The mechanism of the transpassive dissolution of chromium in weakly acidic sulphate solutions

The transpassivity of chromium in 1 M sulphate solution (pH 5) was studied by steady state polarization, rotating ring-disk electrode voltammetry and impedance spectroscopy in combination with the newly developed Contact Electric Resistance (CER) 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 in it. 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(IV) intermediate and a continuous change in the valency state of Cr at the film/solution interface are taken into account. Kinetic equations for both steady state and impedance response were derived and compared to experimental results to determine the reaction rate constants.