Interfacial Tensions and Critical Surface Tensions of Stainless Steels in a Dense CO₂ Atmosphere

Interfacial properties of stainless steel/CO₂ systems are critical to several emerging applications, including geological carbon sequestration, enhanced oil recovery, and development of supercritical CO₂ processes. However, these interfaces have been rarely investigated, particularly under high-pressure and high-temperature conditions. This study aims to determine the critical surface tension (γ_C) and interfacial tension (γ_SF) at the stainless steel/CO₂ interface. Hence, γ_C and γ_SF were determined using Zisman plots and Good's theory, respectively, with the latter requiring the calculation of a molecular interaction parameter. Measurements were carried out on two stainless steels (316 and 316L) under a dense CO₂ atmosphere. The experimental conditions covered pressures from 0.1 to 15.1 MPa and temperatures of 313 and 333 K, representative of the aforementioned applications. Both γ_C and γ_SF decreased with increasing pressure, while the influence of the temperature exhibited more complex trends. This study highlights a correlation between interfacial properties and the thermodynamic state of CO₂, as described by the Widom line, which marks the transition between gas-like and liquid-like regimes in supercritical fluids. Finally, the results show similar values for 316 and 316L, in good agreement with a previous study conducted on 303 stainless steel, supporting the hypothesis that average values can be extended to austenitic stainless steels in general.