Patent Application
20170301929
The present invention relates to a ferritic stainless steel material, and, a separator for polymer electrolyte fuel cells and a polymer electrolyte fuel cell that use the ferritic stainless steel material. The term “separator” herein may also be referred to as a “bipolar plate”.
Fuel cells are electric cells that utilize hydrogen and oxygen to generate a direct current, and are broadly categorized into a solid electrolyte type, a molten carbonate type, a phosphoric acid type, and a polymer electrolyte type. Each type is derived from the constituent material of an electrolyte portion that constitutes the basic portion of the fuel cell.
Nowadays, fuel cells that have reached the commercial stage include phosphoric acid type fuel cells, which operate in the vicinity of 200° C., and molten carbonate type fuel cells, which operate in the vicinity of 650° C. As technological development has moved forward in recent years, attention is given to polymer electrolyte fuel cells, which operate in the vicinity of room temperature, and solid electrolyte fuel cells, which operate at 700° C. or more, as small-sized power sources for automobile use or home use.
As illustrated in
A typical example of the solid polymer electrolyte membrane 2 is a fluorinated ion exchange resin film that has hydrogen ion (proton) exchange groups.
The fuel electrode layer 3 and the oxide electrode layer 4 each include a diffusion layer that is made of carbon paper or carbon cloth constituted by carbon fiber and has a surface on which a catalyst layer is provided that is made of a particulate platinum catalyst, graphite powder, and a fluorocarbon resin with hydrogen ion (proton) exchange groups, and the catalyst layer comes in contact with fuel gas or oxidizing gas that permeates through the diffusion layer.
A fuel gas (hydrogen or a hydrogen containing gas) A is fed through channels 6a formed in the separator 5a to supply hydrogen to the fuel electrode layer 3. An oxidizing gas B such as air is fed through channels 6b formed in the separator 5b to supply oxygen. The supply of these gases causes an electrochemical reaction, whereby direct current power is generated.
A solid polymer fuel cell separator is required to have functions including: (1) a function as a “channel” for supplying a fuel gas with in-plane uniformity on a fuel electrode side; (2) a function as a “channel” for efficiently discharging water produced on a cathode side from the fuel cell out of the system, together with carrier gases such as air and oxygen after the reaction; (3) a function as an electrical “connector” between unit cells that maintains low electrical resistance and favorable electric conductivity as an electrode over a long time period; and (4) a function as an “isolating wall” between adjacent unit cells for isolating an anode chamber of one unit cell from a cathode chamber of an adjacent unit cell.
Although applications of a carbon plate material as a separator material have been earnestly studied at the laboratory level up to now, there is a problem with a carbon plate material in that it easily cracks, and there is also a problem in that machining costs for flattening the surface and machining costs for forming a gas channel are extremely high. Each of these problems is significant and makes the commercialization of fuel cell difficult.
Among carbonaceous materials, a thermally expandable graphite processed product receives the most attention as a starting material for polymer electrolyte fuel cell separators because of its remarkable inexpensiveness. However, several problems remain to be solved in this regard including how to deal with increasingly strict demands for dimensional accuracy, age deterioration of an organic resin binder that arises during application to fuel cells, carbon corrosion that progresses under the influence of cell operation conditions, and unexpected cracking problems that arise when assembling a fuel cell and during use.
As a move in contrast to such studies about applications of a graphite-based starting material, attempts are being made to apply stainless steel to separators with the objective of reducing costs.
Patent Document 1 discloses a separator for fuel cells composed of a metal member, in which a surface making contact with an electrode of a unit cell is directly plated with gold. Examples of the metal member include stainless steel, aluminum, and Ni—Fe alloy, with SUS 304 being used as the stainless steel. According to this invention since the separator is plated with gold, it is considered that contact resistance between the separator and an electrode is reduced, which makes electric conduction from the separator to the electrode favorable, resulting in a high output power of a fuel cell.
Patent Document 2 discloses a polymer electrolyte fuel cell that includes separators made of a metal material in which a passivation film formed on the surface thereof is easily produced by air. Patent Document 2 shows a stainless steel and a titanium alloy as examples of the metal material. According to this invention, it is considered that the passivation film definitely exists on the surface of the metal material used for the separators so as to prevent chemical erosion of the surface, which reduces the degree of ionization of water generated in unit cells of the fuel cell, suppressing the reduction of the electrochemical reactivity in the unit cells. It is also considered that an electrical contact resistance value is lowered by removing a passivation film on a portion making contact with an electrode membrane or the like of a separator and forming a layer of a noble metal.
