Passivation

Passivation is a nitric or citric acid treatment that dissolves free iron and contaminants left on stainless steel by machining, forming or handling, and helps rebuild the chromium oxide film that makes stainless resist corrosion. It does not change dimensions or appearance. It is specified to ASTM A967 or AMS 2700 and verified by water immersion, humidity, salt spray or copper sulfate tests.

Add it to a part

Applies to: 300 and 400 series stainless, precipitation-hardening grades such as 17-4 PH, and powder metal stainless
Adds: 3 days
Cost: Low

Specs

Passivation specifications
Methods	Nitric or citric acid per ASTM A967 or AMS 2700
Dimensional change	None
Appearance	No visible change; may brighten slightly
Verification tests	Water immersion, high humidity, salt spray (ASTM B117), copper sulfate
Free-machining grades	Use processes suited to 303 and 416 to avoid flash attack
Prep	Degrease first; remove scale and heat tint by pickling or mechanically

Call out on the drawing

ASTM A967 (nitric and citric treatments)
AMS 2700 (Method 1 nitric, Method 2 citric)
ASTM A380 (cleaning, descaling and passivation practice)
ASTM F86 (surgical implants)

Materials it suits

Common on

Questions

Is passivation necessary on stainless steel?

It is strongly recommended for machined, formed or welded stainless that will see moisture, food, medical or chemical exposure. Embedded iron from tooling rusts and can start pitting. Many medical, food and aerospace drawings require it.

Nitric vs citric passivation?

Both meet ASTM A967 and AMS 2700. Citric is safer to handle, easier on the environment and works well on most grades. Nitric, sometimes with sodium dichromate, is long established and still preferred by some aerospace specs and for some free-machining grades.

Does passivation remove weld discoloration?

No. Passivation does not remove heat tint or scale. Pickle with a nitric-hydrofluoric acid solution or clean mechanically first, then passivate.

How is passivation tested?

Common ASTM A967 tests are 24 h water immersion or high humidity, salt spray for at least 2 h, and copper sulfate for austenitic grades. The drawing or purchase order selects which test applies.