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.
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.
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.
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.
