Preparation method of high-hydrogen-resistance pure iron nitriding composite diaphragm

Abstract

The invention discloses a preparation method of a high-hydrogen-resistance pure iron nitriding composite diaphragm, which comprises the following steps: putting a matrix into a controllable atmosphere well type nitriding furnace, raising the temperature in the controllable atmosphere well type nitriding furnace to 460-500 DEG C at the temperature raising rate of 5-15 DEG C / min, and carrying out heat preservation for 2-6 hours to nitride the matrix; the base body is placed in a uniform temperature area of a tubular vacuum heat treatment furnace, the tubular vacuum heat treatment furnace is vacuumized, the temperature in the tubular vacuum heat treatment furnace is increased to 480-520 DEG C at the speed of 5-15 DEG C / mi, and heat preservation is conducted for 0.5-2 hours; taking out the matrix after the temperature in the tubular vacuum heat treatment furnace is reduced to room temperature; the hydrogen resistance of the prepared composite diaphragm can reach 13 times that of a pure iron matrix, the performance improvement amplitude is far higher than 3 times that of single gas nitriding treatment, and the composite diaphragm has the characteristics of good mechanical stability and strong deformation resistance.

Description

Technical Field

[0001] This invention relates to the field of surface modification of metallic materials and safety technology of hydrogen energy equipment, and in particular to a method for preparing a high hydrogen-barrier pure iron nitrided composite diaphragm. Background Technology

[0002] Hydrogen atoms are the smallest atoms in the periodic table, possessing extremely high diffusion and permeability. They can easily penetrate and diffuse into the crystal lattice of metallic materials, leading to hydrogen embrittlement. Hydrogen embrittlement causes a sharp decrease in the plasticity and toughness of metallic materials, resulting in sudden brittle fracture under stresses far below the material's yield strength. This poses a serious threat to the long-term safe operation of critical equipment such as hydrogen pipelines, hydrogen storage tanks, fuel cell bipolar plates, valves, and compressors.

[0003] The problems with commonly used techniques for reducing hydrogen embrittlement are as follows:

[0004] Surface ceramic coating technology, such as alumina (Al2O3) and chromium oxide (Cr2O3) ceramic coatings, has a very poor film-substrate adhesion due to the significant difference in the thermal expansion coefficients between the ceramic layer and the metal substrate. Under thermal cycling or mechanical stress, it is very easy to peel off and crack.

[0005] In the gas nitriding technology for iron-based materials, there is a significant abrupt change in performance between the nitride layer and the matrix, with a steep interface transition leading to insufficient bonding strength; moreover, the nitrogen element is unevenly distributed within the nitrided layer, which easily forms coarse needle-like nitrides.

[0006] Therefore, existing technologies have excellent hydrogen barrier properties but extremely poor membrane-substrate adhesion (such as ceramic coatings and PVD coatings), or moderate membrane-substrate adhesion but limited hydrogen barrier properties (such as traditional gas nitriding). Especially under extreme operating conditions (high pressure, high temperature, thermal cycling), the problem of coating peeling and cracking caused by insufficient membrane-substrate adhesion is more prominent, which seriously restricts the safe service life of hydrogen energy equipment. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of existing technologies that cannot simultaneously achieve hydrogen barrier performance and membrane-substrate bonding strength, and to provide a method for preparing a high hydrogen barrier pure iron nitrided composite membrane.

[0008] To achieve the above objectives, the present invention adopts the following technical solution:

[0009] A method for preparing a high hydrogen-barrier pure iron nitrided composite membrane includes the following steps:

[0010] Step 1: Pre-treat the surface of the substrate made of pure iron by grinding, cleaning and drying;

[0011] Step 2: Place the substrate in a controlled atmosphere pit nitriding furnace, and introduce pure ammonia gas into the controlled atmosphere pit nitriding furnace until the gas pressure in the controlled atmosphere pit nitriding furnace reaches 0.02 MPa. Raise the temperature in the controlled atmosphere pit nitriding furnace to 460℃-500℃ at a heating rate of 5℃ / min-15℃ / min, and hold at this temperature for 2 hours-6 hours to nitrid the substrate. After nitriding is completed, allow the substrate to cool naturally to room temperature with the furnace before removing it.

[0012] This invention employs low-temperature nitriding, which allows nitrogen atoms to slowly penetrate into the iron matrix at low temperatures, preventing them from clustering into coarse needle-like nitrides (a problem with traditional high-temperature nitriding). The fine nitride particles are densely packed together, forming a dense protective layer that can block hydrogen atoms without being too brittle and prone to cracking.

[0013] XRD results show that at temperatures between 460℃ and 500℃, the nitrided samples form two key protective phases, ε-Fe. 2-3 N and γ′-Fe4N, ε-Fe 2-3 N and γ′-Fe4N are the most stable and have good hydrogen blocking effects.

[0014] Using a longer time (2-6 hours) allows nitrogen atoms in ammonia sufficient time to diffuse into the iron: at low temperatures, nitrogen atoms do not move quickly and can only stay on the surface for a short time, resulting in a hard surface and a soft interior, making it easy for the joints to delaminate; long-term heat preservation allows nitrogen atoms to slowly diffuse into the interior of the iron, forming a gradient layer that is dense on the surface, uniform in the middle, and gradual in the interior, reducing the difference in internal and external properties and avoiding stress concentration at the joints.

