Stainless steel bipolar plate nitriding / chromium nitride composite coating and preparation method thereof

Abstract

The invention discloses a stainless steel bipolar plate nitriding / chromium nitride composite coating and a preparation method thereof, and relates to the technical field of fuel cells, and the preparation method comprises the following steps: forming a nitrogen-rich transition layer on the surface of stainless steel through tubular furnace plasma assisted nitriding, and then depositing a CrN coating through magnetron sputtering to obtain the stainless steel bipolar plate nitriding / chromium nitride composite coating. And a nitrogen diffusion layer-CrN ceramic layer composite structure is constructed. The novel method of tubular furnace plasma assisted nitriding is used for firstly carrying out nitriding in the first step, a thin layer of chromium nitride is formed on the surface of stainless steel, then reactive sputtering is directly prepared on the basis of a nitriding layer through magnetron sputtering, a compact chromium nitride coating is prepared, and chemical bonding and stress slow release between the coating and a base body are achieved; the interface adhesive force and the corrosion-resistant synergistic performance are obviously improved. The method has the advantages of low-temperature treatment, controllable process, moderate cost and the like, and a new thought is provided for surface modification of the high-performance metal bipolar plate.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of fuel cells, and particularly relates to a stainless steel bipolar plate nitriding / chromium nitride composite coating and a preparation method thereof. BACKGROUND

[0002] As a new generation of clean energy conversion device, the performance and service life of proton exchange membrane fuel cell (PEMFC) are highly dependent on the comprehensive performance of the bipolar plate. The bipolar plate not only bears the key functions of current conduction, reaction gas distribution and product discharge, but also needs to be stably operated for a long time under strong acidity (pH is about 2-3), high humidity and high temperature (70 DEG C) and oxygen / hydrogen atmosphere. Therefore, the bipolar plate material needs to have excellent electrical conductivity, corrosion resistance and reliable interface bonding.

[0003] Although the traditional graphite bipolar plate has good chemical stability, its intrinsic brittleness, low airtightness and processing difficulty limit its lightweight and large-scale application. In comparison, SS316L stainless steel becomes a strong candidate for metal bipolar plates due to its high strength, excellent formability and low cost advantage. However, stainless steel is prone to local corrosion under the working conditions of PEMFC, which leads to an increase in interface contact resistance and is accompanied by the dissolution of metal ions, which poisons the membrane electrode assembly and significantly shortens the service life of the cell.

[0004] Surface modification technology is considered as an effective way to solve the corrosion problem of metal bipolar plates. In order to solve the above problems, a metal-based coating can be prepared on the surface of the metal bipolar plate by physical vapor deposition, chemical vapor deposition, electrochemical deposition, spraying and the like to modify the surface coating of the metal bipolar plate. For example, Chinese Invention Patent CN110718701 A discloses a chromium nitride / chromium carbide composite film composite coating for enhancing the corrosion resistance of a fuel cell metal bipolar plate, which is formed by alternately depositing metal nitride and metal carbide on the surface of the metal bipolar plate to form a surface coating. However, the plating layer often has quality defects, poor corrosion resistance and the like, and therefore needs to be improved. SUMMARY

[0005] The technical problem to be solved by the present application is how to solve the adhesion problem between the existing ceramic phase coating and the hard substrate of stainless steel. The plating layer often has quality defects (such as brittleness, voids, etc.), which can easily cause the plating layer to peel off prematurely, resulting in a decrease in corrosion resistance.

