Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys, zinc, powdered metals, and silver solder. It is used to add mild corrosion resistance, for appearance, and to minimize light reflection.To achieve maximal corrosion resistance the black oxide must be impregnated with oil or wax.One of its advantages over other coatings is its minimal buildup.

There are many advantages of blackening, mainly:
Blackening can be done in large batches (ideal for small parts).
There is no significant dimensional impact (the blacking process creates a layer about 1 µm thick).
It is far cheaper than similar corrosion protection systems, such as paint and electroplating.