The properties of thin anodic passive films potentiostatically formed (1 V I Ef s 5 V us see) on polycrystalline tungsten in aqueous H3P04, HzS04, HNOs and HCl04 solutions (pH z 1.3) were studied using electrochemical impedance spectroscopy at room temperature. The data were analysed with a transfer function using a non-linear fitting routine, assuming that the resistance of the film is coupled in series with the faradaic impedance of the W(O)+W(VI) reaction, with these in parallel with the capacitance of the metal/passive film/electrolyte system.
The relative permittivity (Ed) of the films was found to depend on the acid solution used. Except for the case of films grown in HNOJ, the values of sr are smaller than those of thicker films grown galvanostatically. The film grown in H3P04 solutions presented the lowest value of G, which could be due to the incorporation of phosphate ions into the film.
The number concentration of donors in the films (No) was found to decrease with Er (ie increasing film thickness)
and to be quite dependent on the electrolyte used: No(H3POs) < ND(HC104) < ND(HNO~) c ND(HzSO~). The values for No(HsPOd) are, nevertheless, lo3 times larger than the ones for much thicker WO3 films and are of the same order of magnitude as the ones for annealed WO3 films. The flat band potential of the films grown in HJPO~ was found to be more than 0.1 V smaller than those for films grown in the other solutions.