However, even when a metal material such as a stainless steel coated with a passivation film on the surface thereof as disclosed in Patent Documents 1 and 2 is used as it is for a separator, the metal material exhibit insufficient corrosion resistance and elution of metal occurs, and performance of the supported catalyst deteriorates due to eluted metal ions. Further, since the contact resistance of the separator increases due to corrosion products such Cr—OH or Fe—OH generated after elution, separators made of a metal material are actually plated with a noble metal such as gold, despite the cost thereof.
Under such circumstances, there is also proposed a stainless steel as a separator that is excellent in corrosion resistance and applicable as it is in primary surface without performing expensive surface treatment.
Patent Document 3 discloses a ferritic stainless steel for a polymer electrolyte fuel cell separator that does not contain B in the steel and does not precipitate any of M23C6, M4C, M2C, and MC carbide-based metal inclusions and M2B boride-based metal inclusions as conductive metallic precipitates in the steel, and has an amount of C in the steel of 0.012% or less (in the present specification, the symbol “%” in relation to chemical composition means “mass %” unless specifically stated otherwise). Furthermore, Patent Documents 4 and 5 disclose polymer electrolyte fuel cells to which a ferritic stainless steel including no conductive metallic precipitates precipitating is applied as a separator.
Patent Document 6 discloses a ferritic stainless steel for a separator of a polymer electrolyte fuel cell that does not contain B in the steel and contains 0.01 to 0.15% of C in the steel and precipitates only Cr-based carbides, and discloses a polymer electrolyte fuel cell to which the ferritic stainless steel is applied.
Patent Document 7 discloses an austenitic stainless steel for a separator of a polymer electrolyte fuel cell that does not contain B in the steel, contains 0.015 to 0.2% of C and 7 to 50% of Ni in the steel, and precipitates Cr-based carbides.
Patent Document 8 discloses a stainless steel for a separator of a polymer electrolyte fuel cell in which one or more kinds of M23C6, M4C, M2C, and MC carbide-based metal inclusions and M2B boride-based metal inclusions having electrical conductivity are dispersed and exposed on a surface of the stainless steel, and discloses a ferritic stainless steel that contains C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.04% or less, S: 0.01% or less, Cr: 15 to 36%, Al: 0.001 to 6%, and N: 0.035% or less, in which the contents of Cr, Mo, and B satisfy the expression 17%≦Cr+3×Mo−2.5×B, with the balance being Fe and inevitable impurities.
Patent Document 9 discloses a method for producing a stainless steel material for a separator of a polymer electrolyte fuel cell in which a surface of the stainless steel material is corroded by an acidic aqueous solution to expose, on the surface, one or more kinds of M23C6, M4C, M2C, and MC carbide-based metal inclusions and M2B boride-based metal inclusions having electrical conductivity, and discloses a ferritic stainless steel material that contains C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.04% or less, S: 0.01% or less, Cr: 15 to 36%, Al: 0.001 to 6%, B: 0 to 3.5%, N: 0.035% or less, Ni: 0 to 5%, Mo: 0 to 7%, Cu: 0 to 1%, Ti: 0 to 25×(C %+N %), and Nb: 0 to 25×(C %+N %), in which the contents of Cr, Mo, and B satisfy the expression 17%≦Cr+3×Mo−2.5×B, with the balance being Fe and impurities.
Patent Document 10 discloses a polymer electrolyte fuel cell in which an M2B boride-based metal compound is exposed on the surface, and assuming that an anode area and a cathode area are both one, the area of the anode making direct contact with a separator and the area of the cathode making direct contact with a separator each have a proportion within a range of 0.3 to 0.7, and discloses a stainless steel in which one or more kinds of M23C6, M4C, M2C, and MC carbide-based metal inclusions and M2B boride-based inclusions having electrical conductivity are exposed on a surface of the stainless steel. In addition, Patent Document 10 discloses a stainless steel constituting the separator being a ferritic stainless steel material that contains C: 0.15% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.04% or less, S: 0.01% or less, Cr: 15 to 36%, Al: 0.2% or less, B: 3.5% or less (however, excluding 0%), N: 0.035% or less, Ni: 5% or less, Mo: 7% or less, W: 4% or less, V: 0.2% or less, Cu: 1% or less, Ti: 25×(C %+N %) or less, and Nb: 25×(C %+N %) or less, in which the contents of Cr, Mo, and B satisfy the expression 17%≦Cr+3×Mo−2.5×B.
