JP4523234B2 - Fuel cell separator - Google Patents
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- JP4523234B2 JP4523234B2 JP2003054036A JP2003054036A JP4523234B2 JP 4523234 B2 JP4523234 B2 JP 4523234B2 JP 2003054036 A JP2003054036 A JP 2003054036A JP 2003054036 A JP2003054036 A JP 2003054036A JP 4523234 B2 JP4523234 B2 JP 4523234B2
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
- Y02E60/00—Enabling technologies; Technologies with a potential or indirect contribution to GHG emissions mitigation
- Y02E60/30—Hydrogen technology
- Y02E60/50—Fuel cells
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- Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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- Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
- Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
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Description
【０００１】
【発明の属する技術分野】
本発明は、固体高分子電解質型燃料電池用金属セパレータおよびその製造方法、並びに該セパレータ用ステンレス鋼帯に関する。
【０００２】
【従来技術】
固体高分子電解質型燃料電池用金属セパレータは、複数の単セルが積層された燃料電池スタックを構成する部材であって、十分なガス不透過性と、セル同士の導電をするための電気伝導性が必要である。さらには、電池反応に対して高い耐食性も要する。従来、このような燃料電池用ガスセパレータは、炭素材料あるいは金属材料が用いられてきた。特に金属材料は強度に優れているため、炭素材料を用いるより薄くできることから、近年広く検討されている。
【０００３】
特にステンレス鋼板は、耐食性が高く、安価であるため、チタンやハステロイの様な特殊耐食鋼等の高価な金属に比較して、セパレータ材として適している。しかし、ステンレスはその表面の酸化膜で耐食性を維持しているため、接触抵抗が高く、セル同士の電気抵抗が高くなる不具合がある。これを解決するために接触抵抗の低い貴金属でステンレスを被覆する方法が開発されている。
【０００４】
特開平１０−２２８９１４号（特許文献１）にはSUS304に0.01〜0.06μｍの厚さの金めっき層を形成した技術が紹介されている。
ただし、単に金めっきを施しただけでは酸化膜の影響で抵抗が高いため、酸化膜を除去して金めっきを施す技術が特開平１３−６７１３（特許文献２）に開示されている。
【０００５】
また、めっきではピンホールを代表とする微小欠陥が不可避であり、その部分の耐食性が低下する。前述の特許では問題ないとされているが、特開平１４−２６０６８１（特許文献３）では微小欠陥を減少するためにめっき後に圧延加工して微小欠陥をなくす技術が開示されている。
さらに、簡便に金を被覆する試みとして、特開平１４−２３７３１１（特許文献４）では超音波を利用して金箔を被覆する技術が開示されている。
【０００６】
一方、コネクタ等では、ステンレス鋼にめっきする場合、密着性を高めるためにストライクニッケルめっき後にニッケルめっきを施すことが多いが、前述のように燃料電池で用いられる様な薄い金めっきでは微小欠陥が不可避であるため、下地めっきのニッケルが腐食してしまう欠点があり、直接ステンレスに金めっきを施すダイレクト金めっきの必要があった。
