Hot corrosion of MCrAlY coatings in sulphate and SO2 environment at 900 °C: Is SO2 necessarily bad?

• MCrAlY coatings are tested in molten sulphate environments with or without SO2.
• Without SO2, the high-Al coating is more resistant than the low-Al coating.
• SO2 is beneficial against the fluxing of alumina scale in the high-Al coating.
• SO2 stimulates sulphidation of Cr in the low-Al coating with molten sulphate.

MCrAlY coatings can be corroded due to the basic fluxing (type-I hot corrosion) of the supposed-to-be protective alumina scale in a molten sulphate environment. In this study, two MCrAlY coatings, coating A (10 wt.% Al, 20 wt.% Cr) and coating B (7 wt.% Al, 28 wt.% Cr), were tested in 48-hour cycles at 900 °C in a sodium–potassium sulphate environment with and without SO2 (500 ppm). The aim was to study the effect of SO2 at the typical type-I-hot-corrosion temperature—900 °C. The results showed that the corrosion behavior of the MCrAlY coatings depended not only on the coating composition but also on the corrosion environment. It was found that in coating A alumina scale was more resistant in the sulphate-plus-SO2 condition than that in the sulphate-only condition. Such phenomenon indicated a beneficial effect of SO2. On Coating B, however, mixed oxide layers, consisting of alumina and other oxides and sulfides, formed after a certain number of cycles in the sulphate environments with or without SO2 gas. In this coating, the addition of SO2 in the sulphate environment promoted the formation of non-dense chromia oxides which may form from oxidation of Cr-sulfides.