Electrochemical behavior of Fe in phosphate solutions studied by in situ X-ray absorption near edge structure

The electrochemical behavior of Fe and thin, sputter-deposited films of Fe2O3 in phosphate-containing solutions was studied and compared with the behavior in borate buffer and 0.1 M NaOH. In situ X-ray absorption near edge structure was used to simultaneously monitor changes in the samples' average valency and thickness. The reactions taking plate during reduction of:the passive film on iron are significantly different for all three electrolytes. In borate buffer (pH 8.4), reduction leads to a complete dissolution of the passive film. In alkaline solution, no dissolution takes place, but, instead, the passive film is converted into a lower-valent oxide/hydroxide film during reduction. Similarly, 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. In contrast to the behavior in alkaline solution, where no dissolution takes place independent of the potential, in the phosphate buffer (pH 8.4), dissolution occurs in the potential range of the active/passive-transition. In an alkaline phosphate-containing solution, conversion into metallic iron is possible in a direct reduction step to potential in the stability region of Fe(0): at intermediate potentials, a ferrous layer is formed in the reduction reaction. The presence of phosphate species in the electrolyte enables the further reduction of this ferrous layer into Fe(0) at lower potentials. In pure NaOH solution, the presence of Fe(OH)(2) on the surface hinders further reduction even at very low cathodic potentials. The factors controlling the reactions taking place during the reduction of the passive film on iron are discussed. (C) 1999 The Electrochemical Society. S0013-4651(99)03-074-8. All rights reserved.