Corrosion behavior of 9Cr steel tube in ultra-supercritical power plants under the sulfidation-oxidation coupling effect

Purpose This study aims to elucidate the high-temperature fireside corrosion behavior of T92 steel reheater tubes in ultra-supercritical power plants under sulfidation-oxidation coupling effects. Design/methodology/approach Samples from T92 steel tubes after long-term service (15,479 h and 19,037 h) at 615 °C in flue gas containing H2S (0.3 vol.%) were analyzed. Microstructure, composition and elemental distribution of corrosion scales were characterized using optical microscope, scanning electron microscope with energy dispersive spectroscopy and X-ray diffraction. Findings The corrosion scale had a triplex structure: the outermost layer composed of sulfides and sulfates (FeS and Fe2(SO4)3), the subouter layer composed of Fe2O3 and the inner layer composed of Cr-rich spinel and sulfides. Despite a significant increase of 3,558 h in service time (∼23% longer exposure), the growth of both the subouter and inner layers was remarkably minimal, with an increase of less than 1.1% in thickness, indicating significant inhibition of oxidation behavior. This was attributed to the sulfur accumulation at the inner-layer/substrate interface (reaching ∼44 At.% locally), coupled with the formation of diffusion-blocking phases such as Laves precipitates in the subscale zone. These microstructural characteristics effectively obstructed the transport pathways of the inward migration of the oxidizing species toward the substrate. Originality/value This study provides quantitative evidence that long-term oxidation inhibition arises from sulfur-induced diffusion barrier formation under sulfidation-oxidation coupling effects and a theoretical basis for the high-temperature component reliability assessment in ultra-supercritical power plants.