Decarburization Behavior of Fe–Cr–Ni Alloys under High-Temperature Hydrogen Exposure

The decarburization behavior of Fe–Cr–Ni alloys containing small amounts of carbon was investigated under static high-temperature hydrogen exposure, covering a wide compositional range from ferritic and austenitic stainless steels to Ni-based alloys. Under hydrogen exposure at 800 °C, a clear compositional dependence was observed: decarburization was negligible in the ferritic alloy, whereas the austenitic alloys exhibited noticeable decarburization with a distinct dependence on Ni content. This behavior was rationalized by considering both carbon diffusivity and the formation of an oxide barrier, whose protective effect weakens with increasing Ni content. The results provide fundamental insights into the thermodynamic and kinetic factors governing decarburization in Fe-Cr-Ni alloys, offering a guideline for material selection in hydrogen-containing high-temperature environments.