Method for modifying surface of high-chromium ferritic stainless steel for bipolar plate of fuel cell

Abstract

The invention provides a surface modification method of high-chromium ferritic stainless steel for a fuel cell bipolar plate, and relates to the technical field of fuel cell surface engineering. The method comprises the following steps: annealing a high-chromium ferritic stainless steel cold-rolled substrate at 900-1000 DEG C to obtain a cold-rolled thin strip of which equiaxed ferritic grains are completely recrystallized inside, then removing a surface oxide layer, and carrying out constant-potential anodic passivation treatment in a nitric acid solution. According to the method, the dense gradient passive film rich in Cr2O3 is constructed in situ by utilizing the high oxidation potential of nitric acid electrochemical passivation. The method is suitable for a ferritic stainless steel system with the Cr content of 25 wt%-35 wt%, and especially when a matrix contains trace Ru, the method can induce a surface layer to form conductive RuO2 and construct a metal Ru nano seepage network in the film. According to the bipolar plate material treated through the method, the after-fracture elongation is kept to be larger than or equal to 25%, meanwhile, the interface contact resistance is reduced to 10 m omega.cm or below, the corrosion current density is lower than 10 microamperes / cm, and the strict service requirement of a fuel cell is met.

Description

A method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates Technical Field

[0001] This invention relates to the field of fuel cell surface engineering technology, and in particular to a method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates. Background Technology

[0002] Proton exchange membrane fuel cells (PEMFCs) are considered an ideal power source for future electric vehicles and stationary power plants due to their high energy conversion efficiency, low operating temperature, and zero emissions. Bipolar plates, as the core component of the PEMFC stack, primarily perform functions such as single-cell series conduction, reactant gas distribution, heat management, and support for the membrane electrode assembly (MEA). They account for approximately 60%–80% of the stack's weight and 30%–40% of its cost.

[0003] Currently, bipolar plate materials are mainly divided into three categories: graphite, composite materials, and metallic materials. Among them, stainless steel (especially ferritic stainless steel) has become the preferred material for automotive fuel cell bipolar plates due to its high strength, good ductility (easy to stamp and form), low permeability, and relatively low cost. However, stainless steel bipolar plates face a severe challenge in commercial applications due to the mutual constraints between corrosion resistance and conductivity: ① The internal environment of a fuel cell is acidic (pH 2~4) and has a high operating potential (up to 1.5V vs. SHE during start-up / stop). The stainless steel matrix is ​​extremely prone to corrosion, and the precipitated metal ions (such as Fe) can easily lead to corrosion. 3+ Ni 2+ (1) It will poison the proton exchange membrane and catalyst, leading to battery performance degradation; (2) To prevent corrosion, a chromium-rich oxide passivation film (mainly Cr2O3) naturally forms on the surface of stainless steel. However, this passivation film has semiconductor properties and high surface resistivity, which will increase the internal resistance of the battery and significantly reduce the output power of the stack.

[0004] To address the aforementioned issues, existing technologies primarily employ surface coating modification. Common technical routes include: ① Noble metal plating (such as gold plating, platinum plating): While offering excellent conductivity and corrosion resistance, these are extremely expensive, failing to meet the demands of large-scale commercialization. ② Carbon-based coatings (such as graphite, DLC, conductive polymers): Relatively low cost, but complex preparation processes (typically requiring PVD or CVD), and the difference in thermal expansion coefficients between the coating and the metal substrate, leading to pinholes, cracks, or peeling under long-term thermal cycling and impact, resulting in substrate corrosion failure. ③ Surface nitriding / carburization (such as hot nitriding, plasma nitriding): While improving surface hardness and conductivity, high-temperature treatment easily leads to substrate grain coarsening and the formation of brittle phases (such as Cr2N), severely deteriorating the material's plasticity and making it difficult to meet the requirements of precision stamping of electrode plates.

[0005] Besides applying an external coating, another technical approach is to optimize the properties of the passivation film itself by adjusting the alloy composition and combining it with chemical / electrochemical treatments (i.e., intrinsic modification). For example, increasing the Cr content (>25%) can significantly improve corrosion resistance, but this usually exacerbates the insulation of the passivation film, resulting in contact resistance far exceeding the standard set by the U.S. Department of Energy (DOE) (10 mΩ·cm²). Traditional pickling processes (such as ordinary mixed acid pickling) can only remove the surface oxide scale and cannot fundamentally change the semiconductor band structure of the passivation film; while conventional electrochemical polishing (such as sulfuric acid-phosphoric acid systems) can reduce roughness, the stability of the film at high potentials is still insufficient.

