Black Oxide Coating & Finishing

What it is and when to specify it

Black oxide is a conversion coating that chemically reacts with the surface of steel or stainless steel to form a thin, matte black layer. Unlike paint or plating, it does not build up on the surface, so it changes dimensions only slightly. It is a black oxide finish chosen for appearance, mild corrosion resistance, and dimensional stability.

Specify black oxide when you need a black appearance on screws, bolts, knobs, or tooling without changing fits. It is common on black oxide steel fasteners and on internal components where oil can sit on the surface. It also works on stainless steel, giving a uniform black finish where a bright surface is unwanted.

Black oxide specifications

The two broad classes we specify are hot and room-temperature black oxide. Hot black oxide is the more common process for black oxide coating on steel and stainless, while room-temperature is used where lower heat is preferred. The exact type is confirmed per order against your drawing.

• Color: uniform black, with a matte appearance that can be deepened with a post-treatment oil or wax.
• Materials: carbon steel, alloy steel, and stainless steel.
• Coverage: a thin layer with minimal dimensional growth, which is why it suits threaded and close-fit parts.

Because the coating is thin, black oxide does not carry the corrosion protection of zinc plating on its own. Where rust protection matters, we specify black oxide plus a rust-preventive oil.

Which materials and parts it suits

Black oxide works on both carbon and alloy steel and stainless steel, which makes it a flexible cosmetic and light-protection finish.

• Steel machined parts — black oxide steel fasteners, tooling, and brackets where a thin black layer is wanted.
• Stainless steel machined parts — stainless black oxide for screws and fittings that need a black, non-reflective surface.
• Custom screws, bolts, and knobs where the coating must not disturb thread fits.

Finishing is applied through our vetted partner network, managed under one quality process. We stay the single point of contact, so you do not need to screen black oxide suppliers yourself, and every order ships with a material certificate and pre-shipment photos.

Corrosion, wear and dimensional impact

Black oxide gives light corrosion resistance, not the heavy-duty protection of zinc plating or nickel plating. Its real advantage is dimensional. The layer adds very little thickness, so threads, bores, and press fits stay essentially unchanged, which makes it the finish of choice when a part must turn black without a remachining allowance.

For wear, the surface is not a hard coating, but the matte texture holds oil well, which helps moving parts run smoother. We confirm whether a rust-preventive top coat is needed and check the finished dimensions against the drawing before shipment.