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C h a p t e r 16 / Corrosion and Degradation of Materials S-204 P hotograph showing a bar of steel that has been bent into a ‘‘horseshoe’’ shape using a nut-and-bolt assembly. While immersed in seawater, stress corro- sion cracks formed along the bend at those regions where the tensile stresses are the greatest. (Photo- graph courtesy of F. L. LaQue. From F. L. LaQue, Marine Corrosion, Causes and Prevention. Copyright  1975 by John Wiley & Sons, Inc. Reprinted by per- mission of John Wiley & Sons, Inc.) Why Study Corrosion and Degradation of Materials? With a knowledge of the types of and an under- standing of the mechanisms and causes of corrosion and degradation, it is possible to take measures to prevent them from occurring. For example, we may change the nature of the environment, select a mate- rial that is relatively nonreactive, and/or protect the material from appreciable deterioration. S-205 16.1 I NTRODUCTION To one degree or another, most materials experience some type of interaction with a large number of diverse environments. Often, such interactions impair a material’s usefulness as a result of the deterioration of its mechanical properties (e.g., ductility and strength), other physical properties, or appearance. Occasionally, to the chagrin of a design engineer, the degradation behavior of a material for some application is ignored, with adverse consequences. Deteriorative mechanisms are different for the three material types. In metals, there is actual material loss either by dissolution (corrosion) or by the formation of nonmetallic scale or film ( oxidation ). Ceramic materials are relatively resistant to deterioration, which usually occurs at elevated temperatures or in rather extreme environments; the process is frequently also called corrosion. For polymers, mecha- nisms and consequences differ from those for metals and ceramics, and the term degradation is most frequently used. Polymers may dissolve when exposed to a liquid solvent, or they may absorb the solvent and swell; also, electromagnetic radiation (primarily ultraviolet) and heat may cause alterations in their molecu- lar structure. The deterioration of each of these material types is discussed in this chapter, with special regard to mechanism, resistance to attack by various environments, and measures to prevent or reduce degradation. C O R R O S I O N O F M E T A L S Corrosion is defined as the destructive and unintentional attack of a metal; it is electrochemical and ordinarily begins at the surface. The problem of metallic corro- sion is one of significant proportions; in economic terms, it has been estimated that approximately 5 % of an industrialized nation’s income is spent on corrosion prevention and the maintenance or replacement of products lost or contaminated as a result of corrosion reactions. The consequences of corrosion are all too common.... View Full Document

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