Long-term corrosion of silicon carbide/enamel composite coatings on steel rebars: A microstructure-dependent corrosion behavior

Steel rebar's corrosion greatly affects the durability of reinforced concrete (RC) structures. Recently, enamel coatings have been developed for steel rebar's corrosion protection. Given that the anticorrosion performance of enamel coatings is strongly related to their microstructure, in this study, pure enamel coating (PE) and three silicon carbide/enamel composite coatings with 2.5, 5, and 7.5 wt% silicon carbide addition (S2.5, S5, and S7.5) were prepared on low-carbon steel rebars, representing four different microstructure states, which are severely cracked with porosity of 6%, partially cracked with porosity of 17%, slightly cracked with porosity of 20%, and uncracked with porosity of 26%. The long-term corrosion behavior of different coated rebars was investigated by open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, neutral salt-spray test, and morphological observation to understand the effect of silicon carbide content and coating microstructure on the corrosion behavior. Long-term electrochemical studies (210 days) showed that the Rpor and Rct of S7.5 remained 12 times and 5 times that of PE, respectively, indicating an improved corrosion protection with silicon carbide addition. Combined corrosion studies and microstructure results revealed that the corrosion behavior of enamel coated rebars was closely related to the coating microstructure, especially the pores and microcracks. Furthermore, the competitive mechanism of electrolyte penetration degeneration and “self-healing” like enhancement was proposed to interpret the corrosion evolution modes of different coated rebars. The findings of this work give a deep understanding of the corrosion behavior of silicon carbide/enamel composite coatings and provide a solution for long-life RC structures. •Silicon carbide/enamel composite coatings were found to provide better corrosion protection than pure enamel coating.•Long-term corrosion behavior was investigated by electrochemical test, salt-spray test, and morphological characterization.•The microstructure especially the microcracks was found to dominate the corrosion behavior of enamel coatings.•The competitive mechanism of electrolyte penetration degeneration and “self-healing” like enhancement were discovered.