Effect of material composition on stress accelerated grain boundary oxidation in a high-temperature sulfuric acid environment

Hirota, Noriaki  

In a high-temperature sulfuric acid environment, Alloy600 showed a high corrosion rate of 0.40 mm/year, while Alloy800H and 3Al-Ferrite exhibited much lower rates of 0.03 to 0.01 mm/year. Four-point bending tests revealed the stress to reach 0.2 % yield stress followed the order: 3Al-Ferrite Alloy600 Alloy800H. Bending strain at 80 % of 0.2 % yield stress was highest in Alloy800H and lowest in 3Al-Ferrite, indicating its low deformability. Post-corrosion microstructural analysis revealed that Alloy800H and 3Al-Ferrite formed thin oxide films with no cracks, whereas Alloy600, which developed a thick multilayered oxide film composed of Ni, Fe and Cr, exhibited open cracks. Electron Backscattered Diffraction (EBSD) and Grain Reference Orientation Deviation (GROD) maps confirmed intergranular crack growth and residual tensile stress in Alloy600. These findings indicate that Alloy600, primarily composed of Ni and Cr, formed a spallation and degradation oxide film and subsequently generated oxides that weakened the grain boundaries, thereby promoting the occurrence of Stress Accelerated Grain Boundary Oxidation (SAGBO).