Hard or Clear Anodize
Hard or Clear anodized aluminum refers to aluminum that has gone through the clear anodized process. However, the clear anodization of aluminum does not literally mean that the aluminum becomes clear but rather refers to the fact that the aluminum undergoes an undyed anodization process. The actual appearance of clear anodized aluminum ranges from an extremely light shade of gray to a dark, richer color. The thicker the coating is, the darker the color will also be. The clear anodized aluminum process involves washing the metal with specific chemicals for the process. More specifically, the process of anodization involves aluminum being submerged into solvent baths where chemicals like sodium hydroxide, ammonium bifluoride, and other acids may be used. To sum up, the clear anodized aluminum ends up having a heavier oxide layer as a result.
Most anodizing on machined parts is of commonly specified Type III Hard coat or Hard sulfuric anodizing. The Type III designators come from the common Military specification, MIL-A-8625.