An ellipsometric and rrde study of iron passivation and depassivation in carbonate buffer

The passivation and depassivation of pure iron in carbonateibicarbonate buffer at pH 9.2 shows features that are not found in borate buffers. On a static electrode, the passivation occurs in two stages the first being a dissolution-precipitation process giving a layer of ferrous hydroxide or basic ferrous carbonate. At a slightly higher potential, further oxidation takes place to form a film consisting of a thin inner mixed-valence layer and an outer layer essentially free of ferrous species. This outer layer appears to be hydrated at potentials close to the passivation potential, but the extent of hydration decreases as the potential is raised. Galvanostatic reduction of the film formed at intermediate potentials shows only a single potential arrest corresponding to reduction of the entire film to Fe(II); no further reduction to iron metal was found. For films formed at potentials close to the passivation onset, galvanostatic reduction led to very little Fe(II) being detected at the ring until close to the end of the reduction process, when both RRDE and ellipsometric measurements show a sudden dissolution process, detected in the former technique by a burst of Fe(II) at the ring and in the latter by a sudden change in both A and 1P. For films formed at higher potentials, galvanostatic reduction is found to be a mixture of steady dissolution and, at the end of the process, a burst of dissolution similar to that found in films grown at more negative potentials. Both RRDE and ellipsometric studies have indicated that the amount of charge required to oxidise Fe(II) released in the final surge is independent of formation potential, adding support to a bilayer model for the passive film.