[0006] The invention, titled "A Ferritic Stainless Steel for Fuel Cell Bipolar Plates and Its Preparation Method Thereof" (202510151353.X), details the preparation method of the substrate but does not cover the surface modification method, resulting in deficiencies in conductivity and corrosion resistance.

[0007] Therefore, the key technical challenge to achieve the domestic production of low-cost, high-performance metal bipolar plates is to construct a dense, corrosion-resistant, and highly conductive gradient functionalized passivation film on the surface of ultra-high chromium ferritic stainless steel in situ through a combination of specific preparation processes and surface electrochemical modification, without relying on expensive and easily peelable external coatings, while maintaining the good stamping performance of the substrate. Summary of the Invention

[0008] To address the technical challenges of existing technologies for high-chromium ferritic stainless steel used in fuel cell bipolar plates, such as the trade-off between corrosion resistance and conductivity, poor adhesion and high cost of traditional surface coatings, and the difficulty of achieving a balance between high corrosion resistance, high conductivity, and high formability in conventional surface treatments, this invention provides a method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates. This method, through the synergistic effect of a specific annealing process and nitric acid electrochemical modification technology, combined with specific alloying components of the substrate, constructs a passivation film with excellent electronic conductivity and dense barrier properties in situ on the stainless steel surface, while retaining the good processing and formability of the substrate. The technical solution is as follows:

[0009] A method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates, the method comprising:

[0010] S1. Anneal the high-chromium ferritic stainless steel cold-rolled substrate in the range of 900~1000℃, and then rapidly cool it to obtain a cold-rolled strip with fully recrystallized equiaxed ferrite grains inside.

[0011] S2. The cold-rolled strip obtained in S1 is placed in a nitric acid solution and used as a working electrode for constant potential anodic polarization treatment. A passivation film is grown in situ on the surface of the cold-rolled strip. After cleaning, the modified high-chromium ferritic stainless steel for fuel cell bipolar plates is obtained.

[0012] The high-chromium ferritic stainless steel cold-rolled substrate in S1 has a Cr content of 25.0 wt% to 35.0 wt%. Within this Cr content range, the substrate possesses the basic ability to resist active dissolution in a strongly acidic environment.

[0013] The thickness of the cold-rolled strip obtained in S1 is 0.05~0.5mm.

[0014] The chemical composition of the high-chromium ferritic stainless steel, by mass percentage, also includes one or more alloying elements in the following amounts: Ru: 0.01~0.50%, Sn: 0~0.50%, Sb: 0~0.30%, Cu: 0~0.60%, Nb: 0.15~0.50%, Ti: 0.03~0.20%, Mo: 0~5.00%, and ≤5.0% Ni, with the balance being Fe and unavoidable impurities. The dual stabilization design of Nb and Ti helps to fix interstitial atoms C and N, prevent intergranular corrosion, and improve machinability.

[0015] The key to S1 lies in precisely controlling the annealing process: Too low a temperature or too short a time will result in insufficient recrystallization, residual work hardening, and reduced plasticity; too high a temperature or too long a time will lead to grain coarsening, reduced strength, and deterioration of the surface quality after forming (such as the orange peel effect). Therefore, the annealing time is controlled to be 0.5~5 min. The annealing process used in this invention aims to obtain a fully recrystallized, uniform equiaxed ferrite grain structure. The cold-rolled strip after annealing has an elongation at break ≥25%, and the microstructure consists of uniform recrystallized equiaxed ferrite grains with an average size of 10~40 μm. Simultaneously, it retains an appropriate amount of subgrain structure, exhibiting excellent formability and meeting the requirements of precision stamping of bipolar plates.

[0016] The concentration of nitric acid solution in S2 is 0.5~2.0 mol / L.

[0017] The polarization treatment in S2 is performed at a temperature of 35~65℃, with a constant anodic potential of 0.8~5.0 V (vs.Ag / AgCl) and a treatment time of 0.5~90 minutes.

