Morphological Evolution and Dealloying During Corrosion of Ni20Cr in Molten FLiNaK Salt

Abstract

In this work, the dealloying corrosion behavior of the FCC Ni20Cr (wt.%) in molten LiF-NaF-KF (FLiNaK) salts at 600 °C under varying applied potentials was investigated. Using in-operando electrochemical techniques and a multi-modal suite of characterization methods, we connect electrochemical potential, thermodynamic stability, and electro-dissolution kinetics to the corrosion morphologies. Notably, under certain potential regimes, a micro-scale bicontinuous structure, characterized by a network of interconnected ligaments and pores with the composition of the more noble (MN) element, becomes prominent. At other potentials both MN and less noble (LN) elements dealloy but at different rates. The dealloying process consists of bulk and grain boundary diffusion of Cr to the metal/salt interface, interphase Cr oxidation, accompanied by surface diffusion of Ni to interconnected ligaments. This process is predominately controlled by charge transfer, resulting in the formation of Cr(II) and Cr(III) species at a constant rate of attack for a period of 10,000 seconds. At higher potentials, the bicontinuous porous structure undergoes further surface coarsening. Concurrently, Cr(II), Cr(III), and Ni(II) begin to dissolve, with the dissolution of Ni occurring at a significantly slower rate. When solid state transport of Cr is exceeded by the interfacial rates, dealloying depths are limited.