Effects Of Environmental Factors On Stress Corrosion Cracking Of Dissimilar Metal Weld 16MND5/309L/308L/Z2CND18-12N

Long-term safe operation of nuclear power plants is critical for the development of nuclear power industry. International engineering experience and relevant researches reveal that protection/mitigation of corrosion cracking failures of the key materials is a core problem. Stress corrosion cracking(SCC) has become one of the most significant problems of nuclear key materials in high temperature high pressure water environments. Therefore researches on the SCC behavior of these materials in its service condition are very important. The connection between the low alloy steel pressure vessel nozzle and stainless steel safe end in nuclear power plants is dissimilar metal weld. This part is usually susceptible to SCC because of its complicated microstructure and stress condition due to its special location. In this work, effects of environmental factors on the SCC behavior of two main parts of an advanced dissimilar metal weld 16MND5/309L/308L/316L/Z2CND18-12 N used in nuclear power plants, i.e. 16MND5/309L/308 L and 308L/316 L, were studied respectively, by means of slow strain rate testing(SSRT) and electrode potential measurement/control. Environmental factors were chloride impurity contamination and electrode potential varied in the range from-720 mV to +300mV(vs. SHE) which simulated the electrochemical conditions of the weld in the service conditions from ideal water chemistry to bad water chemistry with significant contamination of oxygen. Most specimens were tested at the strain rate of 5×10-7s-1. Strain rate below and above this were also used to study strain rate effect on SCC. Following research works were carried out and main conclusions were obtained.1) The SCC behaviors of dissimilar metal weld 16MND5/309L/308 L in simulated pressurized water reactor(PWR) primary water environments at 290℃ were investigated. The results showed that the specimens always failed in the bulk zone of the stainless steel 308 L weld metal in ductile appearances when tested in the electrode potential range from-720 mV to +100m V(vs. SHE). This behavior was similar to the result of the specimen tested in inert N2 at the same temperature. When electrode potential was raised to +200mV, the weld exhibited brittle fracture by significant SCC in the area around the 16MND5/309 L interface. Intergranular SCC(IGSCC) occurred in the stainless steel 309 L close to the interface, while transgranular SCC(TGSCC) happened in 16MND5 heat-affected zone close to the interface. This means that a critical potential of SCC, Escc, existed between +100mV~+200mV(vs.SHE). When strain rate was decreased to 1×10-7s-1, the critical potential was still within the range from +100mV to +200mV although the test time increased significantly. When strain rate was raised to 1×10-6s-1, SCC was not observed even at the potentials +200mV and +300mV, which should be attributed to that the time was not long enough for apparent SCC to occur. Contamination of the primary water with 10ppm(i.e. 10 g/m3) chloride increased the SCC susceptibility by decreasing Escc from the range between +100mV~+200mV(vs.SHE) to the range between 0mV to +100m V(vs.SHE). The weld tested at +100mV exhibited brittle fracture by significant SCC in 16MND5/309 L interface. There were IGSCC in 309 L and TGSCC in 16MND5 heat-affected zone. On the fracture surface, the interface layer became an isolated thin layer because of the significant corrosion of the low alloy steel near the interface.2)The SCC behaviors of weld 308L/Z2CND18-12 N in the same high temperature environments at 290℃ were studied. The results showed that the specimens always failed in the bulk zone of 308 L weld metal in ductile appearance when tested in the potential range from-720 mV to +300mV(vs.SHE), which was similar to the result of the specimen tested in N2. There was no apparent critical potential of SCC in the environment. When 10 ppm Cl- was added into the environment, there was also no apparent SCC happened when tested in the range from-720 mV to +300mV(vs.SHE).In this work, SCC occurred only when 16MND5/309L/308 L specimens were tested at high electrode potentials. This phenomenon could be explained by the mechanism of anodic dissolution. It was conclude that SCC would not happen in the two main parts of the dissimilar metal weld in service if the water chemistry is kept in good condition. Contamination of dissolved oxygen and harmful ion such as Cl- should be strictly controlled to avoid SCC.