The basic principle of aluminium hard anodizing

The principle of aluminium hard anodizing is essentially the principle of water electrolysis. When the current is passed, the following reactions will take place:
At the cathode, H2 is released according to the following reaction: 2H + + 2e → H2
On the anode, 4OH - 4e → 2H2O + O2, the precipitated oxygen is not only molecular oxygen (O2), but also atomic oxygen (O) and ionic oxygen (O-2), usually expressed as molecular oxygen in the reaction .
The aluminum used as the anode is oxidized by the oxygen precipitated on it to form an anhydrous Al2O3 film: 2AI + 3[O] = AI2O3 + 1675.7KJ It should be pointed out that the generated oxygen does not all interact with the aluminum, and part of it is precipitated in the form of gas. 
Types of Anodizing Anodizing has long been widely used in industry. There are many methods of naming different names, which can be summed up in the following classification methods:
According to the current type, there are: direct current anodizing, alternating current anodizing, and pulse current anodizing which can shorten the production time to reach the required thickness, the film layer is thick, uniform and dense, and the corrosion resistance is significantly improved.
According to the nature of the film, it is divided into: ordinary film, hard film (thick film), porcelain film, bright modification layer, semiconductor barrier layer and other anodization.
According to the electrolyte, it is divided into: sulfuric acid, oxalic acid, chromic acid, mixed acid and natural color anodizing with organic sulfonic acid solution.