mechanical. Surface preparation is critical to the performance of the coating. One application of thermal spray coatings is for the protection of steel components from heat oxidation at temperatures up to 1095C (2000F) High-temperature corrosion resistance usually requires alloys of aluminum or nickel-chromium alloys Zinc and aluminum coatings are commonly applied to combat corrosion. Stainless steel and Hastelloy alloys are commonly used to combat corrosion in the chemical industry. Sealers or topcoats are commonly used to extend the life of thermal spray coatings and for decorative purposes Conversion coatings are a form of barrier protection in which a portion of the base metal is converted into a protective compound by a chemical or electrochemical treatment. Because the continuity of the conversion coating is more certain than that of an organic coating, conversion coatings treatment often precedes the application of organic coatings to enhance overall corrosion resistance. Proper cleaning of the base metal is essential to ensure this continuous coverage Phosphate conversion coatings are applied to various metal substrates to enhance corrosion resistance and improve paint adhesion. Corrosion protection is not the primary function of phosphates by themselves, but they improve the subsequent corrosion protection treatment The three categories of phosphate coatings are iron phosphates, zinc phosphates, and heavy phosphates. Iron phosphates are used when good paint adhesion and low cost are the primary concern. Zinc phosphates provide good paint adhesion, and they hold oils and waxes well. They provide good painted corrosion resistance. Heavy phosphates(manganese phosphates) are more expensive, but have an increase in unpainted corrosion resistance Chromate conversion coatings are formed by a chemical or electrochemical treatment of metals in solutions containing chromium ions and usually other components. The coatings are applied to enhance bare or painted corrosion resistance, to improve the adhesion of paint or other organic finishes, and to provide the metallic surface with a decorative finish. This process is used on many metals including aluminum, zinc, steel, copper, tin, and nickel. The level of protection is dependent on the substrate metal, the type of chromate coating used and the coating weight. The mechanism of corrosion protection is that of a barrier insulation from the environment that inhibits the cathodic-corrosion reactions Aluminum anodizing is an electrochemical method of converting aluminum into aluminum oxide at the surface of the component. Aluminum anodizing is used extensively. Chromic, sulfuric, and hard-coat anodizing are the most common methods. Chromic is the least used. It is relatively soft, but is useful when precise dimension of the part must be maintained. Sulfuric anodizing is the most widely used due to its light to moderate wear resistance, appearance, and corrosion resistance. Chromic and sulfuric coatings are porous and are typically sealed to close the pores inherent in the process. Hard-coat is typically not sealed, as sealing softens the coating and would reduce its wear resistance Surface modification refers to the modification of the surface composition or structure by using energy or particle beams. Elements may be added to influence the surface characteristics of the substrate by forming alloys, metastable alloys or phases, or amorphous layers. Surface-modified layers have a greater similarity to metallurgical alloying rather than the chemically adhered and mechanical bonds created by conversion or organic coating methods. Ion implantation and laser surface processing are two methods of surface modification. Ion implantation, derived from the semiconductor industry, is a relatively specialized process because of the cost of equipment and high degree of training required of operators. The process involves electrostatically accelerating an ionized species into the substrate. It is more recognized for its improvement of wear resistance than corrosion resistance. although there are corrosion-resistance benefits in certain cases Laser surface modification can modify the metallurgical structure of the surface and change surface properties without adversely affecting the bulk properties. There are generally three forms of laser surface modifications transformation hardening, surface melting, and surface alloying. Laser processing can be used to create corrosion-resistant surface layers. Usefulness of laser processing is limited because it requires a planar substrate and the substrate must be compatible with the thermal conduction requirements of the process References cited in this section 5. P. Elliott, Design Details to Minimize Corrosion, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 339 6. R H. Heidersbach, Cathodic Protection, Corrosion, Volume 13, ASM Handbook, ASM International 1987,p466477
