Oxidation Studies of Alloy 600 in Low Pressure Hydrogenated Steam

The effect of stress on the oxidation behavior of solution-annealed Alloy 600 was investigated by exposing samples to low pressure steam environments containing hydrogen at temperatures up to 480°C. The resultant oxidized surfaces of stressed and unstressed samples were analyzed via several electron microscopy techniques. For hydrogen partial pressures greater than that corresponding to the Ni/NiO equilibrium, the strain-free surface of Alloy 600 was characterized by the presence of Ni nodules. Detailed Analytical Electron Microscopy (AEM) evaluations of the near-surface regions and grain boundaries were performed on site-specific cross-section specimen produced using the focused ion beam (FIB) technique. High resolution scanning transmission electron microscope (STEM) – energy dispersive x-ray (EDX) microanalysis revealed discrete subsurface oxides, as opposed to a continuous oxide film, thus supporting an internal oxidation process. Furthermore, preferential grain boundary oxide penetration was observed. Although no macroscopic correlation between the magnitude of the residual stress and the surface morphology was detected, local stress measurements using a recently developed FIB micro-hole drilling technique revealed residual stress variations at microscopic level. These results correlated well with the presence of intergranular and internal subsurface oxide.