Abstract: The oxidation mechanism and characterization of steam-side oxidation scale formed on the internal surface of the super-heater tubes made of T91 steel removed from the boiler of a power plant are studied by using the testing and analyzing methods of metallurgic inspection, scanning electron microscope-energy dispersive X-ray (SEM-EDX) and X-ray diffraction analysis. The results show that based on the section oxygen elemental distribution and the Fe-Cr elemental diffraction rule, the original steam-side oxidation scale normally consists of three layers. The inner layer is heterosphere oxides, which consists of Cr-enriched and Cr-depleted zones appearing alternatively; the middle layer is the loose and porous Fe3O4 with the columnar crystal microstructure; the outer layer is the thin Fe2O3. In the outer layer, there are small gaps among the oxides and the adhesion is weak, which makes it easy for the oxides to flake off during great temperature variation. The middle layer works better than the outer one.

Key words: T91, steam oxidation, oxidation scale, diffraction, oxidation mechanism