Black Oxide – Bluing

Bluing is a surface process for producing a black protective layer on ferrous surfaces. Its primary purpose is corrosion protection, though it often also produces a decorative, antique appearance.
The process typically involves immersing the workpiece in an acidic or alkaline solution (the bluing agent). This creates a black mixed oxide layer on the surface, also known as “noble rust.” Thus, bluing is not a coating process but a transformation of the material itself.