Effect of SiO2:Na2O molar ratio of sodium silicate on the corrosion resistance of silicate conversion coatings

Passivation treatment by sodium silicate solution is considered as an alternative to chromium chemical conversion treatment to improve the corrosion resistance of hot-dip galvanized (HDG) steels. In this paper, a transparent silicate coating was formed on the surface of HDG steel by immersing in sodium silicate solution with SiO2:Na2O molar ratio in the range from 1.00 to 4.00. The parameter about the SiO2:Na2O molar ratio of silicate solution has been discussed using corrosion resistance and surface morphology. Tafel polarization, electrochemical impedance spectroscopy (EIS) measurements and neutral salt spray (NSS) test show that silicate coatings increase the corrosion resistance of HDG steels. From the results obtained, it is deduced that the optimum SiO2:Na2O molar ratio is 3.50. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction analysis (XRD), and reflectance absorption infrared spectroscopy (RA-IR) show that there are no obvious differences of the chemical composition and structure in various silicate coatings. The silicate coatings mainly consist of zinc oxides/hydroxides, zinc silicate and SiO2. However, atomic force microscopy (AFM) images reveal that the surface of silicate coatings with a molar ratio of 3.50 is more compact and uniform than other silicate coatings.