What is the relationship between the temperature of the anodizing solution and the quality of the oxide film?

[China Aluminum Industry] Anodizing solution temperature is the dominant process parameter that affects the properties of the oxide film. After the temperature rises, the viscosity of the solution decreases, the current density rises (at a constant voltage), or the voltage decreases when the current is constant. At this time, the power consumption can be reduced, and the production efficiency is also improved. When the temperature of the solution exceeds the process specification (24°C), the dissolution rate of the film increases, resulting in a decrease in the film formation rate, film hardness, and film thickness, wear resistance and corrosion resistance, and loosening of the oxide film. A powdery film layer appears, followed by a reduction in the transparency of the film. And affect the dyeing performance, resulting in the color of the dye is not clear enough, or even a haze. This is especially important for the workpiece to be dyed, and every effort should be made to reduce the temperature of the solution. When the temperature of the solution is too low, for example, less than 12°C, the thickness of the oxide film can be increased and the wear resistance is also good, but the brittleness of the film layer is increased, the porosity is small, and it is difficult to dye the dyed part. For this reason, in the anodizing of aluminum and aluminum alloys, the temperature of the solution must be strictly controlled to prevent the temperature of the solution from being too low. Anodizing is an exothermic reaction. The heat generated by the film is very high, and Joule heat is generated. The solution temperature rises rapidly. Therefore, according to the existing conditions, anodizing should take corresponding measures to control the solution temperature in the normal process. Within the specifications, to ensure the quality of the film and the continuous production. When the solution temperature is too low, it is unfavorable to require dyeing after the anodization. At this point, general parts can be processed first, and then when the temperature of the solution slightly rises, the dyeing parts are required for further processing. This is based on experience. What is the relationship between the anodizing time and the oxide film quality? Under the same conditions as the solution concentration and other process conditions, with the prolonged anodizing time, the oxide film formed on the surface of the part will gradually thicken (in a certain range), and the pores will also increase, which is advantageous for dyeing. Under the same process conditions as described above, the anodizing time is shortened, the oxide film becomes thin, the voids become less, and the dark colors are less likely to be stained. For this reason, the anodizing time is not less than the special case. For example, the electrochemical polishing part can maintain a certain brightness after the anodization is maintained, and the anodizing time is not less than 30min, need to dye dark parts still need to be extended, especially when the solution temperature is low, sometimes need to be extended to 45 - 60min. Is it necessary to stir the solution during anodization? It is necessary to adopt a compressed air stirring solution for anodizing. This is because the heat generated during the anodization will accumulate in the solution attached to the surface of the oxide film, which will lead to the accelerated dissolution of the oxide film, thereby deteriorating the overall performance of the parts. The compressed air is used to stir the solution so that the heat generated by the parts can be dissipated quickly, and the cooled solution can be used to cool the surface of the parts, so that the solution can continuously circulate on the surface of the parts. In order to ensure the normal progress of the oxidation process, it is also beneficial to increase the uniformity of the thickness of the oxide film.

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