Thermodynamic and kinetic corrosion behavior of alloys in molten MgCl2–NaCl eutectic: FPMD simulations and electrochemical technologies

Abstract

Electrochemical corrosion of alloy materials is one of the biggest challenges in the development of heat transfer and storage technology by molten salt. Here, the thermodynamic and kinetic properties as well as microstructural characteristics of divalent alloying elements ( M = Ni, Co, Fe, Mn, Cr, and Ti) in molten MgCl 2 –NaCl (MN) eutectic are analyzed based on first principles molecular dynamics (FPMD) simulations and electrochemical experiments, aiming to reveal corrosion rules of alloy and explain corrosion phenomenon. First, special thermodynamic cycles are designed to calculate the redox potentials of M 2+ | M couples in molten MN, which are corrected by the experimental potential of Ni 2+ |Ni reference electrode. Then, ionic/atomic self-diffusion coefficients of molten salt mixtures are evaluated from FPMD simulations, whose credibility is verified by the results from cyclic voltammetry and chronopotentiometry. Studies have shown that the oxidation reaction of Ti are both thermodynamically and kinetically favorable, while the reactions of Fe and Cr are advantageous in thermodynamics and kinetics, respectively. Furthermore, the Ni enrichment on the alloy surface is explained by the larger diffusivity of Ni 2+ than Ni in molten MN, thereby the deposition order of Ni > Mn > Co is deduced. Last, the dynamic coordination numbers and cage volumes of cation-Cl clusters are captured, where the delayed shuttle of Cl − in molten FeCl 2 -MN verifies the strongest stability of [FeCl 6 ] 4− octahedral structure and the short activation time of Cl − in molten CrCl 2 -MN results in the fast corrosion rate of Cr. • Redox potentials of divalent metals in molten MN were studied by FPMD and experiments. • Oxidation reactions of Fe and Cr are separately thermodynamically and kinetically favorable. • Metal deposition or dissolution depends on the ratio of diffusion coefficients of M 2+ and M . • Shuttle intensity of Cl − in M -centered coordination cages determines the corrosion rate of M .