The oxygen electrode. Part 6.-Oxygen evolution and corrosion at iridium anodes

A comparison of the anodic behaviour of iridium and platinum using both current and corrosion-rate measurements at various potentials showed that the rate of oxygen evolution was considerably faster on iridium in both acidic and basic media. Under constant potential conditions a continuous decrease in current with time was observed in the case of platinum, under all conditions, and in the case of iridium, in base. It is assumed that this effect is due to continuous slow oxidation of the metal substrate with consequent inhibition of electron transfer or of radical reaction at the electrode surface. A sharp break on the Tafel plot for iridium in acidic solution was observed at 1.57 V and was attributed to a change in the rate-determining step, the reaction occurring via hydroxyl radical intermediates both above and below the break. A large increase in current was also observed on this plot at 1.8 V and was attributed to a further change in mechanism, the evolution reaction at higher potentials involving a cyclic change in the oxidation state of iridium ions in the oxide film. The latter change was accompanied by the onset of slow corrosion of the electrode - again suggesting a change in the nature of the anodic film. The corrosion current contributed only a very small fraction of the overall current and gave a linear Tafel plot, the slope of which was twice that of the corresponding plot for total current over the same region of potential. It is postulated that the corrosion reaction and the oxygen evolution reaction in this region involve a common intermediate.