Effect of silicate anion distribution in sodium silicate solution on silicate conversion coatings of hot-dip galvanized steels

Abstract

Silicate conversion coatings are prepared by immersing hot-dip galvanized (HDG) steel sheets in sodium silicate solutions with 5wt.% SiO2 and SiO2:Na2O molar ratio in the range of 1.00 to 4.00. The coating with better corrosion resistance is usually obtained in silicate solution with higher molar ratio (3.00–4.00). In this article, the distribution of types of silicate anion in solutions is investigated by transmission infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (29Si NMR), and the surface of the silicate coating is analyzed using reflection infrared spectroscopy (RA-IR). Results indicate that the anion distribution is mainly monomer, linear and less cyclic anions at low molar ratios in very alkaline solutions. With increase in the molar ratio, small anions decrease, two- and three-dimensional anions increase. Accordingly, the silicate coating prepared in the solution with high molar ratio (≥3.00) contains a higher number of large silicate anions, and Si–O–Zn and Si–O–Si bonds of the coating increase, leading to the formation of more dense silicate coating and better corrosion resistance of the coating. Based on the above results and the potential–pH diagram of zinc, probable causes for differences in structure and compactness of silicate coatings, prepared by immersing HDG steels in sodium silicate solutions with different SiO2:Na2O molar ratio, are discussed.