What is passivation, and when does stainless need it?

Passivation explained: the chemical cleaning that keeps stainless from rusting, the ASTM A967 and AMS 2700 standards, and when to spec it on a machined stainless part.

Quick answer: Passivation is a chemical cleaning of stainless steel that removes free iron from the surface and restores the chromium-oxide layer that makes stainless "stainless." It is not a coating and does not change dimensions. Machining, grinding and handling embed free iron that later rusts; passivation to ASTM A967 or AMS 2700 removes it. Spec it on any stainless part headed for medical, food, pharma or corrosion-critical service.

Why machined stainless rusts without it

Stainless resists corrosion because chromium forms a thin passive oxide film. When you machine, grind or even just handle a stainless part with steel tooling, you embed tiny particles of free iron in the surface. That free iron rusts, and it drags the surface down with it. Passivation dissolves the free iron and lets the chromium film reform, clean and continuous.

When to spec it (and it is more often than people think)

If your 316 stainless part is going into a food line, a medical device or a pharma skid and the drawing does not call out passivation to ASTM A967, add it. The machining that made the part is exactly what contaminated the surface. Skipping it is how a "stainless" part shows rust freckles six weeks into a wash-down environment, and then everyone acts surprised that the line has water in it. It is the cheapest insurance in finishing.

It is cheap, so do not skip it

Passivation is a low-cost chemical process priced by part size and quantity, with batch minimums like any finishing step. For a ballpark by size and volume, use the passivation estimator. Compared to the cost of a rejected medical or food part, it is a rounding error. For regulated work, pair it with the right certification, ISO 13485 for medical devices.

What to send for a quote

The part size and quantity, the stainless grade (304, 316), the standard you need (ASTM A967, AMS 2700, or a customer spec), and any test or documentation requirement. Request quotes from Canadian finishing shops and we route it to ones set up for passivation.

FAQ

Does passivation change the part dimensions?

No. It removes only free iron and contaminants, not base metal, so it is dimensionally neutral. That is one reason it is preferred on precision stainless parts.

Nitric or citric acid passivation?

Both meet ASTM A967. Citric is greener and increasingly the default; nitric is the traditional method. Unless your customer spec names one, either is acceptable when done to standard.

Is passivation the same as electropolishing?

No. Electropolishing removes a thin layer of metal to brighten and smooth the surface, and it passivates as a side effect. Passivation is a cleaning step only. Electropolish when you also need a bright, ultra-smooth finish.

Which stainless grades need passivation?

Any grade can benefit, and 300-series (304, 316) parts for food, medical and pharma almost always spec it. 316 for chloride and wash-down service especially, because that is where free-iron rust shows fastest.

Does passivation make stainless rust-proof?

No finish does. It restores the corrosion resistance the base metal is supposed to have by removing surface contamination. Grade selection (316 over 304 for salt and chloride) still matters.

Last reviewed: July 2026. Related reading: what is anodizing and powder coat vs paint.