Metallic alloy with protective oxide layer, method for making the same and apparatus for forming at least an oxide layer at the surface of a metallic alloy

Abstract

The present invention relates to high-temperature oxidation processes for metallic materials. More particularly, it pertains to methods for forming protective, thermally grown oxide layers—such as chromia (Cr₂O₃) and alumina (Al₂O₃)—on various chromium- and aluminum-containing alloys through controlled formation of oxide layers that act as a barrier protecting the alloy substrates, and to coated metals produced by these methods.

Description

[0001] METALLIC ALLOY WITH PROTECTIVE OXIDE LAYER Field of the Invention The present invention relates to high-temperature oxidation processes for metallic materials. More particularly, it pertains to methods for forming protective, thermally grown oxide layers—such as chromia (Cr₂O₃) and alumina (Al₂O₃)—on various chromium- and aluminum-containing alloys through controlled formation of oxide layers that act as a barrier protecting the alloy substrates. The invention is especially applicable to improving corrosion, oxidation, and hydrogen permeation resistance in complex-shaped components without relying on externally applied coatings. Background Hydrogen plays a crucial role in transitioning to a sustainable energy future due to its capacity to store and deliver clean energy, reducing carbon emissions across various hard-to-abate sectors. Its versatility as a fuel and as a feedstock for industrial processes, including steel, cement production, and in transportation, makes it a key component in achieving global carbon neutrality. On a mass basis, hydrogen has nearly three times the energy content of gasoline. Hydrogen, despite its superior energy density by mass, exhibits significantly lower volumetric energy density under ambient conditions. Moreover, hydrogen’s propensity for permeating metal-based materials, in particular austenitic alloys, poses considerable operational and safety concerns, particularly in the context of containment and transportation. The phenomenon, known as hydrogen embrittlement, arises from the diffusion of atomic hydrogen into the metal lattice, which can lead to material degradation and failure. The interaction of hydrogen with metals can be broadly categorized into physical and chemical adsorption processes, followed by absorption into the metal’s crystal lattice. The physical adsorption, or physisorption, involves hydrogen molecules (H₂) being weakly bound to the metal surface without electron transfer. In contrast, chemical adsorption, or chemisorption, involves the dissociation of H₂ molecules into hydrogen atoms (H), which then form stronger bonds with the metal atoms, potentially leading to the absorption of hydrogen atoms into the metal matrix. Over time, the accumulation of hydrogen in the metal can lead to undesirable phenomena including Hydrogen-Induced Cracking (HIC), Hydrogen Environmental Assisted Cracking (HEAC), High- Temperature Hydrogen Attack (HTHA), and ultimately, material failure. The ease with which hydrogen can permeate and dissolve in a material influences its susceptibility to hydrogen-induced damage. Higher hydrogen diffusivity can increase the risk of hydrogen-induced embrittlement and related phenomena. It is therefore desirable to minimize hydrogen permeation into metallic alloys for use in the containment or transport of hydrogen. In addition to hydrogen-related degradation, metallic materials used in the industry are often exposed to corrosive environments. Acidic environments, such as those containing chlorides, can promote localized forms of corrosion, including pitting and crevice corrosion which are particularly detrimental to metallic alloy systems like stainless steels. At elevated temperatures, oxidative degradation becomes a dominant failure mode, as protective oxide layers can spall, crack, or become permeable, and these allow for continued attack by reactive species. Also, the application of existing protective coatings in these demanding environments is often limited by geometric constraints and process-related challenges. Many coating techniques, such as thermal spraying or physical vapor deposition, rely on line-of-sight access to the component surface, making it very difficult to achieve uniform coverage on parts with complex geometries, internal surfaces, or high aspect-ratio features. A further concern is that some coating methods rely on toxic substances, rare earth elements, and Critical Raw Materials (CRMs). These issues have caused supply-chain vulnerabilities and environmental risks. A key example is Chromium VI (Cr6+), which is a regulated carcinogen and is under increasing EU restrictions. All these aspects and limitations highlight the need for novel and sustainable protection methodologies that result in barrier layers that are chemically inert in degrading environments, mechanically and thermodynamically robust, and geometrically adaptable. Summary The present application relates to methods for forming at least one oxide layer at a surface of a metallic alloy, the metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium; wherein the method comprises: introducing the metallic alloy into a heating chamber of a furnace; and performing a heat treatment process to form the at least one oxide layer on the surface of the metallic alloy; wherein the heat treatment process comprises: (a) a temperature ramp-up phase during which the temperature of the heating chamber is increased to a treatment temperature; and then (b) a heat treatment phase during which the temperature of the heating chamber is maintained at the treatment temperature for a treatment time to form the at least one oxide layer; wherein, during temperature ramp-up phase (a) and heat treatment phase (b), the atmosphere within the furnace comprises at least one non-oxidizing gas, hydrogen, and water vapor; wherein, during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 2 (such as at least 5, or at least 10) and during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 0.5; and wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer. Also disclosed is a coated metallic alloy having at least one surface oxide layer produced by the methods disclosed herein. Further, we disclose a coated metallic alloy comprising: a metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium; at least one oxide layer on a surface of the metallic alloy, wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer; wherein the chromia layer and / or the alumina layer has a porosity of less than 2% by volume, for example as measured using scanning electron microscopy (SEM) image analysis. Uses of the coated metallic alloys described herein is also disclosed, for example, in the manufacture of a component for use in hydrogen containment or transport. Finally, the application discloses an apparatus for forming at least one oxide layer at a surface of a metallic alloy, the apparatus comprising: a furnace comprising a heating chamber; a gas supply system in flow communication with the heating chamber via at least one inlet of the heating chamber; and a gas monitoring system in flow communication with the heating chamber via at least one outlet of the heating chamber and configured to monitor the atmosphere within the heating chamber; wherein the gas monitoring system comprises a humidity sensor configured to measure the humidity of gas from the heating chamber. These and other embodiments are described in more detail below. Figures FIGs.1A and 1B: Schematic representations of apparatus in accordance with the present disclosure. FIGs. 2A and 2B: Cross-sectional scanning electron microscopy images revealing the coating formation on Inconel® 625 alloy, see Example 1. FIGs. 3A and 3B: Cross-sectional scanning electron microscopy images revealing the coating formation on the stainless steel type 316L alloy. FIGs. 4A, 4B, and 4C: Cross-sectional scanning electron microscopy images revealing the coating formation on Inconel® 600, Inconel® 625, and Inconel® 718. FIGs.5A and 5B: Comparative analysis of oxidation behavior in stainless steel type 316 alloy without (FIG. 5A) and with (FIG. 5B) furnace purging during the ramp-up phase. FIGs. 6A and 6B: Comparative cross-sectional and surface analysis of two stainless steel type 316L alloy samples subjected to differing H₂ / H₂O ratios during the ramp-up phase. FIGs. 7A and 7B: Influence of heating rate and gas flow rate on humidity control and oxide scale integrity during the ramp-up phase. FIG. 7A humidity profiles as a function of temperature for two ramp-up conditions: (i) low heating rate (5°C / min) and low flow rate (1.2^L / min (retort volume of 24), and (ii) high heating rate (10°C / min) and high flow rate (2^L / min- retort volume of 24). FIG. 7B is a cross-sectional SEM image of stainless steel type 304 alloy subjected to a ramp-up condition outside the specified range of the present disclosure. FIGs.8A and 8B: Effect of initial furnace temperature on oxide scale integrity during high-temperature exposure. FIG.8A initial furnace temperature is room temperature; FIG.8B initial furnace temperature is 230°C. FIGs.9A and 9B: Oxidation morphology as a function of water content during the ramp-up phase. FIGs. 10A, 10B, 10C, and 10D: Cross-sectional SEM micrographs and EDX spectrum of alumina scales formed on FeCrAl-based alloys (composition 1 and composition 2), see Example 9. FIGs. 11A and 11B: XRD patterns of oxide scales formed on FeCrAl-based alloys (FIG. 11A – composition 1; FIG. 11B – composition 2), see Example 9. FIG.12: Luminescence spectra of alumina scales formed on a FeCrAl-based alloy (composition 1), see Example 9. FIG.13: Cross-sectional SEM images of chromia-forming alloys (ferritic, austenitic, and nickel-based), see Example 10. FIG. 14: XRD profiles of chromia scales formed on nickel-based alloys (Inconel® 625), see Example 10. FIG.15: Raman spectra of Cr2O3 scales formed on nickel-based alloy substrates, see Example 10. FIG. 16: Hydrogen permeation and desorption performance of Cr2O3 scales formed on Inconel® 625, see Example 11. FIGs. 17A and 17B: Optical images comparing surface condition of stainless steel type 316 alloy specimens after exposure to 10^wt% FeCl₃ solution, per Example 12. FIG. 17A is a sample treated in accordance with the present disclosure and shows no visible corrosion, surface degradation, or discoloration . FIG.17B shows the uncoated alloy showing severe pitting and surface staining. FIG.18: Weight gain measurements for various chromia-forming alloys following isothermal oxidation at 800^°C for 168 hours, see Example 13. FIG.19A and 19B: Cross-sectional SEM images of Inconel® 625 specimens after exposure to a sulfate salt mixture (30 mol% Fe2(SO4)3, 15 mol% K2SO4, 55 mol% Na2SO4) at 600^°C for 24 hours, see Example 14. FIG.20: Progressive load scratch test results on Inconel® 625 specimens, see Example 15. FIGs.21A and 21B: Pull-off adhesion test results for oxide-coated specimens of (a) Inconel® 625 and (b) stainless steel type 316 alloy, see Example 16. FIGs.22A and 22B: Mandrel bend test specimens of (a) Inconel® 625 and (b) stainless steel type 316L alloy, see Example 17. FIGs. 23A and 23B: SEM micrographs showing cross-sectional SEM images of oxide layers formed on Inconel® 718 after exposure to 950°C in air (FIG.23A) and in the disclosed environment (FIG.23B) for 48 hours, see Example 18. FIGs. 24A and 24B: SEM micrographs showing cross-sectional SEM images of oxide layers formed on Inconel® 600 after exposures to 950°C for 48 hours in air (FIG. 24A) and in the disclosed environment (FIG. 24B), see Example 18. FIGs. 25A and 25B: Cross-sectional SEM images of oxide scales formed on stainless steel type 304 alloy (FIG. 25A, upper panels) and stainless steel type 316 alloy (FIG. 25B, lower panels) following thermal oxidation at 500°C for 12 hours in a 0.01% humidity environment. FIG.26: Cross-sectional SEM images of 316 specimens oxidized at 950°C for 24 hours under elevated humidity (2.2%) conditions. The left image presents a low-magnification view, and the right image shows a corresponding high-magnification image of the boxed region. Detailed Description The method disclosed herein represents a solution to hydrogen permeation into metals, as well as hydrogen uptake, by leveraging the properties of chromium-containing and aluminum-containing austenitic and ferritic alloys. The method disclosed herein is also effective in protecting alloy substrates against a number of different corrosive and degrading environments. Unlike conventional approaches that rely on the deposition of additional layers through processes such as Chemical Vapor Deposition (CVD), the technique disclosed herein transforms a surface portion of the alloy structure itself, creating a dense, notably inward-growing, and substantially pure chromia (Cr₂O₃) and / or alumina (Al₂O₃) layer. The inward-grown Al2O3 and Cr2O3 layers, which also exhibit the desired range of compressive stresses (as measured by XRD), are highly desirable due to the reasons discussed below. This transformation is achieved through a controlled heat treatment process, resulting in a dense chromia or alumina barrier layer with a high purity and a remarkably low porosity of less than 2.0% by volume, or even as low as 0.5% by volume or less, that adheres strongly to the underlying metal, providing exceptional durability and permeation resistance. Chromia and alumina in their most thermodynamically stable phase have a dense, thermodynamically stable oxide structure, which makes them an effective barrier against diffusion processes, such as hydrogen permeation and chloride ions (e.g., from FeCl₃ environments) and high-temperature oxidants (e.g., O₂). In addition to their chemical resistance, these oxides have been demonstrated to possess high mechanical robustness, as evidenced by standardized scratch resistance and pull-off adhesion tests. The effectiveness of a thermally grown chromia or alumina layer with minimal defects according to the present disclosure in considerably slowing down hydrogen permeation and degradation of alloys may be attributed to: 1. Low Porosity: An inward-grown chromia or alumina layer with extremely low defect level according to the present disclosure provides fewer and smaller pathways for hydrogen and corrosive species to diffuse through the material. Porosity and cracks are a critical factor in diffusion processes; lower defects significantly reduce the diffusion coefficient of hydrogen into the material. 2. Strong Chemical Bonds: Inward-grown chromia or alumina forms strong chemical / ionic bonds within its crystalline structure, which contributes to its stability and impermeability. The energy barrier for hydrogen to diffuse through such a tightly bonded structure is significantly higher compared to less dense materials. 3. High Activation Energy for Hydrogen Diffusion: The activation energy for hydrogen to diffuse through chromia or alumina is relatively high due to the material’s dense oxide structure. This means that hydrogen atoms require more energy to migrate, slowing down the permeation process. Hence, quantitatively, a thermally grown chromia or alumina layer according to the present disclosure having compressive stresses, minimal defects and a low porosity can reduce hydrogen permeation substantially compared to inferior chromia / alumina layers. This enhancement not only improves operational safety and efficiency but also extends the lifespan of components exposed to hydrogen and / or other corrosive species, such as heat exchangers, engine components, turbine blades, fuel injectors, medical equipment, pumps, knives, medical implants, burners, filters, storage tanks, and pipelines. Furthermore, the applicability of the disclosed methods to chromium- and / or aluminum- containing austenitic and ferritic alloys facilitates its application across a wide range of hydrogen infrastructure components, including heat exchangers, sheet metal, tubes, storage tanks, pipelines, and filling stations. Furthermore, the versatility and adaptability of the method to different geometries (enabling the treatment of exterior surfaces of metallic components, for example) and operational requirements underscore its potential. Thus, the disclosed methods and coated alloys represent a reliable, efficient, and environmentally sustainable solution to hydrogen permeation challenges. By harnessing the properties of metallic alloys, such as austenitic alloys and ferritic stainless steels, and optimizing the formation of a chromia or alumina barrier layer, the methods disclosed herein can provide improvements compared to current technologies in performance, cost-effectiveness, and ecological impact. The chromia and / or alumina layer is inward-grown in the methods of the present disclosure. That means that the oxide forms by consuming metal atoms (chromium / aluminum) from the substrate alloy rather than being added to the surface from an external source. This can occur when the bond oxygen diffusion rate through the oxide is faster than the outward diffusion rate of metal ions, causing it to grow into the metal. The following factors may affect the inward-growth of metal oxide layers: ● Oxidizing gas partial pressure: Increasing the oxidant (such as oxygen gas) availability by increasing the partial pressure of the oxidizing gas can enhance the driving force for oxygen diffusion into the oxide layer. ● Oxide layer characteristics: The formation of a dense, compact oxide layer can facilitate oxygen hydroxide ion diffusion, i.e., OH-, while obstructing metal ion diffusion. Adjusting the oxidation temperature and atmosphere can influence these properties. ● Temperature Control: Since the diffusion coefficients for bound oxygen (i.e., OH-) and metal atoms are temperature-dependent and both increase with T, operating at a temperature that favorably affects oxygen diffusion more than the metal atom diffusion can promote faster oxygen diffusion relative to metal ion diffusion. Further, the temperature may be optimized to promote chromium / aluminum diffusion relative to other metal species, such as iron or manganese. Advantages of inward-grown chromia / alumina layers include: Improved Adhesion and Compactness: ● Mechanical Interlocking: Inward-grown oxide layers may lead to a better mechanical interlock at the alloy / oxide interface. This enhanced interlocking contributes to improved adhesion, reducing the chance of spallation or delamination under thermal cycling or mechanical stress. ● Increased Surface Area for Bonding: As the oxide layer grows inward, it conforms closely to the microstructure of the substrate. This growth can create a more extensive interface area between the oxide and the metal, increasing the physical contact points. The interfacial area’s intricate nature allows for mechanical interlocking, where the oxide layer fits into the irregularities of the metal surface, much like a key in a lock. ● Enhanced Mechanical Anchoring: Inward growth tends to form oxides that penetrate into the grain boundaries of the metal substrate, leading to mechanical anchoring. This anchoring effect is stronger than mere surface adhesion because it locks the oxide layer into the metal’s microstructure, making it more difficult for the oxide layer to detach. ● Denser Structure: The process of inward growth can result in a denser and more compact oxide structure. This compactness enhances its protective capability. Stress State and Thermal Expansion Mismatch ● Compressive Stresses: Inward-grown oxides can develop compressive stresses due to the volume expansion associated with oxide formation (Pilling-Bedworth ratio considerations). Compressive stresses within the oxide layer can help to close cracks and voids, whereas tensile stresses (more common in outward-grown layers) tend to open up pathways for oxidation and corrosion. Compressive stresses may be measured by, for example, XRD. ● Reduced Thermal Expansion Mismatch: The thermal expansion coefficient mismatch between the oxide layer and the metal substrate can lead to stress accumulation during thermal cycling. Inward-grown layers, due to their closer integration with the substrate, may better accommodate these mismatches, reducing the risk of cracking or spallation. Controlled Growth Kinetics ● Slower Growth Rates: Inward growth is often associated with slower oxide growth rates after the initial formation phase. This slower rate can lead to a more stable and protective layer over time, as rapid growth rates can lead to porous, less protective layers that are more susceptible to cracking. ● Uniform Thickness: Inward growth tends to produce more uniformly thick layers, which is beneficial for consistent protection across the surface of the material. Uniformity helps to avoid thin spots or defects that could serve as initiation points for corrosion or oxidation. Stability and Composition o Purity of the Oxide Layer: Inward-grown layers have a more consistent composition, fewer impurities, and are substantially free of unstable oxides, as described above. o Selective Diffusion of Metal Ions ● Inward Growth: In inward-growing chromia and alumina scales, the growth is predominantly driven by the inward diffusion of hydroxide ions through the existing oxide layer, not outward diffusion of metal ions through the bulk metal. Since this diffusion process occurs through solid metal, it is more selective. The diffusion rates of different metal ions through the solid metal and oxides are not the same, on the other hand, the diffusion rate of hydroxide ions through the solid metal is always constant within one alloy system. This selectivity can lead to a layer that is predominantly composed of the desired oxide, e.g., Cr2O3, with fewer other metal ions reaching the oxide formation front. This inward growth mechanism is actually what makes the scales formed under the conditions described in this patent so protective—they form at the metal / oxide interface rather than at the oxide / gas interface, creating denser, more adherent protective layers. o Environmental Influence ● Outward Growth: Outward-growing layers are more directly exposed to the external environment throughout their formation. This exposure allows for the potential incorporation of environmental impurities (e.g., sulfur, carbon) into the oxide layer as it forms. Additionally, the outward diffusion of metal ions towards the oxide / environment interface might not be as selective due to the interaction with various environmental species, potentially leading to a more heterogeneous oxide composition. o Oxide Layer Density and Compactness ● Dense and Compact Layers: Inward-grown oxide layers tend to be denser and more compact due to the compressive stress involved in their formation and the reduced incorporation of impurities. A denser oxide layer is less porous, which minimizes the pathways for corrosive species to penetrate the layer, enhancing its protective capabilities. o Thermal Dynamics ● Reduced Thermal Gradients: The process of inward oxidation can result in reduced thermal gradients within the oxide layer compared to outward growth. These reduced gradients can minimize the formation of defects (such as voids or cracks) that could serve as paths for impurity migration into the layer, further contributing to the purity and integrity of the oxide. Depending on the oxidizing environment (e.g., temperature), the rate of diffusion of other elements (such as manganese) may increase. This increased diffusion may facilitate formation of mixed oxides such as MnCr2O4 and / or MnAl2O4. This involves the diffusion of manganese (Mn) from the solid solution (ss) through the chromia layer (Cr₂O₃), reacting with oxygen (e.g., from water vapor) to form manganese chromite (MnCr2O4), typically in the spinel structural form. Although manganese is discussed in detail here, similar principles apply to other metals capable of forming a spinel with chromium and / or aluminum (e.g., magnesium, nickel, iron). Elevated temperatures facilitate this process by enhancing the diffusion rates of Mn and oxygen through the chromia layer, promoting the rapid formation and inward-growth of MnCr2O4. However, excessive temperatures may result in a mixed oxide layer where Cr₂O₃ and MnCr2O4 intermingle, obscuring the distinct layers’ formation and potentially compromising the protective qualities of the oxide barrier. The spinel (MnCr2O4) primarily develops at the upper part or surface of the chromia layer (facing away from the substrate alloy), progressing inward and consuming chromia during the process. The rate- limiting step for MnCr2O4 formation is the diffusion of Mn(ss) within the alloy to the alloy / oxide interface. Combining MnCr2O4spinel with a thermally grown Cr2O3layer may still form a dense and structurally complex oxide barrier that can significantly reduce hydrogen permeation, in particular where the spinel forms a distinct layer on or at the upper, or outward-facing, surface of the chromia layer. The chemical interaction between MnCr2O4 and Cr2O3 creates a tightly bonded lattice structure, which increases the activation energy required for hydrogen atoms to diffuse through the material. Such composite oxide layers also lower hydrogen permeation rates compared to singular oxide coatings, and the spinel acts as a protective coating to the chromia / alumina layer. Appropriately controlling the conditions during oxide formation may facilitate a favorable balance and interaction between MnCr2O4and Cr2O3, thereby optimizing the barrier properties. It is noted that certain unwanted metastable oxides, i.e., iron-rich spinels, persist within the thermally grown oxide scale and compromise the structural and chemical integrity of the desired stable oxide layers, such as chromia. These metastable phases can disrupt the continuity, adherence, and protective function of the scale, particularly under thermal cycling or prolonged high-temperature exposure. Methods Described herein is a method of forming at least one oxide layer at a surface of a metallic alloy, the metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium. As used herein, the phrase “base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium” refers to the core metal components of the metallic alloys for use in the methods described herein. Thus, in one embodiment, the base metal comprises iron and aluminum. In one embodiment, the base metal comprises iron and chromium. In one embodiment, the base metal comprises iron, chromium and aluminum. In one embodiment, the base metal comprises nickel and aluminum. In one embodiment, the base metal comprises nickel and chromium. In one embodiment, the base metal comprises nickel, chromium, and aluminum. In one embodiment, the base metal comprises cobalt and aluminum. In one embodiment, the base metal comprises cobalt and chromium. In one embodiment, the base metal comprises cobalt, chromium and aluminum. In one embodiment, the base metal comprises iron, nickel, and aluminum. In one embodiment, the base metal comprises iron, nickel, and chromium. In one embodiment, the base metal comprises iron, nickel, chromium, and aluminum. In one embodiment, the base metal comprises iron, cobalt, and aluminum. In one embodiment, the base metal comprises iron, cobalt, and chromium. In one embodiment, the base metal comprises iron, cobalt, chromium, and aluminum. In one embodiment, the base metal comprises iron, nickel, cobalt, and aluminum. In one embodiment, the base metal comprises iron, nickel, cobalt, and chromium. In one embodiment, the base metal comprises iron, nickel, cobalt, chromium, and aluminum. Specific non- limiting examples of such metallic alloys are provided in the “Definitions” section. According to the methods of the present disclosure, a metallic alloy, for example, a stainless steel, an Ni-base alloy, or a FeCrAl-alloy, is placed inside a heating chamber (e.g., a retort) within a furnace. An inert (non-oxidizing) gas and hydrogen are introduced into the retort, for example, as a gas mixture. The metallic alloy then undergoes a heat treatment process in which the heating chamber within the furnace is heated to a treatment temperature (ramp-up phase), until the heating chamber reaches a treatment temperature, which may be within the range of 800 to 1050°C, more particularly within the range of 850 to 1000°C, such as 900-950°C. The heating chamber is then maintained at the treatment temperature (treatment phase), for example, for a duration of at least 3 hours. Water vapor is also provided to act as an oxidizing agent for oxidizing chromium or aluminum to chromia or alumina during the heat treatment process. As discussed in more detail below, water vapor may be introduced into the retort, either at the beginning of the heat treatment process, or during the heat treatment process. For example, water vapor may be introduced into the heating chamber during the heat treatment process only once the temperature of the heating chamber reaches at least 600°C, or once the treatment temperature is reached. The water vapor may be introduced into the heating chamber by humidifying one or both of the inert gas and hydrogen (e.g., a gas mixture comprising inert gas and hydrogen). Alternatively, the water vapor for oxidizing the chromium / aluminum may be provided by reduction of oxygen gas (dioxygen; O2) by hydrogen within the retort. For example, at elevated temperatures (e.g., higher than 580°C, the ignition temperature for the reaction of hydrogen with oxygen) oxygen may be introduced into the retort, which reacts with the hydrogen to provide the required concentration of water vapor. Water vapor is employed as an oxidizing agent for oxidation of chromium / aluminum instead of relying on free oxygen. This is because the presence of oxygen, a more strongly oxidizing agent than water vapor, in the gas environment surrounding the substrate during heating could lead to the oxidation of metals other than chromium or aluminum that are present in the alloy substrate (e.g., iron), which may detrimentally impact the quality of the resulting oxide layer and its protective effectiveness in reducing hydrogen permeation into the alloy. Also, water vapor is present together with hydrogen (H2) and the H2 / H2O ratio is kept high, higher than 5 during the ramp-up phase and higher than 0.5 during the treatment phase, to reduce the oxidizing power of the gas. This further helps avoid oxidizing other metals present in the alloy, such as iron. The absence of unwanted oxides on the metal surface improves the protective properties of the barrier oxides. Additionally, the use of water as the source of oxygen results in predominantly inward growth of the barrier oxide, which is beneficial because of, e.g., improved adhesion of the barrier oxide to the substrate material. Throughout the heat treatment process, the atmosphere within the heating chamber may be substantially free of free oxygen (O2 gas) (e.g., the concentration of free oxygen in the retort atmosphere may be 500 ppm by volume or less). The hydrogen gas in the system undergoes a reaction with free oxygen (dioxygen) that may be present in the atmosphere within the heating chamber to form water. This is a primary reason for introducing hydrogen to the retort atmosphere. Ideally, the gas composition within the heating chamber should contain zero oxygen, although in practice this is difficult to achieve, and maintaining the free oxygen concentration at 500 ppm by volume or less, optionally 100 ppm by volume or less, optionally 50 ppm or less, is sufficient. When a metal comes into contact with water vapor at elevated temperatures, it undergoes a reaction with the water vapor, resulting in the formation of metal oxides and the release of hydrogen gas. The overall reaction can be expressed as: xM + yH2O → MxOy+ yH2. At the treatment temperature, water is reduced to hydrogen gas and hydroxide ions (OH-) while the metal is simultaneously oxidized at the hydroxide / alloy interface. The two processes are coupled both by electrons traveling outward and by ion currents. When chromia-forming alloys are oxidized in H2O / H2environment, the ion current is carried by oxide ions resulting in inward growth of chromium oxide (Cr₂O₃), as illustrated by the following reaction: 2Cr + 3H2O → Cr2O3+ 3H2For aluminum in the case of alumina-forming alloys (e.g., FeCrAl alloy systems), this results in the formation of Al₂O₃ according to: 2Al + 3H₂O → Al₂O₃ + 3H₂ In austenitic alloys and ferritic stainless steels containing chromium and manganese, a chromium oxide layer is initially established at the interface between the gas and substrate. Gradually, the Cr2O3 layer (also referred to as a chromia scale) undergoes transformation into MnCr2O4 through the following process: At the Cr2O3 / substrate interface, manganese undergoes oxidation, as described by the following equation: Mn(ss) → Mn2++ 2e- At the interface between Cr2O3 and the gas, water reduces to hydrogen gas and hydroxide ions (OH-) by using the generated electrons in the previous equation. Mn2+is then adsorbed at internal interfaces in the chromia scale and migrates toward the Cr2O3 / gas interface, due to the electrochemical potential difference. Upon reacting with the adsorbed oxygen ions, Mn2+converts to manganese oxide, MnO. Later, MnO reacts with chromium oxide, yielding MnCr2O4, a thermodynamically more stable compound compared to MnO. Mn2++ O2-(ads) → MnO MnO + Cr2O3 (s) → MnCr2O4(s) The formation of MnCr2O4 is influenced by the diffusion speed of Mn and Cr atoms within the alloy, both in the bulk and along grain boundaries. Hydroxide ion (OH-) diffusion towards the substrate / oxide interface also impacts oxide formation, affecting MnCr2O4 formation. The pace / rate of chemical reactions, e.g., MnCr2O4and chromia formation, may also affect the thickness of the MnCr2O4and Cr2O3layers. These process parameters can be adjusted by controlling the heat treatment conditions, as taught herein. The heat treatment conditions will affect the diffusion and reaction rates. Appropriate selection of the heat treatment conditions (e.g., temperature, time, oxygen availability, and hydrogen-to-water ratio), as taught herein, may therefore result in a double-layered oxide consisting of an outer spinel (MnCr2O4) layer on the surface and an inner chromia layer. In particular, inward oxide growth to form a dense Cr2O3layer with controlled MnCr2O4spinel formation may be effectively promoted by controlling the volumetric hydrogen-to-water (H2 / H2O) ratio of the heat treatment atmosphere to optimize the oxidizing environment, ensuring sufficient oxygen activity at the metal / oxide interface for chromia formation while preventing conditions that excessively promote spinel or other undesirable oxide (e.g., iron oxide) formation. Such an approach not only supports the development of an effective barrier against corrosion and hydrogen permeation but also tailors the oxide layer’s growth and composition to the specific requirements of inward-growth dynamics. Although the formation of mixed chromium-containing oxides such as MnCr2O4has been described in detail above, similar considerations also apply to the formation of aluminum-containing mixed oxides where the metallic alloy comprises aluminum and the desired oxide layer is an alumina layer. Maintaining a volumetric H2 / H2O ratio of at least 0.5, for example, at least 1, such as at least 2, during the heat treatment process (in particular above 600°C, more particularly above 650°C, or when the temperature of the heating chamber / retort reaches the treatment temperature) promotes precise control of substrate oxidation in the manner desired. In one embodiment, the volumetric H2 / H2O ratio is from 0.5 to 1,000. In one embodiment, the volumetric H2 / H2O ratio is from 10 to 500. In one embodiment, the volumetric H2 / H2O ratio is from 100 to 250. This promotes the development of a dense, continuous, and predominantly pure chromia layer (i.e., a chromia layer comprising less than 5 wt.