[0015] Atmosphere cooling allows for slow temperature reduction and air isolation: After infiltration, cooling is not rushed, and air is prevented from entering. This serves two purposes: firstly, to prevent oxidation of the high-temperature protective film, and secondly, to allow the stress generated during the process to be released slowly, avoiding cracking of the protective film. The temperature of 460℃-500℃ is chosen because excessively high temperatures will cause the nitrides to grow coarse and brittle, and are prone to delamination from the substrate; if the temperature is too low, nitrogen atoms cannot infiltrate, resulting in a thin and uneven protective film. This temperature range forms a dense protective layer and allows for seamless integration with subsequent vacuum treatment. The slightly higher vacuum temperature allows for further optimization of nitrogen atom distribution, resulting in a stronger bond between the film and the substrate.

[0016] Step 3, Vacuuming:

[0017] The substrate is placed in a quartz glass boat, which is then placed into the homogenization zone of a tubular vacuum heat treatment furnace. The furnace is then evacuated until the vacuum level stabilizes at ≤5.0×10-3 Pa, and this vacuum state is maintained for 25-35 minutes.

[0018] Step 4, Vacuum heat treatment:

[0019] The temperature inside the tubular vacuum heat treatment furnace is raised to 480℃-520℃ at a rate of 5℃ / min-15℃ / min and held for 0.5 hours-2 hours; the substrate is removed after the temperature inside the tubular vacuum heat treatment furnace drops to room temperature.

[0020] This invention utilizes heat treatment under vacuum to eliminate stress at the interface between the nitrided film and the substrate, as nitrogen atoms, although incorporated into the iron, may be unevenly distributed. Raising the temperature to 480℃-520℃ (slightly higher than the nitriding temperature) provides sufficient energy to the nitrogen atoms without causing decomposition of the formed nitrides due to excessive heat.

[0021] If there is a large difference in nitrogen concentration between the surface and the interior, it will lead to a significant decrease in the performance of the membrane. The heat preservation time of the present invention can make the originally stagnant nitrogen atoms move from the clustered areas to the empty areas, completely eliminating the unevenness of nitrogen concentration. In a high vacuum environment, without air interference, nitrogen atoms can diffuse freely, and residual impurities on the surface can also be removed, making the nitride particles finer and more uniform.

[0022] This invention releases all the internal stress of the nitriding layer, allowing the membrane and the substrate to adhere more stably together, significantly improving the bonding strength. At the same time, the nitrides are denser and more evenly distributed, greatly enhancing the hydrogen barrier effect.

[0023] The revolutionary performance of composite membranes is reflected in:

[0024] Hydrogen barrier performance: In standard electrochemical hydrogen permeation tests, its steady-state hydrogen permeation current density is reduced to less than 1 / 10 compared to the pure iron matrix, resulting in a more than 10-fold improvement in hydrogen barrier performance. In optimal embodiments, this improvement can reach 13 to 15 times. Hydrogen permeability (J) is not higher than 0.140 × 10⁻⁶. -5 mol·cm⁻¹·s -1 The effective hydrogen diffusion coefficient (Deff) is significantly reduced.

[0025] Mechanical properties: Its surface Vickers hardness is increased by more than 6.17% compared with the sample treated only by gas nitriding, and its internal hardness (at 50 μm from the surface) is increased by more than 1.84%; nanoindentation test shows that it has higher elastic modulus, elastic limit and resistance to plastic deformation.

[0026] Membrane-substrate adhesion: Nanoindentation and scratch experiments show that the indentation morphology is regular, with no obvious cracks or peeling at the edges. The critical load (Lc2) of the scratch test is more than 30% higher than that of the single nitrided sample, demonstrating extremely excellent membrane-substrate adhesion and mechanical stability.

[0027] Microstructure stability: After long-term aging at high temperature (300℃, 100h) or thermal cycling (-40℃ to 150℃, 100 times) tests, its microstructure did not show obvious coarsening or degradation, and the hydrogen barrier performance decay rate was less than 5%, demonstrating excellent long-term service reliability.

[0028] The pure iron nitrided composite membrane of the present invention can be used as a high-efficiency hydrogen barrier in various hydrogen-related equipment, and its application scenarios include, but are not limited to:

[0029] Conventional hydrogen energy equipment includes surface protection for key components such as hydrogen pipelines (especially long-distance high-pressure pipelines), stationary / mobile hydrogen storage containers (Type I-IV cylinders), fuel cell metal bipolar plates, hydrogen compressor cylinders, valves, pump bodies, seals, and welded joints.

[0030] Extreme operating conditions and emerging fields: Hydrogen aviation: hydrogen fuel supply pipelines and turbopump components for jet engines; Deep-sea storage and transportation: hydrogen storage devices and their connection systems operating in deep-sea environments. Chemical processes: internal components of hydrogenation and dehydrogenation reactors involving high temperatures (≤550℃), high pressures (≤100MPa), or corrosive media containing trace amounts of hydrogen sulfide, carbon dioxide, etc.

[0031] Customized components: High-performance hydrogen barrier protection can be achieved by adjusting the process parameters of this invention for ultra-thin (<1mm) flexible diaphragms, complex geometric components, and thick-walled (>10mm) pressure-bearing components.

[0032] Preferably, the pure iron nitriding composite membrane consists of a surface compound layer, an intermediate diffusion layer, and an internal nitrogen diffusion solid solution layer from the outside to the inside.

