Influence of the bath composition and the pH on the induced cobalt–molybdenum electrodeposition

Experimental electrodeposition parameters (pH, cobalt(II), citrate and molybdate concentrations) were varied in order to analyse their influence on induced cobalt–molybdenum codeposition. Voltammetry was the main technique used. In Co–Mo electrodeposition an initial molybdenum oxide layer is formed, over which the alloy is deposited. The process depends on the nature of the species in solution. At quasi-neutral pH, CoCit− is the main Co(II) species when there is sufficient complexing agent. Thus, cobalt is deposited as a result of the reduction of CoCit−, and molybdenum is reduced from MoO42− to MoO2. This intermediate oxide evolves to molybdenum via the formation of [MoO2–CoCit−]ads. In acidic media, molybdate complexes with citrate, so molybdenum oxides are formed from HrMoO4Cit[5−r]. On the other hand, cobalt is deposited mainly by CoHCit reduction. CoCit−/CoHCit and molybdenum oxides are reduced simultaneously, so cobalt nuclei are always needed to induce the codeposition.