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 p yrolysis for study of their electrochemical stability as anodes in chl oride solutions. From reflection high energy electron diffraction and transmission electron microscopy observations, ITO films present a reg ular polycrystalline structure with a very low density of twins. Corro sion was investigated by in situ mass measurements with a quartz cryst al microbalance (QCM) and by direct imaging of corrosion morphology wi th scanning (SEM) and transmission (TEM) electron microscopy. Corrosio n was found to occur at potentials positive of chloride oxidation (0.9 V vs. saturated calomel electrode (SCE)) as long as oxygen is not evo lved (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 ve ry large in acidic HCl solutions (3400 ngs(-1) cm(-2) in 0.1 M HCl). T EM images of corroded ITO membranes reveal an important intergranular attack. The corrosion mechanism is based on the electrochemical format ion of Cl degrees and OH degrees radical species able to withdraw elec trons from In-O surface bonds.