[0033] The surface compound layer consists of ε-Fe 2-3 Composed of N phase and γ′-Fe4N phase, ε-Fe 2-3 N is 60%-70%, γ′-Fe4N phase is 30%-40%, ε-Fe 2-3 The N phase and γ′-Fe4N phase are diffusely distributed;

[0034] Compared with a single gas nitriding layer, the surface compound layer of the present invention has significantly fewer coarse needle-like nitrides, and the structure is more dense and uniform.

[0035] The intermediate diffusion layer is composed of ε-Fe 2-3 Composed of N phase and γ′-Fe4N phase, ε-Fe 2-3 The content of the N phase is less than that of the γ′-Fe4N phase, and the content of the ε-Fe 2-3 The N phase and γ′-Fe4N phase are dispersedly distributed, and the nitrogen content in the intermediate diffusion layer is 1.5%-3%. The nitrogen concentration is distributed in a gentle gradient from the surface of the matrix to the interior of the matrix.

[0036] The intermediate diffusion layer is highly saturated with nitrogen, and contains a large number of fine, diffusely distributed nitride precipitates. The nitrogen concentration is distributed in a gentle gradient from the matrix surface to the matrix interior.

[0037] The internal nitrogen diffusion solid solution layer consists of an α-Fe phase on the substrate surface. Nitrogen atoms are uniformly distributed in the interstices between adjacent α-Fe phases. The solid solution of nitrogen atoms causes a slight distortion in the α-Fe phase lattice, which strengthens the substrate surface.

[0038] In the internal nitrogen diffusion solid solution layer, nitrogen atoms are dissolved in the interstices of adjacent α-Fe phases, resulting in a more uniform distribution. Rapid cooling in existing technologies easily generates thermal stress, leading to a decrease in membrane-substrate adhesion. This invention ensures the stability of the microstructure, allowing the hydrogen barrier layer (surface compound layer - intermediate diffusion layer - internal nitrogen diffusion solid solution layer) to be intact without cracks or peeling. After thermal cycling tests, the hydrogen barrier performance decay rate is <5%. Compared with the cooling methods in existing technologies, the long-term service reliability is significantly improved, which is an important guarantee for the composite membrane to adapt to extreme working conditions.

[0039] Preferably, the process of removing the substrate after the temperature inside the tubular vacuum heat treatment furnace in step 4 has dropped to room temperature includes the following steps:

[0040] The temperature inside the tubular vacuum heat treatment furnace is controlled to drop below 150℃ at a rate of ≤2℃ / min, allowing the substrate to cool naturally to ≤60℃. Nitrogen gas with a purity of ≥99.999% is slowly introduced into the tubular vacuum heat treatment furnace to bring the gas pressure inside the furnace to atmospheric pressure. The substrate is then removed, and a pure iron nitrided composite diaphragm is obtained on the substrate.

[0041] This invention uses ultra-slow controlled cooling at ≤2℃ / min to minimize thermal stress, avoid microcracks caused by sudden temperature drops during cooling, and stabilize the microstructure after vacuum heat treatment.

[0042] Preferably, a controlled atmosphere well-type nitriding furnace pretreatment step is included between step 1 and step 2:

[0043] The substrate was placed in the homogenization zone of a controlled atmosphere pit-type nitriding furnace. After sealing the furnace, nitrogen gas with a purity ≥99.999% was introduced into the controlled atmosphere pit-type nitriding furnace at a flow rate of 0.3 Nm³. 3 / h-0.7Nm 3 / h, continue gas washing until the residual oxygen content in the furnace drops to ≤10ppm.

[0044] The controlled atmosphere pit nitriding furnace is equipped with an oxygen content sensor, which is connected to a display screen located on the furnace. The oxygen content sensor monitors the oxygen content within the furnace, allowing operators to determine the appropriate oxygen level by observing the displayed value and thus decide whether to stop the nitrogen supply.

[0045] This invention utilizes a large flow rate of high-purity nitrogen (0.3 Nm³). 3 / h-0.7Nm 3 ( / h) Long-term gas washing, combined with real-time oxygen content monitoring, reduces the residual oxygen content in the controllable atmosphere well nitriding furnace to ≤10ppm, prevents sample surface oxidation during nitriding, and ensures the purity of the nitriding atmosphere.

[0046] Existing technologies suffer from insufficient gas scrubbing and lack real-time monitoring, which can easily lead to the formation of an oxide film on the surface that hinders nitrogen atom penetration. This invention eliminates oxidation interference and precisely ensures that the environmental atmosphere remains stable during the nitriding process, creating conditions for the formation of fine, dispersed nitrides. This results in a denser oxide film with better hydrogen barrier performance.

[0047] Preferably, the following steps are included before step 1:

[0048] The substrate is placed in a box-type resistance furnace, and argon gas with a purity of ≥99.999% is introduced into the box-type resistance furnace as a protective atmosphere. The temperature inside the box-type resistance furnace is raised to 880℃-920℃ and held for 30-60 minutes. After the substrate cools to room temperature with the protective atmosphere, it is taken out.

[0049] During transportation, the matrix structure may be damaged by external forces, resulting in unevenness. Heating pretreatment can make the initial structure grain size uniform, avoid fluctuations in the thickness of the nitrided layer due to uneven structure during the subsequent nitriding process, and ensure the consistency of hydrogen barrier performance.

