JP2004149920A - Stainless steel for separator of solid polymer fuel cell and its manufacturing method, and solid polymer fuel cell using that stainless steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a stainless steel for the separator of a solid polymer fuel cell having excellent corrosion resistance, and small contact resistance (that is, it excels in electrical conductivity), and the solid polymer fuel cell using it. <P>SOLUTION: The stainless steel for the separator of the solid polymer fuel cell contains ≤0.03mass% C, ≤0.03 mass% N, 20-45mass% Cr and 0.1-5.0mass% Mo, and the sum total of C and N content satisfies ≤0.03mass%, and the balance Fe with inevitable impurities. Moreover, in the stainless steel, a Cr/Fe ratio by atomic ratio computed from Cr and Fe content contained in the passive state film of a surface is 1 or more. <P>COPYRIGHT: (C)2004,JPO

Description

  The present invention relates to a stainless steel for a polymer electrolyte fuel cell separator having excellent durability and a small contact resistance value, a method for producing the same, and a polymer electrolyte fuel cell using the stainless steel separator.

2. Description of the Related Art In recent years, from the viewpoint of global environmental protection, development of a fuel cell which has excellent power generation efficiency and does not emit CO ₂ has been promoted. This fuel cell generates electricity by reacting H ₂ and O ₂ , and depending on the type of electrolyte used, a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid electrolyte fuel cell, an alkaline fuel cell Fuel cells, polymer electrolyte fuel cells, and the like have been developed.

Among these fuel cells, the polymer electrolyte fuel cell is different from other fuel cells,
(a) The power generation temperature is about 80 ° C, and power can be generated at a much lower temperature.
(b) The fuel cell body can be reduced in weight and size.
(c) Can be set up in a short time,
And so on. For this reason, polymer electrolyte fuel cells are the fuel cells that have received the most attention today for use as electric power sources for electric vehicles, stationary generators for home use, and small portable generators.

The polymer electrolyte fuel cell extracts electricity from H ₂ and O ₂ through a polymer membrane, and as shown in FIG. 1, gas diffusion layers 2 and 3 (for example, carbon paper and the like) and separators 4 and 4. 5, the membrane-electrode assembly 1 is sandwiched between them to form a single component (a so-called single cell), and an electromotive force is generated between the separator 4 and the separator 5.

  The membrane-electrode assembly 1 is called an MEA (namely, Membrance-Electrode Assembly), and is formed by integrating a polymer film and an electrode material such as carbon black carrying a platinum-based catalyst on the front and back surfaces of the film. And the thickness is several tens μm to several hundreds μm. The gas diffusion layers 2 and 3 are often integrated with the membrane-electrode assembly 1.

  When the polymer electrolyte fuel cell is applied to the above-mentioned applications, several tens to several hundreds of such single cells are connected in series to constitute a fuel cell stack.

Separators 4 and 5
(A) In addition to its role as a partition separating single cells,
(B) a conductor that carries the generated electrons,
(C) an air flow path, a hydrogen flow path through which O ₂ (ie, air) and H ₂ flow,
(D) It is required to function as a discharge channel for discharging generated water and gas. Further, in order to put the polymer electrolyte fuel cell into practical use, it is necessary to use the separators 4 and 5 having excellent durability and electrical conductivity.

  As for the durability, when used as a power supply for mounting on an electric vehicle, it is assumed to be about 5,000 hours. Alternatively, when used as a stationary generator for home use, it is assumed to be about 40,000 hours. Therefore, the separators 4 and 5 are required to have characteristics such as corrosion resistance that can withstand long-term power generation.

  Regarding the electric conductivity, it is desired that the contact resistance between the separators 4 and 5 and the gas diffusion layers 2 and 3 is as low as possible. The reason is that when the contact resistance between the separators 4, 5 and the gas diffusion layers 2, 3 increases, the power generation efficiency of the polymer electrolyte fuel cell decreases. That is, the smaller the contact resistance, the better the electrical conductivity.

