Black oxide or blackening is a conversion coating for ferrous materials, stainless steel, copper and copper based alloys. It is a conversion coating formed by a chemical reaction produced when parts are immersed in the alkaline aqueous salt solution operated at higher temperatures. The reaction between the iron of the ferrous alloy and the hot oxide bath produces a magnetite (Fe3O4) on the actual surface of the part.
CLASSES
Class 1 : Alkaline oxidizing process for wrought iron, cast and malleable
irons, plain carbon, and low alloy steels.
Class 2 : Alkaline chromate oxidizing process for use on certain corrosion
resistant steel alloys which are tempered at less than 900°F (482°C).
Class 3 : Fused salt oxidizing process for corrosion resistant steel alloys
which are tempered at 900°F (482°C) or higher.
Class 4 : Alkaline oxidizing process for other corrosion resistant steel alloys.