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 H(r)MoO(4)Cit([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. (C) 2003 Elsevier B.V. All rights reserved.