Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd Introduction CORROSION is the deterioration of a material by a reaction with its environment. In a study by the U.s Federal Highway Administration in cooperation with the National Association of Corrosion Engineers(NACE), it was estimated that the annual cost of corrosion was between $121 and $138 billion in 1998 in the United States. These costs included cost of corrosion-control methods, equipment, and services; cost of labor attributed to corrosion management; cost of use of more expensive materials to lessen corrosion damage; and cost of lost revenue, loss of reliability, and loss of capital due to corrosion deterioration. Only selected industrial sectors were analyzed in the study. When extrapolated to all U.S. industries, the total cost estimate is $276 billion, or more than 3% of the U.S. gross domestic product(Ref 1). This great cost is a measure of the importance of corrosion management and an indication of the significance of potential cost saving that corrosion abatement can yield Reference cited in this section 1."Corrosion Costs and Preventive Strategies in the United States, FHWA-RD-01-156, Federal Highway Administration 2002 Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy Ltd Electrochemical Nature of Corrosion The articles in this Section are devoted to the identification and analysis of corrosion- related failures, the categorization of corrosion failures by form and mechanism, and the application of preventive measures. The mechanisms of corrosion are described in more detail in Corrosion, Volume 13 of the asm handbook However, as a brief introduction, the electrochemical nature of corrosion can be illustrated by the attack on zinc by hydrochloric acid. When zinc is placed in dilute hydrochloric acid, a vigorous reaction occurs; hydrogen gas is evolved and the zinc dissolves, forming an acidic aqueous solution of zinc chloride. The reaction is Zn+2HCl→ZnCl2+H (Eq I Since the chloride ion is not involved in the reaction, this equation can be written in the simplified form Zn+2H+→Zn2++H2 (Eq2) Zinc reacts with the hydrogen ions of the acid solution to form zinc ions and hydrogen gas. Equation 2 shows that during the reaction, zinc is oxidized to zinc ions and hydrogen ions are reduced to hydrogen. Thus, eq 2 can be conveniently divided into two reactions: the oxidation of zinc and the reduction of hydrogen ions Oxidation(anodic reaction)Zn-Zn+2 Reduction(cathodic reaction) 2H 2e H2
Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd. Introduction CORROSION is the deterioration of a material by a reaction with its environment. In a study by the U.S. Federal Highway Administration in cooperation with the National Association of Corrosion Engineers (NACE), it was estimated that the annual cost of corrosion was between $121 and $138 billion in 1998 in the United States. These costs included cost of corrosion-control methods, equipment, and services; cost of labor attributed to corrosion management; cost of use of more expensive materials to lessen corrosion damage; and cost of lost revenue, loss of reliability, and loss of capital due to corrosion deterioration. Only selected industrial sectors were analyzed in the study. When extrapolated to all U.S. industries, the total cost estimate is $276 billion, or more than 3% of the U.S. gross domestic product (Ref 1). This great cost is a measure of the importance of corrosion management and an indication of the significance of potential cost saving that corrosion abatement can yield. Reference cited in this section 1. “Corrosion Costs and Preventive Strategies in the United States,” FHWA-RD-01-156, Federal Highway Administration, 2002 Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd. Electrochemical Nature of Corrosion The articles in this Section are devoted to the identification and analysis of corrosion-related failures, the categorization of corrosion failures by form and mechanism, and the application of preventive measures. The mechanisms of corrosion are described in more detail in Corrosion, Volume 13 of the ASM Handbook. However, as a brief introduction, the electrochemical nature of corrosion can be illustrated by the attack on zinc by hydrochloric acid. When zinc is placed in dilute hydrochloric acid, a vigorous reaction occurs; hydrogen gas is evolved and the zinc dissolves, forming an acidic aqueous solution of zinc chloride. The reaction is: Zn + 2HCl → ZnCl2 + H2 (Eq 1) Since the chloride ion is not involved in the reaction, this equation can be written in the simplified form: Zn + 2H+ → Zn2+ + H2 (Eq 2) Zinc reacts with the hydrogen ions of the acid solution to form zinc ions and hydrogen gas. Equation 2 shows that during the reaction, zinc is oxidized to zinc ions and hydrogen ions are reduced to hydrogen. Thus, Eq 2 can be conveniently divided into two reactions: the oxidation of zinc and the reduction of hydrogen ions: Oxidation (anodic reaction) Zn → Zn2+ + 2e (Eq 3) Reduction (cathodic reaction) 2H+ + 2e → H2 (Eq 4)
