CorrosionDOE-HDBK-1015/1-93SPECIALIZED CORROSIONRev. 0CH-02Page 31Pitting and crevice corrosion are a major hazard to a nuclear facility because of the rapidpenetration of the metal with little overall loss of mass. A nuclear facility minimizes pitting andcrevice corrosion by the following actions.Avoiding stagnant or low flow conditions.Using metals and alloys that are less susceptible to the corrosion.Avoiding agents in the medium that cause pitting (for example, chlorides andoxygen).Designing the system and components such that no crevices are present.StressCorrosionCrackingStress corrosion cracking(SCC) is a type of intergranular attack corrosion that occurs at thegrain boundaries under tensile stress. Grain boundaries are discussed in detail in the MaterialScienceHandbook. SCC occurs in susceptible alloys when the alloy is exposed to a particular,specific environment if the alloy is in a stressed condition. Stress corrosion cracking appearsto be relatively independent of general uniform corrosion processes. Thus, the extent of generalcorrosion can be essentially nil, and stress cracking can still occur. Most pure metals areimmune to this type of attack.According to the most widely accepted theory, stress corrosion cracking is caused by a processcalled chemisorption. Unlike relatively weak physical absorption, such as hydrogen gas onplatinum metal, chemisorption may be thought of as the formation of a compound between themetal atoms on the surface as a monomolecular layer of the chemisorbed substance, such as Cl-,OH-, Br-, and some other ions. The formation of this chemisorbed layer greatly reduces theattraction between neighboring metal atoms. A defect initially present then grows as the metalatoms separate under stress, more chemisorption occurs, and the process continues. In verysevere cases, the time required for this cracking to occur is only a matter of minutes.Many stainless steels are susceptible to stress corrosion cracking. Stainless steels containing18 percent chromium and 8 percent nickel are susceptible to cracking in environmentscontaining chloride ions and in concentrated caustic environments (that is, in environmentswhere the hydroxyl ion concentration is high). On the other hand, these types of stainless steelsdo not exhibit any tendency to crack when they are exposed to water environments containingnitrate (NO ), sulfite (SO ), and ammonium (NH ) ions.3 3 3- -- +SCC is of great concern because it can readily crack metal of appreciable thickness. If theenvironment is severe enough, cracking can occur in a very short period of time. The crack canthen lead to a serious failure of the component, or the system, and all the attendant results (forexample, contamination, loss of coolant, and loss of pressure).
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