Black-oxide coatings are formed by a chemical reaction between the metal surface and oxygen. The surface layer formed consists of magnetite (Fe3O4), a form of iron oxide that is black in color and tightly adherent to the metal surface. By contrast, rust is a red oxide of iron called hematite (Fe2O3), which easily flakes off a metal surface and as such is highly undesirable. While some black-oxide-type coatings can be applied at room temperature, most are applied at elevated temperature in the presence of an oxidizing agent in the form of a gas or an aqueous solution, including molten salt baths.
Being a chemical conversion process, one of the unique aspects of black-oxide coatings is that springs and other shapes can be coated and then formed without concern over chipping, cracking or flaking of the surface layer (Fig. 2). Since black-oxide coatings adhere so well, they are often used where tight tolerances are required with minimal change in surface dimensions, typically in the order of 0.013 mm (0.0005 inch). Steels that benefit from this type of coating include low- and medium-carbon steels, alloy steels, stainless steels and tool steels. Certain nonferrous materials, such as brass and aluminum, can also be treated.