JP2005213539A - Ferritic stainless steel for solid polymeric fuel cell separator - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a ferritic stainless steel which is suitable for a solid polymeric fuel cell separator and maintains excellent elution resistance and low surface contact resistance even in the high-humidity and acidic atmosphere in the cell for a long period of time. <P>SOLUTION: The ferritic stainless steel for the solid polymeric fuel cell separator contains ≤0.020 mass% C, ≤0.50 mass% Si, ≤0.40 mass% P, ≤0.50 mass% Si, ≤0.50 mass% Ni, 28 to 32 mass% Cr, 1.5 to 2.5 mass% Mo, ≤0.80 mass% Cu, 0.03 to 0.25 mass% Nb, 0.03 to 0.25 mass% Ti, over 0.20 mass% and ≤1.00 mass% Al, and ≤0.020 mass% N, contains 0.2 to 1.0 mass% V at need, and satisfies ≤0.025 mass% C+N, and ≤0.80 mass% Ni+Cu. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a stainless steel for a separator incorporated in a polymer electrolyte fuel cell that is promising as an in-vehicle power source, a home cogeneration system, and the like.

Fuel cells include phosphoric acid type, molten carbonate type, solid polymer type, solid electrolyte type, etc., but there is little emission of CO ₂ , NOx, SOx, etc., and solid polymer showing very high power generation efficiency Type fuel cells are promising. Solid polymer fuel cells are capable of operating at a low temperature of 100 ° C. or less and can be started up in a short time, and therefore are beginning to be adopted as power sources for automobiles, stationary devices, mobile devices and the like.
The fuel cell is assembled by stacking several tens to several hundreds of unit cells of the minimum unit in order to extract electric power for practical use. Each unit cell utilizes the fact that an ion exchange membrane made of a solid polymer resin having a proton exchange group functions as a proton-conductive electrolyte, fuel gas on one side of the ion exchange membrane, air on the other side, It has a structure for flowing an oxidizing gas such as oxygen.

Specifically, an oxidation electrode 2 and a fuel electrode 3 are joined to both sides of the ion exchange membrane 1, and a separator 5 is opposed to each of the oxidation electrode 2 and the fuel electrode 3 via a gasket 4 (FIG. 1a). An oxidizing gas supply port 6 and a discharge port 7 are formed in the separator 5 on the oxidation electrode 2 side, and a fuel gas supply port 8 and a discharge port 9 are formed in the separator 5 on the fuel electrode 3 side. In addition, a plurality of grooves 5g are formed in the separator 5 to achieve conduction and uniform distribution of the fuel gas g and the oxidizing gas o (FIG. 1b).
In order to increase the ion conductivity of the ion exchange membrane 1, hydrogen humidified by a method of passing warm water heated to around 90 ° C. is used for the fuel gas g. In some cases, the oxidizing gas o may be humidified. When the humidified fuel gas g and oxidizing gas o are sent into the cell, the separator 5 is exposed to a high humidity atmosphere. SO ₄ ²⁻ , F ^{− and the} like generated by decomposition of the resin component of the ion exchange membrane 1 may adhere to the surface of the separator 5. As a result, the separator 5 is placed in a corrosive atmosphere in which corrosion and elution are likely to occur. When corrosion and elution occur, the metal ions dissolved from the separator 5 promote the decomposition of the ion exchange membrane 1 or the catalyst in the electrodes 2 and 3 is contaminated, so that the output and durability of the fuel cell are reduced. .

Since the separator material is required to have chemical stability due to the performance of the fuel cell, a carbon block formed into a predetermined shape by cutting or forming, a carbon resin formed by compression, or the like has been conventionally used. However, the processing cost is high and it is difficult to reduce the thickness required for reducing the weight of the fuel cell. Therefore, it has been studied to use stainless steel that can be molded into a predetermined shape as a separator material for fuel cells (Patent Documents 1 and 2).
Japanese Patent No. 3097690 Japanese Patent No. 3269479

