Hard coating or hard anodizing produces an especially hard and thick protective coating by the electrolytic oxidation of aluminum on the aluminum surfaces. The process is similar to the anodizing process but the processing temperature is kept low. The coat is less porous and thus harder and more resistant to abrasion. The thicknesses achieved with hard anodizing are between 50 and 150µm. This surface features a uniform dark-grey color depending on the pre-treatment.