FERRITIC STAINLESS STEEL MATERIAL, AND, SEPARATOR FOR SOLID POLYMER FUEL CELL AND SOLID POLYMER FUEL CELL WHICH USES THE SAME

Abstract

A ferritic stainless steel material is provided that has a chemical composition containing, by mass %, C: 0.001 to less than 0.020%, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.035% or less, S: 0.01% or less, Cr: 22.5 to 35.0%, Mo: 0.01 to 6.0%, Ni: 0.01 to 6.0%, Cu: 0.01 to 1.0%, N: 0.035% or less, V: 0.01 to 0.35%, B: 0.5 to 1.0%, Al: 0.001 to 6.0%, Sn: 0.02 to 2.50%, rare earth metal: 0 to 0.1%, Nb: 0 to 0.35%, Ti: 0 to 0.35%, and the balance: Fe and impurities, in which a value calculated as {Cr content (mass %)+3×Mo content (mass %)−2.5×B content (mass %)} is from 20 to 45%, and M.sub.2B boride-based metallic precipitates are dispersed in and exposed on the surface of a parent phase composed only of a ferritic phase.

Claims

1. A ferritic stainless steel material having a chemical composition comprising, by mass %, C: 0.001 to less than 0.020%, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.035% or less, S: 0.01% or less, Cr: 22.5 to 35.0%, Mo: 0.01 to 6%, Ni: 0.01 to 6%, Cu: 0.01 to 1%, N: 0.035% or less, V: 0.01 to 0.35%, B: 0.5 to 1.0%, Al: 0.001 to 6.0%, Sn: 0.02 to 2.50%, rare earth metal: 0 to 0.1%, Nb: 0 to 0.35%, Ti: 0 to 0.35%, and the balance: Fe and impurities, wherein: a value calculated as {Cr content (mass %)+3×Mo content (mass %)−2.5×B content (mass %)} is from 20 to 45%, the ferritic stainless steel material further having a parent phase comprising only a ferritic phase, wherein: M.sub.2B boride-based metallic precipitates are dispersed in and exposed on a surface of the parent phase.

2. The ferritic stainless steel material according to claim 1, wherein the chemical composition contains, by mass %, rare earth metal: 0.005 to 0.1%.

3. The ferritic stainless steel material according to claim 1, wherein the chemical composition contains one or more kinds selected from, by mass %: Nb: 0.001 to 0.35% and Ti: 0.001 to 0.35%, and satisfies: 3≦Nb/C≦25, and 3≦Ti/(C+N)≦25.

4. A separator for a solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 1.

5. A solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 1.

6. The ferritic stainless steel material according to claim 2, wherein the chemical composition contains one or more kinds selected from, by mass %: Nb: 0.001 to 0.35% and Ti: 0.001 to 0.35%, and satisfies: 3≦Nb/C≦25, and 3≦Ti/(C+N)≦25.

7. A separator for a solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 2.

8. A separator for a solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 3.

9. A separator for a solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 6.

10. A solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 2.

11. A solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 3.

12. A solid polymer fuel cell comprising the ferritic stainless steel material for a solid polymer fuel cell separator according to claim 6.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0091] FIG. 1 is a multiple-view schematic diagram illustrating the structure of a polymer electrolyte fuel cell, where FIG. 1(a) is an exploded view of a fuel cell (unit cell), and FIG. 1(b) is a perspective view of an entire fuel cell.

[0092] FIG. 2 is a photograph showing an example of the shape of a separator that was produced in Example 3.

DESCRIPTION OF EMBODIMENTS

[0093] Embodiments for carrying out the present invention will be described in detail. Hereinafter, the symbols “%” all refer to “mass %”.

1. M.SUB.2.B Boride-Based Metallic Precipitates

[0094] M.sub.2B contains 60% or more of Cr, and exhibits corrosion resistance that is excellent as compared to that of the parent phase. Because of the concentration of Cr higher than that of the parent phase, a passivation film generated on the surface is also thinner, which makes electrical conductivity (electrical contact resistance performance) excellent.

[0095] By finely dispersing and exposing M.sub.2B boride-based metallic precipitates having electrical conductivity on the surface of the stainless steel, the electrical contact resistance in a fuel cell can be noticeably reduced over a long period in a stable manner.

