Context <p>The durability of concrete structures in marine environments is severely threatened by the penetration of corrosive ions. Although epoxy coatings are widely used, their inherent micro-pores and insufficient barrier properties limit their long-term protective performance. In this study, the performance of g-C₃N₄/h-BN reinforced epoxy coatings applied to concrete surfaces was systematically investigated using first-principles density functional theory (DFT) and molecular dynamics (MD) simulations. The g-C₃N₄/h-BN possesses a stable interfacial structure with a binding energy of − 5.262 eV. The adsorption energy of the heterojunction with epoxy molecules (− 3.542 eV) is higher than that of pure g-C₃N₄ (− 3.432 eV). MD simulation results show that the free volume of the g-C₃N₄/h-BN/EP coating decreases to 8.1 ± 0.9%, and the diffusion rate index of NaCl solution significantly reduces to 13.52 × 10⁻⁴, which is much lower than that of the pure EP coating (19.07 × 10⁻⁴). Furthermore, the g-C₃N₄/h-BN heterojunction improves the mechanical properties of the coating. This theoretical study reveals the barrier mechanism of the composite coating at the atomic scale, providing a solid theoretical foundation for the development of anti-corrosion coatings for concrete infrastructure.</p> Methods <p>First-principles density functional theory calculations were performed using the CASTEP module within the Materials Studio software package. The exchange–correlation energy was described using the Perdew-Burke-Ernzerhof functional under the generalized gradient approximation. All molecular dynamics simulations were performed using the Large-scale Atomic/Molecular Massively Parallel Simulator package.</p>