High Vanadium Research Articles

Wenlanzhangite–(Y) from the Yushui deposit, South China: A potential proxy for tracing the redox state of ore formation

Abstract Mineral phases in which vanadium (V) and heavy-rare-earth elements (HREEs) coexist are rarely documented. Here, we report a new V-HREE-bearing silicate mineral species, wenlanzhangite-(Y), which is a vanadiferous derivate of jingwenite-(Y) [Y2Al2V24+(SiO4)2O4(OH)4] coexisting with jingwenite-(Y) in bedded/massive ores at Yushui, South China. Wenlanzhangite-(Y) forms as a dark brown, 70–100 μm thick rim on a core domain of jingwenite-(Y), which occurs as 100–200 μm columnar crystals. The color, streak, luster, and hardness (Mohs) are dark brown, yellow-gray, vitreous, and ~4, respectively. Compared to jingwenite-(Y), wenlanzhangite-(Y) has higher vanadium and lower aluminum contents. Calculated on the basis of 8 cations, the empirical formula is (Y1.26Dy0.17Er0.11 Gd0.09Yb0.09Nd0.09Sm0.06Sc0.04Ho0.03Ce0.02Tb0.02Tm0.02Pr0.01)Σ2.00(V1.463+Al0.54)Σ2.00V24+(SiO4)2O4(OH)4, which can be simplified to the ideal formula Y2V23+V24+(SiO4)2O4(OH)4. Wenlanzhangite-(Y) is triclinic, with space group P1(#2), Z = 2, and unit-cell parameters a = 5.9632(7) Å, b = 9.599(1) Å, c = 9.9170(9) Å, α = 90.033(8)°, β = 98.595(2)°, γ = 90.003(9)°, and V = 561.28(10) Å3. Wenlanzhangite-(Y) is approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification (IMA2022-142). The structure of wenlanzhangite-(Y) is composed of a-axis-oriented chains of [VO6] octahedra consisting of edge-sharing octahedra linked by insular [SiO4] tetrahedra, leaving open channels occupied by rare earth elements. Observed compositional variation and crystal structure demonstrate that V3+ can substitute for Al3+ in jingwenite-(Y), forming wenlanzhangite-(Y). The occurrence of wenlanzhangite-(Y) indicates a relatively more reducing hydrothermal environment, causing a reduction of V5+ in oxidized fluids to V3+ and thus represents a useful proxy for tracing the redox state of ore formation.

Low-cost and durable polyvinyl alcohol modified polyethylene separator for vanadium redox flow battery

Porous separators are considered a viable alternative to the high cost Nafion membrane for vanadium redox flow battery (VRFB) commercialization. However, the porous separators suffer from high vanadium ion crossover due to the presence of large micron size pores, which leads to decay in capacity of the VRFB system. To tackle the same, a composite separator is synthesized wherein a cross-linked polyvinyl alcohol (PVA) layer is coated on to the porous polyethylene silica separator (PE separator) to mitigate the vanadium ion crossover rate and employed in the VRFB application. Scanning electron microscopy (SEM) analysis confirmed the successful coating of the PVA film on to the PE separator. Synthesized PE-PVA separator showed a significant decrease in the vanadium ion crossover by 31.44 times compared to the pristine PE separator. Consequently, the self-discharge time increases by 8 and 1.66 times compared to the pristine PE separator and the Nafion117 membrane, respectively. VRFB cell equipped with the synthesized separator showed better capacity retention with a capacity fade rate of 0.2 % Ah·cycle−1 than the Nafion117 membrane (0.5 % Ah·cycle−1). In-situ and ex-situ oxidative stability of the separator is explored in detail and a degradation mechanism of the cross-linked PVA membrane is proposed accordingly. The synthesized single side coated PE-PVA´ separator exhibits higher coulombic efficiency of 99 % and comparable energy efficiency of 71 % at a current density of 120 mA·cm−2 and demonstrated excellent lifetime durability tested up to 1600 charge-discharge cycles at 80 mA·cm−2.

