chemical blackingBlack 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 minimise light reflection. To achieve maximum corrosion resistance the black oxide must be impregnated with oil or wax. One of its advantages over other coatings is its minimal buildup.

A hot bath of sodium hydroxide, nitrates, and nitrites, at 285 °F (141 °C),  are used to convert the surface of the material into magnetite (Fe3O4). Water must be periodically added to the bath, with proper controls to prevent a steam explosion.

Hot blackening involves dipping the part into various tanks. These tanks contain, in order, alkaline cleaner, water, caustic soda at 140.5 °C (the blackening compound), and finally the sealant, which is usually oil. The caustic soda bonds chemically to the surface of the metal, creating a porous base layer on the part . Oil is then applied to the heated part, which seals it by “sinking” into the applied porous layer. It is the oil that prevents the corrosion of the workpiece. There are many advantages of blackening, mainly:

  • blackening can be done in large batches (ideal for small parts)
  • no significant dimensional impact (the blacking process creates a layer about a micrometre thick)
  • it is far cheaper than similar corrosion protection systems, such as paint and electroplating