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Black Oxide

Black Oxide is a coating applied to steel in a high temperature alkaline solution. Black oxide is useful because of  black color without any dimensional changes on machined parts. On the other hand, black oxide has very poor corrosion resistance properties; therefore, an oil, wax or other coating is applied to give the coating some corrosion protection. Without this added protection, the black oxide coating would corrode rapidly. It is commonly used in the aerospace industry for transmission and hydrolic systems, where the coating will be constantly coated with oil or other moving parts that can not tolerate the dimensional change of a more corrosion resistant finish.
Specification         Comments
MIL-C-13924C      No dimensional change
Class 1
Alkaline, oxidizing. for wrought iron, cast and malleable irons, plain carbon, and low alloy steels
Class 3                  Fused salt oxidizing.  (Not performed at TPP)
Class 4
Alkaline oxidizing. For 300 series Corrosion resistant steel  alloys only.