Passivation
Also: stainless steel passivation, citric passivation, nitric passivation, ASTM A967, AMS 2700, passivate, chemical passivation
Passivation cleans free iron off stainless steel in a nitric or citric acid bath so its chromium oxide film reforms intact. No coating, no dimensional change: it restores corrosion resistance.
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Stainless steel resists corrosion because chromium in the surface forms a thin, self-healing oxide film. Machining, forming, grinding and handling smear free iron and steel particles into that surface, and each one becomes a rust spot. Passivation removes them: the part is cleaned, immersed in nitric or citric acid for 20-120 minutes, rinsed and dried, and the oxide film reforms over a surface that is now all stainless. Nothing is added; the film is a few nanometres thick.
ASTM A967 and AMS 2700 are the specs. Both offer nitric acid methods (with or without sodium dichromate, at various concentrations and temperatures) and citric acid methods, each keyed to the alloy family. Citric passivation has taken over most commercial work: it is safer to handle, does not etch free-machining and martensitic grades the way nitric can, and it passes the same tests. Both specs finish with a verification test, most often water immersion, high humidity, salt spray, or the copper sulfate test, and a cert.
The limits are worth knowing. Passivation does not remove heat tint, weld scale or embedded scale; those need pickling (nitric-hydrofluoric) or mechanical cleaning per ASTM A380 first, and a passivation over heat tint leaves a part that still rusts. It does not passivate carbon steel, plated steel or titanium (titanium has its own treatments). And a greasy or oxide-shielded part will not passivate: cleaning is most of the job.
At a glance
| Typical tolerances | No coating and no measurable dimensional change; the removed iron and the reformed oxide are far below 1 µm. Aggressive nitric methods can etch free-machining and martensitic grades by a few micrometres and dull the surface, which is why citric or dichromate methods are specified for them. Threads, fits and finishes are unchanged. |
|---|---|
| Size limits | Tank-limited: 1.5-3 m (5-10 ft) is typical at job shops, larger by arrangement. Small parts go in baskets by the thousand. Closed assemblies and tubing are passivated by circulating solution through them. Field passivation of vessels and piping uses gel or spray citric products. |
| Surface finish | Unchanged from incoming, apart from a slight dulling of mirror finishes in nitric methods and a clean, water-break-free surface. No colour change; heat tint and scale remain unless removed first. Hardness and the base metal are untouched. Electropolishing is the process to use when a brighter, smoother, passive surface is wanted. |
| Lead time | 1-3 business days as a batch process, often same day for small lots; add time for verification testing (salt spray takes days) and for pickling or electropolishing when heat tint must come off first. |
Typical of U.S. job shops; confirm with the shop for your part.
Materials
All stainless steels. Austenitic 304, 316 and 321 are routine. Martensitic 410, 420 and 440C and precipitation-hardening 17-4 and 15-5 need the nitric-dichromate or citric variants to avoid etching. Free-machining 303 and 416 are the difficult ones: their sulfides dissolve in nitric acid and pit, so use citric or a nitric-dichromate method with short times. Duplex and superaustenitic grades passivate easily. Not applicable to carbon steel, alloy steel or aluminum; titanium and nickel alloys have different treatments.
What drives the cost
- Lot size: a minimum lot charge dominates; hundreds of parts cost little more than one
- Cleaning: parts that arrive with oil, coolant, marker or adhesive residue need extra degreasing
- The method: citric and nitric-dichromate cost more than plain nitric; AMS 2700 controls more than ASTM A967
- Verification testing (water immersion, humidity, salt spray, copper sulfate) and the certification
- Pickling or mechanical descaling of welded or heat-treated parts before passivation
- Special handling: medical and semiconductor parts with cleanroom packaging and lot traceability
When to use it
- Every machined, ground or formed stainless part that will see moisture, food, medical or marine service
- Stainless parts that have shown rust spots after machining or handling
- After welding or heat treatment, following pickling or blasting to remove scale and heat tint
- Medical devices, food equipment and semiconductor hardware where a documented passive surface is required
- Stainless fasteners and hardware before assembly with other metals
When not to
- Carbon steel, alloy steel, plated or coated parts: the acid will attack them and passivation means nothing there
- Parts with heat tint, weld scale or embedded scale, until pickled or blasted; passivation alone leaves them rusting
- Assemblies with dissimilar metals, adhesives, seals or trapped volumes that the acid will damage or hold
- Free-machining 303 or 416 in a plain nitric bath; specify citric or nitric-dichromate instead
- Expecting a change in appearance or a coating; use electropolishing for brightness
Design tips
- Call out "Passivate per ASTM A967 (citric or nitric, method as applicable)" or "AMS 2700 Method 1 or 2, Type as applicable", and the verification test you want.
- State the alloy on the drawing; the shop picks the method from it, and 303 or 416 get different treatment from 304.
- Sequence passivation last: after all machining, welding, grinding, marking and heat treatment, and after pickling if there is heat tint.
- Avoid blind holes and crevices where acid can be trapped, or specify a rinse and dry step with care; drain holes help.
- Use only stainless or ceramic tooling and clean abrasives after passivation; a carbon-steel wire brush undoes it.
- For weldments, specify pickling or electropolishing to remove heat tint, then passivation; passivation alone is not a heat-tint remover.
- Request a passivation cert with the method and test on medical, food and aerospace jobs.
Passivation by material
Frequently asked questions
- What does passivation do to stainless steel?
- It dissolves the free iron and iron particles left on the surface by machining and handling, so the chromium oxide film that gives stainless its corrosion resistance reforms without rust-starting sites. It adds nothing and changes no dimension.
- Citric or nitric passivation?
- Citric for most commercial work: safer, gentler on 303, 416 and martensitic grades, and it passes the same ASTM A967 and AMS 2700 tests. Nitric remains specified on some legacy aerospace and military drawings and where a customer requires it.
- Does passivation remove heat tint from welds?
- No. Heat tint is a chromium-depleted oxide and must be removed by pickling (nitric-hydrofluoric), electropolishing or mechanical cleaning first. Passivating over heat tint leaves a part that still rusts at the weld.
- Why is my passivated stainless steel rusting?
- Usually embedded iron the passivation could not reach: heat tint, scale, a carbon-steel tool mark after passivation, or oil on the part that shielded the surface. Less often, a free-machining grade (303, 416) that the acid attacked, or an assembly that trapped acid.
- How long does passivation last?
- As long as the surface stays stainless. The passive film reforms on its own in air, so a passivated part stays passive until something contaminates it again, such as contact with carbon steel tools, grinding dust or chlorides.
