JP4214921B2 - Fe-Cr alloy for fuel cell - Google Patents

Description

  The present invention relates to an Fe-Cr alloy for a fuel cell, and in particular, a solid oxide that has excellent oxidation resistance and electrical conductivity even when used at high temperature for a long time, and hardly reduces the power generation characteristics of the fuel cell. The present invention relates to a fuel cell Fe-Cr alloy used for an interconnector for a fuel cell and its peripheral members.

  Fuel cells are expected to be applied to a wide range of power generation systems such as large-scale power generation, cogeneration systems, and automobile power supplies because they emit less harmful gases and have high power generation efficiency. Among them, solid oxide fuel cells (also referred to as solid oxide fuel cells) operate at a high temperature of 700 to 900 ° C., and do not require the use of a catalyst for electrode reaction, It has excellent features such as the ability to use fuel gas and the combination with gas turbines utilizing high-temperature exhaust heat or steam turbine power generation, and has attracted attention as a next-generation energy source.

As shown in FIG. 1, this solid oxide fuel cell is composed of an electrolyte 1, electrodes 2 and 3, and an interconnector 4 (also referred to as a separator). An ion-conducting solid electrolyte such as stabilized zirconia (YSZ) is used, and anodes (air electrode) 2 such as (La, Sr) MnO ₃ and Ni / YSZ (cermet of Ni and yttria stabilized zirconia) are used on both sides. A cathode (fuel electrode) 3 is attached, and electricity is taken out by supplying a fuel gas 5 such as hydrogen gas to one side and an oxidizing gas 6 such as air to the other side using the electrolyte 1 as a partition. The interconnector 4 supports the three layers of the electrolyte 1, the anode 2, and the cathode 3, forms a gas flow path 7, and plays a role of flowing current.

In addition to oxidation resistance, the interconnector of the solid oxide fuel cell is required to have electrical conductivity, thermal expansion matching with other members, and the like. In order to obtain such characteristics, it is effective to adopt a Cr-containing alloy as a material for the interconnector and to produce a film mainly composed of Cr ₂ O _{3 on the} surface, and to obtain the effect of this film conventionally. Ingredient design has been performed. However, when a Cr-containing alloy is used, there is a problem that electrode performance deterioration (hereinafter referred to as “Cr poisoning”) easily occurs due to volatilization and precipitation of Cr-based oxides at a high temperature.

As a metal material for a solid oxide fuel cell that solves the above problems, for example, Patent Document 1 discloses that C: 0.1 wt% or less, Si: 0.5 to 3.0 wt%, Mn: 3.0 wt% or less, Cr : Austenitic stainless steel having 15 to 30 wt%, Ni: 20 to 60 wt%, Al: 2.5 to 5.5 wt%, and the balance being substantially Fe. In Patent Document 2, C: 0.2 wt% or less, Si: 0.2 to 3.0 wt%, Mn: 0.2 to 1.0 wt%, Cr: 15 to 30 wt% and Y: 0.5 wt% or less, REM: 0.2 wt% Hereinafter, Zr: 1 wt% or less containing at least one material, the balance being substantially composed of Fe is disclosed in Patent Document 3 as C: 0.2 wt% or less, Si: 3.0 wt% or less, Mn: 1.0 wt% Hereinafter, a material is disclosed in which Cr: 15 to 30 wt%, Hf: 0.5 wt% or less, and the balance substantially consisting of Fe. Further, Patent Document 4 includes C: 0.03 wt% or less, Mn: 2.0 wt% or less, Ni: 0.6 wt% or less, N: 0.03 wt% or less, Cr: 10.0 to 32.0 wt%, and Si: 2.0 There is disclosed a ferritic stainless steel containing at least one of wt% or less or Al: 6.0 wt% or less in total of 1.5 wt% or more and the balance being substantially made of Fe.
Japanese Patent Laid-Open No. 06-264193 JP 09-157801 A JP-A-10-280103 JP 2003-187828 A