mechanical. Surface preparation is critical to the performance of the coating. One application of thermal spray coatings is for the protection of steel components from heat oxidation at temperatures up to 1095 °C (2000 °F). High-temperature corrosion resistance usually requires alloys of aluminum or nickel-chromium alloys. Zinc and aluminum coatings are commonly applied to combat corrosion. Stainless steel and Hastelloy alloys are commonly used to combat corrosion in the chemical industry. Sealers or topcoats are commonly used to extend the life of thermal spray coatings and for decorative purposes. Conversion coatings are a form of barrier protection in which a portion of the base metal is converted into a protective compound by a chemical or electrochemical treatment. Because the continuity of the conversion coating is more certain than that of an organic coating, conversion coatings treatment often precedes the application of organic coatings to enhance overall corrosion resistance. Proper cleaning of the base metal is essential to ensure this continuous coverage. Phosphate conversion coatings are applied to various metal substrates to enhance corrosion resistance and improve paint adhesion. Corrosion protection is not the primary function of phosphates by themselves, but they improve the subsequent corrosion protection treatment. The three categories of phosphate coatings are iron phosphates, zinc phosphates, and heavy phosphates. Iron phosphates are used when good paint adhesion and low cost are the primary concern. Zinc phosphates provide good paint adhesion, and they hold oils and waxes well. They provide good painted corrosion resistance. Heavy phosphates (manganese phosphates) are more expensive, but have an increase in unpainted corrosion resistance. Chromate conversion coatings are formed by a chemical or electrochemical treatment of metals in solutions containing chromium ions and usually other components. The coatings are applied to enhance bare or painted corrosion resistance, to improve the adhesion of paint or other organic finishes, and to provide the metallic surface with a decorative finish. This process is used on many metals including aluminum, zinc, steel, copper, tin, and nickel. The level of protection is dependent on the substrate metal, the type of chromate coating used and the coating weight. The mechanism of corrosion protection is that of a barrier insulation from the environment that inhibits the cathodic-corrosion reactions. Aluminum anodizing is an electrochemical method of converting aluminum into aluminum oxide at the surface of the component. Aluminum anodizing is used extensively. Chromic, sulfuric, and hard-coat anodizing are the most common methods. Chromic is the least used. It is relatively soft, but is useful when precise dimension of the part must be maintained. Sulfuric anodizing is the most widely used due to its light to moderate wear resistance, appearance, and corrosion resistance. Chromic and sulfuric coatings are porous and are typically sealed to close the pores inherent in the process. Hard-coat is typically not sealed, as sealing softens the coating and would reduce its wear resistance. Surface modification refers to the modification of the surface composition or structure by using energy or particle beams. Elements may be added to influence the surface characteristics of the substrate by forming alloys, metastable alloys or phases, or amorphous layers. Surface-modified layers have a greater similarity to metallurgical alloying rather than the chemically adhered and mechanical bonds created by conversion or organic coating methods. Ion implantation and laser surface processing are two methods of surface modification. Ion implantation, derived from the semiconductor industry, is a relatively specialized process because of the cost of equipment and high degree of training required of operators. The process involves electrostatically accelerating an ionized species into the substrate. It is more recognized for its improvement of wear resistance than corrosion resistance, although there are corrosion-resistance benefits in certain cases. Laser surface modification can modify the metallurgical structure of the surface and change surface properties without adversely affecting the bulk properties. There are generally three forms of laser surface modifications: transformation hardening, surface melting, and surface alloying. Laser processing can be used to create corrosion-resistant surface layers. Usefulness of laser processing is limited because it requires a planar substrate and the substrate must be compatible with the thermal conduction requirements of the process. References cited in this section 5. P. Elliott, Design Details to Minimize Corrosion, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 339 6. R.H. Heidersbach, Cathodic Protection, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 466–477
