Method for producing stainless steel for fuel cell separator, stainless steel for fuel cell separator, fuel cell separator, and fuel cell

Abstract

A stainless steel for use in a fuel cell separator is produced by subjecting stainless steel containing 16 mass % or more of Cr to electrolytic treatment and thereafter to immersion treatment in a solution containing fluorine. The electrolytic treatment is carried out by anodic electrolyzation or by a combination of anodic electrolyzation and cathodic electrolyzation, and an anodic electrolytic quantity Qa and a cathodic electrolytic quantity Qc preferably satisfy QaQc. The solution containing fluorine preferably has a temperature of 40 C. or higher, and hydrofluoric acid concentration [HF] (mass %) and nitric acid concentration [HNO.sub.3] (mass %) satisfying [HF]0.8[HNO.sub.3].

Claims

1. A method of producing stainless steel for use in a fuel cell separator, wherein stainless steel containing 16 mass % or more of Cr comprising subjecting the stainless steel to electrolytic treatment and thereafter to immersion treatment in a solution containing fluorine and an Fe ion concentration of 0.04 g/l or more, wherein the solution containing fluorine is hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid having a hydrofluoric acid concentration [HF] (mass %) and a nitric acid concentration [HNO.sub.3] (mass %) satisfying: [HF]0.8[HNO.sub.3], [HNO.sub.3] including 0 with a proviso that, if [HNO.sub.3] is 0, then [HF] is greater than 0, and wherein electrolytic treatment is carried out in at least one of an acid containing 0.5 mass % or more of sulfuric acid and a solution containing 5 mass % or more of sodium sulfate.

2. The method according to claim 1, wherein the electrolytic treatment is carried out by anodic electrolyzation or by a combination of anodic electrolyzation and cathodic electrolyzation; and an anodic electrolytic quantity Qa in C/dm.sup.2 and a cathodic electrolytic quantity Qc in C/dm.sup.2 satisfy relationship: QaQc having relationship: Qc=0 when electrolytic treatment is carried out by anodic electrolyzation alone with a proviso that, if Qc=0, then Qa>Qc.

3. The method according to claim 1, wherein a temperature of the solution containing fluorine is 40 C. or higher.

4. The method according to claim 1, wherein the solution containing fluorine is hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid having a hydrofluoric acid concentration [HF] (mass %) and a nitric acid concentration [HNO.sub.3] (mass %) satisfying: [HF]1.7[HNO.sub.3], [HNO.sub.3] including 0 with a proviso that, if [HNO.sub.3] is 0, then [HF] is greater than 0.

5. The method according to claim 1, wherein the solution containing fluorine is hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid having a hydrofluoric acid concentration [HF] (mass %) and a nitric acid concentration [HNO.sub.3] (mass %) satisfying: [HF]5.0[HNO.sub.3], [HNO.sub.3] including 0 with a proviso that, if [HNO.sub.3] is 0, then [HF] is greater than 0.

6. The method according to claim 2, wherein a temperature of the solution containing fluorine is 40 C. or higher.

7. The method according to claim 2, wherein the solution containing fluorine is hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid having a hydrofluoric acid concentration [HF] (mass %) and a nitric acid concentration [HNO.sub.3] (mass %) satisfying: [HF]1.7[HNO.sub.3], [HNO.sub.3] including 0 with a proviso that, if [HNO.sub.3] is 0, then [HF] is greater than 0.

8. The method according to claim 3, wherein the solution containing fluorine is hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid having a hydrofluoric acid concentration [HF] (mass %) and a nitric acid concentration [HNO.sub.3] (mass %) satisfying: [HF]1.7[HNO.sub.3], [HNO.sub.3] including 0 with a proviso that, if [HNO.sub.3] is 0, then [HF] is greater than 0.

9. The method according to claim 2, wherein the solution containing fluorine is hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid having a hydrofluoric acid concentration [HF] (mass %) and a nitric acid concentration [HNO.sub.3] (mass %) satisfying: [HF]5.0[HNO.sub.3], [HNO.sub.3] including 0 with a proviso that, if [HNO.sub.3] is 0, then [HF] is greater than 0.

10. The method according to claim 3, wherein the solution containing fluorine is hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid having a hydrofluoric acid concentration [HF] (mass %) and a nitric acid concentration [HNO.sub.3] (mass %) satisfying: [HF]5.0[HNO.sub.3], [HNO.sub.3] including 0 with a proviso that, if [HNO.sub.3] is 0, then [HF] is greater than 0.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view illustrating the fundamental structure of a fuel cell.

(2) FIG. 2 is a figure representing the relationship among [HF]/[HNO.sub.3], a contact resistance value after immersion treatment, and a contact resistance value after durability evaluation.

