Electrochemical Impedance Diagrams of Alloy 600 at Active-Passive Transition Potentials

Electrochemical impedance diagrams of Alloy 600 at active-passive transition potentials have been measured in sulphuric acid media of pH 0.0, 1.0 and 2.0 at 323 K over a wide range of frequencies, 10−3 to 104 Hz. It was found that the diagram for a partly passivated electrode, which can be obtained at a potential slightly lower than the passivation potential in a solution with lower pH, is composed of four semicircles; two of them are capacitive, the others inductive. The capacitive semicircle, which appeared in the highest frequency range in the diagram, was ascribed to the double layer capacity and the charge transfer resistance, the inductive one at higher frequencies to a dissolution reaction, the capacitive one at lower frequencies to a passivation-film-forming reaction, and the inductive one in the lowest frequency range to a passivation-assisting reaction. The degree of proceeding of the passivation in the alloy with increasing potential or solution pH can be evaluated from the vicissitudes of these semicircles in the diagram.