ASTM A967/A967M, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts, is a standard that details effective methods for chemically removing contaminant iron from the surface of stainless steel, as well as methods for verifying the success of the process. The standard was revised at the beginning of 2025 to include more commentary on passivation, namely the materials science behind passivation and how passivation differs to other chemical processes.
Passivation Specifics
ASTM A967 begins by providing multiple definitions of passivation for the context of the document because passivation can refer to multiple processes depending on context. The updated definition used by the standard is Section 3.2.3. It defines passivation as:
The chemical treatment of a stainless steel with a nitric acid or citric acid solution, or other chemical solutions or electrochemical treatments capable of producing similar results, for the purpose of the removal of any combination of contaminant iron, alloyed iron, and possibly other foreign matter from the surface, resulting in an increase of the corrosion resistance of the surface via enabling the formation of a better passive metal oxide film.
Examples of “other foreign matter” from the definition can be metallic inclusions from the steelmaking process or metal oxides embedded on the surface from fabrication. Passivation can also be defined as the spontaneous growth of a metal oxide film on a bare stainless steel surface in the presence of air or moisture, a description like this is stated in Section 3.2.3.1 of ASTM A967 but it is not the definition accepted by the standard. Passivation is not meant to remove heat tint or oxide scale present on a stainless steel surface; that process is called pickling. Pickling chemistries are far more aggressive to the metal surface than passivation ones, as they attack tightly adhered oxides or otherwise adversely affected base metal, instead of the loosely adhered free iron that passivation is targeting.
Two major chemistries are introduced in ASTM A967 for passivation, nitric acid and citric acid. Nitric acid is a common mineral acid used in a variety of chemical treatments other than passivation while citric acid is an organic acid used predominantly in the food and beverage industry. Nitric acid was the original acid choice for chemical passivation, however citric acid use has significantly grown in the last few decades since the organic acid is safer for both humans and the environment to handle than nitric acid.
Passivation Methods
In Section 5 of ASTM A967, the standard refers the reader to ASTM A380 for pretreatment methods to ensure the metal surface to be passivated is free of any nonmetallic contamination that could interfere or inhibit passivation. Section 5.3.2 specifically states, “The resultant pretreated surface shall be substantially free of oil, grease, rust, scale, and other foreign matter.”
“The resultant pretreated surface shall be substantially free of oil, grease, rust, scale, and other foreign matter.”
After preparation is discussed, the standard introduces four distinct process conditions to passivate stainless steel with nitric acid. Various times, temperatures, and nitric acid concentration windows are iterated in each of the four methods. The different process conditions exist due to the large variety of stainless steel grades and families.
For example, ferritic stainless steel grade 409 and martensitic grade 440C are recommended to be processed with Nitric Method 1 or 4, while austenitic stainless steel grade 304 is recommended to be processed with Nitric Method 2 or 3. Also, the first nitric method is the only method to include a hazardous additive, sodium dichromate, in order to further enrich the chromium content of the passive film. Further, the first nitric method is the only method to include a hazardous additive, sodium dichromate, in order to both expedite growth of a passive film and enrich the chromium level of the film. ASTM A967 also contains a fifth method for nitric acid. This is an open-ended process that gives freedom to the operator to determine their own optimal temperature, time, and additives for their component. The standard specifically mentions that the user-developed process must be capable of passing one of the verification tests before being recognized as compliant.
Immediately following the nitric acid methods are a similar five methods for passivating with citric acid. A few things to point out:
- The first three citric methods have fully developed temperatures, times, and concentrations.
- The fourth and fifth methods are open-ended processes.
- Citric Method 5 differs from Citric Method 4 in that Citric 5 involves a specific pH range for the passivating solution while Citric 4 is completely unconstrained.
Additionally, two general differences between Citric Methods 1 – 3 and Nitric Methods 1 – 4 are that:
- There is no mention of additives in the citric methods.
- The citric methods are recommended to run for shorter time than the nitric methods.
Astro Pak's Proprietary Ultra Pass®
Both citric and nitric acid passivation chemistries provide a passive surface that passes all verification tests explained below, however that does not mean both chemicals are equally effective. Astro Pak’s proprietary Ultra Pass® blend, a derivative of citric acid method 4, is more capable of removing free iron than the basic nitric or citric acid chemistries. As a result, a stainless steel surface passivated with Ultra Pass® will have a higher amount of chromium relative to iron in the passive layer than a surface passivated with citri methods 1- 3 and nitric methods 1 -4. Although our recommendation is citric acid, specifically Ultra Pass®, Astro Pak has experience passivating with both citric and nitric acid and can mobilize our crew with the chemistry you prefer.
Rinsing and Neutralization
ASTM A967 prefers all parts are immersed in the passivation solution but allows for other treatment techniques so long as the temperature window of the selected process can be maintained. After passivation with either nitric or citric, ASTM A967 requires parts to be rinsed. The rinse water must have total solid content of less than 200 ppm. Astro Pak passivation procedures include a rinse well beyond the requirement set in A967, we typically rinse to a conductivity value below one microSiemen or until effluent rinse water matches the influent’s conductivity or pH. Neutralization of the passivation solution on the component surface is not required per ASTM A967; however, Astro Pak will typically neutralize our passivation solution to allow for an environmentally safer disposal. The standard then calls for a visual inspection of the passivated component(s). Signs of etching, pitting, or frosting are grounds for a failure of the passivation process.
Acceptance Tests
ASTM A967 defines seven tests for verification that a suitable passive surface has been formed on the component surface. These tests are all qualitative. Although some tests can more precisely detect the amount of free iron remaining on a surface, they are meant to serve as a go, no-go test for passivation.
- Water immersion test
- High humidity test
- Salt spray test
- Copper sulfate test
- Potassium ferricyanide test
- Damp cloth test
- Boiling water immersion test.
Astro Pak operations prefer the copper sulfate test and the potassium ferricyanide test as they are two of the quickest and more precise verification methods.
ASTM A967 and Astro Pak
Astro Pak has decades of experience conducting passivation work on stainless steel components and systems. Our crews mobilize with the necessary hosing and pipe fittings to tackle any stainless steel system, regardless of size and complexity. Our Ultra Pass® chemistry is verified by multiple acceptance tests mentioned in ASTM A967, plus Auger Electron Spectroscopy. Spectroscopy results reveal the Ultra Pass® chemistry provides more chromium enrichment in the passive layer than the nitric acid processes introduced in ASTM A967 can provide. Reach out today to experience a new standard of corrosion protection for your system.
