JP4340448B2

JP4340448B2 - Ferritic stainless steel for fuel cell separator and method for producing the same

Info

Publication number: JP4340448B2
Application number: JP2003034583A
Authority: JP
Inventors: 和白山; 武志宇都宮; 定幸中村; 明弘中; 直人平松; 和加大原田; 敏郎名越
Original assignee: Nisshin Steel Co Ltd
Current assignee: Nippon Steel Nisshin Co Ltd
Priority date: 2002-03-28
Filing date: 2003-02-13
Publication date: 2009-10-07
Anticipated expiration: 2023-02-13
Also published as: JP2004232074A

Description

【０００１】
【産業上の利用分野】
本発明は、固体高分子型等の燃料電池に適し、表面接触抵抗が低く耐食性に優れた燃料電池セパレータ用フェライト系ステンレス鋼及びその製造方法に関する。
【０００２】
【従来の技術】
燃料電池には、リン酸型燃料電池，溶融炭酸塩型燃料電池，固体電解質型燃料電池，固体高分子型燃料電池等がある。なかでも、固体高分子型燃料電池は、１００℃以下の温度で動作可能であり、短時間で起動する長所を備えている。また、各部材が固体であるため、構造が簡単でメンテナンスが容易であり、振動や衝撃に曝される用途にも適用できる。更に、出力密度が高いため小型化に適し、燃料効率が高く、騒音が小さい等の長所を備えている。これらの長所から、車両搭載用動力源としての用途が検討されている。ガソリン自動車と同等の走行距離を出せる燃料電池を自動車に搭載できると、ＮＯ_x，ＳＯ_xの発生がほとんどなく、ＣＯ₂の発生が半減する等、環境に対して非常にクリーンな動力源になる。
【０００３】
固体高分子型燃料電池は、分子中にプロトン交換基をもつ固体高分子樹脂膜がプロトン導電性電解質として機能することを利用しており、他の形式の燃料電池と同様に固体高分子膜の一側に水素等の燃料ガスを流し、他側に空気等の酸化性ガスを流す構造になっている。
具体的には、固体高分子膜１は、両側に酸化極２及び燃料極３が接合され、それぞれガスケット４を介してセパレータ５を対向させている（図１ａ）。酸化極２側のセパレータ５には空気供給口６，空気排出口７が形成され、燃料極３側のセパレータ５には水素供給口８，水素排出口９が形成されている。
【０００４】
水素ｇ及び酸素又は空気ｏの導通，均一分配のため、水素ｇ及び酸素又は空気ｏの流動方向に延びる複数の溝１０がセパレータ５に形成されている。また、発電時に発熱があるため、給水口１１から送り込んだ冷却水ｗをセパレータ５の内部に循環させた後、排水口１２から排出させる水冷機構をセパレータ５に内蔵させている。水素供給口８から燃料極３とセパレータ５との間隙に送り込まれた水素ｇは、電子を放出したプロトンとなって固体高分子膜１を透過し、酸化極２側で電子を受け、酸化極２とセパレータ５との間隙を通過する酸素又は空気ｏによって燃焼する。そこで、酸化極２と燃料極３との間に負荷をかけるとき、電力を取り出すことができる。
【０００５】
燃料電池は、１セル当りの発電量が極く僅かである。そこで、セパレータ５，５で挟まれた固体高分子膜を１単位とし、複数のセルをスタックすることによって取出し可能な電力量を大きくしている（図１ｂ）。多数のセルをスタックした構造では、セパレータ５の抵抗が発電効率に大きな影響を及ぼす。発電効率を向上させるためには、導電性が良好で接触抵抗の低いセパレータが要求され、リン酸塩型燃料電池と同様に黒鉛質のセパレータが使用されている。
黒鉛質のセパレータは、黒鉛ブロックを所定形状に切り出し、切削加工によって各種の孔や溝を形成している。そのため、材料費や加工費が高く、全体として燃料電池の価格を高騰させると共に、生産性を低下させる原因になっている。しかも、材質的に脆い黒鉛でできたセパレータでは、振動や衝撃が加えられると破損する虞が大きい。そこで、プレス加工，パンチング加工等で作製した金属板製セパレータが期待されている（特開2000−239806号公報，特開2000−265248号公報等参照）。
【０００６】
【発明が解決しようとする課題】
酸素又は空気ｏが通過する酸化極２側は、酸性度がｐＨ２〜３の酸性雰囲気にある。このような強酸性雰囲気に耐え、しかもセパレータに要求される特性を満足する金属材料は、これまでのところ実用化されていない。たとえば、強酸に耐える金属材料としてステンレス鋼等の耐酸性材料が考えられる。ステンレス鋼は表面の不動態皮膜によって優れた耐食性を呈するが、不動態皮膜はＣｒの酸化物を主体としＦｅ，Ｓｉ，Ｍｎの酸化物を含む比電気抵抗の高い複合皮膜であるため、電気伝導性に劣っている。比電気抵抗、換言すれば接触抵抗が高くなると、接触部分で多量のジュール熱が発生して大きな熱損失となり、燃料電池の発電効率を低下させる。他の金属板でも、接触抵抗を高くする酸化膜が表面に存在するものがほとんどである。
【０００７】
表面に酸化皮膜や不動態皮膜を形成しない金属材料としてはＡｕが知られている。Ａｕは酸性雰囲気にも耐えうるが、非常に高価な材料であるため燃料電池のセパレータ材としては実用的でない。Ｐｔは酸化皮膜や不動態皮膜が形成されにくい金属材料であり、酸性雰囲気にも耐えうるが、Ａｕと同様に非常に高価な材料であるため燃料電池のセパレータ材としては実用的でない。汎用のステンレス鋼に導電性塗料を塗布したものも用いられているが、コスト高になること，塗料の劣化による接触抵抗の増加等が懸念される。
以上のことから、燃料電池のセパレータに対してはステンレス鋼板を無垢で適用することが好ましい。
【０００８】
【課題を解決するための手段】
本発明は、このような問題を解消すべく案出されたものであり、Ｃｕ含有フェライト系ステンレス鋼板を基材に用い、基材表面の不動態皮膜又は表層を改質することにより、低い表面接触抵抗及び優れた耐食性を長期にわたって維持でき、固体高分子型等の燃料電池に好適なステンレス鋼製燃料電池セパレータを提供することを目的とする。
【０００９】
本発明は、Ｃｒ：１６．０〜４０．０質量％，Ｃｕ：０．０１〜５質量％、Ｃ：０．０８質量％以下，Ｓｉ：１．００質量％以下、Ｍｎ：２．００質量％以下、Ｎｉ：０．０８〜０．１６質量％を含むフェライト系ステンレス鋼板を燃料電池セパレータの基材に使用している。基材・ステンレス鋼板は、Ｍｏ：０．２〜６．０質量％，Ｎｂ：０．０５〜１．０質量％，Ｔｉ：０．０５〜１．０質量％，Ａｌ：０．０１〜１．０質量％，Ｖ：０．０１〜１．０質量％，Ｚｒ：０．０１〜１．０質量％，１種又は２種以上を含み、残部がＦｅおよび不可避的不純物からなる組成をもつ。
【００１０】
基材・ステンレス鋼板の電気伝導性はＣｕリッチ相の分散析出や表層へのＣｕ濃化によって改善され、接触抵抗が低減する。燃料電池用セパレータとして有効な低接触抵抗は、マトリックスにＣｕリッチ相が０.００５体積％以上（好ましくは、０.２体積％以上）の割合で分散析出した金属組織や原子比Ｃｕ／(Ｓｉ＋Ｍｎ)≧０.５にＣｕ濃化した表層で達成される。Ｃｕリッチ相の分散析出及び表層へのＣｕ濃化を併用すると、接触抵抗が一層低下する。
【００１１】
Ｃｕリッチ相は、最終焼鈍までの鋼板製造過程でたとえば８００℃前後の温度で１〜２４時間加熱する時効処理によってマトリックスに分散析出する。時効処理条件に応じてＣｕリッチ相の析出量を０.２体積％以上に調整できる。表層にＣｕを濃化させる手段には、最終焼鈍として露点−３０℃以下の雰囲気で光輝焼鈍する方法，大気焼鈍後にフッ酸−硝酸又は硫酸−硝酸の混酸を用いて酸洗仕上げする方法等が採用される。
