Abstract
Cathodic reduction behaviors of passive films on cobalt have been studied in sodium borate-boric acid buffer solution of pH 8.39. The amount of dissolved Co2+ ion during the catholic reduction has been measured by channel flow double electrodes. On the anodic polarization curve of cobalt there are two passive regions, one of which is passive region I from -0.3 to 0.2V and the other the passive region II from 0.3 to 0.8V. During the cathodic reduction of the passive film formed in region I, which probably consists of a hydrated CoO, no dissolved Co2+ ion is observed. During the cathodic reduction of the passive film formed in region II, which mainly consists of Co3O4, Co2+ ion dissolves from the film. However, the passive film formed at the higher potential of passive region shows an induction period in its cathodic dissolution as Co2+, attributing to the reduction of Co2O3 to Co3O4 in solid phase. The current efficiency for the reductive dissolution from Co3O4 to Co2+ is independent of the applied cathodic current density and increases with increase of electrolyte flow rate. The cathodic reduction mechanism has been estimated from an assumption that a partial diffusion of Co2+ to bulk solution and precipitation of Co(OH)2 onto cobalt simultaneously take place.