In addition, Patent Documents 11 to 15 disclose austenitic stainless clad steel materials in which M2B boride-based conductive metallic precipitates are exposed on the surface, as well as methods for producing the austenitic stainless clad steel materials.
Patent Document 16 discloses a ferritic stainless steel including B in the steel precipitated in the form of M2B boride, and a fuel cell including separators made of the ferritic stainless steel. The ferritic stainless steel is consisting of, by mass %, C: 0.08% or less, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, P: 0.035% or less, S: 0.01% or less, Cr: 17 to 36%, Al: 0.001 to 0.2%, B: 0.0005 to 3.5%, and N: 0.035% or less, with the inclusion of Ni, Mo, and Cu as needed, in which the Cr, Mo and B content satisfy the expression 17%≦Cr+3Mo−2.5B, with the balance being Fe and unavoidable impurities.
Patent Document 17 discloses a stainless steel material for a separator of a solid polymer fuel cell including a conductive substance made of M2B boride-based metal inclusions. For example, as austenitic stainless steel, Patent Document 17 shows stainless steel that consists of, by mass %, C: 0.2% or less, Si: 2% or less, Mn: 3% or less, Al: 0.001% or more and 6% or less, P: 0.06% or less, S: 0.03% or less, N: 0.4% or less, Cr: 15% or more and 30% or less, Ni: 6% or more and 50% or less, and B: 0.1% or more and 3.5% or less, with the balance being Fe and impurities.
Patent Document 18 discloses a ferritic stainless steel plate formed with an oxide film having good electrical conductivity at a high temperature. The ferritic stainless steel plate contains, by mass %, C: 0.02% or less, Si: 0.15% or less, Mn: 0.3 to 1%, P: 0.04% or less, S: 0.003% or less, Cr: 20 to 25%, Mo: 0.5 to 2%, Al: 0.1% or less, N: 0.02% or less, and Nb: 0.001 to 0.5%, with the balance being Fe and inevitable impurities, and satisfies the expression 2.5<Mn/(Si+Al)<8.0. The ferritic stainless steel plate further contains, by mass %, one, or two or more kinds of Ti: 0.5% or less, V: 0.5% or less, Ni: 2% or less, Cu: 1% or less, Sn: 1% or less, B: 0.005% or less, Mg: 0.005% or less, Ca: 0.005% or less, W: 1% or less, Co: 1% or less, and Sb: 0.5% or less.
Patent Document 19 discloses a ferritic stainless steel sheet in which a trace amount of Sn is added to improve oxidation resistance and high temperature strength. The ferritic stainless steel sheet consists of, by mass %, C: 0.001 to 0.03%, Si: 0.01 to 2%, Mn: 0.01 to 1.5%, P: 0.005 to 0.05%, S: 0.0001 to 0.01%, Cr: 16 to 30%, N: 0.001 to 0.03%, Al: more than 0.8% to 3%, and Sn: 0.01 to 1%, with the balance being Fe and unavoidable impurities.
Patent Document 20 discloses a ferritic stainless steel in which a passivation film is modified by addition of Sn to improve corrosion resistance. The ferritic stainless steel contains, by mass %, C: 0.01% or less, Si: 0.01 to 0.20%, Mn: 0.01 to 0.30%, P: 0.04% or less, S: 0.01% or less, Cr: 13 to 22%, N: 0.001 to 0.020%, Ti: 0.05 to 0.35%, Al: 0.005 to 0.050%, and Sn: 0.001 to 1%, with the balance being Fe and inevitable impurities.
Patent Document 1: JP10-228914A
Patent Document 2: JP8-180883A
Patent Document 3: JP2000-239806A
Patent Document 4: JP2000-294255A
Patent Document 5: JP2000-294256A
Patent Document 6: JP2000-303151A
Patent Document 7: JP2000-309854A
Patent Document 8: JP2003-193206A
Patent Document 9: JP2001-214286A
Patent Document 10: JP2002-151111A
Patent Document 11: JP2004-071319A
Patent Document 12: JP2004-156132A
Patent Document 13: JP2004-306128A
Patent Document 14: JP2007-118025A
Patent Document 15: JP2009-215655A
Patent Document 16: JP2000-328205A
Patent Document 17: JP2010-140886A
Patent Document 18: JP2014-031572A
Patent Document 19: JP2012-172160A
Patent Document 20: JP2009-174036A
An objective of present invention is to provide a ferritic stainless steel material that is remarkably excellent in corrosion resistance in an environment inside a polymer electrolyte fuel cell and has contact electrical resistance that is equal to that of a gold-plated material, a separator for polymer electrolyte fuel cells that is made of the stainless steel material, and a polymer electrolyte fuel cell to which the separator is applied.
