Effect of Impurity Sulfur on the Formation of Cr₂O₃ and SiO₂ at the Early Stage of Steam Oxidation in both Ferittic and Austenitic Steels

The dependence of steam oxidation resistance on the sulfur content has been investigated systematically in both ferritic and austenitic steels. It is found that the presence of impurity S, being considered as a harmful element in the past, improves significantly the steam oxidation resistance of high Cr steels. However, such a beneficial S effect is not so remarkable in low Cr steels, indicating that the S effect is related strongly to the Cr content in the steels. However, in case of high Cr ferritic steels containing more than 0.3%Si, the S effect is still less remarkable. These results are probably caused by the difference in the affinity of S for Cr atom and for Si atom. Since S has a strong affinity for Cr, Cr is enriched in the vicinity of the segregated-S on the specimen surface even after the steam oxidation test for a short time. The enriched Cr layer obstructs the formation of any island-like Fe oxides on the surface because of the easy formation of the passive Cr ₂O₃ oxide layer on this area. On the other hand, Si segregates onto the iron surface faster than S, but a part of the segregated-Si on the surface is replaced by S. Therefore, it is considered that the formation of SiO₂ is difficult in the case when the S content is too higher in the steel. As a result, the S effect does not appear so significantly in the Si-containing steels.