Stress corrosion cracking was often observed at dissimilar metal weld with alloy 82/182 in reactor vessel In/Outlet nozzle. Since stress corrosion cracking of dissimilar metal weld is closed related to the tensile residual stress by welding, residual stress should be mitigated using heat treatment, surface treatment, etc. This work focused on the effects of aging and Ultrasonic Nanocrystal Surface Modification(UNSM) on corrosion properties of 316L stainless steel and dissililar metal weld. Therefore, it was conducted into three themes.

[1] Effect of aging and UNSM treatment on intergranular corrosion behavior of 316L stainless steel
Even though chromium carbide was not precipitated, the intergranular corrosion rate of 316L stainless steel was drastically increased with aging time, and it was confirmed that the increased intergranular corrosion rate of slightly-sensitized (not carbide formed) 316L stainless steel was due to the carbon segregation along the grain boundaries. However, UNSM treatment improved the intergranular corrosion resistance of aged stainless steels, and its improvement was due to the reduction of carbon segregation and the grain refinement of the outer surface, including the introduction of compressive residual stress.

[2] Effect of aging and UNSM treatment on passivation behavior of 316L stainless steel
Even though chromium carbide was not precipitated, passivation properties of 316L stainless steel was drastically decreased with aging time, and it was confirmed that the decreased passivation properties of slightly-sensitized (not carbide formed) 316L stainless steel was due to the carbon segregation along the grain boundaries. However, UNSM treatment improved the passivation properties of slightly-sensitized (not carbide formed) specimen and deteriorated it of over-sensitized (carbide formed) specimen. In slightly-sensitized specimen, its improvement was due to the reduction of carbon segregation and the grain refinement of the outer surface, including the introduction of compressive residual stress. In over-sensitized specimen, its deterioration by UNSM treatment was inferred that deterioration the carbide became detached from matrix by UNSM and its voids increased starting points of pitting.(though carbon segregation of grain boundary)

[3] Effect of aging and UNSM treatment on stress corrosion cracking behavior of 316L/alloy 82
1) The SCC test in the secondary corrosion simulation environment was carried out by the U-bend method. In welding specimen, resistance of SCC was reduced and crack mode was intergranular crack. The intergranular cracks are related to the intergranular corrosion properties. It was reduced because intergranular corrosion rate and dos of sensitization were increased by welding. However, UNSM treatment improved resistance of SCC and its improvement was due to the reduction of carbon segregation, Cr carbide, and Cr depletion. And, UNSM treatment reduced residual stress formed by U-bend test. Therefore, the stress corrosion cracking mechanism in the secondary corrosion model environment is affected by the pre-existing active path model.
2) The SCC test in the primary corrosion simulation environment was carried out by SSRT. In welding specimen, resistance of SCC was increased and crack mode was transgranular crack. According to documentary survey, It has been reported that Cr carbide formed by welding increased resistance of SCC. It is also estimated that the change of mechanical properties has affected by welding in alloy 82. However, UNSM treatment reduced resistance of SCC and crack mode was transgranular crack. It is related to the passivation properties. UNSM treatment reduced resistance of the passivation properties and it’s reason estimated that detachment of carbide formed by welding reduced the passivation properties. It is also estimated that residual stress and difference of oxide composition do not affect resistance of SCC. Therefore, the stress corrosion cracking mechanism by UNSM in the primary corrosion model environment is affected by the slip step dissolution model.