However, the austenitic stainless steel of Patent Document 1 needs to add expensive Ni in a large amount of 20 to 60 mass%. In addition, since it contains a large amount of Al and Cr, an oxide film composed mainly of Al-based oxides is produced. Since this Al-based oxide has low electrical conductivity, it can be used as an interconnector for solid oxide fuel cells. Not suitable for use. Furthermore, the austenitic stainless steel has a larger coefficient of thermal expansion than that of the yttria-stabilized zirconia electrolyte (YSZ: 9 to 12 × 10 ⁻⁶ / ° C., austenitic system has a coefficient of thermal expansion from 20 ° C. to 900 ° C. (Stainless steel: 16 to 20 × 10 ⁻⁶ / ° C.) Therefore, there is a possibility that the electrolyte and the electrode may be cracked due to the difference in thermal expansion accompanying the temperature change that occurs when the fuel cell is started or stopped. Further, the materials of Patent Literature 2 and Patent Literature 3 are not materials designed from the viewpoint of preventing Cr poisoning, and when actually used as an interconnector, deterioration of electrode performance is inevitable. Furthermore, when the material of Patent Document 4 is used as an interconnector, it contains a large amount of Si or Al that forms an insulating oxide, so that the electrical resistance increases at high temperatures and the performance of the battery deteriorates. There is a problem.
As described above, conventional metal materials that have been developed for interconnectors of solid oxide fuel cells do not necessarily have sufficient characteristics.

  An object of the present invention is to provide an Fe—Cr alloy suitable for use in a solid oxide fuel cell that ensures basic characteristics such as oxidation resistance and electrical conductivity and is less susceptible to Cr poisoning. There is.

In order to solve the above-described problems of conventional metal materials, the inventors diligently studied a method for suppressing Cr poisoning without significantly reducing oxidation resistance and electrical conductivity. As a result, when 1.1 to 2.0 mass% of Mn is added to the Fe—Cr alloy, Mn ₂ O ₃ is generated near the surface layer of the steel plate at 700 to 900 ° C., and this Mn ₂ O ₃ is a Cr oxide. Newly found that Mn ₂ O ₃ has the effect of preventing electrode performance deterioration by suppressing volatilization of metal, and that Mn ₂ O ₃ has low electrical resistance and little decrease in electrical conductivity due to oxide film formation. It came to do.

The present invention based on the above knowledge is C: 0.20 mass% or less, Si: 1.0 mass% or less, Mn: 1.1 to 2.0 mass%, Cr: 10 to 40 mass%, Al: 1.0 mass% or less, Mo: 0.03 to 5.0 mass %, Nb: 0.1 to 3.0 mass%, and Si and Al are the following conditions;
Si + Al ≦ 1.2mass%
Is a Fe—Cr alloy for fuel cells, characterized in that the remainder is composed of Fe and inevitable impurities.

Further, Fe-Cr-based alloys of the present invention, in addition to the above chemical composition, it is preferable to have 0.005 to 1.0 mass% including the La.

  The Fe—Cr alloy of the present invention is preferably used for a solid oxide fuel cell, particularly an interconnector for a solid oxide fuel cell.

  According to the present invention, by adding Mn to an Fe—Cr alloy, it is possible to obtain a metal material for a fuel cell that is excellent in properties such as oxidation resistance and electrical conductivity and hardly causes Cr poisoning. Therefore, when the Fe-Cr alloy of the present invention is used for an interconnector of a solid oxide fuel cell, deterioration of the power generation performance of the cell can be suppressed even when used at a high temperature and for a long time. Greatly contributes to the increase in size and size.

The reason why the component composition of the Fe-Cr alloy for fuel cells according to the present invention is limited to the above range will be described.
C: 0.20 mass% or less C is an element having an action of forming carbides and increasing high-temperature strength. In order to obtain this effect, 0.001 mass% or more is preferably added. However, if the C content exceeds 0.20 mass%, the workability deteriorates, and the amount of Cr effective for oxidation resistance is reduced by combining with Cr. Therefore, it is necessary to limit it to 0.20 mass% or less. Preferably it is 0.10 mass% or less.

