The galvanized sheet has good corrosion resistance, because the standard electrode potential of zinc is negative than that of iron, so the zinc layer has a sacrificial anode protection effect on the steel base in water and humid air, which can prolong the service life of the steel. In recent years, galvanized steel sheets have gradually replaced traditional steel sheets and have been increasingly widely used in the automotive and home appliance industries. Although the galvanized steel sheet itself has good corrosion resistance, from the point of view of painting, due to its surface finish, its bonding force with the coating is very low. In order to improve the bonding force between the substrate and the coating, before painting Phosphate treatment is also required. Because zinc is more active than iron; during the phosphating process, the zinc surface will corrode and peel off like the iron surface in the phosphating working solution, resulting in the accumulation of zinc ions in the phosphating working solution. The accumulation of zinc ions to a certain extent will cause hardening. Dust hangs on the phosphated surface.
The film forming principle of phosphating of galvanized sheet is different from that of phosphating of iron and steel. Phosphating of steel is formed by electrode reaction deposition to form a phosphating film, while for galvanized sheet, the ions in the phosphating solution and the zinc on the surface of the galvanized sheet Ions are cross-linked and deposited to form a phosphating film.
Therefore, the phosphating of galvanized sheets should adopt a low-zinc, high-manganese-nickel system, and use nitrate as an accelerator to form a phosphating film at a low temperature.