The present inventors have concentrated for many years on the development of a stainless steel material that causes an extremely little metal elution from the surface of a metallic separator and causes almost no progression of metal ion contamination of an MEA (abbreviation of “membrane electrode assembly”) including a diffusion layer, a polymer membrane, and a catalyst layer, and that is hard to cause a reduction in catalyst performance or a reduction in polymer membrane performance, even when used for a long time period as a separator of a polymer electrolyte fuel cell.
Specifically, as a result of studying the application of fuel cells using the conventional SUS 304 and SUS 316L, gold-plated materials thereof, a stainless steel material with M2B and/or M23C6 metallic precipitates, a stainless steel material coated or painted with conductive particulate powder, a surface-modified stainless steel material, and the present invention is completed with the following findings (a) to (c) listed below obtained.
(a) M2B finely dispersed in steel and exposed on the surface of the steel noticeably improves the electrical conductivity (electrical contact resistance) of the surface by functioning as a “passage for electricity” on a stainless steel surface that is covered with a passivation film. However, although the electrical contact resistance performance is as low as that of a gold-plated starting material, there is room for further improvement in stability.
(b) By adding Sn, 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 but also on the surface of M2B with acid solution treatment performed prior to application and gradual melting of the parent phase during application to the fuel cell. This remarkably suppresses elution of metal ions from the parent phase and M2B, reduces the surface contact resistance of the parent phase, and moreover concentrates in the form of metallic tin or a tin oxide on the surface of M2B. This also has an effect that the electrical contact resistance performance of M2B is stable and improved to be as low as that of a gold-plated starting material.
(c) A favorable corrosion resistance is ensured by positively adding Mo. Mo has a relatively minor influence on the performance of a catalyst supported on anode and cathode portions if being elided. That is considered due to the eluted Mo existing in the form of molybdate ions, which are anions and have a small effect that inhibits the proton conductivity of a fluorinated ion exchange resin film having hydrogen ion (proton) exchange groups. Similar behavior can also be expected to V.
The present invention is as described below.
(1) A ferritic stainless steel material having a chemical composition consisting of, 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%,
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 20 to 45%,
the ferritic stainless steel material further having a parent phase comprising only a ferritic phase, wherein: M2B boride-based metallic precipitates are dispersed and exposed on a surface of the parent phase.
(2) The ferritic stainless steel material according to the above (1), wherein the chemical composition contains, by mass %,
rare earth metal: 0.005 to 0.1%.
(3) The ferritic stainless steel material according to the above (1) or (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.
(4) A separator for a polymer electrolyte fuel cell constituted by a ferritic stainless steel material for a polymer electrolyte fuel cell separator according to any one of the above (1) to (3).
(5) A polymer electrolyte fuel cell constituted by a ferritic stainless steel material for a polymer electrolyte fuel cell separator according to any one of the above (1) to (3).
In the present invention, the character “M” in M2B and M23C6 denotes a metallic element, but “M” does not denote a specific metallic element, but rather denotes a metallic element with strong chemical affinity for Cr or B. Generally, in relation with coexisting elements in steel, M is mainly composed of Cr and Fe, and often contains traces of Ni and Mo. Examples of M2B boride-based metallic precipitates include Cr2B, (Cr, Fe)2B, (Cr, Fe, Ni)2B, (Cr, Fe, Mo)2B, (Cr, Fe, Ni, Mo)2B, and Cr1.2Fe0.76Ni0.04B. In the case of carbide, B also has an action as “M”. Examples of M23C6 include Cr23C6, (Cr, Fe)23C6 and the like.
In both of the aforementioned M2B boride-based metallic precipitates and M23C6 carbide-based metallic precipitates, metallic precipitates having part of C replaced by B, such as M23(C, B)6 carbide-based metallic precipitates and M2(C, B) boride-based metallic precipitates, are also precipitated in some cases. The above expressions are assumed to include these metallic precipitates as well. Basically, metal-based dispersants with favorable electrical conductivity are expected to exhibit similar performance.
In the present invention, the subscript “2” in the term “M2B” means that “Between the amount of Cr, Fe, Mo, Ni, and X (where, X denotes a metallic element other than Cr, Fe, Mo, and Ni in steel) that are metallic elements in boride, and the B amount”, such a stoichiometric relation is established that (Cr mass %/Cr atomic weight+Fe mass %/Fe atomic weight+Mo mass %/Mo atomic weight+Ni mass %/Ni atomic weight+X mass %/X atomic weight)/(B mass %/B atomic weight) is approximately two. This style of expression is not specific, and is very general.