[0096] The term “exposure” here means that M.sub.2B boride-based metallic precipitates protrude on the external surface without being covered by the passivation film that is generated on the surface of the parent phase of the stainless steel. The exposure of the M.sub.2B boride-based metallic precipitates causes the M.sub.2B boride-based metallic precipitates to function as passages (bypasses) for electricity, so as to have the effect of noticeably reducing the electrical contact resistance of the surface.

[0097] Although there is a concern that M.sub.2B boride-based metallic precipitates exposed on the surface will fall off, since the M.sub.2B boride-based metallic precipitates are metallic precipitates, the M.sub.2B boride-based metallic precipitates are metallurgically bonded to the parent phase and do not fall off the surface.

[0098] The M.sub.2B boride-based metallic precipitates are precipitated by a eutectic reaction that proceeds at the last stage of solidification, and thus have a composition that is approximately uniform and have a property of being thermally stable in the extreme as well. The M.sub.2B boride-based metallic precipitates do not suffer redissolving, reprecipitation or component changes due to thermal history in the process for producing the steel material. Furthermore, the M.sub.2B boride-based metallic precipitates are extremely hard precipitates. In the processes of hot forging, hot rolling and cold rolling, the M.sub.2B boride-based metallic precipitates are mechanically crushed and finely dispersed uniformly.

2. Metallic Tin and Tin Oxide

[0099] Sn is dissolved in the parent phase by being added as an alloying element at the molten steel stage. When the steel is applied as a solid polymer fuel cell separator, pickling is performed so that M.sub.2B contained in the steel that is located in the vicinity of the steel surface is exposed on the surface to reduce the electrical contact resistance of the steel surface. At this time, tin dissolved in the parent phase concentrates in the form of metallic tin or a tin oxide not only on the surface of the parent phase but also on the surface of M.sub.2B with melting (corrosion) of the parent phase caused by the pickling. In addition, gradual metal elution proceeds in accordance with the environment in the fuel cell immediately after the start of application as a solid polymer fuel cell separator, and the passivation film changes. With elution of the parent phase during such process, tin contained in the steel further concentrates on not only the surface of the parent phase but also on the surface of M.sub.2B, so as to have a behavior of turning into a surface concentration state that is favorable for ensuring the desired properties. Metallic tin and a tin oxide are each excellent in electrical conductivity and act to reduce the electrical contact resistance on the parent phase surface in the fuel cell.

3. Chemical Composition

[0100] (3-1) C: 0.001 to Less than 0.020%

[0101] In the present invention, C is an impurity. It is possible to make the content of C less than 0.001% by applying current refining techniques, which however increases a time for the refinement and costs of the refinement. Therefore, the content of C is set at 0.001% or more. On the other hand, a content of C of 0.020% or more is liable to result in reduction in corrosion resistance due to sensitization, as well as reduction in toughness at normal temperature and reduction in producibility. Therefore, the content of C is set at less than 0.020%. The content of C is preferably 0.0015% or more, and is preferably less than 0.010%.

[0102] (3-2) Si: 0.01 to 1.5%

[0103] Similarly to Al, Si is an effective deoxidizing element in mass-produced steel. A content of Si less than 0.01% leads to insufficient deoxidization. Therefore, the Si content is set as 0.01% or more. On the other hand, a content of Si exceeding 1.5% leads to reduction of formability. Therefore, the content of Si is 1.5% or less. The content of Si is preferably 0.05% or more, more preferably 0.1% or more. Further, the content of Si is preferably 1.2% or less, more preferably 1.0% or less.

[0104] (3-3) Mn: 0.01 to 1.5%

[0105] Mn has an action of fixing S in the steel as an Mn sulfide, and also has an effect of improving hot workability. In order to effectively exert the aforementioned effects, the content of Mn is set at 0.01% or more. On the other hand, a content of Mn exceeding 1.5% leads to reduction of the adhesiveness of a high-temperature oxide scale generated on the surface at a time of heating during production, which is liable to result in scale peeling to be a cause of surface deterioration. Therefore, the content of Mn is set at 1.5% or less. The content of Mn is preferably 0.1% or more, more preferably 0.1% or more. In addition, the content of Mn is preferably 1.2% or less, more preferably 1.0% or less.

[0106] (3-4) P: 0.035% or Less

[0107] In the present invention, P in the steel is the most harmful impurity, along with S, and thus the content of P is set at 0.035% or less. The content of P is preferably as low as possible.

