Formation mechanism of anodic oxide film of aluminum and aluminum alloys

The electrolyte used in the anodizing of aluminum and its alloys is generally an acidic solution with medium solubility, and lead as the cathode only plays a conductive role. When anodizing aluminum and its alloys, the following reactions occur at the anode: the formation and dissolution of the anodizing film are carried out at the same time. In the early stage of anodizing, the formation rate of the film is greater than the dissolution rate, and the thickness of the film is increasing; With the increase of thickness, its resistance also increases, resulting in the growth rate of the film is slowed down, until it is equal to the dissolution rate of the film, the thickness of the film is a certain value. Section A: Formation of non-porous layer. Aluminum oxidation processing chemical oxidation film is thin, the thickness is about 0.5 to 4 microns, and porous, soft, with good adsorption, can be used as the bottom layer of organic coating, but its wear resistance and corrosion resistance are not as good as anodic oxidation film. In section ab of the curve, the voltage increases sharply from zero to the maximum value within a few seconds to tens of seconds at the beginning of power-on, which is called the critical voltage. The results show that a continuous, non-porous film layer is formed on the anode surface. The appearance of this film prevents the further thickening of the film layer. The thickness of the non-porous layer is proportional to the formation voltage and inversely proportional to the dissolution rate of the oxide film in the electrolyte. Second segment B: Formation of porous layer. Polishing oxidation processing and pure aluminum polishing oxidation although they look similar, but in essence the two are different, because in terms of brightness, the latter is much higher than the former. In addition, the composition of raw materials is different. As for polishing and oxidation, the main purpose of polishing is to improve the smoothness and feel of the product surface. In section bc of the curve, after the voltage reaches the maximum value, it begins to decline, and its decline is 10% to 15% of the maximum value. The results showed that the non-porous membrane was dissolved by the electrolyte and a porous layer appeared. Section 3 C: The porous layer thickens. In the cd section of the curve, the oxidation voltage begins to enter a steady and slow rising stage after about 20s. It shows that the non-porous layer is continuously dissolved to form a porous layer, while the new non-porous layer is growing, that is to say, the porous layer is constantly thickening, and the process of film formation and dissolution is carried out at the bottom of each membrane cell. When the formation rate and dissolution rate of the film reach dynamic equilibrium, even if the anodizing time is extended, the thickness of the oxide film will not increase, and the anodizing process should be stopped.