Choosing a Passivation Method Under ASTM A967
September 21, 2026
Passivation shows up on most stainless drawings as a single word. That word is not enough to quote from, and it is not enough to inspect against.
What both methods do
Neither adds a coating. Both are acid baths that dissolve free iron from the surface without dissolving the stainless underneath, so the chromium oxide film can rebuild evenly.
The free iron gets there during manufacture. Carbide inserts, steel fixtures, grinding media, mild steel swarf in the machine and steel brushes all leave iron particles embedded in the surface. That iron rusts. The part then shows rust spots and the customer assumes the wrong grade was supplied.
ASTM A967 is the specification that covers both routes. AMS 2700 is the aerospace equivalent and says much the same thing.
Nitric acid
The original method and still written into many customer standards and older drawings.
How it works. Nitric acid oxidises and dissolves free iron. Sodium dichromate is sometimes added to speed it up on grades that passivate reluctantly.
Where it is strong. Long established, widely available and well understood. Every finisher can do it. It handles a wider range of soil on incoming parts.
Where it causes problems.
- Free-machining grades. Nitric attacks the manganese sulphide stringers in 303 and 416. It dissolves the inclusions and leaves open pits, so the surface can end up more vulnerable than before. ASTM A967 recognises this by listing specific milder nitric methods for these grades.
- High carbon martensitic grades. 440C and 420 can flash attack, a rapid etching that ruins the surface.
- Handling and disposal. Strong acid, fume extraction, hexavalent chromium in some formulations. Real cost and real regulatory burden for the finisher.
Citric acid
The newer route, and the default for medical and food work today.
How it works. Citric acid chelates iron rather than oxidising it. It grabs iron ions and holds them in solution, leaving the chromium untouched.
Where it is strong.
- Gentler on the base metal. It does not attack sulphide inclusions or flash attack high carbon grades, so it is the right call on 303, 416, 420 and 440C.
- Selective. It goes after iron specifically, so there is less risk to the surface finish.
- Easier to handle. Not classed as hazardous in the same way, simpler disposal, no fume extraction requirement.
- Accepted by the standard. ASTM A967 covers it fully and the 2025 revision expanded the commentary around it.
Where it needs care. The part must be genuinely clean going in. Citric is less aggressive, so it will not fight through cutting oil, marker pen or fingerprints. Degreasing before the bath is not optional. Bath temperature and concentration also matter more than with nitric, which means it needs a finisher with process control rather than a tank in a corner.
A simple rule
| Grade | Recommended |
|---|---|
| 304, 304L, 316, 316L | Either. Citric unless a customer standard says otherwise |
| 303, 416, other free-machining | Citric |
| 420, 440C, high carbon martensitic | Citric |
| 17-4 PH, 15-5 PH | Citric, and note the condition on the drawing |
| 2205 and duplex | Either, with process control. Ask the finisher |
If a customer specification or an existing qualified process already names nitric, follow it. Changing a validated process to save a small amount is rarely worth the paperwork.
The part everyone forgets: the acceptance test
A passivation certificate that does not name a test is close to worthless. ASTM A967 lists several and they are not equally severe:
- Water immersion. Alternating immersion and drying, looking for rust. Slow and mild.
- High humidity. Typically 24 hours at high humidity and elevated temperature. Common and reasonable.
- Salt spray. Per ASTM B117, usually 2 hours for passivation acceptance. Severe.
- Copper sulphate. A solution is applied and any copper plating out indicates free iron. Fast, sensitive and the usual choice for austenitic grades. Not valid on grades below about 16 percent chromium, which rules it out on 410, 416 and 420.
- Potassium ferricyanide nitric acid. Very sensitive free iron test, mostly used for investigation.
Pick one and name it. A drawing that says passivate per ASTM A967 citric Method 1, test per copper sulphate, certificate required is quotable, inspectable and comparable between suppliers.
Order of operations
- Deburr first. A burr that breaks off later exposes fresh unpassivated metal. There is more in surface finish.
- Clean and degrease. Especially before citric.
- Passivate.
- Test and document.
- Handle with gloves after. Bare hands put chlorides and oils back onto a clean surface.
If the part is also being electropolished, the electropolishing step leaves the surface passive on its own and a separate passivation is often unnecessary. Say which certificate you need so the shop does not run both. The passivation against electropolishing article covers where each one fits.
What to write
Passivate per ASTM A967, citric acid, Method 1. Acceptance test per copper sulphate. Certificate required with lot traceability.
One line, and every shop that quotes it is quoting the same thing. The full RFQ checklist covers the rest.
Frequently asked questions
Is citric passivation as effective as nitric?
For most grades yes, and ASTM A967 treats both as acceptable routes to the same result. Independent testing over the last two decades has generally found citric equal or better on austenitic grades, which is why medical and food work has moved toward it.
Which method should I use on 303 or 416?
Citric. Nitric can attack the sulphide inclusions in free-machining grades and leave the surface worse than it started. If your part is 303, 416 or another sulphurised grade, say citric on the drawing.
Do I need to specify the acceptance test?
Yes if you want a certificate that means something. ASTM A967 lists several tests including copper sulphate, high humidity, salt spray and free iron checks. Without a named test the shop picks, and you cannot compare one supplier's certificate to another's.
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Get matched with a stainless shopFigures in this guide are published specification limits or published typical values for the alloy, not measurements of any part. Machinability is rated against free-machining carbon steel B1112 at 100 and PREN is chromium plus 3.3 times molybdenum plus 16 times nitrogen. The conventions are set out in thereference section and the editorial standards.