[0108] (3-5) S: 0.01% or Less

[0109] In the present invention, S in the steel is the most harmful impurity, along with P, and thus the content of S is set at 0.01% or less. The content of S is preferably as low as possible. In proportion to coexisting elements in the steel and the content of S in the steel, Most of S is precipitated in the form of Mn-based sulfides, Cr-based sulfides, Fe-based sulfides, or composite non-metallic precipitates with complex sulfides and complex oxides of these sulfides. Furthermore, S may also form a sulfide with a rare earth metal that is added as necessary. However, the non-metallic precipitates of each of these compositions act as a starting point for corrosion in a polymer electrolyte fuel cell separator environment with varying degrees. Therefore, S is harmful in terms of maintaining a passivation film and suppression of metal ion elution. The content of S in usual mass-produced steel is more than 0.005% and at most around 0.008%, but in order to prevent the aforementioned harmful effects of S, the content of S is preferably reduced to 0.004% or less. More preferably, the content of S in the steel is 0.002% or less, and the most preferable content of S in the steel is less than 0.001%. The content of S is preferably as low as possible. Making the content of S less than 0.001% in mass production industrially causes only a slight increase in production costs with present-day refining technology, which is not problematic.

[0110] (3-6) Cr: 22.5 to 35.0%

[0111] Cr is an extremely important basic alloying element for ensuring corrosion resistance of the base material. The higher that the Cr content is, the more excellent the corrosion resistance to be exhibited. In a ferritic stainless steel, a content of Cr exceeding 35.0% makes production of the stainless steel on a mass production scale difficult. On the other hand, a content of Cr less than 22.5% results in failure of securing corrosion resistance that is required for steel used as a polymer electrolyte fuel cell separator even with other elements varied, and furthermore, as a result of precipitating in the form of M.sub.2B boride-based metallic precipitates, the corrosion resistance of the base material may deteriorate due to the amount of Cr in the parent phase that contributes to improving the corrosion resistance reduced as compared to the amount of Cr in the molten steel. Furthermore, Cr in some cases reacts with C in the steel to form M.sub.23C.sub.6 carbide-based metallic precipitates. The M.sub.23C.sub.6 carbide-based metallic precipitates are metallic precipitates that are excellent in electrical conductivity, but are a cause of reduction in corrosion resistance due to sensitization. By exposing M.sub.2B boride-based metallic precipitates on the surface, an electrical surface contact resistance value can be reduced. In order to ensure corrosion resistance in the polymer electrolyte fuel cell, at least an amount of Cr that makes a value calculated as {Cr content (mass %)+3×Mo content (mass %)−2.5×B content (mass %)} from 20 to 45% is required. The content of Cr is preferably 23.0% or more, and is preferably 34.0% or less.

[0112] (3-7) Mo: 0.01 to 6.0%

[0113] Mo has an effect of improving the corrosion resistance with a smaller amount as compared to Cr. In order to effectively exert the corrosion resistance, the content of Mo is set at 0.01% or more. On the other hand, if a content of Mo exceeding 6.0% makes precipitation of intermetallic compounds such as sigma phase during production unavoidable, malting production difficult due to the problem of steel embrittlement. For this reason, the upper limit of the Mo content is set at 6.0%. Furthermore, Mo has a property such that the influence thereof on MEA performance is relatively minor, even if elution of Mo in the steel occurs inside a polymer electrolyte fuel cell due to corrosion. The reason is that because Mo exists in the form of molybdate ions that are anions and does not exist in the form of metallic cations, the influence thereof on the cation conductivity of a fluorinated ion exchange resin film having hydrogen ion (proton) exchange groups is small. Mo is an extremely important element for maintaining corrosion resistance, and it is necessary for the amount of Mo in the steel to be an amount that makes a value calculated as {Cr content (mass %)+3×Mo content (mass %)−2.5×B content (mass %)} from 20 to 45%. The content of Mo is preferably 0.05% or more, and is preferably 5.0% or less.

[0114] (3-8) Ni: 0.01 to 6.0%

[0115] Ni has an effect of improving corrosion resistance and toughness. The upper limit of the content of Ni is set at 6.0%. A content of Ni exceeding 6.0% makes it difficult to form a ferritic single-phase micro-structure even if heat treatment is performed industrially. On the other hand, the lower limit for the content of Ni is set at 0.01%. The lower limit of the Ni content is the amount of impurities that enter when production is performed industrially. The content of Ni is preferably 0.03% or more, and is preferably 5.0% or less.

