Steel Treatments: Blackening. More Steel Treatments The surface of steel can be chemically altered in other ways. Blackening is a process in which the steel surface is coated in black oxide to protect it from rust (red oxide) and corrosion. This delivers a look similar to that of wrought iron, but more precise and without iron's brittleness. The thin black lines of the steel railing against the white wall make for a delicate, minimalist and sculptural stair here.