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 alloy s (CRAs) and sulfide stress cracking (SSC) of high-strength, low-alloy steels. Several SCC tests using a variety of stress application techn iques showed the brine containing thiosulfate exhibited similar severi ty to brine containing H2S in regard to SCC when plastic strain was ap plied to the CRAs. Materials that exhibited SCC susceptibility in brin e containing thiosulfate agreed well the SCC susceptibility of those i n brine containing H2S. Types 304 (UNS S30400) and 316L (UNS S31603) s tainless steels and duplex stainless steel exhibited in both environme nts. However, high-nickel austenitic alloys such as alloys 904L (UNS N 08904) and 825 (UNS N08825) did not. A 10(-3) to 10(-2) mol/l S2O32- a ddition 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-a lloy steels in a 10(-3) mol/l S2O32- + 5% NaCl + 0.5% acetic acid solu tions were close to those in NACE Standard TM0284-86 solution (substit ute ocean water saturated with 0.1 MPa H2S). Results suggested the pos sibility of using thiosulfate ion as an alternative to H2S.