion in stress corrosion cracking is applicable to corrosion fatigue. If. Electro-chemical techniques are generally used to polarise an alloy to an oxidizing potential out of the range that will cause stress corrosion cracking. then the tendency reduces. it tends to induce stress corrosion cracking. Coatings and electro-chemical techniques are also used. Description: A crack propagated at an inclusion present in the material after several months of operation. it is more economical to use cast irons. however. The isocorrosion chart reveals the dip in the curve 14 . which may be satisfactory for oleum service. In concentration of the acid. in others it decreases. Finally. sulphuric acids.CORROSION BY ACIDS The acids most generally used by industry are SULPHURIC ACID sulphuric. although the corrosion rates are higher. Velocity and aeration are factors that must be taken into consideration. In acid is reducing and is better handled by some cases. nitric. materials. In oleum. higher concentrations. Sterling Alloy K26: This is a very widely used alloy for applications involving sulphuric acids. and these cause some of the most severe primarily on the reducing or oxidizing nature corrosion problems. phosphoric and hydrochloric Selection of a metal for this service depends acids. corrosion increases with the materials resistant to reducing conditions. The widespread use of these of the solutions. the acid is known to penetrate the metal along the graphite flakes. The resistance in these alloys is attributed to the graphite network interfering with the reaction between the acid and the metallic matrix. and a little corrosion in these confined areas builds up enough pressure to split the iron. Cast Iron: impurities in the system can cause Cast irons show good resistance in very strong severe problems. Below 85% at room acids places them in an important position with temperature and about 65% up to 66oC. In a number of instances. including oleum. Types 304 and 316 Stainless Steels: These stainless alloys are occasionally utilised for cold. the regard to costs and destruction by corrosion. This wedging action is confined to cast irons and is not apparent in ductile iron. the acid is oxidizing Oxidizing and reducing mixtures of acids and and materials resistant to oxidizing media salts also causes different reactions to different are essential. very dilute sulphuric acid and under conditions that are not strongly reducing in nature. It provides resistance over the entire range of concentration. It is not recommended suited for practically all concentrations and for halogen acids or halogen salt solutions in temperatures. cupric and ferric chlorides. This alloy is also known to corrosion rate within 20 mm per year. the corrosion sulphuric acid and many other media. If the possess good corrosion resistance in the pump.at around 60-80% concentration range at Alloy R52 on the other hand. as opposed to resistance over a wide range of oxidizing and 304. then the temperature limitations of sulphuric acid. known to cause pitting. This will maintain the molybdenum alloy. This alloy is very suitable for used to detect the existence of this condition chlorides. alloy that shows a great deal of thermal stability at high temperatures. The R55 alloy has Types 304 and 316 Stainless Steels: numerous advantages over K26 and is the Type 304 stainless steel exhibits excellent most widely used Sterling alloy for sulphuric resistance to nitric acid at room temperatures acid and most sulphur compounds. This is only an indicative o temperature of 70 C and over the entire test and is not a prediction of definite behaviour. The chromium content in the alloy range) to intergranular attack in nitric acid. but provides stainless steels. is a nickel- approximately 66oC. temperature are increased beyond 50% and 30oC. at a maximum prior to fabrication. alloy is kept in solid solution which is essential for sulphuric acid service. and decrease attack. As the alloy that shows outstanding resistance to chromium content increases. such as up to 30oC. It is better suited to reducing may be increased to 82oC. and even aeration. up to 220 ppm. NITRIC ACID Sterling Alloy R55: One of the most important ingredients for R55 is a nickel-chromium-molybdenum-copper resistance