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Abstract The effect of Cl~- ion on the features of impedance of mild steel and iron electrodes with passive film on their surfaces is studied.In NaNO_2 solution, the polarization resistance and time constant of mild steel electrode are estimated to be 6.35 megohm/cm~2 and 87.5 sec respectively, which are very close to the published data for iron in borax-boric acid solution.As Cl~- ion is introduced into various passivators solutions, the polarization resistance and time constant of the passive-film-covered mild steel and iron electrodes reduce by ca.4 orders of magnitude during pit incubation period,i.e., before the passive film is perforated.In addition, an inductive component is observed on impedance plane in low frequency range for all systems studied.This inductive component is fading away with time gradually.However, even if the inductive are disappears at last, the shrinkage of the real part of the impedance at very low frequencies can still be observed before pits initiate.An equation describing the impedance of passive-film-covered electrodes is proposed, based on which, three cases of the impedance diagrams of passive-film-covered electrodes can be demonstrated.If R_f, which denotes the resistance for the passive-film-forming current density passing through the passive film, is approximately equal to R_s, which denotes the resistance for the passive film dissolution process, a single capacitive arc on the impedance diagram is obtained and the estimated polarization resistance R_p equals R_f, whose magnitude is abont 6 megohm·cm~2 as usually observed for passive electrodes.When R_s>R_f, the faradaic impedance is equivalent to a serial connection of a resistance and a capacitance as observed for valve metals electrodes.If R_s
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Received: 25 August 1989
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