Silicate Conversion Coatings On Hot-dip Galvanized Steel

Hot-dip galvanizing (HDG) can delay the anticorrosion longevity of steel products in various atmospheric environments. These HDG products are suitable for wide applications, such as electrical power, transportation, communication, architectures, automobiles, housewares, and other areas. To overcome the zinc corrosion problems of HDG steel that occur during storage and transportation, chromate conversion coatings are typically used. However, due to its numerous environmental hazards, chromate passivation treatments have been restricted in many countries. Hence, an intense research effort is being undertaken to replace chromate passivation with new chromium-free passivation treatments. In this proposed method, low-concentration silicate salts with very low toxicity and low cost can be deposited to form chemical conversion coating on the zinc layer.?Different SiO₂: Na₂O molar ratios of sodium silicate lead to different degrees of polymerisation of silicate anions in sodium silicate solution that can affect the formation, structure, and corrosion resistance of silicate conversion coating on zinc layer. However, very few works have reported on the relationship between the properties of single silicate coating and SiO₂:Na₂O molar ratio of sodium silicate solution.In this article, sodium silicate solutions with 5 wt.% SiO₂ and SiO₂:Na₂O molar ratios ranging from 1.00 4.00 were prepared. Afterwards, the sodium silicate solutions with different SiO₂:Na₂O molar ratios were investigated by Fourier transform infrared spectroscopy (FT-IR) and nuclear magnetic resonance spectroscopy (29Si NMR). Results showed that the differences in the SiO₂:Na₂O molar ratios of the solutions led to the differences in the Si–O linkages and distribution of silicate anion types in these solutions. With the increase in the SiO₂:Na₂O molar ratio from 1.00 to 3.00, the degree of polymerisation of silicate anions increased; in addition, Si–O–Si bonds increased, simple structures decreased, and complex (from one- to two- and three-dimensional) structures increased. As the molar ratios ranged from 3.00 4.00, the types of Si–O linkages and their distribution were observed to be close.Silicate conversion coatings were prepared by immersing HDG steel sheets in sodium silicate solutions with 5 wt.% SiO₂ and SiO₂:Na₂O molar ratio in the range of 1.00 4.00. The process parameters were as follows: sodium silicate solutions with 5 wt.% SiO₂ and SiO₂:Na₂O molar ratios that range from 1.00 4.00, ambient temperature, time at 1 min, drying temperature of 100±5°C, and drying time of 20 min. The morphology, structure, and composition of silicate coatings were investigated by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), atomic force microscopy (AFM), X-ray diffraction analysis (XRD), X-ray photoelectron spectroscopy (XPS), XPS profile depth analysis, and reflectance absorption infrared spectroscopy (RA-IR). The formation mechanisms of the silicate coatings were also investigated. The corrosion resistance of silicate coatings was evaluated using neutral salt spray (NSS) test, Tafel polarization, and electrochemical impedance spectroscopy (EIS) measurements. The structures and changes of coatings during the corrosion process were observed using a combination of methods consisting of EIS and equivalent circuit. Afterwards, their corrosion resistance mechanisms were also investigated.The silicate conversion coatings on HDG steels consisted of zinc silicate and SiO₂ and were a type of network structure with cross-linked Si–O–Si and Si–O–Zn bonds. The coatings provided an effective physical barrier against the zinc layer. The inner and outer layers were mainly composed of zinc silicate and SiO₂, respectively. With an increase in the SiO₂:Na₂O molar ratios ranging from 1.00 3.00, the formed silicate coating contained larger and more numerous silicate anion groups, and the Si–O–Zn and Si–O–Si bonds also increased. When the SiO₂:Na₂O molar ratios ranged from 3.00 4.00, the amount of dehydration generated by drying and the dehydration of silicate coating significantly decreased, leading to the formation of compact and uniform coating. AFM observations showed that when the SiO₂:Na₂O molar ratio was≤2.00, the surface of coatings was uneven, with some large nodules, pores, and high root-mean-square roughness (rms roughness); however, when the SiO₂:Na₂O molar ratios ranged from 3.00 4.00, the coatings were more uniform, with fine cellular structure (diameter of about 0.3μm) and lower rms roughness.The coatings with higher compactness exhibited higher corrosion resistance. NSS results and electrochemical tests showed that, in comparison with HDG, the corrosion resistance of samples after treatment in sodium silicate solutions was enhanced. By increasing the SiO₂:Na₂O molar ratio in the range of 1.00 3.50, the time at which white rust was generated increased from 8 to 48 h (two cycles), and the icor value decreased by more than one order of magnitude. The EIS fitted parameters showed that the coating resistance Rf reached the maximum, which increased nearly one order of magnitude. Moreover, the Y0 value of the constant phase element (CPE) corresponding to the coating capacitance was at minimum, and the index n value of the CPE was close to 1. This suggests that the surface of the silicate coating with SiO₂:Na₂O molar ratio of 3.50 is most compact and uniform. The charge-transfer resistance Rct reached the maximum, electrical double-layer capacitance Cdl was at minimum, suggesting more difficult occurrence of zinc corrosion, strongest ability of the coating to block electron charge transfer, and optimum corrosion resistance of the coating.EIS analyses of silicate coating with SiO₂:Na₂O molar ratio of 3.50 after immersion in 5 wt.% NaCl solution for different immersion times showed that?the main contribution to the corrosion resistance of the coating came from Rf and Rct?at the early stage after immersion. With increasing immersion time, Rf?and Rct decreased gradually, coating capacitance increased gradually, which indicated that the coating became thinner, pinholes and porosity in the coating increased, and rms roughness for the coating increased. With an increase in immersion time at the last stage after immersion, Rf and Rct values were small; at the same time, the main contribution to the corrosion resistance of the coating coming from diffusion impedance W, and the site of W transferring from the coating diffusion impedance to the coating/substrate interface.Chromate conversion coatings showed excellent anticorrosion properties because of their compact microstructure and their strong self-healing ability. The silicate conversion coatings were scratched with a blunt knife edge and corroded in an NSS chamber for a specific period. Afterwards, the corrosion products in and near the scratched area were investigated by SEM and EDS. The self-healing process and mechanism of the coatings were also discussed. The single silicate conversion coatings showed self-healing abilities under the experimental conditions. During the NSS corrosion process, the silicate anions with Si–OH in the coating which consisted of Si–O–Si and Si–O–Zn bonded through cross-linking, were released from the coating surrounding the scratch and migrated by diffusion to the scratched area. This occurrence led to the formation of a new conversion coating at the scratch surface, through the reaction with Zn–OH. This process also delayed corrosion in the scratched area. Higher SiO₂:Na₂O molar ratios led to improved self-healing abilities. During the test, the duration of self-healing of the silicate conversion coating prepared from sodium silicate solution with SiO₂:Na₂O molar ratio of 3.50 was equivalent to the time of corrosion of a corresponding unscratched coating sample with the same molar ratio. This indicates the apparent self-healing ability of the coating. The silicate coatings with higher SiO₂:Na₂O molar ratios possessed better self-healing abilities (those associated with the compactness of these coatings) and soluble silicate anions that can be?fully released from the more compact coatings and migrate by diffusion to the scratched area. Based on these experimental observations, the model of self-healing process of silicate conversion coatings is hereby proposed.