Ferritic stainless steel for fuel cell bipolar plates, method for controlling surface roughness, method for forming passivation film and use

Abstract

The application discloses a ferritic stainless steel for a fuel cell bipolar plate, a surface roughness control method, a passivation film forming method and use. The ferritic stainless steel comprises C 0.03wt.% or less, N 0.02wt.% or less, Si 0.4wt.% or less, Mn 0.5wt.% or less, Cr 16-23wt.%, Cu 0-2.0wt.%, Mo 1.8-2.5wt.%, Ni 0.2-2.0wt.%, Ti 0.1-0.5wt.%, Nb 0.005-0.5wt.%, P 0.02wt.% or less, S 0.02wt.% or less, and the balance of Fe and other elements unavoidably contained, and the ferritic stainless steel has a grain size of 4-9 levels. The ferritic stainless steel has good corrosion resistance, electric conductivity, elongation and deformation capacity, and has economic and cost advantages.

Description

[0001] This invention is a divisional application of application number 2022101556508, filed on February 21, 2022, entitled Ferritic stainless steel for fuel cell bipolar plates, method for controlling surface roughness, method for forming passivation film and its use. Technical Field

[0002] This invention relates to the field of stainless steel production technology, and specifically to a ferritic stainless steel for fuel cell bipolar plates, a method for controlling surface roughness, a method for forming a passivation film, and its applications. Background Technology

[0003] A fuel cell is a device that directly converts the chemical energy of fuel into electrical energy. Proton exchange membrane fuel cells (PEMFCs) are a type of fuel cell. Due to their high power generation efficiency and environmental friendliness, PEMFCs have received worldwide attention and are developing rapidly. Bipolar plates, as the core component of fuel cells, play crucial roles in supporting the membrane electrode structure, separating hydrogen and oxygen, collecting electrons, conducting heat, providing hydrogen and oxygen pathways, discharging water produced in the reaction, and providing coolant flow channels. With increasing demands for high volumetric power density in fuel cells, metal bipolar plates, due to their high strength and toughness, can be manufactured into thinner bipolar plates through stamping, leading to their increasingly widespread application.

[0004] However, the poor corrosion resistance and high interfacial contact resistance of metal bipolar plates in hydrogen fuel cell environments have always been challenging problems. The industry urgently needs a metal material with strong corrosion resistance, good conductivity, and low cost to meet the large-scale demand brought about by the rapid development of the industry. In 2017, South Korean researchers disclosed a stainless steel with excellent contact resistance for polymer fuel cell separators and its manufacturing method (KR:013918 / 2017; CN:110199047B / 2017). This stainless steel product is currently used in the production of modern fuel cell vehicles, and its conductivity and corrosion resistance meet the standards set by the U.S. Department of Energy. In 2010, Japanese researchers disclosed a corrosion-resistant stainless steel for fuel cells used in JFE and its manufacturing method (CN:102471916 / 2010; JP:062739 / 2010). This material will be used in fuel cell buses.

[0005] With the rapid development of the industry in recent years, the requirements for product consistency have increased accordingly. The main problem with existing ferritic stainless steel bipolar plate manufacturing technologies lies in the poor deformability caused by compositional design issues, leading to difficulties in forming during bipolar plate processing. Bipolar plate deformation also results in poor sealing. Furthermore, the high cost due to the large number of alloying elements further exacerbates the problem. On the other hand, the passivation film on the stainless steel surface, as an important corrosion-resistant functional layer, suffers from thin and discontinuous layers due to manufacturing process issues, resulting in poor corrosion resistance. The uncontrollable growth and composition of the passivation film also lead to unstable conductivity and corrosion resistance. In addition, besides stainless steel, titanium is another mainstream material for metal bipolar plates. However, titanium has relatively poor deformability as a bipolar plate material, making stamping more difficult, and its cost is much higher than stainless steel. These factors all affect key issues such as plate consistency, corrosion resistance, and economy, which urgently need to be addressed. Summary of the Invention

[0006] In view of the above-mentioned problems in the prior art, the purpose of the present invention is to provide a ferritic stainless steel for fuel cell bipolar plates, a method for controlling surface roughness, a method for forming a passivation film, and its application.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] In a first aspect, the present invention provides a ferritic stainless steel for a fuel cell bipolar plate, wherein, based on a mass of 100 wt.% of the ferritic stainless steel, the ferritic stainless steel comprises:

[0009]

[0010]

[0011] The balance is Fe and other elements that are unavoidably present. Among the unavoidable impurities, O is preferably less than 0.02 wt.% and Sn is less than 0.1%.

[0012] The ferritic stainless steel has a grain size of 4 to 9, such as 4, 5, 6, 7, or 8, preferably 6 to 8. This ensures that the stainless steel material has appropriate processability (facilitating rolling, heat treatment, and other processes), which is beneficial for the processing and forming of bipolar plate materials while also being economical.

[0013] Carbon (C) has a solid solution strengthening effect, but its solubility in ferrite is very low. Excess carbon precipitates as carbides. Simultaneously, C reacts with Cr to form Cr carbonitrides, causing intergranular corrosion and chromium depletion at grain boundaries in ferritic stainless steel, affecting the material's mechanical and weldability properties. In the ferritic stainless steel of this invention, the C content is 0.03 wt.% or less, for example, 0.03 wt.%, 0.02 wt.%, or 0.01 wt.%, preferably 0.02 wt.% or less.

[0014] Nitrogen (N) reacts with Cr to form Cr carbonitrides, creating Cr-depleted regions and reducing the corrosion resistance of the stainless steel. In the ferritic stainless steel of this invention, the N content is less than 0.02 wt.%, for example, 0.02 wt.% or 0.01 wt.%.

[0015] Si is a useful element for deoxidation. However, as the content increases, the material's processing performance decreases. In the ferritic stainless steel of the present invention, the Si content is 0.4 wt.% or less, for example, 0.4 wt.%, 0.35 wt.%, 0.3 wt.%, 0.25 wt.%, 0.2 wt.%, 0.15 wt.%, 0.1 wt.%, 0.08 wt.%, 0.05 wt.%, or 0.03 wt.%, etc.

