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

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 Ecorr 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 Ecorr. 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 O2- 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 Ecorr.