Effect of process parameters on phosphate coating formation

The effects of accelerator concentration and bath temperature on the phosphate coating of zinc and galvanised steel have been studied. Additions of 30–130 g L−1 NO3−; to a reference zinc phosphate solution (30 mL L−1 98%H3 PO4+20 g L−1 ZnO) demonstrated that the presence of NO3−; is necessary for complete coating formation. Increasing the bath temperature from 30 to 90°C led to an increase in the thickness and the corrosion protection efficiency of the coating. The microstructure of the coating depended on bath composition and temperature. Addition of F− to a nitrated zinc phosphate solution had little effect on the kinetics of coating formation; however, NO2−; retarded the first stage of coating growth and Ni2+ accelerated this stage and favoured increased coating thickness. The variation of the rest potential of zinc during phosphating correlated closely with final coating thickness and coating corrosion resistance.