[0050] Preferably, step 1 includes the following specific steps:

[0051] Remove oxide scale, scratches, and deformation layers from the substrate surface;

[0052] The substrate undergoes ultrasonic cleaning, plasma cleaning, and drying protection.

[0053] Ultrasonic cleaning: Place the polished substrate in a pH neutral aqueous surfactant solution, use ultrasonic power of 100W-150W, and clean for 10-20 minutes to remove oil and abrasive particles.

[0054] Plasma cleaning: The substrate is placed in a microwave plasma cleaning device, and argon and hydrogen are introduced in a volume ratio of 9:1. The power is 100W-200W, and the plasma is bombarded for 5-10 minutes to remove atomic-level contaminants and extremely thin oxide films.

[0055] Drying and protection: Dry the substrate with nitrogen gas of ≥99.999% purity, and then transfer the substrate into a vacuum drying oven with a vacuum degree ≤10Pa.

[0056] Ultrasonic cleaning removes surface oil and abrasive particles, while microwave plasma (argon-hydrogen mixed atmosphere) bombards and strips atomically adsorbed contaminants and extremely thin oxide films, significantly enhancing surface activity. Traditional acetone or ethanol cleaning cannot remove atomically impurities, easily leading to weak bonding of the nitriding layer. This invention can thoroughly purify the surface, allowing nitrogen atoms to directly contact the base metal atoms, strengthening the initial bonding force of nitriding. At the same time, drying and protection prevent secondary contamination, resulting in smaller performance fluctuations between batches.

[0057] As a preferred method, the vacuuming of the tubular vacuum heat treatment furnace includes the following specific steps:

[0058] Rough pumping: Turn on the mechanical pump and pump until the vacuum level reaches 1Pa-10Pa;

[0059] High vacuum pumping: Start the diffusion pump and continue pumping until the vacuum level stabilizes at ≤5.0×10-3Pa.

[0060] This invention first uses a mechanical pump to quickly remove most of the air in the furnace, reducing the vacuum level to 1Pa-10Pa, thus initially reducing air interference; then a diffusion pump is used to completely remove the remaining trace amounts of air and surface-adsorbed impurities (such as water vapor and oil stains), ultimately achieving a high vacuum of ≤5.0×10-3Pa, with almost no excess molecules in the furnace.

[0061] The benefits of high vacuum: without air obstruction, nitrogen atoms can diffuse freely, eliminating impurities and allowing for smooth atomic migration during subsequent heat treatment; removing surface impurities to avoid affecting the nitride structure formed after nitriding; and isolating oxygen to prevent oxidation of the nitrided layer at high temperatures.

[0062] Therefore, the present invention has the following beneficial effects:

[0063] Vacuum heat treatment promotes nitrogen diffusion and the formation of denser nitrides, resulting in a modified layer with higher hardness on the surface. Furthermore, nitrogen atoms form a nitrogen diffusion solid solution layer within the material. The dissolution of nitrogen atoms in the metal lattice causes lattice distortion, which can be considered a solid solution strengthening mechanism. This further enhances the surface and internal hardness of the composite membrane (surface hardness increased by 6.17%, internal hardness increased by 1.84%). Vacuum heat treatment effectively promotes the diffusion and uniform distribution of nitrogen atoms into the iron matrix, eliminates internal stress generated during nitriding, and optimizes the microstructure of the nitrided layer. This significantly strengthens the bond between the nitride layer and the iron matrix. SEM observation of the N-Fe-HT sample surface morphology after nanoindentation testing revealed that the indentation morphology was regular and crack-free, exhibiting better mechanical stability and resistance to deformation.

[0064] The microstructure of the nitriding layer was precisely controlled. Vacuum heat treatment refined and reduced the coarse needle-like nitrides in the compound layer, making the nitride precipitates in the diffusion layer more dispersed and uniform, forming a dense composite hydrogen barrier layer with a gradient structure, which reduced the channels for hydrogen atom diffusion.

[0065] A revolutionary improvement in hydrogen barrier performance was achieved through a combined process of gas nitriding and vacuum heat treatment, which produced a significant synergistic effect. After low-temperature, long-term nitriding, a fine and dispersed ε-Fe oxide layer was formed on the surface. 2-3 A composite phase layer of N and γ′-Fe4N is constructed to create a preliminary dense hydrogen-blocking microstructure. Vacuum heat treatment allows nitrogen atoms to gain energy for secondary diffusion, penetrating into the inner layer and smoothing the nitrogen concentration gradient between the inner and outer surfaces. Nitrogen atoms in the diffusion layer interact with hydrogen atoms, altering their diffusion path and rate, resulting in a more significant hydrogen-blocking effect. After heat treatment, the microstructure of the diffusion layer differs from the matrix material, containing fine nitride particles or dislocations. These defects scatter and hinder hydrogen diffusion, enabling the final composite membrane to achieve a hydrogen-blocking performance 13 times that of a pure iron matrix, a performance improvement far exceeding the 3-fold increase achieved by single-gas nitriding treatment, thus solving the problem of insufficient hydrogen-blocking performance with single technologies. Attached Figure Description

[0066] Figure 1 This is a cross-sectional gold-to-metal ratio diagram of Fe, N-Fe, and N-Fe-HT samples from Example 1 of this invention;

[0067] Figure 2 This is a comparison of X-ray diffraction (XRD) patterns of Fe, N-Fe, and N-Fe-HT samples from Example 1 of this invention;

[0068] Figure 3This is a comparison diagram of the Vickers hardness distribution of Fe, N-Fe, and N-Fe-HT samples in Example 1 of this invention;

[0069] Figure 4 This is a comparison of nanoindentation hardness distribution diagrams and typical indentation scanning electron microscope (SEM) morphology of Fe, N-Fe, and N-Fe-HT samples from Example 1 of this invention.

