Effect of α-Al₂O₃ particles on the electrochemical codeposition of Co-Ni alloys from sulfamate electrolytes

The effect of α-Al₂O₃ particles on the electrochemical codeposition behaviour of Co-Ni alloys from sulfamate electrolyte was investigated by codeposition experiments, zeta potential and electrochemical methods. The results indicate that the presence of Al ₂O₃ particles in Co-Ni sulfamate electrolytes contributes to the preferential codeposition of Co. Moreover, increasing Co²⁺ concentration also can enhance the codeposition of Al₂O₃ particles in Co-Ni alloy matrix. Zeta potential measurements confirm that Co²⁺ has a stronger tendency to adsorb on Al₂O ₃ particles surface than Ni²⁺ does in sulfamate electrolytes. Steady-state polarization and electrochemical impedance spectroscopy (EIS) show that the effect of Al₂O₃ particles on electrochemical codeposition behaviour of Co-Ni alloy in different Co²⁺/Ni²⁺ ratio electrolytes (1:5 and 5:1) is quite contrary. In nickel-rich electrolytes (Co²⁺/Ni²⁺ = 1:5), the Al₂O₃ particles cause a negative shift in reduction potential and an increase of charge transfer resistance. However, in cobalt-rich electrolytes (Co²⁺/Ni²⁺ = 5:1), Al ₂O₃ particles lead to a positive shift of reduction potential and reduce the charge transfer resistance. Moreover, in EIS, besides one capacitive loop at high frequency and one inductive loop at low frequency, the introduction of Al₂O₃ particles leads to a further inductive loop at low frequency. According to simulated results of EIS, an electrochemical codeposition mechanism of Al₂O₃ particles with Co-Ni alloys is presented and in good agreement with the experimental results.