ELECTROCHEMICAL STUDIES OF STEAM GENERATOR TUBE DEGRADATION IN THE PRESENCE OF THIOSULPHATE

摘要

Sulphur species in oxidation states between S~(2-) and S~(6+) are known to interfere with the protective oxide films that form on steam generator tubing materials. By assisting in the breakdown of passive films, intermediate oxidation state sulphur species can cause intergranular attack and pitting of steam generator tubing over a wide pH range. Intermediate oxidation state sulphur species have also been observed to induce stress corrosion cracking of sensitized Alloy 600 at low temperatures. This work employed electrochemical methods to investigate the effect of thiosulphate on the degradation of steam generator alloys (Alloy 600, Alloy 690, and Alloy 800). The effect of thiosulphate on steam generator tubing degradation was investigated at 150°C in crevice chemistries containing 0.0015 M sodium thiosulphate simulating the local chemistry environments developed in steam generator crevices or under sludge. The detrimental effect of thiosulphate on the boundary conditions of the recommended ECP/pH zone of Alloy 800 was discussed. Accelerated corrosion tests were also performed at selected potentials to confirm the detrimental effects of thiosulphate on the ECP/pH domain.