[0070] Figure 5 This is a comparison of electrochemical hydrogen permeation curves of Fe, N-Fe, and N-Fe-HT samples from Example 1 of this invention. Detailed Implementation

[0071] The present invention will now be further described with reference to the accompanying drawings and specific embodiments.

[0072] Example 1

[0073] This embodiment describes a method for preparing a high hydrogen-barrier pure iron nitrided composite membrane, comprising the following steps:

[0074] Step 1: Select purchased DT4A industrial pure iron plate as the base material. First, perform solution annealing on the iron plate: hold at 900℃ for 30 minutes, then cool with the furnace to obtain a uniform initial structure. Then, wire cut the treated iron plate into circular samples with a diameter of 30mm and a thickness of 3mm. All sample surfaces are successively polished with 400-grit, 800-grit, 1200-grit, and 2000-grit sandpaper, and then mechanically polished with diamond polishing paste until the surface is mirror-like and free of visible scratches. After polishing, the samples are ultrasonically cleaned for 15 minutes each in a mixture of acetone, anhydrous ethanol (volume ratio 1:1), and deionized water to thoroughly remove surface contaminants. After cleaning, the samples are rinsed with anhydrous ethanol and then dried in a drying oven at 50℃ for later use. The samples in this state are labeled as Fe.

[0075] Step 2,

[0076] The Fe sample was placed in a controlled atmosphere well-type nitriding furnace, and high-purity (99.999%) nitrogen gas was introduced into the furnace at a flow rate of 0.5 Nm³. 3 Perform gas scrubbing at a rate of / h for 2 hours to ensure the oxygen content in the furnace atmosphere is reduced to an extremely low level (<10ppm); after scrubbing, shut off the nitrogen supply and switch to pure ammonia (NH3), with the flow rate precisely controlled at 0.2Nm³. 3The furnace pressure was maintained at 0.02 MPa for 1 h. Then, the furnace temperature was raised to 480°C at a rate of 10°C / min and held at this temperature for 4 hours to carry out the nitriding reaction. After the nitriding process was completed, the heating power was turned off and the Fe sample was allowed to cool naturally to room temperature (about 30°C) in the ammonia atmosphere. The sample after this step was called N-Fe.

[0077] Step 3, Vacuum heat treatment:

[0078] The N-Fe sample was placed in a quartz glass boat and sent into the homogenization zone of the tubular vacuum heat treatment furnace. After sealing the furnace tube, the vacuum system was started. First, the mechanical pump was turned on for rough evacuation. After the vacuum level reached 1 Pa, the molecular pump was turned on for high vacuum evacuation. Evacuation was continued for 2 hours to make the vacuum level in the furnace reach and stabilize at 1.3 × 10-3 Pa.

[0079] Start the heating program and raise the furnace temperature to 500℃ at a heating rate of 10℃ / min, and hold the temperature precisely at this temperature for 1 hour. After the holding period, stop heating and allow the N-Fe sample to cool naturally to below 60℃ under high vacuum conditions. Pour high-purity nitrogen into the furnace to atmospheric pressure and remove the sample. Label the sample with the final nitrided composite membrane as N-Fe-HT.

[0080] The pure iron nitriding composite diaphragm consists of a surface compound layer, an intermediate diffusion layer, and an internal nitrogen diffusion solid solution layer from the outside to the inside.

[0081] The surface compound layer consists of ε-Fe 2-3 Composed of N phase and γ′-Fe4N phase, ε-Fe 2-3 N is 65%, and γ′-Fe4N phase is 35%;

[0082] The intermediate diffusion layer is composed of ε-Fe 2-3 Composed of N phase and γ′-Fe4N phase, ε-Fe 2-3 The content of the N phase is less than that of the γ′-Fe4N phase, the nitrogen content in the intermediate diffusion layer is 2%, and the nitrogen concentration is distributed in a gentle gradient from the surface of the matrix to the interior of the matrix.

[0083] The internal nitrogen diffusion solid solution layer consists of an α-Fe phase on the substrate surface. Nitrogen atoms are uniformly distributed in the interstices between adjacent α-Fe phases. The solid solution of nitrogen atoms causes a slight distortion in the α-Fe phase lattice, which strengthens the substrate surface.

