Growth of oxide layers on platinum electrodes in molten alkali nitrates

Abstract

The formation of oxide layers on Pt anodes and on Pt cathodes in each of the molten alkali nitrates, LiNO 3, NaNO 3 and KNO 3, has been investigated under galvanostatic and potentiostatic conditions at 340°C. The coverage by oxygen on the anode increased with the polarization potential and time and reached a limiting value of about 2·6 oxygen atoms per apparent surface platinum atom. No cation effect on the anodic formation of the oxide film was observed. Thick multilayer oxide films grew very rapidly on the “cathode” in the KNO 3 melt and slowly on that in the NaNO 3 melt. In the KNO 3 melt and also in the NaNO 3 melt, the growth rate of the multilayer oxide reached a maximum at −1.65 V ( vs a Ag 0.1 M Ag + in 1 kg KNO 3 reference electrode) and rapidly fell at more negative potentials because of the dissolution of the oxide into the melt. Above −2.0 V, neither the formation of the multilayer oxide nor corrosion of the electrode was observed. In the LiNO 3 melt, no multilayer oxide film was formed at any potentials.