【００１２】
【作用及び実施の形態】
ステンレス鋼板表面の不動態皮膜は、優れた耐食性の発現に有効であるものの、比電気抵抗が高い酸化物、水酸化物等からなるため表面接触抵抗を増大させる原因である。そこで、本発明者等は、電気伝導性に優れた物質を不動態皮膜に含ませ、或いはステンレス鋼表層に濃化させることによって不動態皮膜を改質し、表面接触抵抗を下げる方法を調査検討した。
調査検討の過程で、Ｃｕ含有ステンレス鋼が低い表面接触抵抗を示すことを見出した。なかでも、Ｃｕ含有量が０.０１質量％以上で、０.００５体積％以上の割合でＣｕリッチ相が分散析出しているステンレス鋼板では表面接触抵抗が大幅に低下した。Ｃｕリッチ相が分散析出しているステンレス鋼板でも他のステンレス鋼板と同様に不動態皮膜が鋼板表面に形成されるが、Ｃｕリッチ相が表面に露出した部分では、その上下にＣｒ，Ｓｉ，Ｍｎ等を含む不動態皮膜が形成されずＣｕリッチ相が導通路になるため、表面接触抵抗が著しく低下したものと推定される。
【００１３】
鋼板表面にＣｕリッチ相が露出していない場合でも、最表層又は不動態皮膜中のＣｕ濃度がＳｉ，Ｍｎ濃度に対して高いと、同様に表面接触抵抗が低くなる。比電気抵抗が低いＣｕの酸化物を多く含む不動態皮膜は、比電気抵抗の高いＭｎやＳｉの酸化物を多く含む不動態皮膜に比較して低い表面接触抵抗を示し、後述の実施例からも明らかなように原子比Ｃｕ／（Ｓｉ＋Ｍｎ）≧０．５で燃料電池セパレータに要求される低表面接触抵抗を満足する。
表面接触抵抗の低下に有効なＣｕリッチ相の分散析出又はＣｕ濃化した表層を得る上で、基材・ステンレス鋼板のＣｕ含有量０．０１質量％以上が必要であるが、安定して表面接触抵抗を低下させるには１．０質量％以上のＣｕ含有量が好ましい。Ｃｕ含有量が多くなるほど、Ｃｕリッチ相の分散析出量が増加し、不動態皮膜又は表層のＣｕ濃化が進行する。しかし、過剰量のＣｕ添加は熱間加工性，製造性に悪影響を及ぼすので、Ｃｕ含有量の上限を５質量％に規制する。
【００１４】
最終焼鈍で表層にＣｕを濃化させる場合、露点−３０℃以下の焼鈍雰囲気における光輝焼鈍が有効である。比電気抵抗の高いＭｎ等の酸化物は、焼鈍雰囲気の露点が高くなると増量し、露点が−３０℃を超える焼鈍雰囲気ではＳｉ，Ｍｎ等の酸化進行に応じて母材内部から表層へのＳｉ，Ｍｎ等の拡散が促進され、Ｃｕ濃度が低い最表層又は不動態皮膜が形成される。これに対し、露点−３０℃以下の焼鈍雰囲気では、Ｍｎ等の金属酸化物の増量を抑えられ、結果として金属ＣｕやＣｕの酸化物が最表層又は不動態皮膜に濃化される。
【００１５】
光輝焼鈍に替え大気焼鈍後の酸洗仕上げによってもＣｕリッチ相の分散析出又は表層へのＣｕ濃化が図られる。ステンレス鋼板を大気焼鈍するとＣｒ，Ｆｅ，Ｍｎ，Ｓｉ，Ｃｕ等の酸化物を含むスケールが鋼板表面に形成されるが、スケールは酸洗によって除去され、その後に不動態皮膜が生成する。このとき、焼鈍後のステンレス鋼板を電解酸洗すると、スケール剥離後の鋼板表面に存在するＣｕ又はＣｕリッチ相が電解反応で母材よりも優先的に溶出する。そのため、電解酸洗後の鋼板表面には、Ｃｕ濃度の低い不動態皮膜が形成される。これに対し、フッ酸−硝酸，硫酸−硝酸等の混酸を用いた酸洗では、Ｃｕ又はＣｕリッチ相の優先的な溶出がなく、酸洗後に生成する不動態皮膜にＣｕ濃度の低下がない。
【００１６】
Ｃｕリッチ相の分散析出，表層へのＣｕ濃化により基材・ステンレス鋼板の表面接触抵抗を低下させる条件下で、燃料電池セパレータの腐食環境に耐え得る耐食性を有するステンレス鋼の成分を検討した。燃料電池用セパレータは、ｐＨ２〜３の酸性雰囲気に曝され、起動時に約７０℃に昇温することもある。しかも、セパレータ間に電位差がかかるので、酸性液に対する自然浸漬時の耐食性に加えて電位が貴化した場合の酸性高電位環境における耐食性も要求される。
【００１７】
自然電位環境及び酸性高電位環境における耐食性は、１６.０〜４０.０質量％の範囲にＣｒ含有量を調整することにより向上する。更に、Ｍｏ：０.２〜６.０質量％，Ｎｂ：０.０５〜１.０質量％，Ｔｉ：０.０５〜１.０質量％，Ａｌ：０.０１〜１.０質量％，Ｖ：０.０１〜１.０質量％，Ｚｒ：０.０１〜１.０質量％，Ｂ：０.０００１〜１.０質量の１種又は２種以上を添加すると、自然電位及び酸性高電位何れの環境下における耐食性も向上する。
【００１８】
以上の合金成分の他にＣ，Ｓｉ，Ｍｎ等も含まれるが、燃料電池用セパレータの用途では、Ｃ，Ｓｉ，Ｍｎ等の含有量を低減し、加工性，製造性にとって不利な硬質化を抑制することが好ましい。表面接触抵抗を低減する上でも、Ｃ，Ｓｉ，Ｍｎ等の含有量を低く抑えることが有効である。
たとえば、鋼材を硬質化させるＣを過剰に含ませるとプレス加工が困難になるので、Ｃ含有量を０．０８質量％以下に規制する。Ｓｉは、鋼材の硬質化及び表面接触抵抗の増加に作用するので、１．００質量％以下の含有量に規制する。Ｍｎは、耐食性の低下，接触抵抗の増加に作用するので、２．００質量％以下の含有量に規制する。
【００１９】
Ｃｕリッチ相：０.００５体積％以上
Ｃｕリッチ相を均一微細に析出させ、表面に露出させた部分では、その上下にＣｒ，Ｓｉ，Ｍｎ等を含む不動態皮膜が形成されず、表面接触抵抗が低下する。Ｃｕリッチ相の分散析出と表面接触抵抗の低下との関係を詳細に調査・検討した結果、Ｃｕリッチ相：０.００５体積％以上で燃料電池用セパレータに要求される低表面接触抵抗を示す。表面接触抵抗の低減は、０.５体積％以上のＣｕリッチ相で顕著になる。
Ｃｕリッチ相を分散析出させる熱処理は、ステンレス鋼のＣｕ含有量によっても異なるが、一般的には８００℃前後で１〜２４時間の時効処理を施す条件が採用される。時効処理により微細なＣｕリッチ相が分散析出し、Ｃｕリッチ相の析出量は時効処理条件によって調節される。
【００２０】
表層へのＣｕ濃化：Ｃｕ／(Ｓｉ＋Ｍｎ)≧０．５（原子比）
不動態皮膜又は最表層のＳｉ，Ｍｎに対するＣｕの濃度比を高くすると、表面接触電気抵抗が低下する。表層へのＣｕ濃化は、不動態皮膜又は最表層における原子比Ｃｕ／(Ｓｉ＋Ｍｎ)で定量化でき、原子比Ｃｕ／(Ｓｉ＋Ｍｎ)が０.５以上になると、燃料電池用セパレータとして満足できる程度に表面接触抵抗が低下する。Ｃｕ／(Ｓｉ＋Ｍｎ)≧０.５は、乾燥雰囲気下での光輝焼鈍や大気焼鈍後の混酸酸洗で達成される。
【００２１】
光輝焼鈍：露点−３０℃以下の雰囲気で加熱
焼鈍雰囲気の露点が高くなると、比抵抗の高いＳｉ，Ｍｎの酸化物が多くなり不動態皮膜の表面接触抵抗が増加する。Ｃｕ／(Ｓｉ＋Ｍｎ)≧０.５で表層にＣｕを濃化させる上では、後述の実施例からも明らかなように露点−３０℃以下の焼鈍雰囲気で、好ましくは８５０〜１１００℃にステンレス鋼板を焼鈍する。光輝焼鈍により、マトリックスにＣｕリッチ相が均一に分散析出し、或いは表層にＣｕが濃化して表面接触抵抗の低い表層に改質される。
【００２２】
大気焼鈍後の混酸酸洗：
光輝焼鈍に代えて大気焼鈍後の混酸酸洗によっても、不動態皮膜又は最表層にＣｕを濃化できる。大気焼鈍では、Ｃｕリッチ相の分散析出又は表層へのＣｕ濃化のため、８５０〜１１００℃の温度域にステンレス鋼板を加熱する。大気焼鈍されたステンレス鋼板の酸洗には、酸の種類や濃度に特段の制約が加わるものではないが、一般的に濃度１０体積％程度のフッ酸−硝酸，硫酸−硝酸等が使用される。混酸酸洗では、電解酸洗で生じるＣｕ又はＣｕリッチ相の優先溶解がなく、Ｃｕ濃化の低下がない不動態皮膜が酸洗後に生成する。
【００２３】
【実施例】
表１に示した組成をもつ各種フェライト系ステンレス鋼を真空溶解炉で溶製し、鋳造，熱間鍛造，熱間圧延，焼鈍・酸洗，冷間圧延を経て板厚１.０ｍｍの冷延鋼帯を製造した。一部の冷延鋼帯については、最終焼鈍前までの工程で８００℃×２４時間のＣｕリッチ相析出処理を施した。
【００２４】