to nitric acid is chromium. It is useful over Because of the susceptibility of sensitised Type the entire concentration range and oxidizing 304 (when exposed in the 430oC to 870oC conditions. is to be used intermediate and strong concentration range intermittently. show excellent Sterling Alloys R52 and R53: resistance to red and white fuming nitric acids Alloy R53 is a nickel-chromium-molybdenum at room temperature. however. concentration range. The addition of molybdenum to contact with the material. The minimum amount of withstand the corrosion of both oxidizing and chromium generally accepted is 18%. does not improve corrosion resistance to reducing conditions. It will rate decreases. 15 . The corrosion resistance The copper in these alloys does not discolour decreases as the concentration and the product. This reducing agents to moderately high makes the austenitic stainless steels very well o temperatures. as in type 316. 50% strength. and also to boiling acids up to sulphur dioxide and hydrogen sulphide gases. provides excellent resistance to oxidizing boiling 65% nitric acid (Huey test) is often conditions. Ferric sulphate and conditions. ferric chloride in appreciable concentrations are sulphates. (80 C). Ferric chloride and cupric chlorine. for example. Type 304 does. and is particularly susceptible to copper sulphate in the acid act as inhibitors oxidizing contaminants such as nitric acid. The 4% copper in the nitric acid. when mixed with sulphuric. increases This alloy has outstanding resistance at all the corrosion rates by breaking down the concentrations and at temperatures well above passive film. mixtures of nitric acid with obtained by the electric furnace process is hydrochloric or hydrofluoric acid are corrosive purer in form and is used in the manufacture to stainless steels and their rates depend on of soap. such as sulphates. temperature. the atmospheric boiling points. The presence of oxidizing ions in nitric acid tends to decrease the corrosion resistance of titanium . 16 . aeration. On the other hand. is used in the production of fertilisers. corrosivity to stainless steels and K26 alloys. the hardened condition. Until there is more evidence. The acid On the other hand. plasticisers and concentration and temperatures. but does not increase resistance sufficiently to justify based on the manufacturing process. and the nearly pure services as they are readily corroded. Great care is taken to make sure that the castings are properly solution-annealed. even in initial corrosion of an alloy such as R48 or K26. It shows less than 5 mpy in 65% nitric acid at 180oC. Sterling Alloy K26: K26 is widely used in phosphoric acid service. it has been found that Sterling Alloy R48: the cupric and ferric ions in solution inhibit the This shows excellent resistance and is corrosion of stainless steels in phosphoric probably the only age-hardenable stainless- acid. it would be prudent to use extra low carbon or stabilised alloys for severe services. There is some debate about stainless steels that have been affected by sensitising processes:. detergent. etc. PHOSPHORIC ACID phosphoric. present in acid made by the wet process. The cupric ion may be provided by the type alloy that shows good resistance. acid from the electric furnace process.corroding intergranularly. In a number of studies. but in some cases is the only material that will do the job.maybe its only drawback. insecticides. the additional cost. the addition of chloride or fluoride ions to the phosphoric and Titanium: hydrochloric or hydrofluoric acid.CORROSION BY ACIDS :. the corrosion behaviour and alloy selection are Sterling Alloy R55: This alloy shows good resistance. food. It is an expensive material. or acetic acids. and fluosilicates. Sterling Alloys R52 and R53: These alloys are not suited for nitric acid fluorides. Because of the impurities.Continued Nitric acid. shows reduced Phosphoric acid obtained by the wet process. Other variables include the concentration. Oxidizing agents and minor impurities such as Nickel and Nickel Irons: ferric chloride (or cupric chloride) and nitric Aeration affects these alloys to a great extent. Copper ions at first decrease the at all concentrations and temperatures up to corrosion rate.up to 