Stainless steel material for separator of fuel cell and method for manufacturing same

Abstract

The present invention relates to: a stainless steel material for fuel cell separators, the stainless steel material having a depth of 5 nm or more at which the strength ratio of F- / Fe 2-is 10 or more as determined by deep analysis of the strength of F-, CrO3-, and Fe 2-ions as determined by time-of-flight secondary ion mass spectrometry; and the depth at which the CrO3- / Fe2-intensity ratio is 1 or more is in the range of 6-14 nm.

Description

Technical Field

[0001] This invention relates to stainless steel materials for separators in fuel cells and methods for manufacturing the same. Background Technology

[0002] A fuel cell has a stacked structure consisting of a predetermined number of single cells that generate an electromotive force through the reaction of fuel gas (hydrogen) and oxidant gas (oxygen). Each single cell has: a membrane electrode assembly (MEA) having electrode layers (catalyst layer and gas diffusion layer) with anode and cathode on both sides of an electrolyte membrane, and separators disposed on both sides of the MEA.

[0003] The separator for fuel cells has the function of connecting individual cells in series and also functions as a partition that isolates fuel gas, oxidant gas and coolant from each other.

[0004] Various studies have been conducted on such separators for fuel cells.

[0005] For example, Japanese Patent Application Publication No. 2008-277146 discloses a stainless steel conductive component, characterized in that, in the stainless steel conductive component, the Cr / Fe ratio (atomic %) in the passivation film analyzed by surface X-ray photoelectron spectroscopy (XPS) is 3 or more, the F concentration in the passivation film analyzed by surface X-ray photoelectron spectroscopy (XPS) is 0.1 atomic % or more, and the Li concentration in the passivation film analyzed by time-of-flight secondary ion mass spectrometry (TOF-SIMS) is 0.01 atomic % or more.

[0006] Japanese Patent Application Publication No. 2010-140886 discloses a stainless steel material, characterized in that it is a stainless steel material for the separator of a solid polymer fuel cell, comprising: a stainless steel base material, an oxide film disposed on the surface of the stainless steel base material, a conductive layer disposed on the surface of the oxide film and having a non-metallic conductive material, and a conductive material disposed to penetrate the oxide film and electrically connect to the stainless steel base material and the conductive layer, wherein the non-metallic conductive material disposed on the surface of the oxide film comprises graphitic carbon, and when comparing the peak intensities of the diffraction lines of the atomic planes obtained by wide-angle X-ray diffraction of the graphitic carbon crystal, the ratio of the peak intensity of the diffraction line of the (110) atomic plane to the peak intensity of the diffraction line of the (004) atomic plane is less than 0.1.

[0007] Japanese Patent Application Publication No. 2022-45138 discloses a fuel cell separator comprising: a metal substrate, a corrosion-resistant metal interlayer formed on the metal substrate, and a carbon layer formed on the corrosion-resistant metal interlayer, wherein the peak intensity (I) of the D band in the Raman spectrum of the carbon layer is... D ) and the peak intensity of the G-band (I G The strength ratio of (I)D / I G The value is above 0.70 and less than 0.95. Summary of the Invention

[0008] The separator for fuel cells (also simply referred to as the "separator") also serves to allow the generated current to flow into adjacent cells. Therefore, the substrate constituting the separator requires high conductivity and sufficient corrosion resistance to maintain this high conductivity over a long period of time in the high-temperature, acidic atmosphere inside the fuel cell cells. Here, high conductivity refers to low contact resistance. Contact resistance, on the other hand, refers to the voltage drop that occurs between the electrode and the separator surface due to interfacial phenomena.

[0009] Therefore, pure titanium or titanium alloys, which have excellent conductivity and corrosion resistance, are mostly used as the base material for insulating components. This is one of the important factors that leads to increased costs in the manufacturing of insulating components.

[0010] Therefore, in order to reduce costs, an attempt was made to use inexpensive substrates such as stainless steel as the substrate, and a layer that imparts conductivity and corrosion resistance was formed on the surface of the substrate, thereby manufacturing an insulating component that is both conductive and corrosion resistant.

[0011] A layer that imparts conductivity and corrosion resistance to stainless steel can be formed, for example, by physical vapor deposition (PVD).

[0012] However, such PVD processing requires processing in a vacuum and multiple steps as film formation steps.

