Impact of Impurity Ions in Secondary Circuit Water on Cracking Behaviour of PWR Steam Generator 690TT Tube

Abstract The impact of impurity ions on stress corrosion cracking (SCC) sensitivity of steam generator heat-exchange tube material Alloy 690TT in secondary circuit water (SCW) was studied at 285 °C. The pH of the test solution was adjusted using ammonia to be about 9.7. The dissolved oxygen concentration is less than 1 μg/kg, and the ammonia concentration is 50 to 100 μg/L. A slow strain rate test (SSRT) was employed to evaluate the SCC sensitivity in SCW with various types and concentrations of impurity ions. After the SSRT test, the fracture surfaces of the specimen were observed and analyzed using scanning electron microscope (SEM). In SCW environments containing 3 g/L Cl − or 5 g/L SO 4 2− , local intergranular cracks were found at the fracture surface edges of the specimens, indicating a certain sensitivity to SCC. When the concentration of SO 4 2− was reduced to 5 mg/L, the intergranular crack characteristics of the Alloy 690TT became less apparent, suggesting a high resistance to SCC under low SO4 2− concentrations.