[0084] Performance characterization and results analysis:

[0085] 1. Cross-sectional sections of Fe, N-Fe, and N-Fe-HT samples were cut, mounted, polished, and etched with 3% nitric acid alcohol solution for 10 seconds. Their microstructures were then observed under an optical microscope. The results are as follows: Figure 1 As shown:

[0086] Figure 1 a(Fe): The pure iron matrix exhibits a typical polycrystalline equiaxed crystal morphology with a uniform and dense structure;

[0087] Figure 1 b(N-Fe): A uniformly thick nitride layer can be clearly observed on the sample surface. This nitride layer exhibits a distinct two-layer structure: the outermost layer is a compound layer, inside which numerous black, slender needle-like or dot-like nitrides (mainly ε-Fe) are visible. 2-3 (N phase and γ′-Fe4N phase); Below the compound layer is a diffusion layer, which is darker in color than the matrix, indicating that nitrogen is saturated and a small amount of nitrides have precipitated;

[0088] Figure 1 c(N-Fe-HT): Compared with N-Fe, the microstructure of the material underwent significant evolution after vacuum heat treatment. The most obvious change was the sharp reduction in the coarse needle-like nitrides inside the surface compound layer, resulting in a denser and more uniform microstructure. At the same time, the contrast of the intermediate diffusion layer became more gradual, indicating a reduction in the nitrogen distribution gradient and a significant improvement in the overall microstructure uniformity. This indicates that vacuum heat treatment effectively promoted the further diffusion and rearrangement of nitrogen atoms, achieving "phase structure reconstruction".

[0089] 2. Phase analysis (XRD):

[0090] Phase analysis of Fe, N-Fe, and N-Fe-HT samples was performed using X-ray diffraction, and the results are as follows: Figure 2 As shown.

[0091] As can be seen, obvious ε-Fe was observed in both the N-Fe and N-Fe-HT samples. 2-3 The diffraction peaks of the N phase and γ′-Fe4N phase confirmed the successful formation of the nitride. Compared with pure Fe, the peak intensity and position of the α-Fe phase also showed slight changes, indicating that nitrogen solid solution caused lattice distortion. The differences in intensity, position, and shape of the diffraction peaks fully demonstrate that vacuum heat treatment profoundly altered the crystal structure of iron nitride, affecting the relative content of each phase, grain orientation, and microstrain, which corroborates the observation results of the microstructure.

[0092] 3. Mechanical property testing:

[0093] Vickers hardness and nanoindentation: The hardness distribution was tested along the sample cross-section using a Vickers hardness tester (load 0.98 kg), and the results are as follows. Figure 3 As shown.

[0094] The hardness of the Fe sample remained low (approximately 100 HV) and stable throughout the entire depth range; the hardness of the N-Fe and N-Fe-HT samples increased significantly in the near-surface region and gradually decreased with increasing depth, exhibiting a typical gradient characteristic; importantly, the hardness of the N-Fe-HT sample was generally higher than that of the N-Fe sample at all depths, especially exhibiting the highest hardness value near the surface; quantitative analysis showed that the surface hardness of N-Fe-HT was increased by 6.17% and the internal hardness by 1.84% compared to N-Fe, mainly due to the vacuum heat treatment promoting the formation of denser and more uniform nitrides.

[0095] like Figure 4 As shown, Figure 4 a indicates that the average nanoindentation hardness of N-Fe-HT is higher than that of N-Fe at all depths.

[0096] Indentation morphology analysis: SEM observation of the indentations after nanoindentation ( Figure 4 b- Figure 4 e).

[0097] Figure 4 b (SEM image of the indentation surface of the N-Fe sample): The edge of the indentation is relatively sharp, with visible plastic deformation bulges around it, but no cracks.

[0098] Figure 4 c (SEM image of the indentation depth of the N-Fe sample): The indentation edges are relatively smooth, and the deformation area is larger. I indicates depth.

[0099] Figure 4 d (SEM image of the indentation surface of the N-Fe-HT sample): The indentation edges are very sharp, the shape is regular, and the surrounding deformation area is significantly smaller than that of the indentation. Figure 4 c indicates that the material surface is harder and more brittle, but combined with the characteristic of no cracks, it shows that its strength and toughness are well-balanced.

[0100] Figure 4 e (SEM image of the indentation depth of the N-Fe-HT sample): Indentation morphology ratio Figure 4 c is more regular, and the deformation region is between Figure 4 b and Figure 4 Between c; the above morphological features intuitively demonstrate that vacuum heat treatment not only improves surface hardness, but also optimizes the overall deformation behavior of the material, making it more uniform and stable.

[0101] 4. Hydrogen barrier performance evaluation (electrochemical hydrogen permeation):

[0102] The hydrogen barrier performance of the samples was tested using a dual-electrolysis cell electrochemical hydrogen permeation method. The detection end solution was 0.2 mol / L NaOH, and the hydrogen charging end solution was 0.5 mol / L NaOH + 0.2 g / L CS(NH2)2 (thiourea) solution. The hydrogen charging current density was 15 mA / cm². 2 Before testing, the sample detection end needs to be nickel-plated. The hydrogen permeation curve is shown below. Figure 5 As shown, the calculation results of the key parameters are shown in Table 2.

[0103] Table 1 Hydrogen permeation parameters of Fe, N-Fe, N-Fe-HT and N-Fe-HT-RS samples

[0104]

[0105] like Figure 5 As shown in the hydrogen permeation curves, the current density of Fe increases the fastest and has the highest stability; the current density of N-Fe increases slowly and has a significantly lower stability than that of Fe; while the current density of N-Fe-HT increases extremely slowly and has an extremely low stability.

[0106] In Table 1, i represents the steady-state hydrogen permeation current density, which characterizes the rate at which hydrogen permeates through the material. The smaller the value, the slower the rate and the stronger the hydrogen blocking ability.

[0107] J is hydrogen permeability, which characterizes the amount of hydrogen flux passing through a unit thickness of material per unit time and unit area. The lower the value, the better the hydrogen barrier performance.

