Guide
What does ASTM A967 passivation mean for machined stainless parts?
Passivation is a chemical cleaning step that dissolves free iron and other foreign matter from the surface of a finished stainless steel part, so the chromium-rich oxide film that protects the alloy can form evenly and without interruption. It is not a coating: nothing is deposited, and a correctly passivated part looks the same as it did before the bath. ASTM A967/A967M is the specification most commercial drawings cite for it, and SAE AMS 2700 is the aerospace counterpart. Both leave important choices to the buyer, which is why a bare “passivate per ASTM A967” note can produce very different results from two suppliers.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers and sourcing managers who put a passivation note on turned or milled stainless parts. It works from the published scopes of ASTM A967 and ASTM A380 and from public NASA reports on the nitric and citric treatments; shop practice is labeled as typical guidance. Check the revision your drawing cites before relying on any detail.
On this page
What does passivation actually do to a machined stainless part?
It removes iron and other contamination that machining, handling and tooling leave on the surface, so that corrosion does not start from those foreign particles. The passive oxide itself forms on its own in air once the surface is truly clean.
ASTM A380/A380M-17 is unusually direct about this. It lists three meanings of the word in common use: the spontaneous formation of the passive film in air, the removal of exogenous iron or iron compounds by chemical dissolution, and treatment with a mild oxidant such as nitric acid to encourage the film. It then states that, unless otherwise specified, a requirement for passivation means the second one: removing contaminant iron without significantly affecting the stainless steel itself. It also notes that an oxidizing treatment is generally not needed for the oxide film to form.
The 2017 edition, ASTM A967/A967M-17, cast a wide net in its explanatory note on free iron, taking in iron contamination, iron-tool marks, residual iron salts from pickling, iron dust, atmospheric exposure, iron deposits in welds, embedded iron and iron oxide; the 2025 edition speaks of contaminant iron instead. On a machined part the usual sources are steel chips, shared coolant carrying steel fines, steel fixtures and wire brushes, and tumbling media used on carbon steel. Left in place, any of these can show up as rust spots after a humid ocean voyage, even though the alloy underneath is sound.
Per ASTM A380, descaling is a separate process, and oil and coolant must come off first, because acid cannot reach iron under a film of grease. Passivation therefore starts with degreasing and comes after the last machining, deburring and marking operation.
What does ASTM A967 require, and what does it leave to the buyer?
ASTM A967 defines several families of chemical treatment and seven test practices, and it requires passivated parts to show a chemically clean surface with no etching, pitting or frosting. It does not say which treatment or which test suits your part; the purchase order has to.
The current edition, A967/A967M-25, covers immersion in nitric acid solutions, immersion in citric acid solutions and electrochemical treatment, each followed by thorough rinsing, with or without a separate neutralizing step. Its scope is explicit that the standard does not choose for you: whether a given grade, chemistry or pass/fail test fits a particular application is left open, and the nonmandatory appendixes give general selection guidance by grade. In practice, that means the buyer decides three things: the treatment family (or a specific numbered treatment), the verification test, and how often it is run.
A NASA and United Space Alliance review of the passivation specifications summarized the A967 treatment menu as five nitric solutions, five citric solutions and an open category for other chemical and electrochemical media, and noted that when the purchase order is silent the processor may select any treatment (NASA NTRS, 2011). Some numbered solutions, including Nitric 5 and two of the citric ones, are deliberately open: any process that produces a part passing the specified test is acceptable, so the test is what actually controls the result.
How is AMS 2700 different from ASTM A967?
AMS 2700 is the aerospace passivation specification: it organizes the same nitric and citric chemistry into Methods, Types and Classes, and it sets a default when the order is silent. ASTM A967 is broader and leaves more to the processor.
Per the same NASA summary, which reviewed AMS 2700B, Method 1 is nitric acid (in eight types, several with a sodium dichromate additive), Method 2 is citric acid, and the Class sets the testing regime: Class 1 statistical sampling, Class 2 lot testing and Class 3 periodic testing. If the order does not say otherwise, AMS 2700B implied Method 1, any type, Class 2. Both documents replaced the old federal specification QQ-P-35, which the NASA review records as cancelled in 1997, with the SAE version of QQ-P-35 cancelled in 2005. The NASA review maps the old QQ-P-35 types, still seen on legacy drawings, to their A967 nitric equivalents.