REFERENCE SIGNS LIST

(3) 1 membrane-electrode assembly 2 gas diffusion layer 3 gas diffusion layer 4 separator 5 separator 6 air flow path 7 hydrogen flow path

DETAILED DESCRIPTION

(4) Our methods, stainless steels, separators and fuel cells will specifically be described below.

(5) Initially, stainless steel will be described below.

(6) There is no particular limitation as to what type of stainless steel may be used as a substrate as long as it has corrosion resistance which is required in the operating environment of a fuel cell, and any one of ferritic stainless steel, austenitic stainless steel, and duplex stainless steel can be used. However, to ensure a minimally required corrosion resistance, 16 mass % or more, preferably 18 mass % or more of Cr should be included.

(7) Specifically preferable component compositions with respect to ferritic stainless steel, austenitic stainless steel, and duplex stainless steel are described as follows. Herein, the expression of % with respect to a component refers to mass % unless otherwise specified.

(8) (1) Preferable Component Composition of Ferritic Stainless Steel

(9) C: 0.03% or less

(10) Since C combines with Cr in steel and thereby decreases corrosion resistance, a smaller C content is preferable, but a content of 0.03% or less does not markedly decrease corrosion resistance. Therefore, the C content is preferably 0.03% or less and more preferably 0.015% or less.

(11) Si: 1.0% or less

(12) Si is an element used for deoxidation. However, since excessive content of Si decreases ductility, the Si content is preferably 1.0% or less and more preferably 0.5% or less.

(13) Mn: 1.0% or less

(14) Since Mn combines with S to form MnS and thereby decreases corrosion resistance, the Mn content is preferably 1.0% or less and more preferably 0.8% or less.

(15) S: 0.01% or less

(16) As described above, since S combines with Mn to form MnS and thereby decreases corrosion resistance, the S content is preferably 0.01% or less and more preferable 0.008% or less.

(17) P: 0.05% or less

(18) Since P decreases ductility, a smaller P content is preferable, but a P content of 0.05% or less does not markedly decrease ductility. Therefore, the content is preferably 0.05% or less and more preferably 0.04% or less.

(19) Al: 0.20% or less

(20) Al is an element used for deoxidation. However, since excessive content of Al decreases ductility, the Al content is preferably 0.20% or less and more preferably 0.15% or less.

(21) N: 0.03% or less

(22) Since N combines with Cr in steel and thereby decreases corrosion resistance, a smaller N content is preferable, but a N content of 0.03% or less does not result in a marked decrease in corrosion resistance. Therefore, the content is preferably 0.03% or less and more preferably 0.015% or less.

(23) Cr: 16% or more

(24) Since Cr is a required element for stainless steel to maintain corrosion resistance, the Cr content needs to be 16% or more to obtain this effect. When the Cr content is less than 16%, durability for long-term use as a separator is not realized. In particular, when an environmental change during use is severe, the Cr content is preferably 18% or more and more preferably 20% or more. On the other hand, when the Cr content is more than 40%, workability markedly degrades. Therefore, when workability is emphasized, the Cr content is preferably 40% or less and more preferably 35% or less.

(25) At least one selected from Nb, Ti, and Zr is set to be 1% or less in total. Any one of Nb, Ti, and Zr is a useful element to stabilize C and N in steel as a carbide and a nitride, respectively, or as a carbonitride to improve corrosion resistance. However, when more than 1.0% is included, ductility markedly degrades. Therefore, in any case of single addition or combined addition using these elements, the content of the element or elements is limited to 1.0% or less. Herein, to sufficiently produce an adding effect of these elements, a content of 0.02% or more is preferable.

(26) The required components have been described hereinbefore. In addition thereto, the following components may be appropriately contained.

(27) Mo: 0.02% or more and 4.0% or less

(28) Mo is an element effective to improve corrosion resistance, specifically, localized corrosion of stainless steel. To obtain this effect, a Mo content of 0.02% or more is preferable. On the other hand, when the Mo content is more than 4.0%, ductility markedly decreases, and therefore an upper limit thereof is preferably 4.0% and more preferably 2.0% or less.

(29) Further, to improve corrosion resistance, additionally, 1.0% or less of each of Ni, Cu, V, and W may be contained. Still further, to enhance hot workability, 0.1% or less of each of Ca, Mg, REM (Rare Earth Metals), and B may also be contained.

(30) Fe and unavoidable impurities account for the remainder. Among the unavoidable impurities, the O (oxygen) content is preferably 0.02% or less.

(31) (2) Preferable Component Composition of Austenitic Stainless Steel

(32) C: 0.08% or less

(33) C reacts with Cr in austenitic stainless steel for use in a separator and as a result forms a compound and deposits on a grain boundary as Cr carbide, resulting in a decrease in corrosion resistance. Therefore, a smaller C content is preferable, and a C content of 0.08% or less does not markedly degrade corrosion resistance. Accordingly, the C content is preferably 0.08% or less and more preferably 0.03% or less.