【００２５】
製造された冷延鋼帯を光輝焼鈍仕上げし、或いは大気焼鈍後に酸洗仕上げした。光輝焼鈍では、露点が異なる焼鈍雰囲気で１０００℃に１０秒加熱し、雰囲気の露点が焼鈍材の特性に及ぼす影響を調査した。酸洗仕上げでは、５％硝酸を用いた電解酸洗，６％硝酸＋２％フッ酸の混酸酸洗を採用し、酸洗形態の相違が焼鈍材の特性に及ぼす影響を調査した。
光輝焼鈍又は酸洗仕上げされた冷延鋼帯から切り出された試験片を透過型電子顕微鏡で観察し、マトリックスに分散析出しているＣｕリッチ相の析出量（体積％）を算出した。また、グロー発光分析で分析全元素量に対するＣｕ，Ｓｉ，Ｍｎの濃度（原子％）を求め、原子比Ｃｕ／(Ｓｉ＋Ｍｎ)に従って表層のＣｕ濃度を算出した。更に、純金製の対極及び測定端子を試験片の表面に接触させ、測定端子に１００ｇの荷重を付加した条件下で表面接触抵抗を測定した。
【００２６】
表２の調査結果にみられるように、Ｃｕ含有量が０.０１質量％未満のＦ1や、Ｃｕを０.０１質量％以上含んでいてもＣｕ／(Ｓｉ＋Ｍｎ)＜０.５の表層又はＣｕリッチ相の析出量が０.００５体積％未満の試験番号５〜７は、高い表面接触抵抗を示した。他方、０.０１質量％以上のＣｕを含み、Ｃｕリッチ相の析出量が０.２体積％以上又はＣｕ／(Ｓｉ＋Ｍｎ)≧０.５で表層がＣｕ濃化した試験番号４，８〜19は、低い表面接触抵抗を示した。なかでも、Ｃｕリッチ相の析出量０.００５体積％以上，Ｃｕ／(Ｓｉ＋Ｍｎ)≧０.５にＣｕ濃化した表層の双方を満足する試験番号８，９，11，14は、表面接触抵抗が一層低下していた。
【００２７】
【表２】

