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Rotating ring-disc electrode studies on the anodic dissolution and passivation of iron in potassium carbonate/bicarbonate buffers at 25°C show that in the active anodic dissolution potential range Fe(II) soluble species are generated. This reaction is favoured by the presence of bicarbonate ions in solution and it is explained through the formation of an unstable soluble complex containing Fe(lI) and HCO3 ions. This suggests that the anodic layer at a certain stage of its formation contains some amount of carbonate species. XPS data of surface layers produced at different anodic potentials confirm the presence of the carbonate species in thick anodic layers grown in the prepassive potential region in still solutions, whereas the opposite result is found for the thin passive layers formed at high positive potentials.

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The influence of ionic composition on the electrodissolution and passivation of iron electrodes in potassium carbonate/bicarbonate buffers is studied over the concentration range of anions compatible with the regulating capacity of the buffer, at 25°C, by using voltammetric and rotating disc electrode techniques. Data are discussed on the basis that the interaction between Fe 2÷ and HCO~ ions plays an important role at the outer part of the passivating layer where the local pH resulting from the thickening of the passivating layer is determined by the coupling of different ionic equilibria.