<p>This study examined the high-temperature air oxidation and liquid sodium corrosion of 316H steel at 700&#xa0;℃. The surface and cross-sectional morphology and microstructure of the specimens after the high-temperature testing were examined by multiple characterization techniques. When exposed to high-temperature air, the steel forms a duplex oxide scale with an outer layer dominated by Fe₂O₃ and an inner layer rich in (Fe, Cr)-spinel. Over time, the oxide scale becomes more continuous and compact, improving oxidation resistance without peeling or cracks even after 1500 h. Sodium corrosion results in a dense oxide scale, with a slight increase in thickness near grain boundaries, forming a Cr₂O₃/NaCrO₂ layer that protects the matrix but leads to a Cr-depletion zone, reducing long-term sodium corrosion resistance. The low Cr and high C content in the steel prevent the formation of σ phase but lead to the precipitation of Cr-rich carbides, potentially compromising long-term corrosion resistance and mechanical properties.</p>