% impurities). Additionally, hydrogen reacts with free oxygen present in the heating chamber within the furnace. This ratio mitigates against the formation of undesirable oxides by keeping the oxygen activity in the system at a sufficiently low level as well as changing the oxidation mechanism to promote formation of inward- grown dense chromia or alumina layer. The oxygen activity is dictated by the H2 / H2O ratio, with higher ratios representing a lower oxygen activity and promoting the formation of more stable oxides such as chromia and alumina, in preference to undesirable oxides such as iron oxides. In essence, the balance between hydrogen and water vapor affects the chemical reactions and diffusion processes at the metal / oxide and oxide / environment interfaces. In particular, at 650°C and above, for example, at a temperature within the treatment temperature range of 800 to 1050°C, the formation of iron oxides is thermodynamically unfavorable at H2 / H2O volumetric ratios of 0.5 or greater, thus mitigating against the undesirable formation or iron oxides, which may unfavorably affect the structure and properties of the chromia / alumina layer. Elevated temperatures result in increasing reaction rates. However, this increase in temperature also potentially compromises the heat treatment of the substrate and consequently diminishing tensile strength and other mechanical properties. Higher temperatures also increase the diffusion rate of metals other than aluminum and chromium (e.g., manganese and silicon) to the extent that oxides or mixed oxides of comprising other metals may form to an undesirable extent. A suitable balance may be struck by maintaining a H2 / H2O volumetric ratios of 0.5 or greater, optionally 1 or greater, coupled with a treatment temperature ranging from 800 to 1050°C, more particularly 850°C to 1000°C. The H2 / H2O volumetric ratios may therefore be in the range of 0.5 to 1,000 in combination with a treatment temperature ranging from 800 to 1050°C. Exemplified combinations of H2 / H2O volumetric ratios and treatment temperatures within the heating chamber are summarized in Tables 1a and 1b below: H2 / H2O volumetric ratios treatment temperatures (°C) 0.5 or greater 800-1050 0.5 or greater 830-1030 0.5 or greater 850-950 0.5 or greater 800-1000 0.5 or greater 850-1000 0.5 or greater 850-900 0.5 or greater 900-950 1 or greater 800-1050 1 or greater 830-1030 1 or greater 850-950 1 or greater 800-1000 1 or greater 850-1000 1 or greater 850-900 1 or greater 900-950 Table 1a Vol.% of H2O H2 / H2O volumetric ratios treatment temperatures (°C) 0.3 16.6 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 0.1 50 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 0.6 8.3 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 0.2 25 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 0.8 6.25 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 1 5 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 0.05 100 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 0.02 500 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 0.01 1000 800-1050; 830-1030; 850-950; 800-1000; 850-1000; 850-900; 900-950 Table 1b To avoid the presence of undesirable amounts of free oxygen in the heat treatment atmosphere within the heating chamber, the amount of hydrogen in the atmosphere may be maintained at least 2 vol.%, more particularly at least 4 vol.%, throughout the heat treatment process. For example, the gas mixture flowing into the heating chamber during the heat treatment process may comprise at least 2 vol.%, more particularly at least 4 vol.%, hydrogen. If the H₂ content falls below the threshold, there may be insufficient hydrogen to react with unintentionally introduced oxygen, as in the case of a leak or desorption from surfaces within the heating chamber. This insufficient reaction could elevate oxygen activity, disrupting the desired oxidation process and leading to the formation of undesired oxide phases or inconsistencies in the oxide layer, such as porosity. On the other hand, the use of high amounts of hydrogen gas may result in unsafe operating conditions and is also economically undesirable due to the consumption of large quantities of hydrogen gas. Thus, concentration of hydrogen in the atmosphere within the heating chamber may be no greater than 10 vol.% at any point during the heat treatment process. For example, the gas mixture flowing into the heating chamber during the heat treatment process may comprise no greater than 10 vol.% hydrogen. Thus, a suitable quantity of hydrogen gas in the heat treatment atmosphere may be from about 2 to 10 vol.%, such as from about 4 to about 10 vol.%. In another embodiment, the concentration of hydrogen in the atmosphere within the heating chamber may be greater than 10 vol.% during the heat treatment process. For example, the gas mixture flowed into the heating chamber during the heat treatment process may comprise more than 10 vol.% hydrogen, such as more than 20 vol.% hydrogen, more than 30 vol.% hydrogen, more than 40 vol.% hydrogen, more than 50 vol.% hydrogen, more than 60 vol.% hydrogen, more than 70 vol.% hydrogen, more than 80 vol.% hydrogen or more than 90 vol.% hydrogen, up to a maximum of 99.9 vol.% hydrogen. The heating chamber is then maintained at the treatment temperature (treatment phase) for a treatment time. For example, the treatment time may be at least 3 hours, more particularly at least 6 hours. The use of longer treatment times, especially when coupled with the lower treatment temperatures and higher H2 / H2O volumetric ratios disclosed herein, promotes the slower formation of the desired oxide layers. This slower formation promotes the formation of superior metal oxide layers that are more uniform, denser, and have fewer defects and a lower porosity compared to metal oxide layers formed under different conditions, e.g., higher temperatures and shorter heat treatment times. In general, for a given set of heat treatment conditions, the thickness of the resulting layer increases as the heat treatment time increases. Treatment times of at least 10 hours may also promote the formation of chromia / alumina layers that are at least 1 µm thick, for example, at least 1.5 µm thick, further enhancing the barrier properties of the layer. Furthermore, chromia / alumina layers formed in accordance with the methods disclosed herein have an improved adhesion to the metallic alloy substrate due to the inward-growth mechanism and high quality of the resulting coating, making the formation of chromia / alumina layers of such thicknesses feasible without delamination from the substrate. Suitable treatment times include, but are not limited to, from 3 to 72 hours, 3 to 48 hours, 3 to 24 hours, 3 to 12 hours, 6 to 72 hours, 6 to 48 hours, 6 to 24 hours, 10 to 72 hours, 10 to 48 hours, and 10 to 24 hours. In one embodiment, the treatment time is from 10 to 48 hours. Where referenced herein, the thickness of metal oxide layers may be measured using scanning electron microscopy (SEM) image / micrograph analysis. In particular, the thickness of a metal oxide layer may be measured based on analysis of at least one scanning electron microscopy (SEM) micrograph of a cross-section of the layer. The thickness quoted may represent an average (e.g., arithmetic mean) thickness of the oxide layer, as determined based on measurements taken at several (e.g., at least 5) different locations on the surface of the metallic alloy. The SEM images used to calculate the layer thicknesses are preferably obtained at a resolution of at least 2,000 times to ensure accurate results. To further enhance the quality of the formed metal oxide layers, and, therefore, their protective barrier properties relative to hydrogen permeation, it is desirable to minimize the formation of less desired metal oxides, such as iron oxide, during the temperature ramp-up phase of the heat treatment process in which the temperature of the heating chamber is increased to the intended heat treatment temperature. The present disclosure presents a number of strategies rooted in thermal processing and atmospheric control techniques to mitigate against the formation of less desirable metal oxides during the ramp-up phase of the heat treatment process, as detailed below. These strategies may be employed individually or in any combination. Strategy 1: Optimized Heating Rates To circumvent the formation of unwanted iron oxides during the initial ramp-up phase, the heating rate may be optimized. By using a suitably high heating rate during the temperature ramp-up phase lower temperature ranges prone to iron oxide formation may be bypassed more quickly, thereby enhancing the thermodynamic and kinetic conditions favorable for development of the desired protective oxides. In other words, a rapid transition through critical lower temperature zones in which iron oxide may undesirably form minimizes the window for iron oxide nucleation and growth. For this reason, during the temperature ramp-up phase, the heating chamber of the furnace may be heated at a rate of at least 5°C per minute until the treatment temperature is reached. The most suitable heating rate for a given process may also take into account the thermal gradients induced within the substrate material, namely the metallic alloy, which may lead to thermal stresses, distortion, or even cracking of the substrate material. The choice of heating rate is therefore a balance between mitigating against the formation of undesirable oxides at lower temperatures and the thermal tolerance of the material being treated. In specific high-precision applications, material properties and dimensional tolerances may be critical. For this reason, the heating chamber of the furnace may be heated at a rate not exceeding 50°C, such as not exceeding 20°C per minute during the temperature ramp-up phase. For example, an optimal balance may be achieved at a heating rate of about 10°C per minute during the temperature ramp-up phase. During ramp-up, the total gas flow may be maintained at a combined volumetric rate equivalent to at least three times the chamber volume per hour, and optionally five times. To avoid unnecessary resource use, flow rates may be limited to a maximum of 10-15 times the chamber volume per hour. For example, in a 24-liter chamber, a flow rate between 2-4^L / min may be used. Strategy 2: Purging the Heating Chamber As an initial preparatory step, the heating chamber may be purged to facilitate rigorous control over the heat treatment atmosphere, in particular to reduce the water content and / or the free oxygen content of the atmosphere within the heating chamber to minimize the possibility for undesirable oxide formation during temperature ramp-up and to ensure a high degree of control over the atmosphere during the heat treatment phase. For these reasons, the heating chamber may be purged prior to the ramp-up phase by flowing a purging gas through the heating chamber. The purging gas may comprise, consist, or consist essentially of one or more inert gases. In this context, inert may be taken to mean non-oxidizing. For example, the one or more purging gases may comprise one or more noble gases (e.g., argon) and / or hydrogen. The heating chamber may be purged with sufficient inert gas to reduce the water content of the atmosphere within the furnace to less than 0.3 vol.%, in particular less than 0.2 vol.%, particularly when the atmosphere within the heating chamber comprises 10 vol.% or less of hydrogen, for example, 2 to 10 vol.% of hydrogen, or about 5% vol. % of hydrogen, during the ramp-up phase. If the heat treatment atmosphere comprises higher concentrations of hydrogen, then higher water content values following purging / during the ramp-up phase may be acceptable while still maintaining the required hydrogen-to- water volumetric ratio of at least 5 during the ramp-up phase. For example, if the atmosphere within the heating chamber comprises 83.33 vol.% hydrogen during the ramp-up phase, the water content may be as high as 16.66 vol.% while still maintaining the required hydrogen-to-water ratio. Similarly, if the atmosphere within the heating chamber comprises 10 vol.% hydrogen during the ramp-up phase, the water content may be as high as 2.00 vol.% while still maintaining the required hydrogen-to-water ratio. The purging may therefore reduce the water content of the atmosphere within the furnace sufficiently to permit the ratio by volume of hydrogen to water vapor in the atmosphere within the heating chamber to be maintained at 5 or greater during the temperature ramp-up phase, for example, when the atmosphere within the heating chamber comprises 10 vol.% or less of hydrogen, for example, 2 to 10 vol.% of hydrogen, or about 5% vol. % of hydrogen, during the ramp-up phase. In one embodiment, when the base metal comprises nickel, the ratio by volume of hydrogen to water vapor in the atmosphere within the heating chamber may be at least 2, such as at least 5. The purging may alternatively or additionally reduce the free oxygen content of the atmosphere within the furnace to less than 500 ppm by volume, in particular less than 100 ppm by volume, more particularly less than 50 ppm by volume. For example, the heating chamber’s initial free O2 availability may be reduced to 25-50 ppm by volume of O₂, equivalent to a partial pressure of 2.5-5 x 10⁻⁵ atm. The amount, by volume, of the purging gas supplied to the heating chamber during the purging of the heating chamber may be at least 5 times the volume of the heating chamber, for example, at least about 10 times the volume of the heating chamber. This meticulous atmosphere control not only limits the formation of unwanted oxides and promotes the formation of chromia / alumina but also mitigates safety concerns associated with explosive reactions between hydrogen and oxygen. Continuous monitoring of oxygen partial pressure within exhaust gases may be used to monitor the effectiveness of the purging operation. Strategy 3: Flow Rate Control During the heat treatment process, and in particular during the ramp-up phase, optimization of the flow rate of the gases fed into the heating chamber / retort may also limit the undesirable build-up of oxygen and / or water vapor in the heating chamber atmosphere by continuously flushing the heating chamber. To achieve particularly effective flushing of the heating chamber, during the heat treatment process, in particular during the temperature ramp-up phase, the gases fed into the heating chamber (which may be supplied separately or as a mixture) may be supplied in a combined amount by volume of at least three times the volume of the heating chamber per hour, optionally at least five times the volume of the heating chamber per hour. On the other hand, excessively high flow rates may lead to the needless waste of resources and may be uneconomical. Therefore, the plurality of gases that are supplied to the heating chamber during the heat treatment process may be supplied in a combined amount by volume per hour of no greater than 15 times the volume of the heating chamber per hour, optionally no greater than 10 times the volume of the heating chamber per hour. For example, for a heating chamber that is 24 liters in volume, a suitable flow rate range may be at least 1 liters per minute, such as 2 liters / minute, for example, 1-4 or 2-4 liters / minute. Strategy 4: Limiting Water Vapor Content / Hydrogen-to-Water Ratio During Ramp-Up Water vapor (H₂O) plays an important role in the formation of iron-rich oxides. In particular, the volumetric hydrogen-to-water ratio of the heating chamber atmosphere strongly influences whether iron formation is thermodynamically favorable at any given temperature. The water content of the atmosphere within the heating chamber may be controlled during temperature ramp-up to mitigate against the formation of iron oxides during the ramp-up phase. In particular, during the temperature ramp-up phase (or at least until the temperature of the heating chamber reaches 600°C), the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace may be controlled to be at least 5, optionally at least 10, so as to substantially mitigate against the formation of iron oxides during the temperature ramp-up phase. Suitable H2 / H2O ratios include, but are not limited to at least 5, at least 10, at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 125, at least 150, at least 175, at least 200, at least 250, at least 300, at least 350, at least 400, at least 450, at least 500, etc. Suitable ranges of H2 / H2O ratios include, but are not limited to 5 to 1,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. In one embodiment, the during the temperature ramp-up phase (a), the amount of hydrogen is 2 to 99.9 vol.%. In one embodiment, the during the temperature ramp-up phase (a), the amount of hydrogen is 4 to 40 vol.%. If the gas mixture comprises more than 20 vol. % hydrogen then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 2,000, such as 100 to 2,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises more than 30 vol. % hydrogen then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 3,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises more than 40 vol. % hydrogen then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 4,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises more than 50 vol. % hydrogen then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 5,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises more than 60 vol. % hydrogen then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 6,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises more than 70 vol. % hydrogen then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 7,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises more than 80 vol. % hydrogen then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 8,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises more than 90 vol. % hydrogen then then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 9,000, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100-250. If the gas mixture comprises 99.9 vol. % hydrogen then then suitable ranges H2 / H2O ratios include, but are not limited to 5 to 9,910, such as 100 to 1,000, 500 to 1,000, 5 to 500, such as 100 to 500, or 100- 250. This may be achieved by implementing various of the strategies disclosed herein, for example, strategy 2 (effective purging), and / or by disengaging / deactivating the humidifier(s) that humidify the input gases. These approaches may facilitate maintenance of a dry atmosphere within the furnace, thereby keeping the hydrogen-to-water ratio above the required value of 5, even when the atmosphere comprises relatively low amounts of hydrogen gas (e.g., about 5% by volume). The absence or reduction of H₂O reduces the formation of hydroxyl radicals (OH•) that can accelerate oxidative reactions, thereby minimising iron oxide development. While a significant portion of iron-containing oxides subsequently undergo conversion to chromia under the treatment conditions disclosed herein, without controlling the hydrogen-to-water ratio during ramp-up, e.g., restricting the water vapor content during ramping phase, the unwanted oxides may become detectable within the oxide scale. Furthermore, while most of the iron-rich oxides will be converted to chromia under the heat treatment conditions suggested herein, this conversion leaves behind iron (Fe(s))-particles within the oxide layers, according to the following equation: FeCr2O4(s) + H2(g) → Cr2O3(s) + Fe(s) + H2O(g) By mitigating against the initial formation of iron oxides such as chromite the abundance of iron particles within the oxide layers may be reduced, thus improving coating quality. Strategy 5: Holding the Furnace at Slightly Higher Temperatures The furnace may be held at elevated temperatures, e.g., approximately 100-250°C, for a period of time (e.g., at least 10 minutes, or at least 30 minutes) prior to the heat treatment process and prior to the ramp-up phase and in particular prior to or during the purging to provide control over oxide formation (i.e., “baking”). This strategy expedites the passage through the ramp-up regime, directly approaching the critical conditions favorable for chromia layer formation, and also assists in desorbing oxygen and water from the surfaces within the heating chamber (e.g., insulation materials and internal retort surface areas), thus providing enhanced control over the atmospheric conditions during the heat treatment process, and in particular during the ramp-up phase. The selection of the initial temperature may take into consideration the alloy’s tolerance of heat shock or excessive thermal stresses. The temperatures disclosed herein ensures that while the temperature is sufficiently elevated to minimize undesirable oxide formation, they do not compromise the material’s structural integrity or induce undue thermal gradients. Suitable temperatures include, but are not limited to, about 100°C, about 150°C, about 200°C and about 250°C. Suitable temperature ranges include, but are not limited to, about 100-250°C, about 100-200°C, about 100-150°C, about 150-200°C, about 150-250°C, and about 200-250°C. Implementational Details The disclosed embodiments include systems and methods for producing oxide coatings on metallic substrates (e.g., metals, metallic alloys, etc.). The disclosed embodiments also include oxide coatings grown on the metallic substrates. The disclosed systems for producing oxide coatings may include various configurations, as described in the sections below. For example, in one embodiment, such a system includes an apparatus for forming at least one oxide layer at a surface of a metallic alloy, as shown in Figs. 1A and 1B. The apparatus includes a furnace having a heating chamber; a gas supply system in flow communication with the heating chamber via at least one inlet of the heating chamber; and a gas monitoring system in flow communication with the heating chamber via at least one outlet of the heating chamber and configured to monitor the atmosphere within the heating chamber. In this example, the gas monitoring system includes a humidity sensor configured to measure the humidity of gas from or within the heating chamber. The furnace and its associated heating chamber may include any type of vessel within which a processing temperature may be controlled. In some examples, the heating chamber is a retort (e.g., an airtight vessel in which substances may be heated). The heating chamber may be made from stainless steel, nickel-based alloy, and / or ceramic. In some examples, the heating chamber is configured to operate at a positive pressure relative to atmospheric pressure to prevent backflow of gases into the chamber during cool-down, for example, up to 10% above atmospheric pressure (e.g., up to 0.1 bar gauge pressure). The disclosed coating processes involve control of gases supplied to the heating chamber. As noted, examples of the disclosed coating systems include a gas supply system in flow communication with the heating chamber via at least one inlet of the heating chamber. The gas supply system is configured to provide non-oxidizing gas and hydrogen, and at least one of water vapor and / or oxygen, to the heating chamber. The water vapor required to oxidize the chromium / aluminum may be provided to the heating chamber either as water vapor or may be formed within the heating chamber by reaction of oxygen with hydrogen at elevated temperatures to form water. The gas supply system may, for example, comprise a humidifier in flow communication with the heating chamber and configured to supply water vapor to the heating chamber. In some cases, the gas supply system comprises gas supply tubing for supplying at least one of a non- oxidizing gas and hydrogen to the heating chamber and selectively humidifying the non-oxidizing gas and / or hydrogen. Further, in some examples, the gas supply system comprises a humidifier, and wherein the gas supply tubing for supplying the non-oxidizing gas and / or hydrogen to the heating chamber defines a first gas supply line that passes through the humidifier and a second gas supply line that does not pass through the humidifier. In such cases, the gas supply system is controllable to supply the non- oxidizing gas and / or hydrogen to the heating chamber via either the first or second gas supply lines, for example, using one or more valves. To monitor and control the gas environment(s) associated with the disclosed coating systems, the systems may include a gas monitoring system. As noted, the gas monitoring system may include a humidity sensor configured to measure the humidity of gas from or within the heating chamber. In some examples, the gas monitoring system is arranged in flow communication with the heating chamber via at least one outlet of the heating chamber, the gas monitoring system being configured to monitor the atmosphere within the heating chamber. In some cases, the gas monitoring system comprises a humidity sensor configured to measure the humidity of gas from or within the heating chamber. The gas monitoring system may include an oxygen sensor configured to measure the oxygen content of gas output from the heating chamber, for example. The oxygen sensor may be configured to measure the free oxygen content of the gas output from the heating chamber. The oxygen sensor may also be configured to measure the bound oxygen content of the gas output from the heating chamber. In some cases, the oxygen sensor also serves as the previously mentioned humidity sensor, for example, by measuring the bound oxygen content of the gas. In some examples, the gas monitoring system includes a hydrogen sensor configured to measure the hydrogen content of gas within or output from the heating chamber. The disclosed coating systems may also include various other components, such as one or more pumps to facilitate gas flow through the system. In some cases, one or more vacuum pumps may be provided for evacuating the heating chamber (e.g., to assist in water vapor and free oxygen removal prior to the coating process) and / or for assisting in drawing processing and / or purge gas through the system. The disclosed coating system may also include a closed gas circulation system as shown in Fig 1A. In this system, a pump P1 circulates the gas through the heat exchanger HX1 into the retort to recuperate the heat from the gas that exits the retort. This enables the exiting gas to be cooled to lower temperatures than the temperature in the retort and the inlet gas to be pre-heated prior to entering the retort. The gas may need to be further cooled in a second heat exchanger HX2 to avoid overheating the circulating pump. This system enables the volume flow in the retort to be varied and optimized for best method of forming the oxide layer. The gas is monitored by the sensors measuring humidity H2O, oxygen O2and hydrogen H2content and gas quality and chemical composition is maintained by flowing humidified process gas and dry process gas into the recirculating flow as described above. As new gas enters the process by the control of the flow by the mass flow controllers MFC1 and MFC2, a similar amount is disposed automatically out of the system through valve V5 and the wash bottles WB to maintain constant mass-balance in the system through the gas exit pipe. The exit flow pipe features also a check vave CV2 to ensure that the flow is one-directional. The MFC1 controls the inlet gas flow rate that is humidified in humidifier HT. The flow through the humidifier is also controlled by the shut-off valves V3 and V4 that are normally closed. The MFC2 controls the dry process gas into the process and this flow can be closed by valve V2. The flow direction of the gas from the MFC1 and MFC2 is controlled by a check valve CV1. The circulating pump P1 may also be used as a vacuum pump to facilitate during the initial purging phase, when oxygen is evacuated from the system prior to starting the oxidation phase, by lowering the pressure and disposing the gas out through the valve V8 in the system followed by injecting pure argon gas by opening the valve V1. This is repeated until the required oxygen level in the system is achieved. In the main circuit of the flow there are 2 valves V6 and V7 fitted either side of the pump P1 to facilitate the purging of the system and filling the system with Argon gas. The absolute pressure is measured on either side of the pump P1 and temperature is measured either side of the heat exchanger HX2. The disclosed embodiments include a method for forming at least one oxide layer at the surface of a metallic alloy. Various metallic alloys can be coated using the disclosed methods. In some cases, the metallic alloy comprises a base metal comprising iron, nickel, and / or cobalt. The metal alloy further includes aluminum and / or chromium. In embodiments in which the base metal does not comprise iron, the metallic alloy may nevertheless also comprise iron, for example, as a minor component of the alloy. In the disclosed coating methods, oxide films are grown at exposed surfaces of the metallic alloy by oxidizing aluminum and / or chromium included in the metallic alloy. The at least one oxide layer therefore comprises a chromia (Cr₂O₃) layer and / or an alumina (Al₂O₃) layer, and may additionally comprise one or more other oxide layers, as described herein. As used herein, “at a surface” or “on a surface,” etc. is not to be understood as precluding the presence of intervening layers. For example, the chromia / alumina layer may be formed directly on the surface of the substrate alloy, or one or more additional layers, such as one or more additional oxide layers, may be interposed between the chromia / alumina layer and the substrate alloy. At a high level, the disclosed coating method includes introducing a metallic alloy (e.g., an object or item, such as a component for hydrogen containment, made from the metallic alloy) into the heating chamber of the furnace and, subsequently, performing a heat treatment process to form the at least one oxide layer on or at the surface of the metallic alloy. The heat treatment process includes a temperature ramp-up phase during which the temperature of the heating chamber is increased to a treatment temperature. The heat treatment process also includes a heat treatment phase during which the temperature of the heating chamber is maintained at the treatment temperature for a treatment time to form at least one oxide layer. During both the temperature ramp-up phase and the heat treatment phase, the atmosphere within the furnace comprises at least one non-oxidizing gas, hydrogen, and water vapor. During the temperature ramp-up phase (or at least until the temperature of the heating chamber reaches 600°C), the ratio by volume (i.e., the volumetric ratio) of hydrogen to water vapor in the atmosphere within the furnace is at least 5, optionally at least 10. During the heat treatment phase (or when the heating chamber reaches at least above 600°C), the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 0.5. The metallic alloy onto which the disclosed coating is formed may include various types of materials. In some examples, the metallic alloy includes an austenitic alloy. The metallic alloy may also include an iron-based alloy. For example, the metallic alloy may include a stainless steel, particularly an American Iron and Steel Institute (AISI) grade 316 stainless steel (referred to herein as stainless-steel type 316 alloy), more particularly 316L stainless-steel (i.e., stainless steel type 316L alloy), or 304 (i.e., stainless steel type 304 alloy), or 441 ferritic stainless-steel (i.e., ferritic stainless-steel type 441 alloy). The metallic alloy may also be a nickel-based alloy, such as an Inconel® alloy, particularly Inconel® 625 (UNS N06625). Other suitable Inconel® alloys include Inconel® 600 and Inconel® 718, for example. The metallic alloy may comprise chromium in an amount of at least 10% by weight. For example, the metallic alloy may comprise chromium in an amount in the range of 10 to 30% by weight, more particularly 16 to 26% by weight. The metallic alloy may comprise aluminum in an amount of at least 1.5% by weight. For example, the metallic alloy may comprise aluminum in an amount in the range of 3 to 10% by weight, more particularly 4 to 6% by weight. There is no particular limitation on the shape of the alloys that may be treated in accordance with the methods disclosed herein. As explained above, traditional coating techniques, such as thermal spraying or physical vapor deposition, rely on line-of-sight access to the component surface, as most of the coating materials are applied externally, making it difficult to achieve uniform coverage on parts with complex geometries, internal surfaces, or high aspect-ratio features. In contrast, the methods disclosed herein primarily utilize elements already present within the metallic material to produce a protective layer eliminating the need for line-of-sight access to the alloy surface. This means that the methods may be applied to both simple shapes such as tubes and plates and complex geometries like machine components, additively manufactured parts and the like. After providing the metallic material to be coated into the heating chamber of the furnace, the heating chamber may be purged. For example, in some cases, the heating chamber is purged by flowing a purging gas through the heating chamber prior to the heat treatment process to reduce the water content of the atmosphere within the furnace to less than 0.3 vol.%. The purging may reduce the water content of the atmosphere within the furnace sufficiently to permit the ratio by volume of hydrogen to water vapor in the atmosphere within the heating chamber to be maintained at 5 or greater during the temperature ramp-up phase, for example, when the atmosphere within the heating chamber comprises 10 vol.% or less of hydrogen, for example, 2 to 10 vol.% of hydrogen, or about 5% vol. % of hydrogen, during the ramp-up phase. Such purging may also assist in reducing the free oxygen content of the atmosphere within the heating chamber of the furnace to less than 500 ppm by volume, for example, less than 100 ppm by volume, or even less than 50 ppm by volume. In some examples, the purging may also be accomplished, at least in part, using one or more vacuum pumps in flow communication with the heating chamber. For example, the purging may comprise forming a vacuum or partial vacuum within the heating chamber to effect desorption of oxygen and / or water vapor from the walls of the heating chamber and / or surfaces of the metallic alloy. In other words, the purging may comprise reducing the pressure within the heating chamber below atmospheric pressure using at least one vacuum pump. In some cases, purging of the heating chamber reduces the water content of the atmosphere within the heating chamber of the furnace to less than 0.3 vol.%, optionally less than 0.2 vol.%, particularly when the atmosphere within the heating chamber comprises 10 vol.% or less of hydrogen, for example, 2 to 10 vol.% of hydrogen, or about 5 vol. % of hydrogen, during the ramp-up phase. The purging may reduce the water content of the atmosphere within the furnace sufficiently to permit the ratio by volume of hydrogen to water vapor in the atmosphere within the heating chamber to be maintained at 5 or greater during the temperature ramp-up phase, for example, when the atmosphere within the heating chamber comprises 10 vol.% or less of hydrogen, for example, 2 to 10 vol.% of hydrogen, or about 5% vol.