In situ transmission electron microscopy of high-temperature inconel-625 corrosion by molten chloride salts

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Pragnya, Prachi
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Electronic thesis
Materials engineering
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High-temperature corrosion of molten salt containment materials is of great significance for thermal energy storage systems that are used in concentrated solar power plants (CSP). Mitigating this corrosion is critical for developing cost-effective, energy-efficient systems, which demands comprehensive and thorough understanding of the determinant corrosion reaction mechanisms. So, in this research work, corrosion of Inconel-625 by pure molten chloride salts (MgCl2 − NaCl − KCl) at 500-800 °C has been investigated. It is based on an environmental cell assembly in situ transmission electron microscope (TEM). TEM diffraction and imaging techniques are used to investigate microstructural and compositional evolution at salt-alloy interfaces. A clustering algorithm and a 2D Gaussian fit function are used to determine diffraction spot intensities in in situ diffraction patterns, to quantify alloy corrosion. This facilitates quantitative observation of the evolution of individual grains, in contrast to conventional macroscopic corrosion rate quantification. Procedures are established to minimize incorporation of H2O or O2 from atmosphere in the chloride salts during sample fabrication and corrosion. At first, the corrosive effect of air-exposure on corrosion of Inconel-salt sample is studied by comparing sample corroded with and without air contamination (i.e., vacuum transferred). The Inconel-625 corrosion rate for vacuum transferred samples is 220 ± 20 µm year-1 at 700 °C. Air contamination causes a much more pronounced increase to 1000 ± 150 µm year-1 at 700 °C. Moreover, the individual corrosive effects of the major corroding components present in air that are O2 and H2O vapor are also studied. To perform corrosion in O2 ambient, I employed a top chip that has a gas channel to hold ambient during corrosion. And to study effects of H2O ambient on corrosion, I developed a method to controllably hydrate the salt-stack without exposing it to molecular O2, in a reaction chamber maintained under high vacuum. Then, I investigate corrosion of Inconel-625 by pure molten chloride salts (MgCl2 − NaCl − KCl) at 500-800 °C in 1.0 atm inert N2 or pure O2, or H2O ambient. The isothermal corrosion rates of Inconel-625 are found to be 203 ± 30 μm year-1 for a sample at 700 °C with 1 atm N2 that increased to 463 ± 30 μm year-1 at 800 °C. The corrosion rate increased to 1261 ± 170 μm year-1 for 1 atm O2 ambient at 700 °C. The rate of corrosion in case of a hydrated salt sample at 500 °C is 95 ± 20 μm year-1 that increased to 468 ± 30 μm year-1 at 600 °C and soared to a lower limit of corrosion rate of 3 x 104 μm year-1 at 700 °C. These isothermal corrosion rates indicate that the molten chloride corrosion is significantly accelerated by salt hydration at temperatures above 600 °C. The corrosion is increased at high temperatures due to the generation of increased amounts of corrosive Cl2 and HCl gases coupled with volatile metal compounds. Real time imaging of the microstructure evolution suggests that corrosion is initiated at grain boundaries. Post-corrosion compositional analysis is performed using XPS high resolution scans and AES survey scans and major corrosion products are identified for inert N2 or pure O2, or H2O ambients.
December 2021
School of Engineering
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Rensselaer Polytechnic Institute, Troy, NY
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