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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Morphology selection in dealloying: A phase field study of the coupling among kinetic mechanisms

A phase field model is used to investigate how the corrosion rate and morphology of a dealloying metal is controlled by the kinetic rates of bulk solid diffusion, interfacial diffusivity, and interface dissolution. A specific focus is investigating the manner by which variations in these kinetic parameters synergistically influence the onset of a morphological instability at the solid-liquid interface, which is known to precede rapid dealloying corrosion towards highly topologically complex structures such as bicontinuous porous solids. This work demonstrates that this instability can be suppressed by either an increase of interdiffusion in the solid alloy, or by a decreasing rate of dissolution at the solid-liquid interface. When the instability is suppressed, the result is a relatively planar, passivated dealloying front. Importantly, however, coupled changes in kinetic parameters can also promote the morphological instability and sustain fine-scale ligament and pore formation. Finally, the relevance of the solid-liquid interfacial diffusion for determining the rate of dealloying and ligament formation is highlighted. By elucidating the mechanistic influence of bulk and interfacial kinetic factors on the dealloyed morphology, this work helps to rationalize the contrasting morphologies observed in various dealloying systems.

Diffusion↗
DOE OSTI2025

Electrochemical Behavior of Cerium at an Indium Tin-Doped Oxide Electrode in Acidic Media

The redox behavior and speciation of cerium at mesoporous thin films composed of nanoparticles of indium tin-doped oxide (nITO) electrodes were characterized in pH 4.8, 0.1 M acetate buffer and both 0.1 and 1 M HNO 3 using electrochemical techniques and X-ray photoelectron spectroscopy. Anodic deposition of ceria species from Ce(III) to the nITO electrode was achieved under all solvent conditions via spontaneous condensation of electrochemically generated ceric hydroxide species. In 1 M nitric acid, the rate of CeO 2 dissolution is on the same order as CeO 2 deposition, resulting in negligible amounts of CeO 2 electrodeposited at the nITO surface. The cathodic stripping of CeO 2 from the nITO substrate deposited in 0.1 M nitric acid or pH 4.8 acetate buffer follows a 2-step process where Ce(IV)-oxide is initially reduced to an unstable Ce(III)-oxide species that rapidly undergoes acid catalyzed dissolution to yield soluble Ce(III) (aq) . These findings provide a foundation for the pH and anodic potential controlled deposition of CeO 2 thin films to ITO substrates, which can aid in the development of materials composed of ceria. As a result, they can also be used to infer likely analogous actinide redox behavior and speciation at these electrodes.

Cerium↗
DOE OSTI2022

Radiolysis and Radiation-Driven Dynamics of Boehmite Dissolution Observed by In Situ Liquid-Phase TEM

We report over the last several decades, there have been several studies examining the radiation stability of boehmite and other aluminum oxyhydroxides, yet less is known about the impact of radiation on boehmite dissolution. Here, we investigate radiation effects on the dissolution behavior of boehmite by employing liquid-phase transmission electron microscopy (LPTEM) and varying the electron flux on the samples consisting of either single nanoplatelets or aggregated stacks. We show that boehmite nanoplatelets projected along the [010] direction exhibit uniform dissolution with a strong dependence on the electron dose rate. For nanoplatelets that have undergone oriented aggregation, we show that the dissolution occurs preferentially at the particles at the ends of the stacks that are more accessible to bulk solution than at the others inside the aggregate. In addition, at higher dose rates, electrostatic repulsion and knock-on damage from the electron beam causes delamination of the stacks and dissolution at the interfaces between particles in the aggregate, indicating that there is a threshold dose rate for electron-beam enhancement of dissolution of boehmite aggregates.

