Effect of marine atmosphere severity on degradation morphology and barrier integrity of galvanized coatings

The durability of metallic coatings in marine and coastal atmospheres remains a persistent challenge for infrastructure, renewable energy, and transportation sectors. Conventional zinc (Zn) coatings provide effective sacrificial protection but are prone to rapid red rust formation once the metallic reservoir is consumed, resulting in limited long-term reliability. Alloying with aluminum (Al), magnesium (Mg), and silicon (Si) has been introduced to enhance corrosion resistance through oxide stabilization, microstructural refinement, and compact corrosion product formation; however, most available studies rely on neutral salt spray tests, which poorly replicate real-world wet-dry cycling. Moreover, systematic long-duration comparative datasets across multiple alloys remain scarce. In this work, four representative coatings pure Zn, Zn-Al, Zn-Al-Si, and Zn-Al-Mg were evaluated according to the ASTM G85 standard for 2016 hours. The progression of red rust, evolution of zinc white phases, cut-edge performance, and residual metallic thickness were monitored to establish mechanistic pathways and rank durability. Results revealed a clear performance hierarchy of Zn-Al-Si > Zn > Zn-Al ≈ Zn-Al-Mg. While Zn and Zn-Al-Mg exhibited early and unstable degradation, Zn-Al showed moderate barrier effects but poor edge protection. In contrast, Zn-Al-Si retained residual thickness and delayed red rust formation, reflecting slower metallic depletion and more stable barrier behaviour under cyclic exposure. This study fills a key gap by delivering long-duration comparative data, offering design guidance for coatings in offshore wind, FPV, and marine infrastructure.