Study of anomalous nickel-cobalt electrodeposition with different electrolytes and current densities

Nickel-cobalt electrodeposition was carried out in a simple chloride and a complex ion bath, respectively, containing 0.5 M nickel and 0.05 M cobalt, at current densities from 0.1 to 1000mAcm −2 without using agitation. Anomalous cedeposition only occurred with the simple chloride bath, the content of cobalt (the less noble metal) in the deposits was much higher than in the bath when the current density was lower than 100mAcm − 2. However, the cobalt content in the deposits produced from the complex ion bath was significantly lower than in the electrolyte within the entire range of deposition current density. The anomalous cedeposition could be related to the intrinsically fast kinetics of cobalt deposition. Polarization results showed comparable current densities of individual nickel and cobalt depositions in simple chloride baths at comparable potentials, although the cobalt concentration was only one-tenth nickel in the electrolytes, due to the high exchange current density of cobalt. The anomalous codeposition was also attributable to the dramatic reduction of the nickel deposition rate with cobalt ions in the bath. With the complex ion bath, however, the cobalt contents in the nickel-cobalt deposits were much lower than in the bath, and the effect of plating current density on the deposit composition was negligible.