【０００７】
これらのことから、ステンレスに金めっきをしただけでは、微小欠陥の問題から0.01μｍ程度の薄い金めっきでは耐食性に問題があることが明らかである。
これを解決するためには、金めっき厚を厚くして微小欠陥を減少する必要があった。
事実、特開平１３−３４５１０９（特許文献５）では部分的に金めっき厚を厚くして耐食性を維持し、かつ接触抵抗を低減する技術が開示されている。しかし、厚い金めっき膜の形成は、当然コストに影響し、実用的ではない。
【０００８】
【特許文献１】
特開平１０−２２８９１４号公報
【特許文献２】
特開平１３−６７１３号公報
【特許文献３】
特開平１４−２６０６８１号公報
【特許文献４】
特開平１４−２３７３１１号公報
【特許文献５】
特開平１３−３４５１０９号公報
【０００９】
【発明が解決しようとする課題】
本発明は、かかる問題点を解消するためになされたものであり、貴金属めっきを施してステンレス鋼の接触抵抗を低減するが、薄い貴金属めっき厚でも十分な耐食性を備えた燃料電池用金属セパレータ及びその製造法を提供することを目的とする。また、本発明は、該燃料電池用金属セパレータのためのステンレス鋼帯を提供することを目的とするものである。
【００１０】
【課題を解決するための手段】
本発明者らは、前記貴金属めっきしたステンレス鋼の腐食の原因について鋭意検討した結果、以下に述べるように、この腐食にはステンレス鋼表面に析出している微小な介在物が関与していることを見出した。
【００１１】
ステンレス鋼では、溶解鋳造時の溶湯の脱酸を目的としてシリコンやアルミニウム等を添加する。これが、その後の圧延加工、焼鈍加工を経て板条材となった場合に表面に露出することがある。通常の用途では、これら表面介在物は大きな問題ではないが、燃料電池用セパレータの様にごく薄いダイレクト金めっきを施す場合には問題になることが調査の結果わかった。0.02μｍすなわち20nm前後の厚さの金めっきでは、この介在物を被覆することは不可能で、図１に示す様にめっき後の表面からSEM観察すると黒く見える直径１μｍ以下の微小な介在物が表面に露出しているのが観察された。この黒い部分をAES分析するとシリコン酸化物であった。
【００１２】
そこで、めっき前素材を同様にSEM観察したところ直径１μｍ以下の微小な白い介在物が観察された（図２参照）。この部分もAES分析の結果シリコン酸化物であった。従って、表面の介在物をAuめっきが覆いきれないために、表面にシリコン酸化物が露出するものと考えられる。非導電性のアルミ酸化物でも同様の結果となる。
この介在物は、その表面にめっき皮膜が形成できないため、めっきピンホールと同様の欠陥として働き、その大きさはピンホールが図1で観察できないことからピンホールより非常に大きいことが想定される。従って、この介在物を起点として腐食が進むものと考えられる。さらに、貴金属と介在物もしくは素材金属の間にはガルバニック電池が形成されるため、貴金属が無い場合よりもより腐食が激しくなるものと考えられる。
したがって、これを回避するためには、ステンレス鋼表面に露出している介在物を減少させることが有効であると考え、さらに検討を進めて本発明に至った。
【００１３】
すなわち、本発明は、
（１）ステンレス鋼にめっき厚１０ｎｍ〜６０ｎｍで貴金属めっきを施してなる燃料電池用セパレータに用いるステンレス鋼帯であって、該ステンレス鋼表面の微細介在物の存在比率が面積率で０．１％以下であることを特徴とするステンレス鋼帯、
（２）ステンレス鋼にめっき厚１０ｎｍ〜６０ｎｍで貴金属めっきを施してなる燃料電池用セパレータに用いる前記貴金属めっきを施したステンレス鋼帯であって、該ステンレス鋼表面の微細介在物の存在比率が面積率で０．１％以下であることを特徴とする前記貴金属めっきを施したステンレス鋼帯、
（３）ステンレス鋼にめっき厚１０ｎｍ〜６０ｎｍで貴金属めっきを施してなる燃料電池用セパレータであって、該ステンレス鋼表面の微細介在物の存在比率が面積率で０．１％以下であることを特徴とする燃料電池用セパレータ、
