无保护层激光强化对 316L/Inc600 焊接接头焊缝部位耐腐蚀性的影响

Corrosion wreaks havoc on dissimilar welded joints in the inlet and outlet ports of pressurized water reactors (PWRs) in the first circuit and has become one of the main factors in the development of a large number of cracks before the expiration of the design life. Laser Peening without Coating (LPwC) allows plastic deformation of the surface layer of the material to improve corrosion and has been used by Toshiba in Japan to extend the life of nuclear power plants against corrosion. The work aims to investigate the effect mechanism of LPwC on the corrosion resistance of the weld area of 316L/Inc 600 welded joints. In this study, a 10 mm x 4 mm x 3 mm weld area of 316L/Inc 600 welded joint was used for the study and then polished to a mirror finish with 40 nm silica suspension after ground with metallographic sandpaper (e.g. 400 #, 800 #, 2000 #, and 3000 #). The polished surfaces of the specimens were enhanced by the LPwC enhancement technique. These were two enhancement processes, including 60 mJT1 and 60 mJT1-m. The LPwC-treated specimen was noted as 60 mJT1, and the remelted layer of the other portion of the LPwC-treated specimen was sanded down by 20 µm with 2000 # metallurgical sandpaper and labeled as 60 mJT1-m. Surface roughness was measured with confocal microscopy for both AR and 60 mJT1. The thickness of the remelted layer was measured with an OM microscope. The kinetic potential polarization curves and impedance spectral curves of the specimens with different process parameters in 3.5wt.% NaCl solution were analyzed by an electrochemical workstation to obtain the corrosion resistance of the specimens with different processes, respectively. The corrosion products of the specimens after the electrochemical experiments were washed down by anhydrous ethanol ultrasonication, and the volume and number of pitting pits were measured with a confocal microscope, and the corrosion morphology and elemental distribution were observed with an electron scanning microscope. The results of the experiment showed that LPwC introduced a remelting layer approximately 0.54 μm deep into the metal surface, which increased the surface roughness from 96 nm to 691 nm. After the strengthening and grinding treatment, the charge transfer resistance of the weld increased by 1.7 times and 3.1 times compared to the original state. The passivation current was reduced by 1-2 orders of magnitude, and the pitting voltage increased by more than 100 mV. The mean diameter of pits in the strengthened sample decreased from 33.41 μm to 17.20 μm, and the volume loss decreased from 122 886 μm³ to 49 068 μm³. No element segregation or carbide was observed in the pits. In the original sample, pitting occurred between dendrites, particularly where chromium (Cr) content was low and carbon (C) was enriched around it. After grinding, pits appeared at the grain boundaries, with shallow depth and no element segregation phenomenon. In summary, the thermal load generated by LPwC can eliminate carbides in weld parts, reducing the Cr poor zone and improving the corrosion resistance of the weld. Grinding the surface treated by LPwC can further enhance the corrosion resistance of the weld.