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Carbon steels suffer from 'general' corrosion, where large areas of the surface are affected. Stainless steels in the passive state are normally protected against this form of attack, however, localised forms of attack can occur and result in corrosion problems.
The single most important factor in determining the incidence of corrosion is moisture. (The threshold moisture content below which no appreciable corrosion of embedded metal occurs is the same as the safety limit for wood-rotting fungi, i.e about 20%.) Above this level, wood is always acidic due to the breakdown by water of wood cellulose to acetic acid, through natural ageing processes.
The thermal insulation materials used for stainless steel pipes and vessels contain chlorides. If such insulation materials are exposed to moisture, chlorides may be released into a moisture layer on the pipe or vessel surface and pitting/stress corrosion cracking may result.
Stainless steels are generally very corrosion resistant and will perform satisfactorily in most environments. The limit of corrosion resistance of a given stainless steel depends on its constituent elements which means that each grade has a slightly different response when exposed to a corrosive environment. Care is therefore needed to select the most appropriate grade of stainless steel for a given application. As well as careful material grade selection, good detailing and workmanship can significantly reduce the likelihood of staining and corrosion.
Although stainless steel is much more resistant to corrosion than ordinary carbon or alloy steels, in some circumstances it can corrode. It is 'stain-less' not 'stain-impossible'. In normal atmospheric or water based environments, stainless steel will not corrode as demonstrated by domestic sink units, cutlery, saucepans and work-surfaces.
The most common forms of corrosion in stainless steel discussed.