Committee Members:

Prof. Preet M. Singh, Advisor, MSE

Prof. Chaitanya S. Deo, ME/MSE/NRE

Prof. Hamid Garmestani, MSE

Prof. Joshua P. Kacher, MSE

James R. Keiser, Ph.D., Oak Ridge National Laboratory

Effects of Redox Potential on Corrosion in Molten Fluoride Salt

Abstract:

Over the last decade, there has been an increased interest in Molten Salt Nuclear Reactors (MSR) and Concentrated Solar Power Plants (CSP) as methods of generating clean, safe, and economical energy.  For MSRs, this will be accomplished by using molten salts, commonly molten fluoride salts such as FLiNaK (LiF-NaF-KF), as the coolant and heat transfer agent.  A major challenge in the implementation of these new technologies is the reliable performance of structural materials that must withstand molten fluoride salts at high temperatures.  Molten fluoride salts can be extremely corrosive to the structural alloys, and this corrosion is exacerbated by the presence of various oxidizing impurities in the salt.  These impurities drive up the redox potential of the molten salt, which can lead to the selective dissolution of more active alloying elements from the structural materials in contact with the salt.  As a higher redox potential of the molten salt is the main cause of these changes, redox control methods such as purification of salt, addition of active elements that can react with and consume the oxidizing impurities, and electrochemical cathodic protection can be effective in controlling corrosion of structural materials in the salt.  However, the exact effect of different types of impurities and redox control methods on the redox potential of the salt and the behavior of structural materials in contact with the salt had not been thoroughly studied.   

This research was aimed at understanding the effect of various oxidizing impurities on the corrosion of structural materials and interaction of nuclear graphite with molten fluoride salts and the viability of various redox control methods to control degradation of structural materials.  This work also focused on creating a methodology to reproducibly measure the redox potential of structural materials in a molten fluoride salt system.  The effects of addition of impurities and application of redox control methods on the corrosion of structural materials, mechanical properties of structural materials, and behavior of nuclear graphite were systematically probed by standardized static exposure tests of candidate alloys and nuclear graphite in molten FLiNaK, with manipulation of salt compositions and exposure condition variables to answer specific research questions. The changes in redox potential due to the changes in these variables were probed using electrochemical techniques to quantifiably correlate these variables with the corrosion of the structural materials in molten fluoride salt.  The knowledge gained from this research clarifies the effect and efficiency of different impurities and redox control methods on corrosion in molten fluoride salts in MSR and CSP conditions.