Black oxide is a conversion coating
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 tight tolerances are paramount; mostly as a decorative
coating. Only very limited corrosion protection under mildly corrosive
conditions. Should be kept oiled.
For moving parts which cannot tolerate
the dimensional change of a more corrosion-resistant finish black oxide
coatings 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.
Class 1: Alkaline oxidizing.
For wrought iron, cast and malleable irons, common carbon, and low alloy
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°C)
Class 4: Alkaline oxidizing.
For other corrosion resistant steel alloys.