U.S. patent application number 12/325478 was filed with the patent office on 2009-08-27 for surface treatment method of titanium material for electrodes.
This patent application is currently assigned to Kabushiki Kaisha Kobe Seiko Sho (Kobe Steel, Ltd.). Invention is credited to Jun Hisamoto, Yoshinori Ito, Toshiki Sato, Jun Suzuki, Shinichi Tanifuji, Takashi Yashiki.
Application Number | 20090211667 12/325478 |
Document ID | / |
Family ID | 40527970 |
Filed Date | 2009-08-27 |
United States Patent
Application |
20090211667 |
Kind Code |
A1 |
Suzuki; Jun ; et
al. |
August 27, 2009 |
SURFACE TREATMENT METHOD OF TITANIUM MATERIAL FOR ELECTRODES
Abstract
Disclosed herein is a surface treatment method of a titanium
material for electrodes characterized by including: a titanium
oxide layer formation step S1 of forming a titanium oxide layer
with a thickness of 10 nm or more and 80 nm or less on the surface
of a titanium material including pure titanium or a titanium alloy;
a noble metal layer formation step S2 of forming a noble metal
layer with a thickness of 2 nm or more including at least one noble
metal selected from Au, Pt, and Pd on the titanium oxide layer by a
PVD method; and a heat treatment step S3 of heat treating the
titanium material having the noble metal layer formed thereon at a
temperature of 300.degree. C. or more and 800.degree. C. or
less.
Inventors: |
Suzuki; Jun; (Kobe-shi,
JP) ; Sato; Toshiki; (Kobe-shi, JP) ;
Hisamoto; Jun; (Kobe-shi, JP) ; Ito; Yoshinori;
(Kobe-shi, JP) ; Tanifuji; Shinichi; (Kobe-shi,
JP) ; Yashiki; Takashi; (Takasago-shi, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Kabushiki Kaisha Kobe Seiko Sho
(Kobe Steel, Ltd.)
Kobe-shi
JP
|
Family ID: |
40527970 |
Appl. No.: |
12/325478 |
Filed: |
December 1, 2008 |
Current U.S.
Class: |
148/281 ;
148/537 |
Current CPC
Class: |
H01M 8/0228 20130101;
H01M 8/0215 20130101; H01M 8/0206 20130101; Y02E 60/50
20130101 |
Class at
Publication: |
148/281 ;
148/537 |
International
Class: |
C23C 8/10 20060101
C23C008/10; C21D 9/00 20060101 C21D009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Feb 27, 2008 |
JP |
2008-045384 |
Jul 23, 2008 |
JP |
2008-190011 |
Claims
1. A surface treatment method of a titanium material for
electrodes, comprising: a titanium oxide layer formation step of
forming a titanium oxide layer with a thickness of 10 nm or more
and 80 nm or less on the surface of the titanium material including
pure titanium or a titanium alloy; a noble metal layer formation
step of forming a noble metal layer with a thickness of 2 nm or
more including at least one noble metal selected from Au, Pt, and
Pd on the titanium oxide layer by a PVD method; and a heat
treatment step of heat treating the titanium material having the
noble metal layer formed thereon at a temperature of 300.degree. C.
or more and 800.degree. C. or less.
2. The surface treatment method of a titanium material for
electrodes according to claim 1, wherein the titanium oxide layer
formation step includes heat treating the titanium material at a
temperature of 200.degree. C. or more and 600.degree. C. or less in
an air.
3. The surface treatment method of a titanium material for
electrodes according to claim 1, wherein the titanium oxide layer
formation step includes subjecting the titanium material to an
anodic oxidation treatment at a voltage of 100 V or less.
4. The surface treatment method of a titanium material for
electrodes according to claim 1, wherein the titanium oxide layer
formation step includes subjecting the titanium material to an
oxidation treatment in a passive state forming atmosphere of
titanium.
5. The surface treatment method of a titanium material for
electrodes according to claim 1, wherein prior to performing the
titanium oxide layer formation step, a pickling treatment step of
removing the natural oxide layer on the surface of the titanium
material by a solution containing a non-oxidizing acid is
performed.
6. The surface treatment method of a titanium material for
electrodes according to claim 2, wherein prior to performing the
titanium oxide layer formation step, a pickling treatment step of
removing the natural oxide layer on the surface of the titanium
material by a solution containing a non-oxidizing acid is
performed.
7. The surface treatment method of a titanium material for
electrodes according to claim 3, wherein prior to performing the
titanium oxide layer formation step, a pickling treatment step of
removing the natural oxide layer on the surface of the titanium
material by a solution containing a non-oxidizing acid is
performed.
8. The surface treatment method of a titanium material for
electrodes according to claim 4, wherein prior to performing the
titanium oxide layer formation step, a pickling treatment step of
removing the natural oxide layer on the surface of the titanium
material by a solution containing a non-oxidizing acid is
performed.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a titanium material for
electrodes. More particularly, it relates to a surface treatment
method of a titanium material for an electrode suitable for a
separator made of titanium for a fuel cell.
[0003] 2. Description of the Related Art
[0004] As distinct from the primary cell of a dry cell or the like,
and the secondary cell of a lead storage battery or the like, the
fuel cell capable of continuously extracting electric power by
continuing supplying a fuel such as hydrogen and an oxidant such as
oxygen is high in electric power generation efficiency, is not
largely affected by the size of the system scale, and makes less
noise and vibration. For this reason, the fuel cell is expected as
an energy source covering various uses/scales. Then, the fuel cells
have been developed as, specifically, a polymer electrolyte fuel
cell (PEFC), an alkaline electrolyte fuel cell (AFC), a phosphoric
acid fuel cell (PAFC), a molten carbonate fuel cell (MCFC), a solid
oxide fuel cell (SOFC), a biological fuel cell, and the like.
[0005] A fuel cell will be described by taking a polymer
electrolyte fuel cell as one example thereof. Such a polymer
electrolyte fuel cell is configured as follows. A solid polymer
electrolyte membrane is sandwiched between an anode electrode and a
cathode electrode, resulting in a single cell. A plurality of the
single cells are stacked one on another with electrodes referred to
as separators (or bipolar plates) interposed therebetween.
[0006] The material for the separator for the fuel cell is required
to have a characteristic of low contact resistance (denoting the
occurrence of voltage drop due to the interface phenomenon between
the electrodes and the separator surface), which is kept for a long
period during use as a separator. In terms of the processability
and the strength in addition to this respect, a study has been
conventionally conducted on the application of metal materials such
as aluminum alloys, stainless steels, nickel alloys, and titanium
alloys.
