Localized corrosion such as pitting and crevice corrosion of stainless steels generally occurs in the presence of halide ions, typically chloride (e.g. coastal and deicing chloride salts – sodium, calcium or magnesium chlorides; hydrochloric acid; bleach – sodium or calcium hypochlorite; and other chloride compounds).
Pitting occurs when there is a localized breakdown of the stainless steel’s protective passive layer on an openly exposed surface. Once initiated the growth rate of the pit can be relatively rapid resulting in deep cavities and even through-wall attack. Other metals, such as aluminum, can also exhibit pitting corrosion.
Crevice corrosion occurs at locations where oxygen cannot freely circulate such as tight joints, under fastener heads and in other circumstances where the pieces of metal are in close contact.
Chloride salts, pollutants and moisture from the environment accumulate in the crevice. The environment inside the crevice becomes depleted of oxygen, enriched in chlorides, and acidified which promotes the breakdown of the passive film and anodic dissolution.
The important environmental factors that favor localized attack are higher chloride content, higher temperatures, lower pH, and more noble corrosion potentials.