On the origin of tellurium corrosion resistance of hot-rolled GH3535 alloy
Introduction
Molten salt reactors (MSRs) are characterized by the uranium-bearing molten salts circulating in the primary circuit, which can carry the fuels and transfer heats simultaneously. Such design together with the thermo-physical properties of molten salts give MSRs some unique advantages over the others, including inherent safety, simplified fuel cycle and high power generation efficiency [[1], [2], [3]]. On the other hand, such design makes the whole primary circuit exposed to the corrosive molten salts and the fission products, which brings great challenges to the structural materials [[4], [5], [6]].
Hastelloy N alloy (the China optimized version: GH3535) is a Ni-16Mo-7Cr based solid-solution-strengthened superalloy developed specifically for MSRs [6], which exhibits a good high temperature strength and an excellent corrosion resistance to the molten salts [7]. However, the intergranular cracking of this alloy can be observed in almost all primary circuit components after its four-year service in the Molten Salt Reactor Experiment (MSRE), which was related to the inward diffusion of fission products tellurium (Te) along the grain boundaries [[8], [9], [10]]. When extrapolated to the long lifetime of a commercial reactor, these cracks would not be acceptable for some thin-walled pipes. Moreover, in the Liquid Metal Fast Breeder Reactor (LMFBR), Te corrosion has been also observed in the cladding materials (steels [[11], [12], [13], [14], [15]], Zircaloy 4 [16,17] and nickel based alloy [18]), threatening the mechanical integrity of cladding.
To overcome the Te corrosion problem, the redox potential adjustment in fuel salts [8,19], alloy composition modification [8,9] and alloy microstructure tailoring [12,[20], [21], [22]] have been tried in the previous studies. It has been reported that the small niobium [8] or aluminum [9] additions to Hastelloy N alloy can reduce significantly the degree of embrittlement in the Te corrosion environment. On the other hand, the microstructure tailoring methods were also proved to be effective in several commercial alloys for restrain the intergranular cracking, including grain refinement and precipitates control. In 316 stainless steel corroded by Te vapor, Arima et al. [12] found that the amount of Te penetrating into the grain boundaries was larger in the coarse-grained specimen. Our recent study [20] found that the density and the depth of surface cracking respectively decrease and increase with increasing grain sizes in the Te-corroded GH3535 alloy. It can be concluded that the fine-grained alloys reveal the obvious advantages in resisting Te corrosion and Te-induced cracking. Adamson [21] found that Cr23C6 carbides form along the intragranular slip line in the cold-worked 316 stainless steel and act as diffusion paths for Cs/Te mixtures, which can cause the more uniform corrosion attack zone than the deep intergranular type observed in the annealed alloy. Sasaki et al. [22] have investigated the dependence of Cs/Te corrosion depth on the distribution of Cr23C6 carbides in 10Cr steel, and found that the corrosion is stronger in the specimen with grain boundary carbides than that with intragranular carbides. The positive effects of the grain refinement and carbide redistribution in resisting Te corrosion and Te-induced cracking inspire us to evaluate the possibility of the hot-rolled GH3535 as a Te corrosion-resistant material, which is just characterized by fine grains and a lot of intragranular M6C carbides [23].
In this study, we have investigated systematically the Te corrosion behaviors of the hot-rolled GH3535 alloy with the standard one as a comparison. Based on the microstructure characterizations on the corrosion products and the corroded primary M6C carbides, the mechanism for resisting Te corrosion and cracking in the hot-rolled GH3535 alloy was discussed.
Section snippets
Experiment
Table 1 lists the chemical composition of GH3535 alloy used in this study. The pure raw materials were melted into one ingot by a vacuum induction melt-furnace. Then it was forged into 30 mm × 30 mm square bars and further hot-rolled into round bars with a diameter of 16 mm. As control, a part of rolled bars were solid solution treated at 1220 °C for 40 min to obtain the standard bars. Tensile specimens were machined from the hot-rolled (HR) and solid-solution-treated (ST) bars along the length
Microstructure evolution during Te corrosion
The Te corrosion behaviors are related to the microstructures of alloys. Thus, the initial microstructures of ST and HR specimens and their evolution during Te corrosion process should be clarified before the further discussion. As shown in Fig. 1a and b, both ST and HR specimens contain the primary precipitate particles in the grain interior or at the grain boundaries along the rolling direction, which have been identified as M6C type carbides [26,27]. As compared with the HR specimens, the
Discussion
In this study, the microstructures, Te corrosion behaviors and Te-induced intergranular cracking of the HR specimens were investigated with the ST ones as a control. It can be concluded that the HR specimens are characterized by (a) the denser network of grain boundaries and phase interfaces, (b) the higher proportion of Cr3Te4 and MnTe tellurides in the corrosion scales, (c) the more Te tied to the primary M6C carbides rather than the grain boundaries and (d) the stronger resistance to
Conclusions
- (1)
The shallower and fewer Te penetration and cracks along the grain boundaries can be observed in the HR specimens than that in the ST ones.
- (2)
After exposed in the Te vapor, the more Cr3Te4 and MnTe tellurides can be observed in the HR specimens than that in ST ones, which can be ascribed to the stronger outward diffusion of Cr and Mn due to the grain refinement. These protective telluride scales can help to impede the inward diffusion of Te atoms.
- (3)
The primary M6C carbides can trap the Te atoms to
CRediT authorship contribution statement
Bo-Heng Wu: Writing - original draft, Methodology, Software, Investigation. Li Jiang: Conceptualization, Supervision, Writing - review & editing. Xiang-Xi Ye: Writing - review & editing. Chao-Wen Li: Writing - review & editing. Jian-Ping Liang: Writing - review & editing. Fang Liu: Resources, Supervision, Writing - review & editing, Funding acquisition. Zhi-Jun Li: Resources, Supervision, Writing - review & editing, Funding acquisition.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National key research and development program of China (2016YFB0700404 and 2016YFB0700401), the Natural Science Foundation of Shanghai (19ZR1468200 and 18ZR1448000), Shanghai Sailing Program (Grant No. 19YF1458300), National Natural Science Foundation of China (Grant No.51671154, 51601213, 51671122 and 51971238) and Youth Innovation Promotion Association, Chinese Academy of Science (Grant No.2019264).
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