An oxidation or anodic reaction is indicated by an increase in valence or a release of electrons. a decrease in valence charge or the consumption of electrons signifies a reduction or cathodic reaction. Equations 3 and 4 are partial reactions; both must occur simultaneously and at the same rate on the metal surface. If this were not true, the metal would spontaneously become electrically charged, which is clearly impossible The corrosion of zinc in hydrochloric acid is an electrochemical process. That is, any reaction that can be divided into two or more partial reactions of oxidation and reduction due to the transfer of electrical charge is termed electrochemical. Dividing corrosion or other electrochemical reactions into partial reactions makes them simpler to understand. Iron and aluminum, like zinc, are also rapidly corroded by hydrochloric acid Thus, the problem of hydrochloric acid corrosion is simplified because in every case the cathodic reaction is the evolution of hydrogen gas according to Eq 4. This also applies to corrosion in other acids such as sulfuric, phosphoric, hydrofluoric, and water-soluble organic acids such as formic and acetic. In each case, only the hydrogen ion is active, the other ions such as sulfate, phosphate, and acetate do not participate in the electrochemical reaction When viewed from the standpoint of partial processes of oxidation and reduction, all corrosion can be classified into a few generalized reactions. The anodic reaction in every corrosion reaction is the oxidation of a metal to its ion. This can be written in the general form where n is the number of electrons released A few examples are Ag→Ag+e Zn→Zn2++2e (Eq7) Al→Al+3e In each case the number of electrons produced equals the valence of the ion There are several different cathodic reactions that are frequently encountered in metallic corrosion. The most common cathodic reactions are: Hydrogen evolution 2H+2 H (Eq9) Oxygen reduction(acid solutions) O2+4H+4e→2H2O (Eq10) Oxygen reduction(neutral or basic solutions) O2+2H2O+4e→4OH Metal-ion reduction Metal deposition M+e→M (Eq13) Hydrogen evolution is a common cathodic reaction because acid or acidic media are frequently encountered Oxygen reduction is very common, because any aqueous solution in contact with air is capable of producing this reaction. Metal-ion reduction and metal deposition are less-common reactions and are most frequently found in chemical process streams. All of the above reactions are quite similar; they consume electrons The above partial reactions can be used to interpret virtually all electrochemical corrosion problems. Consider what happens when iron is immersed in water or seawater that is exposed to the atmosphere(an automobile fender or a steel pier piling are examples). Corrosion occurs. The anodic reaction is (Eq14) Since the medium is exposed to the atmosphere, it contains dissolved oxygen. Water and seawater are nearly neutral. and thus the cathodic reaction is: O2+2H2O+4e→4OH (Eq15) Remembering that sodium and chloride ions do not participate in the reaction, the overall reaction can be obtained by adding eq 9 and 12 Thefileisdownloadedfromwww.bzfxw.com
An oxidation or anodic reaction is indicated by an increase in valence or a release of electrons. A decrease in valence charge or the consumption of electrons signifies a reduction or cathodic reaction. Equations 3 and 4 are partial reactions; both must occur simultaneously and at the same rate on the metal surface. If this were not true, the metal would spontaneously become electrically charged, which is clearly impossible. The corrosion of zinc in hydrochloric acid is an electrochemical process. That is, any reaction that can be divided into two or more partial reactions of oxidation and reduction due to the transfer of electrical charge is termed electrochemical. Dividing corrosion or other electrochemical reactions into partial reactions makes them simpler to understand. Iron and aluminum, like zinc, are also rapidly corroded by hydrochloric acid. Thus, the problem of hydrochloric acid corrosion is simplified because in every case the cathodic reaction is the evolution of hydrogen gas according to Eq 4. This also applies to corrosion in other acids such as sulfuric, phosphoric, hydrofluoric, and water-soluble organic acids such as formic and acetic. In each case, only the hydrogen ion is active, the other ions such as sulfate, phosphate, and acetate do not participate in the electrochemical reaction. When viewed from the standpoint of partial processes of oxidation and reduction, all corrosion can be classified into a few generalized reactions. The anodic reaction in every corrosion reaction is the oxidation of a metal to its ion. This can be written in the general form: M → M n+ + ne (Eq 5) where n is the number of electrons released. A few examples are: Ag → Ag+ + e (Eq 6) Zn → Zn2+ + 2e (Eq 7) Al → Al3+ + 3e (Eq 8) In each case the number of electrons produced equals the valence of the ion. There are several different cathodic reactions that are frequently encountered in metallic corrosion. The most common cathodic reactions are: Hydrogen evolution: 2H+ + 2e → H2 (Eq 9) Oxygen reduction (acid solutions): O2 + 4H+ + 4e → 