Corrosion Mechanisms of Candidate Alloys in SCWG of Biomass: Influence of Feedstock Chemistry and Catalysts

Abstract Supercritical Water Gasification (SCWG) offers a promising thermochemical pathway for hydrogen production from wet and lignocellulosic biomass. However, the extreme conversion conditions attributed by high temperatures and pressures, aggressive byproducts and catalysts, and reducing environments pose severe corrosion challenges to reactor materials. In this study, we investigate the corrosion behavior of selected candidate alloys under various SCWG conditions using model compounds, with and without alkaline catalysts such as NaOH. Experimental findings reveal that corrosion is significantly influenced by the synergistic effects of biomass-derived species, catalytic activity, and gas-phase reactions. Notably, UNS N06625, typically resistant to supercritical water, undergoes accelerated degradation under SCWG due to the presence of phenolic intermediates and reducing gases (H2 and CO), which may damage the protective Cr2O3 layers and other oxide layers that initially formed on the surface of alloys. Lignin-rich feedstocks induce pronounced sulfidation via H2S generation, while alkaline catalysts modulate corrosion by altering gas speciation and promoting protective carbonaceous deposits. This work provides mechanistic insights into SCWG corrosion pathways, highlighting the critical roles of feedstock composition, catalyst type, and evolving gas environments. The results suggest the need for proper materials selection and corrosion mitigation strategies to ensure the long-term safe operation of SCWG reactors for sustainable hydrogen production.