Passivation cleans free iron and dirt off the surface of stainless steel parts with a mild acid, usually nitric or citric. That lets the chromium in the steel form a clean, even oxide film, which is what makes stainless resist rust. It is done after machining, grinding or fabrication, when tools and handling leave iron particles that would otherwise show as rust spots.
Quick facts
- Materials
- Stainless steels (300 and 400 series, precipitation hardening grades)
- Acids
- Nitric acid or citric acid solutions
- Changes size
- No (no measurable metal removed)
- Appearance
- Little change; parts look clean and even
- Common standards
- ASTM A967, ASTM A380, AMS 2700
- Usually priced
- Per kg or per batch
How it is done
- Parts are degreased and cleaned so the acid reaches the metal
- They are dipped in a nitric or citric acid bath for a set time and temperature
- They are rinsed well in clean water
- They are dried, and tested if the order asks (water immersion, humidity or copper sulphate test)
Passivation, pickling or electropolishing?
| Passivation | Pickling | Electropolishing | |
|---|---|---|---|
| Removes | Free iron and dirt | Weld scale and heat tint (some metal) | A thin layer of metal |
| Size change | None | Small | Small, controlled |
| Finish | Unchanged | Dull, even | Bright, smooth |
| Typical use | Machined parts | Welded fabrications | Food, pharma, medical parts |
What to tell the passivation shop
- Stainless grade, since some grades need milder treatment
- Standard and method required (for example ASTM A967, citric)
- Whether welds have heat tint that needs pickling first
- Tests and certificate needed
- Threads, blind holes or crevices where acid could stay trapped
- Packing so clean parts are not touched with bare hands or steel tools
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Questions
- Does passivation change the size of a part?
- No. It removes only free iron and contamination from the surface, not a measurable amount of stainless steel, so finished sizes stay the same.
- Nitric or citric passivation?
- Both meet standards such as ASTM A967. Citric is safer to handle and kinder to the environment; nitric is long established and preferred by some older specifications.
- Why do stainless parts rust after machining?
- Iron from cutting tools, fixtures or nearby carbon-steel work sticks to the surface and rusts. Passivation removes it.
The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 22 October 2026.