What Does Black Oxide Mean?
Black oxide is a conversion coating that is used for appearance and to add mild corrosion resistance and minimize light reflection. It is used for ferrous materials, zinc, copper and its alloys, silver solder and powdered metals. Black oxide is impregnated using oil and wax to achieve maximum corrosion resistance. It also provides a deep black color when used with proper post dips.
Black oxides are used where component accuracy and tolerance are paramount, compared to painting methods. This method is used in fasteners, weldments, tooling and casting where oil retention and appearance are essential.
Corrosionpedia Explains Black Oxide
The conversion coating is formed by a chemical reaction formed when a part is immersed in an alkaline aqueous solution of a salt, about 140 degrees Celsius. Iron in the ferrous alloy reacts with the hot oxide bath to produce magnetite (Fe3 O4) at the surface of the immersed part.
Non-ferrous materials also can be oxidized to form black oxides, but only after meeting suitable conditions. Black oxide can be applied at room temperature, although some benefits will not be present. When the cold black oxide process is used, color variations are experienced from one part to another, with the oxide rubbing off onto hands. This does not meet military or automotive industry specifications; thus, is not commonly used.
The steps involved in the conversion coating are cleaning, rinsing, black oxide and after-finish. Pickling or de-scaling also may be applied if the metal surface has traces of rust or scale. Assemblies and other parts with blind holes may be neutralized or rinsed further to eliminate flowering or bleed-out. Plating with zinc, chromium, nickel, cadmium and phosphates must be stripped off in parts on which black oxide is to be produced. The use of this method is recommended over alternative methods, due to the key benefits that come with it.
The use of black oxide is very economical, especially when used with oils. Unlike other methods, an additional benefit is that its use does not cause any dimensional change. It also can achieve more than 100 hours of salt spray with oil dip as a supplement.