% of hydrogen, during the ramp-up phase. The purging also reduces the free oxygen content of the atmosphere within the heating chamber of the furnace to less than 50 ppm. In some examples, the amount by volume of the purging gas supplied to the heating chamber during the purging of the heating chamber is at least 5 times the volume of the heating chamber, more particularly at least about 10 times the volume of the heating chamber. In some cases, the purging gas is a non-oxidizing gas. In some cases, the purging gas comprises or consists of one or more inert gases, such as argon. The purging gas may comprise hydrogen, optionally in combination with one or more inert gases. Thus, in some embodiments, the purging gas may comprise hydrogen and argon. In other embodiments, the purging gas may consist of hydrogen and argon. In addition to (or as an alternative to) purging of the heating chamber prior to initiating coating of the metallic material, the temperature of the heating chamber may be raised above ambient temperature prior to the temperature ramp-up phase. Prior to the temperature ramp-up phase, the heating chamber of the furnace may be raised and then maintained at a temperature of at least 100°C for at least 10 minutes, optionally at least 30 minutes. In other cases, prior to the temperature ramp-up phase, the heating chamber of the furnace is raised and maintained at a temperature of at least 150°C for at least 10 minutes, optionally at least 30 minutes. In some examples, prior to the temperature ramp-up phase and for a time duration after the heating chamber of the furnace reaches at least 100°C or at least 150°C, the internal environment of the heating chamber is exposed to a vacuum pump, as described above. Such pre-heating of the heating chamber can assist in removal of water and / or other substances adhered to the walls of the heating chamber. During the temperature ramp-up phase, various aspects of the heating chamber environment are controlled. For example, during the temperature ramp-up phase, the ratio by volume of hydrogen to water vapor in the atmosphere within the heating chamber of the furnace is controlled to be at least 5, optionally at least 10. During the temperature ramp-up phase, the amount of water vapor in the atmosphere within the heating chamber of the furnace may be controlled to be less than 0.3 vol.%. In other cases, during the temperature ramp-up phase, the amount of water vapor in the atmosphere within the heating chamber of the furnace is controlled to be less than 0.2 vol.%. As described herein, such control of the hydrogen-to-water vapor ratio and / or the absolute amount of water vapor mitigates against the formation of undesirable oxides, such as iron oxides, during the ramp-up phase, resulting in higher- quality protective metal oxide coatings. During the temperature ramp-up phase, the heating chamber of the furnace may be heated at a rate of at least 5°C per minute. As described herein, using heating rates of at least 5°C per minute allows the lower temperature ranges in which iron oxide may be formed to be passed relatively quickly, mitigating against iron oxide formation. In some cases, during the temperature ramp-up phase, the heating chamber of the furnace is heated at a rate of less than 20°C per minute. In some examples, during the temperature ramp-up phase, the heating chamber of the furnace is heated at a rate of about 10°C per minute (e.g., 8 to 10°C per minute). In some cases, during both the temperature ramp-up phase and during the subsequent heat treatment phase, the amount of hydrogen in the atmosphere within the furnace is controlled to be in the range of 2 to 10 vol.%, for example, about 5 vol.%. During both the temperature ramp-up phase and during the heat treatment phase, the atmosphere within the furnace may include oxygen in an amount of less than 500 ppm by volume, optionally less than 100 ppm by volume, or even less than 50 ppm by volume. Various aspects of the heat treatment phase of the heat treatment process are also controlled. For example, during the heat treatment phase, the amount of water vapor in the atmosphere within the heating chamber of the furnace may be at least 0.05 vol.%, optionally at least 0.1 vol.%, more particularly at least 0.3 vol.%. During the heat treatment phase, the amount of water vapor in the atmosphere within the heating chamber of the furnace may be less than or equal to 1.0 vol.%, optionally less than or equal to 0.8 vol.%. For example, during the heat treatment phase, the amount of water vapor in the atmosphere within the heating chamber of the furnace may be within the range of 0.3 to 0.6 vol.%. Regarding the heat treatment temperature, during the heat treatment phase, the treatment temperature within the heating chamber (e.g., at the surface of the metallic substrate, within the internal environment of the heating chamber, etc.) is within the range of 800 to 1050°C. In some cases, the treatment temperature within the heating chamber is within the range of 850 to 1000°C. In other examples, the treatment temperature within the heating chamber is within the range of 900 to 950°C. During the heat treatment phase, the temperature within the heating chamber need not be constant and may vary within the stated ranges. As previously noted, as a result of the disclosed coating process, at least one oxide layer is formed on the surface of the metallic alloy. In some examples, the at least one oxide layer comprises a chromia layer and / or an alumina layer. Notably, the oxide layer can grow on any exposed surface of the metallic substrate, including interior surfaces within voids, cracks, etc. penetrating the metallic substrate. For example, the at least one oxide layer grows into the metallic substrate as oxygen diffuses into metallic substrate to react with aluminum and / or chromium present in the base metal of the metallic substrate. Such an inward-growth process may offer several benefits. For example, all exposed surfaces (even within small surface defects) can be coated, which translates to strong adherence of the coatings to the metallic substrate. Further, the disclosed coatings offer high resistance to permeation, such as from hydrogen and other compounds. In addition to controlling the processing temperature during the heat treatment process, the environment of the heating chamber is also controlled. For example, during the heat treatment process, a plurality of gases can be supplied to the heating chamber, the plurality of gases comprising at least one non- oxidizing gas (e.g., at least one inert gas such as argon) and hydrogen. Once the temperature of the heating chamber has reached a predetermined temperature (e.g., at least 600°C, or the treatment temperature), the plurality of gases supplied to the heating chamber may further include water vapor. The water vapor may be supplied to the heating chamber by humidifying (using at least one humidifier) at least one of the at least one non-oxidizing gas or the hydrogen supplied to the heating chamber. For example, if the at least one non-oxidizing gas and the hydrogen are supplied to the heating chamber as a gas mixture, the gas mixture may be humidified. In some examples, the at least one humidifier may be used to humidify the at least one of the inert gas or the hydrogen supplied to the heating chamber once the temperature of the heating chamber has reached a predetermined temperature (e.g., at least 600°C, or the treatment temperature). Alternatively, the water vapor may be supplied to the heating chamber separately to the at least one non-oxidizing gas and the hydrogen, for example, by a humidifier directly coupled to the heating chamber. Further, during the heat treatment process, in particular during the temperature ramp-up phase, the plurality of gases may be supplied to the heating chamber in a combined amount by volume of at least three times the volume of the heating chamber per hour, optionally at least five times the volume of the heating chamber per hour. In some examples, during the heat treatment process, in particular during the temperature ramp-up phase, the plurality of gases are supplied to the heating chamber in a combined amount by volume per hour of no greater than 15 times the volume of the heating chamber per hour, optionally no greater than 10 times the volume of the heating chamber per hour. In some embodiments, during the heat treatment phase, the heating chamber (e.g., retort) is sealed such that no gases are input into the heating chamber (and optionally no gases are output from the heating chamber) during the heat treatment phase. While not exhaustive, the following provides a few examples of metallic materials that can be coating using the disclosed techniques along with example processing parameters. In one example, the base metal comprises iron, and the treatment temperature does not exceed 1000°C, particularly when the metal oxide layer formed is a chromia layer. In some cases, the base metal comprises nickel and / or cobalt, and the treatment temperature is at least 850°C, particularly when the metal oxide layer formed is a chromia layer. To encourage dense and high-quality film growth at exposed surfaces of the metallic material (e.g., surfaces of the metallic alloy exposed to the atmosphere within the heating chamber during the heat treatment process to form at least one oxide layer at the exposed surfaces), the treatment time may be at least 3 hours, optionally at least 6 hours. In some examples, the base metallic alloy onto which the disclosed coatings are formed comprises at least one metal, M, capable of achieving a +2 oxidation state. For example, the metallic alloy may comprise the at least one metal, M, in an amount of at least 0.1% by weight, optionally at least 0.2% by weight. The metallic alloy may comprise the at least one metal, M, in an amount of no more than 2.5% by weight, optionally no more than 2.0% by weight, for example, when the at least one metal is manganese. In such cases, the at least one oxide layer formed on the base metallic alloy comprises a mixed-oxide layer comprising aluminum and / or chromium together with the at least one metal, M. In some examples, the at least one metal, M, comprises manganese. The mixed-oxide layer may be formed at a top surface of a chromia layer or an alumina layer included in the at least one oxide layer. In some examples, the mixed-oxide layer has a spinel structure. Further, in some cases, the chromia layer and / or the alumina layer has a thickness of at least 500 nm, optionally at least 1 µm, further optionally at least 1.5 µm, for example, as measured using scanning electron microscopy (SEM) image analysis. The chromia layer and / or the alumina layer may have a porosity (pore volume) of less than 2% by volume, for example, as measured using scanning electron microscopy (SEM) image analysis. In examples including a chromia layer in the at least one oxide layer, the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.58, optionally less than 1.53, for example, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis. In examples including an alumina layer in the at least one oxide layer, the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.58, optionally less than 1.53, for example, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis. In additional examples, the mixed-oxide layer comprises MnCr2O4or MnAl2O4. The disclosed systems and methods may be used to form a coated metallic alloy having at least one surface oxide layer. In some examples, the coated metallic alloy includes a metallic alloy comprising a base metal comprising iron, nickel, and / or cobalt together with aluminum and / or chromium. The coated metallic alloy further includes at least one oxide layer on a surface of the metallic alloy, wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer. The chromia layer and / or the alumina layer may have a porosity of less than 4% by volume, more particularly less than 2% by volume, for example, as measured using scanning electron microscopy (SEM) micrograph / image analysis. In particular, the porosity may be measured based on analysis of at least one scanning electron microscopy (SEM) micrograph of a cross-section of the layer. The SEM images used to calculate the porosity values are preferably obtained at a resolution of at least 2,000 times to ensure accurate results. The chromia layer and / or the alumina layer may have a thickness of at least 500 µm, optionally at least 1 µm, further optionally at least 1.5 µm, for example, as measured using scanning electron microscopy (SEM) image analysis. In some cases, the chromia layer has a stoichiometric (atomic or molar) oxygen- to-chromium ratio of less than 1.58, optionally less than 1.53, for example, as measured using energy- dispersive X-ray spectroscopy (EDXS) analysis. In some cases, the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.58, optionally less than 1.53, for example, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis. In general, the coated metallic alloy and the resulting metal oxide coating is as described previously with respect to the related methods. In some cases, the metallic alloy comprises at least one metal, M, capable of achieving a +2 oxidation state, and the at least one surface oxide layer comprises a mixed-oxide layer comprising aluminum and / or chromium together with the at least one metal, M. The mixed-oxide layer may be disposed on top of, or at a top surface of, a chromia layer or an alumina layer included in the at least one surface oxide layer. The mixed-oxide layer may have a spinel structure. The at least one metal, M, may comprise manganese. In other examples, the at least one metal, M, may comprise manganese, magnesium, nickel, and / or iron. The mixed-oxide layer may comprise MnCr2O4 or MnAl2O4. The compressive stresses within the formed chromia and / or alumina layer may exceed 500 MPa, as measured by X-ray diffraction residual stress measurements. The compressive stresses of the formed chromia and / or alumina layers may be at least 30% greater than the compressive stresses within an equivalent chromia and / or alumina layer formed in an atmosphere of ambient air but otherwise formed under the same conditions (e.g., same temperature, heat treatment duration, substrate, etc.). The disclosed coatings and associated coated metallic materials may be useful in a variety of applications. Because of a high density and high degree of surface coverage offered by the disclosed coatings, the coated metallic materials may be especially useful in hydrogen containment or transport applications. For example, the coating can be effectively employed in hydrogen pipeline networks, hydrogen storage tanks, hydrogen fuel cell vehicles, and various high-temperature processes aimed at producing hydrogen. The approach disclosed herein eliminates the need for a direct line of sight to surfaces undergoing oxidation, making it well-suited for the coating of intricate geometries. In particular, the base metal may comprise nickel and / or iron, and the formed oxide layer may be a chromium layer formed from chromium contained in the metal alloy. Throughout the disclosure, where chromium or chromia are referred to, it should be understood that the teachings similarly apply to aluminum and alumina, respectively. Definitions Unless otherwise defined herein, scientific and technical terms used in connection with the present invention shall have the meanings that are commonly understood by those of ordinary skill in the art. Further, unless otherwise required by context, singular terms shall include pluralities and plural terms shall include the singular. The terms “comprising,” “having,” “including,” and “containing,” are to be construed as open-ended terms unless otherwise noted. If aspects of the invention are described as “comprising” a feature, embodiments also are contemplated “consisting of” or “consisting essentially of” the feature. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illustrate the disclosure and does not pose a limitation on the scope of the disclosure unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosure. Other than in the operating examples, or where otherwise indicated, all numbers expressing quantities of ingredients or reaction conditions used herein should be understood as modified in all instances by the term “about” as that term would be interpreted by the person skilled in the relevant art. The term “about” as used herein is equivalent to ± 10% of a given numerical value, unless otherwise stated. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range and each endpoint, unless otherwise indicated herein, and each separate value and endpoint is incorporated into the specification as if it were individually recited herein. For the avoidance of doubt, as used herein “room temperature” refers to a temperature between about 18 and 25°C. As used herein, the term “substantially resistant to hydrogen permeation” means that the hydrogen permeation is so low that is below the detection limit of the available apparatus. As used herein, the term “substantially resistant to corrosion” means that no visual signs of corrosion could be observed using scanning electron microscopy. For example, no evidence of pitting, crevice corrosion, or surface breakdown was observable. In the following alloy definitions, all percentages are by weight (i.e., wt.%): Inconel® 600: Cr: 17.0 to 21.0%, Ni: 50.00 to 55.00%, Mo: 2.80 to 3.30%, Nb+Ta: 4.75 to 5.50%, Ti: 0.65 to 1.15%, Al: 0.20 to 0.80%, C: 0.08% or less, Mn: 0.35% or less, Si: 0.35% or less, P: 0.015% or less, S: 0.015% or less, B: 0.006 or less, Cu: 0.30% or less, Co: 1.0% or less, Fe: balance. Inconel® 625: Cr: 20.0 to 23.0%, Fe: 5.0% or less, Mo: 8.0 to 10.0%, Nb+Ta: 3.15 to 4.15%, C: 0.10% or less, Mn: 0.50% or less, Si: 0.50% or less, P: 0.015% or less, S: 0.015% or less, Al: 0.40% or less, Ti: 0.40% or less, Co: 1.0% or less, Ni: balance. Inconel® 718: Cr: 17.0 to 21.0%, Ni: 50.00 to 55.00%, Mo: 2.80 to 3.30%, Nb+Ta: 4.75 to 5.50%, Ti: 0.65 to 1.15%, Al: 0.20 to 0.80%, Co: 1.00% or less, C: 0.08% or less, Mn: 0.35% or less, Si: 0.35% or less, P: 0.015% or less, S: 0.015% or less, B: 0.006% or less, Cu: 0.30% or less, Fe: balance. Stainless steel type 316 alloy: Cr: 16.0 to 18.0, Ni: 10.0 to 14.0%, C: 0.08% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.015 to 0.03, Si: 0.75% or less, Mo: 2.0 to 3.0%, Ni: 0.1% or less, Fe: balance. Stainless steel type 316L alloy: Cr 16.0 to 18.0, Ni: 10.0 to 14.0%, C: 0.03% or less, Mn: 2.0% or less, P: 0.04% or less, S: 0.015 to 0.03, Si: 0.75% or less, Mo: 2.0 to 3.0%, Ni: 0.1% or less, Fe: balance. FeCrAl alloy composition 1: Cr: 20.0-24.0%, Al: 3.0 to 5.0%, Mn: 0.6% or less, Si: 1.0% or less, Fe: balance. FeCrAl alloy composition 2: Cr: 20.0-24.0%, Al: 5.0 to 6.0%, Mn: 0.6% or less, Si: 1.0% or less, Fe: balance. Stainless steel type 304 alloy: Cr: 18.0 to 20.0, Ni: 8.0 to 12.0%, C: 0.08% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.030 or less, Si: 0.75% or less, N: 0.1% or less, Fe: balance. Stainless steel type 253MA alloy: Cr: 20.5 to 21.5, Ni: 10.5 to 11.5%, C: 0.09% or less, Mn: 0.8% or less, P: 0.040% or less, S: 0.030 or less, Si: 1.5 to 1.7%, N: 0.17% or less, Ce: 0.03 to 0.07%, Fe: balance. Ferritic stainless steel type 441 alloy: Cr: 17.5 to 18.5, C: 0.03% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.015 or less, Si: 1.00% or less, Ti: 0.1 to 0.6%, Nb: 3 × C + 0.3-1.00%, Fe: balance. Stainless steel type 430 alloy: Cr: 16.0 to 18.0, C: 0.12% or less, Mn: 1.0% or less, P: 0.04% or less, S: 0.030 or less, Si: 1.00% or less, Ni: 0.75 or less, Fe: balance. Stainless steel type 310 alloy: Cr: 24.0 to 26.0, Ni: 19.0 to 22.0%, C: 0.25% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.030 or less, Si: 1.5% or less, N: 0.11% or less, Fe: balance. Stainless steel type 254 SMO alloy: Cr: 19.5 to 20.5, Ni: 17.5 to 18.5%, C: 0.02% or less, Mn: 1.0% or less, P: 0.03% or less, S: 0.01 or less, Si: 0.70% or less, Mo: 6.0 to 7.0%, N: 0.18 to 0.25%, Cu: 0.50 to 1.00%, Fe: balance. Embodiments 1. A method for forming at least one oxide layer at a surface of a metallic alloy, the metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium; wherein the method comprises: introducing the metallic alloy into a heating chamber of a furnace; and performing a heat treatment process to form the at least one oxide layer on the surface of the metallic alloy; wherein the heat treatment process comprises a temperature ramp-up phase during which the temperature of the heating chamber is increased to a treatment temperature, and a heat treatment phase during which the temperature of the heating chamber is maintained at the treatment temperature for a treatment time to form the at least one oxide layer; wherein, during the temperature ramp-up phase and the heat treatment phase, the atmosphere within the furnace comprises at least one non-oxidizing gas, hydrogen, and water vapor; wherein, during the temperature ramp-up phase, the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 5, and during the heat treatment phase, the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 0.5; and wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer. 2. The method of embodiment 1, wherein the method comprises purging the heating chamber by flowing a purging gas through the heating chamber prior to the heat treatment process. 3. The method of embodiment 2, wherein the purging reduces the water content of the atmosphere within the furnace to less than 0.3 vol.% and / or reduces the free oxygen content of the atmosphere within the furnace to less than 500 ppm by volume. 4. The method of embodiment 2 or embodiment 3, wherein the purging reduces the water content of the atmosphere within the furnace sufficiently to permit the ratio by volume of hydrogen to water vapor in the atmosphere within the heating chamber to be maintained at 5 or greater during the temperature ramp-up phase. 5. The method of any of embodiments 2 to 4, wherein purging the heating chamber reduces the water content of the atmosphere within the furnace to less than 0.2 vol.%. 6. The method of any of embodiments 2 to 5, wherein purging the heating chamber reduces the free oxygen content of the atmosphere within the furnace to less than 100 ppm by volume, optionally less than 50 ppm by volume. 7. The method of any of embodiments 2 to 6, wherein the amount by volume of the purging gas supplied to the heating chamber during the purging of the heating chamber is at least 5 times the volume of the heating chamber. 8. The method of any of embodiments 2 to 7, wherein the purging gas is a non-oxidizing gas. 9. The method of any of embodiments 2 to 8, wherein the purging gas is an inert gas, optionally wherein the purging gas is argon. 10. The method of any preceding embodiment, wherein, during the temperature ramp-up phase, the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 10. 11. The method of any preceding embodiment, wherein, during the temperature ramp-up phase, the amount of water vapor in the atmosphere within the furnace is less than 0.3 vol.%. 12. The method of any preceding embodiment, wherein, during the temperature ramp-up phase, the amount of water vapor in the atmosphere within the furnace is less than 0.2 vol.%. 13. The method of any preceding embodiment, wherein, during the heat treatment phase, the amount of water vapor in the atmosphere within the furnace is at least 0.05 vol.%, optionally at least 0.1 vol.%. 14. The method of any preceding embodiment, wherein, during the heat treatment phase, the amount of water vapor in the atmosphere within the furnace is less than or equal to 1.0 vol. %, optionally less than or equal to 0.8 vol.%. 15. The method of any preceding embodiment, wherein, during the temperature ramp-up phase and during the heat treatment phase, the amount of hydrogen in the atmosphere within the furnace is in the range of 2 to 10 vol.%. 16. The method of any preceding embodiment, wherein the method comprises, during the heat treatment process, supplying a plurality of gases to the heating chamber, the plurality of gases comprising at least one non-oxidizing gas and hydrogen, and optionally water vapor and / or oxygen. 17. The method of embodiment 16, wherein, only once the temperature of the heating chamber has reached at least 600°C, the plurality of gases supplied to the heating chamber further comprises water vapor. 18. The method of embodiment 16, wherein, only once the temperature of the heating chamber has reached the treatment temperature, the plurality of gases supplied to the heating chamber further comprises water vapor. 19. The method of embodiment, wherein 17 or embodiment 18, wherein the water vapor is supplied to the heating chamber by humidifying at least one of the at least one non-oxidizing gas and the hydrogen supplied to the heating chamber using at least one humidifier, or wherein the water vapor is supplied to the heating chamber separately to the at least one non-oxidizing gas and the hydrogen. 20. The method of embodiment 19, wherein the at least one humidifier is used to humidify the at least one of the inert gas and the hydrogen supplied to the heating chamber only once the temperature of the heating chamber has reached at least 600°C. 21. The method of embodiment 19, wherein the at least one humidifier is used to humidify the at least one of the inert gas and the hydrogen supplied to the heating chamber only once the temperature of the heating chamber has reached the treatment temperature. 22. The method of any of embodiments 16 to 21, wherein, during the heat treatment process, the plurality of gases are supplied to the heating chamber in a combined amount by volume of at least three times the volume of the heating chamber per hour, optionally at least five times the volume of the heating chamber per hour. 23. The method of any of embodiments 16 to 22, wherein, during the heat treatment process, the plurality of gases are supplied to the heating chamber in a combined amount by volume per hour of no greater than 15 times the volume of the heating chamber per hour, optionally no greater than 10 times the volume of the heating chamber per hour. 24. The method of any of embodiments 1 to 21, wherein the heating chamber is sealed such that no gases are input into the heating chamber during the heat treatment phase. 25. The method of any preceding embodiment, wherein, prior to the temperature ramp-up phase, the heating chamber of the furnace is maintained at a temperature of at least 100°C for at least 10 minutes, optionally at least 30 minutes. 26. The method of embodiment 25, wherein, prior to the temperature ramp-up phase, the heating chamber of the furnace is maintained at a temperature of at least 150°C for at least 10 minutes, optionally at least 30 minutes. 27. The method of embodiment 25, wherein, prior to the temperature ramp-up phase and for a time duration after the heating chamber of the furnace reaches 100°C, an environment of the heating chamber is exposed to a vacuum pump. 28. The method of any preceding embodiment, wherein, during the temperature ramp-up phase, the heating chamber of the furnace is heated at a rate of at least 5°C per minute. 29. The method of any preceding embodiment, wherein, during the temperature ramp-up phase, the heating chamber of the furnace is heated at a rate of less than 20°C per minute. 30. The method of any preceding embodiment, wherein, during the temperature ramp-up phase, the heating chamber of the furnace is heated at a rate of about 10°C per minute. 31. The method of any preceding embodiment, wherein, during the temperature ramp-up phase and during the heat treatment phase, the atmosphere within the furnace comprises oxygen in an amount of less than 500 ppm, optionally less than 100 ppm 32. The method of any preceding embodiment, wherein the treatment temperature is within the range of 800 to 1050°C. 33. The method of embodiment 32, wherein the treatment temperature is within the range of 850 to 1000°C. 34. The method of embodiment 33, wherein the treatment temperature is within the range of 900 to 950°C. 35. The method of embodiment 32, wherein the base metal comprises iron, and wherein the treatment temperature does not exceed 1000°C. 36. The method of embodiment 32, wherein the base metal comprises nickel and / or cobalt, and wherein the treatment temperature is at least 850°C. 37. The method of any preceding embodiment, wherein the treatment time is at least 3 hours, optionally wherein the treatment time is at least 6 hours. 38. The method of any preceding embodiment, wherein the surface of the metallic alloy at which the at least one oxide layer is formed is exposed to the atmosphere within the heating chamber during the heat treatment process. 39. The method of any preceding embodiment, wherein the metallic alloy comprises at least one metal, M, capable of achieving a +2 oxidation state, and wherein the at least one oxide layer comprises a mixed-oxide layer comprising the Al and / or Cr and the at least one metal, M. 40. The method of embodiment 39, wherein the mixed-oxide layer is formed at a top surface of the chromia layer or the alumina layer. 41. The method of embodiment 39 or embodiment 40, wherein the mixed-oxide layer has a spinel structure. 42. The method of any of embodiments 39 to 41, wherein the at least one metal, M, comprises manganese. 43. The method of any of embodiments 39 to 42, wherein the mixed-oxide layer comprises MnCr2O4or MnAl2O4. 44. The method of any preceding embodiment, wherein the chromia layer and / or the alumina layer has a thickness of at least 1 µm, optionally at least 1.5 µm, for example as measured using scanning electron microscopy (SEM) image analysis. 45. The method of any preceding embodiment, wherein the chromia layer and / or the alumina layer has a porosity (pore volume) of less than 2% by volume, for example as measured using scanning electron microscopy (SEM) image analysis. 46. The method of any preceding embodiment, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.58, optionally less than 1.53, for example as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis. 47. The method of any preceding embodiment, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.58, optionally less than 1.53, for example as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis. 48. The method of any preceding embodiment, wherein the metallic alloy comprises an austenitic alloy. 49. The method of any preceding embodiment, wherein the metallic alloy is an iron-based alloy. 50. The method of any preceding embodiment, wherein the metallic alloy is a stainless steel, particularly an American Iron and Steel Institute (AISI) grade 316 stainless steel, more particularly 316L stainless steel. 51. The method of any preceding embodiment, wherein the metallic alloy is a nickel-based alloy. 52. The method of any preceding embodiment, wherein the metallic alloy is an Inconel alloy, particularly Inconel 625 (UNS N06625). 53. A coated metallic alloy having at least one surface oxide layer produced by the method of any preceding embodiment. 54. A coated metallic alloy comprising: a metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium; at least one oxide layer on a surface of the metallic alloy, wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer; wherein the chromia layer and / or the alumina layer has a porosity of less than 2% by volume, for example as measured using scanning electron microscopy (SEM) image analysis. 55. The coated metallic alloy of embodiment 54, wherein the chromia layer and / or the alumina layer has a thickness of at least 1 µm, optionally at least 1.5 µm. 56. The coated metallic alloy of embodiment 54 or embodiment 55, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.58, optionally less than 1.53, for example as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis. 57. The coated metallic alloy of embodiment 54 or embodiment 55, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.58, optionally less than 1.53, for example as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis. 58. The coated metallic alloy of any of embodiments 54 to 57, wherein the metallic alloy comprises at least one metal, M, capable of achieving a +2 oxidation state, and wherein the at least one oxide layer comprises a mixed-oxide layer comprising the Al and / or Cr and the at least one metal, M. 59. The coated metallic alloy of embodiment 58, wherein the mixed-oxide layer is disposed on top of the chromia layer or the alumina layer. 60. The coated metallic alloy of embodiment 58 or embodiment 59, wherein the mixed-oxide layer has a spinel structure. 61. The coated metallic alloy of any of embodiments 58 to 60, wherein the at least one metal, M, comprises Manganese. 62. The coated metallic alloy of any of embodiments 58 to 61, wherein the mixed-oxide layer comprises MnCr2O4 or MnAl2O4. 63. The coated metallic alloy of any of embodiments 54 to 62, wherein the compressive stress within the formed chromia and / or alumina layer exceed 500 MPa, for example as measured by X-ray diffraction residual stress measurements. 64. The coated metallic alloy of any of embodiments 54 to 63, wherein the chromia and / or the alumina layer is inward-grown. 65. Use of a coated metallic alloy according to any one of embodiments 54 to 64 in the manufacture of a component for use in hydrogen containment or transport. 66. Use of a component comprising a coated metallic alloy according to any one of embodiments 54 to 64 for hydrogen containment or transport. 67. A component for hydrogen containment or transport comprising a coated metallic alloy of any of embodiments 54 to 64. 68. An apparatus for forming at least one oxide layer at a surface of a metallic alloy, the apparatus comprising: a furnace comprising a heating chamber; a gas supply system in flow communication with the heating chamber via at least one inlet of the heating chamber; and a gas monitoring system in flow communication with the heating chamber via at least one outlet of the heating chamber and configured to monitor the atmosphere within the heating chamber; wherein the gas monitoring system comprises a humidity sensor configured to measure the humidity of gas from the heating chamber. 69. The apparatus of embodiment 68, wherein the gas monitoring system comprises an oxygen sensor configured to measure the oxygen content of gas output from the heating chamber. 70. The apparatus of embodiment 69, wherein the oxygen sensor is configured to measure the free oxygen content of the gas output from the heating chamber and the bound oxygen content of the gas output from the heating chamber. 71. The apparatus of embodiment 70, wherein the humidity sensor is the oxygen sensor. 72. The apparatus of any of embodiments 68 to 71, wherein the gas monitoring system comprises a hydrogen sensor configured to measure the hydrogen content of gas output from the heating chamber. 73. The apparatus of any of embodiments 68 to 72, wherein the heating chamber is a retort. 74. The apparatus of any of embodiments 68 to 73, wherein the heating chamber is configured to operate at a positive pressure relative to atmospheric pressure. 75. The apparatus of any of embodiments 68 to 74, wherein the gas supply system is configured to provide a non-oxidizing gas and hydrogen, and at least one of water vapor and / or oxygen, to the heating chamber. 76. The apparatus of any of embodiments 68 to 75, wherein the gas supply system comprises a humidifier in flow communication with the heating chamber and configured to supply water vapor to the heating chamber. 77. The apparatus of any of embodiments 68 to 76, wherein the gas supply system comprises gas supply tubing for supplying at least one of a non-oxidizing gas and hydrogen to the heating chamber and selectively humidifying the non-oxidizing gas and / or hydrogen. 78. The apparatus of embodiment 77, wherein the gas supply system comprises a humidifier, and wherein the gas supply tubing for supplying the non-oxidizing gas and / or hydrogen to the heating chamber defines a first gas supply line that passes through the humidifier and a second gas supply line that does not pass through the humidifier, wherein the gas supply system is controllable to supply the non-oxidizing gas and / or hydrogen to the heating chamber via either the first or second gas supply lines. Examples In the following Examples, an “FEI Quanta 200 Field Emission Gun (FEG) scanning electron microscope” was used to perform imaging (scanning electron microscopy, SEM). EDXS analysis was carried out using an Oxford Inca EDXS with a silicon drift detector (SDD). Example 1 Figure 2 shows SEM cross-sectional images of a coated Inconel® 625 produced in accordance with the method disclosed in Table 2. Sample dimensions: 40 x 50 x 1.27 mm. After the ramp-up process, the Inconel® 625 sample was heat treated in a heating chamber of a furnace in an atmosphere comprising 4.98 vol.% H2, 0.3 vol.% H2O and 94.71 vol.% Ar (corresponding to a hydrogen-to-water volumetric ratio of 16.6) at 950°C for 24 hours. The temperature ramping rate was 6°C per minute with flow rate of 0.8 L / min during exposure. The heating chamber had a volume of 24 liters. As can be clearly seen from the SEM images shown in FIG. 2, a dense, uniform, and substantially defect-free chromia layer was formed at the surface of the Inconel® 625. FIG.2B is a higher resolution image than FIG. 2A.

