Electrochemical Behavior of Passive Iron in Acid Medium I. Impedance Approach

Wide frequency range impedance measurements are applied to a kinetic investigation of passive iron in various acidic media. Special care was taken in the electrode preparation to reach stable current densities (7 ~A 9 cm-2 in Fe/1M H2SO4, 25 ~ Impedance data were obtained as a function of potential and temperature in the steady state. An original approach to the nonsteady-state behavior is ntroduced by combining transient response to the time dependence of the impedance. Contributions from transport in the film and interfacial impedance are separated. The classical view of high field conduction mechanism refined with respect o the generality of the description and also to the film properties in various environments leading to the concept of chemically dependent high field ion migration. Low frequency be-havior in the millihertz range supported this conclusion by ruling out the idea of diffusion-controlled passive current. The capacitance observed is attributed to the film-growth kinetics. In the last ten years, passivity was mainly investigated by means of in situ optical techniques and non-in situ surface spectroscopies (1-8). These studies resulted in a certain amount of information on the chemistry of passive layers, but their use is often confined to weakl