Material Degradation Studies in Molten Halide Salts
Dsouza, Brendan Harry
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This study focused on molten salt purification processes to effectively reduce or eliminate the corrosive contaminants without altering the salt's chemistry and properties. The impurity-driven corrosion behavior of HAYNES® 230® alloy in the molten KCl-MgCl2-NaCl salt was studied at 800 ºC for 100 hours with different salt purity conditions. The H230 alloy exhibited better corrosion resistance in the salt with lower concentration of impurities. Furthermore, it was also found that the contaminants along with salt's own vaporization at high temperatures severely corroded even the non-wetted surface of the alloy. The presence of Mg in its metal form in the salt resulted in an even higher mass-loss possibly due to Mg-Ni interaction. The study also investigated the corrosion characteristics of several nickel and ferrous-based alloys in the molten KCl-MgCl2-NaCl salt. The average mass-loss was in the increasing order of C276 < SS316L < 709-RBB* < IN718 < H230 < 709-RBB < 709-4B2. The corrosion process was driven by the outward diffusion of chromium. However, other factors such as the microstructure of the alloy i.e. its manufacturing, refining, and heat-treatment processes have also shown to influence the corrosion process. Lowering the Cr content and introducing W and Mo in the alloy increased its resistance to corrosion but their non-uniform distribution in the alloy restricted its usefulness. To slow-down the corrosion process, and enhance the material properties, selected alloys were boronized and tested for their compatibility in the molten KCl-MgCl2-NaCl salt. The borided alloys exhibited better resistance to molten salt attack, where the boride layer in the exposed alloy was still intact, non-porous, and strongly adhered to the substrate. The alloys also did not show any compensation in their properties (hardness). It was also found that the boride layer always composed of an outermost silicide composite layer, which is also the weakest and undesired layer as it easily cracks, breaks, or depletes under mechanical and thermal stresses. Various different grades of "virgin" nuclear graphites were also tested in the molten KF-UF4-NaF salt to assist in the selection of tolerable or impermeable graphites for the MSR operational purposes. It was found that molten salt wettability with graphite was poor but it still infiltrated at higher pressure. Additionally, the infiltration also depended on the pore-size and porosity of the graphite. The graphite also showed severe degradation or disintegration of its structure because of induced stresses.
General Audience Abstract
Molten salts are considered as potential fuel and coolant candidates in MSRs because of their desirable thermophysical properties and heat-transfer capabilities. However, they pose grave challenges in material selection due to their corrosive nature, which is attributed to the impurities and their concentration (mostly moisture and oxygen-based) in the salt. This study focused on purifying the salt to reduce these contaminants without compromising its composition and properties. The influence of purification processes on the corrosion behavior of HAYNES® 230® alloy was studied in the molten chloride salt with different purity conditions. Various nickel and ferrous-based alloys were also studied for their compatibility in the molten chloride salt. This will assist in expediting the material selection process for various molten salt applications. It was observed that several factors such as alloy composition, its microstructure, impurities in the salt attribute to molten salt corrosion. It was also quite evident that corrosion in molten salts is inevitable and hence, the focus was shifted on slowing down this process by providing protective barriers in the form of coatings (i.e. boronization). The borided (coated) alloys not only improved the corrosion resistance but also enhanced and retained their properties like hardness after exposure to molten salts. Since these studies were conducted under static conditions, a more detailed investigation is needed for the selected alloys by subjecting them to extreme flow-conditions and for longer a duration of time. To achieve this objective, a forced circulation molten salt loop was designed and fabricated to conduct flow corrosion studies for alloys in molten chloride salt. Graphite is another critical component of the MSR where it is used as a moderator or reflector. Generally, molten salts exhibit poor wettability with graphite, but they can still infiltrate (graphites) at higher applied pressures, and result in the degradation or disintegration of graphite's structure, and eventually its failure in the reactor. This study provides infiltration data, and understanding of the degradation of various 'virgin' nuclear graphite grades by the molten fluoride salt. This should assist in the selection of tolerable or impermeable graphite grades for MSR operational purposes.
- Doctoral Dissertations