Anodized
Also: Anodizing
An electro-chemical finish that grows a hard, corrosion-resistant oxide layer on aluminum. The standard surface treatment for aluminum rifle hardware.
Anodizing converts the surface of an aluminum part into a hard, integral oxide layer through an electro-chemical bath, rather than laying a coating on top. The result resists corrosion and abrasion and can be dyed, which is why so much aluminum gear comes in matte black or color. Type III, or hardcoat, anodizing is the tough, thick version used where parts take wear.
On a precision rifle, anodizing protects the aluminum that surrounds the steel: the scope tube and rings, the chassis or bottom metal, the bipod, and rail sections. It is the aluminum counterpart to bluing on steel.