It is an iron and chromium alloy that may also contain nickel, molybdenum and other elements, presenting higher physico-chemical properties than ordinary steels, and its main characteristic is high resistance to atmospheric oxidation.

These alloying elements, in particular chromium, impart excellent corrosion resistance when compared to carbon steels. They are, in fact, oxidizable steels. That is, the chromium present in the alloy oxidizes in contact with the oxygen in the air, forming a very thin and stable film of chromium oxide – Cr 2 O 3 – which forms on the surface exposed to the environment. This is called a passive layer and its function is to protect the surface of the steel against corrosive processes. For this a minimum amount of chromium of about 11% by mass is required.

This film is adherent and impermeable by insulating the metal below it from the aggressive environment. Thus, care must be taken not to locally reduce the chromium content of stainless steels during processing. This process is known in metallurgy as passivation.