Corrosion-induced activation of SnO2 films promoted by the electrochemical oxidation of chloride ions

At potentials more positive than the flatband potential, tin dioxide electrodes are normally passivated with respect to charge transfer because of an electron-depleted surface. Nevertheless, such a passivated state can be lost by the occurrence of a corrosion process. In the potential region 0.9-1.3 V/saturated calomel electrode close to the thermodynamic potential for chloride oxidation, sprayed SnO2 thin film electrodes present a loss of mass, determined in situ by quartz crystal microbalance experiments. This phenomenon is characterized by a peak-shaped corrosion rate curve, the amplitude of which depends on the pH, the chloride concentration, the presence of metallic traces, and the type of the dopant. From data obtained at pH 8, and in natural seawater, it is apparent that the corrosion rate is (i) amplified by the presence of metallic traces, and (ii) strongly dependent on the Cl- content, becoming almost negligible for [Cl-] > 0.3 M. These results are tentatively explained by considering the electrochemical production of OH. and Cl-. radicals, able to break Sn-O surface bonds by electron capture (chemical steps), in competition with chlorine and oxygen evolution. The corrosion process is also responsible for a positive shift of bandedges due to charged corrosion intermediates, allowing the SnO2 electrode to become active with respect to anodic charge transfer reactions. (C) 1999 The Electrochemical Society. S0013-4651(98)03-009-2. All rights reserved.