Influence of α-Al₂O₃ nanoparticles on the anomalous electrodeposition of Co-Ni alloys

The interactions between α-Al₂O₃ nanoparticles and metal ions or electrode surface during composite electrodeposition process were investigated by Zeta potential and electrochemical methods. Zeta potential measurements reveal that Al₂O₃ nanoparticles have a stronger tendency to adsorb Co²⁺ on their surface than Ni ²⁺ in sulfamate electrolytes. Steady-state polarization and electrochemical impedance spectroscopy (EIS) indicate that the effect of Al ₂O₃ nanoparticles on the Co-Ni electrochemical codeposition in different electrolytes ([Co²⁺]/[Ni²⁺] = 1/5 and 5/1) is quite contrary. In nickel-rich electrolytes ([Co ²⁺]/[Ni²⁺] = 1/5), the Al₂O₃ particles have an inhibitory action on the reduction of Ni²⁺, while in cobalt-rich electrolytes ([Co²⁺]/[Ni²⁺] = 5/1), Al ₂O₃ particles activate the reduction of Co²⁺. That nanoparticles exhibit different influence on the reduction of Co ²⁺ and Ni²⁺ were explained by considering the adsorption of metal ions on nanoparticle surface. Moreover, it can be seen from EIS that besides one capacitive loop at high frequency and one inductive loop at low frequency, the introduction of Al₂O₃ particles leads to exhibit a further inductive loop at low frequency. According to the simulated results, the electrochemical codeposition mechanism and simulated impedance parameters were analyzed and in good agreement with the experimental results.