（４）ステンレス鋼が機械研磨、電解研磨、化学研磨等の研磨方法を用いることによってその微細介在物の存在比率を0.1%以下にされていることを特徴とする（３）記載の燃料電池用セパレータ、
（５）ステンレス鋼がステンレス鋼の清浄性を高めることによってその微細介在物の存在比率を０．１％以下にされていることを特徴とする（３）記載の燃料電池用セパレータ、
（６）ステンレス鋼の表面微細介在物の面積比率を０．１％以下とし、次いで該表面に貴金属めっきを施すことを特徴とする燃料電池用セパレータの製造方法に関する。
なお、前記（１）のステンレス鋼帯は、これを貴金属めっきした後、成形加工及び切断加工して、あるいは、成形加工及び切断加工した後貴金属めっきして、また、前記（２）のステンレス鋼帯を成形加工及び切断加工して燃料電池用セパレータとすることができる。
【００１４】
【本発明の実施の形態】
本発明の実施の形態について説明する。
表面介在物が0.1％以下の面積率であるSUS304、SUS316Lといったステンレス鋼を電解脱脂→水洗→電解酸洗→水洗の前処理の後、ダイレクト金めっきを施す。
本発明においては、貴金属めっきに先立ち、ステンレス鋼の表面に存在する微細介在物をその面積比率で０．１％以下に低減することが燃料電池セパレータの耐食性を確保するために重要である。さらに、０．０１％以下にすることが好ましい。ステンレス鋼の表面に介在物が面積比率で０．１％を超えて存在する場合には、貴金属めっきを施してもその厚さが０．０２μｍ程度の薄いめっき膜では介在物を被覆することはできないため、耐食性を向上することはできない。また、一般的な中間めっきであるニッケルめっき等を０．５μｍ以上施した場合では介在物を被覆することができるが、中間めっきの耐食性が低く、燃料電池用セパレータとしては使用できない。
【００１５】
本発明において、ステンレス鋼表面介在物の量を制御する方法は、特に制限されるものではない。たとえば、焼鈍等の熱処理で表面に濃化して介在物が存在する場合は、機械研磨や電解研磨、化学研磨等の手法で表層を取り除くことが効果的である。たとえば、砥粒を含んだ不織布いわゆるバフを用いたバフ研磨で表面を研磨する方法がある。このとき、砥粒が表面に残留しないようにバフ後の水洗等の洗浄工程を確実にすることは、本発明では重要なことである。もし、砥粒が残留すると、めっき前処理で取り除けない場合、表面介在物と同様に耐食性を低下させる要因になるからである。
【００１６】
また、電解研磨を用いる方法もある。電解研磨では、通電用の設備が必要なため、めっき前処理の一工程として組み合わせるのが設備コストが低くできるが、専用設備としてもかまわない。電解研磨の場合、機械研磨と異なって研磨工程で付着する粒子等がないことから機械研磨より処理後の洗浄に気を使わなくても良い利点がある。
【００１７】
酸洗を含む化学研磨はステンレス鋼の製造工程で広く用いられている。たとえばふっ酸と硝酸を混合した酸は塩酸で酸洗いしたとき発生する孔食の危険が少なく、良好な酸洗面が得られるため広く用いられている。また、特開平11-140699では塩酸または硫酸で超音波振動とパルス電流を併用して酸化物や硬質物質を除去する方法が開示されている。
【００１８】
前記の化学研磨は、あくまでも表面介在物を取り除くのが目的であるので、特開平１３−６７１３で示されているような酸化膜を取り除くのが目的ではない。ただし、酸化膜を取り除く方法を用いると結果として表面介在物を取り除くことにもなるため、その方法を限定するものではなく、表面介在物の量を制御できればかまわない。さらに、表面介在物が存在せず、酸化膜だけが存在する場合は後述するように、めっきの前処理として一般的に行われる処理が必要なだけであり、あえて酸化膜を取り除く必要は無い。
【００１９】
また、溶解鋳造時の雰囲気を制御して介在物の量を最初から少なくすることも、可能である。たとえば特開2000-273586では、ステンレス箔のエッチング性を向上することを目的として介在物を減少させる技術が開示されており、溶解鋳造時に不活性ガス吹き込みなどによる精錬後の介在物浮上の促進、鋼へのスラグ巻き込みの低減、精錬後鋳造までの時間調整による酸化物系大型介在物の生成抑制などの方法が挙げられている。この様に他の特性を高めるために清浄度を高めたステンレス鋼でも、表面介在物が少ない場合には燃料電池用として問題なく使用できる。
