Passivation treatment is a chemical post-treatment process designed to enhance the inherent anti-corrosion properties of stainless steel. It differs from traditional metal pickling, which removes oxide layers from the surface, and from chemical film treatments applied before coating. After passivation, a protective oxide film forms on the surface of the stainless steel workpiece. This film is extremely thin, with a thickness of less than 0.0000001 inch, roughly 1/100,000th the thickness of human hair. Generally, workpieces treated with passivation are clean, bright, and resistant to rust.

This oxide film naturally forms when the workpieces are exposed to oxygen in the air after manufacturing, polishing, or acid cleaning. Under ideal conditions, the surface is completely covered with this oxide film. However, it can also become contaminated with iron particles from cutting tools, rust, or ferrous particles produced during manufacturing. Although stainless steel items appear smooth and clean, these invisible particles can potentially cause corrosion and degrade the quality of the protective oxide film. In some cases, rusty spots may even be visible.

Additionally, to facilitate the removal of iron chippings from cutting tools, AISI 303, known as free-cutting stainless steel, includes sulfides. However, these sulfides can also lead to rust and rusty spots when exposed to certain environments.