Extraction of Vanadium from High Phosphorus Vanadium Containing Waste Residue via Carbonation: Optimization Using Response Surface Methodology

Vanadium (V) was successfully extracted from a high phosphorus vanadium residue (HPVR) through a carbonation process. Vanadium within HPVR substitutes for Fe in the mineral structure of Ca9(Fe,V)(PO4)7 at elevated temperatures, Na2CO3 reacts with V to form sodium metavanadate (NaVO3), concurrently generating calcium carbonate (CaCO3) through its interaction with Ca9(Fe,V)(PO4)7. Subsequently, V is liberated and leached by water, dissolving in the aqueous phase as metavanadate ions (VO3−). Crucial factors influencing V leaching efficiency include roasting time, roasting temperature, and the amount of Na2CO3 utilized. Response Surface Methodology (RSM) was employed. The optimized parameters determined were as follows: a roasting temperature of 850 °C, a roasting duration of 120 min, a Na2CO3 dosage of 8.01%, a liquid-to-solid ratio (L/S) of 3, and a leaching time of 60 min. Under these conditions, a remarkable V leaching efficiency of 83.82% was achieved. This study underscores the viability of a simplified approach for treating solid waste containing metal slag, which not only mitigates environmental pollution but also yields valuable metals.

Bismuth Single Atoms Regulated Graphite Felt Electrode Boosting High Power Density Vanadium Flow Batteries.

Vanadium flow batteries (VFBs) are considered one of the most promising candidates for large-scale energy storage. However, VFBs suffer from relatively low power density due to severe electrochemical polarization. Herein, we report Bi single atoms supported by an N-doped carbon-regulated graphite felt electrode (Bi SAs/NC@GF) with high electrocatalytic activity and stability, owing to the greatly improved active sites and optimized Bi-N4 configuration. Electrochemical in situ characterization and theoretical calculations elucidate the desolvation process and specific inner sphere reaction mechanism of [V(H2O)6]3+/[V(H2O)6]2+. As a result, a VFB single cell assembled with Bi SAs/NC@GF achieves a much higher energy efficiency of 81.1% at 240 mA cm-2 than NC@GF (70.5%). Moreover, a 5 kW VFB stack equipped with Bi SAs/NC@GF is assembled for the first time and ran stably for over 400 cycles. This work confirms that a single-atom catalyst is efficient for scalable VFBs with high power density and low cost.

A Sustainable Recycling Route from Spent Sulfuric Acid Catalysts to Vanadium Pentoxide Precursor for the Production of Low‐Cost Na3V2(PO4)3@C Na‐Ion Batteries

AbstractThe extraction of vanadium from spent industrial catalysts is a widely practiced process globally due to the large quantities of material available with appreciable vanadium content. However, some of these spent catalysts are unresponsive to established extraction methods because they have different properties. Therefore, separate studies are necessary to deal with specific cases. This work demonstrates how the leaching step can limit the recovery of vanadium to the solution before the coprecipitation step. The NH4Cl‐H2SO4 synergetic system achieved a high vanadium leaching rate of up to 95 %, resulting in the preparation of high‐purity NH4VO3 with 75.45 % V2O5 content. This demonstrates the potential of the prepared V2O5 as a precursor for vanadium materials used in sodium‐ion batteries, allowing for the adjustment of the valence of vanadium. The Na3V2(PO4)3@C particles exhibit a discharge capacity of 110 mAh g−1 at a current density of 0.1 C and 90 mAh g−1 at 1 C. The synergistic leaching system provides a sustainable method for the recovery and reuse of vanadium from spent vanadium catalysts. This contributes to recycling efforts and the development of sodium‐ion batteries.

Polarization-insensitive, wide-angle, high bandwidth vanadium nitride-based infrared nanostructured absorber

Polarization-insensitive, wide-angle, high bandwidth vanadium nitride-based infrared nanostructured absorber

(Invited) Redox Flow Batteries – Exploring Electrolyte Additives and Hybrid Organic/Inorganic Redox Pairs