[0002] Step Description Furnace Condition Criteria for Next Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Flushed with 95% Ar / 5% H₂ Proceed after 15 Ar / H₂ Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C is Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C is Up Ar / 5% H₂ at 2^L / min 950°C reached Purge with 95% Ar / 5% H₂ and Coating 0.3% H₂O vapor, flow rate Maintain 950°C for 24 Process 0.8^L / min hours Proceed after 24 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C is Ramp Down at 0.5^L / min Cool down to 230°C reached Stop — — — Table 2: Process steps performed in a 24 L retort Example 2 A stainless steel type 316L alloy sample was heat treated in a heating chamber of a furnace at 950°C for 12 hours. During the ramp-up phase, the water vapor content of the atmosphere within the heating chamber reached a maximum value of 0.27 vol.% (corresponding to a minimum volumetric hydrogen- to-water ratio of 18.52). During the heat treatment phase (i.e., at 950°C) the water vapor content of the atmosphere within the heating chamber was approx. 0.5 vol.% (corresponding to a volumetric hydrogen-to-water ratio in the range of 10 to 16.7). See Table 3 for experimental setup. Samples dimensions: 30 x 30 x 1 mm. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230^°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230^°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 1.2^L / min Temperature: 230^°C minutes Slow Ramp- Continued flushing with 95% Ramp at 300^°C / h to Proceed when 950^°C Up Ar / 5% H₂ at 1.2^L / min 950^°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.5% H₂O vapor, Flow Maintain 950^°C for Process rate 0.8^L / min 12 hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230^°C is reached Stop — — — Table 3: Process steps performed in a 24 L retort FIG.3 shows SEM cross-sectional images of the resulting coated stainless steel type 316L alloy. Based on the EDXS analysis of the SEM images, the formation of a chromia layer and a spinel layer (MnCr₂O₄) was confirmed, with the spinel layer located on top of the chromia layer. The data reveals the presence of chromia (Cr₂O₃) and Mn-rich spinel structures, indicative of the successful oxidation under the heat treatment conditions. For chromia, the ideal stoichiometric (atomic / molar) oxygen-to-chromium (O / Cr) ratio is 1.5. The EDXS data show an O / Cr ratio of 1.47, closely aligning with the theoretical value and confirming the formation of chromia with high purity. This slight deviation is within the expected range of measurement accuracy, affirming the effectiveness of the heat treatment protocol in promoting chromia formation. The spinel phase, identified as manganese rich (MnCr₂O₄), is characterized by a theoretical O / Cr ratio of 2. The EDXS data reveal an O / Cr ratio of 1.9 for the spinel layer, which, while slightly lower than the theoretical ratio, strongly supports the presence of the MnCr₂O₄ phase. This variance is again within the expected accuracy range for EDXS analysis, suggesting the successful formation of this spinel structure alongside chromia. Example 3 Figure 4 shows SEM cross-sectional images of a coated Inconel® 600, Inconel® 625 and Inconel® 718 samples produced in accordance with Table 2. Sample dimensions: Inconel® 625: 40 x 50 x 1.27 mm Inconel® 600: 40 x 50 x 1 mm Inconel® 718: 40 x 50 x 0.80 mm The Inconel® samples were heat treated in a heating chamber of a furnace in an atmosphere comprising 4.97 vol.% H2, 0.6 vol.% H2O and 94.43 vol.% Ar (corresponding to a hydrogen-to-water volumetric ratio of 8.3) at 950°C for 48 hours. As can be clearly seen from the SEM images shown in FIG. 4, a dense, uniform, and substantially defect-free chromia layer was formed at the surface of the Inconel® samples. Example 4 The effect of furnace purging prior to ramp-up was systematically investigated. Experimental parameters were controlled to isolate the influence of this variable, with all other ramp-up conditions, including heating rate, gas composition, and total gas flow, held constant. In a comparative study, an austenitic stainless-steel alloy (type 316) was exposed to a process atmosphere comprising an H₂ / H₂O ratio of approximately 16 (water content of 0.3 vol.%) during the treatment (barrier forming) the target temperature of 950°C for a duration of 12 hours. These conditions were held constant for both configurations described below. Two comparative configurations were evaluated to assess the influence of furnace purging on resulting oxide microstructure: ● In the first configuration, no purging: the components were inserted into the furnace, and the ramp-up process commenced immediately. ● In the second configuration, a purging step using an inert gas was conducted prior to ramp-up. All other ramp-up parameters, including heating rate, gas composition, and flow rate, were held constant between the two configurations. As shown in FIGs.5A and 5B, the impact of pre-ramp purging on humidity levels was substantial. FIG. 5A (without purging, see details in Table 4) demonstrates elevated humidity levels exceeding 2.0 vol.% during early ramp-up, resulting in non-uniform oxide formation, significant internal oxidation, and the presence of metallic Fe and Ni precipitates, as confirmed by EDX analysis (Fe: 83.36 at.%, Ni: 13.18 at.%, Cr: 1.86 at.%, Mo: 1.61 at.%). FIG. 5B (with purging, produced in accordance with Table 6) shows effective reduction of water vapor concentration (<0.5 vol.%) and formation of a dense, continuous, and high-integrity chromia-based barrier layer with no observable internal oxidation. These results indicate the effectiveness of the purging procedure in removing oxygen and water molecules from the furnace environment. The effect of these differing conditions on oxide scale formation was examined using scanning electron microscopy analysis coupled with energy-dispersive X-ray spectroscopy (SEM-EDXS). A cross- sectional micrograph of the sample processed without purging (FIG. 5a) revealed the presence of extensive internal oxidation and discontinuous oxide layers. The initial high humidity led to the formation of unstable iron- and nickel-rich oxides during early ramp-up. These oxides were later reduced during the treatment (barrier forming) phase, resulting in surface-level metallic Fe and Ni areas. Beneath this outer layer, Mn and Fe-rich spinels were observed, indicative of internal oxidation and chromium depletion. The chromium concentration within the oxidation-affected zone (OAZ) was found to drop below 2 at.%, significantly impairing the alloy’s ability to regenerate and maintain a protective Cr₂O₃ layer. By contrast, in the purged configuration (FIG. 5b), the resulting oxide layer was continuous, defect-free, dense, and uniform, with no signs of internal oxidation or existence of metallic surface areas which are extremely lean in Cr content (below 2 at.%). The low moisture levels during the early stages of heating allowed for the selective formation of a high-purity chromia layer. Thus, these findings demonstrate that the inclusion of a controlled furnace purging step prior to ramp-up is helps to maintain a clean atmospheric environment, substantially reducing residual water vapor and suppressing premature formation of non-protective oxides. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Proceed to Ar / H₂ Insertion retort Temperature: 230^°C Flushing Continued flushing with 95% Ramp at 600^°C / h to Proceed when 600^°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600^°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360^°C / h to Proceed when 950^°C Up Ar / 5% H₂ at 2^L / min 950^°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor, flow rate Maintain 950^°C for Process 0.8^L / min 12 hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230^°C is reached Stop — — — Table 4: Process steps first configuration performed in a 24 L retort; samples dimensions: 30 x 30 x 1 mm Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230^°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230^°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230^°C minutes Continued flushing with 95% Ramp at 600^°C / h to Proceed when 600^°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600^°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360^°C / h to Proceed when 950^°C Up Ar / 5% H₂ at 2^L / min 950^°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor, flow rate Maintain 950^°C for Process 0.8^L / min 12 hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230^°C is reached Stop — — — Table 5: Process steps second configuration performed in a 24 L retort Example 5 Control of H2 / H2O ratio. A Sample of stainless steel type 316 alloy having dimensions 30 x 30 x 1 mm was utilized. In Figure 6A, the H₂ / H₂O ratio during ramp-up was at 4.2. This is the result of an experiment with process steps according to Table 6, where water vapor was inserted in the process from the start. The resulting oxide scale exhibits significant internal oxidation and Fe-containing spinel phases, as confirmed by EDX analysis (O: 61.19 at.%, Cr: 25.30 at.%, Fe: 11.05 at.%, Mn: 2.10 at.%). The surface was dominated by Fe-containing spinels, with no continuous Cr₂O₃ layer detected. The disrupted and non-uniform oxide layer demonstrates the detrimental effect of an insufficiently controlled ramp-up environment. This morphology is characteristic of water vapor exposure exceeding the acceptable range relative to the hydrogen content in the gas environment defined by the enclosed method, which favors iron oxidation during early scale development. In contrast, Figure 6b shows the result of an experiment with process steps according to Table 3 with a H2 / H2O ratio of 18.5. The resulting oxide layer is continuous, defect-free, and free of internal oxidation. EDX analysis of the outer region indicates a Mn-rich spinel phase (EDX analysis from the area designated by box 2 in FIG.6b; O: 57.69 at.%, Cr: 29.93 at.%, Mn: 7.71 at.%, Fe: 3.96 at.%), while the inner layer shows Cr-rich chromia characteristics (EDX analysis from the area designated by box 3 in FIG.6b; O: 54.76 at.%, Cr: 37.22 at.%, Mn: 2.77 at.%, Fe: 3.30 at.%). Thus, a protective chromia layer was successfully formed, with a top and continuous layer mainly composed of spinel MnCr2O4, as identified through elemental analysis. These findings confirm the transition from iron-rich oxides to manganese-rich spinel phases as water vapor levels are suppressed and oxidation potential is more tightly controlled during ramp-up phase. Quantitative EDXS validates the phase composition. The O / Cr atomic ratio from the EDXS analysis from the area designated via box 3 was measured at 1.47, closely matching the theoretical stoichiometric value of 1.5 for Cr2O3, confirming the presence of high-purity chromia. The top-layer spinel, identified as MnCr2O4, exhibited an O / Cr ratio of 1.9, in close agreement with the ideal value of 2.0. Minor Fe content observed in the EDX analysis, from both box 2 and box 3 is attributed to substrate signal overlap due to the interaction volume of the electron beam, and not to actual iron incorporation into the protective scale. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230^°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230^°C level < 10^ppm Flushed with 95% Ar / 5% H₂ Ar / H₂ at 2^L / min, and 0.3% H₂O Proceed after 15 Flushing vapor Temperature: 230^°C minutes Continued flushing with 95% Ar / 5% H₂ at 2^L / min, and Ramp at 600^°C / h to Proceed when 600^°C Fast Ramp-Up 0.3% H₂O vapor 600^°C is reached Continued flushing with 95% Slow Ramp- Ar / 5% H₂ at 2^L / min, and Ramp at 360^°C / h to Proceed when 950^°C Up 0.3% H₂O vapor 950^°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 950^°C for Process 0.8^L / min 12 hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230^°C is reached Stop — — — Table 6: Process steps second configuration performed in a 24 L retort Example 6 Controlling heating and gas flow rates. In a comparative study, the effect of heating and gas flow rates during the ramp-up phase was evaluated. Figure 7A shows two representative configurations demonstrating the effect of varying heating and flow rates on the amount of water content during ramp- up in two distinct cases: ● In one experimental configuration, a lower heating rate (5°C / min) and lower flow rate (1.2 L / min carrier gas / in 24 L retort), see Table 3. ● In an alternative configuration, a higher heating rate (10°C / min) and higher flow rate (2 L / min carrier gas / in 24 L retort), see Table 7. The humidity profiles reveal a distinct divergence between the two conditions. In the first experimental configuration, humidity levels remain modest at lower temperatures but rise sharply beyond 500°C. The slower heating and lower flow condition results in substantial humidity accumulation above 600°C, increasing the risk of non-selective oxidation and interference in the treatment (barrier forming) phase. Without wishing to be bound by theory, this trend is attributed to the longer dwell time at intermediate temperatures, during which time moisture desorbs from furnace walls and sample surfaces without sufficient gas exchange to remove it efficiently. This results in elevated water vapor concentration precisely during the temperature window where oxide nucleation and growth begin, favoring the formation of unwanted Fe- and Ni-rich oxides and promoting internal oxidation of the substrate. In contrast, the second configuration where a higher heating rate and elevated gas flow were employed moisture retention was significantly minimized in the furnace atmosphere. The increased heating rate reduces residence time at intermediate temperatures, while the higher gas flow facilitates rapid displacement of desorbed water vapor, and thus suppressing humidity buildup. This combined approach is shown to maintain a higher hydrogen-to-water vapor (H₂ / H₂O) ratio during ramp-up Fig 7B show SEM cross section of stainless steel type 304 alloy exposure produced in accordance with Table 8. During this exposure heating rate was 6°C / min and gas flow rates was 0.7^L / min during ramp- up phase in 24^L retort, respectively, this caused the H₂ / H₂O ratio to dropped below 5 during ramp-up phase it resulted in the formation of metallic iron and Fe-rich spinels within the chromia layer. SEM- EDXS analysis (Figure 7B) confirmed the presence of these phases, consistent with the reduction (or redox reaction) of FeCr₂O₄: FeCr₂O₄(s) + H₂(g) → Cr₂O₃(s) + Fe(s) + H₂O(g) This reaction results in the generation of metallic iron-rich particles within the chromia matrix. The formation of metallic iron-rich particles within the chromia matrix disrupts the uniformity of the protective oxide layer. These particles act as stress concentrators and fast diffusion paths, leading to crack initiation and reduced barrier effectiveness. The presence of such metallic inclusions disrupts the scale’s uniformity and reduces its long-term protective capability, as Fe is far less oxidation-resistant than Cr₂O₃ and can act as initiation points for further degradation in service. The resulting oxide scale exhibits the presence of iron-rich metallic inclusions formed by the reduction of initially formed iron oxide species. These features indicate excessive moisture retention and uncontrolled oxidation during ramp-up, leading to impaired performance of the final barrier layer. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230^°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230^°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230^°C minutes Slow Ramp- Continued flushing with 95% Ramp at 600^°C / h to Proceed when 950^°C Up Ar / 5% H₂ at 2^L / min 950^°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.5% H₂O vapor, Flow Maintain 950^°C for Process rate 0.8^L / min 12 hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230^°C is reached Stop — — — Table 7: Process steps experimental configuration performed in a 24 L retort; sample dimensions: 30 x 30 x 1 mm