54 ENVIRONMENTAL SCIENCES↗
DOE OSTI2020

Evaluation of Changes to the MTR Fuel Dissolution Flowsheet to Increase the Number of L-Bundles Charged to a Dissolver

Material Test Reactor (MTR) fuels are dissolved in the H-Canyon facility using a Hg-catalyzed, HNO₃ dissolution flowsheet. The flowsheet is based on experimental work in which laboratory-scale experiments were used to provide bounding values for both offgas and H₂ generation rates. The generation of H₂ gas, primarily from the dissolution of Al, is controlled by limiting the surface area exposed to the solution. The number of L-Bundles charged and their submergence level defines the exposed surface area which is directly proportional to the H₂ generation rate. For the MTR fuel dissolution flowsheet, the maximum number of L-Bundles which can be charged to a dissolver was specified as a function of the Al concentration in the solution and the submergence level of the bundle. The MTR fuels are currently dissolved in the 6.1D dissolver. To reach the desired terminal Al concentration, a total of 12 L-Bundles of fuel are dissolved per batch. Depending on the submergence level being used in the dissolver, the 12 bundles are dissolved in three charges of four bundles per charge or in a sequence of three, four, and five bundles. However, if six L-Bundles could be dissolved in the initial charge, a dissolver batch could be completed in two dissolutions instead of three. To assess the feasibility of dissolving six bundles in the first charge, the Savannah River National Laboratory evaluated changes to the MTR flowsheet which increase the number of bundles which can be charged to a dissolver. Since the development of the existing MTR fuel dissolution flowsheet, improved offgas measurement methods have been developed to more accurately calculate the offgas composition and generation rate during laboratory-scale dissolutions. In the first part of this study, improvements in the calibration model for the Raman spectrometer (used for offgas characterization) and a more effective data smoothing routine were applied to existing data. The uncertainty in the offgas composition data used to define the MTR fuel dissolution flowsheet were subsequently re-evaluated which resulted in an approximate 50% reduction in the uncertainty of the H₂ gas concentration measurements. The reduced uncertainty was used in new calculations to define the maximum number of L-Bundles which can be charged to either the 6.1D or 6.4D dissolver as a function of the Al concentration. With the reduced uncertainty in the H₂ concentration measurements, a flowsheet was defined which supported initially charging 6 bundles to a dissolver. This flowsheet is based on the use of 0.002 M Hg, an L-Bundle immersion depth of 54 in., an air sparge/purge flowrate of 40 scfm, an iodine reactor operating temperature of 200 °C, and not exceeding 60% of the H₂ LFL in the dissolver offgas. In the second phase of this study, calculations were performed to assess the impact of changing various operating parameters to increase the number of L-Bundles charged to an H-Canyon dissolver. Applying the improved offgas measurement techniques including the reduced uncertainty in the H₂ concentration measurements to the existing data, the number of bundles of MTR fuel which can be charged to either the 6.1D or 6.4D dissolver were calculated based on varying the bundle immersion depth (44-60 in.), air sparge/purge flowrate (40-60 scfm), iodine reactor operating temperature (150-200 °C), and the percentage of the H₂ LFL which must not be exceeded (60-76%). Qualitatively, the percentage of the LFL had the largest influence on the maximum number of bundles which can be charged to a dissolver followed by the air sparge/purge flowrate, L-Bundle immersion depth, and the iodine reactor operating temperature. During the evaluation of each variable, the other variables were held at the baseline flowsheet conditions.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗
DOE OSTI2025

In-Tank Processing Using Low Temperature Aluminum Dissolution: Year 1 Progress Report

The Hanford Site stores approximately 56 million gallons of radioactive legacy defense waste in underground tanks. This study investigates the feasibility of utilizing in-tank low-temperature aluminum dissolution to favorably alter the transport properties of aluminum-rich southeast quadrant waste sludges, thereby de-risking sludge delivery for Direct Feed of High-Level Waste. Specifically, this study aims to assess the rate and extent of gibbsite dissolution under various conditions, including different particle sizes (~10 to 90 µm), NaOH concentrations (1 to 6 M), and the presence of specific analytes such as NO 2 - , NO 3 - , and other aluminum-rich waste background analytes, using simulated waste solids and liquids. Experimental tests were designed to highlight the impact of aluminum leaching on simulated waste transport properties, quantified by the just-suspended mixing speed (NJS), and were conducted under well-mixed, turbulent conditions, ensuring full suspension of gibbsite particles using an overhead mixer. The results suggest that the presence of NO 2 - and NO 3 - , in conjunction with leaching at relatively low free hydroxide contents, improved transport requirements (i.e., lowered NJS) for pure gibbsite slurries. These findings will inform the design of potential in-tank processing systems, optimizing aluminum dissolution for improved waste management in the Hanford Tank Farms.

