Microstructure and corrosion behavior of alumina-modified chromium-free dacromet coatings

Abstract

To further enhance the zinc-aluminum chromium-free dacromet coatings and find a suitable replacement for chromium, alumina was investigated as a potential additive due to its high hardness. The current study examined the effects of incorporating alumina at varying concentrations (0.1, 0.2, and 0.3 wt%) into chromium-free zinc-aluminum dacromet coatings on St 37 substrate. The addition of alumina significantly improved the uniformity of the coatings by reducing pores and microcracks, which in turn improved adhesion. Increasing the alumina concentration to 0.3 wt% resulted in the most uniform coating with the fewest holes. At the same time, a higher concentration of alumina (0.4 wt%) led to the recreation of cracks on the surface. The microhardness of the coating increased by a factor of 2.6 times when alumina was added up to 0.3 wt% compared to the coating without additives. Moreover, corrosion resistance increased as the alumina concentration reached 0.3 wt%. The corrosion potential and current density were measured as −1079 mV and 0.9 μA/cm², respectively. EIS results showed that as the alumina concentration in the coating increased, the |Z| value rose, indicating an increase in corrosion resistance. The corrosion protection mechanism of the alumina was the barrier effect or physical barrier. Furthermore, salt spray test results validated the electrochemical findings, demonstrating that the 0.3 wt% Al₂O₃ coating formed a dense, protective barrier that effectively resisted Cl⁻ ion penetration.