Advocating for the Integration of Dissolved Sulfur as a Critical Parameter in NACE MR0175/ISO 15156 for Improved Corrosion Severity Evaluation of Ni Alloys

Abstract The recent NACE MR0175/ISO 15156-2020 standards have introduced dissolved H2S concentration as a new sour severity metric alongside the conventional H2S partial pressure. This has been validated through simulations and several laboratory tests. However, our qualification of a sulfur-resistant Ni alloy 716 grade using both conventional low-pressure (2.4 ksi [16.5 MPa]) and field-pressure (15 ksi [103.4 MPa]) lab tests, following NACE TM0177 C-ring testing guidelines, has revealed potential issues when sulfur is present. Despite maintaining consistent sour severity, sulfur severity was found to be five times higher at elevated pressure (15 ksi [103.4 MPa]). A thorough investigation into the fundamental corrosion mechanisms and systematic simulations of the chemical environment across various testing scenarios were conducted. Our study suggests that to accurately reflect real corrosion severity, dissolved sulfur should be included as a key severity metric in the NACE MR0175/ISO 15156 standard for qualifying corrosion-resistant alloys (CRAs) in the presence of sulfur. This work provides the corrosion community with valuable insights into sour corrosion mechanisms and proposes improvements to sour testing methods, facilitating the safe expansion of CRA usage limits in high-temperature, high-pressure (HTHP) sour wells.