Durability and corrosion resistance of defective Zn- and Al-based thermal spray coatings on carbon steel plates in high-chloride environments

• Compared the durability and corrosion resistance of four commonly used thermal spray coatings Zn Al Zn-15Al and Al-5Mg under identical high-chloride environment. • Introduced an X-shaped artificial defect consistent in quality and penetrating into the substrate to simulate initial damage. • Enhanced the durability of Zn coatings by adding Al. • Improved the passivation ability of Al coatings effectively by incorporating Mg. This study systematically assesses the durability and corrosion resistance of metal thermal spray coatings with defects in high-chloride environments, focusing particularly on the long-term performance of four commonly used coating materials in engineering structures: Zn, Al, Zn-15Al, and Al-5Mg. By introducing uniform artificial defects, the study simulated the potential damage scenarios that thermal spray coatings may encounter in actual applications and compares the corrosion resistance of these materials in a highly corrosive environments such as airborne sea salt through accelerated corrosion tests. The results show distinct but complementary corrosion behaviors among the coatings. Zn-based coatings effectively suppressed steel corrosion through sacrificial protection, although their corrosion current density increased from 1.5 to over 100 μA·cm⁻² after 1000 cycles, indicating gradual depletion of the Zn layer. Al-based coatings maintained impedance above 10⁶ Ω·cm² and showed a 70% reduction in corrosion current, reflecting the stability of their passive films. The incorporation of Al into Zn, forming Zn-15Al coatings, reduced the corrosion current by nearly an order of magnitude compared with pure Zn, while Al-5Mg coatings exhibited a synergistic protection mechanism, where early Mg dissolution promoted rapid oxide formation and subsequent Al passivation maintained low current densities of 2-3 μA·cm -2 and stable potentials near -920 mV during prolonged exposure. This study highlights the importance of considering coating damage and defects when selecting thermal spray materials in highly corrosive environments, providing important insights and quantitative evidence for the maintenance and protection of engineering steel structures.