鉄不働態の界面インピーダンス

Abstract

Frequency response of a passive iron electrode was studied in a neutral solution (pH 8.39) at various potentials. An electronic potentiostat with a sinusoidal wave generator was used to control both d.c. and superposed a.c. potential of the electrode. The impedance was determined by measuring the amplitude and the phase shift of a.c. voltage and current on a two-channel osciloscope and recorder. The measurement was made in a frequency range from 10−2 to 2×104 Hz. The frequency response diagrams obtained were classified into three categories according to the corresponding potential regions. In a complate passive state, the frequency response was similar to that of an ideal capacitor, and the electrode behaved as an ideal polarizable electrode, because of the absence of any possible reaction. In the pre-passive and the trans-passive state, however, the frequency responses were simulated by that of a cascade connection of two circuits of capacitor and resistor in parallel connection. It is readily seen that one of the parallel circuits corresponds to the impedance at solution/passive film interface, which represents the transfer resistance and double layer capacitance. Only when the transfer resistance was deduced by the existence of electrochemical adsorption or dissolution, the impedance of the passive film became appreciable. The potential profile across the passive film was discussed and it was concluded that the major part of potential drop exist at the solution/passive film interface.