【００２８】
（実施例２）
表１の鋼種Ｆ３，Ｆ６，Ｆ７，Ｆ１２に実施例１と同じＣｕリッチ相析出処理を施したステンレス鋼板を供試材とし、電気化学試験で燃料電池セパレータ環境における耐食性を調査した。
電気化学試験では、供試材を樹脂に埋め込んで被測定面１０ｍｍ×１０ｍｍの試験片を用意した。ｐＨ２に調整した硫酸水溶液（試験液）を７０℃に保持し、走査速度２０ｍＶ／分で自然電位から１３００ｍＶ，ＳＣＥまでのアノード分極を測定した。燃料電池セパレータが曝される雰囲気には、電位が付加されない自然電位環境，電位が付加される高電位環境がある。自然電位環境における耐食性はアノード分極の不動態化電流密度の低さ、高電位環境における耐食性は１０００ｍＶ，ＳＣＥ時の腐食電流の低さから評価できる。
【００２９】
図２に示したアノード分極曲線の測定結果から、Ｃｒ：１３質量％のＦ３は自然電位側の不導態化限界電流密度が他の供試材の値より高く、耐酸性に不足していることが判る。
Ｃｒ：４１．２質量％のＦ１２は、不動態化限界電流密度が低いものの、高電位側での電流値が他の供試材と比較して極めて高く、セパレータに電位がかかった状態では耐食性に劣っていた。Ｃｒ：１６．６質量％のＦ６，３０．５質量％のＦ７は、不導態化限界電流密度，高電位環境下の電流値共に他の供試材より低く、セパレータ環境で優れた耐食性を示すことが期待できる。
【００３０】
【発明の効果】
以上に説明したように、本発明の燃料電池用ステンレス鋼製セパレータは、Ｃｒ，Ｃｕの添加量を適正管理し、析出量０.００５体積％以上のＣｕリッチ相又はＣｕ／(Ｓｉ＋Ｍｎ)≧０.５でＣｕ濃化した表層により、ステンレス鋼本来の耐食性を確保しながら表面接触抵抗の低い表面状態に改質している。そのため、黒鉛質セパレータに比較して加工性，生産性が格段に優れた燃料電池用セパレータとなり、複数の燃料電池セルをスタックした状態にあっても、表面接触抵抗に起因する内部損失が少なく、発電効率の高い燃料電池が得られる。
【図面の簡単な説明】
【図１】従来の固体高分子膜を電解質として使用した燃料電池の内部構造を説明する断面図（ａ），分解斜視図（ｂ）
【図２】アノード分極曲線の測定結果[0001]
[Industrial application fields]
The present invention relates to a ferritic stainless steel for a fuel cell separator that is suitable for a solid polymer type fuel cell and has low surface contact resistance and excellent corrosion resistance, and a method for producing the same.
[0002]
[Prior art]
Examples of the fuel cell include a phosphoric acid fuel cell, a molten carbonate fuel cell, a solid electrolyte fuel cell, and a solid polymer fuel cell. In particular, the polymer electrolyte fuel cell can operate at a temperature of 100 ° C. or less and has an advantage of starting in a short time. In addition, since each member is solid, the structure is simple and maintenance is easy, and it can be applied to applications that are exposed to vibration and impact. Furthermore, it has advantages such as high power density, suitable for downsizing, high fuel efficiency, and low noise. From these advantages, the use as a power source for mounting on a vehicle is being studied. If a fuel cell that can run the same distance as a gasoline vehicle can be installed in the vehicle, there will be almost no generation of NO _x and SO _{x and} the generation of CO ₂ will be halved. .
[0003]
Solid polymer fuel cells utilize the fact that a solid polymer resin membrane having a proton exchange group in the molecule functions as a proton conductive electrolyte, and like other types of fuel cells, The fuel gas such as hydrogen is allowed to flow on one side and the oxidizing gas such as air is allowed to flow on the other side.
Specifically, the solid polymer membrane 1 has an oxidation electrode 2 and a fuel electrode 3 joined on both sides, and a separator 5 is opposed to each other via a gasket 4 (FIG. 1a). An air supply port 6 and an air discharge port 7 are formed in the separator 5 on the oxidation electrode 2 side, and a hydrogen supply port 8 and a hydrogen discharge port 9 are formed on the separator 5 on the fuel electrode 3 side.
[0004]
For the conduction and uniform distribution of hydrogen g and oxygen or air o, a plurality of grooves 10 extending in the flow direction of hydrogen g and oxygen or air o are formed in the separator 5. In addition, since heat is generated during power generation, the cooling water w fed from the water supply port 11 is circulated in the separator 5 and then a water cooling mechanism for discharging the water from the drain port 12 is built in the separator 5. The hydrogen g fed into the gap between the fuel electrode 3 and the separator 5 from the hydrogen supply port 8 becomes protons that have released electrons, passes through the solid polymer film 1, receives electrons on the oxidation electrode 2 side, and receives the oxidation electrode. It burns with oxygen or air o passing through the gap between the separator 2 and the separator 5. Therefore, when a load is applied between the oxidation electrode 2 and the fuel electrode 3, electric power can be taken out.
[0005]
The fuel cell has very little power generation per cell. Therefore, the solid polymer film sandwiched between the