[0007] For example, in Patent Document 1, there is a description to
the effect that a separator for a fuel cell is manufactured by
using a stainless steel as a base material, and plating the surface
thereof with gold. Whereas, for example, in Patent Document 2,
there is a description to the effect that a stainless steel or a
titanium material is used as a material, and a noble metal or a
noble metal alloy is deposited on the surface, or the oxide film on
the base material surface is removed, followed by deposition of a
noble metal or a noble metal alloy. Further, for example, in Patent
Document 3, there is a description to the effect that a titanium
material is used as the base material, and after removal of the
oxide film on the surface, 1- to 100-nm island-like gold-plated
parts are interspersed.
[0008] Patent Document 1: Japanese Patent Application Laid-Open
Publication No. 10-228914
[0009] Patent Document 2: Japanese Patent Application Laid-Open
Publication No. 2001-6713
[0010] Patent Document 3: Japanese Patent Application Laid-Open
Publication No. 2006-97088
[0011] However, when the materials such as aluminum alloys,
stainless steels, nickel alloys, and titanium alloys are each used
as the base material of a separator for a fuel cell, the materials
are used under extreme conditions in the inside of the fuel cell
such as strong acidity, high temperature, and high pressure, and
hence they tend to be remarkably deteriorated in electrical
conductivity due to the oxide film formed on the base material
surface, and the like. For this reason, even when the separators
using the base materials are low in contact resistance upon start
of use, they cannot keep the contact resistance low for a long
period. Thus, the contact resistance increases with time, causing a
current loss. Further, metal ions eluted from the base material due
to corrosion deteriorate the polymer electrolyte membrane.
[0012] The separators described in Patent Documents 1 to 3 can
reduce the contact resistance upon start of use. However, when the
separators are exposed in the extreme acidic atmosphere in the fuel
cell, the gold plating layer or the like on the surface may peel
off, and thereby the contact resistance may increase, resulting in
deterioration of the performance of the fuel cell. Further, peeling
off of the gold plating layer or the like may cause corrosion, so
that metal ions eluted from the base material may deteriorate the
polymer electrolyte membrane.
[0013] Further, the separators described in Patent Documents 2 and
3 are configured in the following manner. In order to enhance the
electrical conductivity, the oxide film on the surface of the base
material made of titanium is removed. Then, in order to prevent an
oxide film from being formed again, an electrically conductive
layer of a noble metal, an electrically conductive resin, or the
like is provided under prescribed conditions such as a vacuum
atmosphere or a reduction atmosphere. As a result of this, the
electrical conductivity can be enhanced by removal of the oxide
film. On the other hand, hydrogen becomes more likely to enter the
base material, which entails a fear of embrittlement of the base
material due to entering of hydrogen on a long-term basis.
[0014] Herein, the separator for a fuel cell is required to have
both the high electrical conductivity and the high corrosion
resistance. Further, the separator to be used on the hydrogen
electrode side is required not to absorb hydrogen and not to
thereby mechanically embrittle (to have a hydrogen absorption
resistance). However, a pure titanium material or a titanium alloy
material tends to absorb hydrogen and embrittle as the
characteristic of the material. As a common method for suppressing
the hydrogen absorption, there is a method for forming a titanium
oxide layer on the surface. However, the titanium oxide layer is an
insulation layer. For this reason, in accordance with this method,
the electrical conductivity is reduced.
SUMMARY OF THE INVENTION
[0015] In view of the foregoing problem, the present invention has
been completed. It is therefore an object of the present invention
to provide a surface treatment method of a titanium material for
electrodes in order to implement a titanium material for electrodes
excellent in electrical conductivity, corrosion resistance, and
hydrogen absorption resistance.
[0016] The present inventors conducted a close study in order to
solve the foregoing problem. As a result, they found out that
surface treatment of a titanium material under prescribed
conditions improves the electrical conductivity, the corrosion
resistance, and the hydrogen absorption resistance. This has led to
the completion of the present invention.
[0017] A surface treatment method of a titanium material for
electrodes in accordance with a primary aspect of the present
invention, which has solved the foregoing problem, is characterized
by including: a titanium oxide layer formation step of forming a
titanium oxide layer with a thickness of 10 nm or more and 80 nm or
less on the surface of the titanium material including pure
titanium or a titanium alloy; a noble metal layer formation step of
forming a noble metal layer with a thickness of 2 nm or more
including at least one noble metal selected from Au, Pt, and Pd on
the titanium oxide layer by a PVD method; and a heat treatment step
of heat treating the titanium material having the noble metal layer
formed thereon at a temperature of 300.degree. C. or more and
800.degree. C. or less.
[0018] Thus, by forming the titanium oxide layer with a specific
thickness on the surface of the titanium material in the titanium
oxide layer formation step, it is possible to improve the hydrogen
absorption resistance. Then, by forming the noble metal layer in
the noble metal layer formation step, it is possible to improve the
electrical conductivity while improving the corrosion resistance.
Further, by the heat treatment in the heat treatment step, the
noble metal layer is improved in adhesion, and becomes less likely
to be peeled off. Further, a part of oxygen in the titanium oxide
layer is diffused into the titanium material, resulting in an
oxygen deficiency type titanium oxide layer. This can still further
improve the electrical conductivity.
[0019] In the aspect of the present invention, it is preferable
that the titanium oxide layer formation step includes: heat
treating the titanium material at a temperature of 200.degree. C.
or more and 600.degree. C. or less in an air; subjecting the
titanium material to an anodic oxidation treatment at a voltage of
100V or less; or subjecting the titanium material to an oxidation
treatment in a passive state forming atmosphere of titanium. With
any method of these methods, it is possible to impart a higher
hydrogen absorption resistance onto the surface of the titanium
material for electrodes.
[0020] In the aspect of the present invention, it is preferable
that, prior to performing the titanium oxide layer formation step,
a pickling treatment step of subjecting the surface of the titanium
material to a pickling treatment by a solution containing a
non-oxidizing acid. By doing so, it is possible to remove the
contamination present on the surface of the titanium material and
the natural oxide layer formed ununiformly thereon. Therefore, it
is possible to form a uniform titanium oxide layer in the titanium
oxide layer formation step.
[0021] With the surface treatment method of a titanium material for
electrodes in accordance with the aspect of the present invention,
on the surface of the titanium material including pure titanium or
a titanium alloy, a titanium oxide layer is formed, and further a
noble metal layer is formed, and a heat treatment is carried out.
