1 6 JAN 2014
ORIGINAL
DESCRIPTION
TITLE OF INVENTION
TITANIUM ALLOY
TECHNICAL FIELD
[OOO 1 I
The present invention relates to a titanium alloy, and in particular to a titanium
alloy that exhibits high corrosion resistance, e.g., crevice corrosion resistance and acid
resistance while having good workability and economic advantages. The present
invention also relates to a titanium alloy that exhibits high corrosion resistance and good
workability with less likelihood of corrosion growth originating at defects such as flaws.
BACKGROUND ART
[0002]
Titanium has been actively utilized in fields such as the aircraft industry because
of its characteristics of being light and strong. Also, because of its high corrosion
resistance, titanium is increasingly being utilized in a variety of applications such as
construction materials for chemical plants, thermal and nuclear power plants, and
seawater desalination plants.
[0003]
However, although titanium is noted for its good corrosion resistance, the high
corrosion resistance was exhibited only in limited environments such as oxidizing acid
(nitric acid) environments and neutral chloride environments, e.g., a sea water
environment. It was not capable of exhibiting sufficient crevice corrosion resistance in
high temperature chloride environments or sufficient corrosion resistance in a
non-oxidizing acidic solution such as hydrochloric acid (hereinafter also collectively
referred to as "corrosion resistance").
[0004]
In order to solve the above-described problem, titanium alloys formed with a
platinum group metal added to titanium have been proposed, and a number of
standardized products including ASTM grade 7 and ASTM grade 17 are being used in a
variety of applications.
[OOOS]
Specifically, in the chlor-alkali industry, as a material for the anode in
electrolysis, titanium alloys are used for portions where crevice corrosion may occur
due to the use in a chlorine containing hot concentrated brine, e.g., a 20 to 30 percent
brine having a temperature of 100°C or higher.
[0006]
Also, in the nickel or lead refining industry, titanium alloys are used as a
material for reaction vessels or pipes that are exposed to a slurry containing hot
concentrated sulfuric acid solution at a temperature exceeding 100°C.
[0007]
Furthermore, in the field of heat exchangers, titanium alloys are used, for
example, in heat exchanger tubes for salt production that are exposed to a hot
concentrated brine, and heat exchanger tubes for use in incinerators for heat exchange
with the exhaust gas containing chlorine, nitrogen oxides, and sulfur oxides.
[OOOS]
In the petrochemical industry, titanium alloys are used, for example, in
desulfurization reactors that are exposed to crude oil, hydrogen sulfide, ammonium
chloride, or the like at elevated temperatures exceeding 100°C during petroleum
refining.
[0009]
As an alloy having improved corrosion resistance for the above-mentioned
applications, a Ti-O.15Pd alloy (ASTM grade 7) was developed. This titanium alloy
takes advantage of the phenomenon that Pd, included in the alloy, lowers the hydrogen
overvoltage and thus results in maintaining the spontaneous potential within the
passivation range potential. That is, deposition and buildup of Pd leached fiom the
alloy by corrosion causes lowering of hydrogen overvoltage to thereby maintain the
spontaneous potential within the passivation range potential and achieve high corrosion
resistance.
[OO 1 01
However, since ASTM grade 7 having high corrosion resistance contains Pd,
which is a platinum group metal and very expensive (2200 Japanese yen per gram
according to the morning edition of the Nihon Keizai Shimbun dated February 9,201 I),
its fields of use have been limited.
1001 I]
In order to solve this problem, a titanium alloy having a reduced Pd content of
0.03 to 0.1% by mass (ASTM grade 17) has been proposed and put into practical use as
disclosed in Patent Literature 1. Despite the reduced Pd content as compared to that of
ASTM grade 7, ASTM grade 17 exhibits high crevice corrosion resistance.
