Failure mechanisms and corrosion behavior of epoxy-based protective coatings prepared on 20# steel exposed to cooling water

Protective organic coatings are essential for extending the service life and ensuring the safe, low-carbon operation of industrial circulating-cooling-water heat exchangers. Yet prolonged exposure to high-temperature recirculating water accelerates coating degradation and substrate corrosion, while the underlying galvanic mechanisms remain insufficiently understood. Here, we elucidate these mechanisms by immersing epoxy-coated 20# carbon steel in 40 °C cooling water for 40 days, focusing on localized coating damage with an anode-area ratio of 1:2 (small vs. large anode). During immersion, the coating thickness declined to 96.04 µm and 93.12 µm for the small-anode and large-anode specimens, respectively, while blister density surged by day 20–5.1 n cm⁻² and 5.3 n cm⁻². Substrate corrosion rates reached 1.143 mm a⁻¹ (small anode) and 1.048 mm a⁻¹ (large anode). The small anode exhibited a markedly stronger “small-anode effect,” generating higher galvanic current density, deeper pits, and more severe localized attack. Corrosion followed a dynamic “film formation-breakdown-reformation” cycle in which iron-based products such as Fe₂O₃, β-FeOOH, and Fe(OH)₃ initially accelerated anodic dissolution, subsequently compacted to form a temporary barrier, and ultimately underwent delamination. By revealing how localized coating failure and galvanic coupling synergistically intensify corrosion and by clarifying the transient protective role of corrosion products, this study advances the fundamental understanding of galvanic corrosion kinetics and provides a mechanistic framework for designing next-generation coatings and predictive-maintenance strategies, thereby driving progress in corrosion science and engineering.