Black oxide is a coating conversion that causes virtually no dimensional change. It is a uniform continuous conversion of the existing metal to a black form of rust. Used primarily on components where light tolerances are paramount - mostly as a decorative coating. Only very limited corrosion protection under mildly corrosive conditions. Should be kept oiled. For moving parts that cannot tolerate the dimensional change of a more corrosion-resistant finish, black oxide coating should normally be given a supplementary treatment (i.e. oil displacement per MIL-C-16173 Grade 3 or protective treatments of MIL-C-16173). For decorative applications and can be used to decrease light reflection.
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Black Oxide Coating [MIL-DTL-13924D] Classes
Class 1: Alkaline oxidizing. For wrought iron, cast and malleable irons, common carbon and low alloy steels.
Class 2: Alkaline oxidizing. Use on certain corrosion-resistant steel alloys tempered at less than 900°F.
Class 3: Fused salt oxidizing. For corrosion-resistant steel alloys which are tempered at 900°F (482°) or higher.
Class 4: Alkaline oxidizing. For other corrosion-resistant steel alloys.