[0003] Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated with argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 0.7^L / min Temperature: 230°C minutes Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C Up Ar / 5% H₂ at 0.7^L / min 950°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.5% H₂O vapor, Flow Maintain 950°C for 12 Process rate 0.8^L / min hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 8: Process steps alternative configuration performed in a 24 L retort; sample dimensions: 30 x 30 x 1 mm Example 7 Holding the furnace at elevated temperatures. The controlled preconditioning of the furnace at an elevated temperature prior to the insertion of metallic components has been found to improve atmospheric stability during ramp-up. This approach enhances purging efficiency, reduces latent humidity, and suppresses the formation of non-protective oxide species, such as FeCr2O4, during the early stages of thermal exposure. Comparative experiments were conducted to evaluate the influence of initial furnace temperature. Two samples of stainless steel type 316 alloy were exposed to identical oxidation conditions during the treatment phase: 950^°C for 12 hours in a gas mixture with H₂ / H₂O ratio of about 16 at the treatment phase, and same ramp-up process with the only variable being the furnace temperature at the time of sample insertion. The sample introduced at room temperature, see FIG. 8A, produced in accordance with Table 9, exhibits a highly porous and disrupted oxide scale with embedded metallic inclusions. SEM / EDXS analysis confirms a high surface concentration of iron (Fe: 83.78 at.%, Ni: 12.02 at.%, Cr: 2.86 at.%, Mo: 1.64 at.%), indicating the reduction of non-protective iron-rich oxides and poor chromia layer formation. FIG.8B, produced in accordance with Table 5, shows a dense and continuous chromia scale with no embedded metallic inclusions. This shows that samples inserted at room temperature exhibited discrete metallic inclusions within and on top of the oxide scale, attributed to the reduction of transient Fe- and Ni-rich oxides formed during uncontrolled oxidation in the early ramp-up stage. Without wishing to be bound by theory, this behavior is attributed to the release of physisorbed moisture from furnace walls and internal surfaces during the initial heating phase. Water vapor generated in this uncontrolled manner reacts with the alloy surface prior to the establishment of thermodynamic equilibrium, resulting in early-stage formation of undesirable and poor protective oxides. Subsequent reduction of these oxides leads to embedded metallic particles, porosity, and compromised oxide scale integrity. By contrast, samples inserted into a preheated furnace at 230°C formed continuous, homogeneous, and defect-free oxide layers. These improvements arise from the benefits of sufficient desorption of moisture from internal furnace surfaces (e.g., insulation materials and heating chamber walls). This strategy is distinct from traditional furnace baking procedures, where baking is typically performed after purging. In contrast, our approach combines baking and purging simultaneously. In the traditional method, physisorbed water molecules are released during baking, promoting uncontrolled oxidation and the formation of unwanted oxides. However, in our method, simultaneous purging during baking effectively removes these water molecules as they desorb, significantly reducing the risk of oxide formation.

