Failure mechanism of epoxy Zn–Al composite coatings on Q235 steel in industrial marine environments

To investigate the aging failure mechanism of epoxy Zn-Al composite coatings on steel grid supports in industrial marine environments, the corrosion conditions of “high Cl⁻ + high concentrations of industrial acid gases + alternating wet-dry cycles” in the Caofeidian Port Area of Bohai Bay were taken as the testing background. A salt spray/wet-dry alternating cycle test combined with outdoor exposure testing was adopted. Coating performance and morphological evolution were analyzed via thickness measurements, adhesion tests, electrochemical impedance spectroscopy (EIS), 3D laser confocal microscopy, scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). Results indicate a three-stage failure progression: Initial protective stage (Cycles 0–3): The coating remains dense and smooth with minimal color change, gradual thickness increase, and high adhesion. EIS results show |Z| at 0.01 Hz is approximately 10⁸–10⁹ Ω·cm², demonstrating a significant physical barrier function. Localized failure stage (4 cycles): Localized rust spots appear on the coating surface, thickness growth accelerates, adhesion decreases abruptly, |Z| at 0.01 Hz drops to 10⁷ Ω·cm², and the penetration of corrosive media triggers Zn dissolution. Expanded failure stage (≥5 cycles): Corrosion spots expand, the contents of Zn and Al decrease sharply, Fe and O are enriched, |Z| at 0.01 Hz reaches 10⁶ Ω·cm², the coating blisters and peels off, leading to complete failure. This study provides a theoretical basis for optimizing protection strategies in industrial marine environments.