Advanced Characterization of Oxidation Processes and Grain Boundary Migration in Ni Alloys Exposed to 480 °C Hydrogenated Steam

Advanced electron microscopy and surface science techniques were applied to characterize inter- and intragranular oxidation in Ni–Fe–Cr alloys after exposure to 480 °C hydrogenated steam. Intragranular internal Fe and Cr oxidation was observed in all cases while intergranular oxidation, exclusively external or penetrative, varied depending on the Cr content of the alloy. The kinetics and morphology of intragranular internal oxidation and nodule growth were studied through successive short-term exposures with characterization performed between exposures. FIB 3D serial sectioning was used to reconstruct volumes containing oxidized grain boundaries and revealed that diffusion-induced grain boundary migration may play a fundamental role in increasing the outward flux of Cr, Ti, and Al near grain boundaries, depending on the extent of intergranular Cr carbide precipitation. In addition, atom probe tomography was used to study the behaviour of minor impurity elements, Al and Ti, and initial oxidation processes. Further analyses of oxidized samples using three-dimensional ToF-SIMS are also discussed.