[0004] Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Temperature: Room Insertion retort temperature Proceed to evacuation Oxygen Retort evacuated using argon Temperature: Room Continue when O₂ Evacuation gas temperature level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Temperature: room Proceed after 15 Flushing at 2^L / min temperature minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C Up Ar / 5% H₂ at 2^L / min 950°C is reached Equilibration Continued flushing with 95% Hold at 950°C for 120 Proceed after 120 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 950°C for 12 Process 0.8^L / min hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Cool down to room Proceed when room Ramp Down at 0.5^L / min temperature temperature is reached Stop — — — Table 9: Process steps experimental configuration performed in a 24 L retort; sample dimensions: 30 x 30 x 1 mm Example 8 Oxidation morphology as a function of water content during the ramp-up phase. FIGs. 9A shows stainless steel type 316 alloy produced in accordance with Table 10: Due to a small amount of oxygen introduction into the retort to produce water content that exceeds 1.1 vol.%, hydrogen content of about 5 vol.% and the rest argon, (corresponding to a dew point range of approximately 8– 9^°C), continuous chromia as a barrier layer does not form. Instead, internal oxidation results in mainly spinel phases with discontinuous metallic islands on the surface which indicates the formation and subsequent reduction of unstable oxides during ramp-up. A second case (FIG. 9B, stainless steel type 316 alloy produced in accordance with Table 11) is also shown in which the H2 / H2O ratio during ramp- up exceeds 140 during the ramp-up phase resulting in a continuous chromia scale and minimal spinel formation. Criteria for Next Step Description Furnace Condition Step Samples placed in a retort at room temperature. System prepared to insert small Sample amount of oxygen during Insertion process. Temperature: 25°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 25°C level < 10^ppm Flushed with 95% Ar / 5% H₂ Ar / H₂ at 2^L / min and small amount Proceed after 15 Flushing of Oxygen introduced Temperature: 230°C minutes Continued flushing with 95% Slow Ramp- Ar / 5% H₂ at 2^L / min small Ramp at 360°C / h to Proceed when 90 Up amount of Oxygen introduced 950°C 0°C is reached Purge with 95% Ar / 5% H₂ and 0.6% H₂O vapor flow rate Coating 0.8^L / min small amount of Maintain at 950°C for Process Oxygen introduced 24 hours Proceed after 24 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 10: Process steps for FIG. 9A configuration performed in a 24 L retort; sample dimensions: 30 x 30 x 1 mm

[0005] Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C Up Ar / 5% H₂ at 2^L / min 950°C is reached Equilibration Continued flushing with 95% Hold at 950°C for 120 Proceed after 120 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 950°C for 24 Process 0.8^L / min hours Proceed after 24 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 11: Process steps for FIG. 9B configuration performed in a 24 L retort; sample dimensions: 30 x 30 x 1 mm Example 9 Alumina (Al2O3) Scale. Two iron-chromium-aluminum alloys were processed, comprising 20–23 wt% chromium and 4.5–5.5 wt% aluminum, with one alloy further containing 2–3 wt% molybdenum and dispersed thermally stable oxide particles. The first alloy, FeCrAl alloy composition 1, is representative of a commercially available FeCrAl with high-Al concentrations. The second alloy, FeCrAl alloy composition 2, is representative of a commercially available FeCrAl material with slightly lower aluminum content. Samples of each alloy were produced in accordance with process steps described in Table 12. Sample dimensions: 30 x 30 x 1 mm Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 900°C Up Ar / 5% H₂ at 2^L / min 900°C is reached Equilibration Continued flushing with 95% Hold at 900°C for 120 Proceed after 120 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 900°C for 48 Process 0.8^L / min hours Proceed after 48 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 12: Process steps alternative configuration performed in a 24 L retort Scanning Electron Microscopy Cross-sectional SEM analysis of iron-chromium-aluminum alloy subjected to the described oxidation protocol evidence formation of a continuous, dense oxide layer approximately 2-3 µm in thickness. Said alumina scale exhibits structural uniformity throughout the cross-section, without observable cracking, delamination, or porosity, and maintains full adherence to the metallic substrate. As shown in FIGs. 10A, 10B, and 10C, the alumina scale produced via the described process constitutes a thick, dense, thermally grown α-alumina layer. SEM / EDX analysis confirms the oxide layer comprises aluminum and oxygen in stoichiometric ratios consistent with Al₂O₃, with very low amount of iron, chromium, and other minor elements which occasionally falling below detection thresholds within the coating. Said elemental analysis substantiates the high-purity alumina composition, absent secondary phases or contamination. X-ray diffraction XRD analysis was performed on oxide scales formed on FeCrAl alloy composition 1 and FeCrAl composition 2 substrates processed as described above. The measured diffraction pattern exhibited peak positions corresponding to α-alumina (α-Al₂O₃) reference standards, with no detectable peaks attributable to γ-alumina (γ-Al2O3) or other alumina polymorphs. Comprehensive comparison with the DIFFRAC.EVA database confirmed the absence of transient alumina phases (such as δ-, θ-, η-alumina). The crystallographic data thus demonstrates that the scale generated by the claimed process consists solely of thermodynamically stable α-alumina, validating the efficacy of the process for producing protective surface oxide scales (see FIG. 11). Thus, these data verify exclusive composition of thermodynamically stable α-alumina phase. The integrated microstructural and compositional characterization demonstrates that the oxidation process yields a homogeneous, pore-free, fully covering alumina layer exhibiting phase purity and substrate adhesion. Luminescence Spectral Fingerprint Luminescence spectral analysis of alumina scales formed on FeCrAl-based alloys via the disclosed oxidation protocol reveals distinctive and reproducible spectroscopic and microstructural characteristics absent in conventionally produced alumina, for example, in air-oxidized specimens. Air-exposed specimens display sharp, intense R-line emissions at approximately 694-696 nm, characteristic of Cr³⁺ incorporation into the α-alumina lattice. Conversely, samples oxidized via the process disclosed herein demonstrate suppressed Cr³⁺ luminescence intensity, notwithstanding their confirmed α-alumina composition per XRD (see FIG.12). Without wishing to be bound by theory, said optical behaviour differential is attributed to reduced Cr³⁺ lattice incorporation or quenching phenomena resultant from highly ordered, defect-minimized scale microstructure. The lower luminescence intensity of the alumina scales formed by the disclosed treatment suggests reduced Cr incorporation into the growing alumina, consistent with the fact that there is no outward growth of the oxide. The alumina scale formed under the process disclosed herein therefore exhibits differentiated luminescence spectroscopic characteristics when compared to alumina formed under conventional methods. SEM verifies that the scale generated by the disclosed method exhibits thickness exceeding that of air-formed scales by a factor of at least four, while maintaining superior density, substrate adhesion and structural uniformity. Said oxide is continuous, with absence of cracks or porosity (zero identified cracks). The combined spectral signature confirmed α-Al2O3phase, and enhanced microstructure constitute identifiable and reproducible indicators of the oxide scale produced via the process described herein. Example 10 Chromia (Cr2O3) Scale. The evaluated materials include ferritic and austenitic stainless steels (e.g., stainless steel type 304 alloy, stainless steel type 316 alloy, stainless steel type 316L alloy, stainless steel type 253MA alloy, ferritic stainless steel type 441 alloy) and nickel-based alloys, (e.g., Inconel® 600, Inconel® 625). Applicable alloys include ferritic stainless steels (e.g., stainless steel type 430 alloy, ferritic stainless steel type 441 alloy, etc.), austenitic SSs (e.g., stainless steel type 310 alloy, stainless steel type 316 alloy, stainless steel type 304 alloy, stainless steel type 253MA ally, stainless steel type 254 SMO alloy, etc), and nickel-based alloys (e.g., Inconel® 600, 718, 625, etc). Under the described oxidation protocol, all alloys formed a dense Cr2O3-based scales. Microstructural and compositional analyses were conducted to evaluate scale morphology, grain structure, interface quality, and adhesion. The samples of stainless steel type 310 alloy, ferritic stainless steel type 441 alloy, stainless steel type 253MA alloy, stainless steel type 316 alloy, stainless steel type 254 SMO alloy, Inconel® 600 were produced in accordance with process steps described in Tables 13, 14, 15, 16, 17 and 18, respectively. The samples of stainless steel type 310, 316 and 254 alloys, Inconel® 600 were each 30 x 30 x 1 mm. The sample of stainless steel type 253MA was 30 x 30 x 1.5 mm. The sample of Inconel® 625 was 30 x 30 x 1.27 mm. The sample of stainless steel type 441 was 30 x 30 x 0.3 mm. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C Up Ar / 5% H₂ at 2^L / min 950°C is reached Equilibration Continued flushing with 95% Hold at 950°C for 240 Proceed after 240 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.2% H₂O vapor flow rate Maintain 900°C for 20 Process 0.8^L / min hours Proceed after 20 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 13: Ferritic stainless steel type 310 alloy: Process steps performed in a 24 L retort Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 900°C Up Ar / 5% H₂ at 2^L / min 900°C is reached Equilibration Continued flushing with 95% Hold at 900°C for 120 Proceed after 120 Hold Ar / 5% H₂ at 1^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 900°C for 46 Process 1 L / min hours Proceed after 46 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.4^L / min Cool down to 230°C is reached Stop — — — Table 14: Ferritic stainless steel type 441 alloy: Process steps performed in a 24 L retort Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230^°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230^°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230^°C minutes Continued flushing with 95% Ramp at 600^°C / h to Proceed when 600^°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600^°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360^°C / h to Proceed when 900^°C Up Ar / 5% H₂ at 2^L / min 950^°C is reached Equilibration Continued flushing with 95% Hold at 950^°C for 120 Proceed after 120 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 950^°C for Process 0.8^L / min 10 hours Proceed after 10 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230^°C is reached Stop — — — Table 15: Stainless steel type 253 MA alloy: Process steps performed in a 24 L retort Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230^°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230^°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230^°C minutes Continued flushing with 95% Ramp at 600^°C / h to Proceed when 600^°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600^°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360^°C / h to Proceed when 900^°C Up Ar / 5% H₂ at 2^L / min 950^°C is reached Equilibration Continued flushing with 95% Hold at 950^°C for 240 Proceed after 240 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.8% H₂O vapor flow rate Maintain 950^°C for Process 0.8^L / min 20 hours Proceed after 20 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230^°C is reached Stop — — — Table 16: Stainless steel type 316 alloy: Process steps performed in a 24 L retort Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 1000°C Up Ar / 5% H₂ at 2^L / min 1000°C is reached Equilibration Continued flushing with 95% Hold at 1000°C for Proceed after 120 Hold Ar / 5% H₂ at 2^L / min 120 minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 1000°C for 4 Process 0.8^L / min hours Proceed after 4 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 17: Stainless steel type 254 SMO alloy: Process steps performed in a 24 L retort Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C Up Ar / 5% H₂ at 2^L / min 950°C is reached Equilibration Continued flushing with 95% Hold at 950°C for 120 Proceed after 120 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.6% H₂O vapor flow rate Maintain 950°C for 46 Process 0.8^L / min hours Proceed after 46 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 18: Inconel® 600: Process steps performed in a 24 L retort Scanning Electron Microscopy Cross-sectional imaging confirms that chromia-forming alloys develop dense, continuous barrier layers. The resulting chromia layers exhibit uniform thickness, intimate substrate adherence, and absence of cracking, delamination or interfacial porosity across all alloy types, showing the broad applicability of the disclosed protocol to both iron- and nickel-based systems (FIG. 13). The improved morphology is attributed to precise control during the ramp-up and barrier formation phases, which modulate oxidation kinetics, yields porous free oxide layer with improved scale adhesion (FIG. 13). Based on SEM analysis, selected stainless steels exhibit a uniform Mn-rich spinel layer forming on top of the chromia scale under the disclosed protocol; this bilayer architecture is advantageous for applications requiring suppression of chromia volatilization at elevated temperatures. These findings validate that the disclosed processing method enables formation of structurally stable, adherent chromia scales, functioning effectively as oxidation barriers under aggressive conditions both at room temperature and higher. Residual Stresses XRD stress analysis confirms chromia scales formed on nickel-based alloys via the disclosed oxidation protocol exhibit higher compressive residual stresses compared to conventional air oxidation specimens. Two samples processed according to the invention (oxide thicknesses ~2 µm and ~5 µm, respectively) demonstrated residual compressive stresses reaching −4200 MPa, as measured at multiple diffraction angles (110°, 300°, 416° iso, and 416° tilt), see Table 19. Note that the compressive stress values are independent of the thickness of the oxides formed via the disclosed method. Comparative air-formed chromia exhibited substantially lower compressive stresses across identical crystallographic reflections. Compressive stress values in scales formed via the disclosed protocol exceed those of air-formed chromia by approximately 50% in certain measured reflections. These elevated stress values can enhance mechanical stability and durability under external fatigue loads. Despite increased compressive stresses, no delamination, buckling, spallation or microcracking was observed, as verified by SEM, confirming mechanical robustness and substrate adhesion even at increased thicknesses. Additional XRD analyses revealed diffraction peak positions shifted toward lower 2θ values relative to air-formed chromia, which can be due to increased lattice spacing (d-spacing) per Bragg’s Law, indicating elevated compressive residual stress within the oxide lattice. (FIG. 14). XRD-determined residual stress values provide non-destructive quality control metrics for verification of components treated via the disclosed process. 416° iso 416° tilt 300° 110° Sample Description [MPa] [MPa] [MPa] [MPa] Cr2O3 - 2^µm −2800 xx −2800 −4200 - 5^µm −1200 −1300 −2800 −4200 Reference Chromia −735 −910 −2000 −3500 (Air) Note: All values represent compressive stress. “xx” indicates not measured. Table 19: Residual stress measurements of Cr₂O₃ scales by XRD. Raman Spectroscopic Fingerprint Chromia scales formed on a nickel-based superalloy, Inconel® 625, via the disclosed ramp-up and oxidation protocol exhibit distinctive Raman spectral characteristics differentiating said scales from chromia formed under conventional air exposure. In principle, the Raman spectra from the chromia scale could be used as a post-process diagnostic. Raman spectroscopic analysis was carried out using a Horiba LabRAM ARAMIS system. Raman spectroscopic analysis of the resultant scales demonstrates systematic and reproducible shift in the primary Cr2O3 Raman mode proximate to 550 cm⁻¹, consistent with residual compressive stress presence within said scale. Said peak shift is in two samples with different thicknesses produced under invention method conditions (2 µm and 5 µm, respectively). The Raman spectra of both specimens exhibit near-identical characteristics, thereby confirming that the stress state and structural properties of the oxide are independent of scale thickness and intrinsic to the formation conditions. Contrastingly, chromia formed via ambient air oxidation exhibits divergent Raman signature, characterized by an additional prominent peak at approximately 725 cm⁻1. Said peak is absent in all samples formed under the controlled protocol herein described. The absence of said peak in scales produced via the disclosed method evidences high structural order and phase selectivity in the resultant Cr2O3 (FIG.15). Supplementary Raman spectroscopic measurements were conducted on chromia scales produced via the disclosed oxidation protocol, with reference samples prepared via ambient air oxidation. Raman spectra were acquired at randomly selected positions (ten locations per sample side) utilizing a 50× objective, 1800 grooves / mm grating, and 0.5-second acquisition time per cycle for 50 cycles. Instrument calibration verification was performed using silicon wafer, confirming characteristic Si peak at 521 cm⁻¹. The active mode of Cr2O3 at approximately 555-560 cm⁻¹ exhibits consistent blue-shift toward higher wavenumbers in all specimens treated via the disclosed process relative to air-oxidized samples. Said shift is evident in both 2 µm and 5 µm scales, with near-identical peak positions confirming stress state independence from layer thickness and process condition. Without wishing to be bound by theory, the observed upward Raman peak position shift is attributed to compressive residual stress within the oxide scale. Said compressive stress is uniform, reproducible, and constitutes a distinct spectroscopic signature of oxide produced via the disclosed method. Said spectroscopic fingerprint distinguishes the resultant chromia from oxides formed via uncontrolled or ambient oxidation methodologies. Example 11 Hydrogen permeation resistance of oxide scales. To assess the hydrogen permeation resistance of the scales produced by the process according to Table 2, a series of hydrogen uptake and diffusion experiments were conducted on coated according to process described in Table 18 and uncoated Inconel® 625 specimens. These tests were designed to simulate high-temperature, high-pressure service conditions and evaluate the capacity of the formed barrier layer to suppress hydrogen ingress. Two experimental methodologies were employed: (i) Hydrogen uptake and desorption testing Conducted by SWERIM (Sweden), this experiment involved controlled hydrogen charging of both uncoated and coated Inconel® 625 specimens in 200 bar for 16 hours at 300°C, followed by thermal desorption profiling across a temperature range of 25°C to 800°C. The measured data was used to quantify hydrogen uptake, retention, and release behavior. Samples were 40 x 8 x 1.27 mm. (ii) Hydrogen permeation testing Performed by Institute de la Corrosion (France), a membrane-based diffusion cell was employed. One side of the membrane (High Pressure Cell) was exposed to 100 bar of hydrogen gas, while the opposite side (Low Pressure Cell) was connected to a gas chromatograph to detect permeated hydrogen. The setup ensured constant hydrogen exposure at controlled temperatures of 150°C and 300°C, with test durations of up to 66 hours. Circular samples with diameter of 145 mm and thickness of 1.27 mm were utilized. Results: (i) Hydrogen Uptake (Desorption) Tests: Uncoated Inconel® 625 exhibited a desorption peak centered between 400–500°C, indicating significant hydrogen uptake. In contrast, coated specimens processed using the disclosed protocol demonstrated over 80% reduction in hydrogen uptake, with flat and low desorption curves across all temperatures, confirming minimal hydrogen absorption. (ii) Hydrogen Permeation Tests: For coated Inconel® 625 specimens tested at both 150°C and 300°C under 100 bar hydrogen pressure, the measured hydrogen flux was below the detection limit of the analytical equipment. In contrast, uncoated specimens demonstrated measurable hydrogen diffusion coefficients, e.g., 1.3^×^10⁻7^cm² / s at 150°C and 1.9- 2.2^×^10⁻⁶^cm² / s at 300°C. Coated samples showed a dramatic drop in permeation, with values too low to quantify (within the detection limit of the apparatus). These tests show that the barrier layer produced by the methods described herein exhibit hydrogen barrier properties, effectively preventing hydrogen absorption and permeation even under high- temperature and high-pressure conditions. These results confirm the suitability of the disclosed method for applications requiring hydrogen containment or protection from hydrogen-induced degradation (FIG. 16). Example 12 Pitting and crevice corrosion resistance in ferric chloride environment. To evaluate the resistance of scales formed using the methods described to aggressive localized corrosion such as in sea water applications and acidic environments, a standardized ferric chloride test was performed on coated and uncoated stainless steel specimens (FIG. 17). Sample sizes were 30 x 30 x 1 mm. Corrosion testing was conducted in accordance with ASTM G48 G48-11(2020)e1, Standard Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride Solution. The test is designed to simulate highly corrosive environments encountered in acidic process conditions and marine applications. Stainless steel type 316 alloy was used as substrate. The test environment consisted of a 6 wt.% FeCl3 solution maintained at 50°C, with immersion duration of 72 hours. Comparative evaluation was made between: (i) Unprocessed base metal substrate, stainless steel type 316 alloy, and (ii) Stainless steel type 316 alloys processed in accordance with the process steps disclosed in Table 24 to develop dense chromia-based surface layers. Uncoated specimens exhibited severe localized corrosion, including widespread pitting and crevice attack, as well as substantial weight loss. SEM and optical images confirmed extensive surface degradation and coating failure. In contrast, specimens processed in accordance with the methods described herein showed no evidence of pitting, crevice corrosion, or surface breakdown. The oxide layer remained intact and inert throughout the exposure, with no visible defects and no measurable material loss. The protective barrier formed under the protocol demonstrated complete resistance to the aggressive ferric chloride environment. The test results confirm that barrier layer formed via the disclosed method exhibit exceptional resistance to both pitting and crevice corrosion, even under highly accelerated and acidic testing conditions. These results support the application of the disclosed protocol in demanding industrial environments, including offshore, chemical processing, and acidic media, where conventional materials are prone to rapid corrosion failure. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600^°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 900°C Up Ar / 5% H₂ at 2^L / min 900^°C is reached Equilibration Continued flushing with 95% Hold at 900°C for 240 Proceed after 240 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.8% H₂O vapor flow rate Maintain 900°C for 20 Process 0.8^L / min hours Proceed after 20 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 24: Process steps alternative configuration performed in a 24 L retort Example 13 Isothermal oxidation resistance at 800°C. To evaluate the high-temperature oxidation resistance of various chromia-forming alloys, isothermal oxidation testing was performed at 800°C for a continuous duration of 168 hours (one week). The test was designed to assess the comparative performance of untreated alloys versus those processed using the oxidation protocol disclosed herein (FIG. 18). Three alloys were evaluated under two conditions: • Unprocessed base alloy, and • Samples subjected to the process described in Tables 25, 26 and 27, for stainless steel type 316, Inconel® 625 and stainless steel type 253MA alloy, respectively. Sample dimensions: Stainless steel type 316 alloy: 30 x 30 x 1 mm Inconel® 625: 30 x 30 x 1.27 mm Stainless steel type 253Ma alloy: 30 x 30 x 1.5 mm All samples were cleaned and weighed prior to the exposure. After isothermal oxidation in ambient air, the samples were cooled down in furnace and reweighed to determine the extent of oxidation through mass gain measurements. Samples processed in accordance with the disclosed method exhibited negligible mass gain, indicating the formation of a dense, adherent, and protective chromia-based oxide scale with limited growth or volatilization. In contrast, untreated base alloys showed significantly higher mass gains, consistent with the formation of porous or non-uniform oxides. The experimental results demonstrate that the disclosed methods limit oxide scale growth under extended high-temperature exposure. The resulting oxide layer exhibits superior stability and protective function compared to that formed on untreated alloys, confirming the effectiveness of the disclosed methods in enhancing oxidation resistance of chromia-forming alloys. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C Up Ar / 5% H₂ at 2^L / min 950°C is reached Equilibration Continued flushing with 95% Hold at 950°C for 240 Proceed after 240 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.2% H₂O vapor flow rate Maintain 950°C for 20 Process 0.8^L / min hours Proceed after 20 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 25: Process steps alternative configuration performed in a 24 L retort Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 950°C Up Ar / 5% H₂ at 2^L / min 950°C is reached Equilibration Continued flushing with 95% Hold at 950°C for 240 Proceed after 240 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 950°C for 44 Process 0.8^L / min hours Proceed after 44 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 26: Process steps alternative configuration performed in a 24 L retort