separators

5 and 5 is set as one unit, and the amount of electric power that can be taken out is increased by stacking a plurality of cells (FIG. 1b). In a structure in which a large number of cells are stacked, the resistance of the separator 5 greatly affects the power generation efficiency. In order to improve the power generation efficiency, a separator having good conductivity and low contact resistance is required, and a graphite separator is used as in the phosphate fuel cell.
The graphite separator cuts out a graphite block into a predetermined shape and forms various holes and grooves by cutting. For this reason, material costs and processing costs are high, which increases the price of fuel cells as a whole and causes productivity to decrease. Moreover, a separator made of graphite that is brittle in material has a high risk of breakage when subjected to vibration or impact. Therefore, a metal plate separator produced by pressing, punching, or the like is expected (see JP 2000-239806, JP 2000-265248, etc.).
[0006]
[Problems to be solved by the invention]
The oxidation electrode 2 side through which oxygen or air o passes is in an acidic atmosphere with an acidity of pH 2-3. A metal material that can withstand such a strong acidic atmosphere and satisfies the characteristics required for the separator has not been put to practical use so far. For example, an acid resistant material such as stainless steel is conceivable as a metal material resistant to strong acid. Stainless steel exhibits excellent corrosion resistance due to the passive film on the surface, but the passive film is a composite film with a high specific electrical resistance mainly composed of oxides of Cr, including oxides of Fe, Si and Mn. Inferior. When the specific electrical resistance, in other words, the contact resistance is increased, a large amount of Joule heat is generated at the contact portion, resulting in a large heat loss, which reduces the power generation efficiency of the fuel cell. Most of other metal plates have an oxide film on the surface for increasing the contact resistance.
[0007]
Au is known as a metal material that does not form an oxide film or a passive film on the surface. Although Au can withstand an acidic atmosphere, it is a very expensive material, so it is not practical as a separator for fuel cells. Pt is a metal material that is difficult to form an oxide film or a passive film, and can withstand an acidic atmosphere. However, it is an extremely expensive material like Au, so it is not practical as a separator for a fuel cell. A general-purpose stainless steel coated with a conductive paint is also used, but there are concerns about increased costs and increased contact resistance due to paint deterioration.
From the above, it is preferable to apply a stainless steel plate to the separator of the fuel cell.
[0008]
[Means for Solving the Problems]
The present invention has been devised to solve such problems, and by using a Cu-containing ferritic stainless steel plate as a base material, and by modifying the passive film or surface layer of the base material surface, a low surface is obtained. An object of the present invention is to provide a stainless steel fuel cell separator that can maintain contact resistance and excellent corrosion resistance over a long period of time and is suitable for a solid polymer type fuel cell.
[0009]
In the present invention, Cr: 16.0 to 40.0 mass%, Cu: 0.01 to 5 mass%, C: 0.08 mass% or less, Si: 1.00 mass% or less, Mn: 2.00 mass% % Or less, and a ferritic stainless steel plate containing Ni: 0.08 to 0.16% by mass is used as the base material of the fuel cell separator. Substrate stainless steel sheet, M o: 0.2 to 6.0 wt%, Nb: 0.05 to 1.0 mass%, Ti: 0.05 to 1.0 mass%, Al: 0.01 to 1.0% by mass, V: 0.01 to 1.0% by mass, Zr: 0.01 to 1.0% by mass , including one or more kinds , the balance being Fe and inevitable impurities Have.
[0010]
The electrical conductivity of the base material / stainless steel sheet is improved by the dispersion precipitation of the Cu-rich phase and the concentration of Cu on the surface layer, thereby reducing the contact resistance. The low contact resistance effective as a separator for fuel cells is a metal structure in which a Cu-rich phase is dispersed and precipitated in a matrix at a ratio of 0.005% by volume or more (preferably 0.2% by volume or more) or an atomic ratio Cu / (Si + Mn )> 0.5 achieved with Cu-enriched surface layer. When the dispersion precipitation of the Cu rich phase and the Cu concentration on the surface layer are used in combination, the contact resistance is further reduced.
[0011]
The Cu-rich phase is dispersed and precipitated in the matrix by an aging treatment in which, for example, heating is performed at a temperature of about 800 ° C. for 1 to 24 hours in the course of manufacturing the steel sheet until final annealing. The precipitation amount of the Cu rich phase can be adjusted to 0.2% by volume or more according to the aging treatment conditions. As a means for concentrating Cu on the surface layer, there are a method of bright annealing in an atmosphere having a dew point of −30 ° C. or lower as the final annealing, a method of pickling using a mixed acid of hydrofluoric acid-nitric acid or sulfuric acid-nitric acid after annealing, etc. Adopted.
[0012]
[Operation and embodiment]
Although the passive film on the surface of the stainless steel plate is effective in developing excellent corrosion resistance, it is a cause of increasing the surface contact resistance because it is made of an oxide, hydroxide or the like having a high specific electric resistance. Therefore, the present inventors investigated and examined a method for reducing the surface contact resistance by modifying the passive film by including a substance having excellent electrical conductivity in the passive film or concentrating it on the surface of the stainless steel. did.
In the course of the investigation, it was found that Cu-containing stainless steel exhibits low surface contact resistance. In particular, the surface contact resistance of the stainless steel plate in which the Cu content was 0.01% by mass or more and the Cu-rich phase was dispersed and precipitated at a rate of 0.005% by volume or more significantly decreased. Even in a stainless steel plate in which a Cu rich phase is dispersed and precipitated, a passive film is formed on the surface of the steel plate in the same manner as other stainless steel plates. However, in the portion where the Cu rich phase is exposed on the surface, Cr, Si, Mn It is presumed that the surface contact resistance is remarkably lowered because the Cu-rich phase becomes a conduction path without the formation of a passive film including the like.
[0013]
Even when the Cu-rich phase is not exposed on the surface of the steel sheet, the surface contact resistance is similarly lowered if the Cu concentration in the outermost layer or the passive film is higher than the Si and Mn concentrations. The passive film containing a large amount of Cu oxide with a low specific electric resistance shows a lower surface contact resistance than the passive film containing a large amount of Mn or Si oxide with a high specific electric resistance. As is clear, the low surface contact resistance required for the fuel cell separator is satisfied when the atomic ratio is Cu / (Si + Mn) ≧ 0.5.
The Cu content of the base material / stainless steel sheet needs to be 0.01% by mass or more in order to obtain a Cu-rich phase dispersion precipitate or Cu-enriched surface layer effective in reducing the surface contact resistance. To reduce the contact resistance, a Cu content of 1.0% by mass or more is preferable. As the Cu content increases, the amount of dispersed precipitation of the Cu-rich phase increases, and Cu concentration in the passive film or surface layer proceeds. However, excess Cu added hot workability, since an adverse effect on manufacturability, you regulate the upper limit of the Cu content to 5 mass%.