This can implement a titanium material for electrodes excellent in
electrical conductivity, corrosion resistance, and hydrogen
absorption resistance.
[0022] For this reason, for example, when a separator for a fuel
cell is manufactured by using a titanium material for electrodes,
which has been surface treated with the surface treatment method of
a titanium material for electrodes of the present invention, and
used for a fuel cell, the effects of being capable of keeping
excellent electrical conductivity, corrosion resistance, and
hydrogen absorption resistance for a long period are produced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] Embodiment(s) of the present invention will be described in
detail based on the following figures, wherein:
[0024] FIG. 1 is a flowchart for illustrating the flow of a surface
treatment method of a titanium material for electrode in accordance
with the present invention; and
[0025] FIG. 2 is an explanatory view for illustrating the measuring
method of the contact resistance.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Below, by reference to FIG. 1, a surface treatment method of
a titanium material for electrodes in accordance with the present
invention will be described in details. FIG. 1 is a flowchart for
illustrating the flow of the surface treatment method of a titanium
material for electrodes in accordance with the present
invention.
[0027] As shown in FIG. 1, the surface treatment method of a
titanium material for electrodes in accordance with the present
invention includes a titanium oxide layer formation step S1, a
noble metal layer formation step S2, and a heat treatment step
S3.
[0028] Below, the contents of respective steps will be
described.
[Titanium Oxide Layer Formation Step]
[0029] The titanium oxide layer formation step S1 is a step of
forming a titanium layer with a thickness of 10 nm or more and 80
nm or less on the surface of a titanium material including pure
titanium or a titanium alloy (which is hereinafter appropriately
referred to as a titanium material).
[0030] Herein, examples of pure titanium and titanium alloys may
include class-1 to class-4 pure titaniums specified according to
JIS H 4600, and titanium alloys such as Ti--Al, Ti--Ta, Ti-6Al-4V,
and Ti--Pd. Incidentally, class-1 or class-2 pure titanium
specified in JIS H 4600, and a alloys are preferred from the
viewpoints of the cost and the processability. However, pure
titaniums or titanium alloys usable in the present invention are
not limited to these. The ones having compositions equivalent to
those of the pure titaniums or equivalent to those of the titanium
alloys including other metal elements and the like can also be
preferably used.
[0031] The sheet thickness of each such a titanium material has no
particular restriction. However, for example, when the titanium
material is used for a separator for a fuel cell, the sheet
thickness is preferably set at 0.05 to 0.3 mm. This is due to the
following fact: when the sheet thickness of the titanium material
falls within such a range, the material can have the strength and
handling property as a sheet material while the material can be
relatively easily processed into a sheet with such a thickness. Of
course, it is needless to say that the sheet thickness may be set
at less than 0.05 mm, or more than 0.3 mm, if required.
[0032] The formation of the titanium oxide layer on the surface of
the titanium material can be accomplished by any of the following
methods: (1) a heat treatment in an air atmosphere is performed;
(2) an anodic oxidation treatment is carried out at a voltage of
100 V or less; and (3) an oxidization treatment is carried out in a
passive state forming atmosphere of titanium. Below, respective
methods will be described.
(Heat Treatment in Air Atmosphere)
[0033] On the surface of a titanium material including general pure
titanium or titanium alloy, a natural oxide layer with a thickness
of about 10 nm is formed by oxygen in air. However, the conditions
are ununiform. Thus, in order to impart sufficient hydrogen
absorption resistance and corrosion resistance as a titanium
material for electrodes thereto, preferably, a heat treatment is
carried out in an air atmosphere, thereby to form a uniform oxide
layer (titanium oxide layer) with a thickness of 10 nm or more.
When the thickness of the titanium oxide layer is less than 10 nm,
there is a fear that sufficient hydrogen absorption resistance and
corrosion resistance cannot be obtained. On the other hand, when
the thickness of the titanium oxide layer exceeds 80 nm, there is a
fear that sufficient electrical conductivity cannot be obtained.
The thickness of such a titanium oxide layer can be controlled by
the conditions (such as the temperature and the time) of the heat
treatment as described later. The measurement of the thickness of
the titanium oxide layer can be observed by observing the cross
section by means of, for example, a transmission electron
microscope (TEM).
[0034] The heat treatment temperature is set at 200.degree. C. or
more and 600.degree. C. or less. When the heat treatment is carried
out at a temperature of more than 600.degree. C. in an air
atmosphere, the oxidation rate becomes very high. As a result, a
too thick titanium oxide layer is formed in a short time. Thus,
even when the noble metal layer formation step S2 and the heat
treatment step S3 described later are carried out, it becomes
difficult to increase the electrical conductivity. On the other
hand, when the heat treatment temperature is less than 200.degree.
C., the titanium oxide layer is less likely to be formed. The heat
treatment temperature is preferably 250.degree. C. or more and
580.degree. C. or less, and more preferably 300.degree. C. or more
and 550.degree. C. or less. The heat treatment time is required to
be appropriately controlled according to the heat treatment
temperature. Whereas, although depending upon the heat treatment
temperature, the heat treatment time is preferably set at 1 minute
or more and five minutes or less. When the heat treatment time is
less than 1 minute, there is a fear that the thickness of the
titanium oxide layer cannot be set sufficiently large. On the other
hand, when the heat treatment time exceeds 5 minutes, the thickness
of the titanium oxide layer may become too large.
[0035] For example, the relation between the heat treatment
temperature and the heat treatment time and the thickness of the
titanium oxide layer is as follows. When the heat treatment is
carried out at 200.degree. C. for 5 minutes, a titanium oxide layer
with a thickness of about 10 nm is formed. When the heat treatment
is carried out at 300.degree. C. for 5 minutes, a titanium oxide
layer with a thickness of about 15 nm is formed. When the heat
treatment is carried out at 400.degree. C. for 5 minutes, a
titanium oxide layer with a thickness of about 20 nm is formed.
When the heat treatment is carried out at 500.degree. C. for 3
minutes, a titanium oxide layer with a thickness of about 35 nm is
formed. When the heat treatment is carried out at 600.degree. C.
for 3 minutes, a titanium oxide layer with a thickness of about 60
nm is formed. Further, when the heat treatment is carried out at a
temperature of 650.degree. C. exceeding the range of the heat
treatment temperature in the present invention for 3 minutes, a
titanium oxide layer with a thickness of about 85 nm is formed.