[0016] Mn is an unavoidable element in steel, which, in addition to its deoxidizing effect, can also improve the strength of the steel. However, as an impurity, MnS can become the starting point for corrosion, reducing corrosion resistance. In the ferritic stainless steel of this invention, the Mn content is less than 0.5 wt.%, for example, 0.5 wt.%, 0.47 wt.%, 0.45 wt.%, 0.4 wt.%, 0.35 wt.%, 0.3 wt.%, 0.25 wt.%, 0.2 wt.%, 0.15 wt.%, 0.1 wt.%, 0.08 wt.%, 0.05 wt.%, or 0.03 wt.%, etc.

[0017] Cr is the fundamental element determining the corrosion resistance of ferritic stainless steel. Chromium reacts with oxygen in the corrosive medium to form a thin oxide film on the steel surface, which can prevent further corrosion of the steel matrix. However, increasing the chromium content accelerates the formation and precipitation of α and σ phases, resulting in decreased toughness and a significant increase in the brittle transition temperature, which is detrimental to the processing in the stainless steel manufacturing process. In the ferritic stainless steel of this invention, the Cr content is 16–23 wt.%, for example, 16 wt.%, 16.5 wt.%, 16.8 wt.%, 17 wt.%, 17.5 wt.%, 18 wt.%, 18.5 wt.%, 19 wt.%, 19.5 wt.%, 20 wt.%, 20.5 wt.%, 21 wt.%, 21.5 wt.%, 22 wt.%, 22.5 wt.%, or 23 wt.%, etc.

[0018] Cu is an element that improves the corrosion resistance of stainless steel and can also improve the cold working properties of the material. In the ferritic stainless steel of the present invention, the Cu content is 0 to 2.0 wt.%, for example, 0 wt.%, 0.05 wt.%, 0.1 wt.%, 0.2 wt.%, 0.25 wt.%, 0.3 wt.%, 0.35 wt.%, 0.4 wt.%, 0.5 wt.%, 0.55 wt.%, 0.6 wt.%, 0.7 wt.%, 0.8 wt.%, 1 wt.%, 1.2 wt.%, 1.3 wt.%, 1.4 wt.%, 1.6 wt.%, 1.8 wt.%, or 2 wt.%, etc.

[0019] Mo is another key element for improving the corrosion resistance of stainless steel. It promotes the passivation of Fe-Cr alloys and enhances the steel's corrosion resistance in reducing media, particularly its resistance to pitting and crevice corrosion in chloride solutions. However, higher Mo content can lead to the formation of ferrite σ phase and other brittle phases, resulting in decreased toughness and increased strength, which is detrimental to material processing. In the ferritic stainless steel of this invention, the Mo content is 1.8–2.5 wt.%, for example, 1.8 wt.%, 1.85 wt.%, 1.9 wt.%, 2.0 wt.%, 2.1 wt.%, 2.2 wt.%, 2.3 wt.%, 2.4 wt.%, or 2.5 wt.%.

[0020] Ni is an element that improves the corrosion resistance of stainless steel and also reduces contact resistance. In the ferritic stainless steel of this invention, the Ni content is 0.2–2.0 wt.%, for example, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, 0.5 wt.%, 0.6 wt.%, 0.8 wt.%, 1.0 wt.%, 1.1 wt.%, 1.3 wt.%, 1.4 wt.%, 1.5 wt.%, 1.7 wt.%, 1.8 wt.%, or 2.0 wt.%.

[0021] Both Ti and Nb preferentially combine with C and N to form carbonitrides, thereby suppressing the decrease in corrosion resistance caused by the precipitation of Cr carbonitrides. However, if the content is too high, the workability decreases. In the ferritic stainless steel of the present invention, the Ti content is 0.1 to 0.5 wt.%, for example, 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, or 0.5 wt.%. In the ferritic stainless steel of the present invention, the Nb content is 0.005 to 0.5 wt.%, for example, 0.1 wt.%, 0.2 wt.%, 0.3 wt.%, 0.4 wt.%, or 0.5 wt.%, preferably 0.1 to 0.4 wt.%.

[0022] In the ferritic stainless steel of the present invention, the content of P is less than 0.02 wt.%, for example, 0.02 wt.% or 0.01 wt.%.

[0023] In the ferritic stainless steel of the present invention, the content of S is 0.02 wt.% or less, for example, 0.02 wt.% or 0.01 wt.%, preferably 0.01 wt.% or less.

[0024] In addition to the above, for the purpose of improving corrosion resistance, it may contain 0 to 1 wt.% V and / or 0 to 1 wt.% W respectively. To achieve this effect, the content of both elements is preferably 0.1 wt.% or more.

[0025] As a preferred technical solution of the present invention, V and W will preferentially combine with C instead of Cr, thereby improving the corrosion resistance of the material and having a synergistic effect with Nb to a certain extent; at the same time, in order to maintain suitable material processing performance, the amount of Nb added should be appropriately reduced when adding V and W.

[0026] To improve hot workability, it may contain 0.0002 to 1 wt.% rare earth metals, preferably Ce or Y. To achieve this effect, it is preferable to contain 0.0005 wt.% or more.

[0027] The ferritic stainless steel of this invention features a low variety of alloying elements and a low Cr content. Through the design of the types and contents of each element, it achieves excellent corrosion resistance, electrical conductivity, elongation, and deformation capacity, while also possessing economic and cost advantages. Preferably, the Ni content is 0.9–1.2 wt.%, further reducing costs while still satisfying the aforementioned effects.

[0028] The ferritic stainless steel of the present invention has the advantages of lower cost and better forming and processing performance compared with other metal bipolar plates (such as titanium bipolar plates).

[0029] Preferably, the surface roughness of the ferritic stainless steel is between 100 and 700 nm, such as 100 nm, 150 nm, 170 nm, 200 nm, 230 nm, 260 nm, 300 nm, 325 nm, 350 nm, 380 nm, 400 nm, 435 nm, 460 nm, or 500 nm. If the roughness is too small, the interfacial contact resistance increases significantly, the internal resistance of the fuel cell increases, and it cannot achieve good adhesion with the gas diffusion layer, so it is not suitable for application. If the roughness is too large, although the contact resistance is low, the corrosion resistance of the material is significantly reduced, which cannot meet the requirements of bipolar plates in the fuel cell and acidic environment. Preferably, it is 100 to 600 nm, and more preferably 200 to 500 nm.

