Black Oxide
Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper basedalloys, zinc, powdered metals, and silversolder. It isused to add mild corrosion resistance, for appearance and to minimise light reflection. Toachieve maximal corrosion resistance the black oxide must be impregnated with oil orwax. One of its advantages over other coatings is its minimal buildup.
 
The black oxide proces is a chemical conversion coating. This means that the black oxide is not deposited on the surface of the substrate like nickel or zinc electroplating. Instead, the black oxide coating is produced by a chemical reaction between the iron on the surface of the ferrous metal and the oxidizing salts present in the black oxide solution. The seoxidizing salts include penetrates, catalysts, activators and proprietary additives which all take part in the chemical reaction. The result of this chemical reaction is the formation of black iron oxide, magnetite (Fe3O4), on the surface of the metal being coated. Process under the standard ASTM D769-01