| Abstract Scope |
Long-term steam exposure of Sanicro 25 (X7NiCrWCuCoNb25-23-3-3-2) was investigated to clarify how protective oxidation modifies the adjacent metallic microstructure. After 30,000 h at 700 °C, the alloy developed a continuous Cr-rich scale, about 2.6 µm thick, with local Mn enrichment and no macroscopic spallation. Correlative SEM-EDX, TEM/STEM, EELS, 3D FIB-SEM tomography, and CALPHAD/Prisma simulations showed that scale formation was accompanied by a chromium-depleted subsurface zone extending to approximately 6.5 µm. This local chemical gradient changed the thermodynamic stability of secondary phases: Cr-rich M23C6 carbides and the Nb-Cr-N Z phase became unstable when the matrix Cr content decreased sufficiently. Their dissolution is proposed to contribute to a precipitate-depleted region and local grain reconstruction beneath the oxide/metal. |
