Published: Jan 2005
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PITTING CORROSION occurs when discrete areas of a material undergo rapid attack, although the vast majority of the surface remains virtually unaffected. This morphology is in sharp contrast to uniform corrosion in which all parts of the exposed surface recede at approximately the same rate. Essentially, all metals and alloys undergo pitting corrosion under some set of experimental conditions, although the relative susceptibility varies widely. The basic requirement for pitting is the existence of a passive state for the material in the environment of interest. Pitting occurs when portions of the metal surface lose their passivity and dissolve rapidly. This loss of passivity often occurs at heterogeneities in the surface (either physical or chemical). Pitting of a given material depends strongly on the presence of an aggressive species in the environment and a sufficiently oxidizing potential (e.g., Cl- ion in neutral, aerated aqueous solution for Type 304 stainless steel). There are many metal/environment combinations that can lead to pitting, as extensively reviewed by Szlarska-Smialowska  and more recently by Frankel . In many situations, pitting can severely limit the performance of the material.
Associate Professor, University of Virginia, Charlottesvllle, VA