【００２０】
発明者は表面介在物の量をAES分析装置でAES分析と２次電子像観察により同定および面積率を測定したが、これと同様の精度で調査できる測定機器であればよく、測定方法を制限するものではない。
【００２１】
本発明において、表面の介在物を低減させたステンレス鋼の貴金属めっき方法については、基本的には制限されるものではなく、従来の方法により行うことができる。そのめっき厚は、０．０１〜０．０６μｍで十分である。
【００２２】
【実施例】
実施例にて本発明を説明する。
実施例１
厚さ０．３ｍｍのＳＵＳ３１６Ｌを以下の条件で電解研磨を施した。
電解研磨液
燐酸： ６０％
硫酸： ４０％
液温： ６０℃
電流密度：１０Ａ／ｄｍ2
研磨時間 ：１０分
この条件で７μｍ研磨することができた。その表面を観察した結果、介在物は観察できなかった。
【００２３】
このＳＵＳ３１６Ｌを用いて、電解陽極脱脂→水洗→電解陰極酸洗→水洗の前処理を施した後、ダイレクト金めっきを施した。各前処理および金めっき条件は以下の通り。
電解脱脂
パクナＰ１０５：４０ｇ／Ｌ
温度： ６０℃
電流密度：６Ａ／ｄｍ2
陽極電解
電解時間：３０秒
酸洗
硫酸： ２００ｇ／Ｌ
温度： 常温
時間： ３０秒
金めっき
添加剤：ダイレクト金めっき浴
金濃度：１〜４ｇ／Ｌ
浴温： ２０〜４０℃
電流密度：６Ａ／ｄｍ2
めっき厚：２０ｎｍ、５０ｎｍ
この様にして作成したステンレス金めっき材の接触抵抗、アノード分極、硫酸浸漬後の鉄溶出量を以下の条件で調査した。
【００２４】
接触抵抗
山崎試験機製：電気接点シミレータ CRS-1
プローブ： 金
接圧：１０ｇｆ
測定数：４００点
【００２５】
硫酸浸漬試験
溶液：硫酸 ５%
温度：８０℃
液量：５ｃｃ
供試材 １０×５０ｍｍ 浸漬
浸漬時間 〜３０日
測定方法 ICP分析にてFeイオンを定量
その結果、Ａｕめっき厚によらず、接触抵抗は２０ｍΩ以下の低い値を示した。
また、硫酸浸漬後の鉄の溶出量は表１に示すとおり、最大でも５ｐｐｍと低い値であった。
【００２６】
【表１】
実施例２
実施例１で用いたＳＵＳ３１６を電解研磨の代わりに、バフ研磨を施した以外は同様に試験を行った。バフ研磨は砥粒がシリコンカーバイドのものを用いて行い、研磨量は１μｍとした。バフ研磨後はスプレー水洗を実施した。
研磨後の表面をSEM観察した結果、砥粒の残さは認められなかったが、介在物が面積率で０．０５％で存在していた。
この素材を用いて、実施例１と同様にめっき前処理及びＡｕめっきを施し、評価を実施した。
接触抵抗は、実施例１と同様に２０ｍΩ以下の値であった。
硫酸浸漬試験の結果、表１に示す様な溶出量であった。
【００２８】
比較例
実施例で使用したＳＵＳ３１６を研磨処理せずに、実施例１と同様にめっき前処理及びＡｕめっきを施し、評価を行った。表面をＳＥＭ観察したところ、シリコン酸化物の介在物が存在し、その面積率は１５．８％であった。
接触抵抗は実施例１および実施例２と同じく、２０ｍΩ以下であった。
しかし、硫酸浸漬試験では、表1に示す様に多量の鉄イオンが溶出し、３０日浸漬後には、Ａｕめっきはすべて剥離していた。
【００２９】
【発明の効果】
以上説明したように、本発明によれば、薄い貴金属めっきを施しただけで燃料電池用セパレータに要求される接触抵抗が低く、かつ耐食性を備えた該セパレータを得ることができる。また、本発明によれば、該セパレータのためのステンレス鋼帯を得ることができる。
【００３０】
【図面の簡単な説明】
【図１】ステンレス鋼を金めっきした表面のＳＥＭ像。
【図２】同金めっき前のステンレス鋼表面のＳＥＭ像。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a metal separator for a solid polymer electrolyte fuel cell, a method for producing the same, and a stainless steel strip for the separator.