Stainless steel proposed as a separator for fuel cells uses Cr and Mo as main corrosion resistance improving elements, and Cr: 10.5 to 35% by mass, Mo: 0.2 to 6.0% by mass The addition amount is selected. Ti, Nb, etc. may be added. However, high corrosion resistant steels such as SUS436 and SUS444 to which Cr, Mo, Ti, Nb, etc. are added have good corrosion resistance in acid environments, but metal elements are eluted with the progress of corrosion, so the corrosion resistance in separator environments is Not necessarily enough. In fact, in fuel cells incorporating high corrosion resistant steel such as SUS436 and SUS444 as separators, metal ions are leached and output tends to decrease early. Among these, Fe, which is contained most in stainless steel, has an adverse effect of decomposing the ion exchange membrane as Fe ions.
Among ferritic stainless steels, SUS447J1 is listed as the steel type with the most excellent corrosion resistance. SUS447J1 containing 30Cr-2Mo as a basic component is excellent in corrosion resistance and has much less metal ion elution than other steel types. However, SUS447J1 shows higher contact resistance because the steel plate surface is covered with a chromium-based passive film (mixed film of oxide and hydroxide) that is more stable than other ferritic stainless steels. Also, if the passive film is non-uniform in thickness and has defects, the contact resistance is low but the elution resistance is inferior, and Fe in the eluted ions promotes the decomposition of the ion exchange membrane, and the use time becomes longer. Battery performance is degraded.

Therefore, if ferritic stainless steel with SUS447J1 class elution resistance and low contact resistance can be applied to fuel cell separators in a solid state, cost reduction and weight reduction will be achieved, and the practical application of fuel cells will greatly advance. Conceivable. In addition, elution resistance is the most important for stationary fuel cells for home use with a total operating time of tens of thousands of hours, and contact resistance is considered to be the most important for movable fuel cells for automobiles in order to secure space on the vehicle. In addition, a solid stainless steel material most suitable for each fuel cell application is desired.
The present invention is based on knowledge found as a result of investigating and examining the influence of alloy components on corrosion and metal ion elution in the separator environment, and by selecting a ferritic stainless steel having a specific composition as a separator material, An object of the present invention is to provide a fuel cell having improved durability by suppressing the decomposition and deterioration of an ion exchange membrane that causes a decrease in output.

  The ferritic stainless steel for the polymer electrolyte fuel cell separator of the present invention has C: 0.020 mass% or less, Si: 0.50 mass% or less, Mn: 0.50 mass% or less, P: 0.040 mass. %: S: 0.005 mass% or less, Ni: 0.50 mass% or less, Cr: 28-32 mass%, Mo: 1.5-2.5 mass%, Cu: 0.80 mass% or less, Nb: 0.03 to 0.25% by mass, Ti: 0.03 to 0.25% by mass, Al: more than 0.20% by mass and 1.00% by mass or less, N: 0.020% by mass or less, necessary V: 0.2 to 1.0% by mass, the balance is substantially Fe, and is restricted to C + N: 0.025% by mass or less and Ni + Cu: 0.80% by mass or less. And

  In the ferritic stainless steel for the polymer electrolyte fuel cell separator of the present invention, elution resistance is improved by proper management of Cr and Mo contents, and further, the original corrosion resistance of stainless steel is maintained by strict management of P content. However, the surface contact resistance is reduced. Therefore, it becomes a fuel cell separator with much better processability and productivity than graphite separators, and there is little internal loss due to surface contact resistance even when multiple single cells are stacked, and fuel cell with high power generation efficiency Is obtained. In particular, since it has elution resistance at a low level not found in conventional ferritic stainless steel, it is also suitable as a stationary fuel cell separator for home use with a total operation time of tens of thousands of hours.

  Although the passive film on the surface of the stainless steel plate is effective for the development of corrosion resistance, it is a cause of increasing the surface contact resistance because it is composed of a mixture of oxide and hydroxide with high specific electrical resistance. Moreover, although a strong passive film is advantageous for corrosion resistance, it further increases the surface contact resistance, so it can be said that the improvement in corrosion resistance and the reduction in surface contact resistance are contradictory. Therefore, the present inventors first examined an optimal component system that suppresses the elution of metal ions, and further investigated and studied a method for reducing surface contact resistance without significantly degrading the elution resistance by adding a third element. .