[0006] The present application solves the above technical problems by the following technical means: The present application provides a preparation method of a stainless steel bipolar plate nitriding / chromium nitride composite coating, comprising the following steps: (1) Pretreatment: polishing and polishing the surface of the stainless steel; sequentially cleaning with acetone and alcohol; (2) Plasma assisted nitriding by tube furnace: Ar is introduced, vacuum is extracted, and the process is repeated for multiple times until the pressure of the tube furnace quartz tube is 1-1.2 Pa; Ar is introduced again, and the tube furnace is heated; N2-H2 mixed gas is introduced; the radio frequency power source is started to plasma nitride the stainless steel sheet treated in step (1); the mixed gas, the plasma generator and the water cooling system are turned off, and the temperature is cooled to room temperature under the protection of Ar; (3) Depositing CrN coating: the sample treated in step (2) is placed on the sample table of the magnetron sputtering device, vacuum is extracted; at the same time, the Ar and N2 gas flow switches are turned on, the direct current power source power is set, and the main extraction valve position is adjusted; sputtering is performed, and the sample is taken after the chamber is cooled.

[0007] Preferably, in step (1), the stainless steel is 316L stainless steel, 304 stainless steel or 301 stainless steel.

[0008] Preferably, in step (1), the polishing method is polishing with diamond grinding paste.

[0009] Preferably, in step (1), the polishing method is polishing with diamond grinding paste.

[0010] Preferably, in step (1), the cleaning time is 15-20 min.

[0011] Preferably, in step (2), the flow of Ar introduced again is 10-12 mL·min -1 .

[0012] Preferably, in step (2), the temperature is heated to 500-550°C, and further preferably to 550°C.

[0013] Preferably, in step (2), the gas flow ratio of N2-H2 mixed gas is 4-6:1, and further preferably 4:1.

[0014] Preferably, in step (2), the flow of N2-H2 mixed gas is 20-22 mL·min -1 , and further preferably 20 mL·min -1 .

[0015] Preferably, in step (3), the target material height of the sample table is adjusted so that the target distance is 8 cm, the mechanical pump is turned on, the side extraction valve is opened, the side extraction valve is closed below 10 Pa, the electromagnetic valve is opened, and the main extraction valve and the molecular pump are opened.

[0016] Preferably, in step (3), vacuum is extracted for 3-4 h, and the gas pressure in the reaction chamber of the magnetron sputtering device is 1.2×10 - 4 Pa.

[0017] Preferably, in step (3), the Ar gas flow rate is 20-25 mL·min -1 , and more preferably 20 mL·min -1 .

[0018] Preferably, in step (3), the N2 gas flow rate is 15-20 mL·min -1 , and more preferably 15 mL·min -1 .

[0019] Preferably, in step (3), the power of the direct current power supply is set to 120-125 W, and more preferably 125 W.

[0020] Preferably, in step (3), the main valve position is adjusted to make the internal pressure of the magnetron sputtering to 0.8 Pa.

[0021] Preferably, in step (3), the sputtering is performed for 30-40 min.

[0022] The application also provides a stainless steel bipolar plate nitrided / chromium nitride composite coating prepared by the above preparation method.

[0023] The application has the following advantages: 1. Compared with stainless steel without nitriding treatment, the adhesion of the CrN coating is improved.

[0024] 2. Compared with traditional gas nitriding, in the plasma nitriding, N2 and H2 are directly ionized and dissociated in the glow discharge to generate a large number of high-energy particles such as N + , N2 + , and NH x , which are more easily diffused on the surface of the stainless steel and migrated internally at a high temperature of 550℃.

[0025] 3. Compared with chemical vapor deposition, electrochemical deposition, and ion beam assisted deposition, the biggest advantage of the magnetron sputtering device is its low temperature, high density, high film-substrate adhesion, and excellent film uniformity, which is particularly suitable for precise coating treatment of heat-sensitive stainless steel bipolar plates.

[0026] 4. By forming a nitrogen-rich transition layer on the surface of the stainless steel through the tubular furnace plasma-assisted nitriding, and then depositing the CrN coating by the magnetron sputtering, a “nitrogen diffusion layer-CrN ceramic layer” composite structure can be constructed to realize the chemical bonding and stress relief between the coating and the substrate, significantly improve the interfacial adhesion and corrosion resistance performance. This method has the advantages of low-temperature treatment, controllable process, and moderate cost, and provides a new idea for the surface modification of high-performance metal bipolar plates.

