Passivation
Passivation removes free iron from a stainless steel surface with an acid bath so the chromium oxide layer can reform unbroken.
- Part
- Finishing
- Revised
- 2026-08-11
At a glance
- Family
- Additive
- Typical tolerances
- No measurable dimensional change; the passive film is a few nanometers thick
- Surface finish
- Unchanged — passivation neither smooths nor brightens the surface
- Typical volumes
- 1 part to millions; small parts run in bulk baskets at very low cost per piece
- Lead time
- 1–5 business days at a job shop; often same-day when combined with an existing cleaning operation
- Materials
- Metal
What it is
Passivation is a chemical dip that removes free iron and other exogenous contamination from a stainless steel surface so the chromium oxide film that gives stainless its corrosion resistance can reform continuous and unbroken. Nothing is deposited and essentially nothing is removed — the passive film is only a few nanometers thick, so passivation is dimensionally neutral and invisible.
It is a corrective, not a coating. Machining, grinding, blasting with contaminated media, tumbling with steel media and even handling with carbon-steel tooling smear free iron across a stainless surface. That embedded iron rusts, and the rust spots look like the stainless has failed when the substrate is fine. Two chemistries dominate: nitric acid, typically 20–50% by volume at 70–130 °F (21–54 °C), and citric acid, typically 4–10% by weight at 70–160 °F (21–71 °C). ASTM A967 and AMS 2700 define both, along with the tests used to prove the result.
How it works
- Degrease. Cutting oil, coolant, drawing compound and fingerprints must be removed first. An acid bath cannot passivate through a film of oil, and this is the most common cause of a failed passivation.
- Descale if required. Passivation does not remove weld heat tint, mill scale or oxide. Those require pickling, mechanical cleaning or blasting with clean non-ferrous media beforehand. A heat-tinted weld zone that has only been passivated will still corrode, because the chromium-depleted layer under the tint is still there.
- Immerse in the acid. Nitric acid dissolves free iron much faster than it dissolves the chromium-rich stainless matrix. Citric acid works by chelating iron instead and is increasingly preferred because it is not an oxidizer, is easier to handle and dispose of, and does not require the sodium dichromate additions that some nitric baths do. Times run roughly 20–30 minutes for nitric and 4–30 minutes for citric depending on grade and temperature.
- Rinse thoroughly. Residual acid in blind holes, threads or crevices causes the exact corrosion the process was meant to prevent. Deionized water for the final rinse on critical work.
- Dry and let the film form. The passive chromium oxide layer reforms on exposure to air within minutes and continues to strengthen over roughly 24–48 hours.
Choosing the bath
Free-machining grades — 303, 416 and other sulfur- or selenium-bearing steels — and high-carbon martensitic grades can flash-attack in plain nitric acid, etching and darkening instead of passivating. ASTM A967 addresses this with specific nitric types (including dichromate-inhibited baths) and with citric alternatives. Specify the ASTM A967 or AMS 2700 method and the acceptance test rather than just writing "passivate".
Design guidelines
Passivation fixes contamination, not design
If a part corrodes because the grade is wrong for the environment, because a crevice traps chlorides, or because a weld was not descaled, passivation will not save it. Choose the alloy first: 304 for general use, 316/316L where chlorides are present, and duplex or higher grades for aggressive service.
Keep iron off the part in the first place
Specify stainless-only or ceramic blast media, non-ferrous or dedicated stainless tooling, and separate stainless from carbon steel in tumbling, deburring, forming and storage. Passivation removes surface iron but cannot recover iron that has been pressed deep into a soft, worked surface.
Design out acid traps
Blind holes, threads, crimped joints, spot-welded overlaps and unsealed crevices hold acid through rinsing. Provide drain paths and through-features where possible, and specify a deionized final rinse for parts with internal passages.
Do not expect an appearance change
Passivation leaves the surface looking exactly as it went in. Machining marks, dullness, discoloration and weld tint all remain. If the requirement is appearance or cleanability, the surface has to be mechanically finished or electropolished; see the surface finish chart for the Ra ranges each prep achieves.