  In the process of investigation and examination, it was found that the amount of ions eluted from 18Cr austenitic stainless steel containing 3% by mass or more of Cu was large. Among them, Cu and Ni were much eluted, and Fe ions having an adverse effect on the electrolyte membrane were also detected. The cause of the increase in the amount of eluted ions is not always clear, but in stainless steel containing a large amount of Cu and Ni, the passive film on the steel plate surface also contains Cu and Ni, and the passive film is not removed during fuel cell operation. It is presumed that Cu and Ni are dissolved and Fe is eluted from the substrate through defects generated in the passive film due to elution of Cu and Ni.

  On the other hand, as a result of studying a component system that suppresses the elution amount low with a ferritic stainless steel not containing a large amount of Cu and Ni, Cr: 28% by mass or more, Mo: 1.5% by mass or more, Ni + Cu: 0.80% by mass It was found that the required characteristics of the separator material are satisfied below. That is, when Cr and Mo are contained as much as possible within a range that does not adversely affect manufacturability and the Cr concentration of the passive film is increased, excellent elution resistance can be obtained even if Ni and Cu are contained to some extent. It can be maintained.

Furthermore, as a result of detailed examination of the effect of the third element in the component system to which Cr and Mo are added, the level is higher than the level contained in ordinary ferritic stainless steel (specifically, an amount exceeding 0.20% by mass, preferably It has been found that when Al is added in an amount of 0.25 mass% or more, the surface contact resistance is slightly lowered and the deterioration of the ion exchange membrane is suppressed. The reason for this is not necessarily clear, but is presumed as follows.
Al is concentrated on the surface of Al-added stainless steel not used in the separator, and Al is detected on both the stainless steel surface and the electrolyte membrane after use. For this reason, in this component system Al-added stainless steel, the contact resistance is lowered due to inclusion of a compound such as AlN in addition to oxide (alumina (Al ₂ O ₃ )) having a very low electrical conductivity. It is thought to do.

  In addition, Al is an element that is relatively easy to elute in the high humidity and acidic environment of the separator, but the effect of the eluted Al on the decomposition and deterioration of the ion exchange membrane is small. There is a tendency to suppress degradation and deterioration of the exchange membrane. Therefore, Al is substituted in the portion where the molecules constituting the ion exchange membrane are missing, and the substitution with the eluted Fe is also considered to be one factor for preventing the deterioration of the ion exchange capacity.

Hereinafter, the alloy components, contents, and the like of the ferritic stainless steel used in the fuel cell separator of the present invention will be described.
[C, N: 0.020 mass% or less]
C and N are components that lower the workability and low-temperature toughness of ferritic stainless steel. In this component system containing a large amount of Cr and Mo, C and N are reduced as much as possible to ensure workability and low-temperature toughness. It is preferable to do. For this reason, the upper limit of the C and N content is specified as 0.020 mass%, and the total content of C and N is regulated to 0.025 mass% or less. In order to secure a higher level of workability and low temperature toughness, the total content of C and N is preferably regulated to 0.020% by mass or less.
[Si, Mn: 0.50 mass% or less]
Since Si hardens ferritic stainless steel and Mn reduces corrosion resistance, the lower the content of both Si and Mn, the more preferably the upper limit of the content is regulated to 0.50% by mass.

[P: 0.040% by mass or less]
In this component system, it is possible to include P in a range acceptable for ordinary ferritic stainless steel. As the P content increases, the stainless steel hardens and the workability deteriorates, so the upper limit of the P content is set to 0.040% by mass.