[0108] Deff is the effective hydrogen diffusion coefficient, which characterizes the average diffusion rate of hydrogen atoms in a material. The lower the value, the more difficult it is for hydrogen atoms to move, and the better the hydrogen barrier effect.

[0109] cap is the number of hydrogen atoms temporarily stored per unit volume within a material, but which will be released later; a stable value is acceptable.

[0110] As calculated from the data in Table 1, compared to pure iron (Fe):

[0111] The hydrogen barrier performance of N-Fe was improved by about 3 times, and that of N-Fe-HT by about 13 times. After grinding off the surface of N-Fe-HT to obtain N-Fe-HT-RS, its hydrogen permeability was restored to a level close to that of N-Fe, proving that the surface modification layer is the main reason for the significant improvement in hydrogen barrier performance. N-Fe-HT had the lowest effective hydrogen diffusion coefficient, indicating that its microstructure constitutes the strongest obstacle to hydrogen atom diffusion. The apparent hydrogen solubility of N-Fe-HT was also low, which is related to the fact that the uniform diffusion layer inside hinders the interaction between hydrogen and the deeper matrix.

[0112] Example 2

[0113] The difference between this embodiment and Embodiment 1 is that the substrate is a 10mm thick DT4 plate, simulating a thick-walled hydrogen storage component; the nitriding parameters in step 2 are adjusted to: temperature 470℃, time 8 hours, to ensure sufficient time for nitrogen to diffuse to deeper areas; the vacuum heat treatment parameters in step 3 are adjusted to: temperature 490℃, using segmented heat preservation (first 480℃ / 1h, then 500℃ / 1h), total heat preservation time 2 hours, and cooling rate controlled between 0.5℃ / min and 200℃. Testing showed a significant increase in hardness (~150HV) even at 5mm from the surface. Hydrogen permeation tests under simulated deep-sea pressure (70MPa) showed that its hydrogen barrier performance was still 9.5 times that of pure iron substrate, demonstrating excellent deep-sea hydrogen barrier capability and adaptability.

[0114] Example 3

[0115] This embodiment explores performance stability at higher temperatures. In step 2, the nitriding temperature is increased to 520°C for 2 hours to form nitrides with higher thermal stability. In step 3, the vacuum heat treatment temperature is increased to 550°C for 0.5 hours. After the obtained N-Fe-HT sample is exposed to air at 400°C for 100 hours, no obvious oxidation or peeling of the surface compound layer is observed, and the hydrogen barrier performance (test temperature 200°C) decay rate is <8%, demonstrating excellent high-temperature stability.

[0116] Example 4

[0117] This embodiment targets an ultra-thin pure iron membrane with a thickness of 0.5 mm. Pretreatment requires extreme care to prevent deformation. Step 2 employs a lower temperature (450℃) and shorter time (1.5 hours) nitriding process to prevent excessive nitriding leading to brittleness. Step 3 involves vacuum heat treatment at 500℃ for 0.5 hours, with an appropriately accelerated cooling rate. The final product, while maintaining flexibility, achieves hydrogen barrier performance eight times that of the substrate, meeting the application requirements for flexible fuel cell bipolar plates and other applications.

[0118] Comparative Example 1: Nitriding with a single gas

[0119] Step 2 was performed without subsequent heat treatment; the hydrogen barrier performance of the sample N-Fe was only 3.1 times that of pure iron, the hardness improvement was limited, the critical scratch load Lc2 was 35N, and there were obvious needle-like nitrides in the microstructure.

[0120] Comparative Example 2: Single Vacuum Heat Treatment

[0121] The sample Fe-HT underwent a single heat treatment test; its hydrogen barrier properties and hardness showed no statistically significant difference compared to pure iron, and no nitride phase was detected by XRD.

[0122] Comparative Example 3:

[0123] The only difference from Example 1 is that the pretreatment used acetone + ethanol ultrasonic cleaning instead of plasma cleaning; the performance of the resulting N-Fe-HT sample was slightly lower than that of Example 1, with hydrogen barrier performance being 12.8 times that of pure iron, and the performance fluctuation between batches was slightly larger, which proves the positive effect of plasma cleaning on improving performance and consistency.

[0124] Comparative Example 4: Compared with PVD-TiN coating

[0125] A 2 μm thick TiN coating (magnetron sputtering) was prepared on the same pure iron substrate. Its hydrogen barrier performance was approximately 11 times that of pure iron, but the critical load for scratch testing was only 28 N, and the film-substrate adhesion was significantly worse than that of the N-Fe-HT of this invention. In thermal shock testing… After water quenching (20 times), the TiN coating showed localized peeling, while the N-Fe-HT remained intact.

[0126] In summary, through innovative composite process design and in-depth mechanism exploration, this invention has successfully prepared a high-performance, highly reliable, and widely applicable pure iron nitriding composite membrane, providing a highly competitive technical path for solving the hydrogen embrittlement problem in hydrogen energy equipment.