Mn: 1.1-2.0mass%
Mn is an important element having an effect of preventing deterioration of power generation characteristics of the fuel cell by generating Mn ₂ O ₃ on the surface layer of the steel sheet and suppressing volatilization of Cr-based oxides. In order to acquire this effect, it is necessary to add 1.1 mass% or more. More preferably, it is 1.5 mass% or more. However, excessive addition causes an increase in oxidation rate and lowers electrical conductivity, so it is limited to 2.0 mass% or less.

Cr: 10-40mass%
Cr is an important element necessary for producing a Cr ₂ O ₃ film on the surface layer of the steel sheet and ensuring oxidation resistance and electrical conductivity. In order to acquire such an effect, addition of 10 mass% or more is required. On the other hand, excessive addition of Cr promotes volatilization of Cr-based oxides and causes deterioration of workability, so the upper limit is limited to 40 mass%. Preferably it is 10-30 mass%.

Si: 1.0 mass% or less, Al: 1.0 mass% or less, Si + Al ≦ 1.2 mass%
Since Si and Al form an insulating oxide film and reduce electrical conductivity, each is limited to 1.0 mass% or less. In addition, since Si has an affinity for oxygen greater than that of elements such as Fe, Cr, and Mn and smaller than that of Al, it lowers the oxygen partial pressure in the initial stage of oxidation, making it easier to produce an insulating Al ₂ O ₃ film. . However, when the amount of Al is small, an Al ₂ O ₃ film is not generated even if Si is contained to some extent, so the total amount needs to be limited to 1.2 mass% or less. A preferable total amount is 1.0 mass% or less.

Mo: 0.03-5.0mass%
Mo is an element that increases the high-temperature strength and improves the oxidation resistance. To obtain this effect, it is necessary to add 0.03 mass% or more. However, if the added amount exceeds 5.0 mass%, the workability deteriorates, so the content was set to 5.0 mass% or less. Preferably, Mo is 0.1 to 3.0 mass%.

Nb: 0.1-3.0mass%
Nb, like Mo, has the effect of increasing high temperature strength and improving oxidation resistance. In order to obtain this effect, it is necessary to add 0.1 mass% or more. However, if the addition amount exceeds 3.0 mass%, the workability deteriorates, so 3.0 mass% is the upper limit. Preferably it is Nb: 0.1-2.0mass%.

Fe-Cr-based alloys of the present invention, in addition to the above essential components, if necessary, can be added La 0.005 to 1.0 mass%.
La has the effect of improving the adhesion of the oxide film and improving the oxidation resistance. In order to obtain the effect , 0 . It is preferable to add 005 mass% or more. However, excessive addition degrades hot workability, so 1 . Limit to 0 mass% or less. Preferably it is 0.005-0.5 mass%.
Furthermore, the Fe—Cr alloy of the present invention is composed of one or more selected from Sc, Y, Ce, Pr, Nd, Pm, Sm, Zr and Hf having the same effect as La. In addition, it can be added in a range of 0.005 to 1.0 mass% in total.

Furthermore, in addition to the above components, the Fe—Cr alloy of the present invention contains Cu, Ni, V, W, Ta, Ti, Mg and Ca, if necessary, Cu: 0.20 mass% or less, Ni: 1.0 mass %, V: 1.0 mass% or less, W: 3.0 mass% or less, Ta: 2.0 mass% or less, Ti: 0.5 mass% or less, Mg: 0.05 mass% or less, Ca: 0.05 mass% or less Can do.
Components other than those described above are Fe and inevitable impurities. However, the impurities P, S, and N are preferably limited to P: 0.05 mass% or less, S: 0.05 mass% or less, and N: 0.5 mass% or less, respectively.

