Oscillatory phenomena: spontaneous alternance of the occurrence of corrosion and passivation states of an iron electrode in bromate/sulphuric acid medi

Spontaneous oscillations have been found by us in the potential value (vs. a saturated calomel electrode) of an iron electrode in bromate/sulphuric acid media in open electric circuit conditions. We ascribe them to a spontaneous alternance of corrosion and passivation states. Both bromate and H + reductions, in the corrosion state, lead to an increase in the pH value near to the iron surface and to the precipitation of a passivating film of oxide or salt on the electrode surface. The prevalence of H + influx from the bulk of the solution on its disappearance during the passivation state leads to such a low value of pH as to allow the dissolution of the passivating film, and the phenomenon starts again. We have studied the influence of stirring, of the size of the iron electrode surface, and of the bromate and sulphuric acid concentrations on the oscillations.