Effect of zinc clustering, micro-texture, and surface oxide constituents on the electrochemical corrosion of Sn-Zn coatings electrodeposited at different current densities

The Sn-23 wt% Zn coatings were electrodeposited onto mild steel substrate at different current densities (7, 12, 17, and 22 mA/cm²) from sulfate-based electrolytic bath. The electrochemical degradation corrosion mechanism of the coatings in 3.5 wt% NaCl solution were investigated and correlated with evolved grain orientation, grain boundary distribution, solute distribution and corrosion product constituents. The corrosion resistance of the coatings varied non-monotonically with deposition current density. The coating electrodeposited at 12 mA/cm² offered the highest corrosion resistance (R_p = 10409 Ωcm²) due to its lower energy (100) surface grain orientation, higher fraction of low energy grain boundaries, and relatively inert corrosion product consisting of ZnO and SnO₂ compounds. The higher energy (110) surface grain orientation, unstable b-Sn matrix due to embedded Zn-enriched clusters, and weaker corrosion product layer consisting of Zn⁰ led to the lowest corrosion resistance (R_p = 3429 Ωcm²) for coating electrodeposited at 17 mA/cm².