[0027] Of course, implementing any product or method of the application does not necessarily require achieving all the advantages described above at the same time. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the plasma nitriding device in Embodiment 1 of the present invention; Figure 2 This is a diagram of the magnetron sputtering device in Embodiment 1 of the present invention; Figure 3 The images show corrosion resistance test results of the chromium nitride coatings prepared in Example 1 and Comparative Example 1 of this invention; where (a) is the potentiodynamic polarization curve and (b) is the constant potential polarization curve. Figure 4 The following are contact resistance test diagrams of the chromium nitride coatings prepared in Example 1 and Comparative Example 1 of the present invention; wherein (a) is the curve of the surface contact resistance of the sample as a function of the applied force, and (b) is the surface contact resistance of the sample at 1.4 MPa. Figure 5 These are adhesion test images of the chromium nitride coatings prepared in Example 1 and Comparative Example 1 of the present invention. Figure 6 This is an XRD data diagram of chromium nitride formed on the surface of stainless steel by plasma-assisted nitriding in a tube furnace in Embodiment 1 of the present invention. Detailed Implementation

[0029] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Unless otherwise defined, the technical terms used below have the same meaning as understood by those skilled in the art.

[0030] Unless otherwise specified, the test materials and reagents used in the following examples are commercially available or prepared by known methods.

[0031] Unless otherwise specified, all techniques or conditions described in the embodiments can be performed in accordance with the techniques or conditions described in the literature in this field or in the product manual. Unless otherwise specified, the quantitative experiments in the following embodiments are all repeated three times or more, and the results are averaged.

[0032] Example 1: This embodiment provides a method for preparing a nitriding / chromium nitride composite coating on a stainless steel bipolar plate, including the following steps: 1. First, the surface of SS316L stainless steel is sanded with 400-3000 grit sandpaper, polished with diamond polishing paste, and then ultrasonically cleaned with acetone and alcohol solution for 15 minutes to remove surface grease and oxides.

[0033] 2. The sample was treated by tube furnace plasma assisted nitriding The schematic diagram of the plasma nitriding device is shown in Figure 1 : The device is composed of a quartz tube furnace, an inductively coupled plasma generator, a radio frequency matcher, a mechanical pump, a vacuum gauge, a gas supply and flow control system, etc. Among them, the maximum working temperature of the quartz tube furnace is 1200℃; the inductively coupled plasma generator is composed of a copper coil, a water cooling pipeline and an adjustable plasma radio frequency power source, with a maximum output power of 200 W; the mechanical pump can make the vacuum degree in the tube as low as 1 Pa, and the digital vacuum gauge displays the value in real time; the gas supply and flow control system adjusts the flow of N2, H2 and Ar with a high-precision flow controller, and the flow range is 0-100 mL·min -1 .

[0034] Ar was introduced and vacuumed repeatedly for several times until the pressure of the quartz tube was 1 Pa; then, Ar was introduced at a flow rate of 10 mL·min -1 , the tube furnace was raised to 550℃ under Ar atmosphere, then Ar was switched to N2-H2 (4:1) mixed gas, the mixed gas flow was adjusted to 20 mL·min -1 , the radio frequency power source was started, and the stainless steel sheet was plasma nitrided for 6h; finally, the mixed gas, plasma generator and water cooling system were turned off, and the sample was cooled to room temperature under Ar protection.

[0035] 3. The nitrided sample and the original SS316L sample were placed on the sample table of the magnetron sputtering device, and the magnetron sputtering device is shown in Figure 2 : The device is composed of a reaction chamber, a control system, a mechanical pump, a molecular pump, a cooling system, a flow controller and a magnetron target. Adjust the height of the target material to make the target distance 8 cm, first open the mechanical pump, open the side exhaust valve, close the side exhaust valve below 10 Pa, open the electromagnetic valve, main exhaust valve and molecular pump, and vacuumize for 3h to 1.2×10 -4 Pa.