[0018] At this high oxidation potential, nitric acid solution can selectively and preferentially dissolve unstable iron oxides (such as FeO and FeOOH) and loose chromium hydroxides in the surface layer, promoting surface element rearrangement and in-situ growth of a dense passivation film rich in Cr2O3. To ensure the quality stability of the anodic passivation film, it is recommended to remove the oxide layer formed on the surface of the cold-rolled strip during annealing by grinding or other methods before performing the above electrochemical passivation treatment.

[0019] When the Ru content in the high-chromium ferritic stainless steel is 0.01%~0.50%, the S2 will induce the passivation film to form a gradient structure: the outermost layer is enriched with conductive RuO2, which significantly reduces the contact resistance of the outermost layer; the inner layer is a highly dense Cr2O3-rich barrier layer, and the inner layer contains diffusely distributed metallic Ru nanoparticles. These nanoparticles construct an electronic percolation network in the insulating oxide matrix.

[0020] The modified fuel cell bipolar plate obtained in S1 is made of high-chromium ferritic stainless steel at 140 N / cm. 2 The interfacial contact resistance under assembly pressure is ≤10 mΩ·cm², and the steady-state corrosion current density in the simulated fuel cell cathode environment is ≤10 μA / cm²; the potential of the fuel cell cathode environment is 0.8 V vs. RHE.

[0021] When the high-chromium ferritic stainless steel contains Ru, the steady-state corrosion current density after surface modification is ≤1 μA / cm².

[0022] The beneficial effects of the technical solutions provided in the embodiments of the present invention include at least the following:

[0023] 1. It resolves the contradiction between high corrosion resistance and low contact resistance:

[0024] This invention abandons the traditional physical coating approach and reconstructs the band structure and chemical composition of the intrinsic passivation film through nitric acid electrochemical passivation technology. For general high-Cr steel, this method removes the high-resistivity iron oxides / hydroxides on the surface, significantly reducing the interfacial contact resistance (down to below 10 mΩ·cm²). For Ru-containing preferred steel grades, by constructing a gradient conductive channel of "surface RuO2 conductivity + inner layer metallic Ru permeation", the interfacial contact resistance is further reduced to about 4 mΩ·cm², far exceeding the US Department of Energy (DOE) standard.

[0025] 2. Achieved a synergistic effect of excellent formability and corrosion resistance:

[0026] Traditional high-temperature nitriding modification can cause matrix embrittlement. However, the annealing process at 900~1000℃ used in this invention precisely controls the recrystallization behavior, allowing the high-Cr substrate to maintain an elongation at break of ≥25%, which greatly facilitates the stamping and forming of the electrode plate. At the same time, the dense Cr2O3-rich layer formed by electrochemical modification endows the material with excellent corrosion resistance. Under simulated fuel cell cathode environment (0.8 V vs. RHE), the steady-state corrosion current density can be as low as 0.5 μA / cm², which meets the requirements for long-term service.

[0027] 3. The membrane structure is stable and possesses self-healing capabilities:

[0028] Compared to easily peeling external physical coatings, the passivation film prepared in this invention is grown in situ and has extremely strong adhesion to the substrate. In particular, the Ru-containing gradient passivation film can participate in the repassivation process when pitting or mechanical damage occurs, maintaining the conductivity and protective function of the film layer, and significantly improving the overall life cycle reliability of the bipolar plate.

[0029] 4. Low process cost, suitable for large-scale production:

[0030] The nitric acid electrochemical treatment process used in this method is simple and controllable, requiring no expensive vacuum coating equipment (such as PVD) or precious metal electroplating solutions (such as gold plating). Furthermore, the treatment solution can be recycled, making it environmentally friendly and highly promising for industrial application. Attached Figure Description

[0031] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0032] Figure 1 shows the experimental steel treated with different surface treatment processes according to the embodiments of the present invention at 140 N / cm. 2 Comparison of interfacial contact resistance (ICR) under compaction pressure;

[0033] Figure 2 shows the steel of the embodiment provided in this invention after being treated by the method of this invention in a simulated fuel cell environment (pH 2.3 + F). - The potentiodynamic polarization curves in the figure;

[0034] Figure 3 is a current density-time (It) curve of Embodiment 4 of the present invention during 8 hours of constant potential polarization at an anode potential of 0.8 V;

[0035] Figure 4 is a peak fitting diagram of the XPS spectra of Ru 3d at different sputtering times in Example 4 of the present invention. Detailed Implementation

[0036] The technical solution of the present invention will now be described with reference to the accompanying drawings.

