Accelerated corrosion mechanism of galvanized coatings under condensation-drying cycles

A comparative investigation was conducted on the corrosion behavior of galvanized coatings under NaCl-containing condensation-drying cycles and full immersion in NaCl solution. The condensation-drying cycle environment significantly accelerated the corrosion damage of galvanized coatings. The cycles induced spatial redistribution of salts, forming high-salinity salt rings that increased the corrosion tendency of the underlying galvanized coatings. Compared with full immersion, the condensation condition promoted the formation of loose corrosion products with weak interatomic bonding and unstable crystal structures, accelerating the diffusion of aggressive species to the steel substrate. As condensation evolved over time, the internal salt concentration increased and the wetting contact angle decreased. Based on the coffee-ring effect, a functional equation was established in this work to correlate the deposition characteristics of salts and corrosion products inside condensate during the drying process with salt concentration and contact angle. Analysis of the function plot revealed that the coupled effects of increasing salt concentration and decreasing contact angle led to localized high salinity, concentrated loose corrosion products, and the formation of electrochemical corrosion couples between the edge and interior of the condensate. Under the synergistic effects of the inherent microstructural inhomogeneity of the galvanized coating, the structural characteristics of corrosion products, and the evolution behavior of the condensation environment, the corrosion damage of galvanized coatings under condensation-drying cycles was ultimately accelerated.