Renewable energy sources are intermittent and thus require provision to store the energy when available using energy storage devices. These energy storage devices should be safe, efficient, and have a long life cycle for use in grid-scale energy storage systems. Lead-acid, Li-ion, and Redox Flow Batteries (RFBs) are examples of such energy storage devices. Among these energy storage systems, RFBs have unique advantages that make them attractive for large-scale energy storage applications, like a high cycle life of about 10,000 cycles.[1] The current generation of commercial RFBs use vanadium solution in different oxidation states as electrolytes (anolyte and catholyte), carbon-based electrodes and Nafion as the ion-exchange membrane. High cost, risk of electrolyte crossover, low volumetric energy density, and pumping losses limit the vanadium redox flow batteries (VRFBs).[2] Also, the Nafion membrane accounts for about 40% of the system's total cost.[3] To ensure high cycle life, the electrolytes must be stable (active material must remain dissolved in the supporting electrolyte), and vanadium ions should not crossover the Nafion membrane. The VRFBs tend to lose vanadium from electrolyte solutions due to the precipitation of V2O5 during charging, resulting in a significant loss of energy density. Additives help in maintaining the long-term stability of vanadium electrolytes. In this work, we have monitored the solubility and electrochemical characteristics of vanadium electrolyte solutions with V2O5 in the presence of different additives, namely HCl and MSA (methanesulfonic acid), for over three months.[4] The findings of this study provide insight into using these additives for VRFBs. Further research in RFBs has recently shifted towards redox-active aqueous-organic-based electrolytes consisting of Earth-abundant elements (C, H, O, N, S), accommodating the need for green, safe, and low-cost energy storage. We have studied the feasibility of using quinone-based organic electrolytes for RFBs. A proof-of-concept membrane-free two-compartment cell with an auxiliary electrode in each compartment was also demonstrated. In the conventional design, the hydrogen ions move through the Nafion membrane to balance charges but in the auxiliary electrode-based membrane-free setup, the auxiliary electrode in each compartment undergoes redox reactions opposite to the primary electrodes to balance charges.[5]References[1] Y.E. Durmus, H. Zhang, F. Baakes, G. Desmaizieres, H. Hayun, L. Yang, M. Kolek, V. Küpers, J. Janek, D. Mandler, S. Passerini, Y. Ein‐Eli, Side by Side Battery Technologies with Lithium‐Ion Based Batteries, Adv. Energy Mater. 10 (2020). https://doi.org/10.1002/aenm.202000089.[2] B. Turker, S. Arroyo Klein, E.M. Hammer, B. Lenz, L. Komsiyska, Modeling a vanadium redox flow battery system for large scale applications, Energy Convers. Manag. 66 (2013) 26–32. https://doi.org/10.1016/j.enconman.2012.09.009.[3] J. Ye, D. Yuan, M. Ding, Y. Long, T. Long, L. Sun, C. Jia, A cost-effective nafion/lignin composite membrane with low vanadium ion permeation for high performance vanadium redox flow battery, J. Power Sources. 482 (2021) 229023. https://doi.org/10.1016/j.jpowsour.2020.229023.[4] O.H. Nguyen, P. Iyapazham Vaigunda Suba, M. Shoaib, V. Thangadurai, Investigating the Electro-Kinetics and Long-Term Solubility of Vanadium Electrolyte in the Presence of Inorganic Additives, J. Electrochem. Soc. 170 (2023) 110523. https://doi.org/10.1149/1945-7111/ad0a75.[5] S.V. Venkatesan, K. Karan, S.R. Larter, V. Thangadurai, An auxiliary electrode mediated membrane-free redox electrochemical cell for energy storage, Sustain. Energy Fuels. 4 (2020) 2149–2152. https://doi.org/10.1039/c9se00734b.

Structural and chemical evolutions of a magnesium vanadium oxide cathode under electrochemical cycling in magnesium batteries

The design of cathode materials that remain chemically and structurally stable during repetitive ion insertion and extraction poses a significant challenge in developing multivalent batteries. The cycling stability of traditional metal oxide-based cathode is challenged by sluggish diffusion of multivalent cations and parasitic reactivity at interfacial regimes, including the cathode electrolyte interphase layer (CEI). Understanding the reactions at the cathode-electrolyte interface, particularly those induced by non-stoichiometric surface layers, is a crucial design parameter for both cathode materials and electrolytes. In this study, we employed multimodal analysis, including in situ and ex situ X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (TEM) and electrochemical impedance spectroscopy (EIS) to examine the surface reactions and subsequent structural and chemical evolutions of the CEI on high voltage magnesium vanadium oxide (MgV2O4) spinel cathode during the Mg2+ insertion/extraction processes. The results revealed that the presence of non-stoichiometric surface layers in the magnesium vanadium oxide cathode drive the decomposition of bis(trifluoromethanesulfonyl)imide (TFSI-) anion, leading to the formation of the CEI layer. The CEI layer could inhibit the Mg2+ ion transfer processes. Accompanying this reactivity-driven degradation, the magnesium vanadium oxide cathode undergoes pulverization, forming clusters of nanosized particles. This process likely improves cycling ability by creating new intercalation sites and shortening the diffusion pathway for the Mg2+ cations. This study demonstrates that controlling surface stoichiometry and engineering morphological properties are critical design parameters for high performance cathodes for multivalent batteries.

