EFFECTS OF THIOSULFATE ON SUSCEPTIBILITY OF TYPE-316-STAINLESS-STEEL TO STRESS-CORROSION CRACKING IN 3.5-PERCENT AQUEOUS SODIUM-CHLORIDE

Effects of thiosulfate on stress corrosion cracking (SCC) susceptibili ty (S) of type 316 (UNS S31600) stainless steel (SS) were studied. Add ition of sodium thiosulfate (Na2S2O3) to 3.5% sodium chloride (NaCl) i ncreased S of type 316 SS to SCC. A quantitative expression for S was given by: [GRAPHICS] where sigma(max) and t(f) are the maximum stress and time to failure, respectively. Metallographic observations reveale d the fracture mode changed from ductile to brittle for S > similar to 0.2. An increase in potential in the noble direction increased S sign ificantly. An increase in Na2S2O3 concentration increased S at higher potentials but had no tangible effect around the free-corrosion potent ial (similar to 100 mV(NHE)). Na2S2O3 also shifted the pitting potenti al of type 316 SS to more active values and led to formation of wider and more open pits.