D. Schmidt, M.C. Galetz, M. Schütze
in "Proceedings of Corrosion 2012" (2012), NACE
Modern heat resistant ferritic-martensitic steels are of great interest as superheater materials in fossil fuel power plants or as material for interconnectors in solid oxide fuel cells. The environments of such applications contain high amounts of water vapor, which is known to promote the formation of the volatile chromium species CrO2(OH)2 leading to insufficient oxidation resistance of 9% Cr-steels in such atmospheres. Results pertaining to the enrichment of manganese and chromium in metal subsurface regions without altering the bulk phase are presented. The formation of protective scales during oxidation to suppress the evaporation of chromium oxy hydroxide in water vapor containing environments was achieved. Reference oxide samples were prepared to investigate the kinetics of oxidation of chromia, manganese oxide and MnCr2O4-phase without an influence of the substrate material. The diffusion treatment developed was based on thermodynamic considerations for the design of the pack cementation process to reach different compositions in the enriched diffusion zone. The improved oxidation behavior was illustrated by oxidation experiments in an environment with water vapor (1% O2 - 10% H2O - N2) at temperature of 650 degrees C.
