Investigation On Corrosion Mechanism Of Ni-based Alloys In Molten Salt Environments

Molten salt reactor (MSR), one of the Generation-IV nuclear reactors, hasattracted much more attention due to its unique features, such as high thermodynamicefficiency, intrinsic safety, effective utilization of nuclear power, etc. Unlikeconventional nuclear reactors, molten salt reactor is a class of nuclear fission reactorsin which the primary, or even the fuel itself, is a molten salt mixture. However, thehigh temperature molten fluoride salts are highly corrosive, corrosion of the structuralmaterials is one of the main problems that prevent the MSR from long time operation.Due to the excellent molten salt corrosion resistance, nickel-based high temperaturealloy is regarded as the main candidate structural materials of the MSR. Researchshows that the Cr composition of nickel-based high temperature alloys was selectivelyattacked in molten FLiNaK salt. In spite of the known tendency for Cr to beselectively attacked by molten fluoride salts, Cr is still widely used in nickel-basedhigh temperature alloys because it can improve the high-temperature oxidationresistance of materials. Based on the above background, a systematic study wasundertaken to investigate the corrosion mechanism of Cr in molten salt environments.The main contents are summarized as follows:Six kinds of Ni-Cr model alloys (Cr:3~25wt.%) were exposed to moltenFLiNaK salt. The corrosion behaviors of the alloys were analyzed by weight-lossmeasurement, scanning electron microscope (SEM) and electron probe microanalysis(EPMA). The results show that the corrosion degree related to the initial Cr content,and the corrosion of the alloys increased with increasing the Cr content in the alloys.When the Cr content in the alloy is less than11wt.%, the alloys exhibited goodcorrosion resistance, no obvious corrosion signs were observed on the samples afterexposure to molten FLiNaK salt at700°C for400h. When the Cr content in the alloyis higher than15wt.%, the alloys show relatively poor corrosion resistance. Thealloys with higher Cr content were suffered severe attack after exposure to molten FLiNaK salt at700°C for400h, and the corrosion depth of the alloys increased withincreasing the Cr content in the alloys.The effective diffusion coefficient of Cr in various alloys was obtained by fittingthe Cr distribution profile of the Ni-Cr model alloys with the Fick’s law. Using thediffusion coefficient calculated above, it was calculated the Cr distribution profile ofthe alloys by the error function. According to the EPMA line scan of Cr, the errorfunction is modified to minimize the discrepancy between the results of the calculatedprofile and the EPMA results. Based on the above analysis, a modified corrosiondiffusion equation was developed which can be used to predict the Cr elementdistribution profile after corrosion test in molten fluoride salts.To verify the applicability of the corrosion diffusion equation calculated above,six kinds of nickel-based high temperature alloys (Cr:1~25wt.%) were exposed tomolten FLiNaK salt. It was found that the corrosion result is consistent with that inthe Ni-Cr model alloys. For the tested alloys, Hastelloy B-2and Haynes242with lowCr content exhibited better corrosion resistance, meanwhile, the alloys (GH3030,Hastelloy X, GH3044) with high Cr content suffered severe corrosion attack afterexposure to molten FLiNaK salt at700°C for400h. Based on the least squaremethod principle, the validity of the corrosion diffusion equation is testified bycomparison the simulation and the experimental results. The results show that theequation can well predict the distribution of Cr in alloys after the corrosion test inmolten fluoride salts. Additionally, by comparing the corrosion results of the Ni-Crmodel alloys and nickel-based alloys, it was found that Mo can effectively improvethe corrosion resistance of alloys in molten fluoride salts.To investigate interact mechanism of Cr with graphite and Ni in molten FLiNaKsalt environment, corrosion tests of pure metal Cr were performed in molten FLiNaKsalt using graphite crucible and Ni crucible. The results show that galvanic corrosionoccurred when two different electromotive potentials materials are in electricalcontact, and the graphite accelerated the corrosion rate of alloys. Meantime, the Crdissolved into the FLiNaK salt would deposit on the surface of the crucibles. In hightemperature molten salt environments, the Cr deposited on the crucibles in this way would react with the crucible materials to form alloys or Cr-carbides. The Cr-rich filmon the graphite crucible was identified to be Cr7C3with a little amount of Cr23C6. TheCr atoms deposited on the Ni crucible surface and formed Ni-Cr alloy by thermaldiffusion mechanism.The corrosion product of Cr in molten FLiNaK salt was thoroughly investigatedby X-ray absorption near-edge structure spectroscopy (XANES), transmissionelectron microscope (TEM) and X-ray diffraction (XRD). The results demonstratethat the predominant oxidation state of Cr in FLiNaK salt is Cr3+and the maincorrosion product in cooled FLiNaK salt is K2NaCrF6with face centered cubicstructure.