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 creates a strong barrier against humidity and corrosion. Blackening is usually done in a batch operation.
Oxidizingusing a boiling Blackening bath operating at 120°c - 150°c, this system forms a black iron magnetite finish in 20–30 minutes. Overall finish quality is usually quite good, except on cast iron or metal injection moulded parts (red coatings are common) or parts with blind holes or recesses (white salt leaching). In addition, the high temperature and concentration of the bath cause significant splattering and boil-over hazards, and must be operated and maintained very carefully
Process has different heat and chemical requirements that must be considered when designing a blackening system. It is important to choose the black oxide system that offers the best overall combination of coating performance and system operation for each application.