When the heat treatment is carried out at 700.degree. C. for 2
minutes, a titanium oxide layer with a thickness of about 90 nm is
formed. Thus, the short heat treatment time results in an increase
in thickness of the titanium oxide layer.
(Anodic Oxidation Treatment)
[0036] By subjecting a titanium material including pure titanium or
a titanium alloy to an anodic oxidation treatment, it is also
possible to form a titanium oxide layer on the surface. It has been
known that the thickness of the titanium oxide layer formed by the
anodic oxidation treatment roughly depends upon the voltage during
the treatment. When the voltage is 100 V or less, the thickness of
the titanium oxide layer can be controlled at 80 nm or less.
Therefore, the voltage during the anodic oxidation treatment is set
at 100 V or less.
[0037] As the electrolytes for use in the anodic oxidation
treatment, aqueous solutions of sulfuric acid, phosphoric acid,
acetic acid, boric acid, and the like are used alone, or in a mixed
solution thereof. The thickness of the titanium oxide layer to be
formed varies according to the combination of the type and
concentration of the electrolyte, the treatment temperature, the
voltage, and the treatment time. Therefore, the combination is
appropriately controlled so that the thickness becomes 10 nm or
more and 80 nm or less. For example, in the case where the anodic
oxidation treatment is carried out at room temperature (20 to
25.degree. C. ) using 1 mass % phosphoric acid aqueous solution as
the electrolyte, when the treatment is carried out at 20 V for 20
minutes, a titanium oxide layer with a thickness of about 15 nm is
formed; when the treatment is carried out at 50 V for 20 minutes, a
titanium oxide layer with a thickness of about 40 nm is formed; and
when the treatment is carried out at 100 V for 20 minutes, a
titanium oxide layer with a thickness of about 80 nm is formed
(Oxidation Treatment in Passive State Forming Atmosphere)
[0038] By subjecting a titanium material including pure titanium or
a titanium alloy to an oxidation treatment in a passive state
forming atmosphere, it is also possible to form a titanium oxide
layer on the surface. For example, in the case where the titanium
material is immersed in a hydrochloric acid aqueous solution, when
the pH of the solution is less than 0.5, titanium is dissolved
therein. However, at a pH of 0.5 or more, a passivation film is
formed. Such a state is referred to as a "passive state forming
atmosphere". By controlling the temperature and the time of the
oxidation treatment in a passive state forming atmosphere, it is
possible to control the thickness of the passivation film, i.e.,
the titanium oxide film. Namely, it is essential only that the
oxidation conditions are appropriately adjusted so as to implement
the thickness of the titanium oxide layer at 10 nm or more and 80
nm or less.
[0039] For example, in the case of using a hydrochloric acid
aqueous solution with a pH of 2, when the treatment is carried out
at 50.degree. C. for 240 hours, a titanium oxide layer with a
thickness of about 25 nm is formed; when the treatment is carried
out at 100.degree. C. for 240 hours, a titanium oxide layer with a
thickness of about 40 nm is formed; and when the treatment is
carried out at 150.degree. C. for 240 hours, a titanium oxide layer
with a thickness of about 70 nm is formed. Then, when the treatment
is carried out at 200.degree. C. for 240 hours, a titanium oxide
layer with a thickness of about 100 nm is formed. Thus, the
thickness of the titanium oxide layer increases.
[0040] The titanium oxide layers formed by respective methods
described above are different in crystallinity of titanium oxide
from one another according to the method. The titanium oxide layer
formed by a heat treatment in an air atmosphere includes
crystalline titanium oxide. Whereas, the titanium oxide layer
formed by an anodic oxidation treatment and an oxidation treatment
in a passive state forming atmosphere includes amorphous titanium
oxide. Thus, the following has been confirmed. The crystalline
titanium oxide is superior in corrosion resistance and hydrogen
absorption resistance. However, in the atmosphere in the fuel cell,
even amorphous titanium oxide exhibits sufficient performances.
Therefore, the titanium oxide layer formation step S1 may be
carried out with any of the foregoing methods. Further, these
methods are non-limiting, and the titanium oxide formation step S1
can also be carried out with other methods capable of forming a
titanium oxide layer on the surface of the titanium material.
[0041] Preferably, prior to carrying out the titanium oxide layer
formation step S1, a pickling treatment step S11 of subjecting the
titanium material to a pickling treatment is carried out. For
example, when the sheet thickness of the titanium material is set
as small as 0.3 mm or less, a heat treatment and rolling are
carried out until the titanium material is processed into such a
sheet thickness. Therefore, the conditions of contamination on the
titanium material surface and the conditions of the natural oxide
layer thereon often vary diversely. Also in order to form a uniform
titanium oxide layer on the titanium material surface, preferably,
after immersing the titanium material in an acid solution, and
thereby removing the contamination or the natural oxide layer on
the surface, a titanium oxide layer is formed again in the titanium
oxide layer formation step S1 as described above. As the acid
solutions for use in the pickling treatment step S11, for example,
preferred are aqueous solutions prepared by appropriately diluting
non-oxidizing acids such as hydrofluoric acid, hydrochloric acid,
and sulfuric acid, alone or mixtures of two or more thereof, and a
mixture thereof further including one or more of nitric acid and
hydrogen peroxide. For example, a mixed aqueous solution of 1 mass
% hydrofluoric acid and 5 mass % nitric acid can be used. Further,
the pickling treatment may be carried out in a single step or in
two or more divided steps.
[Noble Metal Layer Formation Step]
[0042] The noble metal layer formation step S2 is a step of forming
a noble metal layer with a thickness of 2 nm or more, including at
least one noble metal selected from Au, Pt, and Pd on the titanium
oxide layer formed by the titanium oxide layer formation step S1
with a PVD method.
[0043] It is known that, Au, Pt, and Pd are excellent in corrosion
resistance even though each will not form a passivation film on the
surface, and are excellent in electrical conductivity because they
are transition metals. Therefore, by forming a noble metal layer
including a noble metal appropriately selected from these, it
becomes possible to impart excellent corrosion resistance and
electrical conductivity to the titanium material for
electrodes.
[0044] The thickness of the noble metal layer is required to be set
at 2 nm or more. When the thickness of the noble metal layer is
less than 2 nm, the number of pinholes is too large. For this
reason, during the heat treatment step S3 described later,
oxidation of pure titanium or the titanium alloy of the titanium
material spreads from the pinholes to the underside of the noble
metal layer. Accordingly, the contact resistance is not lowered.
