Electrochemical Properties of the Buffed Surface of Pure Iron in Chromate Aqueous Solutions

Abstract

The relations between electrode potentials of buffed surface of iron in chromate solutions and pH values of the solutions were studied in relation to the corrosion region (pH<4) in the Pourbaix diagram for the Fe-Cr0₃-H₂0 system at 25°C. The results obtained were as follows:
The buffed surface is covered with thin Fe₃0₄ films. The electrode potentials vary with the pH values and the presence of anions, Cl^- and SO^2-₄. The surface reactions are estimated from the observed pH- potential relations as follows:
(1) 0<pH<2;
3Fe+6Fe₃O₄+4CrO^2-_4aq+26H⁺=3Fe²⁺+9γ-Fe₂O₃+4Cr³⁺_aq+13H₂O
(2) 2<pH<4;
3Fe+6Fe₃O₄+4CrO^2-_4aq+10H⁺=3Fe²⁺+9γ-Fe₂O₃+4CrO^-_2aq+5H2O
The electrode potentials of the Fe₃0₄ film depend on the presence of anions, Cl^- and SO^2-₄, and on the structure of chromic ion species, and concerned with the solid state electrolyte reaction, i. e. the dissolution of Fe²⁺ ion from iron crystal surface into the thin Fe₃O₄ film.
The Fe₃O₄ film is rapidly dissolved in the solution by the presence of Cl^- or SO^2-₄, and the surface reactions of pure iron without oxide film are estimated from the observed potentials as follows:
(1) 0<pH<2;
Fe+CrO^2-_4aq+8H⁺_aq=Fe³⁺_aq+Cr³⁺_aq+4H₂O
(2) 2<pH<4;
Fe+CrO^2-_4aq+3H⁺_aq=FeOH²⁺_aq+CrO^-_2aq+H₂O
The dissolution reactions and their electrode potentials of pure iron in chromate solutions depend on the structures of both chromic ion and ferric ion species.