(34) Cr: 16% or more

(35) Cr is an element that is required to secure fundamental corrosion resistance in an austenitic stainless steel sheet. When the Cr content is less than 16%, durability for long-term use as a separator is not achieved. Therefore, the Cr content is set to be 16% or more. On the other hand, when there is a Cr content of more than 30%, it is difficult to obtain an austenitic structure. Accordingly, the Cr content is preferably 30% or less and more preferably 18% or more and 26% or less.

(36) Mo: 0.1% or more and 10.0% or less

(37) Mo is an element effective in inhibiting localized corrosion such as crevice corrosion of austenitic stainless steel for use in a separator. To obtain this effect, the Mo content is required to be 0.1% or more. On the other hand, when the Mo content is more than 10.0%, embrittlement of stainless steel for use in a separator markedly occurs, resulting in a decrease in productivity. Therefore, the Mo content is preferably 0.1% or more to 10.0% or less and more preferably 0.5% or more and 7.0% or less.

(38) Ni: 7% or more and 40% or less

(39) Ni is an element that stabilizes an austenitic phase. When the Ni content is less than 7%, an effect of stabilizing the austenitic phase is not obtained. On the other hand, when the Ni content is more than 40%, Ni is excessively consumed, resulting in an increase in cost. Therefore, the content is preferably 7% or more and 40% or less.

(40) In the austenitic stainless steel for use in a separator, in addition to the above-mentioned C, Cr, Mo, and Ni, the following elements may be included as appropriate.

(41) N: 2.0% or less

(42) N effectively inhibits localized corrosion of austenitic stainless steel for use in a separator. However, since it is industrially difficult to have a N content of more than 2.0%, the content is preferably 2.0% or less. Further, in a common refining method, in the case of a N content of more than 0.4%, it takes a long period of time to add N in a steel making stage for the stainless steel used in a separator, resulting in a decrease in productivity. Therefore, from a cost point of view, the N content is preferably 0.4% or less and more preferably 0.01% or more and 0.3% or less.

(43) Cu: 0.01% or more and 3.0% or less

(44) Cu is an element that improves the corrosion resistance of austenitic stainless steel for use in a separator. To obtain such an effect, the Cu content is preferably 0.01% or more. However, when the Cu content is more than 3.0%, hot workability degrades, resulting in a decrease in productivity. Therefore, when Cu is included, the Cu content is preferably 3.0% or less and more preferably 0.01% or more and 2.5% or less.

(45) Si: 0.01% or more and 1.5% or less

(46) Si is an element that is effective for deoxidation and is added at the refining stage of austenitic stainless steel for use in a separator. To obtain such an effect, the Si content is preferably 0.01% or more. However, excessive content of Si hardens stainless steel for use in a separator, resulting in a decrease in ductility. Therefore, when Si is included, the Si content is preferably 1.5% or less and more preferably 0.01% or more and 1.0% or less.

(47) Mn: 0.001% or more and 2.5% or less

(48) Since Mn combines with unavoidably mixed S and therefore has an effect of reducing solid-solution S in austenitic stainless steel for use in a separator, Mn is an element that is effective in inhibiting grain boundary segregation of S and thus prevents breakage during hot rolling. Such an effect is exhibited when a Mn content is 0.001% or more and 2.5% or less. Therefore, when Mn is included, the Mn content is preferably 0.001% or more and 2.5% or less and more preferably 0.001% to 2.0%.

(49) At least one of Ti, Nb, V, and Zr is set to be 0.01% to 0.5% in total.

(50) Any one of Ti, Nb, V, and Zr combines with C in austenitic stainless steel to form a carbide. In this manner, Ti, Nb, V, and Zr stabilize C and therefore are elements effective to improve the grain boundary corrosion resistance of austenitic stainless steel for use in a separator. In particular, when the C content is 0.08% or less, an effect of improving corrosion resistance in the case of containing at least any one of Ti, Nb, V, and Zr is exhibited in any case of singly containing or multiply containing 0.01% or more in total of Ti, Nb, V, and Zr.

(51) On the other hand, in any case of singly containing or multiply containing more than 0.5% in total of Ti, Nb, V, and Zr, the above effect is saturated. Therefore, in the case of containing Ti, Nb, V, or Zr, the content of at least one of these elements is preferably 0.01% or more and 0.5% or less in total.

(52) Other than the above-mentioned elements, 0.1% or less of each of Ca, Mg, B, and rare-earth elements (so-called REM) may be included to enhance the hot workability of austenitic stainless steel for use in a separator. For deoxidation at the refining stage, 0.2% or less of Al may be included.