The preferred thickness of the noble metal layer is 3 nm or more,
and the further preferred thickness of the noble metal layer is 5
nm or more. On the other hand, the upper limit of the thickness of
the noble metal layer has no particular restriction. However, the
smaller thickness is more preferred from the viewpoint of the cost
because of the necessity of a noble metal. For example, the
thickness is preferably set at 500 nm or less.
[0045] As the method for forming a noble metal layer including at
least one noble metal selected from Au, Pt, and Pd, a PVD method
such as a sputtering method, a vacuum deposition method, or an ion
plating method is used. The PVD method can form a noble metal layer
on the surface of the titanium oxide layer formed on the titanium
material at ordinary temperatures. For this reason, the PVD method
not only can reduce damages given on the titanium material, but
also can form a noble metal layer over a relatively large area,
resulting in an improvement of the productivity.
[Heat Treatment Step]
[0046] The heat treatment step S3 is a step of heat treating the
titanium material having the titanium oxide layer and the noble
metal layer formed thereon in the foregoing manner at a temperature
of 300.degree. C. or more and 800.degree. C. or less.
[0047] Pinholes are present in the noble metal layer. The base
material exposed at the pinholes is in contact with the
environment. Therefore, oxygen is supplied thereto, and oxides are
formed. In the present invention, at the pinhole parts, the
titanium oxide layer formed by the titanium oxide layer formation
step S1 is exposed. In general, titanium oxide formed by a heat
treatment in an air atmosphere, an anodic oxidation treatment, an
oxidation treatment in a passive state forming atmosphere, or the
like is higher in corrosion resistance to the natural oxide layer.
Therefore, high corrosion resistance can also be provided even at
the titanium oxide exposed at the pinholes. Further, when the
material is heat treated in an air atmosphere as the titanium oxide
layer formation step S1, and further, the heat treatment
temperature of the heat treatment step S3 is higher than the heat
treatment temperature, oxidation of titanium further proceeds. As a
result, it is possible to form a titanium oxide layer having higher
hydrogen absorption resistance and corrosion resistance.
[0048] Further, by the heat treatment in the heat treatment step
S3, respective elements mutually diffuse in the noble metal layer
and the titanium oxide layer. Accordingly, the adhesion between
these two layers increases, and the electrical conductivity
increases. Further, the titanium oxide layer is heat treated with
supply of oxygen being cut off by the noble metal layer on the
surface. Therefore, oxygen in the titanium oxide layer diffuses
toward the titanium material, so that a part or the whole thereof
changes into titanium oxide in an oxygen deficient state (titanium
oxide having an oxygen deficient gradient structure). It is
considered as follows: the titanium oxide in the oxygen deficient
state is in the same state as with an n type semiconductor which
increases in the electrical conductivity when oxygen is short of
the stoichiometric ratio, and hence it can be improved in
electrical conductivity.
[0049] In order to obtain such an effect, it is necessary that the
heat treatment temperature in the heat treatment step S3 is set at
300.degree. C. or more and 800.degree. C. or less. When such a heat
treatment temperature is less than 300.degree. C., the elements of
the noble metal layer and the titanium oxide layer are less likely
to mutually diffuse. Thus, the improvement of the adhesion and the
improvement of the electrical conductivity are not sufficiently
carried out. On the other hand, when such a heat treatment
temperature exceeds 800.degree. C., the diffusion rate of the
elements of the noble metal layer and the titanium oxide layer is
too high. Therefore, the noble metal and titanium or a titanium
alloy mutually diffuse too much. For this reason, titanium diffuses
even to the surface of the noble metal layer, and combines with
oxygen of the atmosphere, resulting in the formation of the
naturally oxidized titanium oxide layer. Accordingly, the contact
resistance rather increases, and the electrical conductivity is
degraded. Preferred heat treatment temperature is 330.degree. C. or
more and 750.degree. C. or less. Further preferred heat treatment
temperature is 350.degree. C. or more and 700.degree. C. or less.
Incidentally, when the heat treatment is carried out for a long
time even within such a temperature range, titanium diffuses even
to the surface of the noble metal layer, and as described above,
forms a naturally oxidized titanium oxide layer. For this reason,
the heat treatment time is required to be appropriately controlled
with respect to the heat treatment temperature.
[0050] Out of the noble metals in the noble metal layer, Au or Pt
does not form an oxide film even when subjected to a heat treatment
in an air atmosphere including oxygen present therein. Therefore,
Au or Pt can provide a low contact resistance. Therefore, when Au
or Pt is used for the noble metal layer, the heat treatment can be
carried out in an air atmosphere. Thus, control of the atmosphere
is not required as with the atmosphere heat treatment using an
inert gas or a reducing gas, or a vacuum heat treatment. Therefore,
the layer can be treated in a large quantity for a short time,
which also results in advantages of a high productivity and a low
treatment cost.
[0051] On the other hand, when Pd is used for the noble metal
layer, the heat treatment is preferably carried out in an
atmosphere at 1 Pa or less. Such an atmosphere can make Pd
resistant to oxidation even with the heat treatment, and, as
described above, can allow the titanium oxide layer to include
titanium oxide in an oxygen deficient state. Accordingly, excellent
electrical conductivity can be obtained.
[0052] In the heat treatment step S3, any heat treatment furnace
such as an electric furnace or a gas furnace can be used so long as
the furnace is a heat treatment furnace capable of carrying out a
heat treatment at a heat treatment temperature of at least
300.degree. C. or more and 800.degree. C. or less, and preferably
capable of atmosphere control.
EXAMPLES
[0053] Below, Examples whereby the advantages of the present
invention have been confirmed will be described by comparison with
Comparative Examples not satisfying the requirements of the present
invention.
First Example
[0054] In First Example, the surface treatment method by a heat
treatment in an air atmosphere as the titanium oxide layer
formation step S1 was evaluated.
(Surface Treatment of Test Plate)
[0055] The test plate used was treated in the following manner.
First, a titanium material (2 cm in width.times.5 cm in
length.times.0.2 mm in thickness) formed of class 1 pure titanium
specified according to JIS H 4600 was, as the pickling treatment
step S11, subjected to ultrasonic cleaning in acetone. Then, the
titanium material was subjected to a pickling treatment in a mixed
aqueous solution of 1 mass % hydrofluoric acid and 5 mass % nitric
acid, and cleaned with pure water, and dried. Then, as the titanium
oxide layer formation step S1, a heat treatment in an air
atmosphere was carried out under the heat treatment conditions
(temperature and time) shown in Table 1. The thickness of the
titanium oxide layer formed on the heat treated test plate surface
was measured by observation under a transmission electron
microscope (TEM). The results are shown together in Table 1.