7. P.J. Stone, Corrosion Inhibitors for Oil and Gas Production, Corrosion, Volume 13. ASM Handbook. ASM International, 1987, p 478 8. K B. Tator, Organic Coatings and Linings, Corrosion, Volume 13, ASM Handbook, ASM International 1987,p399 9. R.H. Unger, Thermal Spray Coatings, Corrosion, Volume 13, ASM Handbook, ASM International 1987,p459 Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd Acknowledgments The section"Electrochemical Nature of Corrosion" was adapted from"Corrosion Failures, from the course Principles of failure Analysis(Materials Engineering Institute/ASM International) Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy Ltd References 1."Corrosion Costs and Preventive Strategies in the United States, FHWA-RD-01-156, Federal Highway Administration. 2002 2. Wear and Corrosion, Vol 03.02, Annual Book ofAsTM Standards, ASTM 3.Applying Statistics to Analysis of Corrosion Data, G 16, Annual Book of ASTM Standards, ASTM 4. "Practice for Dealing with Outlying Observations, E 178, Annual Book of ASTM Standards, ASTM 5. P. Elliott, Design Details to Minimize Corrosion, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 339 6. R H. Heidersbach, Cathodic Protection, Corrosion, Volume 13, ASM Handbook, ASM International 1987,p466477 7. P.J. Stone Corrosion Inhibitors for Oil and Gas Production, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p478 8. K.B. Tator. Organic Coatings and Linings Corrosion, Volume 13. ASM Handbook. ASM International 1987,p399 9. R H. Unger, Thermal Spray Coatings, Corrosion, Volume 13, ASM Handbook, ASM International 1987,p459 Thefileisdownloadedfromwww.bzfxw.com
7. P.J. Stone, Corrosion Inhibitors for Oil and Gas Production, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 478 8. K.B. Tator, Organic Coatings and Linings, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 399 9. R.H. Unger, Thermal Spray Coatings, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 459 Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd. Acknowledgments The section “Electrochemical Nature of Corrosion” was adapted from “Corrosion Failures,” from the course Principles of Failure Analysis (Materials Engineering Institute/ASM International). Analysis and Prevention of Corrosion-Related Failures S.R. Freeman, Millennium Metallurgy, Ltd. References 1. “Corrosion Costs and Preventive Strategies in the United States,” FHWA-RD-01-156, Federal Highway Administration, 2002 2. Wear and Corrosion, Vol 03.02, Annual Book of ASTM Standards, ASTM 3. “Applying Statistics to Analysis of Corrosion Data,” G 16, Annual Book of ASTM Standards, ASTM 4. “Practice for Dealing with Outlying Observations,” E 178, Annual Book of ASTM Standards, ASTM 5. P. Elliott, Design Details to Minimize Corrosion, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 339 6. R.H. Heidersbach, Cathodic Protection, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 466–477 7. P.J. Stone, Corrosion Inhibitors for Oil and Gas Production, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 478 8. K.B. Tator, Organic Coatings and Linings, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 399 9. R.H. Unger, Thermal Spray Coatings, Corrosion, Volume 13, ASM Handbook, ASM International, 1987, p 459 The file is downloaded from www.bzfxw.com
Forms of corrosion Introduction corrosion is the electrochemical reaction of a material and its environment This article addresses those forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failur by some other mechanism. In particular, mechanisms of corrosive attack for specific forms of corrosion are described. Other articles in this Volume discuss analysis and prevention of several types of failure in which corrosion is a contributing factor. These articles include"Stress-Corrosion Cracking, " "Liquid-Metal and Solid- Metal Induced Embrittlement, " Hydrogen Damage and Embrittlement, ""Corrosive Wear Failures Biological Corrosion Failures, and"High-Temperature Corrosion-Related Failures The rate, extent, and type of corrosive attack that can be tolerated in an object vary widely, depending on the specific application and initial design. Whether the observed corrosion behavior was normal for the metal used in the given service conditions helps determine what inspection and maintenance action, if any, should be taken All corrosion reactions are electrochemical in nature and depend on the operation of electrochemical cells at the metal surface. This mechanism is discussed in the first section, "Galvanic Corrosion "Galvanic corrosion applies even to generalized uniform chemical attack--described in the section"Uniform Corrosion'-in which the anodes and cathodes of the cells are numerous, small, and close together. The analysis of corrosion failures and the development of