[0006] Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600^°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 900^°C Up Ar / 5% H₂ at 2^L / min 900°C is reached Equilibration Continued flushing with 95% Hold at 900°C for 120 Proceed after 120 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 900°C for 46 Process 0.8^L / min hours Proceed after 46 hours Flushing with 95% Ar / 5% H₂ Proceed when 230^°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 27: Process steps alternative configuration performed in a 24 L retort Example 14 Hot corrosion resistance in sulfate salt mixture. To assess hot corrosion resistance of chromia scale formed via disclosed treatment, Inconel® 625 samples were subjected to sulfate-salts at elevated temperatures. The evaluation compared base material with samples processed according to the protocol in Table 28. Sample size: 30 x 30 x 1.27 mm. A salt mixture consisting of 30 mol.% Fe2(SO4)3, 15 mol.% K2SO4, and 55 mol.% Na2SO4 was applied to the sample surfaces. The specimens were then exposed to isothermal conditions at 600°C for 24 hours in ambient air. Following exposure, cross-sectional analyses were conducted using SEM to evaluate scale morphology, integrity, and corrosion product distribution, see FIG.19 A and 19 B. SEM images of untreated Inconel® 625 samples revealed extensive surface degradation, including: • Formation of a thick, porous corrosion layer composed primarily of sulfides and oxides of iron and chromium. • Evidence of material detachment and void formation at the oxide-metal interface. • Penetration of corrosive species leading to localized structural failure. In contrast, samples processed according to the disclosed method exhibited: • A continuous, adherent oxide scale with minimal porosity. • Absence of corrosion products or interfacial delamination. • No reaction between the salts and protective layer The results confirm that the disclosed method provides coated alloys with enhanced hot corrosion resistance in sulfate-rich environments. The formation of a dense, protective oxide barrier prevents aggressive salt attack and prevents both sulfidation and oxide spallation, thereby preserving structural integrity under hot corrosion conditions. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 1000°C Up Ar / 5% H₂ at 2^L / min 1000°C is reached Equilibration Continued flushing with 95% Hold at 1000°C for 30 Proceed after 30 Hold Ar / 5% H₂ at 2^L / min minutes minutes Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 1000°C for Proceed after 5.5 Process 0.8^L / min 5.5 hours hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 28: Process steps alternative configuration performed in a 24 L retort Example 15 Mechanical durability evaluation via progressive load scratch testing. To assess the mechanical integrity and adhesion strength of oxide scales formed by the disclosed oxidation protocol, progressive load scratch tests were performed on coated metallic substrates (FIG. 20). Sample sizes for the scratch test were 100 x 150 x 1.27 mm. Samples of Inconel® 625 alloy were processed using the oxidation protocol disclosed in Table 2 to form protective oxide scales. Scratch testing was conducted on the coated surfaces using a progressive load method, with normal force incrementally increased from 100^g to 1500^g. The test was performed on the coated surfaces according to ISO 1518:2023 to evaluate for which weight coating penetration occurred. Different weights have been applied to the scratch stylus, i.e., 100, 200, 300, 500, 700, 800, 1000, 1200, 1300 and 1500 g. The scratch path has been observed visually and with a microscope (magnification x10) to determine if penetration occurred. The evaluation focused on coating cohesion, resistance to delamination, and potential substrate exposure. Across the full loading range (100^g to 1500^g), no penetration through the oxide scales to the underlying metallic substrate was observed on any tested specimen. The scales remained adherent and intact, exhibiting no signs of chipping, cracking, or spallation. Surface markings resulting from the applied loads were superficial and did not compromise the protective integrity of the oxide scale. The results confirm that oxide layers formed in accordance with the disclosed oxidation protocol exhibit exceptional mechanical durability and strong interfacial adhesion to the substrate. The ability of the coating to withstand progressively increasing mechanical stress without failure demonstrates its suitability for harsh service conditions where abrasion or mechanical contact may occur. This further substantiates the protective performance and robustness of the oxide scale as developed by the disclosed method. Example 16 Pull-off adhesion testing of oxide scales on processed alloys. To evaluate the adhesive strength and interfacial integrity of oxide scales formed by the disclosed oxidation protocol, a series of pull-off adhesion tests were conducted on coated metallic substrates (FIGs. 21A and 21B). Substrates composed of Inconel® 625 and stainless steel type 316 alloy were processed in accordance with Table 2 and Table 29, respectively to form protective oxide layers. Samples sizes were as follows: Inconel® 625: 150 x 150 x 1.27 mm; stainless steel type 316 alloy: 150 x 150 x 1 mm. Pull-off was performed according to ISO 4624:2023. Aluminum dollies with a diameter of 20 mm were attached to the test surface using the adhesive Scotch-weld Structural Adhesive EC-9323 B / A. Before application of the adhesive, the surface of the dollies and test panels were activated by respectively blasting and extremely slight grinding with ScotchBrite Very Fine. When the adhesive was cured, free cutting of the dollies was performed using a hole saw. The positest equipment was placed onto the dolly head and the force needed to initiate adhesion failure was noted along with the type of failure mode. Six tests per panel were performed. As the panels did not meet requirement regarding substrate thickness according to ISO 4624:2023, all the panels were mounted on a supportive foundation (i.e., glued onto a thick panel of steel) to avoid flexing of the test panels during pull-off testing. Across all tested samples, including multiple replicates per alloy type, no failure occurred at the oxide- substrate interface. All observed failures occurred within the epoxy adhesive or at the dolly-epoxy interface. The oxide scale remained fully intact and adherent to the substrate in all cases, with no evidence of delamination, chipping, or cohesive coating failure. Maximum recorded adhesion values consistently exceeded the threshold typically associated with high-performance protective coatings. The results demonstrate that scales formed via the disclosed protocol show excellent adhesion to the underlying metallic substrate. The robust interfacial bonding observed in both Inconel® 625 and stainless-steel type 316 alloy samples confirms the mechanical integrity and environmental resilience of the disclosed protective barrier, further supporting its applicability in demanding operational environments requiring strong and durable surface protection. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 600°C is reached Slow Ramp- Continued flushing with 95% Ramp at 360°C / h to Proceed when 900°C Up Ar / 5% H₂ at 2^L / min 900°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.3% H₂O vapor flow rate Maintain 900°C for 48 Process 0.8^L / min hours Proceed after 48 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 29: Process steps alternative configuration performed in a 24 L retort Example 17 Flexibility and adhesion evaluation via mandrel bending test. To assess the mechanical flexibility and adhesion integrity of oxide layers formed by the disclosed oxidation protocol, mandrel bend testing was performed in accordance with the ISO 1519-1:2011 bending test standard on specimens of Inconel® 625 and stainless-steel type 316L alloy, which were processed in accordance with Tables 2 and 18 to form protective surface oxide coatings (FIGs. 22A and 22B). Sample sizes were as follows: Inconel® 625: 100 x 50 x 1.27 mm; stainless steel type 316 alloy: 100 x 50 x 1 mm. A type 2 mandrel tester associated with a 4 mm diameter mandrel has been used for this test. Ideally, the panels should be a maximum of 1 mm thick, but as the coated panels of Inconel® 625 and stainless steel type 316 alloy were 1.5 mm thick, bending was harder to perform and, therefore, the bending operation took more time than required in the standard, i.e., 1-2 seconds. Panels were observed directly after the bending operations visually and with a x5 lens. The requirement was no cracking in the coating. This standard specifies a test method to assess the flexibility and adhesion of coatings subjected to bending stresses and thus simulating mechanical deformation encountered in practical applications. The test involved bending coated specimens over cylindrical mandrels of defined radii to induce mechanical strain in the coating and underlying substrate. The purpose was to determine the ability of the oxide scale to accommodate deformation without cracking, delamination, or cohesive failure. Following mechanical deformation, all tested specimens, both Inconel® 625 and stainless-steel type 316L alloy, exhibited no observable cracks, flaking, or delamination of the coating. Visual and microscopic inspections confirmed the continuity and integrity of the oxide layer post-deformation. The laboratory reported that no defects of any kind were detected on the surface of the coated specimens, indicating full retention of protective functionality and adhesion. These results confirm that the oxide scales formed via the disclosed method exhibit excellent flexibility and robust adhesion to the substrate. The coatings maintain structural integrity under mechanical strain, further validating their durability in service environments where bending, thermal cycling, or mechanical stress may occur. The performance under ISO-standardized testing conditions reinforces the practical utility of the disclosed method for producing mechanically resilient surface protection layers. Example 18 Oxide Layer Evolution Under Conventional versus Disclosed Oxidation Protocols. To evaluate the effect of oxidation protocol on oxide layer formation, selected nickel base alloy and stainless-steel specimens were subjected to isothermal oxidation at 950°C in ambient air for 48 hours. The objective was to compare oxide morphologies generated via conventional thermal exposure with those formed using the methods disclosed herein. Samples of Inconel® 718 and Inconel® 600 (Ni-based austenitic alloys were subjected to isothermal oxidation at 950°C in ambient air for 48 hours. Samples sizes were as follows: Inconel® 718: 30 x 30 x 0.8 mm; Inconel® 600: 30 x 30 x 1 mm. FIG.23A depicts oxide scales formed under conventional oxidation conditions. These samples exhibit: • Thick, multi-layered oxide scales with irregular interfaces • Interfacial porosity and discontinuities, suggesting poor adhesion and susceptibility to spallation Samples of Inconel® 718 and Inconel® 600 (Ni-based austenitic alloys) were processed in accordance with Table 2. FIG. 23B shows cross-sections of samples processed according to the process steps described in Table 2. These reveal: • A continuous, uniform oxide layer with sharp, well-adhered interfaces • No porosity and absence of any types of defects in the barrier layers Similar observation is seen for Inconel® 600. FIG. 24A presents SEM cross-section of Inconel® 600 exposed to ambient air at 950°C for 48 hours, while FIG.24B shows the cross-section of Inconel® 600 treated according to the process described in Table 2. This difference in microstructure, evident even under identical thermal conditions, indicates the key role of the methods described herein. The disclosed methods result in the formation of dense, adherent, and structurally stable barrier layers that outperform those generated by conventional oxidation. The approach suppresses scale delamination and internal degradation mechanisms, offering enhanced high-temperature oxidation resistance. These improvements have been consistently observed across a range of chromia- and alumina-forming austenitic and ferritic alloys. Example 19 Effect of prior art temperature condition (500°C) on scale formation on stainless steel type 304 and 316 alloys. A thermal oxidation treatment was conducted at 500^°C on stainless steel type 304 and 316 alloys, in accordance with Table 30. All other processing parameters during both the ramp-up and treatment phases were maintained within the ranges disclosed herein. All sample sizes were 30 x 30 x 1 mm. The resulting oxide scale morphology and thickness were characterized by using cross-sectional SEM at both low and high magnifications. FIGs. 25A and 25B present representative cross-sectional SEM images for treated stainless steel type 304 and 316 alloy specimens, respectively. In both cases, the oxide scale formed at 500°C was found to be discontinuous and extremely thin. For the treated stainless steel type 304 alloy (FIG. 25A), the layer was largely undetectable across the majority of the surface, with isolated regions showing a maximum thickness not exceeding 100^nm. Similarly, the treated stainless steel type 316 alloy (FIG. 25B), showed the layer to be non-uniform and discontinuous, with maximum localized thickness reaching approximately 150^nm. Within the maximum resolution of SEM, the oxide layers lack continuity, and under these conditions, a dense, protective layer failed to form. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Continued flushing with 95% Ramp at 600°C / h to Proceed when 500°C Fast Ramp-Up Ar / 5% H₂ at 2^L / min 500°C is reached Purge with 95% Ar / 5% H₂ Coating and 0.6% H₂O vapor flow rate Maintain 500°C for 12 Process 0.6^L / min hours Proceed after 12 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 30: Process steps alternative configuration performed in a 24 L retort Example 20 To investigate the impact of elevated humidity on the formation of protective oxide scales, a treatment was conducted on stainless steel type 316 alloy in accordance with the method disclosed in Table 31. In this example, the humidity introduced from the beginning to the heating chamber and the relative humidity was maintained at 2.2%, equivalent to dew point of less than 18°C throughout the entire process, including the ramp-up phase. Cross-sectional SEM imaging of the treated specimens (FIG. 26) revealed extensive internal oxidation extending deep into the substrate. EDX analysis inside SEM indicated that the dominant oxide phase was identified as iron-rich spinels, with minor chromia formation localized at the spinel- alloy interface. A distinct, Fe-Ni-rich metallic surface layer was observed on the top. These findings indicate that under lower H2to H2O ratios, here less than 2.3 during ramp-up phase, the formation of a uniform, adherent chromia-dominated surface oxide is suppressed. Instead, the alloy undergoes significant internal oxidation and phase separation which lead to the development of non-protective, heterogeneous surface structures. Criteria for Next Step Description Furnace Condition Step Sample Samples placed in a leak-tight Insertion retort Temperature: 230°C Proceed to evacuation Oxygen Retort evacuated using argon Continue when O₂ Evacuation gas Temperature: 230°C level < 10^ppm Ar / H₂ Flushed with 95% Ar / 5% H₂ Proceed after 15 Flushing at 2^L / min Temperature: 230°C minutes Flushing with 95% Ar / 5% H₂ Ramp at 600°C / h to Proceed when 600°C Fast Ramp-Up and 2.2 vol.% H2O at 2^L / min 600°C is reached Continued flushing with 95% Slow Ramp- Ar / 5% H₂ and 2.2 vol.% H2O Ramp at 360°C / h to Proceed when 950°C Up at 2^L / min 950°C is reached Purge with 95% Ar / 5% H₂ Coating and 2.2% H₂O vapor flow rate Maintain 950°C for 24 Process 0.8^L / min hours Proceed after 24 hours Flushing with 95% Ar / 5% H₂ Proceed when 230°C Ramp Down at 0.5^L / min Cool down to 230°C is reached Stop — — — Table 31: Process steps alternative configuration performed in a 24 L retort The specification is most thoroughly understood in light of the teachings of the references cited within the specification. The embodiments within the specification provide an illustration of embodiments of the invention and should not be construed to limit the scope of the invention. The skilled artisan readily recognizes that many other embodiments are encompassed by the invention. All publications and patents cited in this disclosure are incorporated by reference in their entirety. To the extent the material incorporated by reference contradicts or is inconsistent with this specification, the specification will supersede any such material. The citation of any references herein is not an admission that such references are prior art to the present invention. Any disclosed ranges are considered to be an explicit disclosure of the end points of those ranges, and any point values between said endpoints. Those skilled in the art will recognize or be able to ascertain using, no more than routine experimentation, many equivalents to the specific embodiments of the invention described herein. Such equivalents are intended to be encompassed by the following claims.

Claims

CLAIMS 1. A method for forming at least one oxide layer at a surface of a metallic alloy, the metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium; wherein the method comprises: introducing the metallic alloy into a heating chamber of a furnace; and performing a heat treatment process to form the at least one oxide layer on the surface of the metallic alloy; wherein the heat treatment process comprises: (c) a temperature ramp-up phase during which the temperature of the heating chamber is increased to a treatment temperature; and then (d) a heat treatment phase during which the temperature of the heating chamber is maintained at the treatment temperature for a treatment time to form the at least one oxide layer; wherein, during temperature ramp-up phase (a) and heat treatment phase (b), the atmosphere within the furnace comprises at least one non-oxidizing gas, hydrogen, and water vapor; wherein, during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 2, and during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 0.5; and wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer.

2. A method for forming at least one oxide layer at a surface of a metallic alloy, the metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium; wherein the method comprises: introducing the metallic alloy into a heating chamber of a furnace; and performing a heat treatment process to form the at least one oxide layer on the surface of the metallic alloy; wherein the heat treatment process comprises:(e) a temperature ramp-up phase during which the temperature of the heating chamber is increased to a treatment temperature; and then (f) a heat treatment phase during which the temperature of the heating chamber is maintained at the treatment temperature for a treatment time to form the at least one oxide layer; wherein, during temperature ramp-up phase (a) and heat treatment phase (b), the atmosphere within the furnace comprises at least one non-oxidizing gas, hydrogen, and water vapor; wherein, during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 5, and during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 0.5; and wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer.

3. The method of claim 1 or claim 2, wherein the method comprises purging the heating chamber by flowing a purging gas through the heating chamber prior to step (a), optionally wherein said purging takes place during and / or after baking the furnace at 100-250°C for a period of time (for example, at least 10 minutes, or at least 30 minutes).

4. The method of claim 3, wherein the purging reduces the water content of the atmosphere within the furnace to less than 0.3 vol.% and / or reduces the free oxygen content of the atmosphere within the furnace to less than 500 ppm by volume.

5. The method of claims 3 or 4, wherein the purging reduces the water content of the atmosphere within the furnace sufficiently to permit the ratio by volume of hydrogen to water vapor in the atmosphere within the heating chamber to be maintained at 5 or greater during temperature ramp-up phase (a).

6. The method of any of claims 3 to 5, wherein purging the heating chamber reduces the water content of the atmosphere within the furnace to less than 2 vol.%.

7. The method of claim 6, wherein purging the heating chamber reduces the water content of the atmosphere within the furnace to less than 0.2 vol.%.

8. The method of any one of claims 3 to 7, wherein purging the heating chamber reduces the free oxygen content of the atmosphere within the furnace to less than 500 ppm by volume.

9. The method of claim 8, wherein purging the heating chamber reduces the free oxygen content of the atmosphere within the furnace to less than 100 ppm by volume.

10. The method of claim 9, wherein purging the heating chamber reduces the free oxygen content of the atmosphere within the furnace to less than 50 ppm by volume.

11. The method of any of claims 3 to 10, wherein the amount by volume of the purging gas supplied to the heating chamber during the purging of the heating chamber is at least 5 times the volume of the heating chamber.