[0056] For TEM observation, each test plate was cut out, and the
cut surface was processed by means of a focused ion beam processing
apparatus (FB-2000A manufactured by Hitachi, Ltd.) to be reduced in
thickness to 100 nm or less, resulting in a test piece. The test
piece for observation was observed under the condition of an
acceleration voltage of 200 kV by means of a TEM (HF-2000 field
emission type transmission electron microscope manufactured by
Hitachi, Ltd.), thereby to measure the thickness of the titanium
oxide layer.
[0057] Then, as the noble metal layer formation step S2, on the
titanium material after formation of the titanium oxide layer, a
noble metal layer was formed by a sputtering method. The titanium
material was set on a substrate stage in a chamber of a magnetron
sputtering apparatus, and a target of a noble metal of the type
shown in Table 1 was attached on the electrode in the chamber.
Then, the inside of the chamber was evacuated to vacuum of 0.00133
Pa (1 .times.10.sup.-5 Torr) or less. Then, an argon gas was
introduced into the chamber, and the pressure was controlled so as
to be 0.266 Pa (2 .times.10.sup.-3 Torr). Then, the electrode with
the noble metal target attached thereon was applied with RF (high
frequency). As a result, an argon gas was excited to generate an
argon plasma. Thus, sputtering of the noble metal was carried out,
thereby to form a noble metal layer with the thickness shown in
Table 1 on the surface of each titanium material. Further, the
titanium material was turned upside down. Thus, in the same manner,
the same noble metal layer was also formed on the back side of the
titanium material. As the target of a noble metal, Au, Pt, or Pd
was used. However, formation thereof was carried out under the same
noble metal formation conditions in all cases.
[0058] Finally, as the heat treatment step S3, the titanium
materials each with noble metal layers formed on the opposite sides
were heat treated in the atmosphere (air, or 0.0067 Pa vacuum), at
the temperatures, and for the times shown in Table 1, resulting in
test plates 1 to 12.
[0059] Further, as Comparative Examples, test plates 13 and 14 each
obtained by forming only a noble metal layer on a titanium material
were manufactured. First, as with the test plates 1 to 12, a
titanium material (2 cm in width.times.5 cm in length.times.0.2 mm
in thickness) formed of class 1 pure titanium specified according
to JIS H 4600 was subjected to ultrasonic cleaning in acetone. The
titanium material was set on a substrate stage in the chamber of a
magnetron sputtering apparatus, and the inside of the chamber was
evacuated to 0.00133 Pa (1 .times.10.sup.-5 Torr). Then, an argon
gas was supplied to the ion gun set in the chamber, and the
pressure in the chamber was controlled to 0.0267 Pa (2
.times.10.sup.-4 Torr). Then, an argon ion beam was generated under
the conditions of a filament current of 4.0 A, a discharge voltage
of 60 V, a beam voltage of 500 V, and an acceleration voltage of
500 V. Thus, the argon ion beam was applied at an angle of
45.degree. with respect to the titanium material surface for 5
minutes. As a result, the natural oxide layer on the titanium
material surface was removed.
[0060] Then, the sputtering method was carried out under the same
conditions as those for the test plates 1 and 3 with the titanium
material set in the chamber. As a result, a noble metal layer of Au
or Pt was formed with a thickness of 10 nm on the titanium
material. Further, the titanium material was turned upside down.
Thus, removal of the natural oxide layer and formation of the noble
metal layer were carried out in the same manner, resulting in test
plates 13 and 14.
TABLE-US-00001 TABLE 1 Noble Contact resistance Thickness metal
layer (m.OMEGA. cm.sup.2) Heat treatment of specifications After
1000 h Test conditions in air titanium Noble Thick- Heat treatment
conditions from Hydrogen plate Temperature Time oxide metal ness
Temperature Time Initial immersion in concentration No. (.degree.
C.) (min) layer (nm) type (nm) Atmosphere.sup.Note (.degree. C.)
(min) value sulfuric acid (ppm) Remark 1 200 5 10 Au 10 Air 400 2
3.8 4.1 42 Example 2 300 5 13 Au 5 Vacuum 400 5 8.2 9.5 38 Example
3 300 5 13 Pt 10 Air 500 2 4.5 5.1 28 Example 4 400 5 20 Au 20
Vacuum 400 5 4.2 4.0 38 Example 5 400 5 20 Au 20 Air 400 5 3.8 4.4
31 Example 6 500 3 35 Pd 20 Vacuum 500 5 5.5 5.8 36 Example 7 600 3
60 Au 20 Vacuum 400 5 6.3 7.2 33 Example 8 600 10 75 Au 20 Vacuum
500 5 7.2 8.5 -- Example 9 400 5 20 Au 20 Vacuum 750 3 8.7 8.9 --
Example 10 500 3 35 Pt 10 Vacuum 200 5 12.4 19.2 -- Comparative
Example 11 400 5 20 Au 20 Vacuum 900 3 25.3 50.5 -- Comparative
Example 12 700 2 90 Au 20 Vacuum 500 5 37.2 38.5 -- Comparative
Example 13 -- -- -- Au 10 -- -- -- -- -- 256 Comparative Example 14
-- -- -- Pt 10 -- -- -- -- -- 328 Comparative Example
.sup.NoteVacuum = 0.0067 Pa
(Evaluation of Electrical Conductivity and Corrosion
Resistance)
[0061] The test for confirming the electrical conductivity and the
corrosion resistance were carried out in the following manner. The
test plates 1 to 12 were each measured for the contact resistance
values by measuring the initial value and the value after 1000-hour
immersion in a 80.degree. C. sulfuric acid aqueous solution (pH2),
respectively, and comparing them. The initial values and the
contact resistance values after 1000-hour immersion in a sulfuric
acid aqueous solution are shown in Table 1.
[0062] The contact resistance value of each test plate was measured
by means of a contact resistance measuring apparatus 10 shown in
FIG. 2. Incidentally, FIG. 2 is an illustrative view for
illustrating the measuring method of the contact resistance.
[0063] As shown in FIG. 2, the opposite sides of the test plate
were sandwiched between carbon cloths C, and the outsides thereof
were further applied with a load of 98 N (10 kgf) by copper
electrodes 11 each having a contact area of 1 cm.sup.2,
respectively. Thus, a current of 7.4 mA was passed therethrough by
means of a DC current source 12. Thus, the voltage applied across
the carbon cloths C was measured by means of a voltmeter 13,
thereby to calculate the contact resistance value.
