Abstract
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.
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