Construction and Investigation of Novel Cross-Linked Fluorine-Containing Sulfonated Polyimide Membranes for VFB Application.

Membrane with remarkable proton conductance and selectivity plays a key role in obtaining high vanadium flow battery (VFB) performance. In this work, the trade-off effect between proton conductance and vanadium ion blocking was overcome by the introduction of a cross-linking structure to prepare covalent cross-linked fluorine-containing sulfonated polyimide (CFSPI-PVA) membranes. Herein, the CFSPI-PVA-15 membrane possesses excellent comprehensive properties, including acceptable area resistance (0.21 Ω cm2), lower vanadium ion permeability (0.76 × 10-7 cm2 min-1), and remarkable proton selectivity (3.11 × 105 min cm-3) compared with the commercial Nafion 212 membrane. At the same time, the CFSPI-PVA-15 membrane exhibits higher coulomb efficiencies (97.26%-99.34%) and energy efficiencies (68.65%-88.11%) and a longer self-discharge duration (29.2 h) in contrast with the Nafion 212 membrane. Moreover, 500 cycles of the CFSPI-PVA-15 membrane at 160 mA cm-2 are also stably executed. The internal reasons for the improved chemical stability of the CFSPI-PVA-15 membrane are clarified from theoretical calculations with the mean square displacement value and fractional free volume. Therefore, the CFSPI-PVA-15 membrane exhibits great potential for application in VFB.

Heavily vanadium-doped LiFePO4 olivine as electrode material for Li-ion aqueous rechargeable batteries

Since LiFePO4 batteries play a major role in the transition to safe, more affordable and sustainable energy production, numerous strategies have been applied to modify LFP cathode, with the aim of improving its electrochemistry. In this contribution, a highly vanadium-doped LiFe0.9V0.1PO4/C composite (LFP/C-10V) is synthesized using the glycine combustion method and characterized by x-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Thermogravimetry Differential Thermal Analysis (TGDTA) and Cyclic Voltammetry (CV). It is shown that 10wt.% of vanadium can substitute Fe positions, thus decreasing unit cell volume, which is followed by generation of Li3V2PO4 traces, as detected by CV. High vanadium doping does not change the carbon content in the composite (≈13 wt%) but improves its electronic conductivity and electrochemical performance in both aqueous and organic electrolytes. The reversibility and current response are increasing following the trend: LFP/C, LFP/C -3mol%V, LFP/C - 5 mol % and LFP/C-10 mol %. The best specific capacity is obtained for the most highly doped olivine, which exhibits a reversible process at 1 mV s−1 in an aqueous electrolyte, thus showing a peak-to-peak distance of 56 mV. The high capacity of LFPC-10V is measured in both LiNO3 and NaNO3 electrolytes amounting to around 100 mAh g−1 at 20 mV s−1. Still, the material is only stable in LiNO3 electrolyte, making it more suitable for Li than Na-ion aqueous rechargeable batteries.

Enhanced Post Deposition Annealing Conditions on the Fabrication of High Quality Thermochromic Vanadium Dioxide Films

Enhanced Post Deposition Annealing Conditions on the Fabrication of High Quality Thermochromic Vanadium Dioxide Films

Acid doped branched poly(biphenyl pyridine) membranes for high temperature proton exchange membrane fuel cells and vanadium redox flow batteries