  Until now, polymer electrolyte fuel cells using graphite as the separators 4 and 5 have been put to practical use. The separators 4 and 5 made of graphite have the advantages of relatively low contact resistance and no corrosion. However, since it is easily damaged by an impact, it is difficult to reduce the size, and furthermore, there is a disadvantage that the processing cost for forming the air flow path 6 and the hydrogen flow path 7 is high. These drawbacks of the graphite separators 4 and 5 hinder the spread of polymer electrolyte fuel cells.

  Therefore, attempts have been made to use a metal material instead of graphite as a material for the separators 4 and 5. In particular, various studies have been made toward practical use of the separators 4 and 5 made of stainless steel from the viewpoint of improving durability.

  For example, Japanese Patent Application Laid-Open No. 8-180883 discloses a technique using a metal that easily forms a passive film as a separator. However, the formation of a passive film leads to an increase in contact resistance, which leads to a decrease in power generation efficiency. Therefore, it has been pointed out that these metal materials have problems to be improved, such as higher contact resistance and lower corrosion resistance than carbon materials.

  Japanese Patent Application Laid-Open No. Hei 10-228914 discloses a technique in which the surface of a metal separator such as SUS304 is plated with gold to reduce contact resistance and secure high output. However, it is difficult to prevent the occurrence of pinholes with thin gold plating, and conversely, the problem of cost remains with thick gold plating.

  Japanese Patent Application Laid-Open No. 2000-277133 discloses a method of dispersing carbon powder in a ferritic stainless steel base material to obtain a separator having improved electric conductivity. However, even when carbon powder is used, the surface treatment of the separator requires a considerable cost, so that the cost problem still remains. Further, it has been pointed out that the separator subjected to the surface treatment has a problem that the corrosion resistance is remarkably reduced when a flaw or the like occurs during the assembling.

  Further, attempts have been made to apply the stainless steel as it is to the separator without performing a surface treatment. For example, in JP-A-2000-239806 and JP-A-2000-294255, after adding Cu and Ni positively, impurity elements such as S, P and N are reduced, and C + N ≦ 0.03 mass%. , 10.5% by mass ≦ Cr + 3 × Mo ≦ 43% by mass is disclosed. JP-A-2000-265248 and JP-A-2000-294256 disclose that Cu and Ni are limited to 0.2% by mass or less to suppress the elution of metal ions, and that impurities such as S, P, and N are removed. Disclosed is a ferritic stainless steel for a separator which is reduced and satisfies C + N ≦ 0.03 mass%, 10.5 mass% ≦ Cr + 3 × Mo ≦ 43 mass%.

However, in these inventions, the components of the stainless steel are defined in a predetermined range and the passivation film is strengthened. Based on the idea of reducing the deterioration of the catalytic activity of the catalyst and suppressing the increase in the contact resistance with the electrode due to the corrosion products. Therefore, it is not intended to reduce the contact resistance of the stainless steel itself. Further, it is not possible to ensure the durability (that is, the reduction in output voltage) that can endure power generation for tens of thousands of hours.
JP-A-8-180883 JP 10-228914 A JP 2000-277133 A JP 2000-239806 A JP 2000-294255 A JP 2000-265248 A JP 2000-294256 A

  SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned problems of the prior art, and at the same time has good corrosion resistance and low contact resistance (that is, excellent electrical conductivity), a stainless steel for a polymer electrolyte fuel cell separator. And a polymer electrolyte fuel cell using the same.

  That is, the present invention regulates not only the component of the stainless steel as a material but also the component of the passive film present on the surface within a predetermined range, so that the contact resistance is small even without surface treatment, and the power generation efficiency is excellent. Another object of the present invention is to provide a stainless steel for a polymer electrolyte fuel cell separator having high corrosion resistance of stainless steel itself, and a polymer electrolyte fuel cell using the same.

