Anion-forming Alloying Elements in Metallic Oxidation

Abstract

THE increased resistance of iron to oxidation through the addition of aluminium and silicon is well known, and the effect is marked even at concentrations as low as 1%. Microscopic examination of oxidized iron alloys reveals the formation of multilayered scales¹ (reported by two of us, J. A. von F. and G. A. P., at the Fourth International Congress on Metallic Corrosion, Amsterdam, 1969). Pure iron at 700° C forms a scale consisting only of FeO, Fe₃O₄ and Fe₂O₃. Protected iron alloys do not, however, show FeO formation although multilayer scales are formed and additional phases are shown in all cases (Fig. 1). Electron probe microanalysis clearly demonstrates (Fig. 2) that segregation of the alloying elements occurs within the oxide layer next to the metal-oxide interface. The nature of the oxide layers containing the alloying element is not yet clear. Several workers claim to have detected spinels such as FeAl₂O₄ and Fe₂SiO₄ by X-ray diffraction analysis whereas others suggest mixed oxides are formed between Fe₂O₃ and Al₂O₃ or SiO₂ (refs. 1–7).