[0002]
[Prior art]
The metal separator for a solid polymer electrolyte fuel cell is a member constituting a fuel cell stack in which a plurality of single cells are stacked, and has sufficient gas impermeability and electric conductivity for conducting between cells. is required. Furthermore, high corrosion resistance is required for the battery reaction. Conventionally, carbon materials or metal materials have been used for such fuel cell gas separators. In particular, metal materials have been widely studied in recent years because they are superior in strength and can be made thinner than carbon materials.
[0003]
In particular, a stainless steel plate is suitable as a separator material compared to an expensive metal such as a special corrosion resistant steel such as titanium or Hastelloy because it has high corrosion resistance and is inexpensive. However, since stainless steel maintains corrosion resistance with an oxide film on its surface, there is a problem that contact resistance is high and electrical resistance between cells is high. In order to solve this problem, a method of coating stainless steel with a noble metal having low contact resistance has been developed.
[0004]
Japanese Patent Application Laid-Open No. 10-228914 (Patent Document 1) introduces a technique in which a gold plating layer having a thickness of 0.01 to 0.06 μm is formed on SUS304.
However, since the resistance is high due to the effect of the oxide film simply by performing the gold plating, a technique for removing the oxide film and performing the gold plating is disclosed in Japanese Patent Laid-Open No. 13-6713 (Patent Document 2).
[0005]
Moreover, in plating, the micro defect represented by a pinhole is inevitable, and the corrosion resistance of the part falls. Although it is said that there is no problem in the above-mentioned patent, Japanese Patent Application Laid-Open No. 14-260681 (Patent Document 3) discloses a technique for eliminating micro defects by rolling after plating in order to reduce micro defects.
Furthermore, as an attempt to easily coat gold, Japanese Patent Laid-Open No. 14-237311 (Patent Document 4) discloses a technique for coating a gold foil using ultrasonic waves.
[0006]
On the other hand, when plating on stainless steel, nickel plating is often applied after strike nickel plating in order to improve adhesion. However, as described above, thin gold plating used in fuel cells has minute defects. Since it is inevitable, there is a drawback that the nickel of the base plating corrodes, and there is a need for direct gold plating to directly apply gold plating to stainless steel.
[0007]
From these facts, it is clear that only gold plating on stainless steel has a problem in corrosion resistance in thin gold plating of about 0.01 μm due to the problem of minute defects.
In order to solve this, it was necessary to increase the thickness of the gold plating to reduce micro defects.
In fact, Japanese Patent Laid-Open No. 13-345109 (Patent Document 5) discloses a technique for partially increasing the gold plating thickness to maintain corrosion resistance and reducing contact resistance. However, the formation of a thick gold plating film naturally affects the cost and is not practical.
[0008]
[Patent Document 1]
Japanese Patent Laid-Open No. 10-228914 [Patent Document 2]
Japanese Patent Laid-Open No. 13-6713 [Patent Document 3]
Japanese Patent Laid-Open No. 14-260681 [Patent Document 4]
JP-A-14-237311 [Patent Document 5]
Japanese Patent Laid-Open No. 13-345109
[Problems to be solved by the invention]
The present invention has been made to solve such problems, and reduces the contact resistance of stainless steel by applying precious metal plating, and a metal separator for a fuel cell having sufficient corrosion resistance even with a thin precious metal plating thickness and It aims at providing the manufacturing method. Another object of the present invention is to provide a stainless steel strip for the fuel cell metal separator.