[0036] 4. Turn on the Ar and N2 gas flow switches, the Ar gas flow is 20 mL·min -1 , the N2 gas flow is 15 mL·min -1 , the power of the direct current power supply is set to 125 W, and the sputtering is performed for 30 min by adjusting the main exhaust valve position to 0.8 Pa. After sputtering, the sample is taken out after the chamber is cooled.

[0037] Example 2: The difference between this example and Example 1 is that in step (2), the volume ratio of N2-H2 mixed gas is 6:1. The remaining steps are the same as those of Example 1.

[0038] The prepared stainless steel bipolar plate nitriding / chromium nitride composite coating has similar corrosion resistance and adhesion to that of Example 1.

[0039] Example 3 The difference between this example and Example 1 is that in step (4), the flow rate of Ar is 25 mL·min -1 , and the flow rate of N2 is 20 mL·min -1 . The remaining steps are the same as those in Example 1.

[0040] The prepared stainless steel bipolar plate nitriding / chromium nitride composite coating has similar corrosion resistance and adhesion to that of Example 1.

[0041] Comparative Example 1 The difference between this comparative example and Example 1 is that there is no plasma-assisted nitriding step in the tube furnace, and a CrN coating is directly deposited on the surface of the stainless steel by a magnetron sputtering device. The remaining steps are the same as those in Example 1.

[0042] Comparative Example 2 The difference between this comparative example and Example 1 is that in step (2), the flow rate of Ar is 6 mL·min -1 , the flow rate ratio of N2-H2 mixed gas is 2:1, and the flow rate of N2-H2 mixed gas is 10 mL·min -1 .

[0043] The flow rate of argon in Comparative Example 2 is too low, the removal effect of air inside the tube furnace is reduced, the nitrogen ratio is reduced, and the total gas flow rate is reduced. The density of active particles (such as nitrogen positive ions and excited nitrogen molecules) inside the quartz tube is reduced, the nitriding rate is reduced, and the thickness of the nitriding layer will be significantly thinned. For the application of bipolar plates, a thinner nitriding layer cannot effectively block the penetration of corrosive media into the substrate, which is not conducive to improving corrosion resistance.

[0044] Comparative Example 3 The difference between this comparative example and Example 1 is that in step (2), Ar is introduced and vacuum is repeatedly drawn multiple times until the pressure in the quartz tube furnace is 5 Pa.

[0045] In Comparative Example 3, the vacuum degree is too high and there is a lot of residual air inside the quartz tube. During the process of stainless steel nitriding, the residual O2 will react with Cr and Fe on the surface of the substrate to form Cr2O3 or iron oxide. The presence of the oxide film will hinder the further penetration of nitrogen atoms, resulting in inhibition of the nitriding process.

[0046] Comparative Example 4 The difference between this comparative example and Example 1 is that in step (3), the vacuum is drawn to 4.0×10 -4 Pa.

[0047] The internal reference example 4 of magnetron sputtering has a higher vacuum than example 1, which means that there are still residual gas molecules, such as O2, H2O, CO2, etc. in the cavity. Chromium is a very active metal that is easily oxidized. During the sputtering process, these residual oxygen and water vapor will react with the chromium atoms sputtered out. The final generated coating is not pure chromium nitride, but a mixed phase containing chromium oxide (Cr2O3) or chromium oxynitride. The mixing of this impurity phase will destroy the stoichiometric ratio of the CrN coating and reduce the intrinsic properties of the coating. The columnar crystal structure of the coating becomes coarse or loose, the porosity increases, and the corrosion resistance decreases.

[0048] Comparative example 5: The difference between this comparative example and example 1 is that in step (4), the Ar gas flow rate is 10 mL·min -1 , and the N2 gas flow rate is 5 mL·min -1 .

