The surface treatment methods of aluminum oxidation and anti-oxidation are as follows:
Because the surface oxide film has high porosity and adsorption performance, it is easily contaminated, so after anodizing, the film should be sealed to improve the corrosion resistance, wear resistance and insulation of the film. Commonly used closure methods are:
Boiling water and steam sealing method
The principle is: the hydration of anhydrous alumina at higher temperature:
Al2O3 + nH2O ---> Al2O3·nH2O When γ-AL2O3 with a density of 3.42 is hydrated into a monohydrate, the oxide volume increases by 33%, and aluminum is oxidized. When it is hydrated into a trihydrate, the oxide volume gets an increase of 310%. Therefore, when the oxidized parts are placed in hot water, the barrier layer and the oxide film on the inner wall of the porous layer are first hydrated. After a period of time, the bottom of the hole is gradually closed by the hydration film. When the entire hole is completely closed, the water in the pore stops circulating, and the surface layer of the membrane layer continues to hydrate until the mouth of the entire pore is blocked by the hydration membrane. If steam is used, all pores can be sealed more effectively. However, the equipment and cost of the steam method are higher than those of the boiling water method. Unless there are special requirements, the boiling water method should be used as much as possible.
Dichromate sealing method
This method is suitable for sealing anodized and chemically oxidized films in sulfuric acid solution. The oxide film treated by this method is yellow, 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 dichromate chemically react, the reaction products of basic aluminum chromate and aluminum dichromate precipitate in the film pores, and the hot solution hydrates the surface of the oxide film. Strengthen the sealing effect. Therefore, it can be considered as a double sealing effect of filling and hydration.
The commonly used sealing solution is a 5-10% dichromate aqueous solution, the operating temperature is 90-95°C, and the sealing time is 30 minutes. There should be no chloride or sulfate in the solution.
The influences of Al-Ti-C and Al-Ti-B grain refining master alloys to the passivation film and the related corrosion resistance performance:
TiC nanoparticles (of Al-Ti-C) refine the aluminum alloy structure and mostly distribute inside the grains, which reduces the number of second phases at the grain boundaries, reduces the intergranular corrosion rate, and indirectly improves the continuity of the passivation film, thereby achieving better corrosion performance. While TiB2 nanoparticles (of Al-Ti-B) also refine the aluminum alloy structure, they are mostly distributed at the grain boundaries. The TiB2 nanoparticles distributed at the grain boundaries greatly increase the intergranular corrosion rate and destroy the continuity of the passivation film. As a result, there is no continuous passivation film to protect the substrate during the corrosion process, which leads to the degradation of the corrosion performance.