A Study on the Corrosion Resistance and Service Life Prediction of Water-Based Epoxy-Coated Reinforced Concrete in Harsh Environments

To investigate the corrosion resistance and service life of water-based epoxy-coated reinforcing bars under severe environmental conditions, HRB400 ribbed reinforcing bars were used as the substrate. Four types of water-based epoxy-coated reinforcing bars were prepared, containing 0.3% graphene–polyaniline (PAG), 0.3% iron oxide, 10% zinc phosphate, and 10% zinc–iron powder, respectively, with a bare reinforcing bar control group also included. In accordance with standards such as the ‘Design Standard for Durability of Concrete Structures’, durability tests were conducted under various conditions, including long-term immersion in marine chloride solutions, wet–dry cycling, de-icing salt freeze–thaw cycles, baking and immersion in saline soil, and concrete mixed with seawater. Corrosion current density (Icorr) was monitored using a three-electrode system and the linear polarisation method, and service life was predicted based on the Wiener process. The results indicate that, under all severe environmental conditions, the corrosion current density of the coated reinforcing bars was significantly lower than that of the bare reinforcing bars (BRBs). After 70 cycles of marine wet–dry cycling, the corrosion current density of the bare reinforcing bars reached 0.4569 μA·cm−2, whilst that of the 0.3% PAG coating was 0.1103 μA·cm−2, substantially lower than that of the bare bars (0.4569 μA·cm−2); after 110 freeze–thaw cycles in a de-icing salt environment, the corrosion current density of the bare reinforcing bars was 0.4480 μA·cm−2, whilst that of the PAG-coated bars was 0.1003 μA·cm−2. After 80 cycles of baking and immersion in a saline soil environment, the corrosion current density of the graphene–polyaniline-coated steel increased from 4.97 × 10−3 μA·cm−2 to 0.1021 μA·cm−2 (approximately a 20-fold increase), whilst that of the bare steel rose to 0.4489 μA·cm−2. In concrete mixed with seawater, the corrosion current density of bare reinforcing bars reached as high as 8.60 μA·cm−2 after 120 days, whereas that of coated reinforcing bars was 0.24 μA·cm−2, markedly lower than 8.60 μA·cm−2 for the bare bars. Lifespan predictions indicate that, provided that the specifications for concrete strength and protective layer thickness are met, water-based epoxy coatings have the potential to delay the onset of severe corrosion (Icorr ≥ 1 μA·cm−2) beyond the 50-year design threshold in seawater wet–dry cycling zones and saline soil environments, and are projected to meet the 100-year design requirements in de-icing salt environments. It should be noted that these projections are based on accelerated tests and require validation through long-term field performance data. Graphene-containing polyaniline nanocomposite coatings exhibited the best overall protective performance, whilst zinc phosphate coatings demonstrated outstanding stability in high-chloride environments. For the specific formulations tested in this study, the enhanced corrosion resistance is attributed to the synergistic combination of the epoxy matrix, inorganic fillers (TiO2 and BaSO4) and functional additives; these components collectively provide physical shielding, chemical passivation and dynamic pore-blocking effects. Within the scope of this study, the nanocomposite coating containing 0.3 per cent PAG exhibited the best overall protective performance.