[0010]
[Means for Solving the Problems]
As a result of intensive studies on the cause of corrosion of the noble metal-plated stainless steel, the present inventors, as will be described below, that this corrosion involves minute inclusions precipitated on the stainless steel surface. I found.
[0011]
In stainless steel, silicon, aluminum, or the like is added for the purpose of deoxidizing the molten metal during melt casting. When this becomes a strip material through subsequent rolling and annealing, it may be exposed to the surface. In normal applications, these surface inclusions are not a major problem, but investigations have shown that they are problematic when a very thin direct gold plating is applied, such as a fuel cell separator. In the case of gold plating with a thickness of about 0.02 μm, that is, about 20 nm, it is impossible to cover this inclusion. As shown in FIG. Exposure to the surface was observed. When this black part was analyzed by AES, it was silicon oxide.
[0012]
Therefore, when the pre-plating material was similarly observed by SEM, minute white inclusions having a diameter of 1 μm or less were observed (see FIG. 2). This part was also silicon oxide as a result of AES analysis. Therefore, it is considered that the silicon oxide is exposed on the surface because the Au plating cannot cover the inclusions on the surface. Similar results are obtained with non-conductive aluminum oxide.
Since this inclusion cannot form a plating film on its surface, it acts as a defect similar to a plated pinhole, and its size is assumed to be much larger than the pinhole because the pinhole cannot be observed in FIG. . Therefore, it is considered that corrosion proceeds from this inclusion. Further, since a galvanic battery is formed between the noble metal and the inclusion or the material metal, it is considered that the corrosion becomes more severe than the case where there is no noble metal.
Therefore, in order to avoid this, it was considered effective to reduce the inclusions exposed on the surface of the stainless steel, and further studies were made to arrive at the present invention.
[0013]
That is, the present invention
(1) A stainless steel strip used for a fuel cell separator obtained by subjecting stainless steel to a noble metal plating with a plating thickness of 10 nm to 60 nm , wherein the abundance of fine inclusions on the stainless steel surface is 0.1% in terms of area ratio A stainless steel strip characterized by:
(2) A stainless steel strip subjected to the noble metal plating used for a fuel cell separator obtained by applying a noble metal plating to a stainless steel with a plating thickness of 10 nm to 60 nm , wherein the abundance ratio of fine inclusions on the stainless steel surface is an area stainless steel strip which has been subjected to the noble metal plating, characterized in that at a rate of 0.1% or less,
(3) A separator for a fuel cell obtained by subjecting stainless steel to noble metal plating with a plating thickness of 10 nm to 60 nm , wherein the presence ratio of fine inclusions on the stainless steel surface is 0.1% or less in area ratio A fuel cell separator,
(4) The fuel cell according to (3), wherein the stainless steel has a fine inclusion content of 0.1% or less by using a polishing method such as mechanical polishing, electrolytic polishing, and chemical polishing. Separator,
(5) The fuel cell separator according to (3), wherein the stainless steel has a fine inclusion content of 0.1% or less by enhancing the cleanliness of the stainless steel,
(6) The present invention relates to a method for producing a fuel cell separator, wherein the surface ratio of fine inclusions on the surface of stainless steel is 0.1% or less, and then the surface is plated with noble metal.
The stainless steel strip (1) is precious metal-plated and then formed and cut, or formed and cut and precious-metal plated, and the stainless steel (2) The band can be formed and cut to form a fuel cell separator.
[0014]
[Embodiments of the Invention]
Embodiments of the present invention will be described.
Stainless steel such as SUS304 and SUS316L with surface inclusions of 0.1% or less is subjected to direct gold plating after electrolytic degreasing → water washing → electrolytic pickling → water washing pretreatment.
