Study Of Phosphating And Cathode Electrophoretic Painting Process On AZ61 Magnesium Alloy

Phosphating and cathode electrophoretic painting methods were applied to improving the surface performance of the magnesium alloys in this dissertation. The corrosion resistance of phosphate coating is limited and is generally used as the bottom coating in order to improve the film adhesion and corrosion resistance. Cathodic electrophoretic paint owns good corrosion resistance and high paint efficiency. So the corrosion of magnesium alloys should be controlled effectively by the combination of phosphating and cathode electrophoretic painting.The phosphating process on magnesium alloy was firstly studied in the article. Then, cathode electrophoretic painting was studied after zinc phosphating. Main work, experimental methods and results were separately recounted as follows.During the study of the phosphating process, uniform design method was firstly used to scheme experiment. The Uniform design table U*24(249) provided with use table was chosen to scheme six factors and twenty-four levels experiments. The better phosphate formula and process were selected. Whereafter, the pH value, zinc oxide concentration of the phosphate bath and phosphating tempreture were changed respectively to investigate the influence of these factors on the structure and properties of the phosphate coating. The final optimized conditions for phosphating have been obtained, and the main conditions are H3PO4:7.57mL/L,ZnO:2.0g/L, phosphating temperature:30℃and pH value:2.5.In the process of phosphating research, the structure, surface morphologies and compositions of the phosphate coatings was observed and analyzed by metallogra-phical microscope, SEM and EDS. The thickness of the phosphate coatings was measured by the eddy curreny thickness meter and the corrosion resistance of the phosphate coating was assessed by anodic polarization curves measurement, electro-nical impedance spectra and the experiments of corrosion weight-loss. The various test results have shown that the phosphate coating formed on AZ61 magnesium alloy in the mentioned optimized conditions has more uniform, more detailed micro-structure, more uniform thickness, lower corrosion weight-loss, lower corrosion current density, higher corrosion resistance and lower surface capacitance in the fitting analysis of electrochemical impedance simulations.After ascertaining the phosphating condition, the use and maintenance of the phosphate solution was studied. The use period of the phosphating solution, phosphating sample area and the complement of the phosphating solution were preliminary experimented through analyzing the correlation of phosphate effect with free acidity, total acidity, phosphate ions concentration, zinc ion concentration. At the end of 3rd period new bath should be complemented in time. Here accumulative total of sample area to bath volume was 0.2m2/L.On basis of the above work, phosphate coating was prepared by using the optimized phosphate conditions. The paint is epoxy and acrylic composite cathode electrophoretic paint. In order to optimize the processing parameters of cathode electrophoretic painting, orthogonal design experiment was applied. Through the testing and analysis of the appearance (color, brightness, uniformity, etc.), hardness, thickness, adhesion and performance of the paint film, the optimized electrophoresis conditions were solid 17%, binder ratio 0.25, electrophoresis voltage 160V,and electrophoresis time 3min. The appearance(color, brightness, uniformity) of the electrophoretic coating that was obtained in the optimized process is better than others, the thickness reaches 25um, hardness is 2H, adhesion is at the 1st level and the corrosion resistance is better.