Microstructure And Modification Of Silicate And Lanthanum Conversation Coating On Galvanized Coating

The traditional chromate conversion coatings were used extensively to protect many metallic substrates against corrosion. However, hexavalent chromium presents very high toxicity and has bad environmental impact. To cope with this problem, there is a great need to derive new chromate-free coatings yielding not only anticorrosive properties but also friendly to environment. Recently, the chromium-free passivation has got a quick development, but is still left far behind the chromate passivation. Therefore, the modified chromium-free coatings have become a new bright spot in this area. In this paper, the growth characteristics and microstructure of the silicate conversion coatings and lanthanum salt conversion coatings were investigated using scanning electron microscopy(SEM), energy spectrometer(EDS), X-ray photoelectron spectroscopy(XPS), Electrochemical impedance spectroscopy(EIS) and so on. The objective is to obtain the modified coatings with compact structure and good corrosion performance, by reducing the porosity or eliminating the crack of the coatings, improving the hydrophobic performance and self-healing of the coatings.The silicate solutions with same silica content and SiO2/Na2 O molar ratio prepared by different forms of SiO2(micron-sized or nano-sized silica powders, silicon sol) were investigated using high resolution transmission electron microscopy(HRTEM) and nuclear magnetic resonance spectroscopy(29Si NMR), the distribution proportion of various silicate anions in these solutions were different, the main part of the silicate anions was present as small cluster with a radius of 0.5 to 1.5 nm and monomer silica species. In the best case, there were more than 74% silicate anions with middle degree of polymerization(Q2 and Q3), as well as more than 23% silicate anions with low degree of polymerization(Q0 and Q1) and no more than 3% silicate anions with high degree of polymerization(Q4), such a distribution of silicate anions may be suitable for the formation of a more even and denser silicate coating with better corrosion resistance. On the one hand, the silicate anions with lower degree of polymerization contain more Si-OH, which promote the silicate coating formation, but on the other hand, if the degree of polymerization of silicate anions increases, the amount of dehydration generated by drying and dehydration of silicate coating is decreased, as a result the pores of the coatings would be reduced.The hydrophobic silicate conversion coating can be obtained by adding a small amount of methyl sodium silicate in sodium silicate solution. The results showed that methyl sodium silicate participated in the reactions of the coating formation, and the hydrophobic group(Si-CH3) represented concentration in the surface of the modified silicate coating, as a result the hydrophobic property of the coating was effectively increased. However, the amount of Si-OH groups in the sodium silicate solution is reduced; this leads to the formation of less compact and poorer corrosion resistance coating. The hydrophobic silicate composite coating with good corrosion resistancecan could be obtained by immersing the silicate coatings in the methyl sodium silicate solution; the water contact angle of the silicate composite coating is about 103.8 deg, and AC impedance value of the composite coating is more than 100 kΩ·cm2.A growth process model of lanthanum conversion coating on hot-dip galvanized steel was suggested. At the early stage, lanthanum conversion coating grows more quickly near the zinc grain boundary, where the reactivity is higher than that inside the grain and the zinc anode is dissolved more quickly. The conversion coating nucleates and grows continuously, and quickly covers the surface of the zinc coating. When the thickness of the conversion coating increases to a certain extent, the cracks first occur on the accumulations. Immersed for a long time, some rod-like precipitations are deposited on the conversion coating surface, and then many more rod-like precipitations cover the entire conversion coating. Results of electrochemical impedance spectroscopy(EIS) and equivalent electrical circuits revealed that, the cracks occurred on the lanthanum conversion coating and the electrolyte solution penetrated into the cracks of the coating, the reactions at the bottom of the cracks was accelerated. As a result, the diffusion field and the Warburg impedance W associated to the oxygen diffusion process appeared. Immersed for a long time, there was a layer consisting of many rod-like precipitations on the surface, such as build a double layer structure.After the hot dip galvanized steel pretreated with low concentration of silicate solution, the original highly active areas on the surface of the zinc layer were covered by small amounts of precipitation of silicate, as a result the growth of lanthanum salt coating is homogeneous instead of preferential growth in zinc grain boundary, and leads to the formation of compact and uniform coating with less cracks. Results of electrochemical impedance spectroscopy(EIS) and equivalent electrical circuits revealed that, the Warburg impedance W of modified La coating with silicate pretreatment disappeared. It means less pores or cracks in the coatings, and the corrosion resistance is improved. In addition, the modified La coating showed self-healing ability, some compounds of lanthanum have been transferred to the scratches during corrosion.The lanthanum conversation coatings were post-sealed with silicate or silane solution. The results showed that the lanthanum salt conversion coating and high alkaline silicate solution are incompatible, the corrosion resistance of the La coatings is decreased after the lanthanum salt conversion coating post-sealed with silicate, i.e. the composite coatings with stable structure cannot not be formed. After the lanthanum salt conversion coating post-sealed with silane, the compact composite coatings with the outer layer of silane and inner layer of lanthanum are obtained. Results of electrochemical impedance spectroscopy(EIS) and equivalent electrical circuits revealed that, the negative effects of cracks on coatings could be eliminated by post-sealing with silane; the impedance of the lanthanum/silane composite coating is not decreased although the bottom lanthanum coating cracked. For the lanthanum/silane composite coatings, the synergistic corrosion protection effect of the two layers is exerted; the impedance value of the lanthanum/silane composite coatings is more than twice the total impedance value of the two single coatings.