50oC. However. Sterling Alloy R55: This is usefully resistant to all concentrations o of phosphoric at temperatures up to 90 C. corrosive to most common metals and alloys. but beyond about 10 ppm. Rapid corrosion occurs at pH 4 or 5. Sterling Alloy R48: This alloy works very well at the same Types 316SS and Sterling Alloy K26: temperatures and concentrations as K26. such as shafts and piping. If this alloy chromium in this alloy. including K26 less expensive and has the added advantage are to be used only at very low concentrations of handling abrasives better than K26. are concentrations at which the stainless steels now available. the copper content must be aeration and oxidizing impurities such as nitric controlled to extremely low values. at room temperature. or below. This is excellent in hot concentrated pure phosphoric acid. that dissolved oxygen is not strong Sterling Alloy R52: enough to passivate the material. start to corrode. acid present a very rugged corrosive They are generally not considered to be condition. acid or ferric chloride (when present even in small quantities) is often destructive. It must be kept in mind. they the boiling point. only when specific conditions are definitely known. of the equipment. Pickling solutions which are HYDROCHLORIC ACID sometimes handled by these materials require This is the most difficult of acids to handle inhibitors if the pump is to be handling the from a standpoint of corrosion. Due to its high chromium content. its resistance to must be used. Sterling Alloy R53: This alloy shows good resistance to all Titanium: concentrations of hydrochloric acid at room This alloy is good up to 10% at room temperature and has been used successfully temperature. Increasing the Wrought materials used in conjunction with temperature decreases the critical R48 castings. 17 . it provides better resistance to oxidizing environments. Due to the absence of tend to increase the corrosion rate. The presence of ferric and cupric chlorides actually decreases the corrosion rate of titanium. is The austenitic stainless steels. copper ions (an Sterling Alloy R52: impurity) behave somewhat differently in R52 is widely used to handle hydrochloric acid solution. suitable for hydrochloric acid service because Great care and good judgement is required to they are susceptible to influences other than obtain a balance between service life and cost the acid itself and must be used with caution. Hydrochloric is liquid on a continuous basis. however. 18 . to acetic acid at all concentrations and This increased corrosivity can normally be temperatures. Acetic anhydride usually will increase the corrosive attack.CORROSION BY ACIDS :. especially when the acetic acid is at 100% concentration.Continued ACETIC ACID Chlorides in the stream have been known to Acetic acid is an intermediate chemical used in cause stress corrosion cracking of the the production of cellulose acetate for paints austenitic stainless steels: R53 is usually and acetic anhydride for artificial fibres. formic acid and other solvents which are typical of this process. especially in the case of 304. it has specific problems with corrosion and contamination of the acid. Contaminants such as sulphur dioxide and environment. Various processes are used to produce the If there is a possibility of temperature increase. is often recommended as a qualification test to determine the sensitivity of The effect of contaminants is twofold: the alloy to attack by an acetic acid 1. sulphur trioxide increase the corrosion rate. R52 and R53 both provide excellent resistance Formic acid also increases the corrosion rate. Butane Oxidation: In selection of materials of construction you must consider formation of peroxides. expensive than type 316 stainless steel and 2. B. The presence of aldehydes. formation of peroxides and acetic temperature and time of exposure is anhydride. In this instance. acids. thus a service life-cost justification should esters in the process stream have been known be done before Sterling types R52 and R53 to greatly reduce the corrosion rate. increased. Acetaldehyde Process: The most widely should be evaluated to prevent excessive used process. are utilised. ketones and K26. The regard to material selection due to catalyst iron