A potentiodynamic and SEM study of the behaviour of iron in pH 8.9-11.0 phosphate solutions

The behaviour of iron electrodes immersed in pH 8.9-11.0 Na2HPO4 solutions was studied by potentiodynamic measurements (cyclic voltammetry) and scanning electron microscopy (SEM). The influence of proton and phosphate content, scan rate, anodic switching potential and other devised potential/time perturbation regimes were evaluated in order to obtain an understanding of the reactions involved during the passivation process. The oxidation-reduction peaks were analysed using a thermodynamic approach. It was possible to reveal the most important reactions coming about on the electrode surface, which comprise, initially, the formation of Fe-3(PO4)(2) and Fe(OH)(2) at ca. -0.8V/SCE. Subsequently, these ferrous species are further oxidized to gamma-FeOOH at ca. - 0.36 V/SCE, the latter being transformed into Fe2O3 in the passive region. Depending on the pH value (11.0) and scan rate (slow), Fe3O4 is clearly present as an intermediate oxide structure between the ferrous and ferric species mentioned earlier. In addition, scan rate was discovered to play a remarkable influence on the surface morphology observed during the SEM examinations.