[0030] As a preferred technical solution for the ferritic stainless steel described in this invention, a passivation film is provided on the surface of the ferritic stainless steel, including a p-type passivation film and an n-type passivation film. This passivation film, as an important corrosion-resistant and conductive functional layer, has performance advantages. Its technical principle is as follows: the p-type passivation film can effectively prevent corrosion caused by contact between the solution and the substrate, and the n-type passivation film can effectively prevent the dissolution of metal ions, thereby effectively reducing the adverse effects of metal ions on other core components of the fuel cell (such as proton exchange membranes, catalysts, etc.), and improving the performance and lifespan of the fuel cell stack.

[0031] In this invention, the p-type passivation film corresponds to the p-type semiconductor region, and the n-type passivation film corresponds to the n-type semiconductor region.

[0032] In this invention, p-type passivation film refers to p-type semiconductor passivation film, and n-type passivation film refers to n-type semiconductor passivation film.

[0033] In this invention, the positional relationship between the p-type and n-type passivation films is not specifically limited; for example, the passivation film can be an inner n-type and an outer p-type film. The inner layer refers to the side of the passivation film closer to the stainless steel main material, and the outer layer refers to the side closer to the solution.

[0034] Preferably, the molar ratio of chromium hydroxide to chromium oxide in the p-type passivation film is I. p [Cr(OH)3 / Cr2O3], I p The [Cr(OH)3 / Cr2O3] ratio is not less than 10, for example, 10, 11, 12, 13, 14, 15, 16, 17, 18, 20, 23, or 25, preferably not less than 15. Excellent corrosion resistance can be obtained by optimizing the molar ratio of chromium hydroxide to chromium oxide in the p-type passivation film. The higher the ratio, the higher the proportion of hydroxide and the better the corrosion resistance.

[0035] Preferably, the molar ratio of chromium hydroxide to chromium oxide in the n-type passivation film is I. n [Cr(OH)3 / Cr2O3], I n The [Cr(OH)3 / Cr2O3] ratio is not greater than 10, for example, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1 or 0.5, preferably less than 5. Excellent conductivity can be obtained by optimizing the molar ratio of chromium hydroxide to chromium oxide in the n-type passivation film. The lower the ratio and the higher the oxide ratio, the better the conductivity.

[0036] Preferably, I p [Cr(OH)3 / Cr2O3] / I n [Cr(OH)3 / Cr2O3]>3, preferably I p [Cr(OH)3 / Cr2O3] / I n[Cr(OH)3 / Cr2O3]≥4. For example, 3.5, 4, 5, 6, 7, 8, 9, or 10, etc. If I p [Cr(OH)3 / Cr2O3] / I n If [Cr(OH)3 / Cr2O3]≤3, the overall performance of the passivation film cannot be guaranteed.

[0037] In this invention, I p [Cr(OH)3 / Cr2O3] / I n [Cr(OH)3 / Cr2O3] is abbreviated as I p / I n .

[0038] Preferably, the thickness of the passivation film is 5–20 nm, such as 5 nm, 6 nm, 8 nm, 10 nm, 12 nm, 13 nm, 15 nm, 16 nm, 18 nm, 19 nm, or 20 nm, with 10–15 nm being more preferred. If the passivation film is too thin (less than 5 nm), its corrosion resistance is poor; if it is too thick (greater than 20 nm), its conductivity is poor. The preferred thickness range of 10–15 nm better balances good corrosion resistance and conductivity.

[0039] Preferably, the thickness of the p-type passivation film is t. p The thickness of the n-type passivation film is t. n 0.2 <t p / t n <0.6, t p / t n For example, 0.25, 0.3, 0.35, 0.4, 0.45, 0.5, or 0.55. Under these conditions, excellent interfacial contact resistance can be better guaranteed.

[0040] Preferably, in the passivation film, the inner layer is an n-type passivation film and the outer layer is a p-type passivation film, and it satisfies: 0.2 <t p / t n <0.6.

[0041] In this preferred technical solution, the passivation film structure is an inner n-type (good conductivity) and an outer p-type (good corrosion resistance). The p-type passivation film is thinner and the n-type passivation film is thicker. This structure determines that it has both good corrosion resistance and conductivity.

[0042] Secondly, the present invention provides a method for controlling the surface roughness of stainless steel, the method comprising:

[0043] A stainless steel main material is provided, and the stainless steel main material is electrolyzed in an acid solution. During the electrolysis process, the polarization voltage satisfies the following formula (I):

[0044] E ≥ lg D + 12 + pH (Ⅰ)

[0045] Where E is the polarization voltage, the unit of which is V; D is the grain size of the stainless steel main material, the unit of which is micrometer; and pH is the pH value of the initial acid solution.

[0046] The control method of the present invention uses electrolysis to control the surface roughness of the stainless steel main material, which has the advantage of controllable surface roughness. It can be specifically adjusted according to the characteristics of membrane electrode and gas diffusion in proton exchange membrane fuel cells, so that the bipolar plate and related components can be well bonded, thereby reducing the contact resistance of the battery system and making it more suitable for industry application characteristics.

[0047] In the method of the present invention, if E does not satisfy formula (Ⅰ), the roughness may not change significantly, and the optimal roughness to match the gas diffusion layer cannot be achieved, thereby adversely affecting the performance of the fuel cell stack.

[0048] This control method can be based on the ferritic stainless steel described in the first aspect. It can also be based on other stainless steel materials in the art.

[0049] Wherein, "based on the ferritic stainless steel described in the first aspect" means that the stainless steel main material (e.g., stainless steel sheet) is made of the ferritic stainless steel described in the first aspect, for example, by smelting, hot rolling and cold rolling.