[0037] In embodiments of the present invention, words such as "exemplarily," "for example," etc., are used to indicate that something is an example, illustration, or description. Any embodiment or design described as "exemplary" in the present invention should not be construed as being more preferred or advantageous than other embodiments or designs. Specifically, the use of the word "exemplary" is intended to present the concept in a concrete manner. Furthermore, in embodiments of the present invention, the meaning expressed by "and / or" can be both, or either one.

[0038] In this embodiment of the invention, sometimes a subscript such as W1 may be written in a non-subscript form such as W1. When the difference is not emphasized, the meaning they express is the same.

[0039] To make the technical problems, technical solutions and advantages of the present invention clearer, a detailed description will be given below in conjunction with the accompanying drawings and specific embodiments.

[0040] This invention provides a method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates. The method may include the following steps:

[0041] S1. Annealing a high-chromium ferritic stainless steel cold-rolled substrate in the range of 900~1000℃ to obtain a cold-rolled strip with fully recrystallized equiaxed ferrite grains inside.

[0042] S2. The cold-rolled strip obtained in S1 is placed in a nitric acid solution and used as a working electrode for constant potential anodic polarization treatment. A passivation film is grown in situ on the surface of the cold-rolled strip. After cleaning, the modified high-chromium ferritic stainless steel for fuel cell bipolar plates is obtained.

[0043] The following description, in conjunction with specific embodiments, illustrates this point.

[0044] The following embodiments demonstrate the surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to the method of the present invention. The specific steps are as follows:

[0045] 1. Obtain the bipolar plate substrate according to the preparation method described in "A Ferritic Stainless Steel for Fuel Cell Bipolar Plates and Its Preparation Method" (202510151353.X).

[0046] The ingredients are prepared according to the composition shown in Table 1 below, and the mixture is melted and cast into ingots using a 50kg vacuum induction furnace. The ingots are heated to 1200℃ and held for 2 hours for homogenization treatment, then forged into 35mm thick slabs, and then heated to 1200℃ and held for 2 hours before hot rolling to a thickness of 2.0mm.

[0047] The hot-rolled sheet is annealed at 1020℃ for 10 minutes, followed by pickling to remove surface oxide scale. The pickled sheet is then cold-rolled in multiple passes to obtain a cold-rolled strip of 0.05-0.2 mm. The cold-rolled strip is annealed at 900-1000℃ for 0.5-5 minutes, followed by air cooling to room temperature. This step aims to obtain a fully recrystallized equiaxed ferrite structure with an average grain size of 10-40 μm, while retaining an appropriate amount of substructure to improve plasticity.

[0048] 2. Surface Modification (Electrochemical Passivation): The annealed cold-rolled strip was mechanically polished and then placed in an electrolytic cell as the working electrode. The electrolyte was a 1.6 mol / L nitric acid (HNO3) solution. Polarization was performed at a constant anodic potential of 3.0 V (vs. Ag / AgCl) at 40°C for 5 minutes. After treatment, the strip was rinsed with deionized water and dried.

[0049] Table 1. Composition / wt% and annealing process of the examples

[0050]

[0051] Mechanical properties and 140 N / cm were tested on the stainless steel samples modified by the above method. 2 The results of the interfacial contact resistance (ICR) and corrosion resistance tests under compaction pressure are shown in Table 2.

[0052] Table 2 Electrochemical and Mechanical Properties

[0053]

[0054] As shown in Table 2, Example 4 (containing Ru) exhibits the best overall performance. In contrast, Example 5, due to the addition of a higher Ni content, shows higher strength but significantly reduced corrosion resistance (higher corrosion current density), which may be due to the change in surface film structure caused by Ni. Example 8, due to its lower Cr content and the absence of any modifying elements, exhibits higher contact resistance.

[0055] Mechanical properties (see Table 2): After being treated with the specific rapid annealing process of this invention, all the examples obtained a fully recrystallized equiaxed ferrite structure, and the elongation after fracture reached or exceeded 25%, showing excellent stamping performance.

