Impedance of Iron Electrode Passivated in Borate and Phosphate Solutions

Abstract

The semiconductive properties of the passive film on iron were investigated by measuring the impedance of iron electrode in borate and phosphate solutions at various pH values. The impedance data were compared with the thickness data previously obtained from 3-parameter reflectometry. From the effect of frequency and potential on the impedance an equivalent circuit is postulated for the passivated iron electrode, which consists of a series connection of two parallel R–C circuits. One component R_L–C_L of the two parallel circuits which predominates over frequencies lower than 100 Hz, characterizes the dielectric property of the passive film, whereas the other component R_H–C_H, which appeares at higher frequencies corresponds to the oxide/solution interface. From comparison of C_L with the film thickness, the passive film is found to consist of an inner electron-conductive layer where no potential drop occurs and an outer carrier depletion layer where the electric field is of the order of 1×10⁸ Vm⁻¹. The dielectric constant of the outer layer is also evaluated at about 40. The component R_H–C_H is sensitive to the anionic species present in the solution and is thus likely to be assosiated with an ionic adsorption-desorption process taking place across the electric double layer at the oxide/solution interface.