Electrodeposition of Ni[sbnd]Co alloy from methansulfonate electrolyte. The role of the electrolyte pH in the anomalous codeposition of nickel and cobalt

Abstract

The kinetic characteristics of the electrochemical codeposition of nickel and cobalt from a methanesulfonate electrolyte have been investigated. An analysis of the partial voltammograms of the nickel and cobalt deposition into the alloy showed that the potentials of the metal deposition into the alloy are significantly shifted in a positive direction compared to the deposition of the pure metals. This effect is more pronounced in the case of cobalt, and is the cause of the anomalous codeposition of nickel and cobalt. In the presence of the H3BO3 as a buffering agent in the electrolyte, the electroreduction of nickel(II) and cobalt(II) ions is inhibited; this reveals a significant role of the near-electrode pH in the kinetics of these reactions. The thermodynamic and kinetic aspects of the NiCo alloy electrodeposition were considered. It has been found that the kinetic factor is the determining factor responsible for the anomalous codeposition of nickel and cobalt. It has been shown that an adequate explanation of the observed regularities is possible using a concept that assumes equally accessible electroreduction of alloy components at the catalytic centers formed during the adsorption of their hydroxo complexes on the electrode surface. Based on the proposed mechanism, a kinetic model has been developed that correctly describes the features of nickel and cobalt codeposition.It has been found that the microhardness and internal stresses of NiCo electrolytic alloys obtained from methanesulfonate electrolytes are higher than those of coatings deposited from sulfate electrolytes. This is due to the difference in the degree of distortion of their crystal lattices. The low buffering properties of methanesulfonate electrolytes in comparison with sulfate electrolytes cause the formation of a greater number of hydroxide compounds in the near-electrode layer, which are incorporated into the cathode deposit and affect its structure.