Adrien Couet

In the study, we report an in situ corrosion and mass transport monitoring method developed using a radionuclide tracing technique for the corrosion study of 316L stainless steel (316L SS) in a NaCl–MgCl₂ eutectic molten salt natural circulation loop. This method involves cyclotron irradiation of a small tube section with 16 MeV protons, later welds at the hot leg of the molten salt flow loop, generating radionuclides ⁵¹Cr, ⁵²Mn, and ⁵⁶Co at the salt–alloy interface. By measuring the activity variations of these radionuclides at different sections along the loop, both the in situ monitoring of the corrosion attack depth of 316L SS and corrosion product transport and its precipitation in flowing NaCl–MgCl₂ molten salt are achieved. While 316L SS is the focus of this study, the technique reported herein can be extended to other structural materials being used in a wide range of industrial applications.

Radiation and corrosion can be coupled to each other in non-trivial ways and such coupling is of critical importance for the performance of materials in extreme environments. However, it has been rarely studied in ceramics and therefore it is not well understood to what extent these two phenomena are coupled and by what mechanisms. Here, we discover that radiation-induced chemical changes at grain boundaries of ceramics can have a significant (and positive) impact on the corrosion resistance of these materials. Specifically, we demonstrate using a combination of experimental and simulation studies that segregation of C to grain boundaries of silicon carbide leads to improved corrosion resistance. Our results imply that tunning of stoichiometry at grain boundaries either through the sample preparation process or via radiation-induced segregation can provide an effective method for suppressing surface corrosion.

Refractory high-entropy alloys (RHEAs) are emerging materials with potential for use under extreme conditions. As a newly developed material system, a comprehensive understanding of their long-term stability under potential service temperatures remains to be established. This study examined a titanium-vanadium-niobium-tantalum alloy, a promising RHEA known for its superior high-temperature strength and room-temperature ductility. Using a combination of advanced analytical microscopies, Calculation of Phase Diagrams (CALPHAD) software, and nanoindentation, we investigated the evolution of its microstructure and mechanical properties upon aging at 700°C. Trace interstitials such as oxygen and nitrogen, initially contributing to solid solution strengthening, promote phase segregation during thermal aging. As a result of the depletion of solute interstitials within the metal matrix, a progressive softening is observed in the alloy as a function of aging time. This study, therefore, underscores the need for a better control of impurities in future development and application of RHEAs.

Zirconium oxide formed in high-temperature water conditions is highly heterogeneous in nature, with, for instance, the presence of a high density of grain boundaries and nanopores, secondary-phase precipitates, and microchemical segregations. Irradiation exacerbates these heterogeneities with effects such as radiation-induced segregation and precipitate dissolution/amorphization. The transport of species through the oxide is affected by these heterogeneities, resulting in complex transport mechanisms that are still not well understood. In this study, we focused on chemical heterogeneities in the oxide, specifically the oxide/metal (O/M) interface and how alloying elements are redistributed across the interface as it progresses into the substrate. For the first time, in situ atom probe tomography (APT) experiments, in which the APT needle is oxidized prior to analysis, have been performed on unirradiated and 1-dpa proton-irradiated Zr-Nb-Fe model alloys to characterize chemical redistribution as a function of oxidation temperature and time across the O/M interface. Results show that the niobium and iron contents in the oxide are higher than what can be accounted for only with solute capture. This finding suggests that there is a thermodynamic driving force for the niobium and iron solutes to migrate from the metal into the oxide in the unirradiated system. Under irradiation, niobium-rich irradiation-induced nanoclusters form in the metal matrix, and the iron and niobium solutes are more thermodynamically stable relative to the unirradiated system. We found much less niobium and iron in the oxide formed in the irradiated sample, corroborating the finding that the substrate is more thermodynamically stable. This finding has strong implications relative to unirradiated versus irradiated Zr-Nb oxidation kinetics because niobium solute doping in the oxide is known to significantly affect the alloy oxidation rate.

The molten salt-cooled reactor concept has garnered significant interest and one of the current challenges limiting the deployment of these reactor concepts is the complex corrosion phenomenon observed in molten salt environments. One of these phenomena is activity gradient mass transport, which has been shown to affect dissimilar materials submerged in the same salt medium even when best efforts have been made to electrically isolate dissimilar materials from one another. This mechanism while shown experimentally, has not been predictively studied through a modeling approach. In this study, activity gradients in several 316L-X materials systems have been modeled and the mass transport predicted by the model has been confirmed through static isothermal corrosion testing in a molten fluoride salt medium.

Although zirconium-niobium (Zr-Nb) alloys are commonly used over Zircaloys as fuel claddings in light-water reactors, the fundamental understanding of Nb effects on oxidation and hydrogen pickup mechanisms, especially under irradiation, is still lacking. This study aims at filling this knowledge gap by coupling state-of-the-art experimental and modeling approaches on a set of Zr-xNb (x = 0.5/1.0) model alloys, with different heat treatments and irradiation dose levels to understand the effect of Nb redistribution on corrosion kinetics. To investigate the effect of irradiation on Nb distribution in a Zr-Nb metal substrate, Zr-Nb alloys have been irradiated by 2-MeV protons up to 1 dpa (flat region, about 1.3 × 1019 ions/cm2) at 350°C at the University of Wisconsin Ion Beam Laboratory. Nb redistribution after irradiation has been characterized by scanning transmission electron microscopy and atom probe tomography with a focus on radiation-induced platelets and solute concentrations. Results from atom probe tomography show that indeed solute concentration in solid solution decreases with irradiation dose, which may explain the enhanced corrosion resistance of irradiated Zr-Nb alloys via space-charge effects in the oxide. To understand the correlation between oxide space charge and corrosion kinetics, X-ray absorption near-edge spectroscopy was performed to measure the Nb oxidation states as a function of exposure time and oxide depth in two different annealed Zr-1.0Nb samples, corroded in high-temperature water up to 240 days. X-ray absorption near-edge spectroscopy results show that Zr-1.0Nb with lower, sub-parabolic, corrosion kinetics contains less Nb concentration in solid solution. Lastly, the observed corrosion kinetics are rationalized based on the coupled current charge compensation model in terms of Nb redistribution and its compensation effect on oxide space charges. The effect of irradiation on corrosion kinetics of Zr-Nb alloys is discussed in the light of experimental and modeling results.