MIL-C-13924 & MIL-DTL-13924
Class 1: Alkaline oxidizing process for steel and steel alloys 

Class 4: Alkaline oxidizing process for corrosion resistant steels.

Black Oxide is for decorative use on miscellaneous iron and steel parts. It imparts a black inorganic oxide coating obtained by immersing the part in a chemical bath that operates at a temperature of 270-300ºF followed by a coating of rust preventative oil for additional protection. The black finish varies from dull to lustrous sheen, which is dependent upon the original surface finish of the base metal. The finish is semi-hard and produces no significant dimensional change.

A black oxide finish is especially suitable for parts to be used indoors having close dimensional tolerances. Cast or malleable iron parts do not blacken satisfactorily for a uniform finish.

Resistance to corrosion is largely determined by the supplementary coating applied. This finish is recommended for normal indoor exposure but not for protection against humid or corrosive atmospheres. The black oxide reduces light reflection but does not materially alter surface smoothness. Scratches, tool marks, etc. should be removed before blackening.

Above: Black oxide rifle component.