Stainless steel requires special cleaning and care to keep its luster and resistance to corrosion. Stainless steels are corrosion-resistant by nature, which might suggest that passivating them would be unnecessary. However, stainless steels are not completely impervious to rusting. One common mode of corrosion in corrosion-resistant steels is when small spots on the surface begin to rust because grain boundaries or embedded bits of foreign matter (such as iron from tooling, or from a manufacturing process) or excess carbon build of from high heat exposure allow water molecules to oxidize some of the iron in those spots despite the alloying chromium.
What is passivation? Passivation is "the removal of exogenous iron or iron compounds from the surface of stainless steel by means of a silicon cleaning pad (S.O.S or Scoth brite), a stainless steel wire brush or 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." In addition, it 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 the spontaneous formation of the protective passive film."
In lay terms, the passivation process removes "free iron" contamination left behind on the surface of the stainless steel from machining and fabricating. These contaminants are potential corrosion sites that result in premature corrosion and ultimately result in deterioration of the component if not removed. In addition, the passivation process facilitates the formation of a thin, transparent oxide film that protects the stainless steel from selective oxidation (corrosion). So what is passivation? Is it cleaning? Is it a protective coating? It is a combination of both.
How is passivation performed? The process typically begins with a thorough cleaning cycle. It removes oils, greases, forming compounds, lubricants, coolants, cutting fluids and other undesirable organic and metallic residue left behind because of fabrication and machining processes. General degreasing and cleaning can be accomplished many ways, including degreasing, solvent cleaning and alkaline soaking. Then a course scotch brite pad or stainless wire brush is used to remove the contaminates and restore the stainless to its original form which allows the surface to re- passivate.