[0050] Preferably, the polarization voltage is 5 to 15V, such as 5V, 6V, 8V, 9V, 10V, 12V, 13V or 15V.

[0051] Preferably, the electrolysis time is 10 to 300 s, such as 10 s, 15 s, 20 s, 25 s, 30 s, 35 s, 40 s, 45 s, 50 s, 60 s, 65 s, 70 s, 75 s, 80 s, 85 s, 90 s, 100 s, 120 s, 130 s, 140 s, 150 s, 160 s, 170 s, 180 s, 200 s, 220 s, 230 s, 240 s, 260 s, 280 s, or 300 s, and more preferably 20 to 120 s.

[0052] Preferably, the electrolysis temperature is 25 to 70°C, such as 25°C, 27°C, 30°C, 35°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, or 70°C, with 25 to 40°C being the most preferred.

[0053] Preferably, the acid used in the electrolysis process is sulfuric acid, or a mixture of sulfuric acid and hydrohalic acid. By combining sulfuric acid and hydrohalic acid, the roughness preparation time can be shortened, and the roughness can be appropriately increased.

[0054] In this invention, hydrohalic acid is abbreviated as HX acid, where X is a halogen, such as F, Cl, Br or I.

[0055] Preferably, the hydrohalic acid is at least one of hydrofluoric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid, and more preferably hydrochloric acid and / or hydrofluoric acid.

[0056] Preferably, the concentration of the sulfuric acid is 0.1–14 mol / L, for example, 0.1 mol / L, 0.3 mol / L, 0.5 mol / L, 0.8 mol / L, 1 mol / L, 1.5 mol / L, 1.7 mol / L, 2 mol / L, 2.2 mol / L, 2.5 mol / L, 2.8 mol / L, 3 mol / L, 3.5 mol / L, 3.8 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L, 7 mol / L, 7.5 mol / L, 8 mol / L, 8.5 mol / L, 9 mol / L, 9.5 mol / L, 10 mol / L, 11 mol / L, 12 mol / L, 13 mol / L, or 14 mol / L, and preferably 0.1–7 mol / L.

[0057] Preferably, in the mixed acid solution of sulfuric acid and hydrohalic acid, the concentration of hydrohalic acid is 0-3 mol / L and does not contain 0, for example, 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.25 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.7 mol / L, 1 mol / L, 1.5 mol / L, 1.7 mol / L, 1.9 mol / L, 2 mol / L, 2.2 mol / L, 2.4 mol / L, 2.5 mol / L, 2.8 mol / L or 3 mol / L, etc., preferably not exceeding 0.5 mol / L.

[0058] Thirdly, the present invention provides a method for forming a passivation film on a stainless steel surface, wherein the passivation film is prepared by an electrochemical passivation method, the method comprising the following steps:

[0059] A stainless steel main material is provided, and a three-electrode system is adopted. The stainless steel main material, the counter electrode and the reference electrode are placed in an electrochemical passivation solution and subjected to constant potential polarization to form a passivation film on the surface of the stainless steel main material.

[0060] In a three-electrode system, the stainless steel main material is the sample to be treated, and the other two electrodes are the counter electrode and the reference electrode, respectively. Generally, the reference electrode is located between the sample to be treated and the counter electrode.

[0061] This invention forms a passivation film on the surface of stainless steel through electrochemical passivation. The resulting passivation film exhibits excellent performance, is dense and continuous, and effectively enhances corrosion resistance and reduces contact resistance. Furthermore, the passivation solution and method are environmentally friendly, ensuring excellent contact resistance (meeting US DOE standards) even under ultra-low concentration or hydrofluoric acid-free conditions.

[0062] The electrochemical passivation method of the present invention can control the composition, structure and thickness of the passivation film. The control of the passivation film can be designed based on specific material composition and achieved through the electrochemical passivation process.

[0063] In the method for forming a passivation film on a stainless steel surface, the main stainless steel material involved can be based on the ferritic stainless steel described in the first aspect, or on the stainless steel after the roughness has been adjusted in the second aspect, or on other stainless steel materials in the art.

[0064] In this invention, the preparation method of the stainless steel main material is existing technology. Those skilled in the art can refer to the existing technology for preparation. Exemplarily, but not limitingly, it can be prepared according to the following method:

[0065] The ingot is prepared according to the composition of the ferritic stainless steel described in the first aspect above. The ingot is then roughened to a certain thickness (e.g., 80-120 mm) to obtain a stainless steel plate, which is then hot-rolled. The heating and holding temperature is 1150-1200℃, the holding time is 1.5-2.0 h, the initial rolling temperature is controlled at 1100-1150℃, and the plate is rolled to a certain thickness (e.g., 2-3 mm) in 8-10 passes. The final rolling temperature is controlled above 800℃.

[0066] After hot rolling, the coil undergoes annealing at 950℃~1050℃, with the holding time depending on the size of the hot-rolled coil. It is then pickled after hot rolling. After 8~10 passes of cold rolling to the desired thickness, it undergoes continuous annealing for 1~3 minutes.

[0067] The method of the present invention ensures excellent corrosion resistance and electrical conductivity by preparing a controllable roughness on the stainless steel surface and then forming a passivation film.

[0068] Preferably, the electrochemical passivation solution is a nitric acid solution with a concentration of 0.05–10 mol / L, such as 0.5 mol / L, 1 mol / L, 1.5 mol / L, 2 mol / L, 2.5 mol / L, 3 mol / L, 3.5 mol / L, 4 mol / L, 4.5 mol / L, 5 mol / L, 5.5 mol / L, 6 mol / L, 6.5 mol / L, 7 mol / L, 7.5 mol / L, 8 mol / L, 8.5 mol / L, 9 mol / L, or 10 mol / L, etc., and the concentration of the nitric acid solution is preferably 1.5–5 mol / L.

[0069] Preferably, the electrochemical passivation temperature is 20–85°C, such as 20°C, 25°C, 30°C, 33°C, 35°C, 38°C, 40°C, 45°C, 50°C, 55°C, 60°C, 65°C, 70°C, 75°C, 80°C, or 85°C, and more preferably 35–65°C.

