Passivation
Passivation is a chemical treatment for stainless steel that removes free iron and surface contamination left by machining, and strengthens the natural chromium oxide layer. It restores full corrosion resistance without changing dimensions or appearance.
What it is
Machining, grinding and handling can leave tiny particles of iron from tools on the surface of stainless parts, which later show as rust spots. Passivation dissolves this free iron in a nitric or citric acid solution.
The clean surface then forms a uniform, chromium-rich passive film. The treatment adds no coating, so there is no change in size, and it is standard for stainless parts used in food, pharma, medical and chemical applications.
Typical applications
- Machined stainless steel components and fasteners
- Food, dairy and pharmaceutical equipment parts
- Medical and laboratory components
- Valves, fittings and pump parts
- Welded and fabricated stainless assemblies (after pickling)
Suitable materials
- 300-series stainless (304, 316, 321)
- 400-series stainless (410, 420, 430, with adjusted process)
- Precipitation-hardening grades (17-4 PH)
- Free-machining grades such as 303 (with care)
Points to consider
- Parts must be thoroughly degreased first; passivation will not remove oil or heavy scale.
- Welds and heat tint need pickling before passivation.
- Free-machining (303, 416) and some 400-series grades can be attacked by the wrong acid; the process must match the grade.
- Citric passivation is safer and increasingly preferred, and meets ASTM A967.
Frequently asked questions
Why does stainless steel rust after machining?
Usually because free iron from cutting tools or handling is left on the surface. Passivation removes it and restores the protective oxide film.
Does passivation change the look of stainless steel?
No. It leaves the surface visually the same, with no coating and no dimensional change.
What is the difference between pickling and passivation?
Pickling removes heavy oxide scale and heat tint from welding, and does remove some metal. Passivation is a milder treatment that removes free iron and builds the passive layer.