Black Oxide
Black oxide is a chemical conversion finish that turns the surface of steel a uniform black with almost no dimensional change (about 1 µm). With an oil or wax top coat it gives mild corrosion protection, and it is widely used on tools, fasteners and precision machine parts.
What it is
In the hot process, parts are immersed in an alkaline salt bath at around 135–145 °C, converting the surface into black iron oxide (magnetite). Because the layer is so thin, it does not affect fits, threads or tolerances.
The black layer itself is porous, so a supplementary oil, wax or lacquer is applied afterwards; this top coat provides most of the rust resistance. Room-temperature "cold blackening" is a different, less durable process.
Typical applications
- Precision machined parts and gauges
- Tools, jigs and fixtures
- Socket screws and fasteners
- Gears, shafts and machine components
- Firearm and optical components (low glare)
Suitable materials
- Carbon and alloy steels
- Tool steels
- Cast iron
- Stainless steel (special process)
Points to consider
- Choose black oxide where dimensions must not change; choose zinc plating or zinc-nickel where corrosion protection matters more.
- Specify the top coat (light oil, dry-to-touch oil or wax) based on handling and use.
- Surface finish before treatment determines the final look: polished parts come out glossy black, blasted parts matte.
- Not suitable for outdoor exposure without additional protection.
Frequently asked questions
Does black oxide prevent rust?
Only mildly on its own. With a good oil or wax top coat it protects well indoors, but it is not intended for outdoor or wet conditions.
Does black oxide change dimensions?
Practically no. The layer is only about 1 µm thick, which is why it is used on precision parts, threads and gauges.
What is the difference between black oxide and black zinc plating?
Black oxide is a very thin conversion layer with minimal corrosion protection. Black zinc is a plated zinc coating with black passivation and much better rust resistance, but it adds thickness.