Technical Note: Characterization of Corrosion Films Formed on Austenitic Stainless Steel in Supercritical CO₂ Containing H₂O and O₂

Future technologies require structural alloys resistant to corrosion in supercritical CO₂ (sCO₂) fluids containing impurities such as H₂O and O₂. Traditional pipeline steels are potentially unsuitable for these environments and more corrosion resistant alloys such as stainless steels might be required. Little is known about the corrosion products formed on, and hence the processes which control corrosion of, stainless steels in impure sCO₂ environments. In this study, austenitic stainless steel 347H (UNS S34709) was exposed to sCO₂ containing H₂O and O₂ at 8 MPa and 50°C or 250°C, and separately to the aqueous phase in equilibrium with the sCO₂ at 50°C, to simulate conditions expected in sCO₂-based power cycles and carbon capture and storage pipelines. Only thin (<20 nm) surface films formed after 500 h resulting in small mass changes and corrosion rates <10⁻⁴ mm/y, suggesting the material resists significant degradation in these environments. X-ray photoelectron spectroscopy and transmission electron microscopy were used to characterize the corrosion films in detail. Exposure to the aqueous phase resulted in a thin (<5 nm) Cr-oxide and/or -hydroxide passive film, while exposure to sCO₂ phases resulted in a multilayer Fe-rich oxide structure characteristic of a gas-phase oxidation process.