With the addition of 12% chromium to iron, stainless steel is formed. The chromium protects the iron against most corrosion or red-colored rust; thus the term “stainless” steel. The ability of stainless to form a thin layer of protection on its outside surface, called a “passive film”, is its most important characteristic in preventing corrosion (see PASSIVE FILM). The overriding purpose of stainless steel is to provide corrosion resistance against: (a) atmospheric conditions such as carbon dioxide, moisture, electrical fields, sulfur, salt, and chloride compounds; (b) natural and artificially produced chemical, (c) extremes of weather where cold temperatures cause brittleness and hot temperatures reduce strength and increase corrosion.
For more information, see AUSTENITIC, MARTENSITIC, FERRITIC, and PRECIPITATION HARDENING.