Investigation Of Phosphating Treatment Of Sodium Dihydrogen Phosphate And Adhesive Performance Of AZ31 Mg Alloy

The paper is a part of GM United Fund Project "Joining of Dissimilar Materials with Crash-Toughened Adhesives"which is conducted by General Motors Corporation and Zhengzhou University. The purpose is to research the possibility of the application of AZ31 magnesium alloys in the automobile industry. This paper focuses on the surface pretreatment process of AZ31 magnesium alloys and its effect on the adhesive performance of magnesium. In order to overcome some deficiency of the previous phosphate-potassium permanganate phosphating solution such as longer phosphating time, needing stringently controlling pH value, and so on, we develop a sodium dihydrogen phosphate phosphating solution and optimize the phosphating solution composition and treatment time. According to the analysis of appearance, corrosion resistance of coating, the shear strength of the bonded AZ31 sheets, an optimum phosphating solution is obtained. The appearance and thickness of coating obtained by sodium dihydrogen phosphate phosphating solution were analyzed. The corrosion resistance of phosphate coating was evaluated by pitting experiment, hydrogen-collection experiment and potentialdynamic polarization experiment. The shear strength of the bonded AZ31 sheets and its durability in 3.5%NaCl solution and hot-humid environment after treated by sodium dihydrogen phosphate phosphating solution were analyzed and compared with previous phosphate-potassium permanganate phosphating solution. The results show that:The optimum concentration of complex accelerator, sodium fluoride and sodium dihydrogen phosphate is obtained by the composition optimization. The open circuit potential experiment shows the open circuit potential begins to trend to be stable after 3 min. The electrochemical data shows that when the treatment time is 3 min, the self corrosion current density reached the minimum. So the reasonable treatment time is 3 min. The optimum composition of dihydrogen phosphate phosphating solution 15g/L NaH2PO4, lg/L Na2Mo04,1.5g/L Na2WO4,0.5g/L KNO3,lg/L NaF. Compared with the phosphate-potassium permanganate, the dihydrogen phosphate phosphating solution has the following advantages:shorter treatment time, lower cost and no need of adjust PH value. By investigating the effect of single complex accelerator, single sodium fluoride and their combined addition on the corrosion resistance, adhesive performance, surface appearance of the coating, we find that phosphate coating obtained by the single addition of complex accelerator has many micro cracks. It can improve the corrosion resistance of the coating. The self corrosion current density decline two magnitude when compared with the no treatment AZ31 alloy. But the shear strength of joints is lower. The phosphate coating obtained by single addition of sodium fluoride is complete and no micro crack. It can improve the shear strength of bonded joints, but can not improve the corrosion resistance. The phosphate coating obtained by combined addition of complex accelerator and sodium fluoride is complete and no micro crack also. It can improve not only the corrosion resistance of phosphate coating but the shear strength of the bonded joints also.The shear strength of joints treated by dihydrogen phosphate phosphating solution is 13.0kN, its strength increased by 14%compared with the previous phosphate-potassium permanganate. The corrosion experiments show that although the AZ31 alloys treated by dihydrogen phosphate phosphating solution can improve the corrosion resistance of phosphate coating, its corrosion resistance is lower than the alloys treated by previous phosphate-potassium permanganate process. The shear strength of bonded joints treated by previous process only decreases by 9%in the soak brine and hot-humid experiments. Though, the shear strength of joints treated by dihydrogen phosphate phosphating solution decreases only by 14%also in the soak brine experiment, but its strength decreases by 30%in the hot-humid experiment. These results show that the bonded joints treated by dihydrogen phosphate phosphating solution has low durability in corrosion environment and hot-humid environment even though it has higher initial shear strength.