| Preparation of anodized film on the surface of magnesium alloy is a commonly used method to improve its corrosion resistance at the present stage,but the existing magnesium alloy anodized films still cannot meet the needs of use in corrosive environments.In order to further improve the corrosion resistance of magnesium alloy anodized film,this paper use AZ91D magnesium alloy as the research object,and studied different types of additives to magnesium alloy anode by adding sol and stearic acid additives to NaOH-Na2SiO3 oxidizing solution.The properties of the oxide film and the influence of the anodizing process,and the stearic acid sealing method of the magnesium alloy anodized film were explored.The main findings are as follows:(1)The sol additive can not only increase the film formation voltage,increase the film thickness,reduce the surface porosity and pore size,increase the density of the film layer,change the proportion of each component in the film layer,and can increase the self-corrosion voltage of the oxide film layer,reducing the self-corrosion current density.The experimental results show that the optimal addition amount of silica sol is 2%by volume,titanium sol is 1%by volume,and zirconium sol is 1%by volume.(2)The sol-based additive can affect the formation of the oxide film by adsorption:increase the thickness of the coacervate on the magnesium alloy substrate,increase the electrical resistance,increase the film formation voltage,and reduce the dissolution rate of the oxide film.It is beneficial to formation of the oxide film;hinders the adsorption of other anions,resulting in a decrease in the number of crystal nuclei and making the film layer denser.At the same time,the sol component can reduce the conductivity of the solution and increase the resistance of the solution,resulting in an increase in the output voltage of the output terminal,thereby increasing the thickness of the oxide film layer.Silica sol can also directly participate in the film formation reaction and increase the magnesium silicate content in the film layer.(3)Stearic acid as an additive can reduce the porosity of the surface of the oxide film,reduce the diameter of the pores,and decrease the number of pores;while increasing the thickness of the membrane layer,the defects in the joint between the membrane layer and the matrix are also reduced.The film layer is denser.The self-corrosion current density is reduced,and the self-corrosion voltage is increased by about 0.21V.The optimum amount of stearic acid added is 2g/L.(4)The mechanism of the reaction of stearic acid with the matrix during the anodization process is proposed.The addition of stearic acid increases the film formation voltage of the oxide film,increases the film thickness,enhances the spark discharge effect,and increases the flow of the melt.Thereby resulting in reduced porosity and reduced pore size;at the same time,through the action of a surfactant,promotes the release of 02 from the surface,refines the pores of the membrane,which is beneficial to the improvement of corrosion resistance;The carbon chain on the stearic acid molecule breaks during the reaction,and the broken product binds to Mg2+.The resulting product is deposited in the path caused by the spark and on the surface of the film layer,which changes the composition of the film to a certain extent and forms a denser oxide film layer.(5)The anodized film was sealed by stearic acid sealing process,and the best sealing process was as follows:the stearic acid content was 100%solution,90?-100?,30 min.After sealing,a smooth,dense and flat sealing film is obtained on the surface of the oxide film.The main component of the film is stearic acid;the corrosion current density of the oxide film after sealing is reduced by about 60%compared with the unsealed hole.The immersion time in the 3.5%NaCl solution reached 380 hours,which was extended by about 160 hours compared with the film before treatment,and the corrosion resistance was significantly improved. |
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