  The present inventor has conducted intensive studies on a stainless steel separator for exhibiting high corrosion resistance while keeping the contact resistance low, from the viewpoint of the components of the stainless steel and the components of the passive film. As a result, they found that the contact resistance was significantly reduced by adjusting the composition of the passive film formed on the surface of a high-purity ferritic stainless steel containing Mo as a material.

  First, experimental results that led to the present invention will be described.

In the experiment, C: 0.004% by mass, N: 0.007% by mass, Si: 0.1% by mass, Mn: 0.1% by mass, Cr: 30.5% by mass, Mo: 1.85% by mass, P: 0.03% by mass, S: 0.005% by mass And a cold-rolled ferritic stainless steel (sheet thickness 0.5 mm). Some materials were annealed in air (950 ° C, 2 minutes), and then wet-polished # 600. The other materials were annealed (950 ° C., 2 minutes) in an atmosphere of 75% by volume H ₂ + 25% by volume N _{2 with} a dew point of −60 ° C., so-called BA finishing. Furthermore, these materials are etched at various temperatures and times using an acidic aqueous solution containing 10% by mass of nitric acid, 50% by mass of hydrochloric acid, and 1% by mass of picric acid, followed by washing with pure water and drying with cold air. And used for measurement of contact resistance.

For the measurement of the contact resistance, four test pieces (50 mm × 50 mm) etched under the same conditions were prepared, and as shown in FIG. It is sandwiched between carbon papers (TGP-H-120 made by Toray) 9 alternately, and the electrode 10 that is gold-plated on a copper plate is brought into contact with it, and a pressure of 137.2 N / cm ² (ie, 14 kgf / cm ² ) is applied per unit area. Then, the resistance between the test pieces 8 was measured. The value obtained by multiplying the measured value by the area of the contact surface and dividing by the number of contact surfaces (= 2) was defined as the contact resistance value.

  Note that the contact resistance value was calculated based on the measurement values measured six times while exchanging a pair of test pieces 8, and the average value is shown in Table 1.

  As a reference example, the same measurement was performed on a stainless steel plate (thickness of about 0.3 mm, equivalent to SUS304) and a graphite plate (thickness of 5 mm) with gold plating (thickness of about 0.1 μm) on the surface. Calculated. The results are shown in Table 1.

  The passivation film after the etching treatment was measured by photoelectron spectroscopy, and the spectrum intensities of Fe, Cr, and Al in the passivation film (that is, in the oxidized state) were calculated by the peak separation method, and the Cr intensity was calculated from the spectrum intensity. The content, Fe content, and Al content were determined. Further, the number of Cr and Fe atoms was calculated from the Cr content and the Fe content, and the Cr / Fe ratio was calculated as the ratio of the number of atoms. The number of atoms of Cr, Fe, and Al was calculated from the Cr content, Fe content, and Al content, and the Al / (Cr + Fe) ratio was calculated as the ratio of the number of atoms. Table 1 shows the results.

  Of the O (oxygen) contained in the passive film, oxygen existing in the form of a metal oxide (hereinafter referred to as O (M)) and oxygen existing in the state of a metal hydroxide (hereinafter referred to as O (H) )), The peak intensities are calculated, the respective spectral intensities are calculated, the number of O (M) and O (H) atoms is calculated, and the O (M) / O (H) ratio is calculated by the ratio of the number of atoms. . Table 1 shows the results.

As is clear from Table 1, the contact resistance value is reduced by etching the stainless steel plate. In particular, when the Cr / Fe ratio of the passivation film is 1 or more in terms of the atomic ratio, the contact resistance value of the stainless steel sheet is 10 mΩ · cm ² or less. When the O (M) / O (H) ratio is 0.9 or less in atomic ratio, the contact resistance value is further reduced to 8 mΩ · cm ² or less.

Further, in the stainless steel with the BA finish, Al is contained in the passive film, but when the Al content is reduced by the etching treatment, the contact resistance value also decreases. At this time, if the Al / (Cr + Fe) ratio calculated from the respective contents of Cr, Fe, and Al, which are the main metal elements constituting the passivation film, is less than 0.05 in terms of the atomic number ratio, a BA finish was adopted. The contact resistance value of the stainless steel sheet is 10 mΩ · cm ² or less.

