Abstract
Abstract The impedances of passivated carbon steel in 1.0 M HNO 3 were measured at various polarization potentials. Complex impedance diagrams having two capacitive loops in the high frequency range and a Warburg response at low frequencies were obtained. The charge transfer at the metal|oxide interface was a reversible reaction, and the resistance was independent of the potential. The impedance of the charge transport in passive films can be described by a nonlinear transport equation. The diffusion coefficient in the passive film formed on carbon steel in 1.0 M HNO 3 was about 9.8×10 −17 cm 2 s −1 , and the charge transport in the film followed a diffusion mechanism. The surface layer of the film was a stable n-type semiconductor phase in the low potential range (0.4 to 1.3 V), but it can be converted to a metal-like layer due to degeneration of the surface layer at high potentials (>1.3 V).
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