Trans-twins stress corrosion cracking behaviors of Alloy 690TT in lead-containing caustic solution at 330 °C

Highlights: ► Many twins in Alloy 690TT are cracked in lead containing caustic solution at 330 °C. ► Higher strains cause twins to deviate from the ideal microstructures of ∑3. ► Litharge accelerates the dissolution of Cr and Ni in the oxide film and matrix. ► SCC-resistance of twins is reduced by higher strain and the dissolution of matrix. ► High strain and deleterious impurities in the secondary water should be eliminated. - Abstract: Twin grain boundaries (GBs) are found to be inherently resistant to stress corrosion cracking (SCC), which has become one of the main failure mechanisms of steam generator (SG) tubing since the 1980s and brings huge economic losses to the nuclear power plants. As it is a widely used material for SG tubing, the SCC-resistance of the twins in Alloy 690TT in 10 wt.% sodium hydroxide solution with 100 ppm litharge at 330 °C was studied using C-ring samples. The relationship between the crack paths, twin GBs and the residual strains in the studied areas were analyzed using an environmental scanning electron microscope (ESEM) equipped with electron backscatter diffraction (EBSD) equipment. A continuously stressed C-ring sample without immersion was used to evaluate the effect of residual stress or strain on the microstructure of the twin GBs. The oxides at the crack paths were analyzed by an energy dispersive spectroscopy (EDS). The results show that many twin GBs are cracked during crack propagation. There are more twins with large deviations from the ideal ∑3 twin misorientation in the studied area where the residual strain is high. In situ EBSD analyses verify that higher residual strain causes twins to deviate from the ideal twin microrientation and can even promote twins transiting into random high angle grain boundaries, when the residual strain is high enough. The EDS result illustrates that litharge accelerates the dissolution of the chromium and nickel in the matrix. Overall, the SCC-resistance of the twins in Alloy 690TT in the studied solution is reduced by the destruction of the ideal microrientation of the twin GBs and the preferential dissolution of chromium and nickel at the crack paths. Higher residual strain on the Alloy 690TT and deleterious impurities in the circulating secondary water should be eliminated during the operation of nuclear power plants.