Georgia Institute of Technology Materials Science and Engineering

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.  This is accomplished by using molten salts, most commonly molten fluoride salts, as the coolant and heat transfer agent.  The use of molten fluoride salts lets these reactors operate at much higher temperatures and much lower pressures, which lets them operate with much higher efficiency and with lower risk of accidents.

A major challenge in the implementation of these new technologies is the longevity of materials that must withstand molten fluoride salts at high temperatures.  In MSRs, two main material types are used: alloys as the structural components which comprises heat-exchanger tubes, pipes and vessels, and carbonaceous materials like graphite for reactor core components including fuel elements, moderators, and reflectors.  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 which increase the thermodynamic favourability of corrosion reactions.  These impurities drive up the redox potential of the molten salt, which leads 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 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 behaviour of structural materials in contact with the salt have not been systematically studied.

 The proposed research aims to create a methodology to reproducibly measure the redox potential of structural materials in a molten salt system, and to use the said method to measure the changes in redox potential of the salt with addition of various impurities and redox control methods.  The effects of addition of impurities and application of redox control methods on the corrosion of structural materials will be systematically probed by standardized exposure tests of candidate alloys in molten FLiNaK, with manipulation of salt compositions and exposure condition variables to answer specific research questions. Analysis of post-exposure samples, including weight change, SEM, EDS, XRD etc. will be used to describe and quantify effects of these variables.  The changes in redox potential due to the changes in these variables will be probed using electrochemical techniques to quantifiably correlate these variables with the corrosion of the structural materials in molten fluoride salt.  The knowledge gained from the proposed research will clarify the effect and efficiency of different impurities and redox control methods on corrosion in molten fluoride salts in MSRs and CSPs and will allow engineers to avoid potential problems related to corrosion by informed materials selection and management of coolant salt conditions.