Investigate On Phosphating Of Galvanized Steel

Zinc is a frequently used as a cost-effective method of preventing corrosion, it has a protective effect of sacrificial anode cathodic, extending the service life of steel. In recent decades, a large number of galvanized steel used in the automotive industry. With the appearance of cars on the morphology and corrosion resistance have become increasingly demanding higher, As the galvanized steel with organic coatings poor adhesion, unable to reach the requirements of coating, at the galvanized steel sheet before painting to phosphate treatment, can effectively improve its corrosion resistance, to enhance its binding with the organic coating. Galvanized steel phosphating sequence reveals an essential position before painting.In this paper, The process and performance of phosphating on Galvanized steel in phosphating solution containing different contents of Mn²⁺ was investigated by means of time dependence of open circuit potential, polarization curves, scanning electron microscopy, coating weight– time change, copper sulfate spot test, Cross-cut test and Entire immersion test. Influence of some surfactants on zinc phosphating coating was also investigated.According to the results of orthogonal test, the experimental system in galvanized steel sheet working solution of zinc phosphating the best formula and process parameters: ZnO 10g/L,H3PO4 25ml/ L, Ni(NO3)2 1.4g/L, NaNO3 1.7g/L, tartaric acid 0.2g/L, Mn(NO3)2 0.6g/L, processing temperature: 50-60℃, free acidity: 2.9, total acidity: 49, processing time :3-5min.Through the sudy of the time dependence of open circuit potential found that the process of phosphating galvanized steel can be divided into three stages: the first phase of metal matrix dissolution, while phosphate film began to take shape; the second phase of rapid growth of phosphate film, crystallization and continuous development; third stage of phosphating film grain size increases, was piled up like slice.The study on the phosphating of galvanized steel indicates that Mn²⁺ is a kind of accelerator ,which can be significant increase in phosphating film crystallization, grain refinement, phosphating film crystallization to a more compact, reduce coating weight, the crystallization of phosphate film morphology by the block piled into a thin shape, to improve the corrosion resistance of phosphate coating, and the organic coating to enhance bonding.With the content of Mn²⁺ increasing, it is conducive to the formation of phosphate coating grain, played the role of nucleation agent, and reduce the coating weight in the initial stages of phosphating. But in the latter part, the greater content of Mn²⁺ can make phosphating coating becoming loose and not continuity. The results of experimental show that, the content of 0.6g / L Mn²⁺, the highest since the corrosion potential, copper sulfate experiment has the longest time, coating The best combination of power rating, the grain is more small crystal is more dense, this time to promote the role of Mn²⁺ was the best.The results show that the surfactants in zinc phosphating solution can make phosphating coating weight decrease, and reduce the phosphate film formation time; The phosphating coating formed was more compact and the crystals of the phosphate were finer, thereby increasing Large phosphate grain surface area, makes the phosphate coating and organic coating to enhance their adhesion; and alkali resistance has been enhanced.