Stainless Passivation

What passivation actually does

Passivation is a chemical cleaning step, not a layer. ASTM A967 covers three treatments: immersion in a nitric acid solution, immersion in a citric acid solution, and an electrochemical treatment (source 1). SAE AMS2700 is the other standard a print may name; it covers removing free iron and other less noble contaminants from corrosion-resistant steel parts (source 2). The result is that exogenous iron comes off and the passive oxide layer is restored (source 3), the thin chromium-oxide film that stainless forms on its own and that repairs itself when scratched (source 6).

Nothing is plated, dyed or deposited. A967 asks the passivated surface to be chemically clean, with no etching, pitting or frosting (source 1). Sources differ on whether any metal comes off: one engineering article puts the removal in micro-inches (source 5), and Euro Inox says no metal is removed during acid passivation (source 9).

Why machining leaves iron behind

Iron particles from cutting tools and shop dirt can sit on a stainless surface and act as starting points for corrosion (source 4), and the tests in ASTM A967 are intended to confirm the removal of contaminant iron and other exogenous matter (source 1). Euro Inox names free iron as the cause of rust staining on stainless (source 9).

Passivation removes that iron and leaves the chromium in the steel to protect the whole surface (source 3). The 303/304 stainless and 17-4 PH stainless pages cover those grades.

When to call it out, and on which grades

How clean the surface needs to be depends on the application (source 5). If the part will see moisture, salt, washdown or cleaning chemicals, say so in the quote request.

ASTM A967 covers stainless steel parts in general. Its nonmandatory appendices give guidance on which treatment suits which grade, and it makes no recommendation on which grade or treatment suits a particular application (source 1). A published study passivated 17-4 PH and 15-5 PH in both citric and nitric baths (source 4). 303 carries a minimum of 0.15% sulfur so it machines freely, and the British Stainless Steel Association describes its pitting resistance as significantly lower than 304 (source 7), which is a reason to name the environment on the print. The grade hub on stainless steel parts covers how these grades behave, and the medical and lab parts page covers that industry’s parts.

How it differs from electropolish and pickling

Electropolishing is a separate process. It removes a small amount of material, typically 20-40 micrometers in depth on stainless, and smooths the surface (source 8). Pickling is a more aggressive chemical treatment than passivation and is the one aimed at heavy oxide scale and heat tint (source 5).

Passivation on its own is generally not effective in removing heat tint or oxide scale (source 5), so it does not clear weld discoloration. You can read more on the surface finishes overview, and turned stainless from CNC turning takes the same callout.

How it gets handled on your order

A finish called out on the print is quoted with the part. Passivation is quoted with machined parts, not as a finish-only job on a part you already have. If a part doesn’t match the print, it’s remade or credited.

Sources

All accessed 4 October 2026. Where a figure here differs from your print, the print governs.

1. ASTM International, “A967/A967M-25, Standard Specification for Chemical Passivation Treatments for Stainless Steel Parts”: store.astm.org/a0967_a0967m-25.html (three treatments: nitric acid immersion, citric acid immersion and electrochemical; passivated parts shall show a chemically clean surface with no etching, pitting or frosting; the nonmandatory appendices give general guidelines on selecting passivation treatments for particular grades, and the specification makes no recommendation on the suitability of any grade, treatment or application; tests "intended to confirm the effectiveness of passivation, particularly with regard to the removal of contaminant iron and other exogenous matter").

2. SAE International, “AMS2700, Passivation of Corrosion Resistant Steels” (latest revision AMS2700G, 27 May 2025): saemobilus.sae.org/standards/ams2700-passivation-corrosion-resistant-steels (removal of free iron or other less noble contaminants from the surfaces of corrosion-resistant steel parts).

3. Wikipedia, “Passivation (chemistry),” stainless steel section: en.wikipedia.org/wiki/Passivation_(chemistry) (the process removes exogenous iron and restores a passive oxide layer; acids remove surface iron while sparing the chromium; ASTM A 967 and AMS 2700 are the most prevalent standards; citric acid is gaining in popularity as it is far less dangerous to handle, less toxic and biodegradable).

4. Lara-Banda et al., “Alternative to Nitric Acid Passivation of 15-5 and 17-4PH Stainless Steel Using Electrochemical Techniques,” Materials (Basel), 2020: pmc.ncbi.nlm.nih.gov/articles/PMC7344543/ (passivation removes surface contamination such as small iron particles from cutting tools that can act as initiation sites for corrosion; 17-4PH and 15-5PH tested in citric and nitric solutions).

5. Cherra Meloy, “Passivation and Pickling,” Valve Magazine, 19 December 2017: valvemagazine.com/articles/passivation-and-pickling (nitric and citric acid-based solutions are the most commonly used chemicals for passivation; passivation is generally not effective in removal of heat tint or oxide scale; the material removed is in micro-inches; pickling is a more aggressive chemical treatment than passivation).

6. Wikipedia, “Stainless steel,” corrosion resistance section: en.wikipedia.org/wiki/Stainless_steel (stainless forms a microscopically thin chromium-oxide film by reaction with oxygen; the film is self-repairing).

7. British Stainless Steel Association, “Stainless Steels to Behold: 303,” no date shown: bssa.org.uk/stainless-steels-to-behold-303-%E2%9C%A8/ (0.15% minimum sulphur to improve machinability; pitting corrosion resistance significantly lower than 304).

8. Wikipedia, “Electropolishing”: en.wikipedia.org/wiki/Electropolishing (removes a small amount of material, typically 20-40 micrometres in depth for stainless steel; improves surface finish by levelling micro-peaks and valleys).

9. Euro Inox, Roger Crookes, “Pickling and Passivating Stainless Steel,” reprinted by Safe Food Factory, 13 May 2020: safefoodfactory.com/en/editorials/54-pickling-and-passivating-stainless-steel (free iron is the cause of subsequent rust staining and all traces must be removed; no metal is removed from the surface during acid assisted passivation).