Phosphating And Cathode Electrophoretic On The AZ61 Magnesium Alloy

As a high-quality structural material, magnesium alloys have been used in lots of fields, especially in the aerospace, automobile, electronic and medical equipment. However, the use of magnesium alloy was severely limited by its poor corrosion resisting property. Magnesium alloy phosphating conversion treatment was an effective substrate of organic coating because of its porous and binding force. In this dissertation, phosphating and cathode electrophoretic painting methods were applied to improve the surface performance of the magnesium alloys. The experimental methods, main work and results were recounted as follows separately.In this dissertation, three phosphating systems were selected on the basis of investigation both at home and abroad. We select a optimum phosphating coating for cathode electrophoretic painting by comparing the thickness, weight loss in alkaline solution, surface morphology and elements of three phosphating coatings obtained in different phosphate baths.The influences of each substance in phosphate bath on phosphate coatings were researched with single factor test. The results are as follows:the concentration of Zn(H2PO4)2 has an important effect on thickness of film, the film would be thicker with a higher the concentration of Zn(H2PO4)2; the concentration of NaNO3 has effects on the accelerating of the phosphate process; hydroxylamine sulfate is a good accelerator which has characteristic of low consumption; The concentration of NaF can promote the forming of phosphme film and thin crystalline grain. Tartaric acid and nitrobenzene sulfonate have important effects on the formation of phosphate coatings, but their concentrations have no effects on it.Orthogonal design was used to scheme experiment, the concentrations of four substances were chosen as four factors of orthogonal design table (L9(34)). The optimum orthogonal conditions were CNaF=l.67 g·L-1, Chydroxylamine sulfate=1.5 g·L-1, CznH2PO4=23.1 g·L-1, CNaNO3=5 g·L-1. In the optimized conditions, test results showed that the phosphate coatings formed on magnesium alloys have moderate thickness, lower corrosion weight-loss, more detailed micro-structure and more excellent corrosion resistance than the contrast phosphate coatings. Electrophoretic composite coatings were prepared on basis of optimized phosphate coatings by cathodic electrophoretic. The appearances (color, brightness, uniformity) of the electrophoretic composite coatings obtained in the optimized conditions were better than others, the thickness was 35μm, hardness was 2H-3H, adhesion was on zero level and the corrosion resistance was excellent.The thickness of the phosphate coatings were measured by the eddy curreny thickness meter. The surfaces morphologies of the phosphate coatings were observed by metallographic microscope and SEM; the compositions of the phosphate coatings were analyzed by titrimetry and EDS. The corrosion resistance of the phosphate coatings were assessed by anodic polarization curves measurement, electronical impedance spectra(EIS) and corrosion weight-loss test. The thickness of the organic coatings were measured by the eddy curreny thickness meter. The corrosion resistance of the organic coatings were assessed by EIS. The thickness, weight loss and solvent resistance of organic coatings were tested according to the national standards.