In the present invention, prior to precious metal plating, it is important to reduce the fine inclusions present on the surface of stainless steel to 0.1% or less in terms of area ratio in order to ensure the corrosion resistance of the fuel cell separator. Furthermore, it is preferable to make it 0.01% or less. When inclusions are present on the surface of stainless steel in an area ratio exceeding 0.1%, even if precious metal plating is applied, the inclusion is covered with a thin plating film having a thickness of about 0.02 μm. Therefore, the corrosion resistance cannot be improved. Moreover, when nickel plating etc. which are general intermediate plating etc. are 0.5 micrometer or more, an inclusion can be coat | covered, However, The corrosion resistance of intermediate | middle plating is low and cannot be used as a separator for fuel cells.
[0015]
In the present invention, the method for controlling the amount of stainless steel surface inclusions is not particularly limited. For example, in the case where inclusions are concentrated on the surface by a heat treatment such as annealing, it is effective to remove the surface layer by a technique such as mechanical polishing, electrolytic polishing, or chemical polishing. For example, there is a method of polishing the surface by buffing using a so-called non-woven buff containing abrasive grains. At this time, it is important in the present invention to ensure a washing step such as washing after buffing so that abrasive grains do not remain on the surface. If the abrasive grains remain, if they cannot be removed by the pretreatment for plating, it becomes a factor for reducing the corrosion resistance like the surface inclusions.
[0016]
There is also a method using electropolishing. In electropolishing, since equipment for energization is required, the equipment cost can be reduced by combining it as one step of the plating pretreatment, but it may also be dedicated equipment. In the case of electrolytic polishing, unlike mechanical polishing, there is no particle or the like adhering in the polishing step, so there is an advantage that care is not required for cleaning after processing than mechanical polishing.
[0017]
Chemical polishing including pickling is widely used in the manufacturing process of stainless steel. For example, an acid mixture of hydrofluoric acid and nitric acid is widely used because there is little risk of pitting corrosion that occurs when pickling with hydrochloric acid and a good pickled surface can be obtained. Japanese Patent Application Laid-Open No. 11-140699 discloses a method for removing oxides and hard substances by using ultrasonic vibration and pulse current in combination with hydrochloric acid or sulfuric acid.
[0018]
The purpose of the chemical polishing is to remove the surface inclusions to the last, so the purpose is not to remove the oxide film as disclosed in JP-A No. 13-6713. However, if the method of removing the oxide film is used, surface inclusions are also removed as a result. Therefore, the method is not limited, and the amount of surface inclusions may be controlled. Furthermore, when there is no surface inclusion and only an oxide film exists, as will be described later, it is only necessary to perform a process generally performed as a pretreatment for plating, and it is not necessary to remove the oxide film.
[0019]
It is also possible to reduce the amount of inclusions from the beginning by controlling the atmosphere during melting and casting. For example, Japanese Patent Laid-Open No. 2000-273586 discloses a technique for reducing inclusions for the purpose of improving the etching properties of stainless steel foil, and promoting the floating of inclusions after refining by blowing an inert gas during melting and casting, Methods such as reduction of slag entrainment in steel and suppression of formation of large oxide inclusions by adjusting the time from refining to casting are mentioned. As described above, even stainless steel having high cleanliness to enhance other characteristics can be used without problem for a fuel cell if there are few surface inclusions.
[0020]
The inventor measured the amount of surface inclusions by AES analysis and AES analysis and secondary electron image observation, and measured the area ratio. Not what you want.
[0021]
In the present invention, the noble metal plating method for stainless steel with reduced surface inclusions is not basically limited, and can be performed by a conventional method. A plating thickness of 0.01 to 0.06 μm is sufficient.
[0022]
【Example】
The examples illustrate the invention.
Example 1
SUS316L having a thickness of 0.3 mm was subjected to electrolytic polishing under the following conditions.
Electropolishing liquid phosphoric acid: 60%
Sulfuric acid: 40%
Liquid temperature: 60 ° C
Current density: 10 A / dm 2
Polishing time: 10 minutes Polishing of 7 μm was possible under these conditions. As a result of observing the surface, inclusions could not be observed.
