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Evaluation of the Corrosion Behavior of High Nickel Alloys in Molten Nitrate Salt for Solar Thermal Applications
Materials Science and Engineering
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Increasing population and standard of living around the world has placed significant demand on the production of electricity. The use of fossil fuels for energy production is not a sustainable practice as fossil fuels cannot be quickly replenished. Renewable energy resources are sources of energy that are not depleted when harvested such as solar, wind, hydroelectric, and geothermal power. Concentrated solar power (CSP) involves harnessing the heat energy of sun and generally stores this heat energy to continue energy production after the sun has set. This process of thermal energy storage allows efficient storage of energy without the use of batteries. Intermittency of energy production would be significantly reduced using molten salts as a heat transfer fluid in a CSP plant that utilized a thermal energy storage system. Efficiency is an important factor in the consideration of any process and would be paramount to successfully implementing CSP plants to generate energy from sunlight. A significant barrier to the implementation of this source of renewable energy is stable container materials that can withstand the high heat required for increased efficiency and aggressive corrosion of molten salts. Exposure studies are a common way of testing the corrosion behavior of a material in situ but require the temperature of the molten salt to be maintained throughout the duration of the experiment which can be thousands of hours. Accelerated corrosion testing utilizing electrochemistry was utilized to screen five materials tested in this study – UNS N06230, UNS N06025, UNS N06617, UNS N06625GR1, and UNS N06625GR2 – for further testing through evaluation of electrochemical behavior. The surface morphology of the samples was studied using scanning electron microscopy. The surface oxide chemistry and structure were analyzed using X-ray diffraction, energy dispersive spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy for three of the samples. Electrochemical evaluation of the five materials gave an indication of good corrosion resistance in this medium. All tested samples had current densities on the order of 10-4 A/cm2. UNS N06230 exhibited the lowest average corrosion current density at 0.225 mA/cm2. UNS N06625GR1 exhibited the highest average corrosion current density at 0.431 mA/cm2. Corrosion potential of samples ranged from -66.2 mV vs. Pt to -227 mV vs. Pt. The morphology of the samples surfaces was studied using scanning electron microscopy which showed the formation of a surface film on all samples. Cross-sectional analysis was performed using focused ion beam (FIB) scanning electron microscopy. Pseudopassivity was indicated by the polarization curve of UNS N06625GR1 and was supported through Raman and XPS scans. Raman spectra indicated the formation of a nickel oxide on the surface with varying levels of chromium and iron present. EDS data indicated increases in the oxygen content on the surface of all samples. Raman spectra paired with XPS suggested the formation of various nickel-chromium-iron spinels on UNS N06625. These results show that UNS N06625 has the potential to be a material for use in solar-thermal plants.