Classifications

• • C—CHEMISTRY; METALLURGY
  • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
  • C25D—PROCESSES FOR THE ELECTROLYTIC OR ELECTROPHORETIC PRODUCTION OF COATINGS; ELECTROFORMING; APPARATUS THEREFOR
  • C25D9/00—Electrolytic coating other than with metals
  • C25D9/04—Electrolytic coating other than with metals with inorganic materials
  • C25D9/06—Electrolytic coating other than with metals with inorganic materials by anodic processes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/02—Details
  • H01M8/0202—Collectors; Separators, e.g. bipolar separators; Interconnectors
  • H01M8/0204—Non-porous and characterised by the material
  • H01M8/0206—Metals or alloys
  • H01M8/0208—Alloys
  • H01M8/021—Alloys based on iron
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01M—PROCESSES OR MEANS, e.g. BATTERIES, FOR THE DIRECT CONVERSION OF CHEMICAL ENERGY INTO ELECTRICAL ENERGY
  • H01M8/00—Fuel cells; Manufacture thereof
  • H01M8/10—Fuel cells with solid electrolytes
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
  • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
  • Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
  • Y02E60/30—Hydrogen technology
  • Y02E60/50—Fuel cells

Definitions

• This disclosure relates to corrosion-resistant stainless steel and a method for producing corrosion-resistant stainless steel.
• PEFC polymer electrolyte fuel cells
• Stainless steel which is generally known as a corrosion-resistant material, has a corrosion-resistant layer of trivalent chromium on the steel material, and is easy to process and inexpensive.
• the cathode is polarized to about 0.9 V, and the separator is in contact with the cathode. Therefore, the corrosion-resistant layer of trivalent chromium becomes a higher oxide and dissolves due to transpassive dissolution caused by the application of voltage, and the corrosion resistance is reduced. Therefore, it is difficult to use stainless steel as a separator for a PEFC in an unprocessed state.
• the above potential is a value measured based on a standard hydrogen electrode, i.e., 0.9 V vs. SHE.
• Figure 2 is a graph showing the potential-pH diagram (Pourbais diagram) of chromium.
• Figure 2 shows that chromium is stable as a trivalent oxide over a wide potential range, but at high potentials, such as the above 0.9 V vs. SHE, it is oxidized to hexavalent chromium and loses its corrosion resistance.
• JP 56-156771 A and JP 2016-166394 A propose methods for increasing the durability of stainless steel under certain high-temperature conditions, but there is no mention or suggestion of improving the corrosion resistance of stainless steel under conditions in which a voltage is applied to the stainless steel.
• Means for solving the above problems include the following aspects.
• ⁇ 1> A corrosion-resistant stainless steel having a stainless steel substrate and a surface layer containing tungsten oxide on at least one surface of the stainless steel substrate.
• ⁇ 2> The corrosion-resistant stainless steel according to ⁇ 1>, which is used as a separator for a polymer solid fuel cell.
• a method for producing a corrosion-resistant stainless steel comprising a step of anodically polarizing a stainless steel substrate at a potential of 0.9 V vs. SHE or more in a solution containing tungsten oxide ions and water and having a pH of 2 to 6, to form a surface layer containing tungsten oxide on the surface of the stainless steel substrate.
• ⁇ 5> The corrosion-resistant stainless steel according to ⁇ 1> or ⁇ 2>, wherein the content of tungsten oxide in the surface layer is 4.5 atom% or more in terms of tungsten based on the total mass of the surface layer.
• ⁇ 6> The method for producing the corrosion-resistant stainless steel according to ⁇ 4>, wherein the plating solution contains tungsten oxide ions.
• ⁇ 7> The method for producing a corrosion-resistant stainless steel according to ⁇ 4>, wherein the plating solution contains tungsten oxide in the form of a metal complex.
• a corrosion-resistant stainless steel that has good corrosion resistance and its durability even under conditions where a voltage is applied. Furthermore, another embodiment of the present disclosure provides a method for producing a corrosion-resistant stainless steel having good and durable corrosion resistance.
