A model for potentiostatic current transients during alloy deposition: cobalt–molybdenum alloy

This paper proposes a model for the potentiostatic current transients during induced molybdenum–cobalt alloy deposition. Our model describes the alloy current transient as the sum of three contributions, namely the deposition of molybdenum oxides, the reduction of these oxides to metallic molybdenum, and cobalt deposition. For the oxides of molybdenum alone, our studies suggest that this deposition is not mainly dominated by the nucleation and growth processes. Instead, the oxide species seems to be deposited as a fine dust without having preferred sites onto which to be adsorbed. The reduction of the oxides to metallic Mo is produced with the participation of Co, which means that the complete Mo deposition in the alloy is performed in two steps, the deposition of the oxides and their subsequent reduction. The model describes this behaviour of the Mo deposition as a process controlled first by a charge transference that goes to a mass-transport-controlled situation. For the Co deposition, we use an approach of charge-transfer-controlled systems. Although we assume that the Mo and Co currents are independent, the radii of the deposits of each species have to be equal, RMo(t)=RCo(t), to maintain the homogeneity of the alloy. This link between the radii of the metals imposes a restriction on the Co deposition, which becomes controlled by both the charge transference and by the deposition of the Mo. An expression for the Co deposition under these conditions is developed. Finally, the predictions of the model are compared with the experimental curves.