Sources and scope for this page: Industry convention — no single governing standard (conventional usage, definitions vary between sources), covering the definition of passivation. No independent source check is recorded for this page: the content rests on the sources listed and on the working shown on the page. How sources, methods and updates are recorded.
Definition
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.
Common misuse
Assuming any stainless part is automatically corrosion-resistant. Machined and welded surfaces need the contamination removed before the passive layer is reliable.