[0116] (3-9) Cu: 0.01 to 1.0%

[0117] The content of Cu is 0.01% or more and 1.0% or less. A content of Cu exceeding 1.0% leads to reduction of the hot workability, making mass production difficult. On the other hand, a content of Cu less than 0.01% leads to reduction of corrosion resistance in a polymer electrolyte fuel cell. In the stainless steel according to the present invention, Cu is present in a dissolved state. If Cu is caused to precipitate in the form of a Cu-based precipitate, it becomes a starting point for Cu elution in the cell and reduces the performance of the fuel cell. The content of Cu is preferably 0.02% or more, and is preferably 0.8% or less.

[0118] (3-10) N: 0.035% or Less

[0119] N is an impurity in a ferritic stainless steel. Since N degrades toughness at normal temperature, the upper limit of the content of N is set at 0.035%. The content of N is preferably as low as possible. From an industrial viewpoint, the most preferable content of N is 0.007% or less. However, since an excessively reduction of the content of N leads to an increase in melting costs, the content of N is preferably 0.001% or more, more preferably 0.002% or more.

[0120] (3-11) V: 0.01 to 0.35%

[0121] Although V is not an added element that is intentionally added, V is inevitably contained in a Cr source that is added as a melting raw material used at a time of mass production. The content of V is set at 0.01% or more and 0.35% or less. Although very slightly, V has an effect of improving toughness at normal temperature. The content of V is preferably 0.03% or more, and is preferably 0.30% or less.

[0122] (3-12) B: 0.5 to 1.0%

[0123] In the present invention, B is an important added element. When molten steel is subjected to ingot-making, a eutectic reaction causes all the B in the steel to precipitate as M.sub.2B type boride-based metallic. B is an extremely stably metallic precipitate in terms of thermal properties. M.sub.2B boride-based metallic precipitates exposed on the surface have an action that noticeably lowers electrical surface contact resistance. A content of B is less than 0.5% leads to an insufficient precipitation amount to obtain the desired performance. On the other hand, a content of B exceeding 1.0% makes it difficult to achieve stable mass production. Therefore, the content of B is 0.5% or more and 1.0% or less. The content of B is preferably 0.55% or more, and is preferably 0.8% or less.

[0124] (3-13) Al: 0.001 to 6.0%

[0125] Al is added as a deoxidizing element at the molten steel stage. Since B contained in the stainless steel according to the present invention is an element that has a strong bonding strength with oxygen in molten steel, it is necessary to reduce the oxygen concentration by Al deoxidation. Therefore, it is better to include a content of Al within the range of 0.001% or more and 6.0% or less. Although deoxidation products are formed in the steel in the form of nonmetallic oxides, the residue are dissolved. The content of Al is preferably 0.01% or more, and is preferably 5.5% or less.

[0126] (3-14) Sn: 0.02 to 2.50%

[0127] In the present invention, Sn is an extremely important added element. By containing Sn within a range of 0.02% to 2.50% in the steel, Sn dissolved in the parent phase concentrates in the form of metallic tin or a tin oxide not only on the surface of the parent phase inside the solid polymer fuel cell but also on the surface of M.sub.2B, thereby remarkably suppressing elution of metal ions from the parent phase as well as from M.sub.2B that also proceeds by only a small amount and reducing the surface contact resistance of the parent phase. Furthermore, the Sn concentrates as metallic tin or a tin oxide on the M.sub.2B surface, so that the electrical contact resistance performance of M.sub.2B is also stable and improved to be as low as that of a gold-plated starting material. A content of Sn less than 0.02% results in failure of obtaining the aforementioned effects, and a content of Sn exceeding 2.50% results in reduction in producibility. Therefore, the content of Sn is set at 0.02% or more and 2.50% or less. The content of Sn is preferably 0.05% or more, and is preferably 2.40% or less.

[0128] (3-15) Rare Earth Metal: 0 to 0.1%

[0129] In the present invention, a rare earth metal is an optional added element and is added in the form of a misch metal. A rare earth metal has an effect of improving hot producibility. Therefore, a rare earth metal may be contained at a content of 0.1% as the upper limit. The content of a rare earth metal is preferably 0.005% or more, and is preferably 0.05% or less.

[0130] (3-16) Value Calculated as {Cr Content (Mass %)+3×Mo Content (Mass %)−2.5×B Content (Mass %)}

[0131] This value is an index that serves as a standard indicating the anticorrosion behavior of ferritic stainless steel in which M.sub.2B boride-based metallic precipitates have been precipitated. This value is set within a range of 20% or more and 45% or less. If this value is less than 20%, corrosion resistance within a polymer electrolyte fuel cell cannot be adequately secured, and the amount of metal ion elution is large. On the other hand, if this value exceeds 45%, mass productivity will deteriorate noticeably.