According to the present invention, a ferritic stainless steel material having an excellent metal ion elution resistance property is obtained without performing a high cost surface treatment such as expensive gold plating to reduce the contact resistance of the surface. That is, a ferritic stainless steel material is obtained which is remarkably excellent in corrosion resistance in an environment in a polymer electrolyte fuel cell and has contact electrical resistance that is equal to that of a gold-plated material. The stainless steel material is suitable for use as a separator in a polymer electrolyte fuel cell. For the fully-fledged dissemination of polymer electrolyte fuel cells, it is extremely important to reduce the cost of the fuel cell body, particularly the cost of the separator. It is anticipated that the fully-fledged dissemination of polymer electrolyte fuel cells with metallic separators applied thereto will be accelerated by the present invention.
Embodiments for carrying out the present invention will be described in detail. Hereinafter, the symbols “%” all refer to “mass %”.
M2B 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.
By finely dispersing and exposing M2B 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.
The term “exposure” here means that M2B 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 M2B boride-based metallic precipitates causes the M2B 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.
Although there is a concern that M2B boride-based metallic precipitates exposed on the surface will fall off, since the M2B boride-based metallic precipitates are metallic precipitates, the M2B boride-based metallic precipitates are metallurgically bonded to the parent phase and do not fall off the surface.
The M2B 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 M2B 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 M2B boride-based metallic precipitates are extremely hard precipitates. In the processes of hot forging, hot rolling and cold rolling, the M2B boride-based metallic precipitates are mechanically crushed and finely dispersed uniformly.
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 M2B 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 M2B 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 M2B, 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.
(3-1) C: 0.001 to Less than 0.020%
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%.
(3-2) Si: 0.01 to 1.5%
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.
(3-3) Mn: 0.01 to 1.5%
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.
(3-4) P: 0.035% or Less
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.
(3-5) S: 0.01% or Less
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.
(3-6) Cr: 22.5 to 35.0%
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 M2B 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 M23C6 carbide-based metallic precipitates. The M23C6 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 M2B 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.
(3-7) Mo: 0.01 to 6.0%
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.
(3-8) Ni: 0.01 to 6.0%
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.
(3-9) Cu: 0.01 to 1.0%
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.
(3-10) N: 0.035% or Less
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.
(3-11) V: 0.01 to 0.35%
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.
(3-12) B: 0.5 to 1.0%
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 M2B type boride-based metallic. B is an extremely stably metallic precipitate in terms of thermal properties. M2B 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.
(3-13) Al: 0.001 to 6.0%
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.
(3-14) Sn: 0.02 to 2.50%
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 M2B, thereby remarkably suppressing elution of metal ions from the parent phase as well as from M2B 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 M2B surface, so that the electrical contact resistance performance of M2B 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.
(3-15) Rare Earth Metal: 0 to 0.1%
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.
(3-16) Value Calculated as {Cr Content (Mass %)+3×Mo Content (Mass %)−2.5×B Content (Mass %)}
This value is an index that serves as a standard indicating the anticorrosion behavior of ferritic stainless steel in which M2B 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.
(3-17) Nb: 0 to 0.35%, Ti: 0 to 0.35%
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.
The balance other than the above elements is made up of Fe and impurities.
Next, advantageous effects of the present invention will be specifically described with reference to examples.
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 %
Balance: Fe and Impurities
Balance: Fe and Impurities
indicates data missing or illegible when filed
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.
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.
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.
Final annealing was performed in a bright annealing furnace in a 75 vol % H2-25 vol % N2 atmosphere in which the dew point was adjusted in the range of −50 to −53° C. The annealing temperature was 1060° C.
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.
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 M2B, and the M2B 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.
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.
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.
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 M23C6 could not be confirmed.
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.
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.
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− 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.
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/cm2. 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.
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.
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.
In the steel materials 1 to 11, the precipitation and dispersion of M2B 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 M2B 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 M2B 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 M2B being precipitated and dispersed of and Sn being contained was remarkable.
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.
Separators having the shape shown in the photograph in
A setting evaluation condition for fuel cell operation was a constant-current operation evaluation at a current density of 0.1 A/cm2, 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.
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.
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 M2B 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.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2014-203320 | Oct 2014 | JP | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/JP2015/077750 | 9/30/2015 | WO | 00 |
