Abstract:The morphology and composition of zinc phosphate conversion coatings on hot-dip galvanized steels that were phosphorized in pH 3.0 zinc phosphate solution for 2−600 s were investigated by scanning electron microscopy, energy dispersive spectrometer and X-ray diffractrometry. The growth process of phosphate coatings was also discussed. The results show that the zinc phosphate crystals can be nucleated at the intra-grains and the grain boundaries of zinc. The zinc phosphate crystals firstly are flaky and grown approximately parallel, and latterly develop to several orientations as fan-shaped wafers. With the nucleation and growth of zinc phosphate crystals, the coverage of phosphate coatings increases gradually. However, the pores among the phosphate crystal sheets can not be eliminated completely. The bigger phosphate crystals will be broken off, resulting in the decrease of mass gain of phosphate coatings. The phosphate coatings are mainly composed of Zn3(PO4)2·4H2O. The corrosion resistance of HDG steels is greatly improved by phosphating and increases with the phosphating time and coverage of phosphate coatings.

Key words: hot dip galvanized steel; zinc phosphate conversion coatings; growth process; corrosion resistance; phosphorizing