2H2O (Eq 10) Oxygen reduction (neutral or basic solutions): O2 + 2H2O + 4e → 4OH- (Eq 11) Metal-ion reduction: M 3+ + e → M 2+ (Eq 12) Metal deposition: M + + e → M (Eq 13) Hydrogen evolution is a common cathodic reaction because acid or acidic media are frequently encountered. Oxygen reduction is very common, because any aqueous solution in contact with air is capable of producing this reaction. Metal-ion reduction and metal deposition are less-common reactions and are most frequently found in chemical process streams. All of the above reactions are quite similar; they consume electrons. The above partial reactions can be used to interpret virtually all electrochemical corrosion problems. Consider what happens when iron is immersed in water or seawater that is exposed to the atmosphere (an automobile fender or a steel pier piling are examples). Corrosion occurs. The anodic reaction is: Fe → Fe2+ + 2e (Eq 14) Since the medium is exposed to the atmosphere, it contains dissolved oxygen. Water and seawater are nearly neutral, and thus the cathodic reaction is: O2 + 2H2O + 4e → 4OH- (Eq 15) Remembering that sodium and chloride ions do not participate in the reaction, the overall reaction can be obtained by adding Eq 9 and 12: The file is downloaded from www.bzfxw.com
2Fe+2H2O+O2→2Fe2++4OH→2FeOH2 Ferrous(Fet) hydroxide precipitates from solution. However, this compound is unstable in oxygenated solutions and is oxidized to the ferric(Fe)salt 2Fe(OH)2 +H2O+-O2- 2Fe(OH) The final product is the familiar rust The classic example of a replacement reaction, the interaction of zinc with copper sulfate solution, illustrates metal deposition Zn + Cu Zn-+ Cu (Eq18) or, viewed as partial reactions Zn→Zn2++2e The zinc initially becomes plated with copper and eventually, given enough time and reactants, the products are copper sponge and zinc sulfate solution During corrosion, more than one oxidation and one reduction reaction may occur. when an alloy is corroded its component metals go into solution as their respective ions. More importantly, more than one reduction reaction can occur during corrosion. Consider the corrosion of zinc in aerated hydrochloric acid. Two cathodic reactions are possible: the evolution of hydrogen and the reduction of oxygen. Since the rates of oxidation and solutions containing dis c Increasing the total reduction rate increases the rate of zinc solution. Therefore, acid reduction must be equal solved oxygen normally will be more corrosive than air-free acids. Oxygen reduction simply provides another means of electron disposal. The same effect is observed if any oxidizer is present in acid solutions. A frequent impurity in commercial hydrochloric acid is the ferric ion(Fe), present as ferric chloride. Metals corrode much more rapidly in such impure acid because there are two cathodic reactions hydrogen evolution and ferric ion reduction q21) The anodic and cathodic reactions occurring during corrosion are mutually dependent, and it is possible to reduce corrosion by reducing the rates of either reaction. In the above case of impure hydrochloric acid, it can be made less corrosive by removing the ferric ions and consequently reducing the total rate of cathodic reduction. Oxygen reduction is eliminated by preventing air from contacting the aqueous solution or by removing air that has been dissolved. Iron is nearly inert in air-free water or seawater because there is limited athodic reaction possible If the surface of the metal is coated with paint or other nonconducting film, the rates of both anodic and cathodic reactions will be greatly reduced and corrosion will be retarded. A corrosion inhibitor is a substance that, when added in small amounts to a corrosive, reduces its corrosivity. Corrosion inhibitors function by interfering with either the anodic or cathodic reactions, or both. Many of these inhibitors are organic compounds; they function by forming an impervious film on the metal surface or by interfering with either the anodic or cathodic reactions. High-molecular-weight amines retard the hydrogen-evolution reaction and subsequently reduce corrosion rate. Adequate conductivity in both the metal and the electrolyte is required for continuation of the corrosion reaction. Of course, it is not practical to increase the electrical resistance of the metal because the sites of the anodic and cathodic reactions are not known, nor are they predictable. However it is possible to increase the electrical resistance of the electrolyte and thereby reduce corrosion. Very pure water is much less corrosive than impure or natural waters. The low corrosivity of high-purity water is due to its high electrical resistance and few reducible cations Passivity. Essentially, passivity refers to the loss of chemical reactivity experienced by certain metals and alloys under particular environmental conditions. That is, certain metals and alloys become essentially inert and act as if they were noble metals such as platinum and gold. Fortunately, from an engineering standpoint, the metals most susceptible to this kind of behavior are the common engineering and structural materials, including iron metals such as zinc, cadmium, tin, uranium, and thorium have also been observed to exhibit passivity effect r nickel, silicon, chromium, titanium, and alloys containing these metals. Also, under limited conditions, othe
