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

摘要

MCrAlY coatings can be corroded due to the basic fluxing (Type-I corrosion) of suppose-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 a sodium-potassium sulphate environment with and without SO2 (500 ppm) in air with 100-900 °C thermal cycling up to about 500 hours. The aim was to test the effect of SO2 at the typical Type-I-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, a mixed oxide layer, consisting of alumina and other oxides and sulphides, after a certain cycles in the sulphate environments with and without SO2 gas. In this coating, the addition of SO2 in the sulphate environment changed the corrosion mode from the basic fluxing of alumina to the sulfidation of Cr.