Oxidation, also known as Steaming or Steam Oxidation, is a post-heat-treatment process in which components are exposed to controlled steam at elevated temperatures to form a dense magnetite (Fe3O4) layer on the steel surface.

This black oxide layer improves corrosion resistance, enhances surface appearance and provides additional protection for nitrided or heat-treated components. Unlike decorative coatings, the oxide layer becomes an integral part of the surface and does not significantly affect component dimensions.

The process is widely used after gas nitriding and nitro-carburizing where improved corrosion protection and a uniform black finish are required.