• FIG. 1 is a graph showing the potential-pH diagram (Pourbaix diagram) of tungsten.
• FIG. 2 is a graph showing the potential-pH diagram (Pourbaix diagram) of chromium.
• FIG. 3 is a graph showing potential-current curves of the corrosion-resistant stainless steel of Example 1 and the corrosion-resistant stainless steel of Comparative Example 1.
• a numerical range indicated using “to” means a range that includes the numerical values before and after "to" as the minimum and maximum values, respectively.
• the upper or lower limit value described in a certain numerical range may be replaced with the upper or lower limit value of another numerical range described in stages.
• the upper limit or lower limit of a certain numerical range may be replaced with a value shown in the examples.
• combinations of two or more preferred aspects are more preferred aspects.
• vs. SHE values measured with respect to the standard hydrogen electrode are expressed with the addition of "vs. SHE.”
• SHE Standard Hydrogen Electrode
• NHE Normal Hydrogen Electrode
• the corrosion-resistant stainless steel of the present disclosure has a stainless steel substrate and a surface layer containing tungsten oxide on at least one surface of the stainless steel substrate.
• the corrosion-resistant stainless steel of the present disclosure has the above-described configuration, and therefore has good corrosion resistance even in an environment where a voltage is applied, and the good corrosion resistance is maintained for a long period of time.
• the corrosion-resistant stainless steel of the present disclosure has a surface layer containing tungsten oxide on at least one surface of the stainless steel substrate, as a layer separate from the stainless steel substrate, and therefore differs from the technology disclosed in JP-A-56-156771, which involves vapor-depositing tungsten or tantalum onto an alloy substrate and heating it at a high temperature to cause tungsten or the like to penetrate and diffuse into the substrate, thereby preventing carbon deposition.
• a suitable method for forming a surface layer containing tungsten oxide will be described in detail in the section on the method for producing corrosion-resistant stainless steel.
• the surface layer includes at least tungsten oxide.
• tungsten oxide can take various valence forms such as WO 2 , WO 3 , W 2 O 5 , WO 4 -- , and WO 5 -- , and the term "tungsten oxide" in the present disclosure is used to include all of these modified forms.
• the thickness of the surface layer there is no particular limit to the thickness of the surface layer, and it may be in the range of 1 nm to 1 ⁇ m.
• the surface layer covers at least one surface of the stainless steel substrate, and it is preferred that at least one surface is covered with the surface layer in a state in which the stainless steel substrate is not exposed.
• the surface layer in the present disclosure may be formed by dissolving trivalent chromium from a passive film normally present on a stainless steel substrate and replacing it with tungsten oxide.
• the surface layer covering the surface of the stainless steel substrate can be confirmed, for example, by using Laser Induced Breakdown Spectroscopy (hereinafter also referred to as LIBS method).
• LIBS method the surface of the object is turned into plasma by a laser pulse, and the emitted color is detected in a wide band (deep ultraviolet to near infrared) by a high-resolution spectrometer, so that the elements at the laser irradiation site can be detected and confirmed.
• LIBS method the presence of tungsten on the surface can be confirmed, and thus the formation of a surface layer containing tungsten oxide can be confirmed.
• the LIBS analysis is performed using a laser-induced breakdown spectroscopy instrument (EA-300, Keyence Corporation).
• the content of tungsten oxide in the surface layer is preferably 4.5 atom % or more, and more preferably 5.0 atom % or more, calculated as tungsten, based on the total mass of the surface layer.
• the surface layer formed in Example 1 described below has the following composition as determined by LIBS analysis. (Composition of the surface layer in Example 1) Ni: 66.8 atom% O: 27.2 atom% W: 6.0 atom%
• an untreated stainless steel substrate eg, SUS304
• an untreated stainless steel substrate eg, SUS304
• the surface layer contains tungsten oxide.
• chromium is stable as a trivalent oxide over a wide potential range, but is oxidized to hexavalent chromium at a potential of about 0.9 V vs. SHE, which is the potential during operation of a PEFC, and loses its corrosion resistance.