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2020

A Novel High Energy Density Sorption-based Thermal Battery for Low-grade Thermal Energy Storage

Thermal energy storage (TES) can alleviate peak demand on the electricity grid by offsetting building thermal loads, increasing the grid’s reliability and resilience. However, low energy density and poor energy performance of existing TES technologies limit their applications. Sorption-based thermal battery (STB) system is thus developed using three-phase sorption technology to harvest low-temperature heat, store it with a much higher energy density than common TES systems and dehumidify air or provide space cooling in buildings. Although STB has been experimentally proved to be feasible, influencing factors on its performance are still unknown by far. Therefore, this paper conducted a parametric analysis on crystallization and crystal dissolution performance of a developed STB test rig. The crystallization results showed that the energy density of the STB increased with reducing the solution flow rate and the cooling water temperature. The dissolution results showed that a higher discharge rate of the STB can be achieved with increasing the flow rate and temperature of inlet diluted solution. The work in this study is helpful to the optimal design and operation of the STB system.

Wang, Lingshi↗
DOE OSTI2023

Unraveling Anisotropic and Pulsating Etching of ZnO Nanorods in Hydrochloric Acid via Correlative Electron Microscopy

Despite much technical progress achieved so far, the exact surface and shape evolution during wet chemical etching is less unraveled, especially in ionically bonded ceramics. Herein, by using in situ liquid cell transmission electron microscopy, a repeated two-stage anisotropic and pulsating periodic etching dynamic is discovered during the pencil shape evolution of a single crystal ZnO nanorod in aqueous hydrochloric acid. Specifically, the nanopencil tip shrinks at a slower rate along [0001̅] than that along the $\langle$101̅0$\rangle$ directions, resulting in a sharper ZnO pencil tip. Afterward, rapid tip dissolution happens due to accelerated etching rates along various crystal directions. Concurrently, the vicinal base region of the original nanopencil tip emerges as a new tip followed by the repeated sequence of tip shrinking and removal. The high-index surfaces, such as {101̅m} (m = 0, 1, 2, or 3) and {21̅1̅n} (n = 0, 1, 2, or 3), are found to preferentially expose in different ratios. Our 3D electron tomography, convergent beam electron diffraction, middle-angle bright-field STEM, and XPS results indicate the dissociative Cl – species were bound to the Zn-terminated tip surfaces. Furthermore, DFT calculation suggests the preferential Cl – passivation over the {101̅1} and (0001) surfaces of lower energy than others, leading to preferential surface exposures and the oscillatory variation of different facet etching rates. The boosted reactivity due to high-index nanoscale surface exposures is confirmed by comparatively enhanced chemical sensing and CO 2 hydrogenation activity. In conclusion, these findings provide an in-depth understanding of anisotropic wet chemical etching of ionic nanocrystals and offer a design strategy for advanced functional materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Calcium nitrate: A chemical admixture to inhibit aggregate dissolution and mitigate expansion caused by alkali-silica reaction

Alkali-silica reaction (ASR) is a significant cause of the degradation of concrete structures. ASR results from the dissolution of reactive silicate aggregates and the subsequent formation of damaging, expansive ASR gels. Despite the prevalence of ASR, so far only lithium-based chemical admixtures have been shown to be successful in mitigating its deleterious effects. Herein, dissolved calcium nitrate (Ca(NO 3 ) 2 : CN) is demonstrated to reduce ASR-induced expansion caused by the use of sodium borosilicate (NBS) glass as a model reactive aggregate. The significance of reductions of NBS-glass dissolution and mortar-bar expansion rates is discussed, with special focus on highlighting the mechanism by which calcium nitrate mitigates ASR, i.e., by the inhibition of aggregate dissolution. This study reveals a new and cost-effective pathway to suppress aggregate dissolution and ASR in concrete based on the dosage of soluble alkaline-earth additives.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Dynamic development of geochemical reaction fronts during hydraulic stimulation of shale

