What the specifications require, how citric and nitric differ, which tests prove the result, and why a lot comes back hazed. Written for the engineer holding the print.
Passivation is chemical cleaning. An acid bath dissolves free iron and other exogenous contamination from a stainless surface so the chromium oxide film native to the alloy can reform across the whole area. Nothing is plated, nothing is built up, and it is not a coating.
The practical consequence is why it gets specified on close-tolerance work: no dimensional change. AMS 2700 makes that an inspection criterion, not a claim — treated parts are rejected for etching, pitting, smutting, frosting or dimensional change. A passivated bore is the same bore.
Nor is it corrosion protection: it restores the resistance the alloy already had, no more.
Passivation does not remove heat tint, weld scale or mill scale, and it will not repair pitting or improve surface finish.
Stainless resists corrosion because chromium at the surface forms a thin, self-repairing oxide, and anything that smears carbon steel across it interrupts that. Milling with tool steel, grinding with iron-bearing wheels, forming in carbon steel dies, tumbling with steel media, setting a finished part on a steel bench — all of it embeds free iron, and that iron rusts first. The rust then reads as a failed alloy.
Contamination is rarely uniform, which is why one part in a box stains and the rest do not. Hence the callout on medical device prints, aerospace hardware, food and pharmaceutical equipment, and industrial assemblies where a stained part is a rejected part.
Both are accepted by ASTM A967 and AMS 2700, and they remove free iron by different routes. The choice comes down to the alloy, the chemical restrictions on the program, and what the print says.
Citric chelates iron — it binds it into solution rather than oxidizing it away. Not being a strong oxidizer it is gentler on the base metal, which matters on free-machining and lower-chromium grades that nitric can etch. Temperatures and times are lower and shorter, and there is no nitric acid or hexavalent chromium additive, which is why medical and food-contact work often specifies it outright. ASTM A967 lists Citric 1 through 5; AMS 2700 calls it Method 2. Detail: citric acid passivation.
Nitric dissolves free iron and oxidizes the chromium-rich surface at once. It is the older route and still the default on legacy aerospace and defense prints. It tolerates a part that arrives less than perfectly clean, and for high-carbon and martensitic grades one treatment adds sodium dichromate to limit attack on the base metal. ASTM A967 lists Nitric 1 through 5; AMS 2700 calls it Method 1. Detail: nitric acid passivation.
Neither is universally correct. If you are deciding rather than following a print, the head-to-head is in citric vs nitric passivation under AMS 2700.
ASTM A967 / A967M — Chemical Passivation Treatments for Stainless Steel Parts. The commercial workhorse. It names the bath treatments (Nitric 1–5, Citric 1–5) and the acceptance test practices, and where the purchaser specifies neither, the supplier picks. It covers passivation, not electropolishing.
AMS 2700 — Passivation of Corrosion Resistant Steels. The aerospace document. Method 1 is nitric, subdivided into eight numbered types at different concentrations, temperatures and times; Method 2 is citric. Note the terminology trap: class here means test frequency, not bath chemistry, so “AMS 2700 Method 1” names a chemistry family and nothing else. It supersedes AMS-QQ-P-35 and has been revised repeatedly — put the revision on the print.
ASTM A380 / A380M — Cleaning, Descaling, and Passivation of Stainless Steel Parts, Equipment, and Systems. A practice, not a specification: it describes good procedure for cleaning and descaling but supplies no tests with acceptance criteria, pointing to A967 for those. A print calling A380 alone has told the shop how to work, not how the lot is accepted.
QQ-P-35 — canceled, and still on prints. Revision C was withdrawn on 11 September 1998, directing users to ASTM A967 and to AMS-QQ-P-35, a republication later canceled in favor of AMS 2700. What matters: QQ-P-35 listed Roman-numeral types mapped to specific alloys, and AMS 2700 uses Arabic numbering and dropped that table. Old callouts translate, but not automatically.
| Treatment | Solution | Temperature | Minimum time |
|---|---|---|---|
| Nitric 1 | 20–25% nitric acid with sodium dichromate | 120–130°F | 20 min |
| Nitric 2 | 20–45% nitric acid | 70–90°F | 30 min |
| Nitric 3 | 20–25% nitric acid | 120–140°F | 20 min |
| Nitric 4 | 45–55% nitric acid | 120–130°F | 30 min |
| Nitric 5 | Other nitric combinations, with or without additives | As qualified | As qualified |
| Citric 1 | 4–10% citric acid | 140–160°F | 4 min |
| Citric 2 | 4–10% citric acid | 120–140°F | 10 min |
| Citric 3 | 4–10% citric acid | 70–120°F | 20 min |
| Citric 4 / Citric 5 | Open-parameter citric alternatives in current revisions; Citric 5 carries a pH requirement | As qualified | As qualified |
Figures as published in ASTM A967; confirm against the revision your print calls. Nitric 5, Citric 4 and Citric 5 are deliberately open — a qualified process that passes the acceptance test, rather than a fixed recipe.
