Citric vs. Nitric Acid Passivation: Which Should You Specify?

Reviewed by Tom Harrison, member of the ASME BPE Surface Finish Subcommittee · Updated October 2026

The short answer: Both nitric and citric acid passivation remove free iron from the surface of stainless steel so the protective chromium-oxide layer can re-form, and both are recognized methods under ASTM A967 and AMS 2700. Nitric acid is the traditional process: it's a strong oxidizer, it works on the widest range of alloys, and decades of aerospace and military specifications are written around it. Citric acid is the newer alternative: it's safer to handle, easier to dispose of, gentler on sensitive alloys, and, when properly controlled, produces equivalent corrosion resistance on the common austenitic grades. For most 300-series and PH stainless parts, either will meet the spec; the choice usually comes down to what your specification names, the alloy, and your own environmental and safety preferences.

What passivation is doing, whichever acid you use

Machining, grinding, forming, welding, and contact with carbon-steel tooling all leave free iron on and in the surface of stainless steel. That iron corrodes, and it disrupts the thin chromium-oxide film that makes stainless "stainless." Passivation dissolves the free iron and other surface contaminants without significantly attacking the base metal. When the cleaned surface is exposed to oxygen, the chromium-rich surface re-forms its passive layer, now uninterrupted. The acid doesn't create the passive layer; air does. The acid's job is to clear the way.

For a comparison of passivation with electropolishing, which removes a layer of base metal rather than just cleaning it, see Electropolishing vs. Passivation.

Nitric acid passivation

Nitric acid (HNO₃) has been the standard passivating agent since stainless steel was first produced commercially. It is a strong oxidizing acid, which means it not only dissolves iron but actively promotes the formation of the chromium-oxide film during the immersion itself.

ASTM A967 defines several nitric methods (Nitric 1 through Nitric 5), which vary in acid concentration, temperature, time, and whether sodium dichromate is added as an inhibitor for sensitive grades. Typical ranges run from about 20% to 50% nitric acid by volume, at temperatures from room temperature up to about 140 °F, for 20 to 30 minutes. AMS 2700 Method 1 is nitric passivation, with Types 1 through 8 covering different concentrations and additives for specific alloy families.

Strengths: the widest alloy compatibility, including 400-series martensitic and ferritic grades and precipitation-hardening alloys when the right type is used; the longest track record; and the method most legacy aerospace, defense, and medical specifications call out by name.

Drawbacks: nitric acid is hazardous to handle, produces nitrogen oxide fumes, requires scrubbed ventilation and neutralization before disposal, and can attack free-machining grades (303, 416, etc.) and some high-carbon martensitic grades, causing etching or "flash attack" if the bath is not controlled.

Citric acid passivation

Citric acid passivation came into industrial use in the 1990s, driven by aerospace manufacturers looking for a process that met the same specifications without nitric acid's handling and disposal burden. Citric acid is a weak organic acid; it removes free iron by chelation, binding the iron so it stays in solution, rather than by oxidation.

ASTM A967 defines Citric 1 through Citric 5, typically at 4% to 10% citric acid by weight, at temperatures from room temperature to about 160 °F, for 4 to 30 minutes depending on temperature. AMS 2700 Method 2 is citric passivation.

Strengths: much safer to handle, no toxic fumes, biodegradable, simpler wastewater treatment; gentler on free-machining and sensitive grades, with less risk of flash attack; and, on the common austenitic grades (304, 316, 316L), corrosion-test results equivalent to nitric when the process is properly controlled.

Drawbacks: shorter bath life and more sensitivity to contamination; not every legacy specification accepts it (some older drawings still say "nitric" and require a deviation to use citric); and on certain alloys nitric remains the more proven choice.

Side by side

Which should you specify?

If your specification names the method, use that method. A drawing that says "passivate per AMS 2700 Method 1, Type 2" is asking for nitric, and substituting citric requires customer approval. If it says "passivate per ASTM A967" without naming a method, either is acceptable, and the finisher chooses based on the alloy.

For 304, 316, 316L, and most austenitic parts, either acid works. Many customers now specify citric as a default for environmental and safety reasons, and the results are equivalent.

For free-machining grades (303, 303Se, 416), citric is generally the safer choice; nitric baths for these grades must be inhibited with sodium dichromate and closely controlled to avoid etching.

For 400-series and precipitation-hardening alloys (17-4 PH, 15-5 PH, 410, 440C), nitric with the correct AMS 2700 type or A967 method is the more established route, though citric methods exist and are used successfully.

For medical, pharmaceutical, and food-contact parts, citric is often preferred because it leaves no nitrate residue and aligns with the industry's preference for benign chemistries; ASME BPE and most pharma specifications accept either.

For aerospace and defense, follow the drawing. AMS 2700 recognizes both methods, and many primes have qualified citric, but the drawing controls.

How passivation is verified, regardless of acid

ASTM A967 specifies the acceptance tests: water immersion, high-humidity, salt spray (ASTM B117), copper sulfate, potassium ferricyanide–nitric acid, and damp cloth. The specification or purchase order names which test applies. Harrison performs the ASTM A967 verification tests your specification calls for and issues a certificate of conformance identifying the method used and the test result.

Harrison runs both

Harrison passivates with both nitric and citric acid under ASTM A967 and AMS 2700 at our Houston facility, so the choice is driven by your specification and your part. For installed equipment and fabrications too large to transport, we perform passivation on-site.

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