(53) Fe and unavoidable impurities account for the remainder. Among the unavoidable impurities, the O (oxygen) content is preferably 0.02% or less.

(54) (3) Preferable Component Composition of Duplex Stainless Steel

(55) C: 0.08% or less

(56) C combines with Cr to form a compound and is deposited on a grain boundary as Cr carbide, resulting in a decrease in corrosion resistance. Therefore, a smaller content of C is preferable, and a C content of 0.08% or less does not markedly degrade corrosion resistance. Accordingly, the C content is preferably 0.08% or less and more preferably 0.03% or less.

(57) Cr: 16% or more

(58) Cr is an element that is required to secure fundamental corrosion resistance in a duplex stainless steel sheet. When the Cr content is less than 16%, durability for long-term use as a separator is not achieved. Therefore, the C content is set to be 16% or more. On the other hand, when there is a Cr content of more than 30%, it is difficult to obtain a duplex structure (hereinafter, referred to as a duplex structure having a ferritic phase and an austenite phase unless otherwise specified). Accordingly, the Cr content is preferably 30% or less and more preferably 20% to 28%.

(59) Mo: 0.1% to 10.0%

(60) Mo is an element effective in inhibiting localized corrosion such as crevice corrosion. To obtain this effect, the Mo content is required to be 0.1% or more. On the other hand, when the Mo content is more than 10.0%, embrittlement of stainless steel markedly occurs, resulting in a decrease in productivity. Therefore, the Mo content is preferably 0.1% or more and 10.0% or less and more preferably 0.5% or more and 7.0% or less.

(61) Ni: 1% to 10%

(62) Ni is an element to stabilize an austenitic phase. When the Ni content is less than 1%, the austenitic phase is difficult to form and, as a result, it is difficult to obtain a duplex structure. On the other hand, when the Ni content is more than 10%, it is difficult to form a ferritic phase and, as a result, it is difficult to obtain a duplex structure. Therefore, the Ni content is preferably 1% or more and 10% or less.

(63) In the duplex stainless steel for use in our separator, in addition to the above-mentioned C, Cr, Mo, and Ni, the following elements may be included as appropriate.

(64) N: 2.0% or less

(65) N is an element that inhibits localized corrosion of duplex stainless steel for use in a separator. However, since it is industrially difficult to have a N content of more than 2.0%, an upper limit of 2.0% is preferably set. Further, in a common steel making method, in the case of a N content of more than 0.4%, it takes a long period of time to add N in the stainless steel used in a separator, resulting in a decrease in productivity. Therefore, from a cost point of view, the N content is preferably 0.4% or less and more preferably 0.01% to 0.3%.

(66) Cu: 3.0% or less

(67) Cu is an element that improves the corrosion resistance of duplex stainless steel for use in a separator. To obtain such an effect, the Cu content is preferably 0.01% or more. However, when the Cu content is more than 3.0%, hot workability degrades, resulting in a decrease in productivity. Therefore, when Cu is included, the Cu content is preferably 3.0% or less and more preferably 0.01% or more and 2.5% or less.

(68) Si: 1.5% or less

(69) Si is an element effective for deoxidation and is added at the refining stage of duplex stainless steel for use in a separator. To obtain such an effect, the Si content is preferably 0.01% or more. However, excessive content of Si hardens stainless steel for use in a separator, resulting in a decrease in ductility. Therefore, when Si is included, the Si content is preferably 1.5% or less and more preferably 0.01% or more and 1.0% or less.

(70) Mn: 0.001% or more and 2.5% or less

(71) Since Mn combines with unavoidably mixed S and therefore has an effect of reducing solid-solution S in duplex stainless steel for use in a separator, Mn is an element effective in inhibiting grain boundary segregation of S and thus prevents breakage during hot rolling. Such an effect is exhibited when a Mn content is 0.001% or more and 2.5% or less. Therefore, in the case of containing Mn, the Mn content is preferably 0.001% or more and 2.5% or less and more preferably 0.001% or more and 2.0% or less.

(72) At least one of Ti, Nb, V, and Zr is set to be 0.01% to 0.5% in total.

(73) Any one of Ti, Nb, V, and Zr combines with C in duplex stainless steel to form a carbide. In this manner, Ti, Nb, V, and Zr fix C and therefore are elements effective to improve the grain boundary corrosion resistance of duplex stainless steel for use in a separator. In particular, when the C content is 0.08% or less, an effect of improving corrosion resistance in the case of containing at least any one of Ti, Nb, V, and Zr is exhibited in any case of singly containing or multiply containing 0.01% or more in total of Ti, Nb, V, and Zr.

