Influence of thiosulfate on the pitting behavior of candidate container materials for the disposal of HLW in Boom clay

The localized corrosion behavior of austenitic stainless steels UNS S31603 and UNS N08904 was evaluated in solutions containing chloride, sulfate, and thiosulfate at near-neutral pH simulating deep geological clay environments. These stainless steels are considered as candidate container materials for the final disposal of high-level nuclear waste for the Belgian repository concept in an underground clay environment. Cyclic potentiodynamic polarization measurements were used as an electrochemical technique to compare the pitting behavior of the tested stainless steels. The experiments were conducted at 16 and 90°C in oxidized synthetic interstitial Boom clay water with varying sulfate concentrations of 216 and 5400 mg/L, chloride concentrations ranging from 100 to 10000 mg/L, and thiosulfate concentrations ranging from 2 to 200 mg/L. The surface of each specimen was investigated by Optical Microscopy and Scanning Electron Microscopy after electrochemical testing. Pitting is initiated more easily with increasing temperature. Increasing the chloride concentration lowers E_np for both stainless steels drastically. UNS N08904 type stainless steel is less susceptible to pitting in solutions up to 1000 mg/L CI'. UNS N08904 type stainless steel has a higher resistance to pitting in S₂O₃^2-containing solutions. The values of E_np are -200 mV more noble for UNS N08904 type stainless steel. Thiosulfate only influences the initiation process of pitting, not the growth kinetics. The formed pits have a rose-like appearance due to subsurface growth.