Insights into chloride-induced corrosion mechanisms of iron-based and nickel-based alloys in supercritical water: integrating high-temperature corrosion experiments, machine learning and first-principles calculations

Severe corrosion of reactor materials is a great obstacle to the widespread application of supercritical water gasification (SCWG) of biomass for hydrogen production. This study elucidates the chloride-induced corrosion mechanisms of iron-based and nickel-based alloys in supercritical water (SCW) by integrating machine learning (ML) prediction, experimental investigation and density functional theory (DFT) calculations. SHAP analysis based on Gaussian Process Regression (GPR) identifies chloride concentration as the most influential environmental factor, while Cr, Fe, Ni, and Mo are the dominant alloying elements governing the corrosion behavior. Combining experimental investigation with DFT calculations, we selected three representative candidate materials to further elucidate the mechanisms by which these key factors influence the corrosion behavior. The findings reveal that increasing chloride concentration accelerates general corrosion and pitting corrosion, particularly in 316SS. Chlorine adsorption reduces the affinity between O and the alloy surface and intensifies the depletion of surface electrons, thus promoting selective dissolution of metal and inhibiting protective film formation. Meanwhile, the accumulation of metal cation vacancies at the matrix/passive film interface further promotes passive film rupture. Among the three materials, Inconel 625 shows superior corrosion resistance primarily owing to the synergistic effect between Cr and Mo. This synergy mainly arises from the pronounced O-p and Cr-d hybridization which promotes the development of a stable Cr-rich passive film. Additionally, the strong Mo-O interactions facilitate the formation of Mo-containing oxides which are stable in chloride. Conversely, the limited contribution of Ni to corrosion resistance is mainly due to the chloride-induced Ni depletion in the outer oxide film.