Stainless steel passivation per AMS 2700 and ASTM A967.
Argent provides aerospace passivation services for stainless steel parts — the chemical treatment that removes free iron from the surface and restores the protective chromium oxide layer. Nitric acid and citric acid processes per AMS 2700 and ASTM A967, for 300-series, 400-series, 17-4 PH, and 15-5 PH stainless used in aerospace structural, propulsion, and defense applications.
Why machined stainless steel needs passivation.
Stainless steel resists corrosion because of a thin self-healing layer of chromium oxide on the surface. This passive layer forms naturally when the alloy is exposed to oxygen — it's what makes "stainless" stainless.
Machining, grinding, and forming damage that layer. Cutting tools embed iron particles into the surface, drag chromium-depleted material across freshly cut zones, and contaminate the surface with iron from tooling. Without treatment, those iron particles rust on the part surface within weeks of exposure to humidity. The part looks like it's failing even though the bulk material is still stainless.
Passivation is a chemical treatment that removes the embedded iron and contaminants and chemically restores the protective chromium oxide layer. The process steps:
- Cleaning. Remove machining oils, cutting fluids, and surface contamination. Typically alkaline cleaning followed by deionized water rinse.
- Acid bath. Immerse in nitric acid (traditional) or citric acid (newer, more environmentally friendly) for a controlled time at controlled temperature. The acid dissolves embedded iron particles and any residual contamination without significantly attacking the chromium-rich stainless.
- Rinse and dry. Thorough rinse with deionized water to remove all acid residue, then dry. The chromium oxide layer re-forms naturally during this exposure to oxygen.
- Testing. Passivated parts are tested for residual free iron, typically by water immersion (24-hour wet stain test), copper sulfate, or salt spray per AMS 2700 or ASTM A967 requirements.
Passivation is required on virtually all aerospace stainless steel parts after machining or grinding. It's also commonly required after welding (heat-affected zones can be chromium-depleted) and after surface preparation operations like polishing or blasting.
Passivation processes and standards.
Standards we work to
- AMS 2700 — Passivation of corrosion-resistant steels. The primary aerospace spec.
- ASTM A967 — Chemical passivation treatments for stainless steel parts. Commercial and aerospace baseline.
- QQ-P-35 — Legacy federal spec for passivation; superseded but still referenced on some drawings.
- ASTM F86 — Passivation for medical-grade stainless (some defense applications).
Required testing after passivation
- Water immersion test (high-humidity test). 24-hour wet exposure followed by visual inspection for rust.
- Copper sulfate test. 6-minute exposure to copper sulfate solution — copper deposits indicate free iron contamination.
- Salt spray test (ASTM B117). 2 to 24 hours per drawing requirement, visual inspection for corrosion.
- Ferroxyl test. Chemical test using potassium ferricyanide — blue color indicates free iron.
Which passivation process for your part.
The two passivation chemistries produce equivalent results in most cases, but each has its place:
Nitric acid passivation
- Faster process — typical bath times 20–60 minutes
- Broader alloy coverage, including high-carbon 400-series
- Standard for legacy aerospace programs and most defense work
- Generates nitrogen oxide fumes — requires scrubber-equipped facility
- Hazardous waste disposal cost is significant
Citric acid passivation
- Slower bath times — typically 60–240 minutes at elevated temperature
- Lower-toxicity, less hazardous waste
- Increasingly required on new aerospace programs (Boeing, Lockheed)
- Better for environmentally regulated facilities
- Some restrictions on 400-series alloys depending on carbon content
Argent works with U.S. finishers qualified for both chemistries. Tell us what your drawing specifies — AMS 2700 Method 1 (nitric) or Method 2 (citric) — and we route accordingly. For new program work where you have a choice, we can advise on which chemistry typically performs better for your specific application.
Pairs well with.
Argent customers typically combine multiple capabilities on the same program. These are the most common pairings with this work.