Effects of Tin and Antimony on the High Temperature Oxidation of Iron-Silicon Alloys

The oxidation behaviours of pure Fe, Fe-Si alloys with 0.08∼1.92%Si and Fe and 0.31%Si-Fe alloys containing about 0.05%Sn and Sb were studied at 700°C in air by means of a thermo-balance, X-ray diffraction, spectroscopy, optical microscopy and EPMA. At the same time Pt marker measurements were carried out. The results obtained are summarized as follows:(1) Sn and Sb considerably decrease the oxidation rate of 0.31%Si-Fe alloy, while they have no effect on that of pure Fe.(2) The oxide scales formed on dilute Fe-Si alloys consist of layers of hematite, magnetite, wustite and wustite+fayalite. The composition is not influenced by small additions of Sn and Sb.(3) Pt markers were found on the wustite/wustite+fayalite interface. From this observation it is concluded that oxidizing gases are transported inward in the gas phase across the innermost oxide layer, i.e. wustite+fayalite and oxidize the alloy. The growth of the oxide film on dilute Fe-Si alloys proceeds, therfore, through both the outward diffusion of Fe ions and the inward diffusion of oxidizing gases.(4) Owing to the slower diffusion rate of Fe ions through fayalite than through iron oxides, Si additions improve the oxidation resistance of Fe. EPMA measurements confirmed that the concentration of fayalite in the innermost oxide layer increases with increasing Si content.(5) On the other hand, Sn and Sb additions to 0.31%Si-Fe alloy do not bring about a higher concentration of fayalite, but cause the change of the nature of the innermost oxide layer. Since the diffusion rate of oxidizing gases through this layer probably depends on its nature, e.g. porosity, the protective effects of Sn and Sb can be explained to be due to the change of its nature.