Black Oxide, blackening, oxidizing, oxiding, black passivating, gun bluing . . . these terms all refer to the process of forming a black iron oxide on the surface of ferrous metals.
The black oxide process 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. These oxidizing 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. Black oxide has a range of unique properties and benefits that can be applied to a variety of industries.
Applying a supplementary post treatment after the black oxide finish is commonly used to increase the corrosion protection properties of black oxide coatings. Alone, black oxide offers only a very mild corrosion resistance. A properly applied post treatment, allowing full absorption of the supplementary coating into the pores of the black oxide finish, enhances the corrosion protection to the metal, while producing a deeper black appearance. An appropriate post treatment also increases the resistance to abrasion of the black oxide where break-in of mating parts is required.
Oil post treatments are frequently used after black oxide coating. The oil can be either water-soluble or water displacing and can be controlled so that a fairly heavy oil film is left on the part or so that the part is virtually dry to the touch. Of course, less oil means less corrosion protection. A clear wax or acrylic may also be used as an alternative, dry to the touch post treatment, where absolutely no oil film is desired.
Post Treatments for Improved Corrosion Resistance