suitable corrective measures require not only a basic understanding of electrochemistry, but also the ability to apply fundamental principles of chemistry and metallurgy. It is useful to consider these principles in the context of the form that the corrosion takes. Figure 1 is a schematic of several forms of corrosion, including those described in the sections"Pitting and Crevice Corrosion, Intergranular Corrosion and"Selective Leaching"in this article. The figure also includes external conditions that produce aggressive corrosion such as velocity-affected corrosion, which is addressed in this article, and stress-corrosion cracking (SCC), which is addressed elsewhere in this Volume. These reviews of corrosion forms and mechanisms are intended to assist the reader in developing an understanding of the underlying principles of corrosion; acquiring such an understanding is the first step in recognizing and analyzing corrosion- related failures and in formulating preventive measures / Fig. 1 Forms of corrosion. Adapted from Ref l More detailed discussions of the fundamentals and mechanisms of corrosion are provided in the references listed at the end of this article and in Corrosion volume 13 of ASm Handbook Forms of corrosion Galvanic corrosion Although sometimes considered a form of corrosion, galvanic corrosion is more accurately considered a type of corrosion mechanism, because galvanic action is the basis for, or can accelerate the effects of, other forms of corrosion. Uniform attack, pitting, and crevice corrosion can all be exacerbated by galvanic conditions
Forms of Corrosion Introduction CORROSION is the electrochemical reaction of a material and its environment. This article addresses those forms of corrosion that contribute directly to the failure of metal parts or that render them susceptible to failure by some other mechanism. In particular, mechanisms of corrosive attack for specific forms of corrosion are described. Other articles in this Volume discuss analysis and prevention of several types of failure in which corrosion is a contributing factor. These articles include “Stress-Corrosion Cracking,” “Liquid-Metal and SolidMetal Induced Embrittlement,” “Hydrogen Damage and Embrittlement,” “Corrosive Wear Failures,” “Biological Corrosion Failures,” and “High-Temperature Corrosion-Related Failures.” The rate, extent, and type of corrosive attack that can be tolerated in an object vary widely, depending on the specific application and initial design. Whether the observed corrosion behavior was normal for the metal used in the given service conditions helps determine what inspection and maintenance action, if any, should be taken. All corrosion reactions are electrochemical in nature and depend on the operation of electrochemical cells at the metal surface. This mechanism is discussed in the first section, “Galvanic Corrosion.” Galvanic corrosion applies even to generalized uniform chemical attack—described in the section “Uniform Corrosion”—in which the anodes and cathodes of the cells are numerous, small, and close together. The analysis of corrosion failures and the development of suitable corrective measures require not only a basic understanding of electrochemistry, but also the ability to apply fundamental principles of chemistry and metallurgy. It is useful to consider these principles in the context of the form that the corrosion takes. Figure 1 is a schematic of several forms of corrosion, including those described in the sections “Pitting and Crevice Corrosion,” “Intergranular Corrosion,” and “Selective Leaching” in this article. The figure also includes external conditions that produce aggressive corrosion such as velocity-affected corrosion, which is addressed in this article, and stress-corrosion cracking (SCC), which is addressed elsewhere in this Volume. These reviews of corrosion forms and mechanisms are intended to assist the reader in developing an understanding of the underlying principles of corrosion; acquiring such an understanding is the first step in recognizing and analyzing corrosion-related failures and in formulating preventive measures. Fig. 1 Forms of corrosion. Adapted from Ref 1 More detailed discussions of the fundamentals and mechanisms of corrosion are provided in the References listed at the end of this article and in Corrosion, Volume 13 of ASM Handbook. Forms of Corrosion Galvanic Corrosion Although sometimes considered a form of corrosion, galvanic corrosion is more accurately considered a type of corrosion mechanism, because galvanic action is the basis for, or can accelerate the effects of, other forms of corrosion. Uniform attack, pitting, and crevice corrosion can all be exacerbated by galvanic conditions
