Due to the high porosity and adsorption properties of the surface anodized film, it is easy to be polluted, so after anodizing, the film should be closed to improve the corrosion resistance, wear resistance and insulation of the film. Commonly used sealing methods are: boiling water and steam sealing method
The principle is: hydration of anhydrous alumina at higher temperatures:
Al2O3 + nH2O ---> Al2O3·nH2O
When the density of 3.42 γ-AL2O3 is hydrated into a monohydrate, the oxide volume increases by 33%, aluminum oxidation and when it is hydrated into a trihydrate, the oxide volume increases by 310%, therefore, when the oxidized parts are placed in hot water, the oxide film layer on the inner wall of the barrier layer and the porous layer is first hydrated, after a period of time, The bottom of the pore is gradually closed by the hydration film. When the whole pore is completely closed, the water in the pore will stop circulating, and the surface layer of the membrane will continue to hydrate until the mouth of the entire pore is blocked by the hydration film. If steam is used, all pores can be closed more effectively, but the equipment and cost of the steam method are higher than that of the boiling water method, and unless special requirements, the boiling water method should be used as much as possible.
Dichromate sealing method
This method is suitable for sealing anodic oxidation film and chemical oxidation film in sulfuric acid solution. The oxide film treated by this method has a yellow color, and its corrosion resistance is high, but it is not suitable for decorative use. The principle is: at a higher temperature, the oxide film and bichromate produce a chemical reaction, the reaction products of basic aluminum chromate and aluminum dichromate precipitate in the film hole, while the hot solution makes the surface of the oxide film produce hydration, strengthening the sealing effect. Therefore, it can be considered as a double sealing effect of filling and hydration.
The usually used sealing solution is 5 ~ 10% bichromate brine solution, the operating temperature is 90 ~ 95℃, the sealing time is 30 minutes, and there must be no chloride or sulfate in the solution.
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