Mass-Transfer Effects on the Anodic Behavior of Porous Fe and Co Sintered Electrodes in Aqueous Ammoniacal Solution

A theoretical analysis is presented of the electrochemical conditions within the pore structures of sintered, metallic iron and cobalt subjected to anodic polarization in aqueous ammoniacal solution. The concentrations of the various ionic and molecular species and the distribution of the electrode potential within the pores are calculated. The evaluations of the pore model indicate that the concentrations of the various species are nonuniform within the pore. The ohmic potential drop inside the pore becomes appreciable as the pore depth increases, which promotes the active state in an otherwise passivated sample. The model also shows that the metal ammine complex ions exist only to a certain depth because of increasing depletion of complexing ligand as distance into the pore increases. Experiments with sintered Fe and Co samples show that active dissolution occurs subsurface within the pore structure while the outer surface is in the passive state in accord with the model predictions.