[0049] In comparative example 5, the N2 flow rate is suddenly reduced to 5 mL·min -1 , and the N2 / Ar flow ratio is only 0.5. At this time, the active nitrogen atoms in the reaction chamber decrease. The coating will change from a single CrN phase to a chromium-rich Cr2N (chromium nitride) phase, and even a mixed phase of Cr and CrN, which is usually more brittle than the CrN phase. During the stress or battery stacking process, the coating is more likely to crack due to brittleness, providing a channel for the corrosion medium. The corrosion resistance decreases.

[0050] Comparative example 6: The difference between this comparative example and example 1 is that in step (4), the discharge power is adjusted to 225 W.

[0051] The high-energy particle stream generated by the high power in comparative example 6 will strongly bombard the growing film, and will accumulate a large amount of residual internal stress in the coating. The film layer will produce microcracks, and even peeling and peeling during the cooling process in the reaction chamber. This will result in a decrease in corrosion resistance during application.

[0052] The corrosion resistance of stainless steel bipolar plates is one of the important performance indicators of their performance. Electrochemical testing uses a three-electrode system, and the test is completed by a CHI660E electrochemical workstation. According to the national standard GB / T-20042.6-2011, only a 10 mm x 10 mm square area is exposed as the working electrode during testing, the reference electrode is a saturated calomel electrode, and the auxiliary electrode is a platinum sheet electrode. The corrosion medium is a corrosion solution composed of 80°C, 0.05 mol / L H2SO4 and 2 ppm F-. For example Figure 3As shown in (a), the potentiodynamic polarization curve was tested in the range of -0.6V to 1.2V at a scan rate of 1mV / s. Compared with SS316L stainless steel substrate, the corrosion current density of Comparative Example 1 was 3.73×10⁻⁶. -7 In Example 1, the corrosion current density was further reduced to 1.85 × 10 A / cm². -7 A / cm², exhibiting superior corrosion resistance. The potentiostatic polarization curve was tested over 3 hours, simulating a PEMFC operating environment with a set voltage of 0.6 V (vs. SCE), and oxygen was introduced into the environment. Figure 3 As shown in (b), the nitriding / chromium nitride composite coating of the stainless steel bipolar plate prepared in Example 1, compared with the SS316L stainless steel substrate (corrosion current density of 6.03 × 10⁻⁶), shows a significant difference. -6 The chromium nitride coating significantly improved corrosion resistance (A / cm²). Comparative Example 1 showed a corrosion current density of 3.29 × 10⁻⁶. -7 A / cm²; while in Example 1, after nitriding the substrate and then depositing a chromium nitride coating, the corrosion current density was further reduced to 2.06 × 10⁻⁶. -7 With an A / cm², it exhibits superior corrosion resistance.

[0053] The carbon paper used for the contact resistance test was manufactured by Toray Industries, Japan, model TGP-H-060, with a thickness of 0.19 mm. The results are as follows: Figure 4 As shown, Figure 4 (a) represents the interfacial contact resistance under different pressures; the contact resistance continuously decreases as the applied pressure increases. To better simulate the actual operating environment of a fuel cell, Figure 4 (b) Represents the contact resistance value at the most common test pressure (1.4 MPa). The interfacial contact resistance of the coating prepared in Example 1 is further reduced to 7.718 mΩ at 1.4 MPa compared to Comparative Example 1. cm 2 .

[0054] Adhesion testing was conducted using a nano-scratch analyzer (MCT3, Anton Paar, Austria) to assess the adhesion between the CrN coating and the substrate. Figure 5 As shown, the adhesion of the coating prepared in Example 1 was increased from 8.66743 to 11.39803 compared to Comparative Example 1, representing an increase of 31.51%.