[0056] Interfacial contact resistance (see Figure 1): After electrochemical modification with nitric acid according to the present invention, Example 4 (containing Ru) exhibits the best conductivity, with an ICR as low as 4.0 mΩ·cm², far exceeding the 10 mΩ·cm² standard set by the U.S. Department of Energy (DOE). The ICRs of other examples are close to or lower than 10 mΩ·cm², demonstrating that the process of the present invention can significantly improve the conductivity of ordinary high-Cr stainless steel without relying on precious metals, highlighting the significant advantage of the nitric acid electrochemical passivation process of the present invention in reducing contact resistance.

[0057] Corrosion resistance (see Figures 2 and 3): In a simulated fuel cell cathode environment (pH 2.3 + F), - At a constant potential of 0.8 V, the steady-state corrosion current density of Example 4 remained highly stable during the 8-hour test, without any current fluctuations. This is attributed to the gradient functionalized passivation film formed on its surface. The outer conductive RuO2 layer works synergistically with the inner dense Cr2O3 and metallic Ru nanoparticles (see Figure 4, showing the gradient distribution of Ru elements in the passivation film), achieving a perfect balance between high conductivity and high corrosion resistance. Figure 2 shows the extremely low self-corrosion current density and stable wide passivation range of the material obtained by the method of the present invention, and Figure 3 shows the long-term stability of the passivation film obtained by the present invention under operating conditions.

[0058] In summary, the surface modification method provided by this invention, through the organic combination of annealing microstructure control and nitric acid electrochemical film formation technology, successfully solves the technical problem of balancing corrosion resistance and conductivity of high-chromium ferritic stainless steel in fuel cell bipolar plate applications.

[0059] The above description is merely a specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should be included within the scope of protection of the present invention. Therefore, the scope of protection of the present invention should be determined by the scope of the claims.

Claims

1. A method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates, characterized in that, The method includes: S1, annealing a high-chromium ferritic stainless steel cold-rolled substrate in the range of 900~1000℃ to obtain a cold-rolled strip with fully recrystallized equiaxed ferrite grains inside; S2, placing the cold-rolled strip obtained in S1 in a nitric acid solution as a working electrode and performing constant potential anodic polarization treatment to grow a passivation film in situ on the surface of the cold-rolled strip. After cleaning, the modified high-chromium ferritic stainless steel for fuel cell bipolar plates is obtained.

2. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The Cr content in the high-chromium ferritic stainless steel cold-rolled substrate of S1 is 25.0wt%~35.0wt%.

3. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The thickness of the cold-rolled strip obtained in S1 is 0.05~0.5mm.

4. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 2, characterized in that, The chemical composition of the high-chromium ferritic stainless steel, by mass percentage, also includes one or more alloying elements in the following amounts: Ru: 0.01~0.50%, Sn: 0~0.50%, Sb: 0~0.30%, Cu: 0~0.60%, Nb: 0.15~0.50%, Ti: 0.03~0.20%, Mo: 0~5.00%, and ≤5.0% Ni, with the balance being Fe and unavoidable impurities.

5. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The annealing time in S1 is 0.5~5min, the elongation after fracture of the annealed cold-rolled strip is ≥25%, and the microstructure is uniform recrystallized equiaxed ferrite grains with an average size of 10~40μm.

6. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The concentration of nitric acid solution in S2 is 0.5~2.0 mol / L.

7. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The polarization treatment in S2 is performed at a temperature of 35~65℃, with a constant anodic potential of 0.8~5.0 V (vs.Ag / AgCl) and a treatment time of 0.5~90 minutes.

8. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The oxide layer on the surface of the annealed cold-rolled strip is removed before the polarization treatment in S2.

9. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, When the Ru content in the high-chromium ferritic stainless steel is 0.01%~0.50%, the passivation film in S2 will form a gradient structure: the outermost layer is enriched with conductive RuO2, the inner layer is a Cr2O3-rich barrier layer, and the inner layer contains diffusely distributed metallic Ru nanoparticles.

10. The method for surface modification of high-chromium ferritic stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The modified fuel cell bipolar plate obtained in S1 is made of high-chromium ferritic stainless steel at 140 N / cm. 2 The interfacial contact resistance under assembly pressure is ≤10 mΩ·cm², and the steady-state corrosion current density in the simulated fuel cell cathode environment is ≤10 μA / cm²; the potential of the fuel cell cathode environment is 0.8 V vs. RHE; when the high-chromium ferritic stainless steel contains Ru, the steady-state corrosion current density after surface modification is ≤1 μA / cm².

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