A passivated surface looks identical to an untreated one, so the test is the only evidence. ASTM A967 puts the choice of a practice appropriate to the material and application on the purchaser. Specify passivation without a test and the shop chooses — legitimately, and perhaps not as you would.
| ASTM A967 practice | What it is | What it proves, and where it does not apply |
|---|---|---|
| Practice A — Water immersion | Twelve cycles of one hour immersed in distilled water, one hour drying | Slow and severe; catches marginal free iron. Common on medical and food equipment. |
| Practice B — High humidity | Minimum 24 hours at roughly 97% relative humidity near 100°F | Non-destructive corrosion screen. Widely used for lot acceptance. |
| Practice C — Salt spray | Minimum 2 hours per ASTM B117 | General corrosion resistance. Suited to austenitic grades; may not be appropriate for martensitic or ferritic. |
| Practice D — Copper sulfate | Six minutes minimum surface exposure; copper plates out on free iron | Fast free-iron indicator. Not recommended for martensitic 400 series or ferritic grades below 16% chromium, and not for food-contact parts. |
| Practice E — Potassium ferricyanide–nitric acid | Blue color develops within a 30-second window where free iron is present | Very sensitive free-iron detection. Not recommended for ferritic or martensitic 400 series, and not for food-contact parts. |
| Practices F and G | Damp cloth and boiling water immersion in current revisions | Alternatives where the practices above are unsuitable for the alloy. |
AMS 2700 requires visual inspection plus one of high humidity, water immersion, copper sulfate or salt spray, at the frequency the class sets. Choose with the alloy in mind: copper sulfate on a 416 part reads as a failure that is not one. Our quality system records practice and result against the lot; see also certifications and approvals.
The surface comes out gray, dark or heavily etched instead of unchanged. The cause is usually chloride contamination of the bath — carried in on parts, in rinse water, or from soils not removed first; a commonly cited working limit is about 60 mg/liter. Organic soil dragged into the acid does the same by another route. A process control failure, not an alloy problem.
303, 303Se and 416 carry sulfur or selenium deliberately, to break chips. Those inclusions dissolve during passivation and leave micro-voids that hold residual acid. Unless it is neutralized out, the part passes inspection and bleeds rust weeks later. These grades want alkaline neutralizing steps around the acid, or citric rather than nitric.
410, 416, 420 and the 440 group carry less chromium and more carbon than austenitic grades, so a bath sized for 304 attacks them. These historically run in nitric with sodium dichromate, or at higher concentration, to hold the surface passive. They also break the standard tests: copper sulfate and potassium ferricyanide give false positives on chromium-lean and martensitic material.
Free-machining and high-carbon grades should be run one grade at a time. Two alloys in the same basket touch in solution, and the contact points are where corrosion starts. Mixing 303 with 304 to save a rack can cost the lot. If a shipment holds more than one alloy, say so on the packing list.
Usually one of four things: soils not fully removed and carbonized in the bath, early-stage flash attack, too long or too hot a bath for a sensitive grade, or heat tint present on arrival. Precipitation-hardening grades heat treated after machining arrive with an oxide the acid cannot lift; that has to be descaled, and a second pass will not fix it.
You need pickling or descaling. Weld heat tint, mill scale and heat-treat oxide are not free iron and passivation will not touch them. That is descaling — chemical pickling, which removes base metal and changes finish, or mechanical removal, described in ASTM A380 and covered under media blasting.
You need electropolishing. If the requirement is a smoother, brighter, deburred surface with measurably lower Ra, that is electropolishing — electrochemical metal removal that leaves a passive surface as a by-product, and it moves dimensions. A passivation certificate does not satisfy that callout. For brightening non-ferrous parts we run bright dip.
You need a coating. If the environment is harsher than the alloy can handle, or you need lubricity, controlled torque, electrical performance or paint adhesion, the answer is a deposit: electroless nickel for uniform coverage on complex geometry, silver or tin for conductivity and solderability, dry film lubricant where galling is the failure. Compare the finishing processes, or start at electroplating, anodizing, coatings and capabilities.
We would rather tell you passivation is the wrong operation than run it and hand you a certificate that solves nothing.