• Fig. 1 is a graph showing the potential-pH diagram (Pourbais diagram) of tungsten. As is clear from the graph in Fig. 1, in the case of tungsten, tungsten oxide maintains stability even at a potential of 0.9 V vs.
• the tungsten oxide contained in the surface layer is expected to dissolve and fill the scratches, thereby suppressing the undesirable dissolution of iron, chromium, etc. from the stainless steel located below the surface layer.
• stainless steel substrate There are no particular limitations on the stainless steel substrate used in the corrosion-resistant stainless steel of the present disclosure, and any known stainless steel can be used.
• the stainless steel used for the stainless steel substrate can be used regardless of the content of nickel (Ni) and chromium (Cr) relative to iron (Fe).
• Examples of stainless steel include general-purpose SUS304, SUS316, SUS430, etc., but are not limited to the above examples of stainless steel, and various materials can be used depending on the application of the corrosion-resistant stainless steel.
• the corrosion-resistant stainless steel of the present disclosure has a stainless steel base material with good workability coated with a surface layer with good corrosion resistance, and therefore can be used in a variety of applications.
• the corrosion-resistant stainless steel may be produced by forming a surface layer after processing a stainless steel substrate into a desired shape, or the corrosion-resistant stainless steel may be produced first and then processed into a desired shape.
• Applications of corrosion-resistant stainless steel include separators for polymer solid fuel cells and bipolar plates for various energy devices, because corrosion resistance is maintained for long periods of time even under high temperature conditions and conditions of applied voltage.
• the corrosion-resistant stainless steel of the present disclosure is suitable as a separator for a polymer solid fuel cell.
• the corrosion-resistant stainless steel of the present disclosure has a stable surface layer and maintains the corrosion resistance of the stainless steel substrate even under conditions where a voltage of 0.9 V vs. SHE or more is applied. Therefore, it can be said that the effect is remarkable when it is used as a separator for a PEFC that is placed in contact with the cathode and used for a long period of time. That is, a PEFC separator using the corrosion-resistant stainless steel of the present disclosure has excellent corrosion resistance and maintains its corrosion resistance, and is also able to withstand long-term use.
• the stainless steel substrate has good workability and is inexpensive.
• the corrosion-resistant stainless steel of the present disclosure does not suffer from problems such as difficulty in processing and transpassive dissolution under applied voltage, which have been problems in the past when using stainless steel in PEFCs, and maintains excellent corrosion resistance, has a wide range of applications, and is also useful for reducing the cost of PEFCs.
• the manufacturing method of the corrosion-resistant stainless steel of the present disclosure includes a step of anodically polarizing a stainless steel substrate at a potential of 0.9 V vs. SHE or higher in a solution containing tungsten oxide ions and water and having a pH of 2 to 6, to form a surface layer containing tungsten oxide on the surface of the stainless steel substrate.
• a stainless steel substrate as an anode and a cathode (counter electrode) are immersed in an electrolyte solution containing tungsten oxide ions and water with a pH of 2 to 6, and a voltage is applied at a potential of 0.9 V vs. SHE or higher, which is the transpassive dissolution potential range of chromium, to polarize the anode and form a surface layer containing tungsten oxide on the surface of the stainless steel substrate.
• the anode uses a stainless steel substrate, which may be the same as the stainless steel substrate described in the corrosion-resistant stainless steel of the present disclosure.
• Cathode From the viewpoint of improving the corrosion resistance of the surface layer, it is preferable to use a metal plate containing a metal capable of forming a corrosion-resistant surface layer together with tungsten oxide as the cathode. From such a viewpoint, it is preferable to use, for example, a metallic nickel plate, a metallic tungsten plate, etc. as the cathode, and among them, it is preferable to use a metallic nickel plate.
• a solution containing tungsten oxide ions and water, with a pH of 2 to 6 In the manufacturing method of the present disclosure, a solution containing tungsten oxide ions and water and having a pH of 2 to 6 (hereinafter, sometimes simply referred to as the solution) is used. In preparing the solution, it is preferable to use water with a low impurity content, such as ion-exchanged water or pure water.
