SPECIALIZED CORROSIONDOE-HDBK-1015/1-93CorrosionCH-02Rev. 0Page 36Specialized Corrosion SummaryPitting corrosion occurs where the anodic site becomes fixed in a small area andthe formation of holes in an otherwise unaffected area takes place.Crevice corrosion is a type of pitting corrosion that occurs specifically withinthe low flow region of a crevice.Stress corrosion cracking (SCC) is a type of intergranular attack corrosion thatoccurs at the grain boundaries under tensile stress.Pitting corrosion requires two conditions to occur, low flow and areas of bothhigh and low oxygen concentration. When these conditions are met adifferential aeration cell is established which produces an electron flow from thepoint of low oxygen concentration to the area of high oxygen concentration.The difference in oxygen concentration is usually due to a low flow condition.Pitting corrosion is a hazard due to the possible rapid penetration of the metalwith little overall loss of mass. Pitting corrosion is minimized by:Avoiding stagnant conditionsUsing the correct metals and alloys that are less susceptible to thecorrosionAvoiding agents in the medium that cause pittingDesigning the system and components such that no crevices are presentStress corrosion cracking occurs when three conditions are met; the alloy issusceptible to stress corrosion cracking, the alloy is exposed to specificenvironment, and the alloy is in a stressed condition.Chemisorption is the formation of a monomolecular layer of a compoundbetween the metal's surface atoms. This layer separates the metal's atomsthereby weakening the metal and allowing any existing defects to propagatewhen a stress is applied.SummaryThe important information of this chapter is summarized below.
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