Black oxide is a chemical conversion of the steel surface itself rather than a layer added on top of it, which is why it changes dimensions by almost nothing. That single property makes it the finish of choice on tools, fasteners and precision parts where a plated layer would spoil a fit, and it is also why its corrosion protection depends entirely on what is applied afterwards.
What the process does
The part is cleaned, then immersed in an alkaline oxidising solution at temperature which converts the surface iron into black magnetite. The result is integral with the substrate, so it does not chip or flake the way a coating can. Room temperature and mid temperature processes exist too and produce a similar appearance with different durability.
Dimensions, and why that is the point
The conversion consumes a little of the surface and grows a little, so the net dimensional change is negligible for practical purposes. On a gauge, a die component, a threaded fastener or a sliding fit, that is decisive: plating would need to be allowed for or masked, and black oxide simply does not. It is the reason toolrooms use it so heavily.
Corrosion protection is mostly the oil
Black oxide by itself gives modest corrosion resistance. Its protection comes from the porous oxide holding a supplementary oil, wax or sealant, which is applied immediately afterwards and is genuinely part of the specification. A part that is degreased after blacking has lost most of what it was blacked for, which is a common and avoidable mistake.
Where it is used and where it is not
Tooling, fasteners, firearm components, springs, gauges, machine parts and anything where glare must be reduced without adding thickness. It is not a substitute for zinc plating outdoors, not suitable where the part will be repeatedly degreased, and it works on ferrous material, with different chemistries needed for stainless and non ferrous parts.
Questions people ask about black oxide coating
Does black oxide change part dimensions?
Barely. It converts the surface rather than adding a layer, so the net change is negligible, which is why it is used on gauges, threads and fitted parts.
How much corrosion protection does it give?
Modest on its own. Most of the protection comes from the oil or wax sealant applied afterwards, which is part of the specification rather than an optional extra.
Can stainless steel be black oxided?
Yes, with a different chemistry from the one used on carbon steel. Say the substrate when specifying, because the process is not the same.
Is black oxide better than plating?
Different. Plating gives far better corrosion resistance and changes dimensions. Black oxide gives appearance, glare control and near zero dimensional change.