[0064] It has been assumed that the acceptability criterion for the
electrical conductivity and the corrosion resistance is the one
having a contact resistance value after 1000-hour immersion in a
sulfuric acid aqueous solution of 10 m.OMEGA.cm.sup.2 or less.
[0065] As shown in Table 1, for each of the test plates 1 to 9, the
conditions for the surface treatment fall within the range of the
present invention. Therefore, the initial value of the contact
resistance is low, and the contact resistance value after 1000-hour
immersion in a sulfuric acid aqueous solution is 10
m.OMEGA.cm.sup.2 or less. This indicates that the test plates 1 to
9 are the titanium materials for electrodes having excellent
electrical conductivity and corrosion resistance (Examples).
[0066] On the other hand, for all of the test plates 10 to 12, the
initial value and the value after immersion in a sulfuric acid
aqueous solution of the contact resistance are high (Comparative
Examples). This is considered to be due to the following. As for
the test plate 10, the treatment temperature in the heat treatment
step S3 is lower than the temperature range of the present
invention. For this reason, the electrical conductivity of the
titanium oxide layer does not increase sufficiently. As for the
test plate 11, the treatment temperature in the heat treatment step
S3 is higher than the temperature range of the present invention.
For this reason, in the metal layer and the titanium material, the
elements mutually diffuse too much. As a result, titanium diffuses
to the outermost surface of the test plate to form a naturally
oxidized titanium oxide layer, resulting in a reduction of the
electrical conductivity. Whereas, for the test plate 12, the
treatment temperature in the titanium oxide layer formation step S1
(heat treatment in an air atmosphere) is higher than the
temperature range of the present invention. For this reason, the
titanium oxide layer is formed with a thickness larger than the
range of the present invention, resulting in a reduction of the
electrical conductivity.
(Evaluation of Hydrogen Absorption Resistance)
[0067] The test for confirming the hydrogen absorption resistance
was carried out by measuring the hydrogen concentration of each
test plate for the test plates 1 to 7, 13, and 14. Each test plate
was put in the vapor phase part of a closed container including
water and 0.3 MPa (3 atm) hydrogen. This was heated at 150.degree.
C. As a result, the test plate was exposed in a pure hydrogen
(purity 99.9%) atmosphere humidified to a humidity of about 100%
for 500 hours. Then, in an inert gas (Ar) flow, the test plate put
in a graphite crucible was heat fused by a graphite resistance
heating system together with tin (manufactured by KOJUNDO Chemical
Laboratory Co., Ltd.). As a result, hydrogen was extracted with
other gases. Then, the extracted gases were passed through a
separation column, so that hydrogen was separated from other gases.
The separated hydrogen was transported to a thermal conductivity
detector, and the change in the thermal conductivity by hydrogen
was measured (inert gas fusion--gas chromatography method), thereby
to measure the hydrogen concentrations of the test plates 1 to 7,
13, and 14.
[0068] The hydrogen concentrations of the test plates 1 to 7, 13,
and 14 are shown in Table 1. The acceptability criterion for the
hydrogen absorption resistance was set as the one having a hydrogen
concentration of 70 ppm or less. Incidentally, the concentration of
hydrogen contained in a general titanium material is about 25 to 35
ppm.
[0069] As shown in Table 1, the test plates 1 to 7 are low in
hydrogen concentration, indicating that the test plates 1 to 7
scarcely absorb hydrogen (Examples).
[0070] On the other hand, the test plates 13 and 14, in each of
which no titanium oxide layer is present between the noble metal
layer and the titanium material, are high in hydrogen
concentration, indicating that the test plates 13 and 14 apparently
absorb hydrogen (Comparative Examples) Therefore, there is a fear
that the test plates 13 and 14 each may be embrittled by long-term
hydrogen absorption as the titanium material for electrodes.
Second Example
[0071] In Second Example, evaluation of the surface treatment
method by an anodic oxidation treatment as the titanium oxide layer
formation layer S1 was carried out.
(Surface Treatment of Test Plate)
[0072] Each used test plate was treated in the following manner.
First, a titanium material (2 cm in width.times.5 cm in
length.times.1 mm in thickness) formed of class 1 pure titanium
specified according to JIS H 4600 was, as the pickling treatment
step S11, subjected to ultrasonic cleaning in acetone. Then, the
titanium material was subjected to a first pickling treatment in a
4 mass % hydrofluoric acid aqueous solution. Then, the titanium
material was subjected to a second pickling treatment in a mixed
aqueous solution of 0.5 mass % hydrofluoric acid and 5 mass %
hydrogen peroxide, and then cleaned with pure water, and dried.
Then, as the titanium oxide layer formation step S1, an anodic
oxidation treatment was carried out at the voltage and for the
holding time shown in Table 2 in a room-temperature 1 mass %
phosphoric acid aqueous solution. The thickness of the titanium
oxide layer formed on the test plate surface subjected to the
anodic oxidation treatment was measured by TEM observation in the
same manner as in First Example. The results are shown together in
Table 2.
[0073] Then, each titanium material after formation of the titanium
oxide layer was subjected to the noble metal layer formation step
S2 and the heat treatment step S3 under the conditions shown in
Table 2, respectively, in the same manner as in Example 1. As a
result, test plates 15 to 19 were obtained.
(Evaluation)
[0074] As for each of the test plates 15 to 19, the initial value
and the contact resistance value after 1000-hour immersion in a
sulfuric acid aqueous solution were measured, respectively, in the
same manner as in Example 1 as evaluation of the electrical
conductivity and the corrosion resistance. Further, for each of the
test plates 15 to 19, the hydrogen concentration of the test plate
after 500-hour hydrogen exposure at 150.degree. C. was measured in
the same manner as in Example 1 as evaluation of the hydrogen
absorption resistance. The contact resistance values and the
hydrogen concentrations of the test plates 15 to 19 are shown in
Table 2.