[0070] Preferably, the anode voltage of the electrochemical passivation is not less than 0.45V, such as 0.5V, 0.6V, 0.7V, 0.8V, 0.9V, 1.0V, 1.1V, 1.2V, 1.3V or 1.4V, and more preferably 0.8V to 1.2V.

[0071] Preferably, the electrochemical passivation time is 5 to 120 min, such as 5 min, 10 min, 15 min, 20 min, 25 min, 30 min, 35 min, 40 min, 45 min, 50 min, 55 min, 60 min, 65 min, 70 min, 80 min, 90 min, 100 min, 110 min, or 120 min, and more preferably 50 to 90 min.

[0072] Fourthly, the present invention provides an application of the ferritic stainless steel as described in the first aspect, wherein the ferritic stainless steel is used in fuel cell bipolar plates.

[0073] Compared with existing technologies, the present invention has the following beneficial effects:

[0074] (1) The ferritic stainless steel of the present invention has the characteristics of having few alloying elements and low Cr content. Through the design of the types and contents of each element, good corrosion resistance, electrical conductivity, elongation and deformation capacity are obtained, while also having economic and cost advantages. Among them, the Ni content can be as low as 0.2 to 0.5%, further reducing costs while still meeting the above effects.

[0075] (2) The control method of the present invention uses electrolysis to control the surface roughness of stainless steel main material, which has the advantage of controllable surface roughness. It can adjust the surface roughness of stainless steel according to the characteristics of membrane electrode and gas diffusion in proton exchange membrane fuel cell, so that the bipolar plate and related components can be well bonded, thereby reducing the contact resistance of the battery system, which is more suitable for industry application characteristics.

[0076] (3) This invention forms a passivation film on the surface of stainless steel through electrochemical passivation. The resulting passivation film has excellent performance, is dense and has good continuity, and can achieve the purpose of enhancing corrosion resistance and reducing contact resistance. Moreover, the passivation solution and method are environmentally friendly, and can ensure excellent contact resistance under ultra-low concentration or hydrofluoric acid-free conditions (meeting the US DOE standard). Attached Figure Description

[0077] Figure 1 This is a graph showing the relationship between the roughness of stainless steel and the interfacial contact resistance in one embodiment of the present invention, wherein samples 1-6 correspond to embodiments 16-21 respectively.

[0078] Figure 2 and Figure 3 This is an XPS image of the passivation film of stainless steel in one embodiment of the present invention.

[0079] Figure 4 This is an MS curve of the passivation film of stainless steel in one embodiment of the present invention.

[0080] Figure 5 This is a carrier concentration diagram of the p-type and n-type passivation films of stainless steel in one embodiment of the present invention.

[0081] Figure 6 This is a cross-sectional morphology diagram of the passivation film of stainless steel in one embodiment of the present invention. Detailed Implementation

[0082] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments.

[0083] The chemical composition (mass percentage, wt.%) of the materials used in steelmaking is shown in Table 1.

[0084] Table 1

[0085]

[0086]

[0087] The roles of each element in ferritic stainless steel are as follows:

[0088] Cr is the fundamental element determining the corrosion resistance of ferritic stainless steel. Chromium reacts with oxygen in the corrosive medium to form a thin oxide film on the steel surface, which can prevent further corrosion of the steel matrix. However, an increase in chromium content will accelerate the formation and precipitation of α and σ phases, resulting in a decrease in toughness and a significant increase in the brittle transition temperature, which is detrimental to the processing in the manufacture of stainless steel.

[0089] Mo is another key element for improving the corrosion resistance of stainless steel. It promotes the passivation of Fe-Cr alloys and enhances the steel's corrosion resistance in reducing media, particularly its resistance to pitting and crevice corrosion in chloride solutions. However, higher Mo content can lead to the formation of ferrite σ phase and other brittle phases, resulting in decreased toughness and increased strength, which is detrimental to material processing.

[0090] Carbon has a solid solution strengthening effect. However, its solubility in ferrite is very low, and excess carbon precipitates as carbides. This can cause intergranular corrosion and chromium depletion at grain boundaries in ferritic stainless steel, affecting the material's mechanical and weldability properties.

[0091] Si is a useful element for deoxidation. However, as the content increases, the material's processing performance decreases.

[0092] Mn is an unavoidable element in steel, and besides its deoxidizing effect, it can also increase the strength of steel. However, as an impurity, MnS can become the starting point for corrosion, reducing corrosion resistance.

[0093] N and C react with Cr to form Cr carbonitrides, creating Cr-depleted zones that reduce the corrosion resistance of stainless steel.

[0094] Both Ti and Nb preferentially combine with C and N to form carbonitrides, thereby suppressing the reduction in corrosion resistance caused by the precipitation of Cr carbonitrides. However, if the content is too high, the processability will decrease.

[0095] Cu is an element that improves the corrosion resistance of stainless steel and can also improve the cold working properties of the material.

[0096] Ni is an element that improves the corrosion resistance of stainless steel, and it can also reduce contact resistance.

[0097] The balance is Fe and unavoidable impurities.

[0098] In addition to the above, for the purpose of improving corrosion resistance, it may contain 0 to 1 wt.% V and / or 0 to 1 wt.% W respectively. To achieve this effect, the content of both elements is preferably 0.1 wt.% or more.

[0099] To improve hot workability, it may contain 0.0002 to 1 wt.% rare earth metals, preferably Ce or Y. To achieve this effect, it is preferable to contain 0.0005 wt.% or more.

[0100] Examples 1-21

[0101] Ingots were prepared according to the different grades of ferritic stainless steel shown in Table 1 above. The correspondence between the ingots and the different grades of ferritic stainless steel in each embodiment is shown in Table 2. The ingots were roughed to 100mm to obtain stainless steel plates, which were then hot-rolled. The heating and holding temperature was 1200℃ for 2 hours, with the initial rolling temperature controlled at 1100℃. The plates were rolled to 3mm in 8 passes, with the final rolling temperature controlled at 800℃. After hot rolling, annealing was performed at 1050℃, with the holding time depending on the size of the hot-rolled coil. Pickling was then performed after hot rolling. Afterward, the plates were air-cooled and cold-rolled to produce foil of the required thickness. Finally, annealing was performed at 950℃ for 2 minutes to obtain the final foil sample, i.e., the stainless steel material.

