Black Oxide

Black oxide or blackening is a type of conversion coating that is formed by a chemical reaction between the iron in metal and a chemical solution to create an integral, protective surface. Unlike applied coatings that simply bond to the metal surface, black oxide forms a chemical bond with the metal.

Black oxide is typically applied to ferrous alloys, and involves a blend of caustic, oxidizers, and additives that react with the iron to form a layer of magnetite (Fe3O4). The resulting black oxide coating is used to add mild corrosion resistance, improve the appearance of the metal, and reduce light reflection.

To achieve maximum corrosion resistance, the black oxide coating should be impregnated with oil or wax. One of the advantages of black oxide over other coatings is that it has minimal build-up, and does not affect the part's dimensions, hardness, temper, or surface properties.