2Fe + 2H2O + O2 → 2Fe2+ + 4OH- → 2Fe(OH)2 (Eq 16) Ferrous (Fe2+) hydroxide precipitates from solution. However, this compound is unstable in oxygenated solutions and is oxidized to the ferric (Fe3+) salt: 2Fe(OH)2 + H2O + 1 2 O2 → 2Fe(OH)3 (Eq 17) The final product is the familiar rust. The classic example of a replacement reaction, the interaction of zinc with copper sulfate solution, illustrates metal deposition: Zn + Cu2+ → Zn2+ + Cu (Eq 18) or, viewed as partial reactions: Zn → Zn2+ + 2e (Eq 19) Cu2+ + 2e → Cu (Eq 20) The zinc initially becomes plated with copper and eventually, given enough time and reactants, the products are copper sponge and zinc sulfate solution. During corrosion, more than one oxidation and one reduction reaction may occur. When an alloy is corroded, its component metals go into solution as their respective ions. More importantly, more than one reduction reaction can occur during corrosion. Consider the corrosion of zinc in aerated hydrochloric acid. Two cathodic reactions are possible: the evolution of hydrogen and the reduction of oxygen. Since the rates of oxidation and reduction must be equal, increasing the total reduction rate increases the rate of zinc solution. Therefore, acid solutions containing dissolved oxygen normally will be more corrosive than air-free acids. Oxygen reduction simply provides another means of electron disposal. The same effect is observed if any oxidizer is present in acid solutions. A frequent impurity in commercial hydrochloric acid is the ferric ion (Fe3+), present as ferric chloride. Metals corrode much more rapidly in such impure acid because there are two cathodic reactions, hydrogen evolution and ferric ion reduction: Fe3+ + e → Fe2+ (Eq 21) The anodic and cathodic reactions occurring during corrosion are mutually dependent, and it is possible to reduce corrosion by reducing the rates of either reaction. In the above case of impure hydrochloric acid, it can be made less corrosive by removing the ferric ions and consequently reducing the total rate of cathodic reduction. Oxygen reduction is eliminated by preventing air from contacting the aqueous solution or by removing air that has been dissolved. Iron is nearly inert in air-free water or seawater because there is limited cathodic reaction possible. If the surface of the metal is coated with paint or other nonconducting film, the rates of both anodic and cathodic reactions will be greatly reduced and corrosion will be retarded. A corrosion inhibitor is a substance that, when added in small amounts to a corrosive, reduces its corrosivity. Corrosion inhibitors function by interfering with either the anodic or cathodic reactions, or both. Many of these inhibitors are organic compounds; they function by forming an impervious film on the metal surface or by interfering with either the anodic or cathodic reactions. High-molecular-weight amines retard the hydrogen-evolution reaction and subsequently reduce corrosion rate. Adequate conductivity in both the metal and the electrolyte is required for continuation of the corrosion reaction. Of course, it is not practical to increase the electrical resistance of the metal because the sites of the anodic and cathodic reactions are not known, nor are they predictable. However, it is possible to increase the electrical resistance of the electrolyte and thereby reduce corrosion. Very pure water is much less corrosive than impure or natural waters. The low corrosivity of high-purity water is due to its high electrical resistance and few reducible cations. Passivity. Essentially, passivity refers to the loss of chemical reactivity experienced by certain metals and alloys under particular environmental conditions. That is, certain metals and alloys become essentially inert and act as if they were noble metals such as platinum and gold. Fortunately, from an engineering standpoint, the metals most susceptible to this kind of behavior are the common engineering and structural materials, including iron, nickel, silicon, chromium, titanium, and alloys containing these metals. Also, under limited conditions, other metals such as zinc, cadmium, tin, uranium, and thorium have also been observed to exhibit passivity effects
