Fundamental Understanding of Metal and Alloy Corrosion in Molten LiF-NaF-KF Salts
Committee: Professor Robert Kelly (MSE), Committee Chair
Professor Bicheng Zhou (MSE), Committee Member
Professor Giovanni Zangari (MSE), Committee Member
Professor Chris Paolucci (CHE), Committee Member
Professor John Scully (MSE), Advisor
Molten salt reactor (MSR) technology has become a significant global thrust to realize the promise clean nuclear energy development through a collaborative effort between academia, government laboratories, and industry around the world. MSR is a high-temperature reactor (>500 °C) that utilizes melted metal fluoride salts (also known as molten salts or MS) as the media for nuclear fission and heat transfer. One key advantage of using molten fluorides are that molten salt is water-free and operates at low pressure, preventing steam explosion - a major factor of nuclear disasters. However, molten fluorides were found to be highly corrosive toward structural metals when moisture and metallic cation impurities are present. Corrosion in molten fluorides remains a critical challenge today. The underlying factors that regulate the corrosion process must be well understood to prevent materials failures and ensure the overall sustainability of MSR development.
Published literature adopts a phenomenological approach to characterize the corrosion behavior of candidate materials, which involves the reporting of mass loss, compositional analysis of test solution, cross-sectional microstructure to examine the depth and pathway of corrosion attack exposed to static or flowing MS. This approach often lacks diagnostics, does not assess rates except by serial exposure and makes it difficult to separate variables critical to the corrosion process, leaving questions such as (1) What is the valance of metal dissolution? (2) What are the rate-determining steps governing corrosion rate? (3) What is the role of microstructure and salt chemistry? The real challenge remains that these experiments are often high fidelity and are not adaptable to advanced scanning methods recently developed for aqueous corrosion studies. Therefore, investigation on the corrosion behavior of structural metals should begin with model materials (such as pure metal or high-purity binary alloys); with the focus to target the fundamental principles of corrosion. In this proposal, the focus is to understand the factors that govern the rate-determining steps of dominant corrosion reactions occur at the electrode-salt interface.
The objective of this proposed work is to develop a mechanistic understanding the corrosion process of model metals and alloys (Cr, Ni, Ni-Cr) in molten fluoride salts using the principles of electrochemical mixed potential theory and multi-modal analysis (including a combinatorial approach of corrosion assessment coupling XRD, SEM, EDS, and electrochemical assessment) The proposal will be organized into five tasks:
1. Task 1 aims to illustrate the spontaneity and thermodynamic driving force for metal corrosion by constructing potential-activity diagrams for pure metals in molten fluoride salts and depicting the thermodynamic phase stability of metals and their cations in molten fluorides.
2. Task 2 aims to identify the rate-determining steps (rds) and thermodynamic phase stability regimes where rds changes for of pure metal corrosion (Cr, Ni) in molten fluorides using electrochemical techniques. The effect of crystallographic orientation will also be discussed.
3. Task 3 aims to develop an in-situ, multi-electrode technique to provide temporal description on the corrosion processes and rates of metals and alloys in molten FLiNaK salts.
4. Task 4 aims to understand the corrosion and dealloying behavior of Ni-Cr alloys in molten FLiNaK salts by investigating the effect of alloy composition and electrochemical potential on their morphological evolution. The goal is to identify the parting limit and critical potential for the bicontinuous dealloying of Ni-Cr in molten FLiNaK salts.
5. Task 5 aims to understand how the variation in specific physical and microstructural attributes, including the degree of cold working, grain size and solute ordering, will impact the energy landscape and rates for bicontinuous dealloying in Ni20Cr alloys exposed to FLiNaK salts.
To provide a well-rounded, practical description on the interfacial and transport processes that dictate metal and alloy corrosion in molten fluoride salts, the effect of irradiation will also be explored in Tasks #2, #4 and #5. This proposal targets fundamental mechanism and is moving beyond observational descriptions of materials evolution in molten salts and towards a more fundamentally based scientific yet quantitative understanding of corrosion. Using the principles of understanding the electrochemical thermodynamics and kinetics of model materials as a basis with multi-modal analysis, it made possible to determine dominant variables that control or govern the underlying corrosion mechanism and regulate rates, and enable the development of accurate, quantitative framework to measure the degradation rates, which is a knowledge gap that is not addressed in current research.
All interested persons are invited to attend. Please contact Ho Lun Chan for Zoom information.