Hard anodic coatings covered in this specification are extra thick coatings intended to provide a hard, wear resistant, and corrosion resistant surface for aluminum and aluminum alloy parts. These coatings may be used on structural members, if it is recognized that hard anodizing has an adverse effect on their ductility and may seriously reduce the fatigue strength of such parts. These coatings also may be used to provide electrical insulation but because of large variations in their electrical insulation properties, it is believed that individual tests should be conducted to determine the suitability of these coatings for each application.