Next, the Fe—Cr alloy for fuel cells of the present invention will be described.
Mn ₂ O ₃ in the surface oxide
The Fe-Cr alloy of the present invention prevents the volatility of the Cr-based oxide and reduces the power generation characteristics of the fuel cell even when used for a long time at a solid oxide fuel cell operating temperature of 700 ° C to 900 ° C. It has an excellent feature of suppressing. In order to exhibit the effect, it is necessary to generate Mn ₂ O ₃ in the oxide film formed on the surface layer of the interconnector under the operating environment of the fuel cell. That is, Mn ₂ O ₃ is generated in the oxide generated on the surface layer of the interconnector under a high temperature and long-time use environment of 700 ° C. to 900 ° C. which is the operating temperature of the solid oxide fuel cell. ₂ O ₃ can suppress the volatilization of the Cr-based oxide and prevent the power generation characteristics of the fuel cell from being deteriorated. The presence or absence of the generation of Mn ₂ O ₃ is determined by the following formula when the oxide generated on the surface of the sample held in the atmosphere at 800 ° C. for 1000 hours is X-ray diffracted:
Relative intensity = I (Mn peak intensity of (222) plane of the ₂ O ₃₎ / I ₀ (peak intensity of (222) plane of _{Mn 2 O 3) × 100 (} %)
It can be determined by whether or not the relative intensity defined by is 1% or more. Even if the amount of Mn ₂ O ₃ produced is extremely small, the above effect can be obtained because the ratio of the Cr-based oxide present in the outermost layer is greatly reduced.

Electrical resistance value: 50 mΩ · cm ² or less The electrical resistance value (area resistivity) of the Fe—Cr alloy for fuel cells targeted by the present invention is 50 mΩ · cm ² or less. As shown in FIG. 2, the electric resistance value (area resistivity) here is a cold rolled steel sheet or hot rolled steel sheet with a thickness of 20 mm square sandwiched between Pt plates with a thickness of 1 mm x 20 mm. Bonding Pt wires for current application and voltage measurement to the upper and lower Pt plates, and further applying a 0.2MPa load, this was left in a furnace at 800 ° C in an air atmosphere for 1 hour, and 1.2A It is an electrical resistance value obtained by passing a current and measuring the voltage between the upper and lower Pt plates and multiplying the resistance value by an area of 4 cm ² . When this electrical resistance value exceeds 50 mΩ · cm ² , the performance of the battery is remarkably deteriorated. Therefore, this electrical resistance value is preferably 50 mΩ · cm ² or less. More preferably, it is 45 mΩ · cm ² or less, and further preferably 40 mΩ · cm ² or less.

Voltage drop rate: 2.0% or less The target characteristic of the Fe-Cr alloy for fuel cells of the present invention is that the voltage drop rate is 2.0% when used as a material for an interconnector of a solid oxide fuel cell. It is said. Here, the voltage reduction rate means the rate of reduction of the generated voltage when power is generated at an operating temperature of 700 to 900 ° C. with a current density of 0.2 A / cm ² and generated for 1000 hours with respect to the initial voltage. When the voltage drop rate exceeds 2.0%, it can be evaluated that the long-term durability of 80 to 100,000 hours required for the fuel cell cannot be obtained. Therefore, the target voltage drop rate is 2.0% or less. Preferably it is 1.8% or less.

Next, a method for producing the Fe—Cr alloy of the present invention will be briefly described.
The melting method of the Fe—Cr alloy according to the present invention can be a generally known method, and therefore, it is not necessary to specifically limit it. For example, the steelmaking process is performed as described above using a converter, an electric furnace, or the like. It is preferable to melt molten steel adjusted to the composition range and perform secondary refining by strong stirring and vacuum oxygen decarburization treatment (SS-VOD). The casting method is preferably a continuous casting method in terms of productivity and quality. The steel slab (slab) obtained by casting is reheated as necessary, hot-rolled, hot-rolled sheet annealed at 700-1200 ° C, descaled by pickling or shot blasting, etc. A steel plate is used.

  The hot-rolled steel sheet is suitable as a material for forming an interconnector by forming a gas flow path by cutting, and the thickness in that case is preferably 0.6 to 8.0 mm. On the other hand, when producing an interconnector by press working, the hot-rolled steel sheet is further cold-rolled, or further annealed at a temperature of 700 to 1200 ° C., and pickled and cold-rolled steel sheet is used as a material. Is preferred. In this case, the plate thickness is preferably 0.03 to 2.0 mm. Of course, a hot-rolled steel sheet may be used as a pressing material, and a cold-rolled steel sheet may be used as a cutting material. Further, the groove processing for forming the gas flow path may be performed by a method other than the above-described cutting processing or press processing, for example, a corrugate processing, an etching processing, a coining processing, or the like.

