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. A 10(-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.