Injection of acidic hydraulic fracture fluid (HFF) into shale formations for unconventional oil/gas production results in chemical reactions in the shale matrix that can alter fluid transport. Here, we report the results of set of experiments designed to evaluate the impact of calcite dissolution as a function of carbonate mineral content on matrix chemical reactivity and pore-space modification concomitant with imbibition. In this study, we tracked acidic HFF transport in four samples of Wolfcamp shale with calcite contents varying from 4% to 59% by monitoring the rate and spatial extent of bromide tracer transport using synchrotron-based X-ray fluorescence microprobe (XFM) imaging. Concurrently, we also carried out XFM imaging of the spatial distribution of Ca in the Wolfcamp shale cores (as a proxy of calcite distribution). Our approach thus yields a direct record of time-resolved selective ion transport resulting from the penetration of acidic HFF and the associated mineral transformations in the shale cores. We show that the variability in calcite content of Wolfcamp shale samples can directly affect the rate and spatial extent of imbibition. Although reaction of the acidic HFF with carbonates in shales enhances calcite dissolution and increases porosity, the spatial extent of calcite dissolution in the shale matrix is limited due to a rapid neutralization of pH. The relative abundance and spatial distribution of calcite control the chemical saturation state of the HFF progressing into the matrix. As a result, calcite has a major impact on the spatial extent and rate of matrix alteration and thus on HFF transport during subsurface reservoir stimulation. Consequently, increased calcite content in the shale matrix inhibits the spatial extent of the pore-volume increase and, by extension, the spatial extent and rate of imbibition. Our results thus show that the overall rates of calcite dissolution approach the rates of acidic HFF transport (i.e., Damköhler number ~1), which could contribute to the efficiency of subsurface reservoir stimulation. A better understanding of HFF-calcite reaction rates is crucial for improving the prediction and optimization of fluid transport across HFF-shale interfaces during hydraulic fracturing.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

How do super concentrated electrolytes push the Li-ion batteries and supercapacitors beyond their thermodynamic and electrochemical limits?

Increasing the energy density of energy storage devices is currently the key target of many battery and supercapacitor research activities. For both types of devices, the electrochemical stability window (ESW) determines the effective energy density of the device. ESWs are defined by the effective oxidation and reduction potentials of the electrolyte, which are controlled by many various factors, including the HOMO/LUMO (highest/lowest unoccupied molecular orbital) energies of the electrolyte molecules, the nature of the electrode/electrolyte interphases, and other physicochemical properties. The concentration of the electrolyte would affect the HOMO/ LUMO levels thus also change the ESW. A higher concentration of salt induces specific arrangements among the anion, cation, and solvent molecules of an electrolyte, altering the bulk behavior of the electrolyte, resulting in drastic change in the electrode interfaces. These uniquely modified physicochemical properties extend the ESW in several different ways, including the enhancement in the kinetic stability of the electrodes, as well as the thermodynamic and Nernst shifts of the oxidation/reduction potentials of the electrolyte. For organic electrolytes, it is the reduced amount of free solvent molecules that plays the key role in such changes; whereas for aqueous electrolytes, it is the scarcity of free water molecules and the reduced water activity that control the key properties of the electrolyte. Here, we focus on elucidating the fundamental structural changes occurring within an electrolyte system with increasing salt concentrations. The underlying mechanisms which not only facilitates the extension of the ESW, but also enables higher rate capabilities and mitigates aluminum dissolution for batteries with organic electrolytes, are meticulously explained. Further, we thoroughly discuss the importance of high-voltage stability in aqueous battery systems by exploiting the changed properties observed with higher concentrations of salts. To finish, high-voltage supercapacitors enabled by superconcentrated electrolytes are also explored.

25 ENERGY STORAGE↗
DOE OSTI2024

Real-Time Atomic-Scale Structural Analysis Resolves the Amorphous to Crystalline CaCO 3 Mechanism Controversy

Amorphous calcium carbonate (ACC) occurs as a precursor to geological and biogenic calcium carbonate (CaCO 3 ), yet its transformation pathways and reaction mechanisms remain inconsistent and controversial. In this study, we investigated the transformation of ACC to calcite under both solution and dry conditions, in the presence and absence of impurity ions, utilizing operando time-resolved synchrotron X-ray diffraction (TRXRD) and reactive transport modeling. Results demonstrate that TRXRD techniques allow us to differentiate dissolution-reprecipitation versus solid-state transformation mechanisms for amorphous to crystalline phase transitions. Specifically, we observe that in environments with abundant water, ACC transforms to calcite through a dissolution-reprecipitation mechanism. This features an activation energy of 63 ± 2 kJ/mol and unit cell volume contraction during calcite crystal growth. Conversely, under water-limited conditions, ACC to calcite transformation proceeds via a solid-state transformation mechanism, with an activation energy of 210 ± 2 kJ/mol, three times greater than the dissolution-reprecipitation route, and a unit cell expansion during crystalline calcite growth. Further, to illustrate the magnitude of these effects, the rates of calcite growth were similar during dissolution-reprecipitation at 3 °C [0.00207(35) s –1 ] and solid-state transformation at 280 °C [0.00134(11) s –1 ]. Moreover, the incorporation of an impurity, strontium, significantly retards the rate of calcite growth while expanding its unit cell but whose incorporation is history dependent. Reactive transport modeling of the dissolution–precipitation kinetics suggests that ACC must be dissolving as compact aggregates. These various transformation mechanisms drive diverse geological and biological carbonate formations, impacting their use as paleoenvironmental markers and functional materials synthesis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Phase instabilities in austenitic steels during particle bombardment at high and low dose rates

