Effect Of Sodium Silicate Solution Concentration, Temperature And Sodium Methyl Silicate On Silicate Conversion Coatings On Hot-dip Galvanized Steel

Hot-dip galvanizing(HDG) can improve the corrosion resistance of iron and steel products in the atmosphere, so it is widely applied in many fields. In order to solve the zinc corrosion problems of HDG steel that occurs during storage and transportation, chromate conversion coatings are typically used. However, due to its toxicity, chromate conversion has been restricted during these years, thus, it is quite necessary to find a new passivation technology to substitute chromate conversion. Silicate passivation has many advantages, such as non-toxic, pollution-free, solution stability, low cost and so on, so it has become a research hotspot to replace chromate passivation.Our preliminary study showed that, the Si O2:Na2O molar ratio of the solution has significant effects on the sodium silicate passivation. The solution with the Si O2:Na2O molar ratio of 3.50 containing 5wt.% Si O2 at room temperature can lead to the formation of dense coatings with high corrosion resistance. On this basis, this paper set the Si O2:Na2O molar ratio of sodium silicate solution as 3.50, varying the silica concentration and temperature of solution, and adding sodium methyl silicate to the silicate passivation solution, to further study on the influence of concentration、temperature and sodium methyl silicate on silicate conversion coatings.HDG was immersed in sodium silicate solution with the Si O2:Na2O molar ratio of 3.50, silica concentration were 2wt.%、5wt.% and 8wt.%, respectively, temperature were room temperature、40℃and 60℃, respectively. The sodium silicate coatings were tested by Neutral salt spray、tafel polarization and electrochemical impedance spectroscopy. The results showed that, in the range of testing conditions, with the increase of concentration and temperature, the corrosion resistance of silicate conversion coatings were improved, the polarization resistance Rp were more than 100 k within some certain conditions(5%-60、8%-40 and 8%-60).The silicate solutions were investigated by FT-IR and 29 Si NMR measurements, the results showed that, the sodium silicate solutions with different concentration of silica led to different solution structure, i.e. the silicate anion groups have different Si-O linkage types and distribution. With the increase of the concentration of silica in sodium silicate solution, the degree of polymerization of silicate anion groups increased, simple structure(Q1、Q2) is reduced and the complex structure(Q3、Q4) increased. The composition of silicate conversion coatings were investigated by EDS and XPS, the results showed that, the coatings were containing Si、O、Zn and Na. With the increase of the concentration of silica or the temperature of sodium silicate solution, Si content increased in the coatings, this indicates that the improvement of the concentration of silica or the temperature of sodium silicate solution can promote the formation of silicate conversion coatings.Adding methyl silicate sodium into sodium silicate solution(5wt.% Si O2, M = 3.50) to modify the solutions. The results showed that, the coatings were dense and with high corrosion resistance when the addition amount is 2%; the excessive addition amount of methyl sodium silicate was not good to the formation of conversion coating, the coating was loose and the corrosion resistance was poor. The XPS results showed that, the inner layer of the silicate conversion coating mainly formed by Si-O-Si and Si-O-Zn bond, while the surface layer contains more Si-CH3 bond, which makes the coating has a hydrophobic surface, and the water contact angle increased significantly.