  The hot-rolled steel plate or cold-rolled steel plate can also be used for fuel cell constituent members (for example, heat exchangers, reformers, etc.) other than the solid oxide fuel cell interconnector. In addition, when welding is performed when forming these members, the welding method is not particularly limited. For example, MIG (Metal Inert Gas), MAG (Metal Active Gas), TIG (Tungsten Inert Gas) ), High-frequency resistance welding, high-frequency induction welding, brazing, and the like, such as resistance welding methods such as spot welding and seam welding, and electric key welding methods, can be applied.

Various Fe—Cr alloys having the composition shown in Table 1 were melted in the converter-secondary refining process, and slabs having a thickness of 200 mm were formed by continuous casting. After heating these slabs to 1250 ° C., they were hot-rolled to obtain hot-rolled steel sheets having a thickness of 5 mm, and subjected to hot-rolled sheet annealing at 800 to 1100 ° C. and pickling treatment. Next, a cold-rolled steel sheet having a thickness of 1 mm was formed by cold rolling, annealing was performed at 750 to 1100 ° C., and pickling was performed. A sample was cut out from the cold-rolled annealed plate and subjected to the following test.
<Oxidation test>
A 1mm x 30mm x 30mm sample was cut out from the cold-rolled annealed plate, and an oxidation test was performed by heating and holding in a furnace heated to 800 ° C in an air atmosphere for 1000 hours. Diffracted to identify the product oxide.
<Measurement of electrical resistance value>
As shown in Fig. 2, the electrical resistance value is obtained by sandwiching a sample after oxidation test (20mm x 20mm x 1mm thickness) using a 1mm x 20mm square Pt plate, and applying current to the upper and lower Pt plates. Join a Pt wire for voltage measurement, leave it in a furnace at 800 ° C for 1 hour under a load of 0.2 MPa, measure the voltage when a current of 1.2 A is passed, and obtain the resistance value. The electrical resistance (area resistivity) was determined by multiplying the area of the sample by 4 cm ² . For each sample, n number of 3 was measured and the average value thereof was defined as the electric resistance value.

Moreover, the cold-rolled annealing board obtained as mentioned above was processed into the interconnector, and the power generation characteristics were evaluated in the following manner.
A 0.2 mm x 105 mm thick electrolyte plate sintered with yttria-stabilized zirconia (YSZ) is mixed with NiO powder and YSZ powder on one side so that the mass ratio of NiO and YSZ is 4: 6. The cathode (fuel electrode) material was mixed, and the anode (air electrode) material mixed on the other side with a weight ratio of 8: 2 of La _0.8 Sr _0.2 MnO ₃ powder and YSZ powder. A single cell was produced by screen printing to 50 μm and firing at 1400 ° C. On the both sides of this single cell, a cold-rolled annealed plate cut out to a thickness of 1 mm × 105 mm square, and as shown in Fig. 3, cut a gas channel groove with a depth of 0.5 mm at 5 mm width and 5 mm intervals. A solid oxide fuel cell was manufactured by arranging the interconnector. Ultra high purity hydrogen (purity 99.9999%) as a fuel gas was flowed to the fuel electrode side of the fuel cell thus obtained, and air with a dew point of 30 ° C. was flowed to the air electrode side as an oxidizing gas, and the temperature was 750 ° C. Then, power generation at a current density of 0.2 A / cm ² was performed, and the voltage drop rate after 1000 hours was measured. The above measurement was carried out for each Fe—Cr alloy at an n number of 3, and the average value thereof was determined to evaluate the long-time power generation characteristics.

The results of the above measurements are summarized in Table 2.
The materials No. 1 to 9 and No. 20 to 22 in which Si, Mn, Cr, and Al in the Fe-Cr alloy for interconnectors are in the composition range of the present invention are all included in the oxide film after the oxidation test. The formation of ₂ O ₃ was confirmed, and the voltage drop rate was as low as 1.5% or less, indicating good power generation characteristics. This result is considered to be the result of suppressing the volatilization of Cr-based oxides by the formation of Mn ₂ O ₃ in the oxide film by the appropriate addition of Mn.

