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Black Oxide on Bearings — 1–3 µm, No Dimensional Change

Black oxide is the finish you use when the part must look right, stay dimensionally identical, and resist handling and condensation — not salt water.

1–3 µm24–72 h oiled200 °CNo size change

Short answer. Black oxide (blackening) converts the surface of steel into a thin layer of magnetite, 1–3 µm thick, usually sealed with oil. It adds essentially no dimension, changes no tolerance, and gives mild protection against handling and condensation — typically 24–72 hours of salt spray when oiled. It is not a corrosion barrier in the way plating is.

Key facts — Black oxide

Thickness, protection, temperature and what it does to the fit.

Thickness	1–3 µm (conversion layer, not a deposit)
Salt spray	24–72 h when oiled · a few hours dry
Max service temperature	≈200 °C (the oil film is the limit)
Effect on dimension	Negligible — the layer grows into the surface as well as out
Appearance	Matt to satin black
RoHS / REACH	Compliant; hot blackening baths are caustic, not chromate
Typical use	Bearing rings, exposed steel parts, tools, fixturing

What black oxide actually is

A conversion coating, not a coating.

The part is immersed in a hot alkaline oxidising bath, and the surface steel reacts to form magnetite (Fe3O4). Because the layer consumes some of the base metal and grows partly inward, the external dimension changes by well under a micrometre in most cases — which is why it is the finish of choice for parts whose tolerances cannot absorb a deposit.

On its own, black oxide is a poor corrosion barrier: it is thin and slightly porous. Its performance comes from the oil, wax or dry-film that is applied afterwards, which fills the pores. That is why the specification must state the post-treatment, not just “black oxide”.

Where it earns its place

Dimension-critical parts that must not look like bare steel.

Bearing rings and washers where appearance matters and no dimensional change is allowed.
Machine tool and fixture components, gauge bodies, tooling.
Gun and firearms parts, where the finish is traditional and functional.
Any part that will be handled, stored and assembled — the oil film resists fingerprints and condensation.

Black oxide against phosphate and zinc

Three finishes that are often confused.

Property	Black oxide	Manganese phosphate	Zinc plating
Thickness	1–3 µm	5–15 µm	5–8 µm
Salt spray (oiled)	24–72 h	24–96 h	96–240 h
Dimensional change	Negligible	Moderate	Significant
Friction	Slight reduction with oil	Lowers, retains oil	None
Appearance	Black	Dark grey	Bright / yellow
Best for	Tight tolerance, appearance	Running-in, oil retention	Corrosion

Where it is used

Positions and parts where this finish earns its place.

Rings, washers & accessories

Catalogue

Dimension-critical parts that must not change size.

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Bearing rings

Catalogue

Appearance and handling protection without a dimensional penalty.

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Phosphate instead

Finish

When the surface must also retain oil and aid running-in.

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Passivation for stainless

Finish

The equivalent treatment for 440C and 316 parts.

Read →

Limits and where it fails

What goes wrong, and why.

Poor corrosion protection on its own — salt spray measured in tens of hours.
The oil film is essential and is removed by solvents and cleaning.
Not for stainless (use passivation) or aluminium (use anodising).
Colour varies with alloy and bath condition; matching batches is difficult.
Cannot be used where the part will be in contact with food without a suitable seal.

How to call it out on a drawing

The four lines that stop a supplier guessing.

Say “black oxide per MIL-DTL-13924 Class 1, oiled” or the equivalent commercial standard.
Name the post-treatment: oil, wax or dry film — the corrosion performance depends on it.
Confirm that no dimensional change is acceptable, so the supplier does not substitute phosphate.
For stainless parts, specify passivation instead — black oxide does not work on it.

Frequently asked questions

Does black oxide change dimensions?

By less than a micrometre in practice. The layer partly grows into the base metal, so it is the standard choice where a deposit would consume the tolerance.

Is black oxide rustproof?

No. It resists handling and condensation for a while, especially when oiled, but it is not a corrosion barrier. For salt or water, use zinc-nickel or a stainless grade.

Can black oxide be applied to stainless steel?

Not in the same way. Stainless is passivated instead; there are blackening processes for stainless but they are different chemistries and much less common on bearings.

Why does black oxide sometimes look patchy?

Surface condition before processing — scale, machining marks and different alloys take the conversion at different rates. A uniform surface preparation is what makes a uniform colour.

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