Passivation is a chemical treatment applied to stainless steels. The part is immersed in an acid bath (nitric or citric acid) that dissolves free iron and the contamination left on the surface by machining, such as steel particles picked up from a tool or cutting residues. It then promotes the rebuilding of the thin chromium oxide layer that gives stainless steel its corrosion resistance. No metal is deposited: dimensions and appearance do not change in any measurable way.
It is most useful after an operation that may have contaminated or altered the surface: machining, deburring, polishing, welding. The sectors that require or recommend it are food processing, medical, pharmaceuticals, chemicals and the marine environment. The ASTM A967 standard is a common reference for procedures and verification tests; a workshop that carries out the process will be able to say which method it uses.
It should not be confused with other operations. Pickling, which is more aggressive, removes the oxides and heat tint left by welding. Electropolishing removes a thin layer of material to smooth the surface. By extension, the name passivation is also given to the conversion treatment applied over a zinc deposit, which is a different operation. Finally, a passivated part is not immune to everything: in chloride environments, the choice of grade (316L rather than 304L) remains decisive.
In a quote request
State the exact grade, the standard or specification to follow, the sector of use and the verification test expected. Specify whether the part has been welded or polished, as it may need pickling first, and ask for a treatment certificate if your specification requires one. Plan passivation after machining, deburring and careful degreasing, never before. See also the guide stainless steel 304 or 316L.