[0055] like Figure 6The characteristic peaks of the crystal structure of the original stainless steel are shown, and the characteristic peaks of the original stainless steel are shown as γFe. The characteristic peaks of the crystal structure after nitriding are shown above, and the characteristic peaks of CrN (chromium nitride) after nitriding are shown, that is, nitriding will react with Cr in the stainless steel to form chromium nitride. The S phase represents the S phase characteristic peak formed by the reaction of nitrogen with the original γFe iron during the nitriding process, that is, the nitrogen element will insert into the lattice gap of the γFe iron to form a saturated austenite phase.

[0056] The above test proves that the new method of plasma-assisted nitriding in a tube furnace is used to perform the first step of nitriding to form a thin layer of chromium nitride on the surface of the stainless steel (as shown in Figure 6 The reaction sputtering can be prepared by directly preparing on the basis of the nitriding layer, and a dense chromium nitride coating can be prepared. The adhesion problem between the stainless steel and the coating caused by the hardness difference can be eliminated, and the corrosion resistance and adhesion are significantly improved.

[0057] The above examples are only used to illustrate the technical solutions of the present application, but not to limit it; although the present application has been described in detail with reference to the foregoing examples, those skilled in the art should understand that the technical solutions recorded in the foregoing examples can be modified, or some technical features can be replaced by equivalents; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for preparing a nitriding / chromium nitride composite coating on a stainless steel bipolar plate, characterized in that, Includes the following steps: (1) Pretreatment: Grinding and polishing the stainless steel surface; Clean with acetone and then alcohol in sequence; (2) Plasma-assisted nitriding in a tube furnace: Ar is introduced and vacuum is drawn repeatedly until the pressure in the quartz tube of the tube furnace is 1-1.2 Pa; Ar is introduced again to heat the tube furnace; N2-H2 mixed gas is introduced; the radio frequency power source is started to plasma nitrid the stainless steel sheet after step (1); the mixed gas, plasma generator and water cooling system are turned off and cooled to room temperature under Ar protection; (3) Deposition of CrN coating: Place the sample after step (2) on the sample stage of the magnetron sputtering device and evacuate; at the same time turn on the Ar and N2 gas flow meter switches, set the DC power supply power, and adjust the position of the main pump valve; sputter, and take out the part after the chamber cools down.

2. The preparation method according to claim 1, characterized in that, In step (1), the stainless steel is 316L stainless steel, 304 stainless steel, or 301 stainless steel.

3. The preparation method according to claim 1, characterized in that, In step (1), the grinding method is to grind the surface with 400-3000 grit sandpaper; the polishing method is to polish with diamond polishing paste; the cleaning time is 15-20 minutes.

4. The preparation method according to claim 1, characterized in that, In step (2), the flow rate of Ar introduced again is 10-12 mL·min. -1 Heat to 500-550℃.

5. The preparation method according to claim 1, characterized in that, In step (2), the gas flow ratio of the N2-H2 mixture is 4-6:1; the flow rate of the N2-H2 mixture is 20-22 mL·min. -1 .

6. The preparation method according to claim 1, characterized in that, In step (3), adjust the target height of the sample stage to make the target-to-electrode distance 8 cm. First, turn on the mechanical pump and the side-pump valve. When the pressure drops below 10 Pa, close the side-pump valve and turn on the solenoid valve, the main pump valve, and the molecular pump. Evacuate for 3-4 hours until the gas pressure in the reaction chamber of the magnetron sputtering device is 1.2 × 10⁻⁶. -4 Pa.

7. The preparation method according to claim 1, characterized in that, In step (3), the Ar gas flow rate is 20-25 mL / min. -1 The flow rate of N2 gas is 15-20 mL / min. -1 .

8. The preparation method according to claim 1, characterized in that, In step (3), the power of the DC power supply is set to 120-125W.

9. The preparation method according to claim 1, characterized in that, In step (3), adjust the position of the main pump valve to make the internal gas pressure of the magnetron sputtering reach 0.8 Pa; sputter for 30-40 minutes.

10. A stainless steel bipolar plate with a nitriding / chromium nitride composite coating prepared by the preparation method according to any one of claims 1-9.

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