| Molten salt reactor (MSR) is one of the six most promising Generation IV fissionreactor candidate. It’s also an important part of the future advanced nuclear fissionbelonging to the strategic leading science and technology projects of Chinese academyof sciences. Because of its utilization of nuclear fuel utilization, inherent safety, highthermal efficiency, and so on, MSR has been paid more and more attention. Moltenfluoride salts have been proposed for use as the primary and secondary coolants forMSR. However, the main components of MSR, such as pressure vessel, pipes and heatexchanger, etc probably suffer corrosion in high temperature molten fluoride saltenvironment due to its high corrosivity. The corrosion of the structure materials is oneof the main problems that prevent the MSR from long time operation.Hastelloy N alloy and316L stainless steel (316L) are expected to be two keycandidate structure materials used in MSR. However, there are only a few studies onthe different corrosion resistance of both materials in molten fluoride salt, and therelated corrosion mechanisms are not clarified. This present work mainly investigatedthe long-term corrosion behavior of Hastelloy N alloy and316L in high temperaturemolten FLiNaK salt by means of static corrosion tests, in-situ electrochemicalmeasurement combined with SEM and EPMA analysis. Main attention was paid to thedifference of corrosion resistance for Hastelloy N and316L and the related corrosionmechanisms in molten fluoride salt. The main results and conclusions of corrosionbehavior of Hastelloy N alloy and316L in high temperature molten FLiNaK salt couldbe described as follows.It was found that Hastelloy N and316L all experienced the continuted corrosionduring experimental duration according to the results of static corrosion tests. Thecorrosion resistance of Hastelloy N was superior to that of316L. Hastelloy Nexperienced minor general corrosion, which was mainly due to the outward diffusionof Cr element through the alloy matrix.316L suffered the significant intergranularcorrosion, which was related to the outward diffusion of Cr along its grain boundaries. In addition, the weight loss per unit area of Hastelloy N firstly increased, and thendegraded slowly with exposure time, which was mainly attributed to the deposition ofNi and Fe elements from the molten salt on the surface of Hastelloy N. The weight lossper unit area of316L increased obviously for the first400h, and then became stable.It was found that Hastelloy N and316L all experienced the active dissolutioninstead of the passivation according to the results of in-situ electrochemicalmeasurements. The corrosion potential of Hastelloy N was higher than that of316L,and the corrosion current density and anodic current density were lower than that of316L. After the dynamic polar curve, the weak intergranular corrosion was observedon the local surface of Hastelloy N. However, no significant corrosion was observed onthe cross section of Hastelloy N, and the concentrations of Cr, Fe and Ni elements weresimilar along its cross section. The surface and cross section of316L all experiencedthe serious intergranular corrosion. Fe and Cr elements were dissolved along its grainboundaries.The different corrosion resistance of Hastelloy N and316L in molten FLiNaK saltwas mainly associated with their different chemical compositions. Hastelloy N behavedas the good corrosion resistance due to its Ni base, while316L behaved as the poorcorrosion resistance due to its Fe base. In addition, the different influence of graphiteon Hastelloy N and316L was also responsible for their different corrosion resistance inmolten FLiNaK salts. |
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