An effective coating/substrate interface bond is crucial for impeding the interfacial diffusion of corrosive media and prolonging the service life of the coating. It is therefore of great significance to improve the adhesion of the enamel coating in order to expand the application fields and service conditions of the enamel coating. The adhesion of a carbon steel-based enamel coating is contingent upon the utilization of an adhesive agent, which can facilitate the formation of an adhesion layer through the promotion of interfacial reactions between the metal and the enamel, thereby enhancing the bond strength of the enamel coating. CoO and NiO, as the most commonly used adhesive agents, significantly enhance the adhesion of the enamel coating when added at concentrations of 0.3wt.% to 1.6wt.%. Meanwhile, studies have demonstrated that high-valent oxides can introduce additional electron vacancies and oxygen, which are expected to further enhance the interfacial adhesion reactions and improve the bonding performance of enamel coatings.
Based on this, the work aims to investigate their synergistic effect and the mechanism of effects of oxidation state on the reactive process of the enamel/metal interface from the perspective of nickel and cobalt oxide adhesive agents, in order to further improve the adhesive properties of carbon steel enamel coatings. Four enamel slurries containing 1wt.% Ni2O3, 1wt.% Co2O3, 0.5wt.% Ni2O3 + 0.5wt.% Co2O3 and 0.5wt.% NiO + 0.5wt.% CoO adhesives were prepared with the mill additive method. The coatings were prepared on Q235 steel substrate by a manual one-step enameling method. The frit used was a borosilicate system (SiO2-B2O3-Na2O-SrO), and the sintering temperature was 850 ℃ for 10 minutes. The adhesion properties of the four coatings were evaluated by falling-weight impact and pull-off adhesion test, and the impact craters and pull-off morphologies were analyzed through optical microscope (OM) and scanning electron microscopy (SEM) to study the failure mechanisms. The interfacial reaction process and element diffusion mechanism of the coatings were analyzed with SEM, energy dispersive spectroscopy (EDS) and electron probe X-ray micro analyzer (EPMA) to clarify the effect mechanism of the four adhesive agents (combinations) on the enamel/metal interfacial reaction.
The results indicate that Co2O3 adhesive agent is more prone to forming a deposition layer at the interface compared to Ni2O3 adhesive agent. However, the oxygen released during the sintering process of Ni2O3 promotes the cobalt adhesion reaction. Meanwhile, the diffusion of Co2+/Co3+ facilitates the diffusion of Ni2+ to the interface, resulting in the formation of a deposit layer. This indicates that the simultaneous Co2O3 and Ni2O3 adhesive agents as mill additives have synergistic effects. Co3+ with high standard electrode potential also promotes the galvanic corrosion reaction with the substrate, which in turn increases the number of dendrites at the interface. Overall, cobalt adhesive agents have better adhesion properties than nickel adhesive agents, and high-valent oxide adhesive agents also help to promote adhesion reactions at the enamel/metal interface. The combination of Ni2O3 and Co2O3 adhesive agents exhibits a better interfacial adhesion effect for enamel coatings compared to NiO and CoO adhesive agents.
Key words
enamel coating /
CoO/Co2O3 /
NiO/Ni2O3 /
adhesion /
interfacial behaviour
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Funding
The National Natural Science Foundation of China (W2412069); The "111" Innovation Project of China (D21032)
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