Does 304 stainless need passivation?
Yes, after machining, grinding or welding. Passivation to ASTM A967 removes free iron and restores the chromium oxide layer; citric acid is the modern default.
304 resists corrosion because of a chromium oxide film that forms on its own in air. Machining, grinding, blasting and handling in a shop that also runs carbon steel leave free iron particles and sulfides embedded in the surface, and those spots rust. Passivation is a chemical clean that dissolves the free iron and lets a fresh, uniform passive film form. It removes nothing measurable from the part (no dimensional change), and it does not deposit a coating.
ASTM A967 is the spec to call out; AMS 2700 is the aerospace equivalent. Both allow nitric acid baths (Type 2 with sodium dichromate, or the plain nitric Types 6 through 8) and citric acid baths (Citric 1 through 5). Citric works well on 304, is safer to run and does not have the chromate waste issue, so it is what most job shops now use. Nitric-based passivation is still specified where a legacy drawing or a customer procedure requires it. Either process runs at 20-70 °C for 20-30 minutes, followed by a rinse and a verification test such as water immersion, high humidity or copper sulfate.
Passivation is not pickling. Pickling (nitric-hydrofluoric) removes weld heat tint and scale by dissolving metal; passivation only cleans an already bright surface. A welded 304 assembly usually needs pickling or mechanical cleaning first, then passivation. Electropolishing is another route that both smooths and passivates in one step.
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What to specify
- Passivate per ASTM A967 (or AMS 2700), and state the method if it matters: citric (Citric 2 or 3) or nitric (Type 2 or 6)
- Which verification test the shop must run and record: water immersion, high humidity, or copper sulfate per A967
- Passivate after all machining, grinding, welding and marking, as the last step before packaging
- Pickle or electropolish first if the part is welded or carries heat tint, since passivation will not remove oxide
- Handling: parts must not touch carbon-steel racks or tools afterwards; ask for gloved handling and clean packaging
- Free-machining 303 in the same lot needs a different bath (sodium dichromate or citric); do not mix grades in one basket
Pitfalls
- Passivating before the final machining or deburring operation, which re-contaminates the surface
- Expecting passivation to remove weld heat tint; it does not, pickle first
- Rust spots after passivation almost always trace to blasting with used media or grinding with carbon-steel wheels
- Running 303 in a 304 nitric bath without dichromate and etching the 303
- Skipping the verification test; a passivation cert without a test result proves nothing
- Packing passivated parts in cardboard or carbon-steel bins that leave iron transfer marks
Frequently asked questions
- Is passivation required on 304 stainless?
- Not always, but it is cheap insurance on any machined or welded part. Machined 304 that goes into wet or outdoor service without passivation often shows rust spots from embedded iron within weeks.
- Citric or nitric passivation for 304?
- Citric is the default today: it works on 304, is safer to run and meets ASTM A967. Nitric is specified where an existing procedure or customer requires it. Both give the same result on 304.
- Does passivation change dimensions or appearance?
- No. It removes only surface contamination, well under a micrometre. The part looks the same as it did going in, so the verification test is the only way to know it was done.
