| The zinc phosphate treatment on AZ31 magnesium alloy was investigated in this dissertation. And the effect of benzotriazole(BTA) on the structure and corrosion resistance of the phosphate film on AZ31 magnesium alloy was mainly researched.Firstly, the basic processing parameters of phosphate were discussed, such as the concentration of the components, treatment temperature and the pH value. The optimal processing conditions were as follows: 9 g·L-1 zinc oxide, 39.0 g·L-1 phosphoric acid(85 wt.%), 3.0 g·L-1 potassium nitrate, 1.6 g·L-1 sodium fluoride, 2.2 g·L-1 tartaric acid, the treatment temperature and time was about 45℃and 9 min, respectively. Then BTA was added into the basic solution, and investigation was given on its effect to the zinc phosphate film. Finally, the film was sealed in boiled water and sprayed with paint.The corrosion resistance of the phosphate film was evaluated by Tafel polarization curves, electrochemical impendence spectroscopy(EIS) and immersion tests. The results demonstrated that, the density level and corrosion resistance of the film were both improved with the addition of BTA,and the optimal concentration of BTA was 0.2 g·L-1. The X-ray diffraction(XRD) patterns showed that the film consisted mainly of hopeite Zn3(PO4)2·4H2O. The surface morphology of the film was analyzed by scanning electron microscope(SEM), it indicated that the addition of BTA could be propitious to form new crystal nucleation and make grain refinement. The existence of BTA altered the preferential orientation of Zn3(PO4)2.4H2O crystal and got a more uniform structure. Additionally, the film being sealed in boiled water showed better corrosion resistance due to the filling of some pores. Electrochemical measurements revealed that, the corrosion current density of the paint with a phosphate film as the base layer declined three orders of magnitude compared with the one without a phosphate film in 3.5 wt.% NaCl solution.
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