If the contact resistance value is 10 mΩ · cm ² or less, there is almost no adverse effect on the characteristics of the fuel cell.

  There have been cases where the influence of the Cr / Fe ratio of the passive film has been investigated from the viewpoint of corrosion resistance, but this experiment has shown that contact resistance can be significantly reduced by adjusting the components of the passive film. Obtained knowledge. The present invention completed based on this finding is as follows.

  1. Contains 0.03% by mass or less of C, 0.03% by mass or less of N, 20 to 45% by mass of Cr, 0.1 to 5.0% by mass of Mo, and satisfies 0.03% by mass or less of the total of C content and N content. The balance is a stainless steel having a composition consisting of Fe and unavoidable impurities, and a Cr / Fe ratio calculated from the Cr content and the Fe content contained in the passive film on the surface of the stainless steel is atomic. A stainless steel for a polymer electrolyte fuel cell separator, wherein the number ratio is 1 or more.

  2. The stainless steel for a polymer electrolyte fuel cell separator according to the above item 1, wherein the stainless steel contains, in addition to the composition, at least one selected from the group consisting of the following (1) to (4). .

(1) Si: 1.0 mass% or less
(2) Mn: 1.0 mass% or less
(3) Al: 0.001 to 0.2 mass% or less
(4) 0.01 to 0.5% by mass of Ti or Nb, or 0.01 to 0.5% by mass of Ti and Nb in total
3. Of the oxygen contained in the passive film, the content of oxygen: O (M) existing in the state of metal oxide and the content of oxygen: O (H) existing in the state of metal hydroxide 3. The stainless steel for a polymer electrolyte fuel cell separator according to the above item 1 or 2, wherein the calculated O (M) / O (H) ratio is 0.9 or less in atomic ratio.

  4. The Al / (Cr + Fe) ratio calculated from the Al content, Cr content and Fe content contained in the passive film is less than 0.05 in atomic ratio, 4. Stainless steel for a polymer electrolyte fuel cell separator according to 2 or 3.

  5. A polymer electrolyte fuel cell comprising a polymer electrolyte membrane, an electrode, and a separator, wherein the separator is a polymer electrolyte fuel cell separator stainless steel according to any one of the above items 1 to 4. Polymer electrolyte fuel cell.

  As described above, according to the present invention, a stainless steel for a polymer electrolyte fuel cell separator having a low contact resistance value and excellent corrosion resistance can be obtained. Therefore, it has become possible to provide an inexpensive stainless steel separator to a polymer electrolyte fuel cell which conventionally used an expensive graphite separator due to durability issues.

  The present invention can be widely used not only as a polymer electrolyte fuel cell separator but also as an electrical component made of stainless steel having electrical conductivity.

  First, the reasons for limiting the components of the stainless steel for separator according to the present invention will be described.

C: 0.03% by mass or less, N: 0.03% by mass or less, C + N: 0.03% by mass or less Both C and N react with Cr in stainless steel and precipitate as Cr carbonitrides at grain boundaries, so that corrosion resistance decreases. Bring. Accordingly, the content of each of C and N is preferably as small as possible. If C: 0.03% by mass or less and N: 0.03% by mass or less, the corrosion resistance is not significantly reduced. If the sum of the C content and the N content (hereinafter, referred to as C + N) exceeds 0.03% by mass, cracks generated when the separator is pressed are significantly increased. Therefore, C + N is set to 0.03% by mass or less. Preferably, C: 0.015% by mass or less, N: 0.015% by mass or less, and C + N: 0.02% by mass or less.