Disruption of phase stability by energetic particle bombardment is a major challenge in designing advanced radiation-tolerant alloys and ion beam processing of nanocomposites. Particularly, ballistic dissolution susceptibility of different solute nanocluster species in alloys is poorly understood. Here, low dose rate neutron irradiations were conducted on a Fe-Cr-Ni based austenitic steel in the BOR-60 reactor (9.4 × 10 -7 dpa/s, 318 °C) followed by accelerated dose rate ion irradiations at multiple temperatures ( 10 -3 dpa/s, 380 – 420 °C). Using atom probe tomography, the stability of radiation-enhanced Cu-rich and radiation-induced Ni-Si-Mn-rich nanoclusters was evaluated. During neutron irradiation, Cu-rich clusters nucleated with their core concentrations progressively increasing with dose, while Ni-Si-Mn-rich clusters formed and evolved into G-phase precipitates. Ion irradiations dramatically altered the nanoclusters. Cu-rich clusters were ballistically dissolved, but Ni-Si-Mn-rich clusters remained stable and coarsened with dose at 400 and 420 °C, highlighting that different nanocluster species in a single microstructure can have innately distinct ballistic dissolution susceptibilities. Solute-specific recoil rates were incorporated into the Heinig precipitate stability model, which shows that in addition to radiation-enhanced diffusion, recovery from ballistic dissolution depends on solute concentration gradient near cluster interfaces. Finally, the combined experimental-modeling study quantified the critical temperatures and damage rates where ballistic dissolution dominates for each cluster species.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Nanoporous Fe 2 O 3 and Soluble Fe(II) Intermediates Accelerate the Electrodeposition of Fe in NaOH(aq)

Electrochemical conversion of iron oxide to iron metal can enable low-cost batteries for long duration energy storage and zero-emissions ironmaking for steel. Iron oxides, such as hematite, can be electrochemically reduced to metallic iron in concentrated alkaline electrolytes at modest temperatures, but the relative influences of solid-state and dissolved intermediates at practical reaction rates remains unclear. Here, in this study, we prepare a homologous set of well-defined hematite particles to measure how the nanoscale morphology of oxides controls both their reactivity and apparent reduction mechanism in concentrated hydroxide. Correlated electron microscopy and rotating-ring-disk-electrode measurements revealed that nanoporous hematite and solid intermediates formed iron via a dissolution-redeposition pathway. In contrast, dense hematite particles directly formed iron metal via reactive fracture. While previous studies on iron electrowinning have primarily emphasized the role of particle diameter at the micron scale, these results demonstrate the importance of the dissolution-redeposition pathway to support rapid reaction rates and suggest that nanoscale porosity controls iron oxide reactivity at temperatures <100 °C. Therefore, iron-oxide-to-metal electrolyzers and fast-charging iron-air batteries supported by curtailed electricity can increase the rate of metal formation by accelerating fracture and dissolution in reactant oxides.