[S: 0.005 mass% or less]
Since it adversely affects the corrosion resistance of stainless steel, it is preferable to reduce it as much as possible. In this component system, the upper limit of the S content is defined as 0.005% by mass.
[Ni: 0.50 mass% or less, Cu: 0.80 mass% or less]
Addition of a large amount of Ni and Cu causes degradation of elution resistance. Therefore, the upper limit of the content is set to Ni: 0.50% by mass and Cu: 0.80% by mass, respectively, and the total content of Ni and Cu is set to 0. It was regulated to .80% by mass or less. However, it improves the overall corrosion resistance under acidic atmosphere and greatly improves the low temperature toughness of ferritic stainless steel by adding a small amount, thus improving the overall corrosion resistance and low temperature toughness in the range where metal ions are not eluted. Therefore, the Ni and Cu contents are preferably selected in the range of Ni: 0.15 to 0.35 mass%, Cu: 0.20 to 0.50 mass% or less, and Ni + Cu: 0.50 mass% or less.

[Cr: 28-32% by mass]
In order to ensure corrosion resistance in the separator environment, a Cr content of 28% by mass or more is necessary. Corrosion resistance improves with an increase in Cr content, but excessive addition deteriorates workability and low temperature toughness, so the upper limit of Cr content was regulated to 32 mass%.
[Mo: 1.5 to 2.5% by mass]
In order to ensure corrosion resistance in the separator environment, 1.5% by mass or more of Mo is necessary, but excessive addition hardens stainless steel, so the upper limit of the Mo content was regulated to 2.5% by mass.

[Nb, Ti: 0.03 to 0.25% by mass]
It is an effective component for improving the corrosion resistance and workability of the weld zone. In this component system, C is fixed by Nb and N is fixed by Ti. The action of fixing C and N is observed when Nb and Ti are added in an amount of 0.03 mass% or more, but excessive addition exceeding 0.25 mass% adversely affects workability and low temperature toughness.
[Al: more than 0.20% by mass and 1.00% by mass or less]
It is an effective component for reducing the surface contact resistance and suppressing the deterioration of the ion exchange membrane, and the effect of adding Al is seen in an amount exceeding 0.20% by mass. Al is a component that fixes N and is effective in improving the corrosion resistance of the welded portion. However, excessive addition adversely affects the toughness and manufacturability of ferritic stainless steel, so the upper limit of Al content is set to 1.00% by mass. In order to reduce the surface contact resistance and suppress the deterioration of the ion exchange membrane within a range not causing toughness deterioration and manufacturability deterioration, the Al content is set to 0.20 to 1.00% by mass (further, 0.25 to 0). (80 mass%) is preferable.

[V: 0.2 to 1.0% by mass]
It is a component added as necessary, and exhibits the effect of improving the corrosion resistance of stainless steel in an acidic atmosphere. Under the atmosphere in the cell containing SO ₄ ²⁻ and F ⁻ of the fuel cell, the corrosion resistance is improved by adding 0.2 mass% or more of V. However, excessive addition adversely affects low temperature toughness and manufacturability, so when V is added, the upper limit is regulated to 1.0% by mass.
[Other ingredients]
It is possible to add other components to the ferritic stainless steel used in the present invention as long as the production cost, corrosion resistance, and contact resistance are not significantly impaired. For example, the corrosion resistance of the welded portion is in the range of 0.1 to 0.25% by mass of carbonitride-generating elements such as Ta, Zr, and Hf, or 0.1% of sulfide-generating elements such as Mg, Ca, and Y. It improves when added in the range of less than or equal to mass%. W, Co, Sn of 0.50 mass% or less effective for improving corrosion resistance and 0.1 mass% or less of B effective for improving low temperature toughness are also effective alloy components.

  Ferritic stainless steel with the components shown in Table 1 is melted in a vacuum melting furnace, and after casting and hot rolling, annealing, pickling, and cold rolling are repeated to produce a cold-rolled annealed sheet with a thickness of 0.1 mm. did.

Carbon paper was brought into contact with the surface of a 100 mm square test piece cut out from each cold-rolled annealed plate, and the resistance value when a surface pressure of 15 kgf / cm ² was applied was measured by the four probe method. The measured resistance value was R (mΩ), the cross-sectional area of the test piece was S (cm ² ), and the contact resistance ρ ′ (mΩ · cm ² ) was calculated according to the formula ρ ′ = R × S (mΩ · cm ² ).
Next, the cold-rolled annealed plate was pressed to produce a separator having a gas flow path with a height of 0.4 mm. The separator was assembled in a single cell, and the condensed water obtained by the battery reaction was circulated and re-supplied to the fuel electrode side. A fuel cell using a fluororesin was assembled in the piping and pure water container other than the separator. The fuel cell was generated using hydrogen with a dew point of 90 ° C. as the fuel gas and air as the oxidizing gas.

