Investigating the Effect of CrCl3 on Corrosion Behavior of Ni and Ni-20Cr in Molten ZnCl2 Salt By Electrochemical Noise Measurements

Abstract

Molten chloride and fluoride salts possess various properties such as good thermal conductivity and stability, large specific heat and low viscosity which make them important as fuels and/or coolants for molten salt reactors (MSR). Molten chloride salts are also selected as heat transfer and storage fluids to carry thermal energy from a solar concentrator to a steam generator in concentrating solar power (CSP) technologies. In both the applications the nickel-based alloys are used as structural materials. However high operating temperatures and extremely corrosive nature of molten salts promote faster corrosion of the structural materials. Hence, the corrosion behavior of the structural materials needs to be understood and monitored with time when exposed to the corrosive molten salt environment. Electrochemical techniques are useful for understanding the corrosion mechanism of metals and alloys in molten salts. Among them, electrochemical noise (ECN) is a simple in-situ technique to monitor the corrosion in real time without applying any external signals to the metal/alloy in corrosive environments. A unique advantage of ECN technique is the possibility to detect and analyze the early stages of the localized corrosion process during corrosion initiation as well as a type of corrosion.The corrosion of metals and alloys in molten chloride is strongly affected by impurities (O2, H2O) present in the atmosphere, temperature changes, and concentrations of various chemical impurities (metal oxides and metal chlorides). These impurities have been considered as strong oxidizing agents for metals and alloys in molten chloride salts. Chromium is a major alloying element in nickel-based alloys. Cr can be oxidized into metal chlorides CrClx (x=2,3) due to their favorable Gibbs free energies of the formation as compared to NiCl2 at 623 K. Cr3+ and Cr2+ coexist in the molten salt during corrosion of nickel-based alloys, while Cr3+ is the primary valence state. It is important to study the effect of these multi-valent ions on corrosion of metals/alloys in molten salts. To the best of our knowledge, no study on the effect of Cr3+ on the metals/alloys in molten chloride medium has been reported.The effect of CrCl3 on the corrosion behaviour of Ni and Ni-20 wt %Cr (NiCr) alloy in purified molten ZnCl2 at 623 K was investigated for the first time using ECN measurements in an argon-filled glovebox. ECN measurements were carried out in a three-electrode setup using two working electrodes and platinum as a reference electrode in molten CrCl3-ZnCl2 at 623 K. Ni and NiCr corrosion behavior was examined in two different combinations : 1) identically coupled electrodes (Ni-Ni and NiCr-NiCr vs Pt) and 2) galvanically coupled electrodes (Ni-NiCr vs Pt). Different types of current and potential noise fluctuations have been identified for pitting and other forms of corrosion. Analysis of fluctuations in current and potential noise in Ni and NiCr in all the three combinations concluded that Ni and NiCr exhibited localized corrosion. The addition of CrCl3 to ZnCl2 accelerated the dissolution of Cr and Ni at the metal-salt interface. After ECN studies, the working electrodes were examined by scanning transmission electron microscopy (STEM) coupled with energy dispersive X-ray spectroscopy (EDS) and electron energy loss spectroscopy (EELS). STEM/EDS/EELS observations confirmed pit formation, Ni and Cr dissolution, and Cr depletion at the salt-metal interfaces. Ni-rich and Cr-depleted precipitates were observed in the salt with a galvanically coupled NiCr electrode after ECN studies. The in-depth ECN results will be discussed in conjunction with micro- and nano-scale imaging of dissolution of Ni and Cr elements at the metal-salt interface, and plausible corrosion mechanisms will be proposed. The present ECN results on Ni and NiCr are compared with ECN studies in pure ZnCl2 salt.This work was supported as part of the Molten Salts in Extreme Environments Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science.