Citric acid passivation removes free iron and restores the protective chromium-oxide layer on stainless steel — with a safer, lower-temperature, more environmentally responsible chemistry that many medical and aerospace OEMs now specify. Gleco passivates to ASTM A967 and AMS 2700 at our AS9100D and ISO 9001:2015 certified Texas plants.
| Governing specs | ASTM A967; AMS 2700 (Method 2, citric); ASTM A380; QQ-P-35 (historical) |
|---|---|
| Citric methods (A967) | Citric 1–5 (varying concentration, temperature, and time) |
| Acceptance tests | Water immersion, high humidity, salt spray, copper sulfate, free-iron (potassium ferricyanide-nitric) |
| Dimensional impact | None (no coating is applied) |
| RoHS / environmental | Citric avoids dichromate entirely — the greener passivation route |
| Typical uses | Medical devices/instruments, aerospace and food hardware, post-machining cleanup |
Passivation is not a coating. Nothing is added to the part, and no dimension moves. The acid removes free iron and other exogenous contamination from the surface, and the chromium left behind reacts with oxygen to rebuild the passive chromium-oxide film that makes stainless steel stainless.
Citric acid binds free iron and holds it in solution rather than aggressively dissolving the surface. It is selective toward iron over nickel and chromium, so the alloying elements that build the passive film stay where they are.
Because nothing is deposited and the base alloy is not meaningfully attacked, passivation does not move a tolerance. This is the main practical difference between passivating a part and plating one.
Passivation removes free iron. It does not remove cutting oil, drawing compound, heat tint or weld scale. Those come off first, or the acid never reaches the metal. Descaling guidance lives in ASTM A380.
ASTM A967 defines citric passivation as five treatments. Three carry fixed parameters; the other two are open categories for anything else that passes the required test.
| Treatment | Citric acid | Bath temperature | Minimum immersion |
|---|---|---|---|
| Citric 1 | 4 to 10% by weight | 140 to 160°F | 4 minutes |
| Citric 2 | 4 to 10% by weight | 120 to 140°F | 10 minutes |
| Citric 3 | 4 to 10% by weight | 70 to 120°F | 20 minutes |
| Citric 4 and Citric 5 | Other combinations | As qualified | As qualified |
The pattern is straightforward — the hotter the bath, the shorter the cycle. AMS 2700 takes a simpler line for aerospace work: it defines a single citric method run anywhere from 70 to 160°F for between 4 and 30 minutes, against eight parameter sets on the nitric side. If your print names a specific treatment, send it. If it just says passivate per ASTM A967, tell us the alloy and the acceptance test and we will pick the treatment. Parameters here follow ASTM A967/A967M; always work to the revision your print calls out.
A single citric bath will passivate a wider range of stainless grades than any one nitric bath will. That matters when 304, 17-4 and 416 are on the same traveler and nobody wants three separate setups.
Citric acid is on the FDA GRAS list, and citric passivation was originally developed to passivate the inside of beer kegs. For anything that touches product, that lineage is the reason it gets specified.
No mineral acid, no sodium dichromate, no hexavalent chromium in the waste stream, and a lower operating temperature. For a customer running a REACH or RoHS-driven supply chain audit, that is a shorter conversation.
Citric 1 has a four-minute minimum immersion. The nitric treatments start at 20 minutes. On a hot job that difference is real.
We run both, so we have no reason to sell you the wrong one. There are situations where nitric is the safer bath and we will say so.
Flash attack is a rapid, non-uniform attack that leaves a dark or etched surface instead of a clean one. Chloride contamination is the usual trigger, and excessive bath temperature or extended immersion make it worse. Laboratory work has found citric-passivated parts more prone to it than nitric-passivated parts. If parts arrive carrying chloride-bearing residues, or the geometry traps solution, nitric is the more forgiving choice.
Precipitation-hardening, martensitic and ferritic grades are less corrosion resistant than the austenitics. The traditional route is nitric with sodium dichromate added to raise the oxidizing power — which is exactly what ASTM A967 Nitric 1 is.
QQ-P-35 has been superseded, but it still appears on drawings that have not been revised in twenty years. Send us the print rather than translating it yourself; we will tell you what it maps to in ASTM A967 or AMS 2700 and quote it that way.
Passivation is invisible. A passivated part and a contaminated part look identical, which is why the acceptance test matters as much as the bath.
Water immersion, high humidity, salt spray, copper sulfate, and the potassium ferricyanide-nitric acid test for free iron. The print or purchase order should name which one applies.
ASTM A967 does not recommend the copper sulfate test for martensitic 400-series stainless, or for ferritic 400-series grades with less than 16% chromium. It also is not used on parts intended for food processing. If your part is 416 or 430, expect a different test.
A967 caps final rinse water at 200 ppm total solids. Practical shop guidance is tighter than that on chlorides specifically — under roughly 50 ppm — because chloride is what drives flash attack.
On top of its test requirements, AMS 2700 requires a visual check that finds no etching, pitting, smutting, frosting or dimensional change. A part can pass a chemical test and still fail on appearance.
The acid cannot reach metal it cannot touch. Parts should arrive degreased. If they are coming straight off a screw machine, tell us — we will clean first rather than passivate over film.
Steel wire brushes, carbon steel fixtures, shared tumbling media and grinding dust all embed free iron in a stainless surface. Passivation will remove it, but only if we know it is there and can choose a cycle that gets it all.
The chromium-depleted layer under weld discoloration is not something a passivation bath will fix. That is a descaling or pickling job first, per ASTM A380, and then passivation.
Solution that cannot drain does not rinse. Deep blind holes, tight internal threads and capillary gaps between mated surfaces are the usual sources of after-the-fact staining. Call them out and we will rack accordingly.
Free-machining grades such as 303 and 416 carry sulfide inclusions by design — the sulfur is there to break chips. Those inclusions sit at the surface and behave differently in an acid bath than a clean 304 surface does. Tell us the alloy. It changes the chemistry, the cycle and the test.
No. Passivation restores the corrosion resistance the alloy is capable of. It removes the free iron and contamination that were dragging performance below the alloy specification. If a 304 part is rusting after passivation, the answer is usually contamination, a wrong alloy, or a design that traps moisture — not a thicker film.
The film begins re-forming as soon as the clean surface meets air, and continues to thicken over the following hours and days. This is why parts should not be handled with bare hands, packed wet, or wrapped in materials that outgas, in the window right after processing.
No. Passivation is a stainless steel treatment. If a part has a plated or coated layer on it, that layer is what the acid meets. If you need both, passivation comes first in the sequence, or on the stainless components of an assembly before anything else is applied.
Send it as-is. QQ-P-35 has been superseded, and the modern equivalents live in ASTM A967 and AMS 2700. We would rather read your original drawing and tell you what it maps to than have you guess at a substitution that a source inspector later disputes.
What testing applies is driven by your print, your purchase order and the specification called out. Tell us what evidence you need with the parts — certification, test method, sampling — at quote time rather than at delivery, and it gets built into the routing.
Local turnaround, testing and logistics for citric passivation in the metroplex.
Send the print and we’ll tell you. Quotes typically back within one business day, from either plant.
Precision metal finishing since 1979. Plating, anodizing, coatings and passivation from two Texas plants.
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
