ELECTRON-TRANSFER REACTIONS ON PASSIVE TANTALUM ELECTRODES

The redox system Fe**2** plus **/**3** plus has been reinvestigated in dependence on the concentration of the redox system, the electrode potential, the film thickness d, temperature T and pH. The anodic electron transfer reaction is blocked completely, but cathodic reduction of Fe**3** plus takes place at large overvoltages. In the cathodic Tafel diagram various types of reactions can be distinguished: for very thin, mean and thick films. Activation energies decrease with overpotential and increase with increasing film thickness. The maximum rate constants k//m indicate the contribution of tunnel processes for thin films and classical transitions at the lower edge of the conduction band at the oxide surface for thick films. The influence of the film thickness is less than it is expected from theoretical models. The comparison of all results with theoretical prredictions shows that probably four different processes have to be distinguished.