[0132] (3-17) Nb: 0 to 0.35%, Ti: 0 to 0.35%

[0133] In the present invention, Nb and Ti are both optional added element, and are stabilizing elements for C and N in the steel. Nb and Ti form carbides and nitrides in the steel. For this reason, the contents of Ti and Nb is each set at 0.35% or less. The contents of Nb and Ti are preferably 0.001% or more, and are preferably 0.30% or less. The content of Nb is set so that a value of (Nb/C) is 3 or more and 25 or less, and the content of Ti is set so that a value of {Ti/(C+N)} is 3 or more and 25 or less.

[0134] The balance other than the above elements is made up of Fe and impurities.

[0135] Next, advantageous effects of the present invention will be specifically described with reference to examples.

Example 1

[0136] Steel materials 1 to 17 having the chemical compositions shown in Table 1 were melted in a 180-kg vacuum furnace, and subsequently cast into flat ingots with a maximum thickness of 80 mm. Steel materials 1 to 11 are example embodiments of the present invention, and steel materials 12 to 17 are comparative examples. In Table 1, the symbol “*” indicates that the relevant value is outside the range defined in the present invention, “REM” represents a misch metal (rare earth metal), and “Index” (%)=Cr %+3×Mo %−2.5×B %

TABLE-US-00001 TABLE 1 Steel Chemical Composotion (  Balance: Fe and Impurities Material C Si Mn P S Cr Mo Ni Cu N V B Ai 1 Example 0.002 0.21 0.15 0.022 0.001 26.3 0.08 0.08 0.05 0.007 0.08 0.62 4.02 2 Embodiment 0.003 0.22 0.15 0.022 0.001 26.2 2.07 0.08 0.05 0.009 0.08 0.62 0.018 3 of Present 0.005 0.34 0.50 0.027 0.001 27.9 2.11 0.15 0.08 0.007 0.08 0.53 0.081 4 Invention 0.006 0.34 0.50 0.027 0.001 27.9 2.13 0.15 0.08 0.007 0.08 0.61 0.079 5 0.005 0.35 0.49 0.027 0.002 28.1 2.08 0.14 0.10 0.006 0.09 0.62 0.080 6 0.005 0.36 0.49 0.027 0.002 28.1 2.08 0.14 0.10 0.008 0.09 0.61 0.076 7 0.003 0.50 0.49 0.023 0.001 28.0 4.01 4.10 0.08 0.012 0.08 0.68 0.102 8 0.003 0.50 0.50 0.023 0.001 28.1 3.98 0.08 0.55 0.011 0.08 0.68 0.101 9 0.019 0.51 0.79 0.022 0.001 31.8 2.08 0.03 0.04 0.008 0.09 0.62 0.092 10 0.008 0.35 0.49 0.018 0.001 28.0 2.02 0.08 0.12 0.008 0.10 0.62 0.080 11 0.009 0.35 0.49 0.018 0.001 28.1 2.03 0.08 0.11 0.006 0.09 0.61 0.078 12 Comparative 0.003 0.25 0.31 0.026 0.001 38.8 * <0.01 * 0.08 0.03 0.004 0.05 <0.01 * 0.010 13 Example 0.002 0.19 0.05 0.018 0.001 28.1 2.70 0.15 0.03 0.007 0.08 0.61 0.099 14 0.002 0.19 0.06 0.018 0.001 29.1 4.01 0.14 0.03 0.004 0.04 <0.01 * 0.099 15 0.008 0.35 0.48 0.028 0.001 26.0 4.03 2.02 0.04 0.008 0.08 0.63 0.081 16 0.008 0.37 0.48 0.017 0.001 28.2 2.22 0.13 0.10 0.008 0.11 <0.01 * 0.003 17 0.021 0.51 0.81 0.018 0.003 17.9 * 2.21 7.88 * 0.34 0.145 0.12 <0.01 * 0.004 Steel Chemical Composotion (  Balance: Fe and Impurities Material Sn Nb Ti REM Index 1 Example 0.51 — — — 24.99 2 Embodiment 0.52 — — — 30.65 3 of Present 0.12 — — — 32.65 4 Invention 0.81 — — — 32.70 5 1.22 — — — 32.79 6 2.20 — — — 32.31 7 0.80 — — 0.015 38.33 8 0.88 — — 0.014 38.34 9 0.80 0.21 0.18 0.018 38.40 10 0.66 — 0.014 — 32.40 11 0.65 0.20 — — 32.85 12 Comparative <0.01 * — — — 18.80 * 13 Example <0.01 * — — — 32.87 14 <0.01 * — — — 41.13 15 <0.01 * — — — 38.31 16 0.65 — — — 34.88 17 <0.01 * — — — 24.51 * Means that value deviates from range defined by the present invention. indicates data missing or illegible when filed