Galvanic corrosion occurs when two dissimilar conducting materials(metallic or nonmetallic) are in electrical contact. It usually consists of two dissimilar conductors in electrical contact with each other and with a common conducting fluid(an electrolyte), or it may occur when two similar conductors come in contact with each other via dissimilar electrolytes. The former is the more common condition When two materials are in electrical contact and connected by an electrolyte, a galvanic current flows between them because of the inherent potential difference between the two. The resulting reaction is referred to as couple action, and the electrically coupled system is known as a galvanic cell. The couple consists of an anode (which liberates electrons and corrodes typically by metal dissolution and/or metal oxide formation) and cathode(which gains electrons and typically liberates hydrogen to the electrolyte and/or reduces oxides ). Both reactions at the anode and cathode must proceed simultaneously for galvanic corrosion to occur. The overall reaction results in the corrosion of the anode by metal dissolution or oxidation, and an electrical current flows from the more anodic metal through a conducting medium or electrolyte to the other more cathodic metal (liberating hydrogen and/or causing a" reduction"of oxides). This action is seen in Fig. 2 where a couple is made up of steel and mill scale Electrolyte(water) urrent flo (rust) Cathode anode ste (broken mill scale Fig 2 Breaks in mill scale(Fe3O4)leading to galvanic corrosion of steel The three essential components for galvanic corrosion are Materials possessing different surface potential A common electrolyte A common electrical path A mixed-metal system in a common electrolyte that is electrically isolated will not experience galvanic corrosion, regardless of the proximity of the metals or their relative potential or size Forms of corrosion Factors Affecting Galvanic Corrosion The intensity of galvanic corrosion is affected by the following factors The potential difference between the metals or alloys The nature of the environment The polarization behavior of the metals or alloys The geometric relationship of the component metals or alloys The severity of galvanic corrosion depends on the amount of current flow. The relative potentials and olarization behavior of metals and alloys are key consideration discussed later in this article. Current flow also depends on the physical configuration, the relative area, distance, and shape of the components Geometric factors Thefileisdownloadedfromwww.bzfxw.com
Galvanic corrosion occurs when two dissimilar conducting materials (metallic or nonmetallic) are in electrical contact. It usually consists of two dissimilar conductors in electrical contact with each other and with a common conducting fluid (an electrolyte), or it may occur when two similar conductors come in contact with each other via dissimilar electrolytes. The former is the more common condition. When two materials are in electrical contact and connected by an electrolyte, a galvanic current flows between them because of the inherent potential difference between the two. The resulting reaction is referred to as couple action, and the electrically coupled system is known as a galvanic cell. The couple consists of an anode (which liberates electrons and corrodes typically by metal dissolution and/or metal oxide formation) and a cathode (which gains electrons and typically liberates hydrogen to the electrolyte and/or reduces oxides). Both reactions at the anode and cathode must proceed simultaneously for galvanic corrosion to occur. The overall reaction results in the corrosion of the anode by metal dissolution or oxidation, and an electrical current flows from the more anodic metal through a conducting medium or electrolyte to the other more cathodic metal (liberating hydrogen and/or causing a “reduction” of oxides). This action is seen in Fig. 2 where a couple is made up of steel and mill scale. Fig. 2 Breaks in mill scale (Fe3O4) leading to galvanic corrosion of steel The three essential components for galvanic corrosion are: · Materials possessing different surface potential · A common electrolyte · A common electrical path A mixed-metal system in a common electrolyte that is electrically isolated will not experience galvanic corrosion, regardless of the proximity of the metals or their relative potential or size. Forms of Corrosion Factors Affecting Galvanic Corrosion The intensity of galvanic corrosion is affected by the following factors: · The potential difference between the metals or alloys · The nature of the environment · The polarization behavior of the metals or alloys · The geometric relationship of the component metals or alloys The severity of galvanic corrosion depends on the amount of current flow. The relative potentials and polarization behavior of metals and alloys are key consideration discussed later in this article. Current flow also depends on the physical configuration, the relative area, distance, and shape, of the components. Geometric Factors The file is downloaded from www.bzfxw.com