[0102] For the stainless steel material in Example 1, the elongation after fracture at room temperature was tested according to GB / T 228.1-2010. The specimen was prepared as a sheet tensile test according to the standard and tested. The elongation after fracture of the specimen was 33.5%.

[0103] The stainless steel materials described above were subjected to surface roughness treatments in sequence to produce different surface roughnesses. The specific treatment method was as follows: a sulfuric acid solution was prepared by mixing concentrated sulfuric acid and deionized water. The stainless steel materials described above, with a final length and width of 20 mm, were placed in the sulfuric acid solution for surface roughness treatment under different parameters. The roughness preparation conditions are shown in Table 2.

[0104] Table 2

[0105]

[0106]

[0107] After roughening the stainless steel materials of Examples 16-21, samples were taken, the surface of the material was cleaned with acetone and dried with nitrogen, and the surface roughness of the stainless steel was tested and recorded using a surface profilometer. The surface contact resistance was tested and recorded using an interfacial contact resistance meter at 150 N / cm. 2 The interface contact resistance values ​​are shown in Table 3 and below. Figure 1 As shown.

[0108] Table 3

[0109]

[0110] From Table 3 and Figure 1It is known that within a certain surface roughness range, stainless steel has a low interfacial contact resistance. With appropriate optimization, the material can be used in fuel cells. A roughness between 100 and 700 nm is preferred, and further preferred is 200 to 500 nm. Materials outside this preferred range have higher interfacial contact resistance (Examples 19, 20, and 21), making their application in fuel cells difficult. If the roughness is too small, the interfacial contact resistance increases significantly, increasing the internal resistance of the fuel cell and preventing good adhesion with the gas diffusion layer, thus making it unsuitable for application. If the roughness is too large, although the contact resistance is low, the material's corrosion resistance is significantly reduced, failing to meet the requirements for bipolar plates in the internal and acidic environments of fuel cells. After the above roughness treatment (Examples 16-21 underwent surface roughness and interfacial contact resistance testing), the steel plates of Examples 1-21 were further subjected to electrochemical passivation treatment. The specific treatment method is as follows:

[0111] The obtained steel plate was electrochemically passivated for 1 h in a 1.6 mol / L HNO3 solution at 40 °C with an anodic voltage of 1.1 V.

[0112] After the above electrochemical passivation treatment, the sample was rinsed with deionized water and dried with nitrogen cold air, then placed in a dry environment (air) at room temperature for 24 hours. The sample was then tested, specifically:

[0113] (a) Determine t according to the following method p / t n and I p / I n The results are shown in Table 4:

[0114] (I) The passivation film was subjected to in-depth analysis and narrow-spectrum scanning using X-ray photoelectron spectroscopy (XPS). The X-ray source was an Al Kα micro-focused monochromatic source. The scanning was performed using CAE scanning mode with a pass energy of 30–50 eV and a step size of 0.05–0.1 eV. In-depth analysis was performed using argon ion etching with etching depths of 1 nm, 1 nm, 1 nm, 1 nm, 2 nm, 2 nm, 2 nm, 2 nm, 5 nm, and 5 nm.

[0115] (II) The measured results are processed using software, and the content of each substance is expressed according to the peak area corresponding to each substance. Since the main components of the stainless steel passivation film are Fe and Cr hydroxides and oxides, XPS testing primarily analyzes the Fe and Cr phases and their hydroxide and oxide contents within the passivation film. When the Fe and Cr hydroxide content in the passivation film is higher than the Fe and Cr oxide content, the passivation film is determined to be a p-type semiconductor passivation film; when the Fe and Cr hydroxide content is lower than the Fe and Cr oxide content, it is determined to be an n-type semiconductor passivation film. Based on this, the thickness t of the p-type semiconductor in the passivation film can be determined. p And the thickness t of n-type semiconductor n And the ratio of chromium hydroxide to chromium oxide in the p-type passivation film and the n-type passivation film, thus obtaining t p / t n and I p / I n .

[0116] Figure 2 and Figure 3 This is an XPS image of the passivation film on the stainless steel in Example 1. Figure 2 The figure shows the content of Fe and Cr hydroxides and oxides in the passivation film. It can be seen from the figure that the thickness of the p-type semiconductor passivation film is about 5 nm, while the thickness of the n-type semiconductor passivation film is between 10-15 nm. The ratio of the passivation film thicknesses is t. p / t n In the range of 0.33-0.56, the corrected t p / t n It is 0.55. Figure 3 In the graph, the bar chart represents the content of Cr hydroxide and Cr oxide in different thickness regions of the passivation film, while the curve represents the corresponding ratio of hydroxide to oxide. Figure 2 It can be seen that in the p-type passivation film, the ratio of hydroxide to oxide is above 10, and mostly above 15 (taking 15 as the average); while in the n-type passivation film, the ratio of Cr hydroxide to oxide is around 2-5, and mostly between 2-3 (taking 2.5 as the average). Therefore, it can be determined that I p / I n It is 7.

[0117] (II) Service performance testing under simulated fuel cell operating environment: A 300-hour durability test was conducted at 80°C in a sulfuric acid solution with a pH of 3 and a potential of 0.84V (vs. SHE). The corrosion current density was recorded, and the corrosion current density of the material surface was measured at 150 N / cm². 2 The interface contact resistance values ​​are shown in Table 4.

[0118] Table 4

[0119]

[0120] As shown in Table 4 above, when roughening treatment and electrochemical passivation treatment are performed simultaneously, a contact resistance of less than or equal to 8 mΩ·cm can be obtained. 2 The thickness ratio of p-type and n-type semiconductors is between 0.2 and 0.6, and I p [Cr(OH)3 / Cr2O3] / I n A surface passivation film with a [Cr(OH)3 / Cr2O3] ratio greater than 4, and a current density less than 3 μA·cm. -2 Interface contact resistance less than 8mΩ·cm 2 This indicates that the passivation film has excellent protective and conductive properties.

