Crystalline Phosphating Process And Electrophoresis Technology On The Surface Of Galvanized Steel Sheet For Automobile

With the rapid development of automobile industry, galvanized steel sheet has been applied to the body and other parts of the car increasingly. Its application can not only make the body lightweight, but also increase the resistance to corrosion and prolong the life of the car. Thus the pre-treatment and painting on the surface of galvanized steel sheet also attract more and more attention. Currently, almost all automobile manufacturers still use the traditional ternary phosphating process which contains Zn2+, Mn2+ and Ni2+ and nitrite at middle-temperature. The process not only seriously pollutes the environment, endangers the health of construction workers, but also consumes large amount of energy. This paper develops two different phosphating process with better electrophoresis compatibility for the energy saving and environmental protection, which has a certain application prospect.The main content of this paper can be divided into the following three parts:(1) The phosphating solution without nitrite has been prepared using the orthogonal experiment at room temperature, based on the selection and combination of accelerators and additives. The galvanized steel were phosphated for 2.5 min at 28~33 °C, then fine crystal phosphating film was obtained on the surface and phosphating sediment was little in treatment process. The film forming process and film properties were inveatigated through the potential-time curve, electrochemical Tafel polarization curve, electrochemical impedance spectroscopy(EIS), scanning electron microscopy(SEM), XRD spectrum, and salt spray cycling experiment. The results show that the phosphating film has compact surface with granular structure. The main film components contain Zn3(PO4)2·4H2O, Mn2Zn(PO4)2·4H2O, Zn2Ni(PO4)2·4H2O, Zn2Mn(PO4)2·4H2O. The performance of film can reach the national standard and satisfy the demand of automobile painting industry.(2) The vinyl triethoxy silane(A151) with small molecular weight was selected for surface modification of nanometer SiO2, to improve its dispersibility in the solution. The particle size, Zeta potential and other tests show that the best modification effect can be reached with the nano SiO2 content is 0.3 g and silane concentration is 0.15 g in the ethanol aqueous solution system. Furthermore, the average particle size of nano SiO2 is around 165 nm. The composite modified nano silicon dioxide phosphating film with no nickel can be obtained by adding modified nano SiO2 into the phosphating solution instead of nickel. The corrosion resistance of phosphating film is strongest and film thickness is moderate when the concentration of the modified nano SiO2 in phosphating solution is 0.4 g/L. The film begins to coarse and the thickness increases sharply with increasing modified nano SiO2 concentration in the process. The composite modified nano SiO2 phosphating film were characterized through experiments such as electrochemical Tafel polarization curve and EIS impedance spectroscopy, scanning electron microscope(SEM), XRD spectrum. Results show that the composite coating has excellent corrosion resistance, which can effectively prevent corrosion of the media on the film layer. The film is mainly composed of Mn2Zn(PO4)2·4H2O, Zn2Mn(PO4)2·4H2O, Zn3(PO4)2·4H2O and a small amount of SiO2.(3) The above prepared template in crystal phosphating process was used as substrate for cathode electrophoresis. The effect of electrophoresis voltage, electrophoresis time, electrophoretic temperature and alkali resistance of the film on the matching with electrophoretic paint film was explored. Results show that the two phosphating films perform excellent alkali resistance. Well electrophoresis coating can be obtained with the electrophoresis voltage of 200~240 V, electrophoresis time of 120~180 s, and electrophoresis temperature of 25~30 °C. Measurement such as the adhesion experiment, convex cup experiment and salt spray cross experiment were performed for the electrophoresis paint coating. The results also show that the two phosphating films prepared have good matching with electrophoresis, and the performance of the electrophoresis coating can meet the national and industrial standards.