Black Oxide

Black oxide is a conversion coating formed by a chemical reaction produced when parts are immersed in the alkaline aqueous salt oxidizing solution operated at approximately 290°F. The reaction between the iron of the ferrous alloy and the hot oxide bath produces a magnetite (Fe3O4) on the part surface.

Black oxide is commonly applied to carbon and low alloy steels, but different chemistries can be used to form black oxides on stainless steel and copper based alloys. These coatings are particularly suited for moving parts, particularly for sliding or bearing surfaces, by providing a finish coating that will retain an oil film. These coatings are also suited for parts that cannot tolerate dimensional buildup of a more corrosion-resistant finish. The coatings present a pleasing appearance frequently employed for decorative purposes or decreases in light reflection. Very limited corrosion protection, under mildly corrosive conditions, is obtained with black oxide coating. Usually the parts are oiled or waxed.

Precision gear assemblies benefit from black oxide treatments due to its oil-retaining properties. It also aids in the inspection of wear patterns of mating gears.

As with other plating / conversion processes, hardened steels greater than 40 HRC may require hydrogen embrittlement relief bake dependent on specification requirements.

MIL Black Oxide

Black Oxide