Machinery's Handbook, 31st Edition
Corrosion
553
Effects of Corrosion Corrosion can result in many different effects on a material. The following sections describe some common effects on materials induced at least in part or enhanced by corro- sion, as well as mitigation methods. Crevice Corrosion.—In small gaps, holes, and crevices, such as those between contact - ing parts, the fluid medium can be stagnant and undergo chemical changes with oxygen depletion. This corrosion is difficult to detect due to its location. Where possible, it helps to eliminate features that can trap fluid, use full welds instead of fasteners, and seal joints with grease or nonporous gaskets. Pitting Corrosion.—Localized pitting corrosion is difficult to detect and can be destruc - tive and unpredictable. Pits characteristic of this process may be small or large, may exist singly or in groups, and may be masked by corrosion-protection products. Pitting corro- sion can be initiated by damage to a coating or oxide layer; surface contaminants or inclu- sions; or acid, chloride, or other caustic chemical exposure. Pits can act as stress risers and cause premature part failure due to fatigue or stress corrosion cracking. Microbiologically Influenced Corrosion (MIC).—Also called microbial corrosion or biocorrosion, this process usually takes the form of crevice and/or pitting corrosion under a fouling biofilm that has formed on part surfaces. Chemistry changes due to static conditions under the film, metabolic waste from the microorganisms, and metabolic re - duction of oxygen all contribute to corrosive attack. Many different biological organ - isms, both aerobic and anaerobic, can participate in this corrosion process. Elimination of fouling biofilms is the best defense. Also applicable are the stated methods for reducing pitting or crevices and using corrosion-resistant materials where possible. For example, in seawater applications, titanium is highly resistant to MIC, though it is not immune in all environments. Intergranular Corrosion.—Localized attack along microstructural grain boundaries of a material can occur due to segregation or precipitation of elements at those boundaries. The unique material composition at the grain boundaries can preferentially corrode or cause galvanic corrosion to occur. The resulting changes in material properties can lead to intergranular stress corrosion cracking or disintegration. This type of corrosion is a risk for stainless steel welded in multiple passes, which can cause sensitization of material. Depletion of chromium and/or formation of carbides in the heat-affected zone can lead to a type of intergranular corrosion known as weld decay, or knife-line attacks in alloys stabilized with niobium. Aluminum alloys, particularly ex- truded or heavily worked pieces, also are susceptible to intergranular corrosion; a subtype called exfoliation corrosion may occur where the surface grain is thinner than the rolled grain. Prevention methods include heat treatment of sensitized parts. Selective Leaching (Dealloying).—This process involves preferential corrosion of the least noble element in an alloy. Dezincification of unstabilized brass (leaving porous copper) and iron depletion of gray cast iron (leaving graphite) are examples of selective leaching corrosion. This type of corrosion is difficult to detect visually, but structural weakening can be profound. To reduce the likelihood of selective leaching, vulnerable parts may be heat treated to improve homogeneity. Stress Corrosion Cracking.—Potentially devastating cracks can start and grow slowly in metals and alloys exposed to both static tensile stress (applied and/or residual) and a corrosive environment. Rather than propagating on surfaces, such cracks usually are fine, branching, transgranular, and penetrating, and, thus, difficult to detect. Environmental conditions that induce this process are very specific and depend on materials, applied processes, and part uses. Weldments and heavily worked parts are particularly vulnera - ble, due to the potential for high levels of residual stress. Methods to guard against stress corrosion cracking include using stress-relieving parts with heat treatment or mechanical
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