In contrast, the materials No. 10 to 19 in which Si, Mn, Cr, and Al are not included in the component range of the present invention have a voltage drop rate exceeding 1.5%, and are required for the fuel cell. Has not been achieved. For example, Nos. 17 to 19 are the conventional materials (No. 5 in Table 2 of JP-A-9-15801, No. 3 in Table 1 of JP-A-10-280103, and Table 1 in JP-A-2003-187828). No. 1) of No. 1) shows the same measurement, but No. 17 and 18 do not produce Mn ₂ O ₃ due to the low Mn content and volatilize Cr-based oxides. The voltage drop rate is increased without being suppressed. In No. 19, an Mn content is small and an insulating oxide (SiO ₂ does not form a complete crystal) that cannot be identified by X-ray diffraction due to a large amount of Si is formed. As a result of the increase in resistance, the power generation performance is greatly reduced.

In addition, since the materials No. 10, No. 15 and 16 have any of Si, Al, and Si + Al in the composition range of the present invention, the electrical conductivity is poor and the power generation performance is greatly reduced. Further, when the amount of Mn is insufficient as in No. 11, Mn ₂ O ₃ is not generated, and deterioration of electrode performance due to volatilization of Cr-based oxides cannot be prevented. On the other hand, when Mn is added excessively as in No. 12, the oxidation rate increases and an increase in electrical resistance due to the growth of the oxide film is inevitable. In addition, the No. 13 material had poor oxidation resistance due to the lack of Cr content, and abnormal oxidation occurred in the oxidation test to produce Fe ₃ O ₄ . On the other hand, when Cr is excessively contained as in No. 14, the amount of Cr-based oxide generated increases, and even when Mn ₂ O ₃ is generated, volatilization of the Cr-based oxide cannot be sufficiently suppressed and power generation is performed. The performance is degraded. In addition, since the materials No. 23 and 24 have less Mo and Nb than the component ranges of the present invention, the oxidation resistance is insufficient, and the decrease in electrical conductivity due to the growth of the oxide film cannot be prevented. It is falling.

  The metal material of the present invention can be used for the above-described interconnector for solid oxide fuel cells and heat exchangers and reformers that are peripheral members thereof, as well as other fuel cells and automotive materials, and It can also be suitably used for materials such as boilers and gas turbines in which deterioration of materials due to volatilization of Cr-based oxides is a problem.

It is a schematic diagram for demonstrating the structure of a solid oxide fuel cell. It is a figure which shows typically the apparatus which measures the electrical resistance value of an interconnector. It is a figure which shows typically the apparatus which measures the electric power generation characteristic of a solid oxide fuel cell.

Explanation of symbols

1: Electrolyte 2: Electrode (Anode, Air electrode)
3: Electrode (cathode, fuel electrode)
4: Interconnector (separator)
5: Fuel gas (hydrogen gas)
6: Oxidizing gas (air)
7: Gas flow path (groove)
8: Electronics (electricity)

Claims (4)

C: 0.20 mass% or less,
Si: 1.0 mass% or less,
Mn: 1.1-2.0 mass%,
Cr: 10 to 40 mass%,
Al: 1.0 mass% or less,
Mo: 0.03-5.0 mass%,
Nb: 0.1 to 3.0 mass%, and Si and Al are the following conditions;
Si + Al ≦ 1.2 mass%
Satisfying and containing
A Fe-Cr alloy for a fuel cell, wherein the balance is Fe and inevitable impurities. In addition to the above chemical composition, La and 0.005 to 1.0 mass% for fuel cell Fe-Cr-based alloy according to claim 1, characterized in that it has free. The Fe-Cr alloy for a fuel cell according to claim 1 or 2, wherein the Fe-Cr alloy is used for a solid oxide fuel cell. 3. The Fe—Cr alloy for a fuel cell according to claim 1, wherein the Fe—Cr alloy is used for an interconnector of a solid oxide fuel cell.

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