Anodic corrosion of indium tin oxide films induced by the electrochemical oxidation of chlorides

Conducting tin-doped indium oxide (ITO) films were prepared by spray pyrolysis for study of their electrochemical stability as anodes in chloride solutions. From reflection high energy electron diffraction and transmission electron microscopy observations, ITO films present a regular polycrystalline structure with a very low density of twins. Corrosion was investigated by in situ mass measurements with a quartz crystal microbalance (QCM) and by direct imaging of corrosion morphology with scanning (SEM) and transmission (TEM) electron microscopy. Corrosion was found to occur at potentials positive of chloride oxidation (0.9 V vs. saturated calomel electrode (SCE)) as long as oxygen is not evolved (V < 1.3 V(SCE)). The corrosion rate is very low in the presence of 0.04 M HCl at pH 8 (less than or equal to 5 ngs(-1) cm(-2)), and very large in acidic HCl solutions (3400 ngs(-1) cm(-2) in 0.1 M HCl). TEM images of corroded ITO membranes reveal an important intergranular attack. The corrosion mechanism is based on the electrochemical formation of Cl degrees and OH degrees radical species able to withdraw electrons from In-O surface bonds.