1. Commonly used process of anodizing
Commonly used processes for anodizing aluminum alloys are: sulfuric acid anodizing process, chromic acid anodizing process, oxalic acid anodizing process and phosphoric acid anodizing process. The most commonly used is sulfuric acid anodizing.
2. Sulfuric acid anodizing
At present, the anodic oxidation process widely used at home and abroad is sulfuric acid anodic oxidation. Compared with other methods, it has great advantages in production cost, oxide film characteristics and performance. It has low cost, good film transparency, corrosion resistance and friction resistance. Good performance, easy coloring and other advantages. It uses dilute sulfuric acid as electrolyte to anodic the product. The thickness of the film can reach 5um-20um. The film has good adsorption, colorless and transparent, simple process and convenient operation.
3. Chromic acid anodizing
The film obtained by chromic acid anodization is relatively thin, only 2-5um, which can maintain the original precision and surface roughness of the workpiece; the porosity is low and it is difficult to dye, and it can be used without sealing; the film is soft and has poor wear resistance But the elasticity is good; the corrosion resistance is strong, and the solubility of chromium to aluminum is small, so that the residual liquid in the pinholes and crevices has less corrosion to the parts. It is suitable for castings and other structural parts. This process is used more in the military. At the same time, the quality of the parts can be inspected, and the brown electrolyte will flow out at the cracks, which is obvious.
4. Oxalic acid anodizing
Oxalic acid has little solubility for aluminum oxide film, so the porosity of the oxide film is low, and the film layer has better wear resistance and electrical insulation than sulfuric acid film; but the oxidation cost of oxalic acid is 3~5 times that of sulfuric acid; at the same time, oxalic acid is in the cathode and anode. Will be reacted, resulting in poor electrolyte stability; the color of the oxalic acid oxide film is easy to change with the process conditions, resulting in a color difference in the product, so the application of this process is subject to certain restrictions. But oxalic acid can be used as sulfuric acid oxidation additive more commonly.
5. Phosphoric acid anodizing
The oxide film is more soluble in phosphoric acid electrolyte than sulfuric acid, so the oxide film is thin (only 3um) and the pore size is large. Because the phosphoric acid film has strong water resistance, it can prevent the adhesive from aging due to hydration and make the adhesive bond better, so it is mainly used for the surface treatment of printed metal plates and the pretreatment of aluminum workpieces.