Gleco Plating has been in Texas since 1979. Passivation runs at our plating plants in Rowlett, near Dallas, and McAllen, in the Rio Grande Valley. Citric passivation is a Rowlett process — a print calling AMS 2700 Method 2 or an ASTM A967 citric treatment runs at Rowlett. We are AS9100D certified, ISO 9001:2015 certified, ITAR Registered and DFARS compliant; the AS9100D certificate covers three sites. Local inquiries often start at passivation in Dallas; every plant is on the locations page.
The most common failure is not chemical: a print reading only “passivate” leaves chemistry, treatment and test to the shop. Five items close it, and our design guide covers the wider callout.
1. Alloy and UNS number. “Stainless” is not enough. 304, 316L, 17-4 PH, 303 and 440C are five different processes.
2. Condition. Annealed, cold worked or heat treated — and if heat treated, whether the parts are descaled.
3. Specification and revision. A967, AMS 2700, or the legacy callout exactly as printed.
4. Method, type or treatment. Nitric or citric, and the numbered treatment if the print names one. If it does not, say so and we will recommend one.
5. Test practice. Which A967 practice, or which AMS 2700 test and class. If the part is food-contact, say so — it rules out two tests.
Passivation is often one step in a longer route: descale, passivate, then plate or coat a mating component. With plating, anodizing, passivation, paint and powder inside one company, that route does not become four vendors — see everything under one roof, defense and electronics.
No. Passivation removes surface contamination and deposits nothing, so there is no build-up and no measurable stock removal. AMS 2700 treats dimensional change as a reject condition on inspection, not as an allowance. That is the main reason it is specified on close-tolerance stainless where a plated or converted surface would not be acceptable.
If the print names one, follow the print. If you are choosing: citric is gentler, runs cooler and shorter, and avoids nitric acid and hexavalent chromium additives, which is why medical and food-contact work tends toward it. Nitric is the legacy aerospace and defense default and tolerates parts that arrive imperfectly clean. Both are accepted by ASTM A967 and AMS 2700. The comparison is in citric vs nitric passivation under AMS 2700.
QQ-P-35 Revision C was withdrawn in September 1998 and directed users to ASTM A967 and to AMS-QQ-P-35; AMS-QQ-P-35 was later canceled in favor of AMS 2700. The callout still appears on legacy drawings. QQ-P-35 used Roman-numeral types tied to specific alloys, and AMS 2700 does not carry that alloy table, so the translation is a judgment call we would rather make with you before processing than explain afterwards.
Most often flash attack — chloride contamination in the passivating solution producing a gray, dark or etched surface. Other causes are soils not fully removed before the acid, an over-hot or over-long bath for a sensitive grade, and heat tint or scale that was on the parts before they arrived, which passivation cannot remove. If it is heat tint, the parts needed descaling first, not another pass through the acid.
Yes. A passivated surface is visually identical to an untreated one, so the test is the only evidence. ASTM A967 states that where the purchaser does not specify a practice, the supplier selects from the listed practices — meaning your lot is judged against a criterion you did not choose. Name the practice, and for AMS 2700 name the class, which sets test frequency rather than chemistry.
Yes, with one condition worth knowing. Precipitation-hardening grades are normally heat treated after machining, and heat treatment leaves an oxide film that passivation chemistry will not remove. Those parts need acid descaling or mechanical removal first, otherwise the treatment is being applied over scale. Tell us the condition — H900, H1025, condition A — with the alloy, and whether the parts are already descaled.
Yes, but not as though they were 304. Sulfur and selenium inclusions dissolve out during treatment and leave micro-voids that trap acid, which is what causes parts to pass inspection and rust later. These grades want neutralizing steps around the acid, are often better in citric than nitric, and should be run as a single grade rather than mixed with other alloys. They also give false positives on the copper sulfate and potassium ferricyanide tests, so pick a different practice.
No. Free iron and exogenous contamination, yes. Oxide scale, heat tint and mill scale, no — those need pickling or mechanical descaling, which ASTM A380 describes and which does affect the surface. A weldment usually needs descaling and then passivation, in that order, and the two should be called out separately on the drawing.
Give us the specification, revision, method and required test practice and we will confirm the treatment before the parts ship — or tell you passivation is not the operation you need.
Precision metal finishing since 1979. Plating, anodizing, coatings and passivation from three Texas facilities.
Our paint, powder & Cerakote division: Gleco Paint & Powder Coating
Gleco Rowlett
2220 Grisham Drive
Rowlett, TX 75088
972-475-4300
Gleco McAllen
3800 W. Ursula Ave
McAllen, TX 78503
956-800-4069
