The effects of nano-SiO2 additive on the zinc phosphating of carbon steel

In this paper, nano-SiO2 was used as an accelerator for improving the microstructure and the corrosion resistance of phosphate coating on carbon steel. The chemical composition and microstructure of the coatings were analyzed by X-ray diffraction analysis (XRD) and scanning electron microscopy (SEM). The effects of nano-SiO2 on weight, roughness and corrosion resistance of the phosphate coatings were also investigated. Results show that the compositions of phosphate coating were Zn3(PO4)2·4H2O (hopeite), and Zn2Fe(PO4)2·4H2O (phosphophylite). The phosphate coatings became denser due to the addition of nano-SiO2 which reduced the size of the crystal clusters. The average weight of phosphate coatings approximately linearly increased with the nano-SiO2 content in the bath from 0 to 4 g/L, and the phosphate coatings formed in bath containing 2 g/L nano-SiO2 showed the highest corrosion resistance in 5 wt.% sodium chloride solution at ambient temperature. Nano-SiO2 would be widely utilized as a phosphating additive to replace the traditional nitrite, due to its less pollutant and its better quality of the coating.