[0121] Meanwhile, as shown in Table 4, while keeping the electrochemical passivation treatment conditions unchanged, altering the roughness preparation conditions can change the surface state of stainless steel, thereby ultimately improving the performance of the prepared passivation film.

[0122] As shown in Examples 2-3, under roughness preparation conditions, a hydrohalic acid concentration in the range of 0-3 mol / L is beneficial for improving I. p / I n , make t p / t n Within a suitable range, reduce interfacial contact resistance and corrosion current density.

[0123] As shown in Examples 2-5, in the roughness preparation conditions, a sulfuric acid concentration in the range of 0.1–7 mol / L is beneficial for improving I. p / I n , make t p / t n Within a suitable range, reduce interfacial contact resistance and corrosion current density.

[0124] As shown in Examples 6-9, in the roughness preparation conditions, an electrolysis temperature in the range of 25–70°C is beneficial for improving I. p / I n This reduces interfacial contact resistance and corrosion current density.

[0125] A comparison between Examples 10 and 11, and between Examples 14 and 15, shows that a polarization voltage in the range of 5–15V is beneficial for improving I in the roughness preparation conditions. p / I n , make t p / t n Within a suitable range, reduce interfacial contact resistance and corrosion current density.

[0126] Based on Examples 10-13, the reason for the decrease in passivation film performance in Example 13 may be that the electrolysis time was too long in the roughness preparation conditions, resulting in an undesirable thickness ratio and composition of the p-type and n-type passivation films.

[0127] Examples 22-30

[0128] The difference from Example 1 is that the electrochemical passivation treatment was performed with different parameters, and the analysis and evaluation were carried out in the same way as in Example 1. The electrochemical passivation conditions and test results of Example 1 and Examples 22-30 are shown in Table 5.

[0129] Table 5

[0130]

[0131] Note: Example 30 in Table 5 is a bare sample, which was not electrochemically passivated based on Example 1.

[0132] Table 4 shows that by adjusting the electrochemical passivation parameters, such as nitric acid concentration, temperature, potential, and passivation time, within the specified range, the performance of the passivation film can be further improved. Specifically, the test results of the stainless steel sample in Example 1 after a period of service in a fuel cell environment show that the passivation film can reduce the contact resistance of the stainless steel interface to 8 mΩ·cm. 2 Furthermore, the corrosion current density of stainless steel can be kept at a low level, exhibiting good corrosion resistance and conductivity.

[0133] Mott-Schottky (MS) curves were performed on Examples 1, 25, 29, and 30 to determine the carrier concentration of the passivation film and to assess the effects of electrochemical passivation treatment and voltage on the passivation film performance.

[0134] The specific MS curve testing method is as follows:

[0135] The samples were subjected to MS curve testing in a sulfuric acid solution at pH 3 at 80℃. To increase the conductivity of the sulfuric acid solution, 0.1 mol / L Na₂SO₄ was added to the solution. MS curve testing was performed using an electrochemical workstation with a test range of -1 to 1 V and a test step size of 25 mV / step. The slopes of the linear segments for the p-type and n-type passivation films were obtained by fitting the data. Then, the corresponding carrier concentrations for p-type and n-type passivation films were calculated based on the MS model. The performance of the passivation films was determined by the carrier concentration.

[0136] Figure 4These are MS curves of the passivation films of stainless steel from Examples 1 (electrochemical passivation voltage 1.1V), 25 (electrochemical passivation voltage 0.8V), 29 (electrochemical passivation voltage 0.6V), and 30 (no electrochemical passivation). Figure 4 It can be seen that in the unpassivated sample, the p-type semiconductor region is not obvious, and the passivation film mainly exhibits the characteristics of the n-type semiconductor region. After constant potential polarization at 0.6V, the slope of the straight segment in both the p-type and n-type semiconductor regions increases, but the increase in the p-type region is not significant. However, after constant potential polarization at 0.8V and 1.1V, the slope of the leading-stage of both the p-type and n-type semiconductor regions increases significantly. Overall, with the increase of constant potential polarization voltage, the slope of the straight segment in the p-type semiconductor region is obvious and gradually increases. The slope of the straight segment in the n-type semiconductor region also gradually increases.

[0137] Figure 5 This is an MS carrier concentration plot of the passivation film of stainless steel in Examples 1 (electrochemical passivation voltage 1.1V), 25 (electrochemical passivation voltage 0.8V), 29 (electrochemical passivation voltage 0.6V), and 30 (no electrochemical passivation). Figure 5 It can be seen that the carrier concentration in the p-type semiconductor region of the passivation film on the sample surface gradually decreases after electrochemical passivation (nearly four times lower than before electrochemical passivation at 1.1V). This decrease in carrier concentration indicates an increase in the density of the passivation film, suggesting improved protective performance. The carrier concentration in the n-type semiconductor region decreases somewhat after electrochemical passivation (less than two times lower at 1.1V). This indicates both increased protection in the n-type semiconductor passivation film region and no significant decrease in conductivity. Therefore, overall, the passivation film effectively improves corrosion resistance without significantly increasing contact resistance.

[0138] Meanwhile, from Table 4 above, Figure 4 and Figure 5 It is known that the composition of the passivation film can be altered and its performance optimized by adjusting the parameters of the electrochemical passivation treatment. For example, different voltages can be applied during the electrochemical passivation process to modulate the characteristics of the passivation film (see [reference]). Figure 4 and Figure 5 This causes the thickness ratio t of the p-type semiconductor and n-type semiconductor in the passivation film to increase with increasing potential. p / t n By gradually increasing these parameters, the performance of stainless steel in fuel cell stacks can be improved. In other words, adjusting these parameters can change the composition of the passivation film, optimize its performance, and ultimately improve the adaptability of stainless steel bipolar plates in fuel cells.

