Redox Buffering and Corrosion in Molten Chloride Salts Dictated by Chlorobasicity

This study establishes an in situ synchrotron approach to directly link molten-salt chemistry, redox-buffer speciation, and alloy corrosion in chloride melts. Using a first-of-its-kind corrosion cell coupled to simultaneous X-ray absorption spectroscopy (XAS) and X-ray fluorescence (XRF), we monitor europium redox-buffer speciation and chromium dissolution during corrosion of Ni-20Cr in molten LiCl-KCl and KCl-MgCl2 salts. To build a mechanistic framework, we combine UV-vis optical basicity measurements, in situ electrochemistry, and molecular dynamics simulations to quantify how salt chlorobasicity reshapes europium solvation and shifts the Eu(III)/Eu(II) apparent potential. Increasing MgCl2 content drives the apparent potential of the Eu(III)/Eu(II) couple +0.49 V relative to LiCl-KCl and stabilizes Eu(II)-compatible solvation environments. In situ, time-resolved Eu L3-edge XANES reveals progressive Eu(III) reduction in KCl-MgCl2 melts, while Eu(III) remains predominantly stable in LiCl-KCl. This behavior is consistent with a chlorobasicity-activated reaction with boron nitride (BN) cell components. In less chlorobasic melts, salt redox chemistry enables BN chlorination, allowing BN to act as a sacrificial reductant and chlorinating agent that promotes Eu(III) to Eu(II) conversion, whereas this pathway is not thermodynamically favored in LiCl-KCl. During active corrosion of Ni-20Cr, coupled Eu XANES and Cr XRF measurements show concurrent Eu(III) to Eu(II) conversion and Cr dissolution, confirming that Eu(III) is the oxidizing species driving chromium oxidation. Together, these results demonstrate that chlorobasicity is a central control variable for redox-buffer speciation and corrosion thermodynamics, and more broadly that chlorobasicity can enable or suppress reactive pathways with structural materials in high-temperature molten salts.