Our discussion of black-oxide coatings continues.
The decorative black color imparted by a black-oxide treatment serves many purposes in addition to providing a strong adhesion between the coating and the substrate. It has good lubrication characteristics and will not peel away from the surface under moderate loading. It improves anti-galling properties, decreases the coefficient of (sliding) friction and improves abrasion resistance. In addition, since the process does not involve hydrogen as part of the chemical process, there is no possibility of hydrogen embrittlement.
Additional advantages of black-oxide coatings are an anti-glare surface and that they can be exposed to a temperature up of 900°F (482°C) before their color begins to change. Black-oxide-coated parts can be welded, and no harmful fumes are produced.
One of the misnomers about black-oxide coatings, however, is that they significantly improve corrosion resistance. While slight improvements have been reported, parts in corrosive environments will not generally be protected. A rust-preventative oil or wax can be applied after the black-oxide coating has been applied (and the part thoroughly rinsed) to improve corrosion resistance.
We will finish our discussion next week.