The Research On A New Process Of Anodic Oxidation For Magnesium Alloy AZ31

Magnesium is the lightest structural metal material, with excellent physical and mechanical properties, such as low density and high specific strength. Magnesium and its alloys have low corrosion resistance. While anodic oxidation is an effective surface protective treatment for magnesium and its alloys. In this paper, a new anodic process of AZ31 magnesium alloy based on an environmental friendly electrolyte was developed.This anodic process could improve the corrosion resistance and surface properties of AZ31 magnesium alloy significantly.Firstly, Tafel polarization curves technology was applied to films of AZ31 magnesium alloy in different electrolytes, to evaluate rapidly the corrosion resistance ability, then to select a reasonable electrolyte components. The effect of concentration of additive A on anodizing coating of AZ31 magnesium alloy was investigated, and an ideal concentration of additive A was 20～60ml/L. The results showed that the electrolytic composition including NaOH, Na₂SiO₃, and additive A, phytic acid, and so on, could produce excellent film and increase corrosion resistance.Secondly, the effective concentration and the process parameters of the electrolyte bath were determined by orthogonal tests method. The results showed that the process parameters: current density10 mA/cm² , anodic time 20min, electrolyte temperature 20℃, string speed 3 grade ; the electrolyte components and concentration: 45g/L NaOH, 100g/L Na₂SiO₃, 40ml/L Additive A and 15ml/L Phytic acid. The test results of electrochemical polarization curves showed that the process could significantly improve the AZ31 magnesium alloy corrosion protective properties. It was found that the corrosion resistance ability could be consistent with the polarization curve test through 96 hours the neutral salt spray test. The anodic film of AZ31 magnesium alloy was analyzed by XRD, the main component included Mg₂SiO₄, MgO, Al₂(SiO₄)O and Al₂O₃. The anodic film was analyzed by EDX, the elements included C, O, Na, Mg, Al, Si, P.Thirdly, through the single factor experiment, the effect of current density, time, temperature and stirring on the anodic oxidation of magnesium alloy were studied, It was found that with the increase of the current density, the film grew quickly, while film appearance and corrosion resistance decreased. With the processing time extended, film thickness increased, while surface was rough, the rate of porosity increased and pore size was larger. As the temperature increased, the film became rough and the crack increased. Proper stirring was conducive to the growth of anodic oxidation film.Finally, cell voltage-time of AZ31 magnesium alloy during the anodization was researched.The result suggested that the film-forming process existed three generation steps:â‘ formation of dense layer stage;â‘¡the film rapid growth stage;â‘¢film growth and dissolution balance stage.