Electrode Potentials of Pure Iron and Fe₃O₄ Film in Chromic Acid Solutions

Abstract

The electrode potentials of pure iron covered with Fe₃O₄ film in acidic chromate solutions were measured at 25°C in relation to the Pourbaix diagram of Fe-CrO₃-H₂O system.
The potential at the early stage of immersion showed a stable value corresponding to the oxidizing reaction from Fe₃O₄ to γ-Fe₂O₃. The relation between the potential and pH value of the solution changed with the structural change of chromic ion species: Cr³⁺_ag. (pH<2), CrO^-_2aq. (2<pH<4), and CrO^3-_3aq. (4<pH).
The potential at about 30min. after immersion showed a stable value corresponding to the dissolution of iron. The relation between the potential and pH value changed with the structurall change of ferric and chromic ion species: Fe³⁺_aq., Cr³⁺_aq. (pH<2); FeOH²⁺_aq., CrO^-_2aq. (2<pH<4); and. Fe(OH)⁺_2aq., CrO^3-_3aq. (4<pH).
The observed pH-potential relations were discussed in connection with the electrochemical equilibrium of cathodic and anodic reactions on the surfaces. The electrode potential of the reaction of pure iron with chromic acid solution through the Fe₃O₄ film was found to have a correlation with the anodic reaction of solid electrolyte, i.e., the dissolution of Fe²⁺ from iron crystal surface into the Fe₃O₄ film.