Kazuhisa Azumi1, Toshiaki Ohtsuka1 and Norio Sato1
1Electrochemistry Laboratory, Faculty of Engineering, Hokkaido University, Sapporo
In order to characterize the semiconductive properties of the passivation film on iron, the impedance of iron electrode was investigated in borate and phosphate solutions at various pH values. The results were discussed as compared with thickness data previously obtained from 3-parameter reflectometry.
From the effect of frequency and potential on the impedance an equivalent circuit is postulated for a passivated iron electrode, which consists of a series conection of two parallel RC circuits. One component RLCL of the two parallel circuits which is predominantly responsible for frequencies lower than 100 Hz, characterizes the dielectric property of the passive film, whereas the other component RHCH which appeares at higher frequencies corresponds to the oxide/solution interface.
From comparison CL 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 depleted 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 RHCH is found to be 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.
iron, passivation film, impedance, anion effect, dielectric constant
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