[0013] Therefore, the present invention provides a stainless steel material for separators of fuel cells with sufficient corrosion resistance and low contact resistance, as well as a method for manufacturing the stainless steel material.

[0014] The inventors conducted various studies on methods for solving the above-mentioned problems, and found that by using F measured by time-of-flight secondary ion mass spectrometry (TOF-SIMS)... － CrO3 － and Fe2 － In terms of the depth analysis results of ionic strength, F － / Fe2 － The intensity ratio is greater than 10, and the depth is greater than 5 nm, and CrO3 － / Fe2 －The present invention is completed by forming a conductive film (conductive passivation film) on the surface of a stainless steel substrate in a manner in which the intensity ratio is greater than 1 and the depth is in the range of 6nm to 14nm. This enables the manufacture of stainless steel materials for separators of fuel cells with sufficient corrosion resistance and low contact resistance, i.e., high conductivity.

[0015] That is, the main idea of ​​this invention is as follows.

[0016] (1) A stainless steel material, used for the isolation components of fuel cells, for F measured by time-of-flight secondary ion mass spectrometry. － CrO3 － and Fe2 － In terms of the depth analysis results of ionic strength, F － / Fe2 － The intensity ratio is greater than 10 and the depth is greater than 5 nm, and CrO3 － / Fe2 － The depth with an intensity ratio of 1 or higher ranges from 6 nm to 14 nm.

[0017] (2) According to the stainless steel material described in (1), wherein F － / Fe2 － The intensity ratio is above 10 and the depth is below 14.0 nm.

[0018] (3) A method for manufacturing a stainless steel material for a fuel cell separator, comprising (i) a step of imparting a conductive passivation film to a stainless steel substrate, wherein step (i) includes: (A) a step of injecting fluorine into the passivation film, (B) a step of modifying the passivation film in an alkaline aqueous solution or a non-aqueous solvent, and (C) a step of dissolving iron from the passivation film; step (A) includes a pulse electrolytic treatment of the stainless steel substrate in an aqueous solution containing fluoride ions, wherein the pulse electrolytic treatment of step (A) includes an anolyte current density of 0.05 A / dm³. 2 ~1.0A / dm 2 And cathode current density 0.05A / dm 2 ~5.0A / dm 2 Electrolysis treatment under conditions where the electrolysis time is in the range of 10 seconds to 10 minutes, and step (B) includes pulse electrolysis treatment of stainless steel substrates in alkaline aqueous solution or non-aqueous solvent.

[0019] According to the present invention, a stainless steel material for separators of fuel cells with sufficient corrosion resistance and low contact resistance, and a method for manufacturing the stainless steel material, are provided. Attached Figure Description

[0020] The features, advantages, and technical and industrial significance of exemplary embodiments of the present invention will be described below with reference to the accompanying drawings, wherein the same symbols denote the same components:

[0021] Figure 1 F represents the stainless steel material used for the spacers in Examples 1-3, relative to the sputtering time (seconds). － CrO3 － and Fe2 － A graph of ionic strength (Count).

[0022] Figure 2 F represents the stainless steel material used for the spacers in Examples 4-6, relative to the sputtering time (seconds). － CrO3 － and Fe2 － A graph of ionic strength (Count).

[0023] Figure 3 F represents the stainless steel material used for the spacers in Comparative Examples 1-3, relative to the sputtering time (seconds). － CrO3 － and Fe2 － A graph of ionic strength (Count).

[0024] Figure 4 F represents the stainless steel material used for the spacers in Comparative Examples 4-5, relative to the sputtering time (seconds). － CrO3 － and Fe2 － A graph of ionic strength (Count).

[0025] Figure 5 This refers to the CrO3 content in the stainless steel material used for the spacers in Examples 1-6 and Comparative Examples 1-5. － / Fe2 － The depth with an intensity ratio of 1 or higher relative to F － / Fe2 － A graph showing the relationship between the intensity ratio of a depth greater than 10.

[0026] Figure 6 It is a graph representing the profile of alternating application of cathode and anode currents in pulse electrolysis. Detailed Implementation

[0027] The preferred embodiments of the present invention will be described in detail below.

