The term “stainless steel” is used to describe more than two hundred different grades, with each one tailored to give outstanding performance in various specific applications. All metals react with oxygen in the air or water, to form a film or oxide on the surface cover.
The oxide formed on ordinary steel allows the oxidation to continue producing the typical rusty appearance. However, since stainless steels contain more than 10.5% chromium, the characteristics of the oxide are changed. The chromium reacts with oxygen in the air or water to form the protective passive layer. The passive film is self-healing (in the most situations) if damaged. Nickel improves corrosion resistance and formability, and molybdenum improves resistance to localized corrosion.
Stainless steel surface can show rusty stains if ferrous contamination on the surface prevents the formation of a continuous passive film.
Furthermore, accumulation of dirt may lead to concentrations of corrosive substances that can break down the passive film.