Machinery's Handbook, 31st Edition
548
Corrosion CORROSION
Corrosion is a chemical or electrochemical process by which a material and/or its prop- erties deteriorate due to interaction with one or more external substances. Environmental conditions, surface conditions, and component stresses can initiate or accelerate corro- sion. Effects of corrosion are many, ranging from merely aesthetic changes to interference with electrical connections and weakening of a structural or mechanical part that can lead to failure. Of particular concern are common material changes, such as pitting corrosion and stress corrosion cracking, which are difficult to detect, yet can result in sudden com - ponent failure. Corrosion Types and Methods of Prevention Consideration of material properties—including chemical compatibility, galvanic po - tential, and reaction to anticipated environmental conditions—is important in minimiz - ing corrosion and maximizing the design life of metal and alloy parts and assemblies. However, the mechanisms of corrosion can result in a wide variety of corrosion patterns and processes, and different types and mechanisms of corrosion may occur simultane- ously. Thus, corrosion protection often requires a multipronged approach. Uniform (General) Corrosion.—This process describes either a chemical or electro- chemical attack that uniformly affects an entire exposed surface. The corroding part will gradually thin; corrosive substances may accumulate on the surface. Usually, uniform corrosion can be predicted and measured, so designers can provide for and monitor it to ensure safety. Methods of preventing general corrosion include using appropriate materials, passiv - ation, polishing, coatings and other barriers, and environmental controls, as well as re - ducing stresses on components. To prevent corrosive interaction between incompatible materials, parts can be separated electrically from each other or electrolyte solutions with gaskets or coatings. Gaskets or sealants also can minimize moisture intrusion into joints to further reduce corrosion, though materials interactions should be investigated. For instance, graphite, often found in carbon black rubber and gaskets, should not be used in contact with stainless steels in seawater. Chemical Corrosion.—This type of corrosion generally refers to various chemical in - teractions. In most cases, damage results from the destructive reaction of a contacting substance directly acting on and degrading a material, for instance, an acid affecting a metal. Chemical corrosion also may result in oxide formation or deposition of other sur- face coatings. Dry and High-Temperature Corrosion.—Dry corrosion, also called scaling, generally is caused by chemicals (gases, molten salts, or solids) acting on a surface in a dry atmo- sphere (i.e., no aqueous electrolyte), though particulate abrasion also may be involved. Material selection and effective environmental controls can significantly reduce damage; numerous specialized protective coatings also are available. High-temperature corrosion takes place when dry corrosion occurs and/or accelerates due to a high-temperature environment. Critical temperature and rate of corrosion de - pend on the materials involved and environmental conditions. As this form of corrosion may become a risk at temperatures as low as 194° F (90° C) in some applications (such as oil pipelines), it may simply be identified as dry corrosion. Common results of high- temperature corrosion include oxidation (in air, this is a problem above the material’s scaling temperature), carburization, chlorination, and sulfidation. Electrochemical Corrosion.—This occurs when an electrically active material is ex - posed to an electrolyte, forming an electrical cell in which ionization occurs and elec- trons move from an anodic (active) material to a cathodic (noble) material. As a result, the anode corrodes through oxidation, while the cathode absorbs the free electrons through a reduction reaction. With sufficient oxygen, the cathode often will develop a protective
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