Process · Passivation
A chemical treatment that strips free iron and contaminants from machined stainless and lets its chromium oxide film reform — the difference between stainless that resists corrosion and stainless that rusts.
Stainless steel is only as corrosion-resistant as its surface. Machining and handling embed free iron and leave contaminants that prevent the protective chromium oxide film from forming evenly — so a freshly machined stainless part can actually rust. Passivation chemically removes those contaminants and restores the film.
Process & standard
| Aspect | Detail |
|---|
| Standard | ASTM A967 (and AMS 2700) |
| Methods | Citric acid (preferred) or nitric acid |
| Effect | Removes free iron; reforms Cr oxide film |
| Dimensional change | None (chemical clean, not a coating) |
| Best for | 304 / 316 machined, welded or ground parts |
When to specify it
- After machining or grinding stainless, especially with steel tooling nearby.
- After welding, to clean the heat-affected zone (often with pickling first).
- For medical, food and marine parts where full corrosion resistance is required.
Applies to 304 and 316 parts; for appearance, combine with brushing or bead-blasting from the finishing guide.
Frequently asked questions
Why does stainless steel need passivation?
Machining, grinding and handling can leave free iron and contaminants on a stainless surface that rust and break down the protective film. Passivation removes them and lets the chromium oxide layer reform fully.
Citric or nitric acid passivation?
Both meet ASTM A967. Citric is safer and more environmentally friendly and is increasingly the default; nitric is the traditional method. The corrosion result is comparable for most grades.
Does passivation change appearance or dimensions?
No — it is a chemical cleaning, not a coating. It does not measurably change dimensions or color; the part looks the same but resists corrosion far better.
Is passivation the same as pickling?
No. Pickling is more aggressive and removes scale and a surface layer (e.g. after welding); passivation is a milder treatment that restores the oxide film on an already-clean surface.
