Due to the high porosity and adsorption performance of the aluminum oxide film on the surface of Changshu hard film, it is easy to be polluted. Therefore, after anodic aluminum oxidation, the film should be closed to improve the corrosion resistance, wear resistance and insulation of the film.

The common sealing methods of Changshu dura mater are: < 1 > boiling water and steam sealing method: its principle is: hydration of anhydrous aluminum oxide at high temperature: Al2O3 + nH2O -- > Al2O3 · nH2O when the density is 3.42 γ- When Al2O3 is hydrated into monohydrate, the volume of aluminum oxide increases by 33%, while when it is hydrated into trihydrate, the volume of aluminum oxide increases by 310%. Therefore, when the aluminum oxidized parts are placed in hot water, the aluminum oxide film layer on the inner wall of the barrier layer and porous layer is first hydrated, and after a period of time, the hole bottom is gradually closed by the hydration film, When the whole pore is completely closed, the water in the pore stops circulating, and the surface layer of the film continues to hydrate until the mouth of the whole pore is blocked by the hydration film.

If steam is used, all pores can be sealed more effectively, but the equipment and cost of steam method are higher than that of boiling water method. Unless special requirements, boiling water method should be used as far as possible< 2> Dichromate sealing method: this method is suitable for sealing anodic aluminum oxidation film and chemical aluminum oxidation film in sulfuric acid solution. The aluminum oxide film treated by this method shows yellow and has high corrosion resistance, but it is not suitable for decorative use.

The principle of Changshu dura mater is: at high temperature, aluminum oxide film and dichromate produce chemical reaction, and the reaction products aluminum basic chromate and aluminum dichromate precipitate in the film hole. At the same time, the hot solution hydrates the surface of aluminum oxide film and strengthens the sealing effect. Therefore, it can be considered as the double sealing effect of filling and hydration.