Does black oxide change the size of a part?

Practically no: the layer is only about 1 µm thick, so threads, bores and fits stay as machined. That is the main reason to choose it over plating or painting.

Is blackodising rust proof?

No. It gives mild protection indoors, and only when the parts are oiled or waxed afterwards. For outdoor parts use zinc plating, powder coating or stainless steel.

Can stainless steel be blackened?

Yes, but it needs a different bath from carbon steel. Tell the plater the exact grade so they use the right process.

Blackodising (black oxide) job work: finish and uses

Blackodising (black oxide) is a chemical treatment that turns the surface of steel parts black. The layer is extremely thin, so it does not change the size of threads and fits. With oil or wax after treatment it gives mild rust protection and a neat black look, which is why it is common on fasteners, tools and machine parts.

Quick facts

For: Carbon and alloy steels; special baths for stainless steel
Layer thickness: About 0.5–1.5 µm (no size change in practice)
Process: Hot alkaline bath at about 135–145 °C
Rust protection: Mild indoors, only with oil or wax afterwards
Look: Matt to semi-gloss black
Usually priced: Per kg or per piece

How it is done

Degreasing and cleaning (and pickling if the parts are rusty)
Dipping in a hot blackening salt bath for about 10–20 minutes
Rinsing in water
Dipping in rust-preventive oil, dewatering oil or wax

Good uses

Fasteners, washers and dowel pins
Tool-room parts, jigs, fixtures and gauges
Machine parts where size cannot change
Gun parts and hand tools

Limits

Not for outdoor or wet use: choose zinc plating or powder coating there
Does not hide rough or scratched surfaces; the finish shows the machining underneath
Hardened parts can blacken unevenly; tell the plater the hardness

Need blackodising (black oxide) done?

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Questions

Does black oxide change the size of a part?: Practically no: the layer is only about 1 µm thick, so threads, bores and fits stay as machined. That is the main reason to choose it over plating or painting.
Is blackodising rust proof?: No. It gives mild protection indoors, and only when the parts are oiled or waxed afterwards. For outdoor parts use zinc plating, powder coating or stainless steel.
Can stainless steel be blackened?: Yes, but it needs a different bath from carbon steel. Tell the plater the exact grade so they use the right process.

The figures on this page are typical values for guidance, written by Diemeco from standard engineering practice. They are not a specification: confirm grades and properties with the relevant standard or the supplier's mill test certificate, and tolerances with your workshop. Last checked 15 October 2026.