Black Oxide
The parts are immersed in a bath of oxidizing salts which creates a porous base layer that is black in color (black iron oxide). The parts are then typically dipped in oil which absorbs into that base layer. Black oxide doesn't add much corrosion protection, however parts do not suffer from white corrosion, so it is a preferred coating for the electronics industry as white corrosion can cause electrical shorts. Also, because it does not require acid or electroplating, there is virtually no risk of hydrogen embrittlement (case hardened parts will need to be baked).
Application: Standard finish on socket head cap screws. Parts used in the electronics industry.
Appearance: Black with oil topcoat