Blackening of Steel

Chemical and electrochemical processes for blackening steel were investigated and the coatings compared for corrosion resistance, abrasion resistance, abrasion resistance, smut formation and light or infrared reflectance. The alkaline oxidizing process is an economical means for chemically applying a black oxide coating and is valuable where dimensional change cannot be tolerated however, minimal corrosion resistance is provided. Phosphate coated steel provided greater corrosion resistance than the black oxide coating and excelled in its low degree of light reflectance and freedom from smut. The black oxide and manganese phosphate coatings were approximately equal in abrasion resistance and superior in this regard to the other black coatings tested. Zinc or cadmium plated steel with a chemical black chromate film and proprietary black chromium plated steel provided greatest corrosion resistance. Black nickel deposits were somewhat inferior to black chromium in most characteristics tested but are more easily and economically applied.