For humidification of the fuel gas, hydrogen gas was fed into a humidifier with a capacity of 10 liters containing pure water, and pure water reduced by evaporation or the like was replenished every 500 hours. All pure water was replaced in 2500 hours, and metal ions contained in the pure water were analyzed by ICP-MASS method. The elution amount of metal ions was evaluated by the sum of the five elements of Fe, Cr, Ni, Mo, and Cu.
In the constant current operation of 0.5 A / cm ² , the initial battery output was 0.58 to 0.60 V in all cases. During the continuous operation for 5000 hours, the output voltage was measured every 2500 hours, and the output voltage reduction degree was calculated as output voltage reduction degree (%) = (voltage at each measurement point / initial voltage) × 100. In addition, the pH value of the moisture discharged from the fuel cell (humidification moisture and the sum of moisture generated by the cell reaction) was measured, and the corrosion state of the separator taken out from the fuel cell after the test was observed.

As can be seen from the test results in Table 2, the stainless steel separator satisfying the component conditions defined in the present invention is maintained at a low pH value for battery drainage, and the battery output is hardly reduced. It can be said that the elution of the metal components is small, the environment in the cell is mild, and the decomposition and deterioration of the ion exchange membrane are suppressed. In fact, the separator after operation for 2500 hours and 5000 hours was very slightly corroded, and the output voltage was hardly lowered.
On the other hand, the separator of the comparative example has high contact resistance (steel type F5) or insufficient elution resistance, and red rust is generated (steel types F1, F3, F5), and the durability required for the fuel cell separator is insufficient. Was.
As is clear from this comparison, when ferritic stainless steel satisfying the component conditions specified in the present invention is used for the fuel cell separator material, the decomposition and deterioration of the ion exchange membrane are suppressed, and the output voltage is maintained at a high level. It can be seen that the durability of the battery is improved.

  As described above, when ferritic stainless steel of the specified component design is used for the separator material, low contact resistance is maintained over a long period of time even when exposed to an acidic and high-humidity cell atmosphere, and ion exchange is performed. A fuel cell separator in which elution of metal ions that contaminate the membrane is suppressed can be obtained. Therefore, there is provided a fuel cell with little decrease in output voltage and excellent durability. Moreover, since the stainless steel separator is easier to process than graphite, the fuel cell can be made thinner and lighter.

Sectional view (a) and exploded perspective view (b) illustrating the internal structure of a fuel cell using a solid polymer membrane as the electrolyte

Explanation of symbols

1: Ion exchange membrane 2: Oxidizing electrode 3: Fuel electrode 4: Gasket 5: Separator 5g: Groove 6: Oxidizing gas supply port 7: Oxidizing gas discharge port 8: Fuel gas supply port 9: Fuel gas discharge port g: Fuel gas o: Oxidizing gas

Claims (3)

  C: 0.020 mass% or less, Si: 0.50 mass% or less, Mn: 0.50 mass% or less, P: 0.040 mass% or less, S: 0.005 mass% or less, Ni: 0.50 % By mass or less, Cr: 28 to 32% by mass, Mo: 1.5 to 2.5% by mass, Cu: 0.80% by mass or less, Nb: 0.03 to 0.25% by mass, Ti: 0.03 ~ 0.25 mass%, Al: more than 0.20 mass% and 1.00 mass% or less, N: 0.020 mass% or less, the balance is substantially Fe composition, C + N: 0.025 mass% Hereinafter, ferritic stainless steel for solid polymer fuel cell separator, characterized in that it is regulated to Ni + Cu: 0.80% by mass or less.   The ferritic stainless steel for a polymer electrolyte fuel cell separator according to claim 1, further comprising V: 0.2 to 1.0 mass%.   A polymer electrolyte fuel cell separator using the ferritic stainless steel according to claim 1 or 2 as a material.

Images