[0139] The cross-sectional morphology of the passivation film of the stainless steel in Example 1 was characterized. Specifically, transmission electron microscopy (TEM) was used to characterize the cross-section of the sample cut by focused ion beam, obtaining a cross-sectional morphology image of the passivation film (to protect the passivation film from damage during focused ion beam sample preparation, a carbon film was first deposited on the outermost layer). The results are as follows: Figure 6 As shown, the top part is the deposited C layer, the middle part is the passivation film, and the bottom part is the stainless steel substrate. The figure shows that the passivation film is continuous, dense, and uniform overall, with no obvious defects, and its thickness is approximately 12–20 nm. The unevenness of the outer passivation film is due to the dynamic growth and dissolution process of the outermost passivation film in the acid solution, resulting in an uneven surface on the passivation film surface in acid solutions.

[0140] Examples 31-37

[0141] The difference from Example 1 is the steel grade, whether surface roughening treatment is performed, and whether electrochemical passivation treatment is performed. The results are shown in Table 5. Specifically, surface roughening treatment can be performed directly after step (1) without electrochemical passivation treatment; electrochemical passivation treatment can be performed directly after step (1) without surface roughening treatment; or surface roughening treatment and electrochemical passivation treatment can be performed sequentially after step (1).

[0142] The roughness treatment conditions were as follows: room temperature (25℃), polarization of the sample in a 3 mol / L H2SO4 solution at 10V for 50s.

[0143] The conditions for chemical passivation are as follows: the steel plate obtained after the above roughness treatment is electrochemically passivated for 1 hour in a 1.6 mol / L HNO3 solution at 40°C with an anodic voltage of 1.1 V.

[0144] Table 6

[0145]

[0146]

[0147] As shown in Table 6 above, setting a passivation film after roughness treatment can further enhance the protective effect and improve conductivity, thereby further improving performance.

[0148] Simultaneously, the stainless steel was subjected to roughening treatment followed by the application of a passivation film. The passivation films on the stainless steel surface all exhibited good performance, with the p-type and n-type semiconductor thickness ratio between 0.2 and 0.6, and I... p [Cr(OH)3 / Cr2O3] / I n [Cr(OH)3 / Cr2O3] is greater than 4; and after a certain period of operation, its interfacial contact resistance is less than 8 mΩ·cm. 2Current density less than 3 μA / cm 2 The performance is good. In addition, regardless of whether a passivation film is applied after roughening treatment or a passivation film is applied directly without roughening treatment, the performance of the surface passivation film is significantly improved compared to the sample without roughening treatment or passivation film application. Subsequent long-term service tests also show that its interface contact resistance and current density are reduced.

[0149] The applicant declares that the detailed method of the present invention is illustrated by the above embodiments, but the present invention is not limited to the above detailed method, that is, it does not mean that the present invention must rely on the above detailed method to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A method for controlling the surface roughness of stainless steel for a fuel cell bipolar plate, characterized by, The control method includes: A stainless steel main material is provided, and the stainless steel main material is electrolyzed in an acid solution. During the electrolysis process, the polarization voltage satisfies the following formula (I): E ≥ lg D + 12 + pH (Ⅰ) Where E is the polarization voltage, the unit of polarization voltage is V, D is the grain size of the stainless steel main material, the unit of grain size is micrometer, and pH is the pH value of the initial acid solution. The roughness is between 100 and 700 nm; The stainless steel main material is made of ferritic stainless steel, and based on 100 wt.% of the ferritic stainless steel, the ferritic stainless steel comprises: C0.03wt.% or less; N0.02wt.% or less; Si0.4wt.% or less; Mn ≤ 0.5wt.%; Cr 16~23wt.%; Cu 0~2.0wt.%; Mo 1.8~2.5 wt.%; Ni 0.2~2.0 wt.%; Ti 0.1~0.5 wt.%; Nb 0.005~0.5wt.%; P < 0.02 wt.%; S < 0.02wt.%; The balance is Fe and other elements that are unavoidably present. Among the unavoidable impurities, O is less than 0.02 wt.% and Sn is less than 0.1%.

2. The method for controlling the surface roughness of stainless steel for fuel cell bipolar plates according to claim 1, characterized in that, The polarization voltage is 5~15V.

3. The method of claim 1, wherein the stainless steel is a 300 series stainless steel. The electrolysis time is 10~300s.

4. The method of controlling the surface roughness of a stainless steel for a fuel cell bipolar plate according to claim 3, characterized by, The electrolysis time is 20~120s.

5. The method for controlling the surface roughness of stainless steel for a fuel cell bipolar plate according to Claim 1, characterized by, The electrolysis temperature is 25~70℃.

6. The method of controlling the surface roughness of a stainless steel for a fuel cell bipolar plate according to claim 5, characterized by, The electrolysis temperature is 25~40℃.

7. The method for controlling the surface roughness of stainless steel for a fuel cell bipolar plate according to Claim 1, characterized by, The acid used in the electrolysis process is sulfuric acid, or a mixture of sulfuric acid and hydrohalic acid.

8. The method for controlling the surface roughness of stainless steel for fuel cell bipolar plates according to claim 7, characterized in that, The hydrohalic acid is at least one of hydrofluoric acid, hydrochloric acid, hydrobromic acid, and hydroiodic acid.

9. The method of controlling the surface roughness of a stainless steel for a fuel cell bipolar plate according to claim 8, characterized by, The hydrohalic acid is hydrochloric acid and / or hydrofluoric acid.

10. The method for controlling the surface roughness of stainless steel for a fuel cell bipolar plate according to claim 7, characterized by, The concentration of the sulfuric acid is 0.1~14 mol / L.

11. The method for controlling the surface roughness of a stainless steel for a fuel cell bipolar plate according to claim 10, characterized by, The concentration of the sulfuric acid is 0.1~7 mol / L.

12. The method for controlling the surface roughness of stainless steel for fuel cell bipolar plates according to claim 7, characterized in that, In the mixed acid solution of sulfuric acid and hydrohalic acid, the concentration of hydrohalic acid is 0~3 mol / L and does not contain 0.

13. The method for controlling the surface roughness of a stainless steel for a fuel cell bipolar plate according to claim 12, characterized by, In the mixed acid solution of sulfuric acid and hydrohalic acid, the concentration of hydrohalic acid does not exceed 0.5 mol / L.

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