Stainless steel is a generic term for a family of corrosion resistant alloy steels containing 10.5% or more chromium.
All stainless steels have a high resistance to corrosion. This resistance to attack is due to the naturally occurring chromium-rich oxide film formed on the surface of the steel. Although extremely thin, this invisible, inert film is tightly adherent to the metal and extremely protective in a wide range of corrosive media. The film is rapidly self repairing in the presence of oxygen, and damage by abrasion, cutting or machining is quickly repaired.
Stainless steel is susceptible to certain types of corrosion despite the fact that one of the main reasons for using stainless steel is its resistance to corrosion. The different kinds of corrosion affecting stainless steel are:
General Corrosion, pitting corrosion, crevice corrosion, stress corrosion cracking, sulphide stress corrosion cracking, intergranular corrosion, galvanic corrosion and contact corrosion.
Corrosion in stainless steel occurs when the protective layer of chromium-oxide (Cr2O3) is broken down. This can spread as microscopic or visibly spots over the surface of the steel.