[0028] In this specification, the features of the invention will be described appropriately with reference to the accompanying drawings. It should be noted that the stainless steel material for the separator of the fuel cell of the present invention and its manufacturing method are not limited to the embodiments described below, and can be implemented in various ways, including modifications and improvements, that can be made by those skilled in the art, without departing from the spirit of the invention.

[0029] This invention relates to a stainless steel material, specifically a stainless steel material for separators in fuel cells, having a specific coating.

[0030] In this invention, the base material of the stainless steel material (also referred to as "stainless steel material") used as the separator of the fuel cell is not limited to stainless steel. Examples of stainless steel include austenitic, ferritic, martensitic, austenitic-ferritic (two-phase), and precipitation-curing stainless steels. Specific examples include SUS301, SUS304, SUS304L, SUS316, SUS316L, SUS430, SUS430J1L, SUS434, SUS444, SUS447, and SUS631. Furthermore, examples of surface finishes include bright annealing (BA), pickling (2D), light rolling after pickling (2B), and quenching and tempering rolling.

[0031] By choosing stainless steel as the base material for stainless steel materials, raw material costs can be reduced.

[0032] The thickness of the substrate is not limited, but is typically in the range of 0.05 mm to 0.2 mm, and in one embodiment, it is in the range of 0.06 mm to 0.12 mm.

[0033] By making the thickness of the substrate within the above range, raw material costs can be suppressed.

[0034] The stainless steel material of the present invention has a conductive passivation film.

[0035] Here, a conductive passivation film refers to a film that imparts conductivity and corrosion resistance to a passivation film typically formed on the surface of stainless steel as a substrate by injecting fluorine and further increasing the Cr ratio.

[0036] For the conductive passivation film in this invention, when performing F... － and Fe2 － During the depth analysis of ionic strength, F － / Fe2 － The depth at which the intensity ratio is 10 or higher is 5 nm or higher, and in one embodiment, it is 6 nm or higher. － / Fe2 －There is no upper limit to the depth at which the intensity ratio is 10 or higher, but it is typically 14 nm or less, and in one embodiment, it is 11 nm or less. Here, in this specification, "the depth at which the intensity ratio is X or higher is Y or higher (in the range of Z or lower)" means that in this measurement, the intensity ratio of X or higher is the time that continues from the beginning of the measurement to a depth equivalent to Y or higher (in the range of Z or lower), that is, it means that the intensity ratio of X or higher continues from the surface of the object being measured to a depth of Y or higher (in the range of Z or lower).

[0037] For the conductive passivation film in this invention, when CrO3 is subjected to time-of-flight secondary ion mass spectrometry (TOF-SIMS)... － and Fe2 － During in-depth analysis of ionic strength, CrO3 － / Fe2 － The depth with an intensity ratio of 1 or higher is in the range of 6 nm to 14 nm, and in one embodiment, it is in the range of 7 nm to 13 nm.

[0038] It should be noted that the depth resolution of various ion intensities in the conductive passivation film based on TOF-SIMS can be determined using the following measurement conditions.

[0039] The TOF-SIMS 5 device manufactured by IONTOF GmbH can be used as a TOF-SIMS measurement apparatus. TOF-SIMS measurement uses Bi3. ＋＋ A cluster ion beam (accelerating voltage 60 keV, current measured using a Faraday cup of 0.4 pA) was used as the primary ion source to irradiate a 100 × 100 μm area at the center of the region irradiated by the sputtered ion beam used for depth-direction analysis, as described later. 2 The region is used to detect the secondary ions generated therefrom.

[0040] Using Cs ＋ Ion beams were used as sputtering ion sources for depth-oriented analysis to irradiate 500×500μm. 2 The depth distribution was measured in the region while alternating sputtering and TOF-SIMS measurement as described later. In Examples 1-6 and Comparative Examples 2-5, which are described in detail below, irradiation was performed at an accelerating voltage of 0.5 keV and a current value of 40 nA measured using a Faraday cup. In Comparative Example 1, irradiation was performed at an accelerating voltage of 0.25 keV and a current value of 10 nA measured using a Faraday cup. The sputtering rate under these conditions is expressed as a converted value derived from the sputtering time of the thermal oxide film on a thermally oxidized silicon substrate (100 nm thick thermal oxide film (SiO2)). The sputtering rate in Examples 1-6 and Comparative Examples 2-5 was 0.0225 nm / s, and the sputtering rate in Comparative Example 1 was 0.0045 nm / s.

