Alternative for Evaluating Sour Gas Resistance of Low-Alloy Steels and Corrosion-Resistant Alloys

Thiosulfate ion was used as a substitute for hydrogen sulfide (H₂S) to simulate stress corrosion cracking (SCC) of corrosion-resistant alloys (CRAs) and sulfide stress cracking (SSC) of high-strength, low-alloy steels. Several SCC tests using a variety of stress application techniques showed the brine containing thiosulfate exhibited similar severity to brine containing H₂S in regard to SCC when plastic strain was applied to the CRAs. Materials that exhibited SCC susceptibility in brine containing thiosulfate agreed well the SCC susceptibility of those in brine containing H₂S. Types 304 (UNS S30400) and 316L (UNS S31603) stainless steels and duplex stainless steel exhibited in both environments. However, high-nickel austenitic alloys such as alloys 904L (UNS N08904) and 825 (UNS N08825) did not. A10⁻³ to 10⁻² mol/l S₂O₃²⁻ addition in 20% NaCl aqueous solution at 353 K corresponded to H₂S of 0.1 to 1 MPa at 473 K. The SSC susceptibilities of high-strength, low-alloy steels in a 10⁻³ mol/l S₂O₃²⁻ + 5% NaCl + 0.5% acetic acid solutions were close to those in NACE Standard TM0284-86 solution (substitute ocean water saturated with 0.1 MPa H₂S). Results suggested the possibility of using thiosulfate ion as an alternative to H₂S.