High-Temperature Oxidation Behaviors of Structural Materials for Very High Temperature Reactors

Abstract

The high-temperature oxidation behavior of 316L stainless steel, Alloy 617, and Incoloy 800H—candidate structural materials for advanced energy systems—was systematically investigated in a dry air environment over a wide temperature range relevant to very high temperature reactor (VHTR) applications. Oxidation kinetics were evaluated using thermogravimetric analysis (TGA), while oxide scale morphology, elemental distribution, and phase evolution were characterized by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD), respectively. The results demonstrate that oxidation behavior is strongly temperature dependent and differs significantly among the investigated alloys. 316L stainless steel exhibits the poorest oxidation resistance, characterized by the formation of porous and poorly adherent Fe-rich oxides (Fe₂O₃ and Fe₃O₄) and severe scale degradation at elevated temperatures. Incoloy 800H shows intermediate oxidation performance, forming a protective Cr₂O₃-based scale at temperatures up to approximately 1173 K; however, this protection deteriorates at higher temperatures due to chromium depletion and the increasing dominance of Fe-rich oxides. Alloy 617 forms a continuous and chromia-dominated oxide scale over a broader temperature range, resulting in lower oxidation rates and improved scale stability under the investigated conditions. Parabolic oxidation rate constants were quantitatively determined, confirming predominantly diffusion-controlled oxidation kinetics within the parabolic regime. Arrhenius analysis revealed distinct apparent activation energies for the three alloys, reflecting differences in the temperature sensitivity of oxidation processes. However, the results also indicate that activation energy alone does not directly represent overall oxidation resistance, which is strongly influenced by oxide scale morphology, adhesion, and phase stability. Based on a combined assessment of oxidation kinetics, microstructural evolution, and phase analysis, the oxidation resistance ranking under the specific dry air and short-term isothermal conditions investigated is determined to be Alloy 617> Incoloy 800H > 316L stainless steel. These results provide a comparative assessment of high-temperature oxidation behavior under controlled dry air conditions.