Sulphur diffusion through a growing chromia scale and effects of water vapour

Fe-30Cr (wt%) alloy was exposed to Ar-0.5%SO2 and Ar-10%H2O-0.5%SO2 gases at 650 °C, forming Cr2O3 scales. In dry SO2 for 20 h, sulphur enrichment and scattered Cr-sulphides were observed at the scale-alloy interface, together with internal Cr-sulphide precipitates. After 100 h, sulphur disappeared from the interface, while more internal Cr-sulphides were produced. In wet SO2, the sulphur enrichment and Cr-sulphides were maintained in both times, but no internal sulphidation was seen. Atom probe analysis revealed sulphur enrichment on chromia grain boundaries in both gases, to a greater extent in wet SO2. These effects are discussed in terms of chromia grain boundary diffusion.