Impedance Measurements and Analysis of the Corrosion of Chromium

Abstract

Impedance measurements, performed in the active and active‐passive transition range, are explained by a simple mechanism. Four different time constants encountered in the impedance diagrams can be explained by diffusion controlled dissolution of chromium, hydrogen evolution, blocking of the surface by hydrogen, and passivation of the surface. In the passive region the potential dependent change in the "character" of the passive film results in another time constant in the impedance diagram. The surface can be seen as composed of two different areas. At one area diffusion controlled dissolution of chromium takes place, while at the other area blocking of the surface by hydrogen and passivation was found to occur. From the proposed mechanism the admittance expression was calculated and the parameters in the mechanism were determined.