Impedance Diagrams of 18-8 Stainless Steel in Passive and Transpassive States in Na₂SO₄ Solutions

Abstract

Electrical properties of the interface of 18-8 stainless steel electrode in the passive and transpassive states have been studied by using complex impedance diagrams measured in 1 M-Na₂SO₄ with pH 2.0 and 6.4.
The diagrams obtained in the passive region always showed distorted hemicircles which were equivalent to the impedance loci of electrical circuits having several parallel RC circuits connected in series. It is thought, therefore, that passive films are composed of several layers whose electrical properties are different from one another. Using the resistance at zero frequency (R_ω→0) of the diagram and the film thickness which was obtained by ellipsometry, the apparent specific resistance of the passive film was estimated to be ca. 2×10¹² Ω·cm.
In the transpassive region, the diagrams measured exhibited an inductive loop at extremely low frequencies in addition to a capacitative hemicircle which appeared at high frequencies. The appearance of an inductive loop means that the transpassive dissolution process contains a relaxation process with a large time constant.