Published: Jan 2005
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STAINLESS STEELS ARE iron-based alloys containing at least 10 % chromium . Although iron may corrode in an ordinary rural environment, the chromium gives stainless steel its ability to form a protective or passive film that resists corrosion. It is this ability to resist the formation of rust that led to the name “stainless steel.” Generally, it is thought that the passive film consists of hydrated chromium oxide. Other alloy elements such as iron, molybdenum, silicon, etc., have also been detected in the passive film. The passive film also tends to incorporate anions and cations that may be present in the environment in which the film forms. Stainless steels are autopassivating in the sense that the passive film is formed spontaneously upon exposure to air, moisture, or an oxidizing acid. When stainless steel is exposed to such an environment, its corrosion resistance approaches that of noble metals; however, when exposed to an environment that damages, reduces, or inhibits the formation of the passive film, corrosion resistance of the alloy is compromised.
Specialist, Stainless Alloy R&D, Carpenter Specialty Alloys, Reading, PA