Irradiation Assisted Stress Corrosion Cracking (IASCC) is one of the critical concerns when stainless steel components have been in service in a light water reactor for a long period. It is known that IASCC occurs by the synergistic effects of many factors under the particular environment found in nuclear power plants, including neutron irradiation and high temperature water. Especially, the degradation of corrosion resistance resulting from neutron irradiation on austenitic stainless steel is considered the main cause of IASCC.
However, it is difficult to evaluate corrosion behavior on irradiated materials by conventional methods. Corrosion resistance degradation occurs at chromium (Cr) depleted areas formed by radiation induced segregation. It is in the nanometer scale and occurs along grain boundaries or in the grain matrix. Therefore, we developed a new evaluation method using an Atomic Force Microscope (AFM) to evaluate the grain boundary corrosion behavior on irradiated materials.
Fig. 1-8 shows the AFM observations of an ion irradiated stainless steel surface after a corrosion test. Steps are observed at grain boundaries. Moreover, preferential corrosion was detected at some random grain boundaries. IASCC occurred at the preferentially corroded grain boundaries, as seen in this figure.
Under neutron irradiation conditions, helium (He) is produced in some materials by nuclear transmutation. Fig. 1-9 shows the effect of He implantation on corrosion behavior of ion irradiated stainless steel, which was obtained by the AFM evaluation method. Corrosion was suppressed by He implantation at 300 degree Celsius or higher.
In the future, the mechanism of IASCC will be investigated by using AFM, and the results will be applied to implement a new method to suppress IASCC in nuclear power plants. This will produce higher integrity of internal core components.
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