[0137] The cast surface of the respective ingots was removed by machining, and after being heated and held in a town gas heating furnace that was heated to 1170° C., the respective ingots were forged into a slab for hot rolling having a thickness of 60 mm and a width of 430 mm, at the surface temperature of the ingot being in a temperature range from 1170° C. to 930° C. The slab for hot rolling having a surface temperature of 800° C. or more was recharged as it was into the town gas heating furnace that remained heated to 1170° C. to reheat the slab, and after being soaked and held, the slab was subjected to hot rolling to have a thickness of 30 mm with a two-stage upper and lower roll-type hot rolling mill, and gradually cooled to room temperature.

[0138] After cutting was performed on the surface and the end faces by machining, the steel materials 1 to 17 were heated and held once more in the town gas heating furnace heated to 1170° C., and thereafter subjected to hot rolling to have a thickness of 1.8 mm, being formed into coils having coil widths of 400 to 410 mm and individual weights of 100 to 120 kg.

[0139] After making the coil widths 360 mm by slitting, surface oxide scale was grinded using a coil grinder at normal temperature, and after undergoing intermediate annealing at 1080° C., each coil was finished to a cold rolled coil with a thickness of 0.116 mm and a width of 340 mm while sandwiching steps of an intermediate coil pickling process and end face slitting in the process.

[0140] Final annealing was performed in a bright annealing furnace in a 75 vol % H.sub.2-25 vol % N.sub.2 atmosphere in which the dew point was adjusted in the range of −50 to −53° C. The annealing temperature was 1060° C.

[0141] For all the steel materials 1 to 17, noticeable end face cracking, coil rupturing, coil surface defects or coil perforation were not observed in the course of the present experimental production.

[0142] The micro-structures were ferrite single-phase micro-structures, and it was confirmed that in all of the steel materials to which B was added, the added B precipitated in the steel in the form of M.sub.2B, and the M.sub.2B was finely crushed in sizes ranging from 1 μm for smaller precipitates to around 7 μm for larger precipitates, and was dispersed uniformly including the plate thickness direction, from a macroscopic viewpoint.

[0143] Cleaning was performed after removing a bright annealing coating film on the surface by polishing with 600-grade emery paper, and an intergranular corrosion resistance evaluation was performed by a copper sulfate-sulfuric acid test method in accordance with JIS-G-0575.

[0144] The results are summarized in Table 2. The steel material 17 shown in Table 2 is a material that is equivalent to a commercially available austenitic stainless steel, and the steel material 18 is a material obtained by performing gold plating with respect to the steel material 17.

TABLE-US-00002 TABLE 2 Principal Iron ion concentration Conductive (ppm) in immersion Metallic liquid after immersion Precipitates for 1000 hours at Confirmed in Electrical Surface Contact Resistance (mΩ .Math. cm.sup.2): 90° C. in sulfuric Steel Applied Load is 10 kgf/cm.sup.2 acid aqueous solution of (excluding oxide- Measurement Starting Measurement Starting Material II: pH 3 containing 80 ppm based non- Material I: Surface after immersion for 1,000 hours F.sup.− ions which simulated metallic Intergranular Surface after at 90° C. in sulfuric acid aqueous solution inside of electric cell: precipitates and Corrosion spray etching of pH 3 containing 80 ppm F.sup.− ions which Immersion of two 80-mm Steel sulfide-based non- Resistance with 43° Bsume ferric simulated environment inside an electric square test places, liquid Material metallic precipates) JIS-G-0575 chloride aqueous solution cell, diagonally leaning in Teflon holder volume 800 ml 1 Example M.sub.2B No Cracking 5.5 4.3 34 2 Embodiment M.sub.2B No Cracking 3.4 3.3 31 3 of Present M.sub.2B No Cracking 8.5 4.4 89 4 Invention M.sub.2B No Cracking 5.3 5.3 32 5 M.sub.2B No Cracking 4.2 5.3 34 6 M.sub.2B No Cracking 3.5 4.3 35 7 M.sub.2B No Cracking 3.4 4.4 36 8 M.sub.2B No Cracking 4.3 5.3 41 9 M.sub.2B No Cracking 3.3 3.3 39 10 M.sub.2B No Cracking 4.5 5.3 53 11 M.sub.2B No Cracking 4.4 4.5 52 12 Comparative — (None) No Cracking 89.98 202.198 8965 13 Example M.sub.2B No Cracking 16.18 21.23 2895 14 — (None) No Cracking 38.64 143.185 1895 15 M.sub.2B No Cracking 13.15 21.25 1564 16 — (None) No Cracking 8.8 192.215 85 17 — (None) No Cracking 56.35 136.186 3075 18 Reference — (None) No Cracking 2.3 2.3 31 Example

