The behaviour of iron electrodes immersed in pH 8.9‐11.0 Na 2 HPO 4 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 (PO 4 ) 2 and Fe(OH) 2 at ca. −0.8 V/SCE. Subsequently, these ferrous species are further oxidized to γ‐FeOOH at ca. −0.36 V/SCE, the latter being transformed into Fe 2 O 3 in the passive region. Depending on the pH value (11.0) and scan rate (slow), Fe 3 O 4 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.
Anti-Corrosion Methods and Materials – Emerald Publishing
Published: Jun 1, 2004
Keywords: Passivation; Iron; Electrodes