[0023]
Using this SUS316L, after pretreatment of electrolytic anode degreasing → water washing → electrolytic cathodic pickling → water washing, direct gold plating was performed. Each pretreatment and gold plating conditions are as follows.
Electrolytic degreasing pakuna P105: 40 g / L
Temperature: 60 ° C
Current density: 6 A / dm 2
Anodic electrolysis time: 30 seconds Pickling sulfuric acid: 200 g / L
Temperature: Normal temperature Time: 30 seconds Gold plating additive: Direct gold plating bath Gold concentration: 1-4 g / L
Bath temperature: 20-40 ° C
Current density: 6 A / dm 2
Plating thickness: 20 nm, 50 nm
The contact resistance, anodic polarization, and iron elution after immersion in sulfuric acid of the stainless gold plating material thus prepared were investigated under the following conditions.
[0024]
Contact Resistance Yamazaki Test Machine: Electric contact simulator CRS-1
Probe: Gold contact pressure: 10 gf
Number of measurements: 400 points
Sulfuric acid immersion test solution: sulfuric acid 5%
Temperature: 80 ° C
Liquid volume: 5cc
Specimen 10 × 50 mm Immersion immersion time ˜30 days Measurement method Quantitative determination of Fe ions by ICP analysis As a result, the contact resistance showed a low value of 20 mΩ or less regardless of the Au plating thickness.
Moreover, as shown in Table 1, the elution amount of iron after immersion in sulfuric acid was a low value of 5 ppm at the maximum.
[0026]
[Table 1]
Example 2
A test was performed in the same manner except that SUS316 used in Example 1 was subjected to buffing instead of electrolytic polishing. The buffing was performed using silicon carbide abrasive grains, and the polishing amount was 1 μm. After buffing, spray water washing was performed.
As a result of SEM observation of the polished surface, no residue of abrasive grains was observed, but inclusions were present at an area ratio of 0.05%.
Using this material, pre-plating treatment and Au plating were performed in the same manner as in Example 1 and evaluated.
The contact resistance was a value of 20 mΩ or less as in Example 1.
As a result of the sulfuric acid immersion test, the elution amount was as shown in Table 1.
[0028]
Comparative Example SUS316 used in Examples was subjected to pre-plating treatment and Au plating in the same manner as Example 1 without polishing, and evaluated. When the surface was observed by SEM, inclusions of silicon oxide were present, and the area ratio was 15.8%.
The contact resistance was 20 mΩ or less, as in Examples 1 and 2.
However, in the sulfuric acid immersion test, a large amount of iron ions were eluted as shown in Table 1, and after immersion for 30 days, all the Au plating was peeled off.
[0029]
【The invention's effect】
As described above, according to the present invention, it is possible to obtain a separator having a low contact resistance required for a fuel cell separator and having corrosion resistance only by performing thin noble metal plating. Moreover, according to this invention, the stainless steel strip for this separator can be obtained.
[0030]
[Brief description of the drawings]
FIG. 1 is an SEM image of a surface of stainless steel plated with gold.
FIG. 2 is an SEM image of a stainless steel surface before the gold plating.
Claims (6)
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2003054036A JP4523234B2 (en) | 2003-02-28 | 2003-02-28 | Fuel cell separator |
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| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP2003054036A JP4523234B2 (en) | 2003-02-28 | 2003-02-28 | Fuel cell separator |
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| JP2004265694A JP2004265694A (en) | 2004-09-24 |
| JP4523234B2 true JP4523234B2 (en) | 2010-08-11 |
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| JP5403642B2 (en) * | 2003-11-07 | 2014-01-29 | 大同特殊鋼株式会社 | Corrosion resistant conductive material |
| JP2007095318A (en) * | 2005-09-27 | 2007-04-12 | Daido Steel Co Ltd | Anti-corrosion member, separator for fuel cell, and fuel cell |
| WO2019082591A1 (en) * | 2017-10-25 | 2019-05-02 | Ｊｆｅスチール株式会社 | Method for producing stainless steel plate for fuel cell separator |
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