[0145] As shown in Table 2, sensitization was not observed in the steel materials 1 to 11. Furthermore, extracted residue analysis was performed, but precipitation of Cr-based carbides represented by M.sub.23C.sub.6 could not be confirmed.

Example 2

[0146] Cut plates having a thickness of 0.116 mm, a width of 340 mm and a length of 300 mm were extracted from the steel materials 1 to 18, and a spray etching process using a 43° Baume ferric chloride aqueous solution was performed at 35° C. simultaneously on the entire top and bottom faces of the cut plates. The time period of the etching process by spraying is 40 seconds. The etching amount was set at 8 μm for a single face.

[0147] Immediately after the spray etching process, spray washing with clean water, washing by immersion into clean water, and a drying treatment using an oven were performed consecutively. After the drying treatment, 60-mm square samples were cut out and adopted as starting material I for electrical surface contact resistance measurement.

[0148] Further, 60-mm square samples that were separately extracted from the steel materials 1 to 18 were subjected to immersion treatment for 1000 hours at 90° C. in a sulfuric acid aqueous solution of pH 3 containing 80 ppm F.sup.− ions which simulated the inside of a polymer electrolyte fuel cell, and adopted as starting material II for electrical surface contact resistance measurement which simulated the environment during fuel cell application.

[0149] Electrical surface contact resistance measurement was performed while the starting material for evaluation was held between platinum plates in a state in which the starting material for evaluation was sandwiched with carbon paper TGP-H-90 manufactured by Toray Industries, Inc. Measurement was performed by a four-terminal method that is commonly used for evaluating separator materials for fuel cells. The applied load at the time of measurement was 10 kgf/cm.sup.2. The lower the measurement value that was obtained, the greater the degree to which the measurement value indicated a reduction in IR loss at the time of power generation, and also a reduction in energy loss due to heat generation. The carbon paper TGP-H-90 manufactured by Toray Industries, Inc. was replaced for each measurement. Note that, measurement was performed twice at different places on the respective steel materials.

[0150] The electrical contact resistance measurement results and the amount of iron ions that eluted into the sulfuric acid aqueous solution of pH 3 which simulated an environment inside an electric cell are summarized in Table 2. In the metal ion elution measurement, although Cr ions and Mo ions and the like were also determined at the same time, since the amount thereof was very small, the behavior of such ions is indicated by comparison with the Fe ion amount for which the elution amount was largest.

[0151] Note that, as described above, the steel material 18 is a starting material obtained by performing a gold-plating process to an average thickness of 50 nm on the starting material I and II for surface contact resistance measurement of the steel material 17, and the gold-plated material is considered to be the ideal starting material that has the most excellent electrical surface contact resistance performance. Therefore, the steel material 18 is additionally shown as a reference example.

[0152] In the steel materials 1 to 11, the precipitation and dispersion of M.sub.2B and of also containing Sn, so that the electrical surface contact resistance was stable and as low as that of a gold-plated material, and eluted iron ions were also of the same level as that of a gold-plated material. With the exception of the steel materials 12 to 15 and 17 to which Sn was not added, the presence of metallic tin and a tin oxide was confirmed on the surface of the starting material I for electrical surface contact resistance measurement after the spray etching process using the ferric chloride aqueous solution, and on the surface of the starting material II that simulated an environment during fuel cell application using sulfuric acid aqueous solution of pH 3. It was found that, in comparison with the steel materials 12, 14, and 17 in which M.sub.2B metallic precipitates did not precipitate as well as the steel materials 13 and 15 in which metallic tin and a tin oxide were not present on the surface because Sn was not added thereto, the steel materials 1 to 11 that are example embodiments of the present invention being materials to which B and Sn were added, were distinctly decreased in electrical surface contact resistance values, proving that the improvement effect is remarkable. Furthermore, in comparative examples in which Sn was contained but M.sub.2B was not precipitated and dispersed, such as the steel material 16, the electrical surface contact resistance increased as compared with the steel materials 1 to 11 that are example embodiments of the present invention which were materials to which B and Sn were added. Consequently, in the steel materials 1 to 11, the improvement effect brought by M.sub.2B being precipitated and dispersed of and Sn being contained was remarkable.

