STRESS-CORROSION CRACKING OF SUS316L STAINLESS-STEEL IN THE CHLORIDE SOLUTION CONTAINING THIOSULFATE ION BY THE SLOW STRAIN RATE TECHNIQUE

Stress corrosion cracking (SCC) behavior of SUS316L stainless steel in the chloride solution containing thiosulfate has been investigated by using a slow strain rate technique (SSRT). The susceptibility of SCC was found to depend on the strain rate, the concentration of thiosulfate and chloride, and pH. The highest susceptibility was obtained at the condition of 20 mass%NaCl containing 10(-2) kmol.m-3 Na2S2O3 at pH 5.3 and 2.2 X 10(-7) s-1. Observations of fractured specimens showed that cracks perpendicular to the tensile axis generated at the pits. The initial fracture mode of the crack was intergranular, followed by transgranular and dimple with propagation of the crack. Intergranular fracture initiated from intergranular corrosion on the bottom of the pit, in which pH decreased less than 2. Immediately after immersion, pits generated easily because the dissolved oxygen with thiosulfate rapidly raise free corrosion potential upto pitting potential in cooperation with the action of thiosulfate to lower the pitting potential. Decrease in pH caused lowering of pitting potential and increase of the number of cracks, while the highest SCC susceptibility was obtained at pH 5.3. At conditions showing the high SCC susceptibility, corrosion potential was always in the range between -400 and -370 mV (Ag/AgCl). Therefore, the SCC behavior in this system is strongly controlled by the electrochemical factors.