Which one to cite follows the end market. Aerospace primes and their flow-downs usually name AMS 2700; industrial, food-equipment and many medical drawings name ASTM A967; ASTM A380 is a practice, not a product specification, and its scope reaches beyond parts to equipment and installed systems. Revisions have moved on since the documents summarized here, so confirm the current AMS 2700 revision letter with SAE before writing it on a drawing.
| Point | ASTM A967 | SAE AMS 2700 | ASTM A380 |
|---|---|---|---|
| Document type | Specification for chemical passivation treatments of stainless parts | Aerospace material specification for passivation of corrosion-resistant steel | Practice for cleaning, descaling and passivating parts, equipment and systems |
| Nitric treatments | Five solutions; about 20–55% nitric acid, 70–140 °F | Method 1, eight types; about 20–55% nitric acid, 70–155 °F | Acid treatments covered as guidance; refers to A967 for passivation |
| Citric treatments | Five solutions; about 4–10% citric acid, 70–160 °F | Method 2; about 4–10% citric acid, 70–160 °F | Citric named among cleaning and passivating options |
| Other media | Electrochemical and other chemical treatments allowed if the part passes | Optional additives and applied voltage within Method 1 | Wide range of cleaners and descaling methods |
| Testing | Practices A–G; lot testing when the order specifies it | Class 1 sampling, Class 2 lot, Class 3 periodic | Cleanness tests, from visual and water-break to ferroxyl |
| If the order is silent | Processor may choose the treatment | Method 1, any type, Class 2 | Passivation means removal of contaminant iron |
Should you specify nitric or citric acid passivation?
For most austenitic grades either works, so the practical answer is to specify the result and the test, and allow citric unless your customer or grade rules it out. Name nitric only where a flow-down, a legacy approval or a sensitive grade demands it.
Citric acid is the easier chemistry to run: NASA’s evaluation cites worker safety, lower environmental impact and lower operating cost as its advantages over nitric acid, whose fumes and wastewater are regulated (NASA NTRS, 2013). That paper records that QQ-P-35 gave way to A967 and AMS 2700, both of which permit citric acid. Two of the five A967 citric solutions are open-ended, accepted on test results rather than fixed settings, and NASA and the European Space Agency ran a parameter study to find optimum settings for specific alloys: 304, 316, 321, several precipitation-hardening grades, A286 and AL-6XN. That study varied concentration from 4% to 10%, temperature from 38 °C to 82 °C and time from 60 to 120 minutes, then judged the panels by salt spray to ASTM B117 (NASA NTRS, 2016).
Neither acid is automatically better; the grade and process window matter more than the label. Where no approval locks in nitric, a note allowing either family plus a named test lets the processor pick the bath that suits the grade, while the test protects you.
Why do 303 and other free-machining grades need special care?
Free-machining stainless grades such as 303 and 416 contain added sulfur that forms sulfide inclusions, and an aggressive acid bath can dissolve those inclusions and leave pits or a dark, etched surface. Passivate them with a treatment chosen for the grade, and never mix them in one bath load with standard grades.
These grades exist because they machine so well; ASTM A582 covers free-machining stainless bars designed for optimum machinability. The same sulfides that break the chip are the weak spots in corrosion. When an acid removes an inclusion from a freshly cut surface, it leaves a small cavity that can hold solution and start a pit, a failure often called flash attack in shop practice. ASTM A967 requires passivated parts to show no etching, pitting or frosting, so a 303 part that comes out of the bath dull or speckled has failed the specification, not just the cosmetic check.
As typical industry guidance, processors handle free-machining grades with milder or inhibited treatments, tight control of bath contamination and time, and in some cases an alkaline pre-treatment and post-neutralization sequence; the A967 appendixes carry grade-selection guidance for this reason. On the design side, ask whether the part needs a free-machining grade at all: 304 or 316 machines more slowly but avoids the sulfide problem on wetted, medical or outdoor parts.
How do you verify that a part was passivated?
You verify it with one of the ASTM A967 test practices, named on the purchase order with how often it is run. A passivated part cannot be identified by eye, so without a named test the certificate only records that a bath was used.
ASTM A967/A967M-25 describes its tests as qualitative checks that confirm the removal of contaminant iron and other foreign matter. As typical guidance, the copper sulfate and ferricyanide tests are quick and sensitive but chemically mark the surface, so they are run on sample parts or on areas where a trace does not matter; humidity and salt spray take longer and are better suited to lot or qualification testing.
| Practice | Test | Principle | Typical use |
|---|---|---|---|
| A | Water immersion | Wetting and drying in clean water; rust marks reveal iron | Simple lot check on parts that tolerate immersion |
| B | High humidity | Exposure in a humidity cabinet; rust marks reveal iron | Lot or qualification testing |
| C | Salt spray | Neutral salt fog per ASTM B117; rust marks reveal iron | Qualification and aerospace or defense flow-downs |
| D | Copper sulfate | Copper plates out where free iron is present | Fast shop check on austenitic grades; marks the part |
| E | Potassium ferricyanide–nitric acid | Blue color develops where iron is present | Very sensitive spot test; marks the part |
| F | Damp cloth | Contact with a wet cloth; rust marks reveal iron | Large parts that cannot go in a cabinet |
| G | Boiling water immersion | Short boil and dry; rust marks reveal iron | Quick lot check where parts tolerate heat |
What should the drawing note and the certificate say when you buy overseas?
