Influence of Alloy Grain Size on High Temperature Corrosion Behavior of 2.25 Cr-1 Mo Steel in SO2+O2 Atmosphere

The investigation was primarily aimed to study the influence of grain size on high temperature corrosion behavior of 2.25 Cr-1 Mo steel in SO2+O-2 (ratio 2: 1) environment. The various grain sizes (18 mu m, 26 mu m, 48 mu m, and 72 mu m) were obtained by different annealing treatment. Isothermal corrosion studies were carried out in different grain size specimens at 923 K for 8 hours. The corrosion film growth rate and the reaction kinetics were studied by weight gain method. The external scales of the post corroded specimen were characterized in Scanning Electron Microscope (SEM), Energy dispersive Spectroscopy (EDS) and X-Ray Diffraction analysis (XRD). Finally, it is concluded that the corrosion rate of 2.25 Cr-1 Mo steel strongly depends on grain sizes of the specimens. The corrosion rate increases with the decreases in grain size. The finer grains (18 mu m) show higher corrosion rate than the coarse grains (72 mu m). The reaction kinetics follows the parabolic growth rate. The scale analysis of finer grain size specimen (18 mu m) shows the thicker scale and extensive scale cracking and spallations, whereas the coarse grain specimen (72 mu m) shows adherent and compact scale without any scale cracking and spallations. The mechanism of the corrosion has been explained on the basis of scale characterizations in SEM, EDS and XRD for different grain sizes.