Cr: 20 to 45 mass%
Cr is an element necessary for ensuring the corrosion resistance of the stainless steel sheet, and if the Cr content is less than 20% by mass, it cannot withstand long-term use as a separator. If the Cr content is less than 20% by mass, it is difficult to adjust the Cr / Fe ratio of the passivation film to 1 or more to make the contact resistance 10 mΩ · cm ² or less. On the other hand, when the Cr content exceeds 45% by mass, toughness is reduced due to precipitation of the σ phase. Therefore, the Cr content was set to 20 to 45% by mass. Preferably it is 22 to 35% by mass.

Mo: 0.1 to 5.0 mass%
Mo is an element effective for improving the crevice corrosion resistance of a stainless steel sheet. In order to exhibit this effect, the content needs to be 0.1% by mass or more. On the other hand, if it is added in excess of 5.0% by mass, the stainless steel becomes extremely brittle and production becomes difficult. Therefore, the Mo content was set to 0.1 to 5.0% by mass. Preferably it is 0.5 to 3.0% by mass.

  In the stainless steel for separator according to the present invention, in addition to the above-described composition, it is desirable to define the contents of the following components.

Si: 1.0 mass% or less
Si is an element effective for deoxidation, and is added during the smelting stage of stainless steel. However, if it is excessively contained, the stainless steel plate becomes hard and the ductility decreases. Therefore, the upper limit of the Si content is preferably set to 1.0% by mass. More preferably, it is 0.01 to 0.6% by mass.

Mn: 1.0 mass% or less
Mn is an element effective in suppressing the grain boundary segregation of S and preventing cracking during hot rolling, since Mn has an effect of reducing S dissolved in stainless steel by being combined with S. When the Mn content is 1.0% by mass or less, this effect is sufficiently exhibited. Preferably it is 0.001-0.8 mass%.

Al: 0.001 to 0.2 mass%
Al is an element effective for deoxidation in the steelmaking process, and 0.001% by mass or more is required to obtain the effect. On the other hand, if it is added in excess of 0.2% by mass, the effect is saturated and the cost increases. Therefore, the Al content is preferably set to 0.001 to 0.2% by mass.

0.01 to 0.5% by mass of Ti or 0.01 to 0.5% by mass of Nb, or a total of 0.01 to 0.5% by mass of Ti and Nb
Ti and Nb are elements effective for fixing C and N in stainless steel as carbonitrides and improving press formability. When the C content and the N content satisfy the above ranges and Ti or Nb is added, the effect is exhibited when the Ti content is 0.01% by mass or more or the Nb content is 0.01% by mass or more. When Ti and Nb are added, the effect is exhibited when the total content of Ti and Nb is 0.01% by mass or more. On the other hand, when Ti or Nb is added, if the Ti content exceeds 0.5% by mass or the Nb: content exceeds 0.5% by mass, the effect is saturated. When Ti and Nb are added, the effect is saturated when the total content of Ti and Nb exceeds 0.5% by mass. Therefore, when adding Ti or Nb, it is preferable to contain 0.01 to 0.5% by mass of Ti or 0.01 to 0.5% by mass of Nb, and when adding Ti and Nb, it is preferable to add 0.01 to 0.5% by mass of Ti and Nb in total. preferable.

  In the present invention, in order to improve the hot workability of the stainless steel sheet used as the material of the separator, in addition to the above-described elements, each of Ca, Mg, REM (that is, a rare earth element) and B is 0.1% by mass or less, or a stainless steel sheet. May be added for the purpose of improving the toughness of Ni. Further, in order to reduce the contact resistance value, Ag: 1% by mass or less, Cu: 5% by mass or less may be added, and V: 0.5% by mass or less may be added for the purpose of finely dispersing Ag.

  Other elements are the balance Fe and inevitable impurities.

  Next, the characteristics that the stainless steel for separator according to the present invention should have will be described.

Cr / Fe ratio of the passive film on the surface of the stainless steel sheet: 1 or more in terms of the atomic ratio The component of the passive film is an important factor in reducing the contact resistance value. It is necessary to increase the Cr / Fe ratio calculated from the Cr content and the Fe content of the active film. As described in the above experimental results, in order to obtain a contact resistance value of 10 mΩ · cm ² or less, the Cr / Fe ratio needs to be 1 or more in atomic ratio.