Specify the test, not just the process
ASTM A967 pairs each treatment with acceptance practices — high humidity, water immersion, salt spray, copper sulfate and free-iron tests. A drawing note that reads "passivate per ASTM A967, citric 3, test per Practice E" is verifiable; "passivate" alone is not.
Sequence it last
Passivation belongs after all machining, welding, forming, marking and deburring. Any subsequent operation with steel tooling reintroduces free iron, and laser marking can locally disturb the passive layer, which is why medical device routings often place passivation after marking.
| Feature | Recommended | Limit | Why |
|---|---|---|---|
| Nitric acid bath | 20–50 vol%, 70–130 °F (21–54 °C) | Not for free-machining grades uninhibited | Sulfur-bearing grades flash-attack |
| Citric acid bath | 4–10 wt%, 70–160 °F (21–71 °C) | — | Non-oxidizing, safer handling and disposal |
| Dimensional change | None measurable | — | The passive film is a few nanometers thick |
| Pre-cleaning | Full degrease mandatory | — | Acid cannot passivate through oil |
| Weld heat tint | Remove mechanically or by pickling first | — | Passivation does not restore a chromium-depleted layer |
| Position in routing | After all machining and marking | — | Steel tooling recontaminates the surface |
Cost drivers
Passivation is among the cheapest metal finishing operations. It is priced per lot or per pound, and the bath cost per part is small, so the price is driven almost entirely by handling and paperwork.
- Part handling. Small parts run in baskets at very low cost per piece. Large weldments must be racked, immersed individually or spray-treated.
- Bath choice. Citric baths generally carry lower disposal and handling cost than nitric; nitric with dichromate carries the highest.
- Testing and certification. Copper sulfate, high humidity, water immersion or salt spray testing per ASTM A967, plus a certificate of conformance, is often the largest single line on a small order.
- Pre-cleaning exceptions. Parts arriving with weld tint, mill scale, heavy oil or steel-media embedment need pickling, blasting or descaling first — a separate operation.
- Rework. A failed copper sulfate test means re-cleaning and re-running the whole lot.
Cost-reduction tactics:
- Keep carbon steel away from stainless throughout the shop; contamination avoided is passivation rework avoided.
- Batch parts so the lot minimum is spread over as many pieces as possible.
- Specify citric acid unless a customer specification requires nitric.
- Combine passivation with an existing cleaning step in the routing rather than adding a separate handling cycle.
- Where the part also needs a bright, low-particle surface, price electropolishing once instead of mechanical polishing plus passivation.
Questions
6 questionsWhat does passivation actually do?
It removes free iron and other surface contamination from stainless steel so the chromium oxide film can reform continuous and unbroken. Nothing is deposited and essentially no metal is removed — the passive film is only a few nanometers thick, so the process is dimensionally neutral and visually invisible.
Citric or nitric acid passivation — which should I specify?
Citric acid, typically 4–10% by weight at 70–160 °F (21–71 °C), is increasingly the default because it is non-oxidizing, easier to handle and cheaper to dispose of. Nitric acid at 20–50% by volume remains common and is required by some customer specifications. Both are defined in ASTM A967 and AMS 2700.
Does passivation remove weld heat tint or scale?
No. Passivation dissolves free iron, not oxide. Heat tint, mill scale and weld scale must be removed by pickling, blasting with clean non-ferrous media, or mechanical cleaning first. A passivated but still heat-tinted weld will corrode because the chromium-depleted layer beneath the tint remains.
Does passivation change part dimensions?
No measurable amount. This is the main practical difference from electropolishing, which removes 0.0002–0.001 in (5–25 µm) per surface. Passivation can therefore be applied to finished, toleranced parts without any allowance.
Why did my 303 stainless part come out etched and dark after passivation?
Free-machining grades such as 303 and 416 contain sulfur or selenium, and they can flash-attack in an uninhibited nitric bath — etching and darkening instead of passivating. ASTM A967 addresses this with dichromate-inhibited nitric types and with citric alternatives; specify the method rather than just writing 'passivate'.
Where in the routing should passivation go?
Last, after all machining, welding, forming, deburring and marking. Any operation using steel tooling or steel media reintroduces free iron. Medical device routings often place passivation after laser marking specifically because marking can disturb the passive layer.