Effects of Tin, Antimony, Arsenic and Tellurium on the High Temperature Oxidation of Carbon Steel

The oxidation behaviours of carbon steels containing 0.01∼0.5%Sn, Sb, As and Te at 700°C in air were studied by using a thermo-balance, EPMA, X-ray diffraction and optical microscope. The results obtained are summarized as follows: (1) Small additions of Sn, Sb, As and Te considerably improve the oxidation resistance of carbon steel, their effects remaining unaltered with higher concentrations of the additives than a certain level. This is not due to the formation of a protective oxide film containing much of these elements. (2) These elements are enriched in the metal phase near the metal-oxide interface, as can be expected from the free energies of formation of their oxides. (3) The oxide scales formed consisted of layers of hematite, magnetite, wustite and wustite+fayalite, as is also found in the oxidation of a pure Fe-Si alloy. Sn, Sb, As and Te additions were found to change the nature of the innermost oxide layer, i.e. wustite+fayalite. Since the diffusion of the reacting atoms through this layer determines the overall oxidation rate, the protective effect of these elements can be explained to be brought about by the change of the nature of this layer.