Passivation is the removal of iron compounds from the surface of a stainless steel by means of a chemical dissolution, most typically by a treatment with an acid solution that will remove the surface contamination but will not significantly affect the stainless steel itself.

The standard also describes passivation as the chemical treatment of stainless steel with a mild oxidant, such as a nitric acid solution, for the purpose of enhancing spontaneous formation of the protective passive film.

The purpose is to remove the “free iron” on the surface of stainless steel left behind from machining and fabrication. Left attached, this free iron becomes a site for potential corrosion and deterioration. During passivation, a thin, oxide film is also formed that protects stainless steel from corrosion.