Both high temperature proton exchange membrane fuel cell (HT-PEMFC) and vanadium redox flow battery (VRFB) are represented as two advanced energy conversion and energy storage devices. They have a same core component of the separator membrane, which still faces several intractable scientific and industrial issues. For HT-PEMFC, the increase in conductivity is normally as the expense of mechanical strength; while for VRFB, the improvement in proton transport always brings in serious vanadium ion crossover. Meanwhile, the membrane also should possess an excellent chemical stability towards the attack by radicals or high valence vanadium ions. The above questions can be well solved by the preparation of triphenylbenzene (TPB) branched poly(biphenyl-4-acetylpyridine) membranes (x%TPB-PBAP), which are synthesized by one-step Friedel-Crafts polymerization. Amounts of alkaline pyridine groups equip x%TPB-PBAP membranes with good phosphoric acid and sulfonic acid absorption capability, resulting in high proton conductivity in both HT-PEMFC and VRFB. Meanwhile, the construction of the branched structure, i.e. a kind of covalently crosslinked network, can improve the mechanical strength and chemical stability. Consequently, the 1.5 %TPB-PBAP membrane displays large potential in both HT-PEMFC and VRFB. A single H2-O2 cell based on the 1.5 %TPB-PBAP/263 %PA membrane shows a peak power density of 1010 mW cm−2 at 180 °C without any back pressure. Meanwhile, the VRFB based on above membrane also depicts better battery efficiencies and cycle durability than that with Nafion 212.

Effect of tempering temperature on stress corrosion resistance of a low alloy high strength steel with high vanadium content

The stress corrosion resistance of a low alloy high strength steel with high vanadium content, tempered at different temperatures, was examined by electrochemical measurements and slow strain rate tensile test under different environments. Combined with scanning electron microscopy (SEM), X-ray diffraction (XRD) and transmission electron microscopy (TEM), the results show that three different tempering temperatures had an influence on the microstructure of this steel as well as its electrochemical behavior and stress corrosion sensitivity. With the increase of tempering temperature, the ferrite grains coarsened, the average dislocation density lowered as well as the number of vanadium carbide also decreased, though the size of vanadium carbide increased. However, the interface distortion between vanadium carbide and matrix exacerbated, resulting in the increase of the local dislocation density. Therefore, the corrosion potential and impedance decreased, meantime, the stress corrosion sensitivity aggravated.

Quinone convertible sulfated ion conductive side chain for highly selective vanadium redox flow batteries

In this work, the concept of a quinone convertible sulfated ion conductive side chain is first proposed with tunable Donnan effect and strong proton conducting ability to achieve high vanadium redox flow battery performance.

Supported H8PV5Mo7O40 on activated carbon: Synthesis and Investigation of influencing factors for catalytic performance.

Polyoxometalates (POMs), in particular the Keggin-type HPA-5 (H8PV5Mo7O40) are widely established as effective catalysts for acid- and redox-catalyzed reactions. Yet, they are mainly used as homogeneous catalysts, which poses challenges regarding catalyst separation. This study explores the synthesis of supported HPA-5, and its application as a heterogeneous catalyst for biomass conversion, focusing on activated carbons with diverse chemical and physical properties as support materials. Characterization of these carbons gives insights into the influence of surface area, oxygen content, and acidity on HPA adsorption and stability. Activated carbon CW20, was found to be the best support due to its high vanadium loading and effective preservation of the HPA-5 structure. It underwent various pre- and post-treatments, and the obtained supported catalysts were evaluated for their catalytic performance in converting glucose under both oxidative (OxFA process) and inert (Retro-Aldol condensation) conditions. Notably, HPA-5 supported on CW20 emerged as an exceptional catalyst for the retro-aldol condensation of glucose to lactic acid, achieving a selectivity of 15% and a conversion rate of 71%, with only minimal vanadium leaching.

Preparation and wear properties of high-silicon high-vanadium wear-resistant alloy with nano pearlite matrix and carbides composite structure

Combining high hardness carbide of high vanadium cast iron with nano pearlite, which has high strength and toughness, represents an innovative attempt to explore a new technology for preparing high vanadium alloy. In this study, a high-silicon high-vanadium wear-resistance alloy (HSHVWRA) was designed, incorporating a nano pearlite matrix and carbides composite structure. By increasing the vanadium content, numerous high hardness micrometer vanadium carbides were obtained The CCT curve was shifted to the left by increasing the silicon content in the alloy. Subsequently, the nano pearlite matrix was obtained by air cooling after austenite at 1100 °C. After heat treatment, the inter lamellar spacing of pearlite decreased from 500 to 1500 nm in the as-cast state to 50–200 nm. The heat treatment process significantly improved the wear resistance of HSHVWRA by obtaining a nano pearlite matrix. The weight loss due to abrasive wear in the nano pearlite matrix-based HSHVWRA was reduced by 26.9 % compared to the traditional micro-pearlite matrix-based alloy. In the impact wear test, the wear weight of the nano pearlite material was reduced by 63.7 % compared to the cast sample, resulting in a maximum relative wear resistance of 2.75. Nano pearlite has the ability to prevent crack initiation between the matrix and carbides and inhibit the propagation of fatigue cracks, thereby enhancing the fatigue resistance of the matrix. Additionally, with a higher number of layered sheets, nano pearlite can effectively withstand impact loads. Meanwhile, submicron carbides dispersed in the nano pearlite matrix contribute to a second phase strengthening role and cooperate with nano pearlite to enhance the wear resistance of the materials.

