CATHODIC REDUCTION OF PASSIVE FILMS ON IRON IN BORATE AND PHOSPHATE BUFFER pH 8.4: DIFFERENT MECHANISMS REVEALED BY IN SITU TECHNIQUES

The electrochemical behavior of passive Fe and thin, sputter-deposited films of Fe2O3 was studied in borate and phosphate buffer pH 8.4 solutions. Cyclic voltammograms and in situ light absorption measurements which enable the monitoring of the oxide film thickness indicate a similar behavior of the Fe electrode in both pH 8.4 solution, especially a presence of a oxide-free surface at low cathodic potentials. However, X-ray absorption near edge structure (XANES) studies which allow a simultaneous monitoring of changes in the samples' average valency and thickness reveal that the reactions taking place during reduction of the passive film on iron are completely different for the two electrolytes. In borate buffer (pH 8.4), reduction leads to a complete dissolution of the passive film and the end product of reduction is soluble Fe(2+). In phosphate buffer (pH 8.4), there is no dissolution in a direct step to low cathodic potentials, but the resulting reduction product is metallic iron. Hence, the formation of the bare oxide-free metal surface at cathodic potentials takes place by different mechanisms in the two pH 8.4 solutions, depending on the type of anion present in the solution.