contamination is greatly increased as the carry-overs. K26 is well-utilised. recommended if this is the case. These alloys are more tolerated by type 316 stainless steel. Ferroxyl testing of pumps servicing a meticulous grade of acetic acid is often done before shipment. Practice “A” of ASTM A262 (Oxalic Acid Etch). which include: the low carbon grades such as 316L and 304L A. whereas nickel and nickel alloys are this alloy is to be used with nickel and nickel- used at higher temperatures. at concentrations determines the purity that is obtainable. and including molten caustic soda.CORROSION BY ALKALIS Of all the available alkaline materials. It is produced along with chlorine by the such as types 304 and 316 are used up to 10% electrolysis of sodium chloride. The type of concentration and up to the boiling point of electrolytic cell used for production caustic soda. In iron are both used where minimum such instances. Oxidizable sulphur compounds also tend to increase the corrosion rate of cast nickel at elevated temperatures. cast nickel does an Sterling Alloy R55: excellent job. When temperatures above This alloy is used in similar situations to K26. above 10% the critical temperature decreases. In concentrations above 75%. These alloys may be used up to 70% agents such as sucrose or dextrim may be caustic soda concentrations. caustic Types 304 and 316 Stainless Steels: soda (sodium hydroxide) is the most widely The cast versions of austenitic stainless steels used. Stress relief of added to minimise corrosion and product these alloys may help minimise stress corrosion contamination. chemical. which is further purified cracking of these alloys and consideration before sale. They have been used up to 50% at the boiling point. caustic soda produced by contamination of the product by copper is other methods should be utilised. It o 320 C are to be considered. the cast nickel provides better stress corrosion cracking pump castings should be solution-annealed to resistance than K26. Galvanic effects must be considered if detrimental). Mercuric cells produce 50% grade caustic. cracking of these alloys. minimise the possibility of graphite precipitation at grain boundaries and a Sterling Alloy R52 and R53: resultant loss in ductility. The thermal decomposition at 260oC of impurities such as chlorates (present in caustic Ni-Resist Type 2 Cast Iron: soda produced by the diaphragm cell method) Ni-Resist Type 2 cast iron and spheroidal nickel increases the corrosion rate of cast nickel. Chlorides in the process stream have been whereas the diaphragm cells produce 9% to known to contribute to stress corrosion 15% grade caustic. pulp and paper. must be given to the stress and temperature The major users of caustic soda are the limitations of these alloys. The data available for these alloys is not Velocity and aeration have little effect. 19 . However. or reducing desired. Sterling Alloy K26: Pumps made of this material have been used Iron and steel are widely used at low for handling caustic soda up to 70% and temperatures (if iron contamination is not 120oC. sufficient to make any indicative statements. based alloys. except at high temperatures such as above 540oC. and aluminium industries. besides being hazardous. Types 304 and 316 Stainless Steels: These alloys may be used to pump sodium and sodium-potassium mixtures. leading to destruction. The liquid metals and fused salts employed are usually high thermal conductors. dissolves in the liquid metal. In addition. care should be taken to prevent Liquid metals cause different types of carburisation by carbonaceous material. where its solubility is less. They do not undergo as many rupture failures as do the nickel chromium steels. They have a temperature limitation of 540oC. For example. The tendencies. In the mercury. the lower is the corrosion rate. This Grey cast iron: is also good for some of these liquid metals. mercury requires very high pumping power. the lack of electrochemical reactions. Water and steam require high pressure equipment which. bismuth and bismuth-lead alloys up simplest type of attack. they save a great deal in initial heat-up during the start-up of a power plant. thallium. and This kind of uniform thinning also occurs when solid metal may dissolve at a