Study on the corrosion behavior of Inconel 625 in molten chloride salt

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Feng, Jinghua
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Electronic thesis
Nuclear engineering
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Concentrated solar power (CSP) has been used as one of the most important clean and renewable energy sources. Molten chloride salts, which possess excellent thermal-physical properties with low economic cost, have become the most promising heat transfer media and thermal energy storage media for new generation CSP in recent years. However, molten chloride salts are very corrosive to the thermal components and containment materials such as piping, storage tanks and heat exchangers. Extensive studies have been devoted to the corrosion behaviors of nitrate and fluoride salts which are commonly used in traditional solar power plants and molten salt nuclear reactors respectively. But there has been relatively little research on the corrosion behavior of molten chloride salts to Ni-based alloy film. The goal of this study is to study the corrosion behaviors of the Inconel 625 film in molten chloride salt, and to explore methods of improving the corrosion resistance of the film by combining both high-temperature corrosion experiments and cellular automata (CA) simulations. In order to achieve this goal, three task have been conducted in this study: (i) Explore the microstructure of the Inconel 625 film obtained by magnetron sputtering; (ii) Investigate the corrosion behavior of molten chloride salt to Inconel 625 film and improve the corrosion resistance of Inconel 625 by growing the grain size of the film; (iii) Develop a cellular automata program to mimic the diffusion-reaction process of molten salt. Surface morphology and film phase identification are studied by scanning electron microscope (SEM) and X-ray diffraction (XRD) respectively. The SEM scan shows that the grain size of the films obtained by magnetron sputtering grows with the rising deposition temperature. Based on the XRD patterns, all these films display planes of (111) and (222) of Ni3Cr2, but the film deposited at 600 °C exhibits a unique peak at 51° corresponding to (200) of Ni3Cr2. XRD rocking curves show a decreasing FWHM value with increased grain size, which indicates that film deposited at higher temperature has relatively higher degree of crystallinity. A seven-slab reflectometry model (Sapphire - Inner contamination - Inconel sublayer - Principal Inconel - Oxide - Outer contamination - Air) has been successfully developed and verified to study the layered structure of Inconel 625 films with atomic-scale spatial resolution along the surface normal. The model reveals that ~2 nm thick Inconel sublayer is found underneath the principal Inconel film. A thin NiO oxide layer is found on top of the principle Inconel. The thickness of NiO layer is observed to decrease with growing deposition temperature. Two very thin contaminations are identified both on the substrate and on the oxide film respectively. The corrosion tests are conducted to study the corrosion behaviors of Inconel 625 in molten salt, which indicate that Cr is the most readily attacked element in Inconel 625. It is also found that Mo enrichment occurs in most Cr-depleted regions. Among salt ingredients, Mg and Cl are two most commonly observed elements in the corroded films. The comparison of corrosion tests for films with different grain size demonstrates that the increase of the film grain size alleviates the Cr depletion and consequently improves the corrosion resistance of Inconel 625. We successfully developed a CA model with a graphical user interface to simulate the corrosion processes of films with different grain size. It is found that the film with larger grain size is more corrosion resistant and the ratio of Cr and Ni decreases with generation, which are consistent with experimental results. In addition, all surface roughness curves exhibit very large fluctuations in the course of corrosion. The sharp roughness rise is usually caused by the corrosion along vertical grain boundaries while the sudden drop is attributed by the detachment of grains from the film. These findings are beneficial in predicting and improving the material properties of Ni-based alloys, and thus provide guidance for the materials selection in third generation CSP.
August 2021
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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