Passivity, although difficult to define, can be quantitatively described by characterizing the behavior of metals that show this unusual effect. first consider the behavior of what can be called an active metal that is a metal that does not show passivity effects. The lower part of the curve in Fig. I illustrates the behavior of such a metal. Assume that a metal is immersed in an air-free acid solution with an oxidizing power corresponding to 6,int a and a corrosion rate corresponding to this point. If the oxidizing power of this solution is increased, say, by adding oxygen or ferric ions, the corrosion rate of the metal will increase rapidly. Note that for such a metal the corrosion rate increases as the oxidizing power of the solution increases. This increase in rate is exponential and yields a straight line when plotted on a semilogarithmic scale as in Fig. 1. The oxidizing power of the solution is controlled by both the specific oxidizing power of the reagents and the concentration of these reagents. Oxidizing power can be precisely defined by electrode potential, but is beyond the scope of this discussion Transpassive 6asN8oco60 Passive Active 100010.000 Corrosion rate Fig. 1 Corrosion characteristics of an active-passive metal as a function of solution oxidizing power(electrode potential) The behavior of this metal or alloy can be conveniently divided into three regions: active, passive, and transpassive. In the active region, slight increases in the oxidizing power of the solution cause a corresponding rapid increase in the corrosion rate. However, at some point, if more oxidizing agent is added the corrosion rate shows a sudden decrease. This corresponds to the beginning of the passive region. Further increases in oxidizing agents produce little if any change in the corrosion rate of the material in the passive region. Finally at very high concentrations of oxidizers or in the presence of very powerful oxidizers, the corrosion rate again increases with increasing oxidizing power. This region is termed the transpassive region It is important to note that during the transition from the active to the passive region, a 10 to 10 reduction in corrosion rate is usually observed. Passivity is due to the formation of a surface film or protective barrier that is stable over a considerable range of oxidizing power and is eventually destroyed in strong oxidizing solutions Under conditions in which the surface film is stable. the anodic reaction is stifled and the metal surface is protected from corrosion. For example, stainless steel owes its corrosion-resistant properties to a passive surface film. The naturally occurring passive film is usually enhanced with immersion in a hot nitric acid solution or steam. For example, stainless steel surgical implants develop this passive layer when the implants are sterilized in steam The exact nature of this barrier that forms on the metal surface is not well understood. It nay be a very thin, transparent oxide film or a layer of adsorbed oxygen atoms. However, for the purposes of engineering application, it is not necessary to understand completely the mechanism. To summarize, metals that possess an active-passive transition become passive(very corrosion-resistant) in moderately to strongly oxidizing environments. Under extremely strong oxidizing conditions, these materials lose their corrosion- resistant properties Thefileisdownloadedfromwww.bzfxw.com
Passivity, although difficult to define, can be quantitatively described by characterizing the behavior of metals that show this unusual effect. First, consider the behavior of what can be called an active metal, that is, a metal that does not show passivity effects. The lower part of the curve in Fig. 1 illustrates the behavior of such a metal. Assume that a metal is immersed in an air-free acid solution with an oxidizing power corresponding to point A and a corrosion rate corresponding to this point. If the oxidizing power of this solution is increased, say, by adding oxygen or ferric ions, the corrosion rate of the metal will increase rapidly. Note that for such a metal, the corrosion rate increases as the oxidizing power of the solution increases. This increase in rate is exponential and yields a straight line when plotted on a semilogarithmic scale as in Fig. 1. The oxidizing power of the solution is controlled by both the specific oxidizing power of the reagents and the concentration of these reagents. Oxidizing power can be precisely defined by electrode potential, but is beyond the scope of this discussion. Fig. 1 Corrosion characteristics of an active-passive metal as a function of solution oxidizing power (electrode potential) The behavior of this metal or alloy can be conveniently divided into three regions: active, passive, and transpassive. In the active region, slight increases in the oxidizing power of the solution cause a corresponding rapid increase in the corrosion rate. However, at some point, if more oxidizing agent is added, the corrosion rate shows a sudden decrease. This corresponds to the beginning of the passive region. Further increases in oxidizing agents produce little if any change in the corrosion rate of the material in the passive region. Finally, at very high concentrations of oxidizers or in the presence of very powerful oxidizers, the corrosion rate again increases with increasing oxidizing power. This region is termed the transpassive region. It is important to note that during the transition from the active to the passive region, a 103 to 106 reduction in corrosion rate is usually observed. Passivity is due to the formation of a surface film or protective barrier that is stable over a considerable range of oxidizing power and is eventually destroyed in strong oxidizing solutions. Under