This study focused on the kinetic data measurements, such as diffusion coefficient D and exchange current density i_0 of the electrochemical reactions of corrosion products (Fe, Cr and Ni ions) and corrosive species (OH-), and corrosion studies of structural materials (SS 316H and Alloy 617), including static corrosion and galvanic corrosion, in molten MgCl2-NaCl-KCl and/or NaF-KF-UF4-UF3 salts in a temperature range of 600 to 800C. The study applied the semi-differential (SD) analysis method and innovative fitting method for the kinetic property data measurements in the multicomponent system of NaF-KF-UF4-UF3 salts. In molten MgCl2-NaCl-KCl salts, the measured D_(OH^- ) has the largest value followed by D_(〖Cr〗^(2+) ), D_(〖Fe〗^(2+) ), D_(〖Cr〗^(3+) ) and D_(〖Ni〗^(2+) ) at the studied temperatures, and none of the diffusion coefficients depends on the ion concentration in the studied concentration range and all of them followed the Arrhenius law. At the same temperature, the measured D_(Fe^(2+) ) and D_(〖Cr〗^(2+) ) values in molten NaF-KF-UF4-UF3 salts were slightly smaller than those obtained in molten MgCl2-NaCl-KCl salts. The non-linear curve fitting technique was applied to determine the exchange current density i_0, charge transfer coefficient α, limiting current density i_L and standard rate constant k^0 values. i_0 and k^0 followed the Arrhenius law. The obtained fundamental data can be applied to corrosion models which make the corrosion rate prediction possible in a static system from the experimental kinetic data.
Corrosion studies of SS 316H and Alloy 617 in thermal purified molten NaF-KF-UF4-UF3 salts were performed for 120 hours. Based on the post-test analysis, the major metal species corrosion products were Cr, Fe and Mn in SS 316H tests, and Cr, Co, Ni in Alloy 617 tests. The measured UF4/UF3 ratio increased after corrosion tests because some of the U3+ was oxidized to U4+ by corrosive impurities and corrosion products during tests. Cr depletion and salt penetration were observed at grain boundaries (GBs) for both SS 316H and Alloy 617. For Alloy 617 specimens, the corroded area could be divided into two parts: the first part (near the surface) where Cr was completely depleted, and the second part (underneath the first part) where Cr was partially depleted. For SS 316H specimens, the average attack depth was larger than that of Alloy 617. Mo segregation was observed in the matrix of SS 316H specimens but was found to be enriched at GBs in the second part of Alloy 617 specimens. The corrosion study of Alloy 617 with time was also conducted for 72 hours and 32 hours, respectively. A thin layer composed of Fe, Co, Ni and Mo was found on the surface of the specimen, which was different from the previous 120-hour tests. In the salt, the concentration of Cr kept increasing with time, while for the other identified corroded elements, i.e., Fe, Co, Ni and Mo, their concentrations increased first, then decreased until becoming zero or stable.
In the galvanic corrosion study of Alloy 617/graphite in molten NaF-KF-UF4-UF3 salts, the galvanic corrosion rate of Alloy 617 at 750C was about four times of that at 650C in the 2-hour tests, which indicated that temperature has a significant effect on the galvanic effect. In the 120-hour galvanic corrosion test, the galvanic corrosion rate became slightly larger with time in the studied system. Similar to the previous 120-hour Alloy 617 corrosion test, the corroded area of the post-test specimen was divided into two parts. The measured attack depth in both parts were much smaller compared with that in the 120-hour Alloy 617 test. This was because of the lower corrosive impurity concentrations in the salt used in the test. The salt in the galvanic corrosion test has been used in the previous corrosion test, during which the corrosive impurities were consumed, which made the salt less corrosive.
Finally, it is necessary to point out that all the salts used in the present work were only thermally purified, which is effective in the removal of moisture but not in the removal of oxide impurities. Therefore, further studies are needed to understand the oxides' impacts on the corrosion behavior, especially on the salt penetration. / Doctor of Philosophy / Molten salt is a promising candidate that can be used as fuel and coolant in the molten salt reactors (MSRs). Besides, it can also be used as thermal energy storage, heat transfer fluid in the concentrated solar power plants, because it has high heat capacity, low vapor pressure, and high thermal conductivity. However, materials corrosion is a key concern of molten salt applications, and it is known that the corrosion by molten salts is mainly impurity driven. The impurities, such as moisture in the salts, can make the salt more oxidized, thus becoming more corrosive to corrode the structural materials. The present work focus on the kinetic property of metal specie corrosion products and non-metal impurity in the molten fluoride and chloride salts, which were directly related to the mass transfer and charge transfer process during the corrosion. Especially in the measurements of fluoride salts, innovative methods were applied which were confirmed to perform well in the multicomponent system (Fe and Cr ions coexisted). The static corrosion tests of SS 316H and Alloy 617 were conducted in molten fluoride salt at high temperatures. The main purpose was to study their corrosion behavior and understand the corrosion mechanisms. The corrosion rate of SS 316H was also estimated, which could be a crucial criterion in the material selection. In addition, the corrosion of Alloy 617 with time was also investigated. The metal specie corrosion product concentration change trends were obtained, and the corrosion behavior over the different corrosion stages was analyzed. Different corrosion phenomenon was observed in different corrosion test. Thus, they shed lights on the study of how the corrosion was developed during the corrosion process. Moreover, galvanic corrosion was another major corrosion type when two or more dissimilar materials were electrically contacted. The galvanic corrosion of Alloy 617/graphite was studied in the molten fluoride salts. The galvanic corrosion rate increased with the rise of temperature, which verified that temperature was a key factor that affected the galvanic corrosion. And the galvanic effect was also turned out to increase with time in the present study.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/112075 |
| Date | 04 October 2022 |
| Creators | Yang, Qiufeng |
| Contributors | Mechanical Engineering, Zhang, Jinsuo, Bai, Xianming, Liu, Yang, Pierson, Mark Alan |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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