Stainless steel is formed on the surface of a thin and strong and stable chromium-rich oxide film (protective film), to prevent the continued penetration of oxygen atoms, continue to oxidize, and get the ability to resist corrosion. Once for some reason, this kind of film is destroyed constantly, oxygen atoms in the air or liquid will continuously infiltrate or the iron atoms in the metal will continue to separate out to form loose iron oxides, and the metal surface will be continuously rusted. This type of surface film is damaged in many forms, and is often found in everyday life.