What makes stainless steel “stainless”?
Stainless steel must contain at least 10.5 % chromium. It is this element that reacts with the oxygen in the air to form a complex chrome-oxide surface layer that is invisible but strong enough to prevent further oxygen from “staining” (rusting) the surface. Higher levels of chromium and the addition of other alloying elements such as nickel and molybdenum enhance this surface layer and improve the corrosion resistance of the stainless material. See the “Stainless Steel Primer” for more information.
Compared to plain carbon steel, stainless steels (there are more than 300 grades) contain at least 12% Chromium – a metal that is intrinsically corrosion resistant. Because of this alloying, stainless steels form a tightly-adherent and protective oxide layer which is so thin that the metallic coloration of the underlying steel is visible. The oxide layer that forms in ambient environments on plain carbon steels (i.e. “rust”) is a different oxide, which is nearly opaque, orange, thick, loose and porous.
