The mechanism of spontaneous dissolution of the air-formed oxide film on iron in a deaerated neutral phosphate solution

Abstract

Pure iron was oxidized in air at room temperature for 3–14 days and was then immersed in a deaerated 0.1 mol l^-1 phosphate solution at pH 7.0. The corrosion potential E_corr and the rate of dissolution of the oxide were measured as functions of the immersion time. It is proposed that the oxide film formed in air, which consists of an outer γ-FeOOH layer and an inner Fe₃O₄ layer, dissolves according to the following cell reactions: an outer cathodic reaction γ-FeOOH+H₂PO₄^-+3H⁺+e→FeH₂PO₄⁺+2H₂O with E°=1.049 V, an inner cathodic reaction Fe₃O₄+3H₂PO₄^-+8H⁺+2e→3FeH₂PO₄⁺+4H₂O with E°=1.177 V, and the anodic reactions Fe+2H₂O→Fe(OH)₂+2H⁺+2e with E°=-0.104 V and/or 3Fe+4H₂O→Fe₃O₄+8H⁺+8e with E°=-0.085 V in which the cathodic reaction determines E_corr. The cathodic reaction occurs at the oxide-solution interface by accepting electrons transported through the oxide, and the anodic reaction occurs at the metal-oxide interface to form oxide by reacting with OH^- and/or O^2- ions migrating through the oxide layer. Thus the proposed mechanism is rather different from the local action cell model. After the dissolution of the oxide film, the iron dissolves according to the following cell reactions: Fe+H₂PO₄^-→FeH₂PO₄⁺+2e with E°=-0.505 V and 2H⁺+2e→H₂ with E°=O V, where the anodic reaction determines E_corr.