Stainless steel having excellent surface electrical conductivity for fuel cell separator and manufacturing method therefor
A stainless steel a fuel cell separator including a surface oxide layer having a bandgap energy of 2 eV or less. A value of the surface oxide atomic ratio ((sum of atomic concentrations of metal elements in metal oxide (MO)/(sum of atomic concentrations of metal elements in total oxides and hydroxides))) is 0.44 or less, as measured on the surface of a stainless steel containing 15 wt % or more of Cr by X-ray angle-resolved photoemission spectroscopy using an Al-Kα X-ray source under the condition where a take-off angle of photoelectrons is from 12° to 85°. The metal oxide (MO) includes a mixed oxide: M represents an alloying element other than Cr and Fe or a combination thereof in the matrix; and O represents oxygen; and the total oxides and hydroxides include a Cr oxide, a Cr hydroxide, an Fe oxide, an Fe hydroxide, and the metal oxide (MO).
1 . A stainless steel for a fuel cell separator, comprising:
a surface oxide layer having a bandgap energy of 2 eV or less,
wherein a value of the following surface oxide atomic ratio (1) is 0.44 or less, as measured on a surface of the stainless steel containing 15 wt % or more of Cr by X-ray angle-resolved photoemission spectroscopy using an Al-Kα X-ray source under the condition where a take-off angle of photoelectrons is from 12° to 85°,
sum
of
atomic
concentrations
(
at
%
)
of
metal
elements
in
metal
oxide
(
MO
)
sum
of
atomic
concentrations
(
at
%
)
of
metal
elements
in
total
oxides
and
hydroxides
(
1
)
wherein the metal oxide (MO) includes a mixed oxide: M represents an alloying element other than Cr and Fe or a combination thereof in the matrix; and O represents oxygen, and
the total oxides and hydroxides comprise a Cr oxide, a Cr hydroxide, an Fe oxide, an Fe hydroxide, and the metal oxide (MO).
2 . The stainless steel according to claim 1 , wherein the surface oxide layer of the stainless steel forms an ohmic contact with the matrix.
3 . A method for manufacturing the stainless steel according to claim 1 , the method comprising:
a primary surface treatment performed by immersing a cold-rolled stainless steel sheet in a nonoxidizing acid solution, or conducting an electrolytic treatment after the immersing, and
a secondary surface treatment performed by immersing the cold-rolled stainless steel sheet in an oxidizing acid solution.
4 . The method according to claim 3 , wherein the primary surface treatment comprises immersing the cold-rolled stainless steel sheet in the nonoxidizing acid solution for 5 seconds or more, or performing an electrolytic treatment with a current density of 0.1 A/cm 2 or more for 5 seconds or more after the immersing, and
the nonoxidizing acid solution is a 5 wt % or more hydrochloric acid (HCl) or sulfuric acid solution at 50° C. or higher.
5 . The method according to claim 3 , wherein the secondary surface treatment comprises immersing the cold-rolled stainless steel sheet in the oxidizing acid solution for 5 seconds or more, and
the oxidizing acid solution is a 5 wt % or more nitric acid solution at 50° C. or higher.
6 . The stainless steel according to claim 1 , wherein the value of the surface oxide atomic ratio (1) is 0.39 or less.