O (M) / O (H) ratio of passive film on stainless steel sheet surface: 0.9 or less in atomic ratio The bonding state of O (oxygen) contained in the passive film is also important in reducing the contact resistance Factor. In order to reduce the contact resistance value, O (O (M)) existing in the state of a metal oxide and O (H) calculated from the oxygen existing in the state of a metal hydroxide (ie, O (H)) are used. It is effective to lower the (M) / O (H) ratio. As described in the above experimental results, if the O (M) / O (H) ratio is 0.9 or less in atomic ratio, a contact resistance of 8 mΩ · cm ² or less can be obtained.

The Al / (Cr + Fe) ratio of the passive film on the surface of the stainless steel plate: the atomic number ratio is less than 0.05. When the passive film contains Al oxide, the contact resistance increases. Compared to the pickled or polished stainless steel plate, the passivation film contains more Al in the BA finished stainless steel plate. Therefore, in order to reduce the contact resistance value of the BA-finished stainless steel sheet, it is necessary to reduce the amount of Al in the passive film. As described in the above experimental results, if the Al / (Cr + Fe) ratio calculated from the Cr content, Fe content, and Al content of the passive film is less than 0.05 in atomic ratio, the BA-finished stainless steel sheet , A contact resistance value of 10 mΩ · cm ² or less can be obtained.

  In order to adjust the Cr content, Fe content, Al content and the bonding state of O (oxygen) of the passive film in this way, etching using an acid, immersion in an acidic aqueous solution, electrolytic etching, etc. Can be used.

  Such adjustment of the composition of the passive film may be performed before processing the stainless steel plate into the separator, or may be performed after processing the stainless steel plate into the separator. However, in order to stably maintain the Cr / Fe ratio, O (M) / O (H) ratio, and Al / (Cr + Fe) ratio within predetermined ranges, it is necessary to perform pickling after processing into a separator. Is preferred.

  When a polymer electrolyte fuel cell is manufactured using the stainless steel separator thus manufactured, a polymer electrolyte fuel cell having low contact resistance, excellent power generation efficiency, and high corrosion resistance can be manufactured.

[Example 1]
A stainless steel having the components shown in Table 2 was melted by a converter and a strong stirring vacuum oxygen decarburization method (SS-VOD), and a slab having a thickness of 200 mm was formed by a continuous casting method. After heating this slab to 1250 ° C, a hot-rolled stainless steel sheet having a thickness of 4 mm was formed by hot rolling, and further subjected to annealing (850 to 1100 ° C) and pickling. Next, the thickness was reduced to 0.3 mm by cold rolling. After annealing (850 to 1100 ° C.) and pickling, temper rolling was performed to obtain a cold-rolled stainless steel sheet having a so-called 2B finish.

  Four 200 mm × 200 mm test pieces were cut out from the center of the obtained cold rolled stainless steel sheet in the width direction center and the center in the length direction. Four test pieces cut out from cold-rolled stainless steel sheets of steel numbers 1 to 9 were each subjected to press working to obtain a separator having a predetermined shape. Then, for each steel number, the composition of the passive film was adjusted on some of the separators to adjust the Cr / Fe ratio. Here, when performing the composition adjustment treatment of the passive film, A: a solution containing 10% by mass of nitric acid, 50% by mass of hydrochloric acid and 1% by mass of picric acid (50 ° C., 120 seconds) or B: 5% of nitric acid A solution containing 50% by mass and 20% by mass of hydrofluoric acid (50 ° C., 300 seconds) was used.

