Next in our series of commentaries on cleaning and passivation standards is ASTM A967/A967M – Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts. This standard details effective methods to chemically remove free iron from the surface of stainless steel, plus methods to verify successful operation of the process.
ASTM A967 begins by providing a basic definition of passivation for the context of the spec as there are multiple often-used definitions of the term. The standard defines passivation as the specific chemical process that removes free iron and other surface contaminants (such as sulfides, organics or metal inclusions) from a stainless steel surface. 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, this is the definition ASTM A967 does not refer to. 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 is the more established chemical for metal surface treatment compared to citric acid, however it also poses a greater safety concern and requires stricter handling and use precaution.
Before passivation, ASTM A967 refers the reader to ASTM A380 to ensure the metal surface to be passivated is free of any contamination.
After preparation is discussed, the spec 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, a ferritic stainless steel will be more effectively passivated with a different nitric method than an austenitic stainless steel. 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 chrome 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 spec 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:
- The first three citric methods have fully developed temperatures, times, and concentrations.
- The fourth and fifth methods are open-ended processes.
- Citric method five differs from citric method four in that citric five involves a specific pH range for the passivating solution while citric four is completely unconstrained.
Additionally, two general differences between citric methods one through three and nitric methods one through four 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.
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 UltraPass blend, a derivative of citric acid method four, is more capable of removing free iron than the basic nitric or citric acid chemistries. As a result, a stainless steel surface passivated with UltraPass will have a higher amount of chromium relative to iron than a nitric passivated surface. An increased amount of the more chemically resistant element chromium provides the increased corrosion resistance benefitting citric acid over nitric acid. Although our recommendation is citric acid, Astro Pak has experience passivating with both citric and nitric acid and can mobilize our crew with the chemistry you prefer.
The spec 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 spec 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.
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 UltraPass chemistry is verified by multiple acceptance tests mentioned in ASTM A967, plus Auger Electron Spectroscopy. Spectroscopy results reveal our passivation chemistry provides more chrome enrichment in the passive layer than nitric acid process can provide. Reach out today to experience a new standard of corrosion protection for your system.