Effect of Surface Area. The relative size of the anodic and cathodic surfaces is important. The intensity of galvanic attack is related to the relative size of the metals in electrical contact. Large cathodic areas coupled to small anodic areas will aggravate galvanic corrosion and cause severe dissolution of the more active metal. The reverse situation-large anodic areas coupled to small cathodic areasproduces very little galvanic current This is why imperfections or holidays in protective coatings may lead to severe galvanic corrosion in the localized region of the coating imperfection. It is extremely dangerous to coat the anodic member of a couple because this may only reduce its active area, which severely accelerates the attack at these holidays in the otherwise protective coating. If inert barrier coatings are employed, both the anode and cathode must be protected Effect of Distance. Dissimilar metals in a galvanic couple that are in close physical proximity usually suffer greater galvanic effects than those that are farther apart. The distance affects the resistance of the current path in the solution and the external circuit. Thus, if dissimilar pipes are butt welded with the electrolyte flowing through them, the most severe corrosion will occur adjacent to the weld on the anodic membe Effect of Shape. The geometry of the circuit elements determines the electrical potential gradient, which causes the current to flow Galvanic series Because galvanic corrosion is directly related to the electrical current caused by the natural potential difference potentials for a given electrolytic solution. For example, the galvanic series of potentials for metals o or electromotive force(emf) between different metals, it is useful to rank metals according to their relati chloride-containing aqueous solution (i.e, seawater)is often used for purposes of general comparison. This series is shown in Fig 3. When two metals of a galvanic cell are widely separated in the galvanic series, there is a greater flow of current than for metals with small difference in potential. The galvanic series also allows one to determine which metal or alloy in a galvanic couple is more active Metals that are more anodic in a given ell are prone to corrode by metal dissolution or oxidation. The more cathodic material is more corrosion resistant(i.e, more noble)
Effect of Surface Area. The relative size of the anodic and cathodic surfaces is important. The intensity of galvanic attack is related to the relative size of the metals in electrical contact. Large cathodic areas coupled to small anodic areas will aggravate galvanic corrosion and cause severe dissolution of the more active metal. The reverse situation—large anodic areas coupled to small cathodic areas—produces very little galvanic current. This is why imperfections or holidays in protective coatings may lead to severe galvanic corrosion in the localized region of the coating imperfection. It is extremely dangerous to coat the anodic member of a couple because this may only reduce its active area, which severely accelerates the attack at these holidays in the otherwise protective coating. If inert barrier coatings are employed, both the anode and cathode must be protected. Effect of Distance. Dissimilar metals in a galvanic couple that are in close physical proximity usually suffer greater galvanic effects than those that are farther apart. The distance affects the resistance of the current path in the solution and the external circuit. Thus, if dissimilar pipes are butt welded with the electrolyte flowing through them, the most severe corrosion will occur adjacent to the weld on the anodic member. Effect of Shape. The geometry of the circuit elements determines the electrical potential gradient, which causes the current to flow. Galvanic Series Because galvanic corrosion is directly related to the electrical current caused by the natural potential difference or electromotive force (emf) between different metals, it is useful to rank metals according to their relative potentials for a given electrolytic solution. For example, the galvanic series of potentials for metals in a chloride-containing aqueous solution (i.e., seawater) is often used for purposes of general comparison. This series is shown in Fig. 3. When two metals of a galvanic cell are widely separated in the galvanic series, there is a greater flow of current than for metals with small difference in potential. The galvanic series also allows one to determine which metal or alloy in a galvanic couple is more active. Metals that are more anodic in a given galvanic cell are prone to corrode by metal dissolution or oxidation. The more cathodic material is more corrosion resistant (i.e., more noble)