12. The method of claim 11, wherein the amount by volume of the purging gas supplied to the heating chamber during the purging of the heating chamber is at least 10 times the volume of the heating chamber.

13. The method of any of claims 3 to 10, wherein the amount by volume of the purging gas supplied to the heating chamber during the purging of the heating chamber is about 5 to15 times the volume of the heating chamber.

14. The method of claim 13, wherein the amount by volume of the purging gas supplied to the heating chamber during the purging of the heating chamber is about 10 times the volume of the heating chamber.

15. The method of any of claims 3 to 14, wherein the purging gas is a non-oxidizing gas.

16. The method of claim 15, wherein the purging gas comprises hydrogen.

17. The method of claim 16, wherein the purging gas comprises hydrogen in combination with an inert gas.

18. The method of claim 17, wherein the at least one purging gas comprises hydrogen in combination with argon.

19. The method of any preceding claim, wherein the non-oxidizing gas present during temperature ramp-up phase (a) and heat treatment phase (b) is argon.

20. The method of any preceding claim, wherein, during the temperature ramp-up phase (a), the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 10.

21. The method of claim 20, wherein during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 20, at least 30, at least 40, at least 50, at least 60, at least 70, at least 80, at least 90, at least 100, at least 125, at least 150, at least 175, at least 200, at least 250, at least 300, at least 350, at least 400, at least 450, or at least 500.

22. The method of any one of claims 1 to 19, wherein during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is 5 to 1,000.

23. The method of claim 22, wherein during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is 100 to 1,000.

24. The method of claim 23, wherein during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is 500 to 1,000.

25. The method of claim 22, wherein during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is 5 to 500.

26. The method of claim 23, wherein during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is 100 to 500.

27. The method of claim 26, wherein during temperature ramp-up phase (a) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is 100 to 250.

28. The method of any preceding claim, wherein, during temperature ramp-up phase (a), the amount of hydrogen in the atmosphere within the furnace is 10 vol.% or less.

29. The method of any one of claims 1 to 27, wherein, during temperature ramp-up phase (a), the amount of hydrogen in the atmosphere within the furnace is 2 to 99.9 vol.%.

30. The method of claim 29, wherein, during temperature ramp-up phase (a), the amount of hydrogen in the atmosphere within the furnace is 4 to 40 vol.%.

31. The method of any preceding claim, wherein, during temperature ramp-up phase (a), the amount of water vapor in the atmosphere within the furnace is less than 0.3 vol.%.

32. The method of claim 31, wherein, during temperature ramp-up phase (a), the amount of water vapor in the atmosphere within the furnace is between 0.05 and 0.3 vol.%.

33. The method of claim 32, wherein, during temperature ramp-up phase (a), the amount of water vapor in the atmosphere within the furnace is less than 0.2 vol.%.

34. The method of claim 33, wherein, during temperature ramp-up phase (a), the amount of water vapor in the atmosphere within the furnace is between 0.05 and 0.2 vol.%.

35. The method of any one of claims 1 to 34, wherein, during heat treatment phase (b), the amount of water vapor in the atmosphere within the furnace is at least 0.05 vol.% 36. The method of claim 25, wherein, during heat treatment phase (b), the amount of water vapor in the atmosphere within the furnace is at least 0.1 vol.%.

37. The method of any one of claims 1 to 34, wherein, during heat treatment phase (b), the amount of water vapor in the atmosphere within the furnace is less than or equal to 1.0 vol. %.

38. The method of claim 37, wherein, during heat treatment phase (b), the amount of water vapor in the atmosphere within the furnace is less than or equal to 0.8 vol.%.

39. The method of any one of claims 1 to 34, wherein, during heat treatment phase (b), the amount of water vapor in the atmosphere within the furnace is 0.3 to 0.6 vol.%.

40. The method of claim 28, wherein, during heat treatment phase (b), the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is between 0.5 and 10,000.

41. The method of any one of claims 1 to 27 and 29 to 38, wherein, during heat treatment phase (b), the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is between 0.5 and 1,000.

42. The method of claim 41, wherein, during heat treatment phase (b), the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is between 0.5 and 500.

43. The method of claim 27, wherein, during temperature ramp-up phase (a) and during the heat treatment phase (b), the amount of hydrogen in the atmosphere within the furnace is in the range of 2 to 10 vol.%.

44. The method of claim 43, wherein, during temperature ramp-up phase (a) and during the heat treatment phase (b), the amount of hydrogen in the atmosphere within the furnace is in the range of 4 to 10 vol.%.

45. The method of any preceding claim, wherein during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 1.

46. The method of any preceding claim, wherein during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is at least 2.

47. The method of any one of claims 1 to 44, wherein during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is in the range of 0.5 to 1,000.

48. The method of claim 47, wherein during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is in the range of 10 to 500.

49. The method of claim 48, wherein during heat treatment phase (b) the ratio by volume of hydrogen to water vapor in the atmosphere within the furnace is in the range of 100 to 250.

50. The method of any preceding claim, wherein the method comprises, during the heat treatment process, supplying a plurality of gases to the heating chamber, the plurality of gases comprising at least one non-oxidizing gas and hydrogen.

51. The method of claim 50, wherein the plurality of gases additionally comprises water vapor and / or oxygen.

52. The method of claim 50 or 51, wherein, only once the temperature of the heating chamber has reached at least 600°C, the plurality of gases supplied to the heating chamber further comprises water vapor.

53. The method of claim 50 or 51, wherein, only once the temperature of the heating chamber has reached the treatment temperature, the plurality of gases supplied to the heating chamber further comprises water vapor.

54. The method of claim 52 or 53, wherein the water vapor is supplied to the heating chamber by humidifying at least one of the at least one non-oxidizing gas and the hydrogen supplied to the heating chamber using at least one humidifier.

55. The method of claim 52 or 53, wherein the water vapor is supplied to the heating chamber separately to the at least one non-oxidizing gas and the hydrogen.

56. The method of claim 54, wherein the at least one humidifier is used to humidify the at least one of the inert gas and the hydrogen supplied to the heating chamber only once the temperature of the heating chamber has reached at least 600°C.

57. The method of claim 54, wherein the at least one humidifier is used to humidify the at least one of the inert gas and the hydrogen supplied to the heating chamber only once the temperature of the heating chamber has reached the heat treatment temperature.

58. The method of any one of claims 50 to 57, wherein, during the heat treatment process, the plurality of gases are supplied to the heating chamber in a combined amount by volume of at least three times the volume of the heating chamber per hour.

59. The method of claim 58, wherein the plurality of gases are supplied to the heating chamber in a combined amount by volume of at least five times the volume of the heating chamber per hour.

60. The method of any one of claims 50 to 59, wherein, during the heat treatment process, the plurality of gases are supplied to the heating chamber in a combined amount by volume per hour of no greater than 15 times the volume of the heating chamber per hour.

61. The method of claim 60, wherein, during the heat treatment process, the plurality of gases are supplied to the heating chamber in a combined amount by volume per hour of no greater than 10 times the volume of the heating chamber per hour.

62. The method of any of claims 1 to 49, wherein the heating chamber is sealed such that no gases are input into the heating chamber during the heat treatment phase.

63. The method of any one of claims 3 to 62, wherein, prior to temperature ramp-up phase (a) and prior to the purging, the heating chamber of the furnace is maintained at a temperature of at least 100°C for at least 10 minutes.

64. The method of claim 63, wherein, prior to temperature ramp-up phase (a), the heating chamber of the furnace is maintained at a temperature of at least 150°C for at least 10 minutes.

65. The method of claim 63, wherein, prior to temperature ramp-up phase (a) and prior to the purging, the heating chamber of the furnace is maintained at a temperature of at least 150°C for at least 10 minutes.

66. The method of claim 64, wherein, prior to temperature ramp-up phase (a), the heating chamber of the furnace is maintained at a temperature of about 230°C for at least 10 minutes.

67. The method of claim 63, wherein, prior to temperature ramp-up phase (a) and for a time duration after the heating chamber of the furnace reaches 100°C, an environment of the heating chamber is exposed to a vacuum pump.

68. The method of claim 67, wherein exposure to the vacuum pump is continued until the level of oxygen gas in the atmosphere is below 10 ppm.

69. The method of any preceding claim, wherein, during temperature ramp-up phase (a), the heating chamber of the furnace is heated at a rate of at least 5°C per minute, such as 50°C per minute.

70. The method of any one of claims 1 to 68, wherein, during temperature ramp-up phase (a), the heating chamber of the furnace is heated at a rate of less than 20°C per minute.

71. The method of any preceding claim, wherein, during temperature ramp-up phase (a), the heating chamber of the furnace is heated at a rate of about 10°C per minute.

72. The method of any preceding claim, wherein, during temperature ramp-up phase (a) and during heat treatment phase (b), the atmosphere within the furnace comprises oxygen in an amount of less than 500 ppm.

73. The method of claim 72, wherein, during temperature ramp-up phase (a) and during heat treatment phase (b), the atmosphere within the furnace comprises oxygen in an amount of less than 100 ppm.

74. The method of any preceding claim, wherein the heat treatment temperature in the heating chamber during heat treatment phase (b) is within the range of 800 to 1050°C.

75. The method of claim 74, wherein the heat treatment temperature in the heating chamber during heat treatment phase (b) is within the range of 830-1030°C.

76. The method of claim 74, wherein the heat treatment temperature in the heating chamber during heat treatment phase (b) is within the range of 850-950°C.

77. The method of claim 74, wherein the heat treatment temperature in the heating chamber during heat treatment phase (b) is within the range of 800-1000°C.

78. The method of claim 74, wherein the heat treatment temperature in the heating chamber during heat treatment phase (b) is within the range of 850 to 1000°C.

79. The method of claim 74, wherein the heat treatment temperature in the heating chamber during heat treatment phase (b) is within the range of 850 to 900°C.

80. The method of claim 74, wherein the heat treatment temperature in the heating chamber during heat treatment phase (b) is within the range of 900 to 950°C.

81. The method of any one of claims to 1 to 73, wherein the base metal comprises iron, and wherein the heat treatment temperature in the heating chamber during heating phase (b) does not exceed 1000°C.

82. The method of any one of claims to 1 to 73, wherein the base metal comprises nickel and / or cobalt, and wherein the heat treatment temperature in the heating chamber during heating phase (b) is at least 850°C.

83. The method of any preceding claim, wherein the duration of heat treatment phase (i.e., step (b)) is at least 3 hours.

84. The method of any preceding claim, wherein the duration of heat treatment phase (i.e., step (b)) is at least 6 hours.

85. The method of any one of claims 1 to 82, wherein the duration of heat treatment phase (i.e., step (b) is about 3 to 72 hours, 3 to 48 hours, 3 to 24 hours, 3 to 12 hours, 6 to 72 hours, 6 to 48 hours, 6 to 24 hours, 10 to 72 hours, 10 to 48 hours and 10 to 24 hours.

86. The method of any preceding claim, wherein the metallic alloy comprises at least one metal, M, capable of achieving a +2 oxidation state, and wherein the at least one oxide layer comprises a mixed-oxide layer comprising the Cr and the at least one metal, M.

87. The method of claim 86, wherein the mixed-oxide layer is formed at a top surface of the chromia layer.

88. The method of claim 86 or claim 87, wherein the mixed-oxide layer has a spinel structure.

89. The method of any of claims 86 to 88, wherein the at least one metal, M, comprises manganese.

90. The method of any of claims 86 to 89, wherein the mixed-oxide layer comprises MnCr2O4.

91. The method of any preceding claim, wherein the chromia layer and / or the alumina layer has a thickness of at least 0.5µm, as measured using scanning electron microscopy (SEM) image analysis.

92. The method of claim 91, wherein the chromia layer and / or the alumina layer has a thickness of at least 1 µm, as measured using scanning electron microscopy (SEM) image analysis.

93. The method of claim 92, wherein the chromia layer and / or the alumina layer has a thickness of at least 1.5 µm, as measured using scanning electron microscopy (SEM) image analysis.

94. The method of any preceding claim, wherein the chromia layer and / or the alumina layer has a porosity (pore volume) of less than 2% by volume, as measured using scanning electron microscopy (SEM) image analysis.

95. The method of any preceding claim, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.58 as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

96. The method of any preceding claim, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.53 as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

97. The method of any preceding claim, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.52 as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

98. The method of any preceding claim, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.51 as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

99. The method of any one of claims 1 to 94, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.58 as measured using energy-dispersive X- ray spectroscopy (EDXS) analysis.

100. The method of claim 99, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.53 as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

101. The method of claim 100, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.52 as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

102. The method of claim 101, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.51, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

103. The method of any preceding claim, wherein the metallic alloy comprises an austenitic alloy.

104. The method of claim 103, wherein the metallic alloy is an iron-based alloy.

105. The method of claim 104, wherein the metallic alloy is an iron-based alloy, having a chromium content of more than 10 wt.%.

106. The method of claim 105, wherein said iron-based alloy is an FeCrAl alloy.

107. The method of claim 106, wherein said FeCrAl alloy is selected from composition 1: Cr: 20.0- 24.0%, Al: 3.0 to 5.0%, Mn: 0.6% or less, Si: 1.0% or less, Fe: balance; and composition 2: Cr: 20.0- 24.0%, Al: 5.0 to 6.0%, Mn: 0.6% or less, Si: 1.0% or less, Fe: balance.

108. The method of any one of claims 1 to 104, wherein the metallic alloy is a stainless steel.

109. The method of claim 108, wherein the metallic alloy is a stainless steel selected from American Iron and Steel Institute (AISI) grade 304, 316, 316L, 253MA and 441 stainless steels, particularly grade 316 stainless steel, more particularly 316L stainless steel.

110. The method of claim 109, wherein the metallic alloy is an austenitic stainless steel type alloy selected from stainless steel type 310 alloy, stainless steel type 316 alloy, stainless steel type 430 alloy, stainless steel type 253MA alloy and stainless steel type 254 alloy.

111. The method of claims 1 to 102, wherein the metallic alloy is a nickel-based alloy.

112. The method of claim 111, wherein the nickel-based alloy is an Inconel® alloy.

113. The method of claim 112, wherein the nickel-based alloy is an Inconel® alloy selected from Inconel® 625, Inconel® 600, Inconel® 718.

114. The method of claim 113, wherein the nickel-based alloy is Inconel® 625 (UNS N06625).

115. A coated metallic alloy having at least one surface oxide layer produced by the method of any preceding claim.

116. The coated metallic alloy of claim 115, wherein the alumina and or the chromia layer is substantially resistant to corrosion, as measured by scanning electron microscopy (SEM).

117. The coated metallic alloy of claim 116, wherein said surface oxide layer is substantially resistant to corrosion by chloride ions or sulfate salts.

118. The coated metallic alloy of claim 117, wherein said surface oxide layer is substantially resistant to corrosion by sulfate salts selected from Fe2(SO4)3, K2SO4, and Na2SO4.

119. The coated metallic alloy of any one of claims 115 to 118, wherein said surface oxide layer is substantially resistant to hydrogen permeation, as measured by desorption testing.

120. The coated metallic alloy of any one of claims 115 to 119, wherein said alloy has a hydrogen diffusion coefficient less than 2.5^×^10⁻⁶^cm² / s at 300^°C and at 100 bar hydrogen pressure.

121. A coated metallic alloy comprising:a metallic alloy comprising: a base metal comprising iron, nickel, and / or cobalt; and aluminum and / or chromium; at least one oxide layer on a surface of the metallic alloy, wherein the at least one oxide layer comprises a chromia layer and / or an alumina layer; wherein the chromia layer and / or the alumina layer has a porosity of less than 2% by volume, as measured using scanning electron microscopy (SEM) image analysis.

122. The coated metallic alloy of claim 121, wherein the chromia layer and / or the alumina layer has a thickness of at least 0.5 µm.

123. The coated metallic alloy of claim 122, wherein the chromia layer and / or the alumina layer has a thickness of at least 1 µm.

124. The coated metallic alloy of claim 123, wherein the chromia layer and / or the alumina layer has a thickness of at least 1.5 µm.

125. The coated metallic alloy of any one of claims 121 to 124, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.58 as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

126. The coated metallic alloy of claim 125, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.53, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

127. The coated metallic alloy of claim 126, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.52, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

128. The coated metallic alloy of claim 127, wherein the chromia layer has a stoichiometric (atomic or molar) oxygen-to-chromium ratio of less than 1.51, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

129. The coated metallic alloy of any one of claims 121 to 124, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.58, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

130. The coated metallic alloy of claim 129, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.53, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

131. The coated metallic alloy of claim 130, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.52 as measured using energy-dispersive X- ray spectroscopy (EDXS) analysis.

132. The coated metallic alloy of claim 131, wherein the alumina layer has a stoichiometric (atomic or molar) oxygen-to-aluminum ratio of less than 1.51, as measured using energy-dispersive X-ray spectroscopy (EDXS) analysis.

133. The coated metallic alloy of any of claims 121 to 132, wherein the metallic alloy comprises at least one metal, M, capable of achieving a +2 oxidation state, and wherein the at least one oxide layer comprises a mixed-oxide layer comprising the Cr and the at least one metal, M.

134. The coated metallic alloy of claim 133, wherein the mixed-oxide layer is disposed on top of the chromia layer.

135. The coated metallic alloy of claim 133 or claim 134, wherein the mixed-oxide layer has a spinel structure.

136. The coated metallic alloy of any one of claims 133 to 135, wherein the at least one metal, M, comprises Manganese.

137. The coated metallic alloy of any one of claims 133 to 136, wherein the mixed-oxide layer comprises MnCr2O4.

138. The coated metallic alloy of any one of claims 133 to 137, wherein the compressive stress within the formed chromia and / or alumina layer exceed 500 MPa, as measured by X-ray diffraction residual stress measurements.

139. The coated metallic alloy of any one of claims 133 to 138, wherein the chromia and / or the alumina layer is inward-grown.

140. The coated metallic alloy of any one of claims 121 to 139, wherein said surface oxide layer is substantially resistant to corrosion, as measured by scanning electron microscopy (SEM).

141. The coated metallic alloy of claim 140, wherein said surface oxide layer is substantially resistant to corrosion by chloride ions or sulfate salts.

142. The coated metallic alloy of claim 141, wherein said surface oxide layer is substantially resistant to corrosion by sulfate salts selected from Fe2(SO4)3, K2SO4, and Na2SO4.

143. The coated metallic alloy of any one of claims 121 to 142, wherein said surface oxide layer is substantially resistant to hydrogen permeation, as measured by desorption tests.

144. The coated metallic alloy of any one of claims 121 to 143, wherein said alloy has a hydrogen diffusion coefficient less than 2.5^×^10⁻⁶^cm² / s at 300^°C and at 100 bar hydrogen pressure.

145. Use of a coated metallic alloy according to any one of claims 115 to 144 in the manufacture of a component for use in hydrogen containment or transport.

146. Use of a component comprising a coated metallic alloy according to any one of claims 115 to 144 for hydrogen containment or transport.

147. A component for hydrogen containment or transport comprising a coated metallic alloy of any of claims 1151 to 144.

148. An apparatus for forming at least one oxide layer at a surface of a metallic alloy, the apparatus comprising: a furnace comprising a heating chamber; a gas supply system in flow communication with the heating chamber via at least one inlet of the heating chamber; and a gas monitoring system in flow communication with the heating chamber via at least one outlet of the heating chamber and configured to monitor the atmosphere within the heating chamber; wherein the gas monitoring system comprises a humidity sensor configured to measure the humidity of gas from the heating chamber.

149. The apparatus of claim 148, wherein the gas monitoring system comprises an oxygen sensor configured to measure the oxygen content of gas output from the heating chamber.

150. The apparatus of claim 149, wherein the oxygen sensor is configured to measure the free oxygen content of the gas output from the heating chamber and the bound oxygen content of the gas output from the heating chamber.

151. The apparatus of claim 150, wherein the humidity sensor is the oxygen sensor.

152. The apparatus of any of claims 148 to 151, wherein the gas monitoring system comprises a hydrogen sensor configured to measure the hydrogen content of gas output from the heating chamber.

153. The apparatus of any of claims 148 to 152, wherein the heating chamber is a retort.

154. The apparatus of any of claims 148 to 153, wherein the heating chamber is configured to operate at a positive pressure relative to atmospheric pressure.

155. The apparatus of any of claims 148 to 154, wherein the gas supply system is configured to provide a non-oxidizing gas and hydrogen, and at least one of water vapor and / or oxygen, to the heating chamber.

156. The apparatus of any of claims 148 to 155, wherein the gas supply system comprises a humidifier in flow communication with the heating chamber and configured to supply water vapor to the heating chamber.

157. The apparatus of any of claims 148 to 156, wherein the gas supply system comprises gas supply tubing for supplying at least one of a non-oxidizing gas and hydrogen to the heating chamber and selectively humidifying the non-oxidizing gas and / or hydrogen.

158. The apparatus of claim 157, wherein the gas supply system comprises a humidifier, and wherein the gas supply tubing for supplying the non-oxidizing gas and / or hydrogen to the heating chamber defines a first gas supply line that passes through the humidifier and a second gas supply line that does not pass through the humidifier, wherein the gas supply system is controllable to supply the non-oxidizing gas and / or hydrogen to the heating chamber via either the first or second gas supply lines.

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