[0041] By injecting Fe, which acts as an electron carrier, into the passivation film, the conductivity of the passivation film can be improved, significantly reducing the contact resistance of conventionally formed passivation films. Furthermore, by modifying the passivation film to have a composition primarily composed of Cr oxide and Cr hydroxide, corrosion resistance can be improved; even after prolonged exposure to the atmosphere, the film will not deteriorate, preventing or suppressing the time-series degradation of surface contact resistance. Therefore, in this invention, by adjusting the concentration balance of F, Cr, and Fe in the passivation film along its thickness direction, sufficient corrosion resistance and low contact resistance of the stainless steel material are achieved.

[0042] The thickness of the conductive passivation film is not limited. The thickness of the conductive passivation film, expressed as an average thickness, is typically in the range of 1 nm to 10 nm, and in one embodiment, it is in the range of 2 nm to 6 nm. The average thickness of the conductive passivation film can be determined, for example, by TEM cross-sectional observation.

[0043] By making the thickness of the conductive passivation film within the above range, sufficient conductivity and corrosion resistance can be ensured.

[0044] The present invention also relates to a method for manufacturing the stainless steel material of the present invention.

[0045] The method for manufacturing the stainless steel material of the present invention includes (i) a conductive passivation film formation process.

[0046] (i) The process includes: (A) a process of injecting fluorine into the passivation film, (B) a process of modifying the passivation film in an alkaline aqueous solution or a non-aqueous solvent, and (C) a process of dissolving iron from the passivation film, and, as appropriate, (D) a heat treatment process.

[0047] In process (i), the substrate used is as described above.

[0048] The thickness of the substrate is not limited, but is typically in the range of 0.05 mm to 0.2 mm, and in one embodiment, it is in the range of 0.06 mm to 0.12 mm.

[0049] By making the thickness of the substrate within the above range, raw material costs can be suppressed.

[0050] In process (A), as a method for injecting fluorine into the passivation film, an example is the electrolytic treatment (electrochemical treatment) of stainless steel in an aqueous solution containing fluoride ions.

[0051] As the fluoride ion source used for fluoride implantation, there is no limitation on the use of hydrofluoric acid or any fluoride compound that is soluble in water and generates fluoride ions; any compound can be used. Examples include alkali metal fluorides (such as sodium fluoride, potassium fluoride, etc.), ammonium fluoride, antimony trifluoride, copper fluoride, sodium hydrogen fluoride, potassium hydrogen fluoride, etc. In one embodiment, the fluoride ion source is an alkali metal fluoride, such as sodium fluoride or potassium fluoride.

[0052] To electrochemically inject fluoride, a stainless steel substrate is subjected to pulsed electrolysis (anode and cathode polarities) in an aqueous solution of hydrogen fluoride or in an acidic aqueous solution obtained by adding nitric acid, sulfuric acid, phosphoric acid, etc., to the aforementioned fluoride ion source. The pH of the treatment solution is typically in the range of 0–3, and in one embodiment, it is in the range of 0–2. The fluoride concentration is typically 0.001 kmol / m³. 3 A wide range of saturation concentrations is possible. Aqueous solutions do not require heating; for example, they can be used typically in the range of 10°C to 30°C, and in one embodiment at room temperature (e.g., 20°C). Pulse electrolysis is a method of modifying metal surfaces by alternately applying anodic and cathode potentials. Either anodic or cathode potential can be applied first. Figure 6 This curve represents the alternating application of cathode and anodic currents in pulse electrolysis. The pulse electrolysis condition is an anodic current density of 0.05 A / dm³. 2 ~1.0A / dm 2 The range, in one embodiment, is 0.050 A / dm. 2 ~1.0A / dm 2 The range, in one embodiment, is 0.10 A / dm. 2 ~0.50A / dm 2 The range is such that the cathode current density is 0.05 A / dm². 2 ~5.0A / dm 2 The range, in one embodiment, is 0.050 A / dm. 2 ~5.0A / dm 2 The range, in one embodiment, is 0.10 A / dm. 2 ~4.0A / dm 2 The electrolysis time refers to the total time of the anode pulse time (the time for which the anode potential is applied) and the cathode pulse time (the time for which the cathode potential is applied), typically ranging from 10 seconds to 10 minutes, and in one embodiment from 20 seconds to 5 minutes. Furthermore, the anode pulse time and cathode pulse time can be arbitrarily varied as long as they fall within the range of the electrolysis time.

