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Oxidation process of anodized aluminum plate
The sulfuric acid anodizing process is performed by placing aluminum as an anode in a sulfuric acid electrolyte, and the process of forming an anodized film on the aluminum surface by electrolysis is called anodization of aluminum sulfate. Since the sulfuric acid alternating anodizing current is inefficient, the oxide film is poor in corrosion resistance and low in hardness, so it is rarely used. At present, widely used at home and abroad is DC anodizing of sulfuric acid. Compared with other acid anodizing, it has obvious advantages in production cost, oxide film characteristics and performance: 1. Low production cost; 2. High transparency of film; 3. Good corrosion resistance and wear resistance; 4. Electrolytic coloring and chemical dyeing are easy.
The chromic acid anodizing process The chromic acid anodizing process was first developed by Bengough and Stuart in 1923. The chromic acid oxide film is much thinner than the sulfuric acid oxide film and the oxalic acid oxide film, and the thickness is generally only 2 to 5 um, which can maintain the accuracy and surface roughness of the original parts. The film layer is opaque, has a low porosity, is difficult to dye, and can be used without sealing. The solubility of chromic acid solution on aluminum is small, so that the residual solution in the pinhole and the gap has little influence on the corrosion of the parts, and is suitable for surface treatment of castings, rivets and machined parts, and the process is also used in military equipment. Many, the surface treatment of aluminum alloy aerospace parts in the United States, Russia and the United Kingdom generally uses this process. In addition to the protective effect of the chromic acid oxide film, it can also be used as a means of checking the quality of the parts, such as pinholes and cracks on the parts. In the processing operation, the eye-catching brown electrolyte will flow out from it. It is easy to be discovered by people in time.
Oxalic Acid Anodizing Process The oxalic acid anodizing process was widely adopted in Japan and Germany as early as 1939. Since the oxalic acid electrolyte has low solubility to aluminum and the oxide film, the porosity of the oxide film is low, and the corrosion resistance, wear resistance and electrical insulation of the film are better than those of the sulfate film. However, the oxalic acid anodizing cost is high, generally 5 to 10 times that of sulfuric acid anodizing; oxalic acid is reduced to glycolic acid at the cathode, and the anode is oxidized to carbon dioxide, so that the electrolyte stability is poor; the color of the oxalic acid oxide film is easy to follow The process conditions change and change, which causes the product to have chromatic aberration. Therefore, the process is limited in application, and is generally only used under special requirements, such as making electrical insulation protection layer, surface decoration of daily necessities, and the like. As an additive in sulfuric acid electrolyte, oxalic acid is commonly used to relax the anodizing temperature and facilitate the production of thick film. Generally, in the sulfuric acid electrolyte, the amount of oxalic acid added is 10~15g/L, which can make the oxidation temperature not exceed 20°C. When it is relaxed to 22 ° C, the addition of oxalic acid makes the dissolution ability of the electrolyte to the film relatively weak, and thus the film formation speed can be improved to some extent.