conditions in which the surface film is stable, the anodic reaction is stifled and the metal surface is protected from corrosion. For example, stainless steel owes its corrosion-resistant properties to a passive surface film. The naturally occurring passive film is usually enhanced with immersion in a hot nitric acid solution or steam. For example, stainless steel surgical implants develop this passive layer when the implants are sterilized in steam. The exact nature of this barrier that forms on the metal surface is not well understood. It may be a very thin, transparent oxide film or a layer of adsorbed oxygen atoms. However, for the purposes of engineering application, it is not necessary to understand completely the mechanism. To summarize, metals that possess an active-passive transition become passive (very corrosion-resistant) in moderately to strongly oxidizing environments. Under extremely strong oxidizing conditions, these materials lose their corrosionresistant properties. The file is downloaded from www.bzfxw.com
Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy Ltd Analysis of Corrosion-Related Failures Corrosion can be categorized as being uniform or localized Uniform corrosion is the most common form of corrosion. Its mechanism is electrochemical and is identified as a consistent loss of material over the entire exposed surface of the material in question. This is the type of corrosion that affects the greatest number of structures, such as buildings, bridges, pipelines, and outdoor equipment. It attacks the most surface area and the most costly. Uniform corrosion is also the easiest to analyze, to predict corrosion rates for, and to establish preventive measures and maintenance schedules for. Preventive techniques include painting(coatings) inhibition, and cathodic or anodic protection. The service life of the affected component can normally be estimated with a reasonable degree of accuracy, and catastrophic failures can be avoided Localized corrosion, such as crevice and pitting corrosion, intergranular corrosion, selective leaching, erosion- corrosion, and stress-corrosion cracking (SCC), act on a small portion of a component. The rate of localized corrosion is often orders of magnitude greater than that of uniform corrosion. Evidence of localized corrosion by virtue of its physical size and location, is often difficult to detect. These factors combine to make localized corrosion mechanisms insidious. If undetected they can lead to catastrophic system failures Certain environments induce specific corrosion failure modes. Hydrogen damage, liquid- and solid-metal- induced embrittlement, biologically and microbiologically influenced corrosion(MIC), and high-temperature corrosion are also addressed in articles in this section While categorizing local corrosion by form, mechanism, or environment aids understanding, failures are often attributed to a combination of modes. The combination of failure modes makes prediction of localized corrosion failures more difficult Corrosion failure involves the same general steps as other failures, as described in the article" Practices Failure Analysis"in this Volume. However, one potential difference between a general failure and a failure related to corrosion is the need with the latter for immediate preservation and protection of all evidence Corrosion failure also may require sampling and testing of corrosion products immediately, such as in the case of MIC, where viable cultures can provide the most meaningful result. When possible and cost effective, a site visit is most desirable. A site visit may provide the investigator with information that otherwise may have been omitted or overlooked Corrosion failures often relate to the material selection and the environment. details of the material pecifications, quality-assurance records, installation and maintenance records, and a history of the environment are all useful resources in resolving corrosion failures. Information regarding system upsets or diversions from the normal operating environment should be provided. A comparison of the actual material utilized with the material design specifications should also be performed History of Failed Parts. Obtaining a history of the operation of the failed component is crucial to solving the cause of failure. The operating environment, any changes in the environment, and temperature excursions should all be obtained. Any notation of previous failures or operating anomalies is useful. When possible, engineering drawings and sketches should be reviewed Information should be provided regarding any testing that was performed by the plant personnel. For instance, liquid-penetrant examination can result in chemical contamination of a surface. The use of paints or dyes to mark components may also alter the corrosion resistance of a part and the chemical composition of a corrosion product. It is also important that the investigator retains pertinent corrosion evidence before movement or disassembly of a component. The chain of events leading to, and occurring after, the failure should be documented On-Site Examination and Sampling. On-site examination should include a walking tour of the failure area Photographic documentation should be made to depict the conditions after the failure. If applicable, viewing a similar undamaged assembly or operation on the site where the failure occurred may be instructive Photographic documentation must be performed with special attention given to capturing the true colors of the corrosion products. Macrographs of the corrosion deposit should include any layering effects in the deposits
Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd. Analysis of Corrosion-Related Failures Corrosion can be categorized as being uniform or localized. Uniform corrosion is the most common form of corrosion. Its mechanism is electrochemical and is identified as a consistent loss of material over the entire exposed surface of the material in question. This is the type of corrosion that affects the greatest number of structures, such as buildings, bridges, pipelines, and outdoor equipment. It attacks the most surface area and is the most costly. Uniform corrosion is also the easiest to analyze, to predict corrosion rates for, and to establish preventive measures and maintenance schedules for. Preventive techniques include painting (coatings), inhibition, and cathodic or anodic protection. The service life of the affected component can normally be estimated with a reasonable degree of accuracy, and catastrophic failures can be avoided. Localized corrosion, such as crevice and pitting corrosion, intergranular corrosion, selective leaching, erosioncorrosion, and stress-corrosion cracking (SCC), act on a small portion of a component. The rate of localized corrosion is often orders of magnitude greater than that of uniform corrosion. Evidence of localized corrosion, by virtue of its physical size and location, is often difficult to detect. These factors combine to make localized corrosion mechanisms insidious. If undetected they can lead to catastrophic system failures. Certain environments induce specific corrosion failure modes. Hydrogen damage, liquid- and solid-metalinduced embrittlement, biologically and microbiologically influenced corrosion (MIC), and high-temperature corrosion are also addressed in articles in this Section. While categorizing local corrosion by form, mechanism, or environment aids understanding, failures are often attributed to a combination of modes. The combination of failure modes makes prediction of localized corrosion failures more difficult. Corrosion failure involves the same general steps as other failures, as described in the article “Practices in Failure Analysis” in this Volume. However, one potential difference between a general failure and a failure related to corrosion is the need with the latter for immediate preservation and protection of all evidence. Corrosion failure also may require sampling and testing of corrosion products immediately, such as in the case of MIC, where viable cultures can provide the most meaningful result. When possible and cost effective, a site visit is most desirable. A site visit may provide the investigator with information that otherwise may have been omitted or overlooked. Corrosion failures often relate to the material selection and the environment. Details of the material specifications, quality-assurance records, installation and maintenance records, and a history of the environment are all useful resources in resolving corrosion failures. Information regarding system upsets or diversions from the normal operating environment should be provided. A comparison of the actual material utilized with the material design specifications should also be performed. History of Failed Parts. Obtaining a history of the operation of the failed component is crucial to solving the cause of failure. The operating environment, any changes in the environment, and temperature excursions should all be obtained. Any notation of previous failures or operating anomalies is useful. When possible, engineering drawings and sketches should be reviewed. Information should be provided regarding any testing that was performed by the plant personnel. For instance, liquid-penetrant examination can result in chemical contamination of a surface. The use of paints or dyes to mark components may also alter the corrosion resistance of a part and the chemical composition of a corrosion product. It is also important that the investigator retains pertinent corrosion evidence before movement or disassembly of a component. The chain of events leading to, and occurring after, the failure should be documented. On-Site Examination and Sampling. On-site examination should include a walking tour of the failure area. Photographic documentation should be made to depict the conditions after the failure. If applicable, viewing a similar undamaged assembly or operation on the site where the failure occurred may be instructive. Photographic documentation must be performed with special attention given to capturing the true colors of the corrosion products. Macrographs of the corrosion deposit should include any layering effects in the deposits