[0053] As a step (B), an example is the method of electrolytically treating (electrochemically treating) a stainless steel substrate in an alkaline aqueous solution or a non-aqueous solvent.

[0054] There are no limitations on the alkaline aqueous solution; any alkaline aqueous solution can be cited, such as sodium hydroxide solution, potassium hydroxide solution, calcium hydroxide solution, magnesium hydroxide solution, barium hydroxide solution, lithium hydroxide solution, ammonium hydroxide solution, etc. There are also no limitations on the non-aqueous solvent; examples include ethanol, methanol, dimethyl ether, diethyl ether, methyl ethyl ether, etc. A mixture of water and a water-miscible non-aqueous solvent can be used.

[0055] The pH of the alkaline solution can be above 10, and in one embodiment above 11. The solution does not require heating and is typically in the range of 10°C to 30°C, with room temperature (e.g., 20°C) in one embodiment. In the case of electrochemical treatment, the stainless steel substrate is subjected to pulse electrolysis in an alkaline aqueous solution or a non-aqueous solvent. In the case of pulse electrolysis, the pulse electrolysis density (both anode and cathode) is 0.001 A / dm³. 2 ~10A / dm 2 The range, in one implementation, is 0.1 A / dm. 2 ~5A / dm 2 The electrolysis time typically ranges from 5 seconds to 10 minutes, and in one embodiment, it ranges from 10 seconds to 5 minutes.

[0056] The method for effectively implanting fluoride ions into the passivation film is to repeatedly perform steps (A) and (B) described above. The order of steps (A) and (B) is not critical. In one embodiment, step (A) is performed first, followed by step (B).

[0057] In step (C), an example of a method to preferentially dissolve iron from the passivation film is to perform an immersion treatment in an aqueous solution containing fluoride ions. It should be noted that prior to this treatment, performing step (D) of heat treatment in the atmosphere or inert gas atmospheres such as nitrogen or Ar is effective. This is believed to be because the Fe concentrated to the outermost layer of the passivation film through heat treatment readily forms a complex with fluoride ions during the subsequent immersion treatment in an aqueous solution containing fluoride ions, thus dissolving from the passivation film. By preferentially dissolving Fe from the passivation film, the film can be modified to have a composition primarily composed of Cr oxides and Cr hydroxides.

[0058] The iron in the passivation film can be dissolved by impregnating stainless steel in an aqueous solution containing fluoride ions. Examples of aqueous solutions include hydrofluoric acid, or an acid can be added to the aforementioned fluoride ion source to create an acidic aqueous solution. The pH is typically in the range of 0 to 3, and in one embodiment, it is in the range of 0 to 2. The fluoride concentration is typically 0.001 kmol / m³. 3 A wide range of saturated concentrations. Examples of acids used for pH adjustment include nitric acid, sulfuric acid, and phosphoric acid. The concentration is typically 0.01 kmol / m³.3 ~10kmol / m 3 The range, in one embodiment, is 0.1 kmol / m 3 ~5kmol / m 3 The temperature of the aqueous solution is typically in the range of 10°C to 80°C, and in one embodiment, it is in the range of 20°C to 60°C. The immersion time is typically in the range of 5 seconds to 20 minutes, and in one embodiment, it is in the range of 5 seconds to 10 minutes.

[0059] Furthermore, as part of heat treatment step (D), in order to effectively dissolve the iron in the passivation film, heat treatment can be performed in the atmosphere or inert gas atmospheres such as nitrogen or Ar before step (C), i.e., immersion treatment in an aqueous solution containing fluoride ions. The heat treatment temperature is typically in the range of 100°C to 700°C, and in one embodiment, it is in the range of 400°C to 700°C. The treatment time is typically in the range of 30 seconds to 30 minutes, and in one embodiment, it is in the range of 30 seconds to 5 minutes.

[0060] This heat treatment forms an iron concentration layer on the outermost surface of the passivation film. Subsequent immersion in an aqueous solution containing fluoride ions allows Fe to readily form complexes with the fluoride ions, dissolving them into the solution. Through this treatment, the passivation film becomes composed primarily of Cr, thus improving corrosion resistance. It is believed that even after prolonged exposure to the atmosphere, the film will not deteriorate, and the time-series degradation of surface contact resistance is reduced.