  If the passivation film composition was not adjusted, the Cr content, the Fe content, the Al content, the O content existing as metal oxide, and the metal hydroxide of the passivation film after pressing. The O content was measured, and the number of atoms of each element was calculated. Then, the Cr / Fe ratio, the O (M) / O (H) ratio, and the Al / (Cr + Fe) ratio were calculated. In the case of passivation film composition adjustment processing, after press working, after further passivation film composition adjustment processing, it is present as the Cr content, Fe content, Al content, and metal oxide of the passive film. After measuring the O content, the O content existing as a metal hydroxide, and calculating the number of atoms of each element, the Cr / Fe ratio, the O (M) / O (H) ratio, and the Al / ( Cr + Fe) ratio was calculated. The content of each element was determined by separating the peaks of the spectral intensity using photoelectron spectroscopy.

The power generation characteristics of each of the separator subjected to the composition adjustment treatment of the passive film and the separator not subjected to the treatment were examined. In order to evaluate the power generation characteristics, using a membrane-electrode assembly 1 (FC50-MEA manufactured by Electrochem) having an effective area of 50 cm ² in which the polymer membrane, the electrodes, and the gas diffusion layers 2 and 3 were integrated, A single cell having the shape shown in FIG. 1 was prepared. The air flow path 6 and the hydrogen flow path 7 of the single cell were each a rectangle having a height of 1 mm and a width of 2 mm, and were arranged in 17 rows in total. Air is supplied to the cathode side, and ultrapure hydrogen (purity 99.9999% by volume) is supplied to the anode side after being humidified by a bubbler maintained at 80 ± 1 ° C., and the current density is 0.4 A / cm ² (condition 1). An output voltage of 0.7 A / cm ² (condition 2) was measured.

After operating continuously for 2000 hours under the condition of a current density of 0.4 A / cm ² , the output voltages under the conditions 1 and 2 were measured. During the power generation experiment of the single cell, the temperature of the single cell body was kept at 80 ± 1 ° C. The membrane-electrode assembly 1, carbon paper 9, etc. were replaced with new ones each time the test piece was replaced.

As a reference example, a stainless steel plate (equivalent to SUS304) was processed into the same shape as the above steel Nos. 1 to 9, and then the surface was plated with gold (about 0.1 μm thick), and a 3 mm thick graphite plate The output voltage at a current density of 0.4 A / cm ² and 0.7 A / cm ² was measured using a separator in which 17 rows of grooves having a width of 2 mm and a height of 1 mm were arranged on one side of the substrate at intervals of 2 mm. The method of measuring the output voltage is the same as that of the steel numbers 1 to 9 described above.

  Table 3 shows the results.

  As is clear from Table 3, the stainless steel satisfying the component range of the present invention (that is, steel Nos. 3 to 6 and 9) was treated with the solution of A or B to adjust the components of the passivation film to adjust the Cr content. A single cell using a separator with a / Fe ratio of 1 or more can obtain an output voltage equivalent to that of a gold-plated separator or a graphite plate separator, both in the initial output voltage and in the output voltage after 2,000 hours. The level was sufficiently practical.

  In addition, a single cell using a separator having an O (M) / O (H) ratio of 0.9 or less has further improved performance. Both the initial output voltage and the output voltage after 2,000 hours have passed, the gold-plated separator has been used. And an output voltage equivalent to that of a graphite plate separator.

  On the other hand, in the case of the stainless steels (ie, steel numbers 1, 2, 7, and 8) out of the component range of the present invention, the initial output voltage and the output voltage after 2,000 hours have passed regardless of the presence or absence of the passivation film composition adjustment treatment. Decreased in comparison with gold-plated separators and graphite plate separators.

  Further, even if stainless steel satisfying the component range of the present invention (that is, steel numbers 3 to 6 and 9) is not subjected to the passivation film composition adjustment treatment, the passive film has a low Cr / Fe ratio, The initial output voltage was lower than that of a gold-plated separator or a graphite plate separator.

[Example 2]
The hot-rolled stainless steel sheet used in Example 1 was cold-rolled to a thickness of 0.2 mm, and further annealed (850-1050 ° C.) in ammonia decomposition gas having a dew point of −60 ° C., so-called BA-finished cold-rolled stainless steel sheet. A steel plate was used.

