Practices in Failure Analysis Corrosion failures Corrosion is traditionally defined as the destructive chemical or electrochemical reaction of a metal with its environment. A broader, more modern definition of corrosion is the deterioration of a material or its properties due to reaction with its environment. The latter definition makes three important points. It does not limit corrosion to chemical or electrochemical processes, because some forms of corrosion do not involve these processes. It also characterizes corrosion damage as the deterioration of the material or its properties, because some forms of corrosion weaken the material without visible changes in appearance or measurable weight loss. Finally, the modern definition of corrosion acknowledges that nonmetallic materials may corrode. Examples of corrosion of nonmetallic materials are the aging of rubber due to the effects of heat and/or oils; swelling, oxidation, stress cracking, and ultraviolet deterioration of plastics; destruction of the binder in concrete by environmental agents; and the biological attack or rotting of wood This section focuses on analysis of metal corrosion There are many forms of metallic corrosion. These include: uniform corrosion, localized attack(e.g, crevice corrosion and pitting), galvanic attack, cracking phenomena(e.g, stress-corrosion cracking, hydrogen embrittlement, liquid metal embrittlement, and corrosion fatigue), velocity phenomena (e.g, erosion, cavitation, and impingement), fretting, ntergranular attack, and dealloying or selective leaching. These forms are discussed in more detail in the article Forms of Corrosion''in this volume The type of corrosion, the corrosion rate, and the severity and extent of corrosion are influenced by the nature of the environment, the metal surface in contact with the environment, and the mechanical stresses(magnitude and direction) These factors do not necessarily remain constant as corrosion progresses. They are affected by externally imposed changes and those resulting from the corrosion process itself. Other factors that greatly affect corrosion processes include temperature and temperature gradients at the metal/environment interface, crevices in the metal part or assembly, relative motion between the environment and the metal part, and the presence of dissimilar metals in an electrically conductive environment. Processing and fabrication operations such as surface grinding, heat treating, welding, cold working, forming, drilling, and shearing produce local or general changes on metal parts that, to varying degrees, affect their usceptibility to corrosion. When a corrosion failure has occurred, several means of preventing or minimizing future failures are available. Very often more than one method is used at the same time. The more important corrective and preventive measures are Change in alloy, heat treatment, or product form(e.g, solution annealing of austenitic stainless steels minimizes the risk of intergranular attack and stress- corrosion cracking) Use of resin coatings(e.g, acrylics, epoxies, phenolics, furanes and urethanes in the form of paints, potting compounds, adhesives, coatings, and linings Use of inert lubricants(chemically inert resins such as silicones, esters, and fluorocarbons that sometimes can serve both as effective lubricants and as corrosion-resistant coatings and linings Use of electrolytic and chemical coatings and surface treatments(e.g, anodizing of aluminum and aluminum alloys for protection in natural,"nonaggressive environments) Use of metallic coatings(e.g, zinc-rich coatings) Use of galvanic protection(cathodic or anodic) Design changes for corrosion control Use of inhibitors Changes in pH and applied potentia Continuous monitoring of variables Corrosion Failure analys Complete investigations of corrosion failures can be very complex, although not all corrosion failures require a comprehensive, detailed failure analysis. Often the preliminary examination will determine the extent of investigation required. In general, the investigation should consider various possibilities without being unnecessarily costly or time consuming. Routine checks to determine that the specified material is used is important. Such checks have shown that preferentially at the welds. Forgings that failed in service actually were castings in which failure was initiated byPractices in Failure Analysis Corrosion Failures Corrosion is traditionally defined as the destructive chemical or electrochemical reaction of a metal with its environment. A broader, more modern definition of corrosion is the deterioration of a material or its properties due to reaction with its environment. The latter definition makes three important points. It does not limit corrosion to chemical or electrochemical processes, because some forms of corrosion do not involve these processes. It also characterizes corrosion damage as the deterioration of the material or its properties, because some forms of corrosion weaken the material without visible changes in appearance or measurable weight loss. Finally, the modern definition of corrosion acknowledges that nonmetallic materials may corrode. Examples of corrosion of nonmetallic materials are the aging of rubber due to the effects of heat and/or oils; swelling, oxidation, stress cracking, and ultraviolet deterioration of plastics; destruction of the binder in concrete by environmental agents; and the biological attack or rotting of wood. This section focuses on analysis of metal corrosion. There are many forms of metallic corrosion. These include: uniform corrosion, localized attack (e.g., crevice corrosion and pitting), galvanic attack, cracking phenomena (e.g., stress-corrosion cracking, hydrogen embrittlement, liquid metal embrittlement, and corrosion fatigue), velocity phenomena (e.g., erosion, cavitation, and impingement), fretting, intergranular attack, and dealloying or selective leaching. These forms are discussed in more detail in the article “Forms of Corrosion” in this Volume. The type of corrosion, the corrosion rate, and the severity and extent of corrosion are influenced by the nature of the environment, the metal surface in contact with the environment, and the mechanical stresses (magnitude and direction). These factors do not necessarily remain constant as corrosion progresses. They are affected by externally imposed changes and those resulting from the corrosion process itself. Other factors that greatly affect corrosion processes include temperature and temperature gradients at the metal/environment interface, crevices in the metal part or assembly, relative motion between the environment and the metal part, and the presence of dissimilar metals in an electrically conductive environment. Processing and fabrication operations such as surface grinding, heat treating, welding, cold working, forming, drilling, and shearing produce local or general changes on metal parts that, to varying degrees, affect their susceptibility to corrosion. When a corrosion failure has occurred, several means of preventing or minimizing future failures are available. Very often more than one method is used at the same time. The more important corrective and preventive measures are: · Change in alloy, heat treatment, or product form (e.g., solution annealing of austenitic stainless steels minimizes the risk of intergranular attack and stress-corrosion cracking) · Use of resin coatings (e.g., acrylics, epoxies, phenolics, furanes and urethanes in the form of paints, potting compounds, adhesives, coatings, and linings) · Use of inert lubricants (chemically inert resins such as silicones, esters, and fluorocarbons that sometimes can serve both as effective lubricants and as corrosion-resistant coatings and linings) · Use of electrolytic and chemical coatings and surface treatments (e.g., anodizing of aluminum and aluminum alloys for protection in natural, “nonaggressive” environments) · Use of metallic coatings (e.g., zinc-rich coatings) · Use of galvanic protection (cathodic or anodic) · Design changes for corrosion control · Use of inhibitors · Changes in pH and applied potential · Continuous monitoring of variables. Corrosion Failure Analysis Complete investigations of corrosion failures can be very complex, although not all corrosion failures require a comprehensive, detailed failure analysis. Often the preliminary examination will determine the extent of investigation required. In general, the investigation should consider various possibilities without being unnecessarily costly or time consuming. Routine checks to determine that the specified material is used is important. Such checks have shown that “seamless” tubes that failed in service by developing longitudinal splits were actually welded tubes that corroded preferentially at the welds. Forgings that failed in service actually were castings in which failure was initiated by