Effects of CO and O₂ Impurities on Supercritical CO₂ Corrosion of Alloy 625

Corrosion of alloy 625 was investigated in three supercritical CO₂ (sCO₂) environments: research-grade (RG) (99.999 pct pure), RG + 100 ppm O₂ (at.), and RG + 1 pct CO (at.). Samples were exposed to each condition for a total of 1000 hours at 750 °C and 20 MPa. Each sample was analyzed using mass change, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), time-of-flight-secondary ion mass spectroscopy (TOF-SIMS), and Raman spectroscopy. Samples exposed to RG CO₂ produced a compact uniform chromia layer. The samples exposed to oxygen-rich CO₂ showed a less-uniform oxide with nodules characterized by Ni- and Fe-rich oxides on the surface with enhanced chromia formation beneath. The oxide grown in 1 pct CO exhibited the least protective oxide consisting of a duplex chromia scale with inner equiaxed grain structure and an outer porous layer. RG + 1 pct CO was the only environment in which carbon was observed throughout the oxide, although carbon enrichment was observed at the M-O interface of the oxides produced in RG and RG + 1 pct CO environments. A model for explaining carburization of chromium in sCO₂ environments was produced by treating the oxide as a diffusion barrier and considering the chemical equilibrium of CO₂ through the Cr₂O₃ phase.