Localized Corrosion Behavior Of Stainless Steel Welded Plate Used For Spent Fuel Pool

In this work,the pitting corrosion,crevice corrosion,intergranular corrosion and stress corrosion cracking?SCC?behaviors of stainless steel liners for spent fuel pools of nuclear power plants in several relevant environments were studied by performing chemical immersion tests and electrochemical tests.The test materials included the base metals and welded joints of the 304L/ER316L austenitic stainless steel welded plate?304L for the base metal and ER316L for the welding wire?for the early second-generation NPPs?nuclear power plants?and the S32101/ER2209 duplex stainless steel welded plate?S32101 for the base metal,ER2209 for the welding wire?for the third-generation NPPs,as well as the welded joints after ultrasonic impact treatment?UIT?.The simulated environments were divided into two types:one was pure boric acid solution containing 2700 mg/L B³⁺and another was chlorion-doped solution containing2700 mg/L B³⁺+200 mg/L Cl^-.In addition,standard tests were carried out using 6%FeCl₃ and 3.5%NaCl solutions recommended by the relevant national standards.The test temperatures were 30,40 and 60?.The main research works and results are described as follows:1.The pitting behaviors of the two welded plates were studied by tests of FeCl₃immersion,polarization curves in various environments and electrochemical impedance spectroscopy?EIS?.?1?The pitting corrosion resistance ranking of the parts of two welded plates was the weld metal>base metal>heat affected zone,mainly due to higher contents of Cr,Ni and Mo in the weld metal but relatively imperfect microstructure in the heat affected zone.The pitting resistance of weld metal and base metal of S32101/ER2209 was better than that of 304L/ER316L,but the heat affected zone was poor.Above rank in pitting resistance of the steels and welds were consistant to the rank in pitting resistanc equivalent number?PREN?calcaulted based on the chemical compositions.?2?No apparent pitting was found in the parts of the weld plates even at high potentials in the electrochemical tests of the pure boric acid solution at 30,40 and 60?.After doping 200 mg/L Cl^-in the pure boric acid solution,the pitting tendency increased significantly.The pitting sensitivity increased with the rise of temperature.?3?UIT had no significant effect on the pitting resistance of weldedments.2.The crevice corrosion behaviors of the two welded plates were studied by FeCl₃immersion and critical potential method.?1?The crevice corrosion resistance ranking of the parts of two welded plates was the weld metal>base metal>heat affected zone,the reasons were same as the pitting.The crevice corrosion resistance of base metal and heat affected zone of S32101/ER2209was worse than that of 304L/ER316L,but the weld metal was better than the latter.?2?No apparent crevice corrosion was found in the parts of the weld plates at high potentials in the electrochemical tests of the pure boric acid solution at 30 and 40?,and the repassivation potentials E_rp>800 mV_SCE.?3?UIT had no significant effect on the resistance to crevice corrosion of weldments.3.The intergranular corrosion resistance of two welded plates were tested by method of copper sulfate-sulfuric acid test for stainless steel.?1?The intergranular corrosion resistance of 304L/ER316L welded plates was better than S32101/ER2209 welded plates.No intergranular corrosion cracks had occurred in the base matels and welded joints of 304L/ER316L after testing,which may be related to low carbon content.The base metals and welded joints of S32101/ER2209all emerged cracks after testing,which may be related to lower plasticity and the precipitates near the grain boundaries.?2?UIT had no significant effect on the intergranular corrosion resistance of weldments.4.The SCC resistance of the base metals,welded joints included as welded and with UIT of two welded plates were studied by four-point bending and U-bending tests.In the solution containing 2700 mg/L B³⁺+200 mg/L Cl^-at 40?,the samples'surface were uniform corrosion except SCC crack after immersed for 100 days and 90 days respectively.5.It can be believed that the pure boric acid environment that meets the design parameters is good enough for service,but when the solution contains aggressive Cl^-,the localized corrosion resistances of the weldedments will be adversely affected in different degrees.What's more,the corrosion tendency increased with the raise of temperature.Therefore,the water temperature and water quality should be strictly controlled to avoid localized corrosion during the service period of the spent fuel pool.