Abstract
The electrokinetics of chromium-modified passive iron electrodes in alkaline chromate solutions are reported. Cyclic voltammetric studies at different pHs, chromate concentrations and temperatures have shown two different types of pseudocapacitive behaviour: the reductive precipitation of hydrous Cr(III) species on the top of the iron passive layer and redox transformations of chromium ions entrapped in the passive film matrix. The latter process has been analysed in terms of a thin layer redox-modified electrode model. A simple kinetic behaviour for the multi-electron Cr(VI)/Cr(III) reduction with a Cr(VI)/Cr(V) rate-de-termining step involving a protonation step is apparent with a Tafel-type temperature dependence. The electrochemical results are discussed in connection with XPS chromium concentration-depth profiles in the modified surface matrix.
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