Electrochemical reactions by tunnel current through thin insulating anodic oxide films on tantalum

Tantalum electrodes covered with oxide films, the thickness of which ranged from 40–90 Å, were investigated. Indium or gold was deposited on the oxide film by the vacuum deposition technique to prevent any change of oxide film properties by polarization. Polarization curves of In¦oxide film¦Ta diodes in a chloride buffer solution of pH 2.3 almost coincided with theI/V curves of them in the solid state. A redox reaction of a ferricyanide/ferrocyanide couple at Au ¦oxide film¦Ta was also perfectly controlled by electron transfer through the insulating oxide layer. Electron conduction with anodic polarization was mainly brought about by the electron tunnelling, while the Schottky emission possibly predominated in the conduction with cathodic polarization. Actual rates of corrosion of indium deposited on the films did not accord with those predicted by extrapolation of polarization curves to the open circuit potential, owing to the high electric resistance of the oxide films.