• tungsten oxide salt that dissolves in water to produce tungsten oxide ions, such as WO 4 2 ⁇ .
• a salt of tungstic acid is preferable, for example, a sodium salt of tungsten oxide, a potassium salt of tungsten oxide, etc.
• Na 2 WO 4 .2H 2 O, K 2 WO 4 , Li 2 WO 4 , etc. can be used.
• the content of the tungsten oxide salt compound in the solution can be selected depending on the purpose.
• the content of the tungsten oxide salt compound in the solution can be in the range of 0.1 M (molar) to 3 M.
• the solution preferably has a pH of 2 to 6, more preferably 4 to 5.
• the pH of the plating solution can be measured using a pH meter (HM-41X, DKK-TOA Corporation).
• the pH in this disclosure is a value measured at room temperature (25° C.).
• the solution may further contain, in addition to water, a raw material for tungsten oxide ions, and a complex-forming compound to be contained as desired, for example, known additives used in plating solutions depending on the purpose.
• known additives include pH adjusters.
• the pH adjuster may be, for example, an alkaline agent such as ammonia.
• the solution may contain metal ions contained in the metal plate forming the cathode.
• the solution may further contain nickel ions.
• the nickel ions may be contained in the form of complex ions.
• a known nickel compound can be used, such as NiSO 4 .6H 2 O.
• the content of the nickel ion raw material in the solution can be selected depending on the purpose. For example, when the nickel ion raw material is contained in the solution, the content of the nickel ion raw material can be in the range of 0.1 M to 3 M, which is approximately equimolar to the tungsten oxide ion raw material, relative to the solution.
• the nickel ions may be present in the form of complex ions.
• a complex-forming compound can be further added depending on the nickel ion content.
• the solution is preferably a plating solution.
• the solution used in the production of the corrosion-resistant stainless steel is preferably a plating solution containing tungsten oxide ions (also called tungstate ions) from the viewpoint of enabling the formation of a surface layer more efficiently.
• the plating solution contains nickel ions.
• the nickel ions can be contained in the form of complex ions, which are the reaction product of nickel ions and a complex-forming compound in a molar ratio of 1:1.
• a plating solution can be prepared by adding 0.1 M to 3 M NiSO4.6H2O , 0.1 M to 3 M Na2WO4.2H2O , which is an equimolar amount to the NiSO4.6H2O , and 0.2 M to 6 M citric acid, which is an equimolar amount to the total amount of the NiSO4.6H2O and the Na2WO4.2H2O , to ion-exchanged water, and adjusting the pH to 2 to 6, preferably 3 to 5, to obtain a plating solution.
• the stainless steel substrate as the anode electrode and the cathode electrode for example, a metallic nickel plate
• the plating solution which is an electrolytic solution
• a voltage of 0.9 V vs. SHE or higher which is the transpassive dissolution potential range of chromium
• Means for maintaining the temperature of the solution within the above range include a temperature sensor, a temperature control device equipped with a heater that is driven by data from the temperature sensor, a thermostatic bath, etc.
• the potential applied in the anodic polarization treatment is 0.9 V vs. SHE or more, and preferably 1.5 V vs. SHE or more. There is no particular upper limit to the potential, but from the viewpoint of the formability of the surface layer, it can be 3.0 V vs. SHE or less.
• the current density is preferably from +100 mA/cm 2 to +150 mA/cm 2 , and more preferably from +110 mA/cm 2 to +130 mA/cm 2 .
• the treatment time is preferably from 3 to 10 minutes, more preferably from 4 to 8 minutes.
• a surface layer containing tungsten oxide is formed on the surface of the stainless steel substrate used as the anode electrode.
• the method for confirming the formation of the surface layer is as described above.
• the solution preferably the plating solution
• the solution may be stirred.
• Stirring improves the formation of the surface layer.
• the stirring method include a method in which a bath filled with the solution is connected to a flow path equipped with a pump to circulate the solution, and a method in which a stirring device equipped with stirring blades or a stirring device such as a stirrer is used.
