Stainless steel does not rely solely on its surface to resist rust; instead, it prevents further oxidation through the formation of a chromium-rich oxide layer. This very thin yet strong oxide film on its surface blocks oxygen atoms from penetrating the metal, thus preventing further oxidation. However, if this layer is damaged-such as by scratching or wear-oxygen atoms from the air can infiltrate the metal, causing iron atoms to precipitate and form loose iron oxide, commonly known as rust.
: If there are dust particles or foreign metal fragments on the surface of stainless steel, in humid air, these contaminants can create a micro-battery with condensed water between them and the stainless steel, triggering an electrochemical reaction that damages the protective film.