[0014]
When Cu is concentrated on the surface layer by final annealing, bright annealing in an annealing atmosphere having a dew point of −30 ° C. or lower is effective. An oxide such as Mn having a high specific resistance increases when the dew point of the annealing atmosphere increases, and in an annealing atmosphere where the dew point exceeds −30 ° C., Si from the inside of the base material to the surface layer according to the progress of oxidation of Si, Mn, etc. , Mn and the like are diffused, and the outermost surface layer or passive film having a low Cu concentration is formed. On the other hand, in an annealing atmosphere having a dew point of −30 ° C. or less, an increase in the amount of metal oxide such as Mn can be suppressed, and as a result, metal Cu or Cu oxide is concentrated on the outermost layer or the passive film.
[0015]
In place of bright annealing, Cu-rich phase is dispersed and precipitated or Cu is concentrated on the surface layer by pickling after atmospheric annealing. When a stainless steel plate is annealed to the atmosphere, a scale containing oxides such as Cr, Fe, Mn, Si, and Cu is formed on the surface of the steel plate, but the scale is removed by pickling, and a passive film is formed thereafter. At this time, if the annealed stainless steel plate is subjected to electrolytic pickling, Cu or Cu-rich phase present on the surface of the steel plate after scale peeling elutes preferentially over the base material due to the electrolytic reaction. Therefore, a passive film having a low Cu concentration is formed on the surface of the steel sheet after electrolytic pickling. On the other hand, in pickling using a mixed acid such as hydrofluoric acid-nitric acid or sulfuric acid-nitric acid, there is no preferential elution of Cu or Cu-rich phase, and there is no decrease in Cu concentration in the passive film formed after pickling. .
[0016]
The components of stainless steel having corrosion resistance that can withstand the corrosive environment of the fuel cell separator were examined under the condition that the surface contact resistance of the base material / stainless steel sheet was reduced by dispersion precipitation of Cu rich phase and concentration of Cu on the surface layer. The fuel cell separator is exposed to an acidic atmosphere having a pH of 2 to 3, and may be heated to about 70 ° C. during startup. Moreover, since a potential difference is applied between the separators, corrosion resistance in an acidic high potential environment when the potential becomes noble is required in addition to corrosion resistance during natural immersion in an acidic liquid.
[0017]
Corrosion resistance in a natural potential environment and an acidic high potential environment is improved by adjusting the Cr content in the range of 16.0 to 40.0 mass%. Furthermore, Mo: 0.2-6.0 mass%, Nb: 0.05-1.0 mass%, Ti: 0.05-1.0 mass%, Al: 0.01-1.0 mass%, When one or more of V: 0.01 to 1.0% by mass, Zr: 0.01 to 1.0% by mass, B: 0.0001 to 1.0% by mass are added, the natural potential and high acidity are increased. Corrosion resistance under any potential environment is also improved.
[0018]
In addition to the above alloy components, C, Si, Mn, and the like are also included, but in the application of fuel cell separators, the content of C, Si, Mn, etc. is reduced, which is disadvantageous for workability and manufacturability. It is preferable to suppress. In order to reduce the surface contact resistance, it is effective to keep the content of C, Si, Mn, etc. low.
For example, the inclusion in the excess C to harden the steel material since the press working is difficult, the C content 0. The amount is restricted to 08 mass% or less. Since Si acts on hardening of steel and increase in surface contact resistance , 1 . The content is regulated to 00% by mass or less. Since Mn acts on a decrease in corrosion resistance and an increase in contact resistance , 2 . The content is regulated to 00% by mass or less.
[0019]
Cu rich phase: 0.005% by volume or more Cu-rich phase is uniformly and finely deposited and exposed on the surface, a passive film containing Cr, Si, Mn and the like is not formed on the upper and lower sides, and surface contact resistance Decreases. As a result of detailed investigation and examination of the relationship between the dispersion precipitation of the Cu-rich phase and the decrease in the surface contact resistance, it shows a low surface contact resistance required for the fuel cell separator when the Cu-rich phase is 0.005% by volume or more. The reduction of the surface contact resistance becomes remarkable in a Cu-rich phase of 0.5% by volume or more.
The heat treatment for dispersing and precipitating the Cu-rich phase varies depending on the Cu content of the stainless steel, but generally, conditions for performing an aging treatment at around 800 ° C. for 1 to 24 hours are employed. The fine Cu-rich phase is dispersed and precipitated by the aging treatment, and the amount of precipitation of the Cu-rich phase is adjusted by the aging treatment conditions.
[0020]
Concentration of Cu on the surface layer: Cu / (Si + Mn) ≧ 0.5 (atomic ratio)
When the concentration ratio of Cu to Si or Mn in the passive film or the outermost layer is increased, the surface contact electric resistance is lowered. Cu concentration in the surface layer can be quantified by the atomic ratio Cu / (Si + Mn) in the passive film or the outermost layer, and when the atomic ratio Cu / (Si + Mn) is 0.5 or more, it is satisfactory as a fuel cell separator. The surface contact resistance decreases. Cu / (Si + Mn) ≧ 0.5 is achieved by mixed pickling after bright annealing or atmospheric annealing in a dry atmosphere.
[0021]
Bright annealing: heating in an atmosphere with a dew point of −30 ° C. or lower If the dew point of the annealing atmosphere is increased, the oxides of Si and Mn having a high specific resistance increase, and the surface contact resistance of the passive film increases. In concentrating Cu on the surface layer with Cu / (Si + Mn) ≧ 0.5, as is clear from the examples described later, a stainless steel plate is preferably used at an annealing atmosphere with a dew point of −30 ° C. or lower, preferably 850 to 1100 ° C. Annealing. By bright annealing, the Cu-rich phase is uniformly dispersed and deposited on the matrix, or Cu is concentrated on the surface layer to be modified to a surface layer with low surface contact resistance.
[0022]
Mixed pickling after atmospheric annealing:
Instead of bright annealing, Cu can be concentrated in the passive film or the outermost layer also by mixed acid pickling after atmospheric annealing. In the atmospheric annealing, the stainless steel plate is heated to a temperature range of 850 to 1100 ° C. for the dispersion precipitation of the Cu rich phase or the concentration of Cu on the surface layer. The pickling of the stainless steel plate that has been annealed to the atmosphere does not impose any particular restrictions on the type and concentration of the acid, but hydrofluoric acid-nitric acid, sulfuric acid-nitric acid, etc. with a concentration of about 10% by volume are generally used. . In mixed pickling, there is no preferential dissolution of Cu or Cu-rich phase that occurs in electrolytic pickling, and a passive film with no decrease in Cu concentration is formed after pickling.
[0023]
【Example】
Various ferritic stainless steels having the composition shown in Table 1 are melted in a vacuum melting furnace, and then cold rolled to a thickness of 1.0 mm through casting, hot forging, hot rolling, annealing / pickling, and cold rolling. A steel strip was produced. Some cold-rolled steel strips were subjected to Cu rich phase precipitation treatment at 800 ° C. for 24 hours in the process before the final annealing.
[0024]