[0127] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a high hydrogen-barrier pure iron nitrided composite diaphragm, characterized in that, Includes the following steps: Step 1: Pre-treat the surface of the substrate made of pure iron by grinding, cleaning and drying; Step 2, The substrate is placed in a controlled atmosphere pit nitriding furnace, and pure ammonia gas is introduced into the furnace until the pressure reaches 0.02 MPa. The temperature inside the furnace is raised to 460℃-500℃ at a heating rate of 5℃ / min-15℃ / min, and held at that temperature for 2-6 hours to nitrid the substrate. After nitriding, the substrate is allowed to cool naturally to room temperature with the furnace before being removed. Step 3, Vacuuming: The substrate is placed in a quartz glass boat, which is then placed into the homogenization zone of a tubular vacuum heat treatment furnace. The furnace is then evacuated until the vacuum level stabilizes at ≤5.0×10-3 Pa, and this vacuum state is maintained for 25-35 minutes. Step 4, Vacuum heat treatment: The temperature inside the tubular vacuum heat treatment furnace is raised to 480℃-520℃ at a rate of 5℃ / min-15℃ / min and held for 0.5 hours-2 hours; the substrate is removed after the temperature inside the tubular vacuum heat treatment furnace drops to room temperature.

2. The method for preparing the high hydrogen-barrier pure iron nitrided composite diaphragm according to claim 1, characterized in that, The pure iron nitriding composite diaphragm consists of a surface compound layer, an intermediate diffusion layer, and an internal nitrogen diffusion solid solution layer from the outside to the inside. The surface compound layer consists of ε-Fe 2-3 Composed of N phase and γ′-Fe4N phase, ε-Fe 2-3 N is 60%-70%, γ′-Fe4N phase is 30%-40%, ε-Fe 2-3 The N phase and γ′-Fe4N phase are diffusely distributed; The intermediate diffusion layer is composed of ε-Fe 2-3 Composed of N phase and γ′-Fe4N phase, ε-Fe 2-3 The content of the N phase is less than that of the γ′-Fe4N phase, and the content of the ε-Fe 2-3 The N phase and γ′-Fe4N phase are dispersedly distributed, and the nitrogen content in the intermediate diffusion layer is 1.5%-3%. The nitrogen concentration is distributed in a gentle gradient from the surface of the matrix to the interior of the matrix. The internal nitrogen diffusion solid solution layer consists of an α-Fe phase on the substrate surface. Nitrogen atoms are uniformly distributed in the interstices between adjacent α-Fe phases. The solid solution of nitrogen atoms causes a slight distortion in the α-Fe phase lattice, which strengthens the substrate surface.

3. The method for preparing the high hydrogen-barrier pure iron nitrided composite diaphragm according to claim 1, characterized in that, Step 4 involves removing the substrate after the temperature inside the tubular vacuum heat treatment furnace has dropped to room temperature. This includes the following steps: The temperature inside the tubular vacuum heat treatment furnace is controlled to drop below 150℃ at a rate of ≤2℃ / min, allowing the substrate to cool naturally to ≤60℃. Nitrogen gas with a purity of ≥99.999% is slowly introduced into the tubular vacuum heat treatment furnace to bring the gas pressure inside the furnace to atmospheric pressure. The substrate is then removed, and a pure iron nitrided composite diaphragm is obtained on the substrate.

4. The method for preparing the high hydrogen-barrier pure iron nitrided composite diaphragm according to claim 1, characterized in that, Between step 1 and step 2, there is also a pretreatment step using a controlled atmosphere pit nitriding furnace: The substrate was placed in the homogenization zone of a controlled atmosphere pit-type nitriding furnace. After sealing the furnace, nitrogen gas with a purity ≥99.999% was introduced into the controlled atmosphere pit-type nitriding furnace at a flow rate of 0.3 Nm³. 3 / h-0.7Nm 3 / h, continue gas washing until the residual oxygen content in the furnace drops to ≤10ppm.

5. The method for preparing the high hydrogen-barrier pure iron nitrided composite diaphragm according to claim 1, characterized in that, The following steps precede step 1: The substrate is placed in a box-type resistance furnace, and argon gas with a purity of ≥99.999% is introduced into the box-type resistance furnace as a protective atmosphere. The temperature inside the box-type resistance furnace is raised to 880℃-920℃ and held for 30-60 minutes. After the substrate cools to room temperature with the protective atmosphere, it is taken out.

6. The method for preparing the high hydrogen-barrier pure iron nitrided composite diaphragm according to claim 1, characterized in that, Step 1 includes the following specific steps: Remove oxide scale, scratches, and deformation layers from the substrate surface; The substrate undergoes ultrasonic cleaning, plasma cleaning, and drying protection. Ultrasonic cleaning: Place the polished substrate in a pH neutral aqueous surfactant solution, use ultrasonic power of 100W-150W, and clean for 10-20 minutes to remove oil and abrasive particles. Plasma cleaning: The substrate is placed in a microwave plasma cleaning device, and argon and hydrogen are introduced in a volume ratio of 9:

1. The power is 100W-200W, and the plasma is bombarded for 5-10 minutes to remove atomic-level contaminants and extremely thin oxide films. Drying and protection: Dry the substrate with nitrogen gas of ≥99.999% purity, and then transfer the substrate into a vacuum drying oven with a vacuum degree ≤10Pa.

7. The method for preparing a high hydrogen-barrier pure iron nitrided composite diaphragm according to any one of claims 1, 2, 3, 4, 5, or 6, characterized in that, The specific steps for evacuating a tubular vacuum heat treatment furnace are as follows: Rough pumping: Turn on the mechanical pump and pump until the vacuum level reaches 1Pa-10Pa; High vacuum pumping: Start the diffusion pump and continue pumping until the vacuum level stabilizes at ≤5.0×10-3Pa.

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