HIGH RESISTANCE ANODIC FILM FORMATION ON BISMUTH WITH OR WITHOUT APPEARANCE OF INDUCTION PERIOD

The galvanostatic oxidation of bismuth in aqueous oxalic acid solutions has been studied at various anodic current densities, J, electrolyte concentrations, N, and temperatures, T. An induction period was found to appear always when, J is lower than a critical value, specific for given N and T. Relations considering the duration of the induction period as a function of J, N, and T, and the dependence of the linear potential rise on time in absence of induction period or after its expiration have been established. These relations are explained on the basis of a model considering the precipitation of crystals of a slightly soluble bismuth compound which isolates a part of electrode surface. When the current density on the rest (uncovered) surface reaches the critical value, an anodic film begins to grow on it, causing corresponding increase in the anodic potential.