[0025]
The manufactured cold-rolled steel strip was brightly annealed or pickled after atmospheric annealing. In bright annealing, heating was performed at 1000 ° C. for 10 seconds in an annealing atmosphere with different dew points, and the influence of the dew point of the atmosphere on the properties of the annealed material was investigated. In the pickling finish, electrolytic pickling using 5% nitric acid and mixed pickling of 6% nitric acid + 2% hydrofluoric acid were employed, and the effect of differences in pickling form on the properties of the annealed materials was investigated.
A specimen cut from a cold-rolled steel strip that had been brightly annealed or pickled was observed with a transmission electron microscope, and the precipitation amount (% by volume) of the Cu-rich phase dispersed and precipitated in the matrix was calculated. Further, the concentration (atomic%) of Cu, Si and Mn relative to the total amount of elements analyzed was determined by glow emission analysis, and the Cu concentration of the surface layer was calculated according to the atomic ratio Cu / (Si + Mn). Further, the surface contact resistance was measured under the condition that a pure gold counter electrode and a measurement terminal were brought into contact with the surface of the test piece and a load of 100 g was applied to the measurement terminal.
[0026]
As can be seen from the results of the investigation in Table 2, the surface layer of Cu / (Si + Mn) <0.5 or Cu even if the Cu content is less than 0.01% by mass, or Cu is contained in an amount of 0.01% by mass or more. Test Nos. 5 to 7 in which the precipitation amount of the rich phase was less than 0.005% by volume showed high surface contact resistance. On the other hand,

test numbers

4, 8 to 19 containing 0.01% by mass or more of Cu, the precipitation amount of the Cu-rich phase being 0.2% by volume or more, or Cu / (Si + Mn) ≧ 0.5, and the surface layer being Cu-concentrated. Showed low surface contact resistance. Among them,

test numbers

8, 9, 11, and 14 satisfying both the Cu rich phase precipitation amount of 0.005% by volume or more and the Cu / (Si + Mn) ≧ 0.5 Cu-enriched surface layer are the surface contact resistance. Was even lower.
[0027]
[Table 2]