  From the obtained cold-rolled stainless steel sheet, a test piece was collected in the same manner as in Example 1 to obtain a separator having a predetermined shape. Further, the Cr / Fe ratio, O (M) / O (H) ratio and Al / (Cr + Fe) ratio of the passive film of this separator were calculated. The shape and forming method of the separator, the process for adjusting the composition of the passive film, and the method for measuring the content of each element are the same as those in Example 1, and therefore description thereof is omitted.

  Further, similarly to Example 1, the power generation characteristics of the separator were examined. Table 4 shows the results.

  As is clear from Table 4, even in the stainless steel plate with the BA finish, the stainless steel satisfying the component range of the present invention (that is, steel numbers 3 to 6 and 9) is treated with the liquid A or B, A single cell using a separator with the composition of the passive film adjusted to have a Cr / Fe ratio of 1 or more can be used for both the initial output voltage and the output voltage after lapse of 2,000 hours. An output voltage equivalent to that obtained was obtained, which was a level that could sufficiently withstand practical use.

  A single cell using a separator having an O (M) / O (H) ratio of 0.9 or less, or a separator having an Al / (Cr + Fe) ratio of less than 0.05 has further improved performance, an initial output voltage and 2,000 hours. With respect to the output voltage after the lapse of time, an output voltage equivalent to that of a gold-plated separator or a graphite plate separator was obtained.

  On the other hand, in the case of the stainless steels (ie, steel numbers 1, 2, 7, and 8) out of the component range of the present invention, the initial output voltage and the output voltage after 2,000 hours have passed regardless of the presence or absence of the passivation film composition adjustment treatment. Decreased in comparison with gold-plated separators and graphite plate separators.

FIG. 2 is a perspective view schematically illustrating an example of a polymer electrolyte fuel cell. It is sectional drawing which shows typically the sample used for the measurement of contact resistance.

Explanation of reference numerals

DESCRIPTION OF SYMBOLS 1 Membrane-electrode assembly 2 Gas diffusion layer 3 Gas diffusion layer 4 Separator 5 Separator 6 Air flow path 7 Hydrogen flow path 8 Test piece 9 Carbon paper
10 electrodes

Claims (5)

  Contains 0.03% by mass or less of C, 0.03% by mass or less of N, 20 to 45% by mass of Cr, 0.1 to 5.0% by mass of Mo, and satisfies 0.03% by mass or less of the total of C content and N content. The balance is a stainless steel having a composition consisting of Fe and unavoidable impurities, and a Cr / Fe ratio calculated from the Cr content and the Fe content contained in the passive film on the surface of the stainless steel is atomic. A stainless steel for a polymer electrolyte fuel cell separator, wherein the number ratio is 1 or more. The stainless steel for a polymer electrolyte fuel cell separator according to claim 1, wherein the stainless steel contains, in addition to the composition, at least one selected from the following groups (1) to (4). steel.
(1) Si: 1.0 mass% or less
(2) Mn: 1.0 mass% or less
(3) Al: 0.001 to 0.2 mass% or less
(4) 0.01 to 0.5% by mass of Ti or Nb, or 0.01 to 0.5% by mass of Ti and Nb in total   Of the oxygen contained in the passive film, the content of oxygen: O (M) existing in the state of metal oxide and the content of oxygen: O (H) existing in the state of metal hydroxide The stainless steel for a polymer electrolyte fuel cell separator according to claim 1 or 2, wherein the calculated O (M) / O (H) ratio is 0.9 or less in terms of atomic ratio.   2. The Al / (Cr + Fe) ratio calculated from the Al content, Cr content and Fe content contained in the passive film is less than 0.05 in atomic ratio. 4. The stainless steel for a polymer electrolyte fuel cell separator according to 2 or 3.   A polymer electrolyte fuel cell comprising a polymer electrolyte membrane, an electrode, and a separator, wherein the separator comprises the stainless steel for a polymer electrolyte fuel cell separator according to any one of claims 1 to 4. Polymer electrolyte fuel cell.

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