NSUF 19-1742: Investigation of the mechanism behind irradiation-decelerated corrosion of Ni-20Cr in molten fluoride salt
Motivation: The degradation of materials in molten salt reactors (MSRs) must be understood before they can be constructed on a commercial scale. Among different degradation modes, corrosion in molten salts has been extensively studied, proceeding mainly via selective dissolution of Cr from the alloys into the salts. The corrosion rate is influenced by a variety of parameters, such as temperature, salt impurity content, and radiation flux. Radiation, in particular, will influence most of the parameters. however, its effects on corrosion of MSR-relevant alloys have rarely been studied.
Preliminary Results: To fill this knowledge gap, a simultaneous corrosion/irradiation facility for testing molten salt-facing materials was constructed at MIT. 3 MeV protons are used as a radiation source to bombard sample foils 30 microns thick, allowing the proton beam to pass through while producing relatively uniform radiation damage. On the back side of the sample foil, a heated molten fluoride salt reservoir serves as a corrosion medium. By masking the beam, each sample is divided into two regions, an irradiated/corroded region in the center, and a corrosion only region surrounding it, which enables us to directly target the influence of radiation on corrosion using one sample. Ni-20Cr irradiated in 650°C FLiNaK+5EuF3 for 4 hours showed that proton irradiation decelerates corrosion under the testing conditions. This conclusion is made from the fact that the region under corrosion was corroded through, while the region under both corrosion and irradiation was not. This was further verified by cross-sectional images on multiple sample foils.
Hypothesized Mechanism: Radiation induced segregation of Ni and Cr atoms is proposed as the mechanism behind radiation-decelerated corrosion. Cr atoms segregate away from GBs, “hiding” them from the salt, while Ni atoms segregate toward the GBs, making the GBs more “inert,” thus delaying corrosion voids from connecting.
Proposed Experiments: To back up the proposed mechanism, scanning transmission electron microscope (STEM) with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) capabilities will be used to detect the proposed segregation of Ni and Cr within 10 nm scale under proton irradiation. For the segregation of Ni, spots in the corrosion and irradiation region will be sufficient. However, it is noticed that the corrosion attack of GBs is related to the grain boundary type. To obtain a valid comparison of Cr profiles with and without radiation, Electron Backscatter Diffraction (EBSD) will be required to first select two grain boundaries of the same type and similar orientations on these two regions. TEM lamellas of three representative grain boundaries (low angle grain boundary, high angle grain boundary, and Sigma3 grain boundary) will be prepared for regions exposed to irradiation/corrosion and corrosion only (totally 6 TEM lamella).
Impact: In addition to interesting fundamental science, this effect has the potential to derisk molten salt reactor materials significantly. If the irradiated environment is found to be equally or less aggressive than the unirradiated environment via radiation-decelerated corrosion, then the path to commercialization of MSRs can be greatly shortened.
Additional Info
| Field | Value |
|---|---|
| Abstract | Motivation: The degradation of materials in molten salt reactors (MSRs) must be understood before they can be constructed on a commercial scale. Among different degradation modes, corrosion in molten salts has been extensively studied, proceeding mainly via selective dissolution of Cr from the alloys into the salts. The corrosion rate is influenced by a variety of parameters, such as temperature, salt impurity content, and radiation flux. Radiation, in particular, will influence most of the parameters. however, its effects on corrosion of MSR-relevant alloys have rarely been studied. Preliminary Results: To fill this knowledge gap, a simultaneous corrosion/irradiation facility for testing molten salt-facing materials was constructed at MIT. 3 MeV protons are used as a radiation source to bombard sample foils 30 microns thick, allowing the proton beam to pass through while producing relatively uniform radiation damage. On the back side of the sample foil, a heated molten fluoride salt reservoir serves as a corrosion medium. By masking the beam, each sample is divided into two regions, an irradiated/corroded region in the center, and a corrosion only region surrounding it, which enables us to directly target the influence of radiation on corrosion using one sample. Ni-20Cr irradiated in 650°C FLiNaK+5EuF3 for 4 hours showed that proton irradiation decelerates corrosion under the testing conditions. This conclusion is made from the fact that the region under corrosion was corroded through, while the region under both corrosion and irradiation was not. This was further verified by cross-sectional images on multiple sample foils. Hypothesized Mechanism: Radiation induced segregation of Ni and Cr atoms is proposed as the mechanism behind radiation-decelerated corrosion. Cr atoms segregate away from GBs, “hiding” them from the salt, while Ni atoms segregate toward the GBs, making the GBs more “inert,” thus delaying corrosion voids from connecting. Proposed Experiments: To back up the proposed mechanism, scanning transmission electron microscope (STEM) with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS) capabilities will be used to detect the proposed segregation of Ni and Cr within 10 nm scale under proton irradiation. For the segregation of Ni, spots in the corrosion and irradiation region will be sufficient. However, it is noticed that the corrosion attack of GBs is related to the grain boundary type. To obtain a valid comparison of Cr profiles with and without radiation, Electron Backscatter Diffraction (EBSD) will be required to first select two grain boundaries of the same type and similar orientations on these two regions. TEM lamellas of three representative grain boundaries (low angle grain boundary, high angle grain boundary, and Sigma3 grain boundary) will be prepared for regions exposed to irradiation/corrosion and corrosion only (totally 6 TEM lamella). Impact: In addition to interesting fundamental science, this effect has the potential to derisk molten salt reactor materials significantly. If the irradiated environment is found to be equally or less aggressive than the unirradiated environment via radiation-decelerated corrosion, then the path to commercialization of MSRs can be greatly shortened. |
| Award Announced Date | 2019-05-14T15:56:04.843 |
| Awarded Institution | Center for Advanced Energy Studies |
| Facility | Microscopy and Characterization Suite |
| Facility Tech Lead | Alina Zackrone, Yaqiao Wu |
| Irradiation Facility | None |
| PI | Weiyue Zhou |
| PI Email | [email protected] |
| Project Type | RTE |
| RTE Number | 1742 |