The Study On Corrosion Behavior Of 316L Stainless Steel Weld Seam In High-speed Flowing Liquid Pb-Bi

In the nuclear industry,lead bismuth eutectic?LBE?is a commonly used as coolant of lead-cooled fast reactor?LFR?in Generation IV nuclear energy system and accelerator driven sub-critical system?ADS?,and it is also the candidate material for spallation target of ADS.However,the cladding structure material will suffer corrosion when exposed in high-speed liquid LBE for a long time,which can reduce the service life of the cladding structure material.316L stainless steel is preferred as a kind of nuclear steel because of its excellent properties,especially its resistance to radiation damage and corrosion resistance,and it has been widely used as cladding structural material for the nuclear reactor.The welding technology is widely used in the cladding structure parts of the nuclear reactor,such as the main pump impeller,pipe joints and coolant storage tanks,etc.Because of the weak area caused by the uneven microstructure and welding residual stress of the weld,which makes the weld be corroded in high-speed flowing liquid LBE preferentially and the corrosion process is more complex.Therefore,the study on corrosion behavior of 316L stainless steel welds in high-speed flowing liquid LBE is of great significance,which can provide theoretical basis for improving the corrosion resistance and service life of the cladding material of the nuclear reactorIn this paper,the 316L stainless steel plates were firstly butt-welded by Tungsten Inert Gas Welding,and one-side welding with back formation.Then,some of the welded steel plates were subjected to different surface treatments and solution treatment.Finally,all experimental materials were cut into T-shaped samples,and then put in the corrosion experimental device about high-speed flowing liquid metal developed independently by our research group.All experiments were carried out in saturated oxygen concentration liquid LBE at 550?.The specific experiments are as follows:1)Corrosion experiments with low relative flow rates?1.70 m/s,2.31 m/s,2.62 m/s,2.98 m/s,3.69 m/s and 4.77 m/s?at 500 h were performed.which were to study the corrosion mechanism of flowing high-speed liquid LBE and the effect of the relative flow rate on the corrosion behavior of welds.And the corrosion time was extended to 1500 h at different relative flow rates?1.70 m/s,2.31 m/s and 2.98 m/s?,which were to study the effect of corrosion time on corrosion behavior of welds.2)Corrosion experiments of the samples with different surface states?180#sandpaper,1000#sandpaper,mechanical polishing and electrolytic polishing?were performed at a relative flow rate of 2.98 m/s for 500 h.which were to study the effects of the surface state on the corrosion resistance of welds.3)The corrosion experiment of the sample with solution treatment was performed at a relative flow rate of 2.98 m/s for500 h,which was to study the effects of solution treatment on the corrosion resistance of the weld.The results show that the double oxide layer was formed in all samples after corrosion,and the inner oxide layer is composed by dense FeCr₂O₄,which grows into the matrix.The outer oxide layer is composed by loose Fe₃O₄ and PbFe₄O₇,which grows outwards and is easily corroded.The corrosion process of the flowing high-speed liquid LBE mainly involves elemental migration?dissolved corrosion and oxidation corrosion?and wear corrosion.Cr,Fe,and O are the key to the migration of elements.Wear corrosion can exfoliate the oxide layer and promote element migration.The effect of the relative flow rate on the corrosion process of the weld is complicated.In general,the oxide layer on the weld surface is thicker when the relative flow rateis faster,which means that the corrosion is more serious.As the relative flow rate increases,the element migration increases first,and then it becomes slower because of the retardation of the thickened oxide layer.However,the wear corrosion is continuously strengthened until the weld is damaged.Therefore,the relative flow rate should be set considering corrosion and cooling effect.The sample with smaller surface roughness can reduce the degree of the wear corrosion.At the same time,the strengthening layer formed on the surface during the treatment can also hinder the element migration,which improve the corrosion resistance of the sample as a whole.Among them,the corrosion resistance of the electropolished sample is the best.Solution treatment can dissolve carbides and impurities,homogenize the microstructure,and reduce the weak areas,thereby reducing the element migration rate and improving the corrosion resistance of the sample.