Corrosion Behavior of High Nickel Alloys in Molten Nitrate Solar Salt

Abstract

Electrochemical corrosion testing was used to evaluate the corrosion resistance of four different nickel-based alloys, namely UNS N06230, UNS N06025, UNS N06617, and UNS N06625, in a molten nitrate salt that is used as a heat transfer media and thermal energy storage material in solar-thermal power plants. The tests were conducted at 500 °C to simulate near-maximum service temperature in the 3:2 by weight sodium nitrate: potassium nitrate mixture. All tested samples had corrosion current densities on the order of 10−4 A cm−2 with corrosion potentials varying from −227 mV to −66.2 mV vs Pt. The tested samples exhibited low corrosion current density in the range of 0.225 mA cm−2 to 0.431 mA cm−2. 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 scanning electron microscopy. The surface chemistry was ascertained using energy dispersive spectroscopy, X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). 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.