| Pb-Bi eutectic alloy(LBE) is considered as the promising candidate coolant material for Accelerator Driven Sub-critical System(ADS) and lead cooled fast reactor(LFR) in the fourth generation nuclear energy systems due to the excellent properties. Austenitic stainless steel is also the important candidate structural of the cladding of the fourth-generation nuclear reactors and loop pipes due to its excellent corrosion resistance. However, long-term exposure to structural materials in liquid Pb-Bi alloys will lead to the dissolution and complex chemical reactions with the impurity oxygen in Pb-Bi alloys to generate new compounds to change the solid metal of the original property, which will lead to the formation of dissolution and oxidation corrosion. Welding technology is essential in the pipe connections and the cladding material. But the corrosion behavior becomes more complex due to segregation, coarse grains and welding residual stress of welded joints, many corrosion often occurs preferentially in welded joints nearby. So the study of austenitic stainless steel welded joints at high velocity liquid Pb-Bi alloy corrosion behavior is significant, it can provide theoretical and experimental basis for the further optimization of austenitic stainless steel welded joints corrosion resistance aspect.316L and 310 austenitic stainless steel which are commonly used in the nuclear industry were selected as the research object in this paper. Under the rotary corrosion test apparatus designed and manufactured by independent research group, The 316 L stainless steel welded joints with different welding heat input had a dynamic corrosion in the liquid Pb-Bi alloy which was 550? for 1000 h and the relative flow rate is 1.7m/s. Effect of heat input on the corrosion resistance of welded joints material was analyzed.The corrosion condition of welded joint and parent metal was compared at the same time. The difference of corrosion behavior of the welded joints of 316 L stainless steel under different relative flow rates was researched and were compared with parent metal. The relative velocity on the corrosion resistance of welded joints of stainless steels was analyzed. The difference between the corrosion behavior of 310 and 316 L stainless steel welded joints and parent metal and at the same flow rate was analyzed and the corrosion mechanism was analyzed.The results showed that double oxide layers were formed on the surface of samples of 316 L stainless steel under different welding heat input(7.6KJ/cm, 12.8KJ/cm, 18.7KJ/cm), which were corroded in the liquid Pb-Bi alloys with the temperature of 550?. The relative flow rate was 1.7m/s. The oxide layer and the substrate had a good combination and there was no significant loss. The outer oxide layer which growed from the initial surface to outside mainly composed of Fe3O4 and some Pb took part in the reaction and generated PbFe4O7. The inner oxide which growed from the initial surface to inside was mainly composed of FeCr2O4 and the compactness of the inner oxide is greater than the outer oxide. The infiltration of Pb-Bi alloy occurred in the outer layer only which means the double oxide layer has a protective effect on the matrix. The corrosion resistance of parent metal was the best compared with weld zone and heat effect zone.So the welded joints were the Key protected areas. The study found that the oxidation speed of samples with higher heat input were higher and the corrosion weightless rates was higher. Heat input mainly affects the size of the austenite grain size and the size of the weld zone of ferrite. Austenite grain size on the corrosion resistance of welding joint area play a crucial role. The smaller the grain size was the better the corrosion resistance was.The corrosion resistance of welding joint is the best when the heat input is 7.6 KJ/cm.In addition, 316 L stainless steel welded specimen were corrode for 1000 h in 550? liquid Pb-Bi alloys at different relative rates(1.7m/s,2.31m/s,2.98m/s), double oxide layers were formed on the surface. The corrosion process with high flow rates mainly due to the oxidation corrosion and erosion corrosion.With the relative velocity increase, mass transfer process of elements became faster, so that the oxidation corrosion of the stainless steel welded joint was more serious.Meanwhile, The improvement of relative velocity increased liquid Pb- Bi alloy abrasion on the surface of the oxide layer. Show the internal oxidation layer growth rate is greater than the outer oxide layer. The surface roughness of the outer oxide layer is gradually increasing with the increase of relative velocity. At the same relative rates(2.31m/s), the corrosion resistance of 310 stainless steel welded joint is better than 316 L stainless steel welded joint.Studies show that the different Cr content is the main cause of corrosion resistance differences. The increase of Cr content provides more nucleation particles which conducive to continuous protective layer of Fe-Cr spinel oxide earlier and benefical to the compactness of inner oxide layer and the corrosion resistance of stainless steel welded joints in this test. |
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