The natural or applied formation of a protective oxide layer on a metal surface.
Stainless steel's corrosion resistance comes entirely from a chromium oxide layer a few nanometres thick that forms instantly in air and reforms if damaged. It is not that stainless does not corrode — it is that it repairs itself.
Machining can compromise it: embedded iron particles from tooling, heat tint from welding, and free iron from contamination all sit on the surface and become corrosion initiation sites. Passivation treatment removes them and lets the oxide reform cleanly.
Assuming any stainless part is automatically corrosion-resistant. Machined and welded surfaces need the contamination removed before the passive layer is reliable.
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