Study On Preparation And Corrosion Resistance Of Rare Earth Passive Films On ADC12 Aluminum Alloy

The passivation treatment can effectively improve the corrosion resistance of aluminum and its alloy. Although the industrial use of chromate passivation technology to obtain the ideal effect, the toxicity of hexavalent chromium and bring a serious environment pollution. Therefore, the exploitation of chromate-free passivation which has better practicability and environmental friendly is the developing direction.This paper mainly studied the rare earth passivation for ADC12 aluminum alloy. Firstly, we study the effect of various process parameters on the corrosion resistant performance of passive films, which are obtained in H₂O₂-Ce?NO₃?₃ passivation system and KMnO₄-Ce?NO₃?₃ passivation system, with electrochemical method. On the basis of the single factor experiment, through orthogonal experiment to determine the optimum parameters of passive films, respectively. And then, using the means of the electrochemical test, weight loss experiment, contact angle test, scanning electron microscopy?SEM? and energy dispersive spectroscopy?EDS?, for the two kinds of passivation systems, characterization of the cerium-based passive films which obtained under the optimized technological condition respectively. And compared with the films' performance of their corresponding single cerium nitrate passivation system. Finally, to analysis the formation mechanism and corrosion mechanism of films. The main results are listed as follows:1) The optimum parameters of H₂O₂-Ce?NO₃?₃ passivation system was as follows: Ce?NO₃?₃ 5g/L, H₂O₂ 80ml/L, passivation temperature 25?, passivation time 20 min, pH=2.8. Through weight loss experiment, we found that the average corrosion rate of aluminum alloy which after treated in optimum H₂O₂-Ce?NO₃?₃ system and single Ce?NO₃?₃ system in 3% NaCl solution are 0.0677 g·m-2·h-1 and 0.2213g·m-2·h-1, respectively, which passivation rate were 86.80% and 56.86% compared with the untreated one. By contact angle test, we can observe that the contact angle of bare aluminum alloy is 66.6°, whereas the angle on the surface of only Ce?NO₃?₃ system treatment is 86.7°, and for the H₂O₂-Ce?NO₃?₃ treated is 102.4°. SEM measurement indicated, more pits are observed on the surface of the alloy treated in Ce?NO₃?₃ solution. And alloy surface which treated in H₂O₂-Ce?NO₃?₃ solution appears to be more even and smoother than that treated in Ce?NO₃?₃ solution. The EDS analysis reveals that the surface treated in H₂O₂-Ce?NO₃?₃ solution have higher cerium content. Finally, we concluded the main ingredient of passive film is Ce???/Ce???/Al oxide/hydroxide. The corrosion mechanism: passive film hindered the erosion of Cl—and the migration of oxygen and electronic, thus inhibited the corrosion reaction and improved the corrosion resistance of aluminum alloy.2) The optimum parameters of KMnO₄-Ce?NO₃?₃ passivation system was as follows: Ce?NO₃?₃ 5g/L, KMnO₄ 1.5g/L, passivation temperature 40?, passivation time 10 min, pH=2.0. Through weight loss experiment, we found that the average corrosion rate of aluminum alloy which after treated in optimum KMnO₄-Ce?NO₃?₃ system and single Ce?NO₃?₃ system in 3% NaCl solution are 0.0364 g·m-2·h-1 and 0.1536g·m-2·h-1, respectively, which passivation rate were 92.90% and 70.06% compared with the untreated one. By contact angle test, we can observe that the contact angle of bare aluminum alloy is 66.6°, whereas the angle on the surface of only Ce?NO₃?₃ system treatment is 90.3°, and for the KMnO₄-Ce?NO₃?₃ treated is 103.5°. SEM measurement indicated, alloy surface which treated in KMnO₄-Ce?NO₃?₃ solution appears to be more even than that treated in Ce?NO₃?₃ solution. And there are almost no obvious corrosion phenomenon can be observed, after the sample treated in KMnO₄-Ce?NO₃?₃ solution.The EDS analysis reveals that the surface treated in KMnO₄-Ce?NO₃?₃ solution have higher cerium content. Finally, we concluded the main ingredient of passive film is Ce???/Ce???/Al oxide/hydroxide. The corrosion mechanism: passive film hindered the erosion of Cl—and the migration of oxygen and electronic, thus inhibited the corrosion reaction and improved the corrosion resistance of aluminum alloy.