An English Metallurgist, working on a project to improve rifle barrels, accidentally discovered that adding chromium to low carbon steel gives it stain resistance.In addition to iron,carbon and chromium, modern stainless steel may also contain other elements such as nickel, niobium, molybdenum and titanium.Nickel, molybdenum, niobium and chromium enhance the corrosion resistance of stainless steel.Its the addition of a minimum 12% of chromium to the steel that makes it resist rust, or stain 'less' than other types of steel.The chromium in the steel combines with oxygen in the atmosphere to form a thin, invisible layer of chrome-containing oxide, called the passive film.The sizes of chromium atoms and their oxides are similar, so they pack neatly together on the surface of a metal,forming a stable layer only a few atoms thick.If the metal is cut or scratched and the passive film is disrupted, more oxide will form quickly and recover the exposed surface, protecting it from the oxidation corrosion.Iron, on the other hand rusts very quickly because atomic iron is much smaller than its oxide.The passive film requires oxygen to self-repair, so stainless steels have poor corrosion resistances in low-oxygen and poor circulation environments.
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