Stainless steel bar rotating on a turntable. Stainless steel is a carbon steel that contains at least 10.5% chromium, and commonly more than that. Normal steel, which is iron with up to 2% carbon, readily corrodes in moist air, forming rust (iron oxide) that flakes off, exposing more metal. The addition of chromium to the steel protects against rusting as the chromium forms a very thin surface layer of chromium dioxide, which prevents atmospheric oxygen from reaching lower layers. This is called passivation.