• the surface layer contains tungsten oxide, which provides good corrosion resistance even when a voltage of 0.9 V vs. SHE is applied to the surface layer, and this corrosion resistance is maintained for a long period of time, protecting the stainless steel substrate and providing good corrosion resistance.
• the manufacturing method disclosed herein does not require high energy compared to forming a film containing tungsten oxide by a deposition method or the like, can be carried out using known simple equipment, and can efficiently produce corrosion-resistant stainless steel that has good corrosion resistance even when a voltage is applied.
• the surface layer containing tungsten oxide formed by anodic polarization treatment in the passive dissolution potential region has good adhesion to the stainless steel substrate and is also durable.
• the manufacturing method of the present disclosure may include steps (other steps) other than the steps described above as necessary.
• Other steps include a polishing step of the stainless steel substrate as the raw material, a cleaning step of the stainless steel substrate, and a cleaning and drying step of the obtained corrosion-resistant stainless steel.
• Example 1 (1) Preparation of Electrodes
• a stainless steel substrate to be used as an anode electrode working electrode
• a SUS304 plate (length 5 cm, width 2 cm, thickness 0.1 mm) was prepared.
• the SUS304 plate was polished with diamond paste to remove impurities from the surface.
• a cathode electrode counter electrode
• a metallic nickel plate (length 5 cm, width 2 cm, thickness 1 mm, manufactured by Nilaco Corporation) was prepared.
• Example 2 An electrolyte solution was prepared in the same manner as in Example 1, except that the components added to the ion-exchanged water were changed as follows, and an anodic polarization treatment was carried out under the following conditions to obtain the corrosion-resistant stainless steel of Example 2. 0.15 M Na 2 WO 4 ⁇ 2H 2 O, and 0.3 M citric acid. The measured pH of the prepared electrolyte was 5.
• Example 3 An electrolyte solution was prepared in the same manner as in Example 1, except that the components added to the ion-exchanged water were changed as follows, and an anodic polarization treatment was carried out in the same manner as in Example 2, to obtain the corrosion-resistant stainless steel of Example 3. 0.15 M Na 2 WO 4 .2H 2 O, and 0.3 M citric acid. The measured pH of the prepared electrolyte was 2.3.
• Example 4 An electrolyte solution was prepared in the same manner as in Example 1, except that the components added to the ion-exchanged water were changed as follows, and an anodic polarization treatment was carried out in the same manner as in Example 2, to obtain the corrosion-resistant stainless steel of Example 4. 0.40 M Na 2 WO 4 .2H 2 O, and 0.3 M citric acid. The measured pH of the prepared electrolyte was 4.5.
• Comparative Example 1 As Comparative Example 1, a SUS304 plate, which was used as the raw material in Example 1 but had no surface layer formed thereon, was prepared.
• SSE written on the horizontal axis of FIG. 3 means a saturated KCl/silver-silver chloride electrode, which is a reference electrode having a potential difference of 0.199 V with respect to SHE at 25°C.
• SUS304 plate of Comparative Example 1 having no surface layer transpassive dissolution was observed, and in the region exceeding a potential of 0.5 V vs. SHE, the current density was higher than that of the corrosion-resistant stainless steel of Example 1, and it is understood that corrosion resistance cannot be expected.
• Comparative Example 1 Elements detected on the surface of SUS304 plate: Fe: 75.3 atom%, Cr: 18.5 atom%, Ni: 6.2 atom%
• the surface layers of Examples 1 to 4 contained nickel and tungsten.
• chromium and iron, which are inferior in corrosion resistance were not detected in the surface layer, and it was found that the surface layer was formed over the entire surface of the stainless steel base material. From the evaluation results of the corrosion-resistant stainless steels of Examples 2 to 4, it can be seen that, at the same level of evaluation, by using an electrolyte with a higher tungsten oxide content, the current density becomes smaller and the corrosion resistance is further improved.

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• 2023
  • 2023-09-26 JP JP2024550357A patent/JPWO2024071142A1/ja active Pending
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