Thermophysical and Molten Salt Corrosion Behavior of Structural Materials for Next-Generation Clean Energy Systems
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Brankovic, Sonja
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Abstract
As next-generation clean energy technologies like concentrated solar power (CSP) and molten salt reactor (MSR) systems operate at ever higher temperatures to increase efficiency and thermal energy storage capabilities, using molten chloride salt as a heat transfer and energy storage fluid can provide many benefits, including high-temperature operation, a low operating pressure, extended storage time, and increased safety. In this extreme environment, it is essential to understand the temperature-dependent thermophysical properties and molten salt corrosion behavior of candidate structural alloys and aluminosilicate refractories for salt storage tanks, piping, and heat exchangers. Though these types of materials have been used in established applications (for example, aerospace and gas turbine engine components in the case of Ni-alloys, hot-face furnace liners for aluminosilicate refractories), corrosion studies of these types of alloys are not easily comparable in the literature; for several alloys and many of the refractories studied in this project, published corrosion data does not exist. High-temperature thermophysical data of the candidate alloys and refractories are more widely available, though not consistently in the temperature range of interest (600–800°C). The purpose of this thesis project is first to characterize the high-temperature thermophysical properties (thermal diffusivity, specific heat, and thermal conductivity) of the candidate materials. This data, combined with published results from the literature, is then used to down-select materials for temperature-dependent immersion corrosion testing.
Twelve Ni- and Fe-based alloys and seven aluminosilicate refractories were initially selected for experimental testing and sourced from commercial manufacturers. The temperature-dependent thermal diffusivity and specific heat of these candidate alloys and refractories were determined via light flash analysis (LFA) and differential scanning calorimetry (DSC), respectively. These experimental results were compared with available manufacturer data of the materials’ high-temperature thermophysical properties. A subset of high-performing and commercially viable alloys and refractories, with the addition of two alumina-forming alloys, were selected for molten chloride salt corrosion testing. Samples were immersed in purified 45.98 MgCl2–38.91 KCl–15.11 NaCl wt% salt for 100 hours at 650°, 725°, and 800°C. Corrosion rates were calculated based on nominal sample densities and measured weight changes after the immersion test; comparisons of pre- and post-test surface elemental and phase compositions were performed using X-ray fluorescence (XRF) and X-ray diffraction (XRD), respectively. A more detailed cross-section analysis was performed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS).
For the Ni-based alloys, the measured specific heat and thermal diffusivity were approximately linear as a function of temperature (which is commonly seen in manufacturer data sheets) but did see some evidence of second-order phase changes in the DSC data. In situ HT-XRD testing of several down-selected alloys showed that the alloys’ crystal structure was expanding as a function of temperature in a roughly linear manner, though there was no clear appearance of new phases or decrease in material stability. The aluminosilicate refractories exhibited no obvious phase changes in the DSC or LFA runs; this was confirmed by the in situ XRD tests.
After the 100-hour immersion testing, uniform corrosion was visible on many of the samples’ surfaces and increased as a function of temperature, based on the measured mass loss of each sample. The temperature-dependent increase was most apparent in the alloys with a significant base iron content. This trend was confirmed by SEM imaging and EDS linescans of the sample cross-sections. XRF testing of the corroded alloys’ surfaces showed several compositional changes that are commonly seen in molten halide corrosion, including depletion of active metals like iron and chromium, and a corresponding enrichment in more noble elements like nickel and molybdenum. For several of the alloys, XRD testing of the corroded surfaces showed some evidence of oxide contamination. For the pre-oxidized alloys, no significant difference in performance was observed compared to the bare alloys; the developed oxide layer provided no measurable corrosion protection after 200 hours of chloride salt corrosion testing.
Corrosion testing of the aluminosilicate refractories revealed no consistent, temperature-dependent trend in mass gain after 200 hours of chloride salt immersion at three temperature points. However, at higher test temperatures (725° and 800°C), vaporized chloride salt penetrated the refractories’ surface above the immersion line. A “transition line” was also observed, marking the highest level of the molten salt; this line was darker than the vapor and immersed regions of the refractories, indicating that any residue or contaminants floated on the surface of the molten salt.
This thesis work is significant because it provides a broad, high-temperature thermophysical characterization of candidate alloys and aluminosilicate refractories for the next-generation solar and nuclear industries. Compared to the provided manufacturer data, the temperature-dependent runs from this thesis work provides a much finer dataset and elaborates on trends that are more subtle in the published archive. The results from the immersion molten chloride salt testing of down-selected alloys and refractories contribute important data for these same industries where understanding material corrosion resistance is critical for safe and economic performance.
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Date
2024-05-01
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Dissertation