Salt potential control by titanium chloride to mitigate the corrosion in molten chloride salts

Abstract

This study examines a thermodynamic approach to mitigating metal corrosion in molten chloride salts, focusing on lowering the salt's potential to protect structural materials. Thermodynamic diagrams show that the constituents of stainless steel and Ni-based alloys, i.e. Fe, Ni, and Cr, are susceptible to oxidation due to their open circuit potential (OCP) relative to the potential of the salt. Mo is an exception, as it is inert. The addition of TiCl₂ to the molten salt effectively reduces its potential, buffering it at - 1.07 V, below the oxidation threshold of Cr. This reduction is achieved by in situ synthesis of TiCl₂ by ZnCl₂, which prevents excessive oxidation of Ti to higher states, and maintains the stability of the salt potential over time.

Corrosion tests on 304 L stainless steel and Inconel®600 show significant corrosion without TiCl₂, as the unbuffered salt potential exceed the immunity domain of these alloys. However, the addition of TiCl₂ shifts the salt potential into the immunity domain of Fe, Ni, and Cr, preventing corrosion. The results that a controlled, lower salt potential is crucial for ensuring material stability in molten chloride environments. The results highlight the importance of maintaining a zero redox potential difference between the salt and the metals to achieve inertness. Therefore, controlling the molten salt potential appears to be essential when considering structural integrity in harsh molten salt environments.