TABLE-US-00002 TABLE 2 Anodic Contact resistance oxidation Noble
metal layer (m.OMEGA. cm.sup.2) treatment Thickness specifications
After 1000 h Test conditions of titanium Noble Heat treatment
conditions from Hydrogen plate Voltage Time oxide metal Thickness
Temperature Time Initial immersion in concentration No. (V) (min)
layer (nm) type (nm) Atmosphere.sup.Note (.degree. C.) (min) value
sulfuric acid (ppm) Remark 15 20 20 15 Pt 10 Air 400 5 3.8 5.2 39
Example 16 50 20 41 Au 10 Air 400 5 4.1 5.5 33 Example 17 80 20 68
Au 10 Vacuum 400 5 6.1 6.8 32 Example 18 100 20 80 Au 20 Vacuum 400
5 8.2 8.5 28 Example 19 120 20 95 Au 20 Vacuum 400 5 12.5 14.2 28
Comparative Example .sup.NoteVacuum = 0.0067 Pa
[0075] As shown in Table 2, for each of the test plates 15 to 18,
the conditions for the anodic oxidation treatment fall within the
range of the present invention. Therefore, the initial value of the
contact resistance is low, and the contact resistance value after
1000-hour immersion in a sulfuric acid aqueous solution is 10
m.OMEGA.cm.sup.2 or less. This indicates that the test plates 15 to
18 are the titanium materials for electrodes having excellent
electrical conductivity and corrosion resistance (Examples).
[0076] On the other hand, for the test plate 19, the voltage in the
anodic oxidation treatment is higher than the temperature range of
the present invention. For this reason, the titanium oxide layer is
formed with a thickness larger than the range of the present
invention. As a result, the electrical conductivity is reduced, and
the contact resistance is more than 10 m.OMEGA.cm.sup.2 in initial
value (Comparative Example).
[0077] Further, as shown in Table 2, for all of the test plates 15
to 18, the hydrogen concentration is the value as low as less than
40 ppm. This indicates that each test plate scarcely absorbs
hydrogen even by the titanium oxide layer formed by the anodic
oxidation treatment, and becomes a titanium material for electrodes
excellent in hydrogen absorption resistance.
Third Example
[0078] In Third Example, evaluation of the surface treatment method
by a oxidation treatment in a passive state forming atmosphere as
the titanium oxide layer formation step S1 was carried out.
(Surface Treatment of Test Plate)
[0079] The test plate used was treated in the following manner.
First, a titanium material (2 cm in width.times.5 cm in
length.times.1 mm in thickness) formed of class 1 pure titanium
specified according to JIS H 4600 was, as the pickling treatment
step S11, subjected to ultrasonic cleaning in acetone. Then, the
titanium material was subjected to a pickling treatment in a mixed
aqueous solution of 1 mass % hydrofluoric acid and 5 mass % nitric
acid, and cleaned with pure water, and dried. Then, as the titanium
oxide layer formation step S1, an oxidation treatment was carried
out in a passive state forming atmosphere. In a container made of
titanium and with a volume of 0.3 L, and applied with Teflon
(registered trademark) lining, the titanium material and a 0.2 L
hydrochloric acid aqueous solution (pH2) were put, and the
container was closed. Then, the container was immersed in pure
water put in an autoclave made of titanium. Thus, the autoclave was
closed, and held at the temperature and holding time shown in Table
3. Then, the autoclave and the container made of titanium were
opened. Then, the titanium material was taken out therefrom, washed
with pure water, and dried, resulting in a titanium material
subjected to the oxidation treatment. The thickness of the titanium
oxide layer formed on the test plate surface subjected to the
oxidation treatment was measured by TEM observation in the same
manner as in First Example. The results are shown together in Table
3.
[0080] Then, each titanium material after formation of the titanium
oxide layer was subjected to the noble metal layer formation step
S2 and the heat treatment step S3 under the conditions shown in
Table 3, respectively, in the same manner as in Example 1. As a
result, test plates 20 to 23 were obtained.
(Evaluation)
[0081] As for each of the test plates 20 to 23, the initial value
and the contact resistance value after 1000-hour immersion in a
sulfuric acid aqueous solution were measured, respectively, in the
same manner as in Example 1 as evaluation of the electrical
conductivity and the corrosion resistance. Further, for each of the
test plates 20 to 23, the hydrogen concentration of the test plate
after 500-hour hydrogen exposure at 150.degree. C. was measured in
the same manner as in Example 1 as evaluation of the hydrogen
absorption resistance. The contact resistance values and the
hydrogen concentrations of the test plates 20 to 23 are shown in
Table 3.
TABLE-US-00003 TABLE 3 Conditions for oxidation treatment Noble
Contact resistance in passive Thickness metal layer (m.OMEGA.
cm.sup.2) state forming of specifications After 1000 h Test
atmosphere titanium Noble Thick- Heat treatment conditions from
Hydrogen plate Temperature Time oxide metal ness Temperature Time
Initial immersion in concentration No. (.degree. C.) (min) layer
(nm) type (nm) Atmosphere.sup.Note (.degree. C.) (min) value
sulfuric acid (ppm) Remark 20 50 240 25 Au 10 Air 400 5 4.0 4.9 38
Example 21 100 240 40 Au 10 Vacuum 400 5 5.9 7.4 35 Example 22 150
240 68 Au 10 Vacuum 400 5 8.8 9.2 30 Example 23 200 240 97 Au 20
Vacuum 400 5 11.8 13.5 27 Comparative Example .sup.NoteVacuum =
0.0067 Pa
[0082] As shown in Table 3, for each of the test plates 20 to 22,
the initial value of the contact resistance is low, and the contact
resistance value after 1000-hour immersion in a sulfuric acid
aqueous solution is 10 m.OMEGA.cm.sup.2 or less. This indicates
that the test plates 20 to 22 are the titanium materials for
electrodes having excellent electrical conductivity and corrosion
resistance (Examples).
[0083] On the other hand, for the test plate 23, with the oxidation
treatment in a passive state forming atmosphere, the titanium oxide
layer is formed with a thickness larger than the range of the
present invention. As a result, the electrical conductivity is
reduced, and the contact resistance is more than 10
m.OMEGA.cm.sup.2 in initial value (Comparative Example).
[0084] Further, as shown in Table 3, for all of the test plates 20
to 22, the hydrogen concentration is the value as low as less than
40 ppm. This indicates that each test plate scarcely absorbs
hydrogen even by the titanium oxide layer formed by the oxidation
treatment in the passive state forming atmosphere, and becomes a
titanium material for electrodes excellent in hydrogen absorption
resistance.
[0085] Up to this point, the surface treatment method of the
titanium material for electrodes in accordance with the present
invention was described specifically by way of the best mode for
carrying out the invention and Examples. However, the gist of the
present invention is not limited to the description, and must be
construed broadly based on the description of the appended claims.
Further, it is needless to say that various changes, modifications,
and the like based on the description are included in the gist of
the present invention.
* * * * *