Hard anodizing is typically applied to heavy wear industrial parts intended for use in aggressive or highly corrosive applications. This type of coating is typically far thicker and harder than normal anodizing. Again, this type of anodize will lend the parts a durability approaching that of hard faced or case hardened steel. Hard coated metal usually becomes a dark gray to a black finish. This can vary depending on the aluminum alloy. One of the main purposes of hard anodizing is to make the aluminum more resistant to corrosion. The thicker oxidized layer protects the finished part from being exposed to moisture, oxygen, and various other factors. Sealed items are even more corrosion resistant.
The outer coating is also extremely hard, typically much harder than the original metal. In many cases, a thick hard anodized coating can be as hard as tool steel. The oxide layer is part of the metal, it won't peel off and the surface finish will increase. Hard anodized metals are usually very well insulated and they don't conduct heat or electricity well. Thus this process being especially useful for applications that require the part be used at high temperatures.