TEM↗
DOE OSTI2022

Atomistic thermodynamics and kinetics of dicalcium silicate dissolution

Low-temperature cement manufacturing has garnered academic and industrial attention for its low environmental footprints. However, the sluggish hydration kinetics of the resultant cement affects their early-age strength development. This motivates fundamental studies to unravel the mechanistic picture of the dissolution process and discover science-informed pathways to accelerate hydration. Standard atomistic simulations seldomly exceed a microsecond making them impractical to study slow dissolution processes. We are using rare event sampling techniques, we provide the mechanistic picture of Ca 2+ ion dissolution from a kink site on the dicalcium silicate surface. The Ca 2+ ion dissolution is comprised of two sequential stages: breaking restraints from the kink sites to form a ledge adatom and detaching from the ledge/terrace adatom sites into the solution. The first and second stages feature free energy barriers of ~63 kJ/mol and ~ 29 kJ/mol respectively, making the first stage the rate-limiting step of the entire Ca 2+ dissolution kinetics. Using the reactive flux method, the rate and equilibrium constants for each reaction step are calculated, which yield the Ca 2+ ion activity of ~1.03 × 10 -5 . The diffusion calculations indicate that the surface effects lower the self-diffusion coefficient of Ca 2+ ions at the solid-water interface.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Thermodynamic equilibrium and kinetic fundamentals of oxide dissolution in aqueous solution

Dissolution of oxides in aqueous solutions is fundamentally important for a range of applications and a critical process that determines the chemical durability of industrial ceramics, the performance of nuclear waste forms, and the chemical weathering of minerals. The thermodynamic equilibrium and kinetics of dissolution reactions are key to determining the rate at which oxides dissolve. The increase in collaborative research across disciplines in materials research necessitates a common background to tackle shared scientific problems across different fields. This review selectively examines the fundamentals of dissolution theories that have been developed in chemistry, geochemistry, and materials science, and assembles them into a single collective document for the broader materials science community. Applications of the theories are highlighted using examples from specific areas, but can be similarly applied to other areas. Challenges and future research needs for a predictive-level understanding are discussed in light of the current literature.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2025

Geochemical Assessment for Carbon Sequestration in the Conasauga Group, Northwest Georgia, USA

Sedimentary geological formations are known to be great candidates for geological carbon sequestration. Published studies suggest the southeast of the United States contains many formations suitable for carbon storage. The Cassville 1 Stratigraphic Borehole well could act as a potential carbon reservoir for nearby energy resource facilities in Georgia, United States. Although studies have shown that porous formations are adequate for geological carbon sequestration, it is important to understand possible geochemical reactions between CO 2 and the targeted geological formation before injecting any fluids. In this study, a sandstone sample from the Cassville 1 well is being considered for geological carbon sequestration in the Conasauga Group in Northwest Georgia. Here, the collected sandstone sample, consisting of quartz, K-feldspar, micas, kaolinite, and carbonate minerals such as calcite and dolomite, has a 6% porosity. Leveraging the formation composition and porosity, a one-dimensional continuum reactive transport model was built using CrunchFlow to assess possible geochemical reactions between injected CO 2 and the geological formation. Simulation results show that the carbonate minerals, calcite and dolomite, dissolve during the injection period of 10,000 days, increasing formation porosity from 6% to as much as 30%. The rate and extent of carbonate mineral dissolution and resulting porosity increase are highly sensitive to mineral reactive surface area values. No evidence of mineral precipitation was observed, suggesting that dissolution reactions will control porosity evolution during the CO 2 injection period.

42 ENGINEERING↗
DOE OSTI2026

Electro-carburization of mild steel through two-step reaction in molten-salt electrolyte

The rapid carburization of mild steel is demonstrated via the electrochemical deposition of solid carbon, followed by thermochemical iron-carbon (Fe-C) reaction using a molten lithium-potassium carbonate (LiKCO 3 ) salt electrolyte and carbon dioxide as the carbon source. Depth-dependent microstructural and compositional changes in the steel are shown to be controlled by the applied cell voltage and reaction temperatures. Reaction temperatures above the Fe-C eutectoid temperature promote faster carburization due to carbon dissolution into austenite. The carbon deposition rate is regulated by the applied cell voltage: At - 2.4 V, carbon deposition occurs much faster than its diffusion into the steel, resulting in the accumulation of a carbon-rich layer exceeding 1 mm, which does not diffuse into the steel. In contrast, at - 1.8 V, all deposited carbon effectively diffuses into the steel, leaving a minimal external deposit. In both cases, carbon ingress forms a carbide-rich surface layer and precipitates additional carbides along the grain boundaries within the steel electrode. This method offers a potential approach to increase the carbon content of mild or low-carbon steel feedstocks, such as scrap or direct reduced iron, for industrial applications.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