Nitric acid passivation is the traditional method for removing free iron and restoring the corrosion resistance of stainless steel — specified where a drawing calls for a nitric process. Gleco passivates to ASTM A967 and AMS 2700 at our AS9100D and ISO 9001:2015 certified, ITAR Registered Texas plants.
| Governing specs | ASTM A967; AMS 2700 (Method 1, nitric); ASTM A380; QQ-P-35 (historical) |
|---|---|
| Nitric methods (A967) | Nitric 1–4 (varying concentration and temperature; some with sodium dichromate) |
| Acceptance tests | Water immersion, high humidity, salt spray, copper sulfate, free-iron (potassium ferricyanide-nitric) |
| Dimensional impact | None (no coating is applied) |
| Alloy note | Correct type selected for free-machining, high-carbon martensitic, and PH grades |
| Typical uses | Aerospace and defense stainless hardware, machined components, legacy nitric-specified parts |
Nitric acid passivation dissolves free iron and other exogenous contamination from a stainless surface, then leaves an oxidizing environment in which the chromium that remains rebuilds the passive chromium-oxide film. Nothing is deposited. No dimension moves. What changes is that the surface finally behaves like the alloy it is made of.
Nitric acid is a strong oxidizer as well as an acid. It removes iron and simultaneously drives the formation of the oxide film. That dual action is why it has been the default passivation chemistry for seventy years.
A passivated part measures the same as an unpassivated one. Free iron sits at the surface in microscopic quantity; removing it does not move a tolerance.
Cutting oil, drawing compound, heat tint and weld scale all have to come off first. Descaling and cleaning guidance is in ASTM A380; passivation follows it, it does not replace it.
ASTM A967 defines five nitric treatments. Four carry fixed parameters. The fifth is the open category for any other combination that passes the required acceptance test.
| Treatment | Nitric acid | Additive | Bath temperature | Minimum immersion |
|---|---|---|---|---|
| Nitric 1 | 20 to 25% by volume | 2.5% by weight sodium dichromate | 120 to 130°F | 20 minutes |
| Nitric 2 | 20 to 45% by volume | None | 70 to 90°F | 30 minutes |
| Nitric 3 | 20 to 25% by volume | None | 120 to 140°F | 20 minutes |
| Nitric 4 | 45 to 55% by volume | None | 120 to 130°F | 30 minutes |
| Nitric 5 | Other combinations | As qualified | As qualified | As qualified |
Nitric 1 is the one people ask about. The sodium dichromate is there to make the bath more strongly oxidizing, which is what less corrosion-resistant grades need in order to build a good film — precipitation-hardening, martensitic and ferritic stainless in particular. It is also the reason some customers cannot accept it: dichromate means hexavalent chromium. If that is a constraint in your supply chain, say so at quote time and we will look at Nitric 4 or at citric instead. Parameters here follow ASTM A967/A967M; always work to the revision your print calls out.
Austenitic stainless is easy. Everything else needs a conversation. The single most useful thing you can put on an RFQ is the alloy and condition, because that is what drives the bath.
The straightforward case. A standard nitric bath without additive builds a good passive film on chromium-nickel grades, and these are what the middle treatments in A967 were written around.
17-4 PH, 15-5 PH, 410, 440C and similar are less corrosion resistant than the austenitics and are the classic case for a more oxidizing bath — nitric with sodium dichromate. Heat treat condition matters here too; tell us the condition, not just the alloy.
Same reasoning as the martensitics, with the added wrinkle that some of these grades sit below 16% chromium, which changes which acceptance test can be used on them.
Sulfur is added to these grades deliberately, to break chips. The resulting sulfide inclusions sit at the surface, and an aggressive bath can strip them out and leave microscopic discontinuities that hold residual acid. These parts need the right cycle and a genuinely thorough rinse, and they are worth flagging on the RFQ.
We run both chemistries, so there is no reason for us to push one. The trade is real and it goes in both directions.
Flash attack — a rapid, non-uniform attack that leaves a dark or etched surface — is usually triggered by chloride contamination and made worse by excessive bath temperature or extended immersion. Laboratory work has found citric-passivated parts more prone to it than nitric-passivated parts. When parts arrive dirty, or the geometry traps solution, nitric is the safer bath.
Citric 1 has a four-minute minimum immersion against a twenty-minute floor on the nitric side. It uses no mineral acid and no dichromate, it is on the FDA GRAS list, and it keeps hexavalent chromium out of the waste stream entirely.
One citric bath will passivate a wider range of stainless grades than any single nitric bath will — which is worth real money when a job carries three alloy families on one traveler.
Neither is a substitute or a downgrade. A967 defines both, AMS 2700 defines both, and a part passivated to either and verified against the specified test is a passivated part. If your print names one, we run that one.
A passivated part and a contaminated part look identical. The acceptance test is what separates them, and it belongs on the print.
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 the one that 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, and it 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. Shop practice on chlorides specifically is tighter — under roughly 50 ppm — because chloride is what drives flash attack.
Aerospace work carries an appearance requirement on top of the chemical test: no etching, pitting, smutting, frosting or dimensional change. A part can pass the chemistry and still be rejected on looks.
Not just "stainless." 17-4 PH in H900 and 17-4 PH in H1150 are not the same passivation problem, and 303 is not 304.
ASTM A967 Nitric 2, AMS 2700 Method 1, or an OEM spec that points at one of them. If the print just says "passivate," tell us the alloy and the acceptance test and we will select the treatment.
Certification, test method, sampling. Agree it at quote time rather than at delivery and it gets built into the routing instead of bolted on afterward.
Machining, welding, grinding, tumbling, heat treat, blasting. Anything that could have embedded iron or left heat tint changes what has to happen before the passivation tank.
Blind holes, tight internal threads, capillary gaps between mated surfaces. Solution that cannot drain does not rinse, and that is where after-the-fact staining comes from. Call it out and we will rack for it.
QQ-P-35 has been superseded, but it still appears on drawings that have not been revised in decades. Send us the print as it is. We would rather read the original and tell you what it maps to in ASTM A967 or AMS 2700 than have you substitute something a source inspector later disputes.
No. The problem under weld discoloration is a chromium-depleted layer, and a passivation bath will not restore it. That is a descaling or pickling operation first, per ASTM A380, and then passivation. If your parts are welded, tell us — it changes the routing and the quote.
It should not. Passivation is not a brightening or polishing operation, and AMS 2700 explicitly requires that the surface show no etching, pitting, smutting or frosting afterward. If a part comes back looking different, something in the process was out of control, and that is worth a phone call rather than a shrug.
Nitric acid is an oxidizing acid and is not the classic embrittlement risk that acid pickling or electroplating on high-strength steel is. That said, high-strength martensitic and precipitation-hardening grades warrant a conversation about the whole process route rather than passivation in isolation. If your part is high-strength and has been through other chemical processing, raise it and we will talk through the sequence.
The passive film starts forming immediately and continues to build over the following hours. Parts should be fully dry before packing, handled with clean gloves rather than bare hands, and kept away from packaging materials that can outgas or hold chloride. Most staining complaints trace back to what happened after the tank, not in it.
Local turnaround, testing and logistics for nitric 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 three Texas facilities.
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