Anodic Behaviors of Iron and Its Oxides in Dilute Sulfuric Acid and Akaline Solutions

Abstract

Anodic behaviors of pure iron and iron oxides in dilute sulfuric acid and alkaline solutions were investigated to clarify the significance of passivation and other oxidizing potentials of iron. The anodic polarization curve of pure iron indicated four current peaks at potentials of about −0.1, 0.3, 0,6 and 1.4 V in 1 N H2SO4 and −0.9, −0.7, −0.4 and 0∼0.4 V (vs S.C.E.) in 1 N KOH. The anodic polarization curves of FeO1.05, FeO1.1 and Fe3O4 indicated three, two and one peaks, respectively, and the potentials of the peaks of the oxides agreed with those of pure iron. From these anodic behaviors of the oxides, it was suggested that the four potentials at the current peaks in the potential-current curve of iron show four redox potentials of Fe/FeO1.05, FeO1.05/FeO1.1, FeO1.1/Fe3O4 and Fe3O4/γ Fe2O3, where FeO1.05 and FeO1.1 are two Wüstites containing minimum and maximum combined oxygen, respectively. These potentials were also confirmed from the measurements of potential decay curves of iron and iron oxides. From these results, it was concluded that the passivation potential of iron in 1 N H2SO4 corresponded to the redox potential of FeO1.05/FeO1.1, whereas that in 1 N KOH corresponded to the redox potental of Fe/FeO1.05.