(74) On the other hand, in any case of singly containing or multiply containing more than 0.5% in total of Ti, Nb, V, and Zr, the above effect is saturated. Therefore, in the case of containing Ti, Nb, V, or Zr, the content of at least one of these elements is preferably 0.01% to 0.5% in total.

(75) Other than the above-mentioned elements, 0.1% or less of each of Ca, Mg, B, and rare-earth elements (so-called REM) may be included to enhance the hot workability of duplex stainless steel for use in a separator, and for deoxidation at the refining stage, 0.2% or less of Al may be included.

(76) Fe and unavoidable impurities account for the remainder. Among the unavoidable impurities, the O (oxygen) content is preferably 0.02% or less.

(77) Stainless steel having excellent conductivity and durability for use in a fuel cell separator is obtained by subjecting the above-mentioned stainless steel to electrolytic treatment and immersion treatment in a solution containing fluorine.

(78) It is important to carry out electrolytic treatment prior to immersion treatment in a solution containing fluorine. The electrolytic treatment reforms a film formed in a process of producing stainless steel to easily exhibit an effect of reducing contact resistance via the immersion treatment in a solution containing fluorine. In addition, even when Fe ions are mixed in the solution containing fluorine, the effect of reducing contact resistance tends not to disappear. The electrolytic treatment and the immersion treatment are preferably carried out continuously, but cleaning to an extent that a surface is not markedly deteriorated may be carried out between the electrolytic treatment and the immersion treatment. Further, after the immersion treatment, cleaning to the extent that the surface is not markedly deteriorated may be carried out. The cleaning includes immersion in an alkali or acid.

(79) The electrolytic treatment is preferably carried out by anodic electrolyzation or by a combination of anodic electrolyzation and cathodic electrolyzation. Further, anodic electrolytic quantity Qa and cathodic electrolytic quantity Qc preferably satisfy the relationship: QaQc. When the electrolytic treatment is carried out by anodic electrolyzation alone, the relationship: Qc=0 is satisfied. Qa is the product of current density and treating time in anodic electrolyzation, and Qc is the product of current density and treating time in cathodic electrolyzation. The electrolytic treatment preferably includes anodic electrolyzation and this method is not limited. Alternating electrolyzation may be applied, but in the case of Qa<Qc, due to re-adhesion of eluted components, an effect of reducing contact resistance by the following immersion treatment tends to be insufficient. Therefore, the relationship: QaQc is preferably satisfied.

(80) In the immersion treatment, the temperature of a solution containing fluorine is preferably 40 C. or higher. In the case where the temperature is lower than 40 C., it is difficult to realize an effect of reducing contact resistance and therefore increased treating time is required to obtain a sufficient effect. An upper limit of the temperature of the solution is not specifically limited, but is, however, preferably 90 C. or lower from, for example, a safety point of view.

(81) Further, the solution containing fluorine is preferably hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid due to its marked effect, and the hydrofluoric acid concentration [HF] (mass %) and the nitric acid concentration [HNO.sub.3] (mass %) preferably satisfy the relationship: [HF]0.8[HNO.sub.3]. A mixture of nitric acid and hydrofluoric acid refers to a mixed liquid of hydrofluoric acid and nitric acid. When no nitric acid is contained in the solution containing fluorine, [HNO.sub.3] is designated to be 0. FIG. 2 is a figure representing the relationship among [HF]/[HNO.sub.3], a contact resistance value after immersion treatment, and a contact resistance value after durability evaluation. In FIG. 2, the measurement method and evaluation criteria for the contact resistance value after immersion treatment and the contact resistance value after durability evaluation are the same as in EXAMPLE 1 to be described later. As is evident from FIG. 2, the contact resistance value after immersion treatment and the contact resistance value after durability evaluation are evaluated as (good) and (good) in the case of [HF]0.8[HNO.sub.3]; (good) and (excellent) in the case of [HF]1.7[HNO.sub.3]; and (excellent) and (excellent) in the case of [HF]5.0[HNO.sub.3], respectively. The reason for this is thought to be that in the case of [HF]<0.8[HNO.sub.3], stainless steel is passivated and, as a result, it is difficult to realize an effect of reducing contact resistance. According to the above results, [HF]0.8[HNO.sub.3] is satisfied, [HF]1.7[HNO.sub.3] is preferably satisfied, and [HF]5.0[HNO.sub.3] is more preferably satisfied.

(82) With respect to the evaluation of the contact resistance value here, less than 5 m.Math.cm.sup.2, 5 m.Math.cm.sup.2 or more and less than 10 m.Math.cm.sup.2, and 10 m.Math.cm.sup.2 or more are determined to be excellent (), good (), and poor (X) in the case prior to durability evaluation, respectively; and less than 10 m.Math.cm.sup.2, 10 m.Math.cm.sup.2 or more and less than 15 m.Math.cm.sup.2, 15 m.Math.cm.sup.2 or more and less than 20 m.Math.cm.sup.2, and 20 m.Math.cm.sup.2 or more are determined to be excellent (), good (), fair (), and poor (X) in the case after durability evaluation, respectively.

