Anodic oxidation is a process in which the metal is placed in the electrolyte as an anode, so that the metal surface forms an oxide film of dozens to hundreds of microns. The formation of this oxide film makes the metal have anti-corrosion and wear-resisting properties. A typical and common anodic oxidation of aluminum and alloy is taken as an example to illustrate its principle.
Aluminum and aluminum alloy workpiece after surface oil removal pretreatment process, as the anode, other aluminum plate as the cathode, with dilute sulfuric acid (or chromic acid) solution as electrolyte. After electrification, the anodic reaction is OH- discharge to release oxygen, which quickly reacts with aluminum on the anode to form oxide, and releases a lot of heat, that is, the oxide film in the anodic oxidation process, which is composed of Al2O3 and Al203·H20 on the side near the electrolyte, and the hardness is relatively low. Due to the inhomogeneity of the membrane and the dissolution of the membrane by acidic electrolyte, loose pores are formed, namely, porous layers are formed. The electrolyte reaches the aluminum surface through the loose hole (right figure), making the oxide film on the aluminum matrix grow continuously.
The oxide film obtained by anodic oxidation binds firmly to the metal crystal, thus greatly improving the corrosion resistance of the metal and its alloy, and can improve the resistance of the surface and enhance the insulation performance. The oxidized aluminum wire can be used as the winding coil of the motor shaft transformer. In addition, due to the porous metal aluminum oxide film, adsorption performance is strong, so it can be dyed with a variety of bright colors, aluminum products for decoration. For the surface pores that do not need staining, it is necessary to close the pores to reduce the size of the pores, improve the corrosion resistance of the oxide film, and prevent corrosive media from entering the pores and causing corrosion.
