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.