Mechanistic insights into oxidation and carbon transport in 316 H stainless steel under creep in high pressure CO/CO2 environment

The influence of applied mechanical stress on oxidation and carburisation behaviour in a high-pressure CO/CO 2 gaseous environment, representative of Advanced Gas-cooled Reactor (AGR) conditions, was investigated using Type 316H stainless steel. Specimens were subjected to varying stress levels ranging from 210 to 350 MPa for exposure durations between 62 and 1000 hours. A duplex oxide structure was formed and characterised, with particular emphasis on the development of the inner oxide layer. Stress levels exceeding a threshold of approximately 290 MPa promoted enhanced grain boundary oxidation, whilst higher stresses also induced localised oxide instability within grains, attributed to creep-driven microstructural degradation. Quantitative carbon concentration profiles, obtained via EPMA from cross-sectional analyses, revealed elevated peak carbon levels and fluxes under active stress, while the depth of carbon penetration remained largely unaffected. These profiles were fitted using diffusion–trapping models, indicating that stress enhanced carbon flux primarily through increased trapping rather than changes in diffusivity. This behaviour was attributed to dislocation structures and grain boundary cavitation acting as dominant trapping sites. An engineering model was developed to describe Vickers hardness (Hv) elevation and carbon uptake as functions of stress and time, providing a mechanistic framework for stress-assisted degradation in AGR environments. • Stress fundamentally alters both oxidation and carburisation mechanisms, promoting grain boundary oxidation and carbon segregation and evidence of intragranular oxidation and enhanced bulk carburisation through creep‑induced cavities and dislocation networks. • Carburisation becomes dominated by stress‑assisted carbon trapping, rather than enhanced bulk diffusion, leading to severe near‑surface hardening without an increase in penetration depth. • A predictive master equation linking stress, carbon concentration, hardness, depth, and exposure time has been proposed, providing a practical tool for life assessment of AGR components.