(83) Under conditions other than the above ones, electrolytic treatment is preferably carried out in an acid containing 0.5 mass % or more of sulfuric acid. Electrolytic treatment in the acid containing sulfuric acid is advantageous to reform a film of stainless steel, and the concentration of sulfuric acid is preferably 0.5 mass % or higher. When the concentration of sulfuric acid is lower than 0.5 mass %, the film of stainless steel tends to be inadequately reformed. The upper limit of the concentration of sulfuric acid is not specifically limited. However, since the above effect is saturated with excessive addition of sulfuric acid, the concentration of sulfuric acid is preferably 50 mass % or lower and more preferably 1.0 to 40 mass %.

(84) Further, electrolytic treatment in a solution containing a salt is also advantageous to reform a film of stainless steel, and the concentration of the solution containing a salt is preferably 5 mass % or higher. At a concentration lower than 5 mass %, the film tends to be inadequately reformed. As the salt, for example, sodium sulfate is advantageously applicable. However, other than this, any salt having large solubility in water is usable. The upper limit of the concentration of the salt is not specifically limited, and the salt may be included up to the upper limit of its solubility. However, since even with excessive addition of the salt, the above effect is saturated, the salt concentration is preferably 40 mass % or lower and more preferably 8 to 30 mass %.

(85) The method of producing ferritic stainless steel, austenitic stainless steel, or duplex stainless steel serving as a substrate is not specifically limited by production conditions, and may be based on a conventionally well-known method. However, the following will be described as preferable production conditions.

(86) A steel ingot having a preferably prepared component composition is heated to 1100 C. or higher and then hot-rolled, followed by annealing at a temperature of 800 to 1100 C. and repetitive cold rolling and annealing to produce a stainless steel sheet. The sheet thickness of the thus-obtained stainless steel sheet is preferably about 0.02 to 0.8 mm. It is efficient to carry out final annealing, electrolytic treatment, and immersion in a solution containing fluorine online continuously. However, on the other hand, it is also possible to carry out a part or all of the processes off-line independently and perform cleaning between these processes.

EXAMPLES

Example 1

(87) Steel having the chemical composition listed in Table 1 was refining in a vacuum melting furnace and the thus-obtained steel ingot was heated to 1200 C., followed by hot rolling to produce a hot-rolled sheet having a sheet thickness of 5 mm. The thus-obtained hot-rolled sheet was annealed at 900 C. and descaled by pickling, followed by repetitive cold rolling and annealing/pickling to produce a cold-rolled sheet having a sheet thickness of 0.7 mm. Thereafter, in sulfuric acid of 30 C. having a concentration of 2 mass %, a part of the sample was subjected to electrolytic treatment in the sequential order of +2 A/dm.sup.21 sec.fwdarw.2 A/dm.sup.21 sec.fwdarw.+2 A/dm.sup.21 sec.fwdarw.2 A/dm.sup.21 sec.fwdarw.+2 A/dm.sup.21 sec (+ represents anodic electrolyzation, and represents cathodic electrolyzation) and thereafter to immersion treatment (treatment time: 90 sec) in a mixed solution of a mixture of nitric acid and hydrofluoric acid of 55 C. having 5 mass % HF+1 mass % HNO.sup.3 in which Fe was present in an amount of 0 to 1.0 g/l. The Fe concentration in the mixed solution of a mixture of nitric acid and hydrofluoric acid is changed to increase the immersion treatment quantity and thereby, an increase of an amount of Fe mixed in the solution is simulated.

(88) Treatment conditions of samples and contact resistance values after immersion treatment are shown in Table 2. Further, a test piece having a size of 30 mm30 mm was cut out from a sample after undergoing contact resistance measurement and degreased with acetone, and thereafter subjected to a durability evaluation test by being held for 20 hours at 0.8 V vs. SHE (standard hydrogen electrode) in sulfuric acid (80 C.) of pH 3 containing 2 ppm of Cl where an operating environment of a fuel cell was simulated to evaluate a contact resistance value after the durability evaluation test. The obtained results are shown in Table 3.

