Inhibitory Effect of Boric Acid on Intergranular Attack and Stress Corrosion Cracking of Mill-Annealed Alloy 600 in High-Temperature Water

The inhibitory effect of boric acid (H₃BO₃) on the intergranular attack and stress corrosion cracking (IGA/SCC) propagation of steam generator (SG) tubing was studied under accelerated test conditions. Based on an analysis of stress intensity factors at IGA/SCC crack tips, notched C-ring tests were carried out to evaluate the effect of stress intensity and H₃BO₃ on the IGA/SCC crack propagation. Auger electron spectroscopy (AES) and alternating current (AC) impedance measurements were also conducted to clarify the inhibitory effect of H₃BO₃. Notched C-ring test results indicated that IGA/SCC crack velocity of mill-annealed Alloy 600 (UNS N06600) increased gradually with increasing stress intensity factor in the range from 4 MPa√m to ~26 MPa√m, which could apply to IGA/SCC crack tips of actual SG tubes under pressurized water reactor (PWR) secondary conditions. IGA/SCC crack velocities were lower in nearly neutral solutions than in alkaline or acidic solutions. Adding H₃BO₃ slightly retarded the crack velocity in both volatile treatment (AVT) water and caustic solutions. Furthermore, AES analysis revealed that boron was incorporated into the oxide films formed in alkaline and acidic solutions. AC impedance studies showed that the polarization resistances of oxide films formed in H₃BO₃ solutions were higher than those of films formed in alkaline and acidic solutions. Good agreement was obtained between the IGA/SCC inhibitory effect of H₃BO₃ and the formation of the stable oxide films containing boron.