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Passivation for Stainless Bearings — Restoring the Passive Layer

Passivation does not coat anything. It removes the free iron left by machining, lapping and handling, and lets the stainless steel rebuild its own chromium oxide layer.

0 µmFree iron removal440C & 316No size change
Short answer. Passivation is an acid treatment (typically nitric or citric) that dissolves free iron and iron contamination from a stainless surface, allowing the chromium oxide passive layer to re-form uniformly. It adds no thickness, changes no dimension, and is the correct treatment for 440C and 316 parts that show rust spots after machining, lapping or handling.

Key facts — Passivation

Thickness, protection, temperature and what it does to the fit.
Thickness0 µm — a chemical treatment, not a coating
MechanismDissolves free iron; chromium oxide layer re-forms
AcidsCitric (safer, widely accepted) or nitric (traditional, ASTM A967)
Effect on dimensionNone
Max service temperaturen/a — the steel's own limit applies
StandardsASTM A967 / AMS 2700
Typical use440C and 316 bearing rings, machined parts, welded assemblies

Why stainless rusts at all

Free iron, not a bad alloy.

Stainless resists corrosion because chromium at the surface reacts with oxygen to form a passive oxide layer a few nanometres thick. Machining, lapping, blasting and even handling with carbon-steel tools embed free iron particles in the surface. Those particles rust, and the rust stain looks like a material failure even though the alloy is correct.

Passivation removes that free iron. It does not remove scale, weld discolouration or embedded particles of a different alloy — those need pickling or mechanical cleaning first. The two processes are often confused; pickling dissolves scale, passivation removes free iron.

Where passivation belongs

Any machined stainless part.
  • 440C bearing rings after grinding and lapping.
  • 316 components after machining, welding or abrasive blasting.
  • Parts that will be stored, shipped or assembled in humid conditions.
  • Food, medical and pharmaceutical equipment where a clean passive surface is required.
  • Any stainless part that showed rust spots in incoming inspection.

Passivation against pickling and electropolishing

Three stainless treatments that are routinely confused.
PropertyPassivationPicklingElectropolishing
Removes free ironYesYesYes
Removes scale / heat tintNoYesPartly
Material removedNone measurableSome5–25 µm
Changes dimensionNoSlightlyYes
StandardsASTM A967 / AMS 2700ASTM A380ASTM B912

Where it is used

Positions and parts where this finish earns its place.

440C bearings

Material

The grade most often passivated after grinding.

Read →

316 bearings

Material

Marine and chemical parts after machining or welding.

Read →

Authenticity checks

Help

Rust on a “stainless” bearing is often a passivation issue, not a fake.

Read →

Black oxide for carbon steel

Finish

The equivalent treatment where the part is not stainless.

Read →

Limits and where it fails

What goes wrong, and why.
  • Does not remove scale, heat tint or embedded foreign material — that requires pickling.
  • Cannot fix a wrong alloy: if the part is 304 rather than 316, passivation will not make it seawater-resistant.
  • Nitric acid passivation is losing ground to citric for environmental and safety reasons.
  • No effect on mechanical properties, so it cannot compensate for a soft or mis-heat-treated part.
  • Poor practice — dirty bath, short time — leaves the surface worse than it started.

How to call it out on a drawing

The four lines that stop a supplier guessing.
  1. State the standard: “passivate per ASTM A967 Nitric 2” or “Citric 1”, with the immersion time.
  2. Say whether pickling is also required — scale and heat tint need it.
  3. Require a post-treatment verification: water-break test or copper sulphate test.
  4. Specify that the parts are handled with stainless or clean tools afterwards.

Frequently asked questions

Does passivation change the dimensions of a bearing?

No. It removes free iron at a microscopic level and adds nothing. It is the only “finish” here that is dimensionally neutral.

Why did my 440C bearing rust if it is stainless?

Almost always free iron contamination from machining or handling, or service in chlorides. Passivation fixes the first; nothing fixes the second except a different material.

Passivation or electropolishing?

Passivation for free iron removal with no dimensional change. Electropolishing if the surface must also be brightened or deburred at a microscopic level, and a small dimensional change is acceptable.

How do I check that passivation worked?

The water-break test (a film of water should stay unbroken for 30 seconds) or the copper sulphate test for free iron. Both are quick and should be part of the specification.

Need Passivation on the drawing?

Model numbers, quantities and the application — we reply with brand options, price and lead time within 30 minutes on working days.

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