A treatment used on aluminum to put a heavy stable film of protective oxide on the metal surface.
It is similar to the passivated layer that forms on stainless steel. Once this layer is formed the oxidization (corrosion) rate slows down to less than 0.002 inches (0.05 mm) per year, which is the definition of corrosion resistance.
In recent years the aluminum industry has learned how to color this oxide or ceramic layer. Needless to say it should never be polished, or rubbed off.