Stainless steels are steel alloys that contain more than 10.5% chromium with excellent corrosion resistance. Chromium reacts strongly with oxygen to form a very thin, invisible, stable oxide film on the surface of the stainless steel. This film is called the passive layer and forms rapidly in ordinary atmospheres. If it is damaged, the film usually heals spontaneously. It is this passive layer that gives stainless steel its corrosion resistance.