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.

Corrosion in Molten Salts for Solar Thermal Power

“…4 The solubility of Cr corroborates findings that corrosion of Fe-Cr-Ni steel alloys in thermal convection loops exposed to thermal cycling exhibits a three-layer corrosion film consisting of a Crdepleted outer layer, a Fe and Cr-rich oxide mid-layer, and a Ni-rich in contact with the base metal. 16,[18][19][20][21] Several studies of the oxide layer formed on Fe-Cr-Ni alloy systems generally show the formation of Fe-Cr spinels in duplex and/or triplex films with the formation of Ni-Fe-Cr oxide phases at the surface. [17][18][19][22][23][24][25] High Ni-content alloys, such as Incoloy 800H, form a thicker Ni-Fe spinel compared to stainless and carbon steels as the Ni in the spinel composition are correlated to the Ni content of the alloy.…”

“…[17][18][19][22][23][24][25] High Ni-content alloys, such as Incoloy 800H, form a thicker Ni-Fe spinel compared to stainless and carbon steels as the Ni in the spinel composition are correlated to the Ni content of the alloy. 19,20…”