THE ELECTROCHEMICAL-BEHAVIOR AND IMPEDANCE CHARACTERISTICS OF THE PASSIVE FILM ON CARBON-STEEL IN NITRIC-ACID SOLUTIONS

The impedances of passivated carbon steel in 1.0 M HNO3 were measured at various polarization potentials. Complex impedance diagrams having two capacitive loops in the high frequency range and a Warburg respons e at low frequencies were obtained. The charge transfer at the metal\o xide interface was a reversible reaction, and the resistance was indep endent of the potential. The impedance of the charge transport in pass ive films can be described by a nonlinear transport equation. The diff usion coefficient in the passive film formed on carbon steel in 1.0 M HNO3 was about 9.8 x 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 deg eneration of the surface layer at high potentials (> 1.3 V). (C) 1998 Elsevier Science S.A. All rights reserved.