Specialized Corrosion Summary

SPECIALIZED CORROSION DOE-HDBK-1015/1-93 Corrosion CH-02 Rev. 0 Page 36 Specialized Corrosion Summary Pitting corrosion occurs where the anodic site becomes fixed in a small area and the formation of holes in an otherwise unaffected area takes place. Crevice corrosion is a type of pitting corrosion that occurs specifically within the low flow region of a crevice. Stress corrosion cracking (SCC) is a type of intergranular attack corrosion that occurs at the grain boundaries under tensile stress. Pitting corrosion requires two conditions to occur, low flow and areas of both high and low oxygen concentration. When these conditions are met a differential aeration cell is established which produces an electron flow from the point 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 metal with little overall loss of mass. Pitting corrosion is minimized by: Avoiding stagnant conditions Using the correct metals and alloys that are less susceptible to the corrosion Avoiding agents in the medium that cause pitting Designing the system and components such that no crevices are present Stress corrosion cracking occurs when three conditions are met; the alloy is susceptible to stress corrosion cracking, the alloy is exposed to specific environment, and the alloy is in a stressed condition. Chemisorption is the formation of a monomolecular layer of a compound between the metal's surface atoms. This layer separates the metal's atoms thereby weakening the metal and allowing any existing defects to propagate when a stress is applied. Summary The important information of this chapter is summarized below.