2275.     MICHEL, A., BENARD, J., AND CHAUDRON, G.  [Mechanism of the Superficial Oxidation of Iron.]  Bull. soc. chim., vol. 1, 1944, pp. 175-179; Chem. Abs., vol. 40, 1946, p. 2370.

        Specimens of electrolytic Fe were heated at temperatures from 250°-900° in air.  The structure of the separated oxide scale was determined by use of the Debye-Scherrer method of X-ray diffraction and the thermomagnetic method of analysis.  At 250°, the oxide consists largely of γ Fe₂O₃ and a small quantity of Fe₃O₄.  The reaction mechanism is given by the equation Fe→Fe₃O₄→γ-Fe₂O₃.  In the interval of 400°-600° variable mixtures of γ Fe₂O₃ and Fe₃O₄ are formed.  The mechanism is given by the equation Fe→Fe₃O₄→γFe₂O₃→a-Fe₂O₃.  Above 600° FeO is the major component.  At 900° after 3 hr. of oxidation and a rapid cooling the oxide consists largely of FeO.  A small quantity of Fe₃O₄ is found in the superficial layer and a film of a-Fe₂O₃ on the surface.  The mechanism of oxidation is Fe→FeO→Fe₃O₄→a-Fe₂O₃.  The FeO is assumed to decompose on cooling by the reaction 4FeO→Fe₃O₄+Fe.  It is concluded that both Fe and O₂ diffuse in the oxidation reaction.  The distribution of reaction products is determined by the magnitude of each process.