| Molten salt nuclear energy systems are operated in the harsh environment of hightemperature and strong fluoride salt corrosion. Among them, the primary loop andreactor vessel of molten salt reactors are additionally exposed to a strong neutronirradiation. Hastelloy-N alloy is one of the candidate materials closest to the servicerequirements of the molten salt nuclear energy systems. However, it is difficult toimport a large amount of Hastelloy-N alloy from abroad.Additionally, no experimentdataindicates that the existing performance of Hastelloy-N alloy can meet the highertemperature requirements of next-generation nuclear energy systems.Consequently, it isnecessary to develop the similar new alloy. GH3535alloy produced byShenyangInstitute of Metal Research is similar to Hastelloy-N alloy. Firstly, as thevessel wall material touched with high-temperature molten salt directly, GH3535alloymust resist high-temperature molten salt corrosion. Secondly, as the pipe material,GH3535alloy must resist high-temperature oxidation and have good strength. However,the existing performance of GH3535alloy can not meet the higher temperaturerequirements of next-generation nuclear energy systems.Based on the above background, this article focuses on the high-temperaturecorrosion resistance optimization approach of GH3535alloy. There are many ways suchas the processing technology of alloy, fine the composition of alloy, coating on thesurface of alloy and so on. However, the effect is not very significant or persistence isnot very good. In this thesis, many literature investigation and lots of experiments showthat this problem is expected to be solved by rare earth micro-alloyingmodification.Trace of rare earth yttrium can modify the performance of the alloy.Yttrium is considered to be one of the elements which has great application prospect.Therefore, this thesis aims to investigate the high-temperature corrosion resistance ofGH3535alloy containing various yttrium concentrations.This thesis relies on the synchronous radiation analysis and testing techniques,combining with other conventional methods, such as weight loss method, scanning electron microscope, transmission electron microscope and so on, to in-depth study theeffects of rare earth element yttrium on microstructure and high-temperature corrosionresistance. The main results of this study are as follows:1. The yttrium additions change the microstructure of GH3535alloy significantly.Firstly, the type and amount of precipitates changed. In the alloy without yttriumaddition, only M6C precipitate appeared, a large amount of which presented chain shapeand distributed along grain boundary. When the concentration of yttrium in the alloywas0.05wt%, another new precipitate Ni17Y2appeared, distributing in the grain interior,the amount of which is very little and the morphology of which presented sleek granule(Ф≈100nm). The number of M6C decreased and the morphology also changed todispersed particles and distributed along the gain boundary. However, with furtherincreasing yttrium concentration, Ni17Y2and M6C grew larger and the amount of whichalso increased. Moreover, the gain was fined by yttrium addition.2. The high-temperature oxidation tests were conducted in the muffle furnace at1000℃. The oxide scale was characterized systematically and the mechanism of theeffect of yttrium on the oxidation resistance of GH3535alloy was discussed. TheGH3535alloy containing0.05wt%yttrium exhibited the best oxidation resistanceamong the five GH3535alloys, the oxide scale of which was only5-8μm and two-layerstructure, a compact Cr2O3and YCrO3, appearing in the internal layer of the oxide scale,inhibited the further oxidation. However, the oxide scale of GH3535alloy containingother yttrium concentrations is10microns to27microns thick, containing three layers.3. The molten salt corrosion tests were performed at850oC for620h in sealedgraphite crucibles encapsulated in the outer304stainless steel containment. The resultsdemonstrated that the alloy showed the best corrosion resistance among the five alloyswhen the concentration of yttrium was0.05wt%. This thickness of corrosion scale wasonly several microns, in spite of this, it contained a compact YF3layer which can inhibitthe element diffusion effectively. Nevertheless, the corrosion resistance decreasedsharply with further increasing yttrium concentration. The corrosion oxide was verythick and a large amount of corrosion holes appeared in it. Y is a active element, whichis very easy to be combined with F. However, the atomic radius of Y is large, leading to the diffusion coefficient of Y is small. Further more, the content of Y in the solidsolution of the alloy containing0.05wt%yttrium was the highest. Therefore, the YF3layer could be formed in the corrosion layer of GH3535alloy containing0.05wt%yttrium.4. The mechanical property was tested in this thesis. The alloy containing0.05wt%Y concentration showed the best high-temperature corrosion resistance. Simultaneously,the mechanical properties were also improved at0.05wt%Y concentration. This worklaysa solid foundation for the development of nickel-based alloys which are suitable forapplying to a new generation of molten salt reactors, cooling system of advanced hightemperature reactor and molten salt heat transfer systems of reactor-hydrogen plants. |
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