[0153] Based on the results of analyzing the iron ions in the immersion liquid that simulated the inside of a fuel cell that are shown in Table 2, it is clear that the addition of Sn brings an effect of suppressing the elution of metal ions. Note that the reason the steel material 17 being a gold-plated material is favorable is because of a covering effect of a gold plating film that is excellent in corrosion resistance. It could be determined that the steel materials 1 to 11 that are example embodiments of the present invention are equivalent to gold plating, and it was thus determined that a surface covering effect of the same level as gold plating inside a fuel cell can also be expected of metallic tin and a tin oxide.

Example 3

[0154] Separators having the shape shown in the photograph in FIG. 2 were press-formed using the coil starting materials prepared in Example 1, and application thereof to actual fuel cells was evaluated. The area of a channel portion of the separators was 100 cm.sup.2.

[0155] A setting evaluation condition for fuel cell operation was a constant-current operation evaluation at a current density of 0.1 A/cm.sup.2, and this is one of the operation environments for a stationery-type fuel cell for household use. The hydrogen and oxygen utilization ratio was made constant at 40%. The evaluating time was 500 hours.

[0156] The evaluation results for the steel materials 1 to 18 are summarized in Table 3. Note that, for the steel materials 12, 14, 16 and 17 in Table 3, there was a marked decline in performance, and evaluation was ended after less than 400 hours.

TABLE-US-00003 TABLE 3 Cell resistance value (mΩ) behavior Fe ion concentration Fe ion concentraation during unit cell fuel cell (ppb) in outlet gas (ppb) in outlet gas operation: 0.1 mA/cm.sup.2 contant-current condensate liquid from condensate liquid from Fe ion concentration operation, gas utilization ration 40% cathode electrode of fuel anode electrode side of fuel (μG) in MEA poylmer Steel After 50 hours from After 500 hours from cell stack: 400 hours cell stack: 400 hours membrane after end of Material start of operation start of operation after start of operation after start of operation operation 1 Example 0.76 0.79 2.7 28 72 2 Embodiment 0.76 0.78 3.2 26 70 3 of Present 0.75 0.79 3.0 28 72 4 Invention 0.75 0.77 3.1 26 74 5 0.72 0.73 3.2 24 68 6 0.71 0.72 2.3 22 68 7 0.75 0.77 2.6 26 68 8 0.75 0.77 2.5 28 70 9 0.74 0.78 2.6 28 69 10 0.74 0.78 2.8 26 70 11 0.75 0.78 3.0 28 72 12 Comparative 1.53 >2.0 (Stopped at 183 hours) — — — 13 Example 0.75 0.83 3.5 32 96 14 1.38 >2.0 (Stopped at 350 hours) — — — 15 0.74 0.83 3.4 33 90 16 0.74 >2.0 (Stopped at 333 hours) — — — 17 1.45 >2.0 (Stopped at 315 hours) — — — 18 Reference 0.69 0.72 2.6 22 64 Example

[0157] As shown in Table 3, remarkable differences were recognized in cell resistance values measured using a commercially available resistance meter (model 3565) manufactured by Tsuruga Electric Corporation, and thus the precipitation and dispersion effect of M.sub.2B and the Sn addition effect were confirmed. In addition, as shown in Table 3, deterioration in performance over time in the steel materials 1 to 11 of the present invention was also small. After operation ended, the stack was disassembled and the applied separator surface was observed, and it was confirmed that there was no rusting from the separator and that the amount of metal ions in the MEA also did not increase.

REFERENCE SIGNS LIST

[0158] 1 Fuel Cell [0159] 2 Solid Polymer Electrolyte Membrane [0160] 3 Fuel Electrode Layer (Anode) [0161] 4 Oxide Electrode Layer (Cathode) [0162] 5a, 5b Separator [0163] 6a, 6b Channel