Intergranular attack and stress corrosion cracking (IGA/SCC) has been experienced within crevices between tubes and tube support plates and within crevices between tubes and tube sheets of steam generators (SG) in pressurized-water reactor (PWR) plants. IGA/SCC has resulted in plant shutdowns caused by leakage and has extended outages for plugging and sleeving of a large number of tubes. Various remedial actions, such as boric acid (H3BO3) injection, Na/Cl molar ratio control, and highhydrazine (H2NNH2) operation, have been applied to PWR plants.1 In particular, H3BO3 injection has been assumed to exert an inhibitory effect on IGA/SCC initiation of mill-annealed alloy 600 (alloy 600MA, UNS N06600),(1) and the effect of H3BO3 in high-temperature alkaline solutions has been investigated by many researchers.2-3 In addition, IGA/SCC initiation has been controlled by H3BO3 injection at Japanese power plants. However, the propagation of IGA/SCC has not been suppressed adequately, and the inhibitory mechanism has not been clarified. The stress intensity factor (K) at the crack tip and the environmental conditions are considered key parameters in IGA/ SCC propagation. The emphasis of the present study was clarification of the IGA/SCC propagation behavior. The effects of K at the crack tips on the IGA/SCC propagation behavior of SG tubes were studied using the double-cantilever beam (DCB) and SG model boiler test methods. Auger electron spectroscopy (AES) was
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 MPam to 26 MPam, 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
steam cracked Key Generator
Intergranular attack and stress corrosion cracking (IGA/SCC) have been experienced within crevices between tubes and tube support plates, and within crevices between tubes and tubesheets of steam generators (SG) in pressurized water reactor (PWR) plants. IGA/SCC has resulted in plant shutdowns due to leakage and extended outages for plugging and sleeving large numbers of tubes. Various remedial actions, such as boric acid (H3BO3) injection, sodium chloride (NaCl) molar ratio control, and a high-concentration hydrazine (H2NNH2), have been applied to PWR plants.1 In particular, H3BO3 injection has been assumed to exert an inhibitory effect on IGA/SCC initiation of mill-annealed Alloy 600 (Alloy 600MA; UNS N06600)(1). The effect of H3BO3 in high-temperature alkaline solutions has been investigated by many researchers.2-3 It is reported that IGA/SCC initiation has been adequately controlled by H3BO3 injection.1-3 However, the inhibitory effect of H3BO3 on IGA/SCC initiation and the inhibitory mechanism on IGA/SCC propagation have not been clarified as of yet. The stress intensity factor at the crack tip and environmental conditions are considered the key parameters in IGA/SCC propagation.
A high-temperature slow strain-rate facility was developed to evaluate the stress corrosion resistance of alloys considered for the construction of liquid metal fast breeder reactor (LMFBR) steam generators. The primary concern was stress corrosion that might occur on the water side during steam generator operation in locally faulted caustic environments. Most of the studies were performed at 316C in 5 or 10 percent sodium hydroxide (NaOH) solutions with 2Cr-1Mo steel and Incoloy-800.
Unlike 2Cr-1Mo, Incoloy-800 was susceptible to caustic cracking at 316C. The susceptibility depended on the metallurgical condition and to some extent, the oxidizing potential. The cold-worked and Grade 1 conditions exhibited excellent resistance to caustic cracking in the slow strain-rate test. However, limited studies in the straining electrode test showed the Grade 1 condition could be readily cracked at anodic oxidizing potentials.
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