Graphene-modified Epoxy Coatings for Passive Corrosion Protection of Reinforcing Steel in Concrete: Mechanisms, Performance Evaluation, and Dosage Optimization

Chloride-induced corrosion of reinforcing steel is a major durability concern for reinforced concrete structures, especially in marine, coastal, and salt-affected areas. A common passive corrosion mitigation method is the use of conventional epoxy-coated reinforcement; however, its durability is often degraded by coating imperfections, microcracking, inadequate adhesion in humid environments, and the progressive ingress of water, oxygen, and chloride ions. Graphene-modified epoxy coatings have recently attracted considerable attention as advanced nanocomposite systems due to the exceptional impermeability, high aspect ratio, hydrophobicity, and mechanical reinforcement potential of graphene-based materials. This review critically evaluates the progress in graphene-modified epoxy coatings for passive corrosion protection of reinforcing steel in concrete, discussing protection mechanisms, coating application, electrochemical performance, hydrophobicity, microstructural features, dispersion quality, and optimum dosage. Electrochemical impedance spectroscopy, salt spray test, potentiodynamic polarization, water contact angle measurement, and microscopic analysis data reveal that well-dispersed graphene can enhance the coating resistance, decrease corrosion current density, delay rust formation, improve moisture resistance, and enhance barrier stability in comparison with traditional epoxy coatings. The improvements are mainly due to the tortuous diffusion paths, lower permeability, higher hydrophobicity, crack-bridging mechanisms, and the good interfacial compatibility between graphene, epoxy, and the steel substrate. The coating performance is greatly affected by the type of graphene, filler content, dispersion method, coating thickness, application method, curing conditions, and exposure environment. Promising laboratory results notwithstanding, there is still a lack of full-scale field validation, extensive testing on reinforced concrete, analysis of the bond between coating and concrete, environmental evaluation, and scalable dispersion techniques. This review presents a framework for improving graphene-modified epoxy coatings as long-lasting, sustainable protective systems for reinforced concrete structures.