Mechanical, hydrothermal, and interfacial performance of electrostatic spray-applied fusion epoxy coating on ASTM A252 steel

Fusion Bonded Epoxy (FBE) coatings are widely used for the protection of structural steels exposed to aggressive environments. However, despite their established use in pipeline systems, there is limited literature addressing the behaviour of FBE specifically applied to ASTM A252 steel, a material extensively used in deep foundations, port structures, and tubular components subjected to combined mechanical loading and continuous moisture exposure. This study presents an integrated evaluation of the influence of dry film thickness on the mechanical, tribological, hydrothermal, and interfacial performance of the FBE/ASTM A252 system, treating thickness as a structural variable rather than merely a geometric parameter. Coatings with thicknesses between 200 and 500 µm were applied by electrostatic spraying onto surfaces prepared to Sa 2½, followed by controlled preheating and thermal curing. Characterisation included dry film thickness measurement, discontinuity inspection, pull-off adhesion testing before and after immersion in water at 80 °C for 48 h, abrasion resistance under 5000 wear cycles, Shore D hardness, differential scanning calorimetry, and scanning electron microscopy coupled with energy-dispersive spectroscopy. The results show that system performance cannot be explained solely by the thermally determined degree of cure. Adhesion stability after hydrothermal conditioning and wear resistance are governed by the interaction between thickness, film architecture, stress redistribution, and interfacial anchoring quality. Thinner coatings showed greater susceptibility to interfacial degradation after thermal–moist exposure, whereas thicker films provided increased tolerance to diffusive transport and subsurface stress concentration. The primary scientific contribution lies in experimentally demonstrating that the functional response of the FBE/ASTM A252 system arises from coupled mechanisms involving cure kinetics, film microstructure, and interface integrity, establishing mechanistically grounded criteria for defining thickness ranges in structural applications exposed to severe environments.