A roll coating tungstate passivation treatment for hot-dip galvanized sheet steel

A tungstate passivation treatment was performed on hot-dip galvanized steel via a roll coating process. The effect of anion (phosphate and nitrate) in the tungstate solution on the microstructure and corrosion resistance of the passive film was explored. The surface morphology of the GI steel slightly changed after roll coating using tungstate/phosphoric and tungstate/nitric solutions. Cross-sectional transmission electron microscope characterization revealed that the passive film prepared by the tungstate/phosphoric solution consisted of a thin compact layer, whereas that prepared by the tungstate/nitric solution was relatively thick and porous. The reduction of tungstate anion to low-valent tungsten oxide and metallic tungsten was also observed. Compared with nitrate anion, the presence of phosphate anion in the solution resulted in zinc phosphate, less metallic tungsten, and more hexavalent tungsten species in the passive film. As a result, the passive film prepared by the tungstate/phosphoric solution displayed a better corrosion resistance than that prepared by the tungstate/nitric solution, as evaluated by polarization measurement and salt spray test.