[0061] As described above, by injecting F, which acts as an electron carrier, into the passivation film, the conductivity of the passivation film can be improved, significantly reducing the contact resistance of previously formed passivation films. Furthermore, by modifying the passivation film to have a composition dominated by Cr oxide and hydroxide, corrosion resistance can be improved, and the film will not deteriorate even after prolonged exposure to the atmosphere, thus preventing or suppressing the time-series degradation of surface contact resistance.

[0062] It should be noted that (i) the process can be performed on both sides of the substrate or only on one side of the substrate.

[0063] The manufacturing method of the present invention enables the production of stainless steel materials with sufficient corrosion resistance and electrical conductivity.

[0064] The stainless steel material of the present invention can be used as a material for separators for fuel cells. For example, separators for fuel cells manufactured by directly using the stainless steel material of the present invention, or by further coating it with a corrosion-resistant metal intermediate layer (e.g., a titanium layer) and a conductive layer (e.g., a carbon layer), are components of a fuel cell cell (single cell) and are disposed on both sides of a membrane electrode assembly (electrolyte membrane, electrode layers of anode and cathode disposed on both sides of the electrolyte membrane).

[0065] The separator for fuel cells made using the stainless steel material of the present invention is manufactured into a fuel cell cell by bonding, for example, with the constituent elements of a fuel cell cell known in the art, such as membrane electrode assembly, sheet components, etc., using an adhesive.

[0066] Fuel cell units manufactured using fuel cell separators incorporating the stainless steel material of this invention can be used in various electrochemical devices, such as solid polymer fuel cells.

[0067] The following describes several embodiments of the present invention, but it is not intended to limit the invention to the configuration shown in these embodiments.

[0068] I. Manufacturing of Stainless Steel Materials

[0069] Stainless steel materials with conductive passivation films were manufactured according to the methods and electrolysis conditions shown in Table 1. TOF-SIMS depth analysis and contact resistance were measured on the stainless steel materials.

[0070] <TOF-SIMS In-Depth Analysis>

[0071] TOF-SIMS depth analysis was performed on the conductive passivation film formation surface of each stainless steel material to determine the F value relative to sputtering time (depth). － CrO3 － and Fe2 － Ion strength. The detailed conditions for TOF-SIMS depth analysis are as follows.

[0072] Sputtering rate:

[0073] For Examples 1-6 and Comparative Examples 2-5, the sputtering rate was 0.0225 nm / s (SiO2 conversion) (sputtering time 4440 s, depth direction 100 nm), and for Comparative Example 1, the sputtering rate was 0.0045 nm / s (SiO2 conversion) (22200 s, depth direction 100 nm).

[0074] The TOF-SIMS 5 device manufactured by IONTOF GmbH was used as the TOF-SIMS measurement apparatus. The TOF-SIMS measurement was performed using Bi3. ＋＋ A cluster ion beam (accelerating voltage 60 keV, current measured using a Faraday cup of 0.4 pA) was used as the primary ion source to irradiate a 100 × 100 μm area at the center of the region irradiated by the sputtered ion beam used for depth-direction analysis, as described later. 2 The region is used to detect the secondary ions generated therefrom.

[0075] Using Cs ＋ Ion beams were used as sputtering ion sources for depth-oriented analysis to irradiate 500×500μm. 2The depth distribution was measured in the region while alternating sputtering and TOF-SIMS measurement (described later). In Examples 1-6 and Comparative Examples 2-5, irradiation was performed at an accelerating voltage of 0.5 keV and a current of 40 nA measured using a Faraday cup. In Comparative Example 1, irradiation was performed at an accelerating voltage of 0.25 keV and a current of 10 nA measured using a Faraday cup. The sputtering rates under these conditions are expressed as converted values ​​derived from the sputtering time of the thermal oxide film on a thermally oxidized silicon substrate (100 nm thick thermal oxide film (SiO2)). The sputtering rates for Examples 1-6 and Comparative Examples 2-5 were 0.0225 nm / s, and the sputtering rate for Comparative Example 1 was 0.0045 nm / s.