hot zone of a pump and precipitate on the walls of a cool zone. This causes rapid dissolution without saturation. and the activity of the solid efficiency of a power plant is increased by metal ions. the greater is the amount of solid metal that can be held in solution. Contact of dissimilar metals with the same liquid metal can cause transfer of one solid metal through the liquid metal to the other solid metal. the wetting for use in the power plant industries. If used intermittently. resulting in uniform thinning or preferential leaching of a selective constituent from the solid metal. The most salient feature is of these alloys can handle lithium. . operating at higher temperatures. such as cadmium and Bi-Pb-Sn alloys. tin.CORROSION BY LIQUID METALS Heat-transfer characteristics of low boiling Finally. whereas sodium requires low pumping power. 20 their alloys. bismuth. This is because the greater the liquid volume. impurities such as dissolved gases can point metals make them particularly attractive change the solubility limits. the solid metal to various temperatures. The greater the ratio. they require lower pumping power due to their lower density. is also expensive. dissolution may result in the formation of Cast nickel: possesses the greatest resistance brittle alloy phases. Due to The surface area to volume ratio is of utmost importance. to stress cracking in lead. their low melting points. Both corrosive attack. Mo 4%. 21 Titanium Titanium Alloy Zirconium N270 Rubber Lined RT6 Rubber lined TITANIUM TITANIUM ALLOY ZIRCONIUM RUBBER LINED RUBBER LINED Ebonite (rigid) Ebonite (flexible) Hf 4. The information is advisory only and the use of the materials and methods is solely at the risk of the user.08% max.08% max. Sn 7%.08% max.6026 GERMANY:W-Nr G-X 3 CrNiMoCuN 26 6 3 G-X 2 NiCrMoCu 25 20 G-X5 CrNiMoNb 18 10 X 5 CrNiMo 17 12 2 G-X 2 CrNiMo 18 10 G-X 5 CrNiNb 18 9 G-X6CrNi 18 91 X 2 CrNi 19 11 GERMANY:DIN .08% max.4% BS1400 LG4 C . Mo 28%. Cr 17%. Cr 18%. Cr 23%.4401 1.4552 1.03% max. Nb 12xC Standard Material TYPE AUSTENITIC (ALLOY 20 TYPE) STERLING ALLOY K26 STERLING MATERIAL B752 702C/704C/705C B367 (Ti-Pd 8A) B367 (C-3) NONE A494 CW-12MV A494 N-12MV A351/A743/A744 CD4-MCU A351/A743/A744 CN-7M A351/A743/A744 CF-8M A351/A743/A744 CF-8M A351/A743/A744 CF-3M A351/A743/A744 CF-8 A351/A743/A744 CF-8 A351/A743/A744 CF-3 A216 WCA/B/C A436 Type 2 A395 A278 Class 35/40 USA:ASTM Type 904 Type 316 Type 316 Type 316L Type 347 Type 304 Type 304L Carbon Steel USA:AISI COMPARABLE MATERIAL DESIGNATIONS GX6 CrNiMoNb 20 11 X5 CrNi Mo17 12 GX2 CrNiMo 19 11 X6 CrNi N6 18 11 X5 CrNi 18 10 GX2 CrNi 19 10 FEG42 GS400-12 ITALY:UNI Z3 CNDU 26-05-M Z6 NCDU 25-20-04M GX5 NiCrCuMo 29 21 Z4 CNDNb 18-12M Z7 CND 17-12-02 Z3 CND 18-12-02 Z6 CNNb 18-10M Z6 CN 18-10M Z2 CN 18-10M A420 ARM FGS 400-15 FRANCE:AFNOR SCS 22 SCS 14 / SUS 316 SCS 16 / SUS 316L SCS 21 / SUS 347 SCS 13 / SUS 304 SCS 19 / SUS 304L JAPAN:JIS STERLING FLUID SYSTEMS PUMP MATERIALS OF CONSTRUCTION 3.4810 1. Ni 8% STERLING ALLOY R52 BS1504 Gr304c15 AUSTENITIC (304) Ni-Cr-Mo (Hastelloy C type) BS1504 Gr304c12 AUSTENITIC (304L) C 0.4308 1. V . Nb 8xC Ni-Cr-Mo-Cu-Si ALLOY BS1504 Gr316C12 AUSTENITIC (316L) STERLING ALLOY R53 BS1504 Gr347c17 AUSTENITIC (304 + Nb) C .1090 2.4819 2. Ni 5%. Cr 22%.7055 2.4404 1. Cr 18%. Ni 10% Mo 2% C . Cu 3%. Cr 18%.4515 1.Neither Sterling Fluid Systems. Ni 10% Mo 2% Nb 8xC LEADED GUNMETAL BS1504 Gr318C17 AUSTENITIC (316 + Nb) C .12% min.25% max Ni-Mo (Hastelloy B type) BS1504 Gr161-430 CARBON STEEL Ni 20%.5% Pd 0.7040 0.4536 1.4% Ni 62%. Mo 2% BS3468 L-Ni Cr 20 2 NI-RESIST STERLING ALLOY R48 BS2789 Gr420/12 SG IRON (DUCTILE) DUPLEX STAINLESS BS1452 Gr 250 GRAY IRON SHORT FORM COMPOSITION Ni 26%.03% max. Zn 3% Ni 58%. Cr 2% Cr 26%. W 4%. Commercially pure Cu bal.4581 1. Cr 18%. nor any of its officers. Cr 18%. Ni 8%. Si 4%. W 1% Ni 54%.0446 0. Ni 10% Mo 2% STERLING ALLOY R55 BS1504 Gr316C16 AUSTENITIC (316) C . Cu 4%.6660 0. Ni 8% C . Mo17%. Cr 18%. Mo 3%.4306 1. directors or employees accept any responsibility for the use of the methods and materials discussed herein. Reproduction of the contents in whole or part or transfer into electronic or photographic storage without permission of copyright owner is expressly forbidden. Cu2%. V 0. Pb 4%.