Stainless steels are a special class of steel alloys known primarily for their corrosion resistant properties. The stainless characteristics associated with these alloys are achieved through the formation of an invisible and adherent chromium-rich oxide surface film that, when damaged, has the unusual ability to heal itself in the presence of oxygen. In order for an alloy to be classified as a stainless steel it must contain at least 11.5 wt% Cr, with at least 12 wt% Cr required for aqueous corrosion resistance. At 12 wt% Cr, they become passive in aqueous solutions. There are five main classes of stainless steels, designated in accord with their crystallographic structure. Each class consists of several alloys of somewhat differing composition having related physical, magnetic, and corrosion properties.