Study of the scale formed on super 304H boiler tube steels after long-term steam oxidation at high temperatures

Steam oxidation of a Super 304H boiler tube was performed at steam temperatures of 650 °C and 700 °C for time durations up to 5000 h. The oxidation scale outcomes which formed on as-received untreated and shot-peened (SP) tubes were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). γ-Grain nanocrystallization on the surface of the SP specimen generated an abundant fraction of grain boundary that guaranteed the formation of a continuous amorphous SiO2 layer at the metal-scale interface that enhanced the formation of dense Cr2O3 and FeCr2O4 in the scale. Scale breakaway along defects in the outer layer of the scale was accelerated by the steam temperature in the untreated specimens, whereas the scale that formed on the SP specimens appeared to be stable.