Cost estimation and performance evaluation of 3D printed membranes for vanadium redox flow batteries

AbstractThe membrane is a central component in the commercialization of vanadium redox flow batteries (VRFB), with Nafion being the preferred material for those membranes. Nafion suffers however from high vanadium crossover and high cost, thus limiting its wider commercial application in VRFB. To address this, studies have focused on sulfonated polyether ether ketone (SPEEK) as an alternative material. This work presents an economic study on the production costs of 3D printed SPEEK (3D‐SPEEK) membranes for VRFB, with multiple production scenarios evaluated. This analysis relies on the performances of 3D‐SPEEK membranes prepared in our laboratory, which have shown higher Coulombic efficiency and lower vanadium ions diffusion than Nafion membranes. Results from the economic analysis revealed that the cost of the 3D‐SPEEK membrane varies from 567.77 to 79.35 €/m2 depending on the production scheme and scale used. Production costs lower than commercial Nafion membranes are achieved when a large production scheme is used. Further analysis shows that 3D‐SPEEK membranes might even be cheaper than conventional SPEEK membranes if their printing time is reduced.

Consideraciones generales sobre la génesis de la ocurrencia de uranio y vanadio en las rocas sedimentarias Cretácicas del Sinclinal de Berlín Cordillera Central (Andes Colombianos)

The mineralization at the Berlin project in Colombia presents a huge interest based on its relatively high uranium and vanadium concentrations (0.11% U3O8) and (0.45% V2O5), with a suite of other economically interesting elements, all of them necessary to achieve the energy transition, including Y, Re, Ag, and P. The present research suggests a hypothesis for the U and V mineralization: an epigenetic origin. The results indicate that the source for U, V, and the other economic elements associated, were released from the black shales of the Abejorral Formation that is overlying a carbonate succession of Cretaceous wackestone. Those elements were transported in diagenetic fluids under specific physical and geochemical conditions. Efficient strata-bound trap occurred when the carbonate succession triggered a redox front that interacted not only with mineralized fluids, but organic matter and H2S.

Selective recovery of vanadium from high-chromium vanadium slag by a mechanically activated low-sodium salt roasting-water leaching process

Stabilizing the toxic element chromium and improving vanadium recovery are crucial for the economic and efficient treatment of high chromium-bearing vanadium slag. This study employed mechanical activation to enhance vanadium extraction from high chromium-bearing vanadium slag through a low-sodium salt roasting-water leaching process. The effects of Na2CO3 addition, roasting temperature, and roasting time were investigated to determine the optimal roasting parameters. Subsequently, the effects of mechanical activation on the morphological and microstructural changes were characterized using a laser diffraction particle size analyzer, a micropore physisorption analyzer, scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction. The effects of mechanical activation on the extraction of vanadium and chromium were determined by leaching the roasted samples after varying activation times. The results indicated that mechanical activation decreased particle size, increased surface area, and enhanced the activity of the samples. The leaching results demonstrated that mechanical activation enhanced the leaching efficiency of vanadium by 5 %, reduced the optimum roasting temperature by 50 ℃, and slightly increased the leaching efficiency of chromium by 0.5–1 %.

Revealing the oxidation mechanism of high vanadium high-speed steel using multi-scale characterization

The microstructure evolution of the oxide scales formed in high vanadium high-speed steel after exposure to 500 °C and 700 °C for up to 120 min was investigated using multi-scale characterization. The oxide scale showed a triplex structure consisting of internal oxide layer, inner oxide layer, and outer oxide layer. The internal and outer oxides formed were Cr2O3 and Fe3O4, respectively. The progressive increase in oxygen partial pressure resulted in the conversion of Cr2O3 to FeCr2O4 in the inner oxide layer with nanocavities. The “available space model” and the “internal oxidation model” were identified as controlling oxidation mechanisms.