Stress corrosion cracking of Alloy X-750 in a CO-CO₂reducing gas mixture

Primary water stress corrosion cracking (PWSCC) of Alloys 600 and X-750 is believed to occur via a preferential intergranular oxidation (PIO) mechanism, in which only the alloying elements are oxidized, leaving Ni metallic. Superheated H₂-steam has been used as an accelerated environment for PWSCC studies, supported by comparable Arrhenius activation energies. Since the PIO model does not require an aqueous solution, this study investigates the occurrence of PIO and SCC in Alloy X-750 exposed to a 480 °C CO–CO₂ mixture. Post-experiment nanoscale characterization of fractured samples revealed characteristics consistent with PWSCC, including PIO and Ni enrichment, accompanied by cavity formation.