Effects of thiosulfate on susceptibility of type 316 stainless steel to stress corrosion cracking in 3.5% aqueous sodium chloride

Effects of thiosulfate on stress corrosion cracking (SCC) susceptibility (S) of type 316 (UNS S31600) stainless steel (SS) were studied. Addition of sodium thiosulfate (Na2S2O3) to 3.5% sodium chloride (NaCl) increased 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 revealed the fracture mode changed from ductile to brittle for S > similar to 0.2. An increase in potential in the noble direction increased S significantly. An increase in Na2S2O3 concentration increased S at higher potentials but had no tangible effect around the free-corrosion potential (similar to 100 mV(NHE)). Na2S2O3 also shifted the pitting potential of type 316 SS to more active values and led to formation of wider and more open pits.