Improving mechanical properties and corrosion resistance of Zn-Ni-Co ternary alloy coatings by exploring the optimum anomalous co-deposition process

Abstract

Establishment of optimum electrodeposition process parameters is essential to enhance the mechanical properties and corrosion resistance of Zn-Ni-Co ternary alloy anomalous co-deposited coatings. This study employs Hull cell experiments to delineate compositional evolution in ternary coatings, followed by systematic direct-current electrodeposition on carbon steel to quantify the [Ni²⁺]/[Co²⁺] molar ratio effects on performance metrics. Zn deposition dominated at higher current densities, while Co accumulation increased at lower currents. Ni catalyzed the deposition of both Zn and Co, while the reduction of itself was always inhibited. The increased Co content improved corrosion resistance and refined the grains, but decreased the cathodic current efficiency and coating thickness. Hardness enhancement might be attributed to grain refinement combined with phase-boundary strengthening. The best comprehensive performance was achieved by the coating with a [Ni²⁺]/[Co²⁺] ratio of 3:7, which obtained mechanical hardness of 5.24 GPa, deposition efficiency of 77.4 % and the i_corr of 2.301 μA/cm² in 3.5 % NaCl solution at 30 ℃, suggesting high resistance to uniform corrosion for the Zn-Ni-Co coating. Compared to Zn-Ni coatings, the ternary alloy showed an 82.3 % lower i_corr and 53.5 % reduction in mass loss in NaCl + NaHSO₃ corrosive media.