Partial processes during cobalt and molybdenum codeposition from solutions containing oxyacids

Abstract

Distribution of citrate complexes of Co(II) and Mo(VI) in weakly acidic solutions is analyzed. It is found that the primary cathodic process is hydrogen evolution. When cathodic polarization is increased, Co(II) reduction is possible, while more negative potentials may result in full reduction of Mo(VI) to the metallic state. Reduction of H3O+ ions in the solutions of oxyacids occurs with participation of a ligand playing the role of proton donor. Hydrogen evolution also remains the main electrochemical process in the molybdate solutions. It is assumed that Mo(VI) reduction occurs with participation of atomic hydrogen, but not all molybdate is reduced to metallic state. Co(II) reduction in solutions containing Mg(II) occurs to a higher extent but the degree of Mo(VI) reduction depends on the Co(II) concentration: increase in the latter results in a decrease in the content of metallic molybdenum in coatings.