(89) With respect to the contact resistance, a sample was sandwiched between two pieces of carbon paper (TGP-H-120, produced by Toray Industries, Inc.) and further an electrode in which a copper sheet was subjected to gold plating was brought into contact on both sides thereof. Then, current was allowed to flow with a pressure of 20 kgf/cm.sup.2 per unit area and the potential difference between the sample and one electrode was measured to calculate electric resistance. A value obtained by multiplying the measured value by an area of a contact face was designated as the contact resistance. In the case prior to durability evaluation, less than 5 m.Math.cm.sup.2, 5 m.Math.cm.sup.2 or more and less than 10 m.Math.cm.sup.2, and 10 m.Math.cm.sup.2 or more were determined to be excellent (), good (), and poor (X), respectively; and in the case after durability evaluation, less than 10 m.Math.cm.sup.2, 10 m.Math.cm.sup.2 or more and less than 15 m.Math.cm.sup.2, 15 m.Math.cm.sup.2 or more and less than 20 m.Math.cm.sup.2, and 20 m.Math.cm.sup.2 or more were determined to be excellent (), good (), fair () and poor (X), respectively.

(90) In our methods and stainless steels, in any one of the cases prior to durability evaluation (after immersion treatment) and after durability evaluation, lower contact resistance and favorable conductivity are realized and also excellent durability is achieved.

(91) As is evident from FIG. 2, the contact resistance of a sample having been subjected to no electrolytic treatment became poor when the Fe ion concentration in a mixture of nitric acid and hydrofluoric acid was 0.04 g/l or higher.

(92) In addition, as is evident from FIG. 3, the contact resistance of a sample having been subjected to no electrolytic treatment also became poor after durability evaluation when the Fe ion concentration in a mixture of nitric acid and hydrofluoric acid was 0.04 g/l or higher.

Example 2

(93) Steel having the chemical composition listed in Table 4 was melted in the vacuum melting furnace and the thus-obtained steel ingot was heated to 1200 C., followed by hot rolling to produce a hot-rolled sheet having a sheet thickness of 5 mm. The thus-obtained hot-rolled sheet was annealed at a temperature of 900 to 1100 C. and descaled by pickling, followed by repetitive cold rolling and annealing/pickling to produce a cold-rolled sheet having a sheet thickness of 0.7 mm. Thereafter, electric treatment was carried out under various conditions and thereafter immersion treatment (treatment time: 90 sec) in hydrofluoric acid or a mixture of nitric acid and hydrofluoric acid containing 1.0 g/l of Fe ions was carried out. Conditions for the electrolytic treatment and the immersion treatment are shown in Table 5. Contact resistance values after immersion treatment are shown in Table 6.

(94) Further, a test piece having a size of 30 mm30 mm was cut out from a sample after contact resistance measurement and degreased with acetone, and then subjected to a durability evaluation test by being held for 20 hours at 0.8 V vs. SHE in sulfuric acid (80 C.) of pH 3 containing 2 ppm of Cl where an operating environment of a fuel cell was simulated to evaluate a contact resistance after the durability evaluation test. The obtained results are shown in Table 6. Herein, the measurement method of the contact resistance is the same as in EXAMPLE 1.

(95) As is evident from Tables 5 and 6, in the scope of our methods and stainless steels, in any one of the cases prior to durability evaluation (after immersion treatment) and after durability evaluation, lower contact resistance and favorable conductivity are realized and also excellent durability is achieved.

(96) TABLE-US-00001 TABLE 1 Steel sym- Chemical composition (mass %) bol C Si Mn P S Ni Cr N Remarks a 0.009 0.15 0.21 0.022 0.004 0.1 21.2 0.01 Compliant steel

(97) TABLE-US-00002 TABLE 2 Conductivity Fe ion Contact Electrolytic concentration in resistance Sample Steel treatment nitric-fluorinated value No. symbol (Yes or No) acid (g/l) (m .Math. cm.sup.2) Evaluation Remarks 1 a Yes 0.00 4.1 : Excellent Inventive example 2 a Yes 0.04 4.2 : Excellent Inventive example 3 a Yes 0.10 4.0 : Excellent Inventive example 4 a Yes 0.50 3.7 : Excellent Inventive example 5 a Yes 1.00 4.3 : Excellent Inventive example 6 a No 0.00 4.4 : Excellent Reference example 7 a No 0.04 54.9 X: Poor Comparative example 8 a No 0.10 55.2 X: Poor Comparative example 9 a No 0.50 55.4 X: Poor Comparative example *Current conditions (+ represents anodic electrolyzation and represents cathodic electrolyzation) +2 A/dm.sup.2 1 sec.fwdarw.2 A/dm.sup.2 1 sec.fwdarw.+2 A/dm.sup.2 1 sec.fwdarw.2 A/dm.sup.2 1 sec.fwdarw.+2 A/dm.sup.2 1 sec Qa: 6 C/dm.sup.2, Qc: 4 C/dm.sup.2

