Stainless steel material for a separator of a solid polymer fuel cell and a solid polymer fuel cell using the separator

1. A stainless steel member for a separator of a solid polymer fuel cell, the member comprising:

a stainless steel base metal;

an oxide film located on a surface of the stainless steel base metal;

an electroconductive layer located on a surface of the oxide film opposite to the oxide film surface located on the stainless steel base metal, and comprising a nonmetallic electroconductive material, wherein the nonmetallic electroconductive material comprises graphite and the value of a lattice spacing of d002 of the graphite is at most 3.390 angstroms; and

an electroconductive material which is located so as to penetrate the oxide film and which electrically contacts the stainless steel base metal and the electroconductive layer;

wherein when comparing peak intensities of diffraction lines of atomic planes of a crystal of the graphite located on the surface of the oxide film as measured by wide-angle X-ray diffraction, the ratio of a peak intensity of a diffraction line of a (110) atomic plane to a peak intensity of a diffraction line of a (004) atomic plane is less than 0.1,

wherein the electroconductive material is formed of a boride-type metallic inclusion which has a M 2 B-type structure and which is partly embedded in the stainless steel base metal, wherein M is a metal.

2. The stainless steel member according to claim 1 , wherein the electroconductive layer is located on a surface of the electroconductive material.

3. The stainless steel member according to claim 1 , wherein the boride-type metallic inclusion contains a chromium boride.

4. The stainless steel member according to claim 1 , wherein the electroconductive layer is prepared by sliding a member containing graphite on a surface consisting of the surface of the oxide film and a surface of the boride-type metallic inclusion exposed on the stainless steel base metal.

5. The stainless steel member according to claim 4 , wherein the surface consisting of the surface of the oxide film and the surface of the boride-type metallic inclusion exposed on the stainless steel base metal has an Ra roughness of at least 0.10 micrometers.

6. The stainless steel member according to claim 1 , wherein the electroconductive layer is prepared by coating a composition comprising the graphite and an adhesive agent on a surface consisting of the surface of the oxide film and an exposed surface of the boride-type metallic inclusion, the content of the adhesive agent in the composition being at most 2% by weight of the content of the graphite in the composition.

7. The stainless steel member according to claim 6 , wherein the adhesive agent contains at least one of PVDF (polyvinylidene difluoride) and PTFE (polytetrafluoroethylene).

8. A solid polymer fuel cell having a stack of unit cells with individual separators disposed between them, each of the unit cells being prepared by laminating a fuel electrode film, a proton-exchange membrane, and an oxidant electrode film in this order, DC power being generated by supplying a fuel gas and an oxidant gas to the stack of unit cells, wherein each separator is obtained from the stainless steel member described in claim 1 .