THE IMPEDANCE CHARACTERISTICS CAUSED BY THE PASSIVE FILM FORMED ON CARBON-STEEL IN NITRIC-ACID SOLUTION

A kinetic study of the passive film on carbon steel in 1.0 mol . dm(-3) HNO3 solution was performed based on the impedance measurement method. A complex diagram with two capacitive loops in high frequency and Warburg-type impedance spectrum in low frequency were obtained in the potential range of 0.5 to 1.4 V (vs. SCE). In this range, (1) the charge- transfer-resistance (Rt) was roughly constant at metal/oxide interface, (2) the capacitance C-HF followed Mott-Schottky relation in the lower range (<1.3 V), (3) the deviation from the relation was observed in the higher range (>1.3 V). In this range (>1.3 V), it is suggested that the surface layer of the passive film can be transformed from a n-type semiconductor layer to a metal-like layer by the degeneration of electron energy levels at the most external surface layer of the film.