Hard oxidation is called hard anodic oxidation treatment. Generally, the thickness of hard anodic oxide film is required to be 25- 150um, mostly 50-80um, and the thickness is less than 25um. The thickness of anodic oxide film used for tooth bonding, spiral and other applications is about 50um. Under some special technological conditions, it is required to produce a hard anodic oxide film with a thickness greater than 125um, but the bath solution of hard anodic oxidation is generally sulfuric acid solution and sulfuric acid with organic acids, such as oxalic acid and sulfamic acid. In addition, hard anodizing can be achieved by reducing the anodizing temperature or sulfuric acid concentration. For deformed aluminum alloy with copper content greater than 5% or silicon content greater than 8%, or aluminum alloy with high silicon pressure casting, some special anodic oxidation measures should be considered. For example, 2XXX series aluminum alloy, in order to avoid the aluminum alloy from being burned during anodic oxidation, 385g/L sulfuric acid and 15g/L oxalic acid can be used as electrolytic bath solution, and the current density should be increased to 2. Above 5A/dm.