The mechanism of the transpassive dissolution of chromium in acidic sulfate solutions

The transpassivity of chromium in 1 M sulfate solutions (pH 0 and 5) was studied by steady-state polarization, rotating ring-disk electrode voltammetry, and impedance spectroscopy in combination with the contact electric resistance technique. Ring-disk electrode measurements in the transpassive region indicated the dissolution of Cr mainly as Cr(VI) and partly as Cr(III). The conductivity of the anodic film strongly increased in the transpassive potential range, probably due to the formation of Cr(VI) species. A kinetic model for the transpassive process was advanced to explain the observed experimental features. It comprises two dissolution paths of Cr as Cr(VI) and one as Cr(III). The formation of a Cr(TV) intermediate and a continuous change in the valence state of Cr at the film/solution interface are considered. Kinetic equations for both steady state and impedance response were derived and compared to experimental results to determine the reaction rate constants.