Corrosion of Directed Energy Deposited Cantor Alloy in Molten KNO3-NaNO3 for Solar Thermal Applications

Abstract

High entropy alloys (HEA) with 4-5 component elements are being considered for various applications including nuclear and solar thermal energy generation due to desirable mechanical properties and corrosion resistance. Advanced heat transfer systems in both nuclear and solar thermal energy generation are being designed to utilize molten salts as the working fluid, allowing for a significant increase in hot-leg temperatures leading to increased efficiency. The use of additively manufactured HEAs may be beneficial to these systems but corrosion properties of HEAs have not yet been established for many of the common molten salt systems. Additionally, the unique microstructure of HEA components produced via additive manufacturing can potentially alter the corrosion properties of the component relative to a cast or forged microstructure. This study will focus on CoCrFeMnNi HEA Cantor alloy prepared via an additive manufacturing pathway using directed energy deposition (DED); this method will allow for the development of complex geometries not available through traditional manufacturing methods. To understand the corrosion of additively-manufactured HEAs, a sample of DED-manufactured Cantor alloy was exposed to a 40/60 weight percent mixture of molten KNO3-NaNO­3 at 500 °C. This environment is relevant to solar thermal power plants. Accelerated corrosion testing using potentiodynamic polarization was carried out. Also, the sample subsequently underwent 50 hours of exposure to the molten salt. The sample surface was analyzed with x-ray diffraction, x-ray photoelectron spectroscopy, and focused ion beam milling coupled with scanning electron microscopy/energy dispersive spectroscopy. The molten salt electrolyte was analyzed post-exposure via inductively-coupled plasma – optical emission spectroscopy. Cantor alloy exposed to this nitrate salt mixture developed an oxide film with small, densely packed, vertically oriented fin-shaped grains and an inhomogeneous composition profile through its depth. The results of these studies will be discussed. Acknowledgments: The authors thank Zachary Karmiol, research scientist at Materials Characterization Nevada, for technical input. One of the authors, JM, acknowledges the Graduate Research Fellowship from the US National Science Foundation. This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1447692.