On the Relative Fitness-For-Service of Alloy 925 Compared to Stainless Steels 22Cr and 25Cr in CCUS Well Completions

Abstract Alloy 925 (UNS N09925) is a well-established Ni-Cr-Mo alloy in completion equipment that combines an attractive minimum yield strength of 110 ksi (758 MPa), proper general and localized corrosion resistance, and a strong resistance to environmentally assisted cracking (EAC). Despite its greater cost, its use in equipment surpasses that of 22Cr duplex (UNS S32205) and 25Cr super duplex (UNS S32750) stainless steels. For underground CO2 storage, where trace impurities may lead to strong oxidizing and/or acidic conditions, questions arise about the corrosion performance of Alloy 925 versus 22Cr and 25Cr stainless steels. Concerns apply to general and localized corrosion, and the necessity for higher-grade Ni-Cr-Mo alloys. This study (1) evaluates the performance of Alloy 925, 22Cr, and 25Cr through eight autoclave tests at 70°F (21°C), 175°F (79°C), and 250°F (121°C) in environments with traces of SO2, NO2, H2S, O2, and at least 300 ppmv H2O, and (2) supplements these findings with immersion tests in inorganic acids. Results indicate that under oxidizing conditions, such as those involving O2 or elemental sulfur (S8), derived from gas impurities, Alloy 925 experiences slightly more corrosion than the stainless steels. However, this corrosion remains negligible, confirming the general fitness-for-service of Alloy 925 in CO2 sequestration.