Aluminum oxide is grown out of the surface during anodizing and then becomes aluminum hydrate that is extremely hard. The porous nature of the anodized layer allows the product to be dyed any color that is required. 

Anodize Per Mil-A-8625 Type II Class I and II.

Hard Anodize:

Hard anodize for aluminum is mostly used for its resistance to wear and corrosion Characteristics. Hard anodizing penetrates into the part and also builds up a layer on the parts surface.

Hard Anodize Per Mil-A-8625 Type III class I and II.