(98) TABLE-US-00003 TABLE 3 Durability Contact resistance value after durability Sample evaluation No. (m .Math. cm.sup.2) Evaluation Remarks 1 6.2 : Excellent Inventive example 2 6.2 : Excellent Inventive example 3 6.3 : Excellent Inventive example 4 5.4 : Excellent Inventive example 5 6.9 : Excellent Inventive example 6 7.1 : Excellent Reference example 7 151.3 X: Poor Comparative example 8 167.4 X: Poor Comparative example 9 153.0 X: Poor Comparative example

(99) TABLE-US-00004 TABLE 4 Steel Chemical composition (mass %) symbol C Si Mn P S Ni Cr Mo N Remarks a 0.009 0.15 0.21 0.022 0.004 0.1 21.2 0.01 Compliant steel b 0.012 0.32 0.94 0.023 0.003 8.5 17.9 0.03 Compliant steel c 0.062 0.31 0.75 0.028 0.005 4.3 24.1 1.81 0.08 Compliant steel d 0.020 0.37 0.53 0.023 0.003 0.1 15.3 0.03 Comparative example

(100) TABLE-US-00005 TABLE 5 Electrolytic treatment Immersion treatment Sample Steel Current Qa Qc Liquid Temperature No. symbol Liquid composition conditions *1 (C/dm.sup.2) (C/dm.sup.2) composition *2 ( C.) Remarks 10 a 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF + 1% HNO.sub.3 30 Inventive example 11 a 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF + 1% HNO.sub.3 40 Inventive example 12 a 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF 55 Inventive example 13 a 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF + 3% HNO.sub.3 55 Inventive example 14 a 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF + 7% HNO.sub.3 55 Inventive example 15 a 2 mass % H.sub.2SO.sub.4 + A 6 4 5% HF + 1% HNO.sub.3 55 Inventive example 15 mass % Na.sub.2SO.sub.4 16 a 15 mass % Na.sub.2SO.sub.4 A 6 4 5% HF + 1% HNO.sub.3 55 Inventive example 17 a 2 mass % H.sub.2SO.sub.4 B 150 140 5% HF + 1% HNO.sub.3 55 Inventive example 18 a 2 mass % H.sub.2SO.sub.4 C 60 0 5% HF + 1% HNO.sub.3 55 Inventive example 19 a 2 mass % H.sub.2SO.sub.4 D 4 6 5% HF + 1% HNO.sub.3 55 Inventive example 20 b 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF 55 Inventive example 21 b 15 mass % Na.sub.2SO.sub.4 B 150 140 5% HF 55 Inventive example 22 b 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF + 3% HNO.sub.3 55 Inventive example 23 c 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF 55 Inventive example 24 d 2 mass % H.sub.2SO.sub.4 A 6 4 5% HF + 1% HNO.sub.3 55 Comparative example *1: Current conditions (+ represents anodic elecvtrolyzation and represents cathodic electrolyzation) A: +2 A/dm.sup.2 1 sec.fwdarw.2 A/dm.sup.2 1 sec.fwdarw.+2 A/dm.sup.2 1 sec.fwdarw.2 A/dm.sup.2 1 sec.fwdarw.+2 A/dm.sup.2 1 sec B: (+5 A/dm.sup.2 2 sec.fwdarw.5 A/dm.sup.2 2 sec) 14 times.fwdarw.+5 A/dm.sup.2 2 sec C: +2 mA/dm.sup.2 30 sec D: 2 A/dm.sup.2 1 sec.fwdarw.+2 A/dm.sup.2 1 sec.fwdarw.2 A/dm.sup.2 1 sec.fwdarw.+2 A/dm.sup.2 1 sec.fwdarw.2 A/dm.sup.2 1 sec *2: % in the table refers to mass %.

(101) TABLE-US-00006 TABLE 6 Durability Contact Conductivity resistance value Contact after durability Sample resistance evaluation No. (m .Math. cm.sup.2) Evaluation (m .Math. cm.sup.2) Evaluation Remarks 10 9.2 : Good 18.4 : Fair Inventive example 11 4.9 : Excellent 11.9 : Good Inventive example 12 4.0 : Excellent 6.1 : Excellent Inventive example 13 6.3 : Good 9.6 : Excellent Inventive example 14 9.4 : Good 14.8 : Good Inventive example 15 3.9 : Excellent 5.9 : Excellent Inventive example 16 4.1 : Excellent 6.2 : Excellent Inventive example 17 4.0 : Excellent 6.9 : Excellent Inventive example 18 4.1 : Excellent 6.0 : Excellent Inventive example 19 9.3 : Good 17.7 : Fair Inventive example 20 3.8 : Excellent 7.3 : Excellent Inventive example 21 4.0 : Excellent 7.6 : Excellent Inventive example 22 7.5 : Good 9.3 : Excellent Inventive example 23 4.4 : Excellent 8.4 : Excellent Inventive example 24 8.7 : Good 131.0 X: Poor Comparative example