[0076] <Contact Resistance Measurement>

[0077] For each stainless steel material, the peak intensity (Ig) of the D-band in the Raman spectrum of the carbon layer was measured, as shown in Japanese Patent Application Laid-Open No. 2022-45138, for a material cut from a fuel cell separator (having a metal substrate, a corrosion-resistant metal interlayer formed on the metal substrate, and a carbon layer formed on the corrosion-resistant metal interlayer). D ) and the peak intensity of the G-band (I G The strength ratio of (I) D / I G The contact resistance between the fuel cell separators (with a value of 0.70 or higher and less than 0.95) was measured initially. It should be noted that the load pressure was 1.0 MPa. Subsequently, each stainless steel material was immersed in an FCC solution (ethylene glycol:water = 50:50) and placed at 85°C for 1000 hours (after FCC). Next, for each sample after the durability test, the contact resistance between the material and the material cut from the fuel cell separator was measured in the same manner as above. For practicality, the acceptable (○) standard for contact resistance was set at less than 1 mΩ·cm initially. 2 Less than 3mΩ·cm after FCC 2 If the contact resistance does not meet the acceptable standard, it is marked as unacceptable (×).

[0078] Table 1

[0079]

[0080] Table 1 lists the electrolysis conditions for process (A) as follows. Pulse electrolysis is as follows: Figure 6 The process involves alternating application of cathode and anode currents every 1 to 10 seconds. It should be noted that in the electrolysis conditions shown in Table 1, A represents the anode current density and C represents the cathode current density. For example, in the case of Example 1, it is shown that in step (A), according to... Figure 6 The curve shown is applied with an anodic current density of 0.36 A / dm. 2Cathode current density 3.0 A / dm 2 The current.

[0081] II. Results

[0082] exist Figures 1-4 The figure shows F relative to sputtering time (depth). － CrO3 － and Fe2 － Ion strength, shown in Table 1, is based on Figures 1-4 of, F － / Fe2 － Depth with an intensity ratio of 10 or higher (F) － / Fe2 － ) and CrO3 － / Fe2 － Depth with an intensity ratio greater than 1 (CrO3) － / Fe2 － ), Figure 5 F is shown in the middle － / Fe2 － The strength ratio of depth with CrO3 is greater than 10. － / Fe2 － The relationship between the strength ratio and the depth of the contact resistance, and the determination result based on the contact resistance.

[0083] Depend on Figures 1-5 As shown in Table 1, by making F － / Fe2 － The intensity ratio is greater than 10, and the depth is greater than 5 nm, and CrO3 － / Fe2 － The depth with an intensity ratio of 1 or higher is in the range of 6nm to 14nm, thus reducing the contact resistance in both the initial stage and after FCC.

[0084] In summary, by adjusting the concentration balance of F, Cr, and Fe in the conductive passivation film on the surface of stainless steel, sufficient corrosion resistance and low contact resistance can be achieved.

Claims

1. A stainless steel material, used for the separator of a fuel cell. For F measured by time-of-flight secondary ion mass spectrometry － CrO3 － and Fe2 － In terms of the depth analysis results of ion strength, F － / Fe2 － The intensity ratio is 10 or higher, and the depth is 5 nm or higher. CrO3 － / Fe2 － The depth with an intensity ratio of 1 or higher ranges from 6 nm to 14 nm.

2. The stainless steel material according to claim 1, wherein, F － / Fe2 － The intensity ratio is above 10 and the depth is below 14.0 nm.

3. A method for manufacturing a stainless steel material for a fuel cell separator, comprising (i) a step of imparting a conductive passivation film to a stainless steel substrate, (i) includes the following steps: (A) The process of injecting fluorine into the passivation film, (B) The process of modifying the passivation film in an alkaline aqueous solution or a non-aqueous solvent, and (C) The process of dissolving iron from the passivation film. (A) The process includes pulse electrolytic treatment of stainless steel in an aqueous solution containing fluoride ions. (A) The pulse electrolysis process includes an anode current density of 0.05 A / dm³. 2 ~1.0A / dm 2 And cathode current density 0.05A / dm 2 ~5.0A / dm 2 Electrolytic treatment under conditions where the electrolysis time is in the range of 10 seconds to 10 minutes. (B) The process includes pulse electrolytic treatment of stainless steel substrates in alkaline aqueous solutions or non-aqueous solvents.

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