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Pitting-attack behavior of cold-rolled 321 stainless steel in sodium chlorine solution at 250 oC HAN Yun-tao1, LV Jin-long2, and LUO Hong-yun3
1. College of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, 100191 China (hanyuntao1985@163.com)
2. College of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, 100191 China (ljlhit@126.com)
3. College of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing, 100191 China (luo7128@163.com)
Abstract: Pitting-attack in steam generator tubes is one of the main degradation mechanisms that affect the life of steam generators in pressurized water reactors (PWR). In order to investigate the pitting-attack resistance of cold-rolled 321 stainless steel under severe conditions, a four-point loaded bentbeam stresscorrosion test in a 5 wt.%chloride aqueous solution (pH 8.63 at 25 oC ), with immersing periods of 240 hr and 660 hr, at 250 oC, was carried out. The material microstructure was observed and analyzed with optical microscopy (OM), and the phase structure with X-ray diffraction (XRD). The morphology of the specimens after the immersing test was examined using scanning electron microscopy (SEM), and the corrosion products on the specimens’ surface were analyzed through X-ray photoelectron spectroscopy. The results obtained indicated that the microstructure of the material was composed of equiaxed austenitic grains with typical annealing twins. There was an approximately 10.19 vol.%α’-martensite phase in the austenitic matrix. After the immersing test, specimens to which different levels of stress were applied showed different patterns of attack. When the stress applied to the specimens was below 250 MPa, for either 240 hr or 660 hr, only pitting-attack was detected on the surface. But when the stress was 300 MPa or more, pitting-attack and pitting-induced stress corrosion cracks were recognized. It was also observed that the distribution of stress on the specimens had a large influence on the pitting-attack behavior. Pitting-attacks mainly occurred around the specimens’ V-notch area where the stress was concentrated. The stress distribution of the specimen was analyzed through the finite element method.The paper ends with a discussion on the mechanisms of pitting attack.
Keywords: Stress corrosion cracking; pitting attack; 321 stainless steel; Four-point loaded bentbeam stresscorrosion test; Sodium chloride |
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