[0028]
(Example 2)
A stainless steel plate obtained by subjecting the steel types F3 , F6, F7, and F12 of Table 1 to the same Cu-rich phase precipitation treatment as in Example 1 was used as a test material, and the corrosion resistance in a fuel cell separator environment was investigated by an electrochemical test.
In the electrochemical test, a test piece having a surface to be measured of 10 mm × 10 mm was prepared by embedding a test material in a resin. A sulfuric acid aqueous solution (test solution) adjusted to pH 2 was maintained at 70 ° C., and anodic polarization from natural potential to 1300 mV, SCE was measured at a scanning speed of 20 mV / min. The atmosphere to which the fuel cell separator is exposed includes a natural potential environment to which no potential is added and a high potential environment to which a potential is added. The corrosion resistance in the natural potential environment can be evaluated from the low anodic polarization passivating current density, and the corrosion resistance in the high potential environment can be evaluated from the low corrosion current at 1000 mV, SCE.
[0029]
From the measurement results of the anodic polarization curve shown in FIG. 2, Cr: 13 mass% F3 has a higher desensitization limit current density on the natural potential side than the values of other test materials, and lacks acid resistance. I understand that.
Cr: 41.2% by mass F12 has a low passivation limit current density, but the current value on the high potential side is extremely high compared to other test materials, and the corrosion resistance is obtained when a potential is applied to the separator. It was inferior to. Cr: 16.6% by mass F6, 30.5% by mass F7 are lower in deactivation limit current density and current value under high potential environment than other test materials, and have excellent corrosion resistance in the separator environment. You can expect to show.
[0030]
【The invention's effect】
As described above, the stainless steel separator for a fuel cell of the present invention appropriately manages the addition amount of Cr and Cu, and a Cu-rich phase having a precipitation amount of 0.005% by volume or more or Cu / (Si + Mn) ≧ 0. The surface layer enriched with Cu in .5 is modified to a surface state with low surface contact resistance while ensuring the inherent corrosion resistance of stainless steel. Therefore, it becomes a fuel cell separator with significantly superior processability and productivity compared to a graphite separator, and even when a plurality of fuel cells are stacked, internal loss due to surface contact resistance is small, A fuel cell with high power generation efficiency can be obtained.
[Brief description of the drawings]
FIG. 1 is a cross-sectional view (a) and an exploded perspective view (b) illustrating the internal structure of a fuel cell using a conventional solid polymer membrane as an electrolyte.
Fig. 2 Measurement result of anodic polarization curve

Claims

Cr: 16.0 to 40.0 mass%, Cu: 0.01 to 5 mass%, C: 0.08 mass% or less, Si: 1.00 mass% or less, Mn: 2.00 mass% or less, Ni : 0.08 to 0.16% by mass , Mo: 0.2 to 6.0% by mass, Nb: 0.05 to 1.0% by mass, Ti: 0.05 to 1.0% by mass, Al: 0.01-1.0% by mass, V: 0.01-1.0% by mass, Zr: 0.01-1.0% by mass, or one or more of them, with the balance being Fe and A ferritic stainless steel made of inevitable impurities , wherein the second phase mainly composed of Cu is dispersed and precipitated in a matrix at a ratio of 0.005% by volume or more. steel.

Cr: 16.0-40.0 mass%, Cu: 0.01-5 mass%, C: 0.08 mass% or less, Si: 1.00 mass% or less, Mn: 2.00 mass% or less, Ni : 0.08 to 0.16% by mass , Mo: 0.2 to 6.0% by mass, Nb: 0.05 to 1.0% by mass, Ti: 0.05 to 1.0% by mass, Al: 0.01-1.0% by mass, V: 0.01-1.0% by mass, Zr: 0.01-1.0% by mass, or one or more of them, with the balance being Fe and A ferritic stainless steel made of unavoidable impurities, characterized in that the atomic ratio Cu / (Si + Mn) of Cu to Si and Mn is 0.5 or more and the surface layer is enriched with Cu. Stainless steel.

Cr: 16.0-40.0 mass%, Cu: 0.01-5 mass%, C: 0.08 mass% or less, Si: 1.00 mass% or less, Mn: 2.00 mass% or less, Ni : 0.08 to 0.16% by mass , Mo: 0.2 to 6.0% by mass, Nb: 0.05 to 1.0% by mass, Ti: 0.05 to 1.0% by mass, Al: 0.01-1.0% by mass, V: 0.01-1.0% by mass, Zr: 0.01-1.0% by mass, or one or more of them, with the balance being Fe and A ferritic stainless steel composed of inevitable impurities, in which a second phase mainly composed of Cu is dispersed and precipitated in a matrix at a ratio of 0.005% by volume or more, and the atomic ratio of Cu to Si and Mn is Cu / (Si + Mn ) Is 0.5 or more and the surface layer is enriched with Cu, a ferrite for fuel cell separators Stainless steel.

A fuel cell separator comprising the ferritic stainless steel according to any one of claims 1 to 3 as a base material.

Cr: 16.0-40.0 mass%, Cu: 0.01-5 mass%, C: 0.08 mass% or less, Si: 1.00 mass% or less, Mn: 2.00 mass% or less, Ni : 0.08 to 0.16% by mass , Mo: 0.2 to 6.0% by mass, Nb: 0.05 to 1.0% by mass, Ti: 0.05 to 1.0% by mass, Al: 0.01-1.0% by mass, V: 0.01-1.0% by mass, Zr: 0.01-1.0% by mass, or one or more of them, with the balance being Fe and A method for producing a ferritic stainless steel for a fuel cell separator, comprising subjecting a ferritic stainless steel plate made of inevitable impurities to bright annealing in an atmosphere having a dew point of -30 ° C or lower.

Cr: 16.0-40.0 mass%, Cu: 0.01-5 mass%, C: 0.08 mass% or less, Si: 1.00 mass% or less, Mn 2.00 mass% or less, Ni: 0 0.08-0.16% by mass , Mo: 0.2-6.0% by mass, Nb: 0.05-1.0% by mass, Ti: 0.05-1.0% by mass, Al: Contains one or more of 0.01 to 1.0 mass%, V: 0.01 to 1.0 mass%, Zr: 0.01 to 1.0 mass%, the balance being Fe and inevitable A method for producing a ferritic stainless steel for a fuel cell separator, comprising subjecting a ferritic stainless steel plate made of impurities to air annealing and then pickling using a mixed acid of hydrofluoric acid-nitric acid or sulfuric acid-nitric acid.