The drawing should name the specification and revision, the allowed treatment family, the test practice and the sampling, and the certificate should report each of those back for the lot shipped. A note that stops at “passivate per ASTM A967” leaves every one of them to the supplier.
Generic wording to adapt, not any supplier’s standard:
- PASSIVATE PER ASTM A967/A967M-25, NITRIC OR CITRIC TREATMENT AT PROCESSOR’S OPTION. VERIFY PER PRACTICE [B OR D], EACH LOT. General industrial turned and milled parts.
- PASSIVATE PER SAE AMS 2700 [REV], METHOD [1 OR 2], CLASS 2. Parts that flow down from an aerospace customer.
- MATERIAL 303 (FREE-MACHINING). PASSIVATE PER ASTM A967/A967M-25 USING A TREATMENT SUITED TO FREE-MACHINING GRADES. NO ETCHING OR PITTING. High-sulfur grades.
For the certificate of conformance, ask for the part number and lot, the specification and revision, the treatment actually used (for example Citric 2 or Nitric 2), the test practice and its result, the date, and the name of the processing shop if it is not the machining supplier. That last line matters when parts are made in Vietnam or elsewhere in Southeast Asia, where machining and chemical finishing are often done by different companies: the certificate should come from whoever ran the bath, or be backed by that shop’s own record. Ask also how parts are packed, since carbon-steel racks or bins can put iron straight back on the surface.
For implants, ASTM F86 adds surface preparation and marking requirements on top of passivation; the medical, dental and veterinary page covers that sector. As typical guidance, a correctly passivated surface shows no measurable dimensional change, so a tight CNC turning tolerance does not need a finish allowance for it. The surface finishing page lists the finishes our partner plants apply, and the quote names where each step is done; raise any passivation callout in the RFQ so the finishing route is confirmed before quoting, and send the drawing for a quote with the note written as you intend it. The RFQ preparation guide covers the rest of the package.
Frequently asked
Does passivation change the color or size of a stainless part?
It should not. ASTM A967 requires no etching, pitting or frosting, so a visible change in appearance is a sign that something went wrong, often on a free-machining grade.
Is electropolishing the same as passivation?
No. Electropolishing removes a thin layer of metal to smooth and brighten the surface, while passivation removes contamination without removing metal. ASTM A967 does allow electrochemical passivation treatments, so check which one a quote means.
Can passivated parts rust later?
Yes, if iron is put back on them. Contact with carbon-steel tools, bins or packaging, or later grinding and marking, can recontaminate the surface, so passivate last and pack with that in mind.
My drawing still says QQ-P-35. What should I do?
QQ-P-35 has been cancelled, and ASTM A967 and AMS 2700 replaced it. Update the note to the specification your customer accepts, or state on the order which current specification the supplier should use in its place.
Do 400-series stainless parts need passivation?
They can be passivated, but martensitic and ferritic grades are less corrosion resistant and more sensitive to aggressive baths than 304 or 316, so the treatment and any hardening sequence should be agreed with the processor.
Sources
- ASTM A967/A967M-25, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts, ASTM International, 2025 (published abstract and scope), and the 2017 edition scope, Note 1 (free iron)
- ASTM A380/A380M-25, Standard Practice for Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems, ASTM International, 2025, and the 2017 edition scope, section 1.1.1 (meanings of passivation)
- Citric Acid Passivation of Stainless Steel, United Space Alliance and NASA, presented at the Aircraft Airworthiness and Sustainment Conference, 2011 (summaries of QQ-P-35, ASTM A967 and AMS 2700B requirements)
- Alternative to Nitric Acid for Passivation of Stainless Steel Alloys, NASA Technology Evaluation for Environmental Risk Mitigation, 2013
- NASA and ESA Collaboration on Alternative to Nitric Acid Passivation: Parameter Optimization of Citric Acid Passivation for Stainless Steel Alloys, final test report, NASA, 2016
- ASTM A582/A582M, Standard Specification for Free-Machining Stainless Steel Bars, ASTM International, 2021 edition scope
- ASTM F86, Standard Practice for Surface Preparation and Marking of Metallic Surgical Implants, ASTM International, 2021 edition scope