[0012]
Patent Literature 2 discloses a titanium alloy that is capable of being
manufactured at a reduced cost while its corrosion resistance is prevented from
decreasing. The titanium alloy of Patent Literature 2 contains 0.01 to 0.12% by mass
in total of at least one of platinum group metals and 5% or less by mass of at least one
of Al, Cr, Zr, Nb, Si, Sn and Mn. In typical applications, titanium alloys exhibit
adequate properties such as corrosion resistance if Pd is present in an amount of 0.01 to
0.12% by mass. However, to meet the need for further improvement in properties in
recent years, the Pd content, particularly when reduced to less than 0.05%, is not
sufficient for a titanium alloy to exhibit adequate properties such as corrosion resistance.
I
I ~ Moreover, even in typical applications, the demand for further cost savings is
increasing.
[00 1 31
Patent Literatures 3 and 4 disclose titanium alloys containing a combination of a
platinum group metal, a rare earth metal, and a transition metal, as inventions belonging
to different fields of art from that of the present invention. These inventions relate to
an ultra high vacuum chamber and a titanium alloy for use in ultra high vacuum
chambers, respectively.
[00 1 41
In these inventions, the addition of a platinum group metal and a rare earth metal
is intended to achieve the advantage of inhibiting, in ultra high vacuum environment,
the diffusion and release of the gas components forming a solid solution in the material
into the vacuum. These patent lieteratures state that the platinum group metal acts to
trap hydrogen and the rare earth element acts to trap oxygen in the titanium alloy.
[00 151
Furthermore, these inventions specify, as an essential element, a transition metal
selected from the group consisting of Co, Fe, Cr, Ni, Mn, and Cu in addition to the
platinum group metal and the rare earth metal. These patent lieteratures state that the
transition metal acts to fix the hydrogen atoms adsorbed on the surface of the vacuum
chamber by the platinum group metal. However, it is not clear whether or not the
titanium alloys of Patent Literatures 3 and 4 have corrosion resistance because there are
no disclosures or suggestions in this regard.
[00 1 61
Non-Patent Literature 1 states that Pd must be present in an amount of 0.05% or
more by mass to ensure the crevice corrosion resistance of a Ti-Pd alloy, and that
addition of Co, Ni, or V as a third element improves the crevice corrosion resistance.
[OO 171
As described above, conventional art techniques are becoming less adequate to
meet the need for hrther improvement in properties if the Pd content is below 0.05 %
by mass.
[OO 1 81
Furthermore, even a Ti-Pd alloy with a Pd content of 0.05% or more by mass
had a problem in that when defects such as flaws occur in the surface due to the service
environment, corrosion originating at the defects is likely to develop.
CITATION LIST
PATENT LITERATURE
[00 1 91
PATENT LITERATURE 1 : Japanese Patent Publication No. H04-57735
PATENT LITERATURE 2: International Publication No. W02007/077645
PATENT LITERATURE 3: Japanese Patent Application Publication No. H06-64600
PATENT LITERATURE 4: Japanese Patent Application Publication No. H06-65661
NON-PATENT LITERATURE
[0020]
Non-Patent Literature 1 : The Society of Materials Science, Committee on Corrosion and
Protection, "Low Alloy Titanium Having Good Crevice Corrosion Resistance,
SMI-ACE", September 12,2001.
SUMMARY OF INVENTION
TECHNICAL PROBLEM
[002 11
The present invention has been made in view of the foregoing problems.
Accordingly, an object of the present invention is to provide a titanium alloy having
corrosion resistance comparable to or better than that of the conventional art as well as
good workability, and also having economic advantages afforded by a reduced content
of a platinum group metal such as Pd as compared to the conventional art. Another
object of the invention is to provide a titanium alloy that has a Pd content similar to that
of the conventional art but has advantages of corrosion resistance comparable to or
better than that of the conventional art and good workability, and what is more, less
likelihood of corrosion growth originating at defects such as flaws that occurred in the
surface.
SOLUTION TO PROBLEM
[0022]
In order to achieve the above object, the present inventors have developed a
better understanding of the mechanism for improvement of the corrosion resistance of a
Ti-Pd alloy, and conducted studies on the following: enhancing the corrosion resistance
of a Ti-Pd alloy by including a non-conventional element that facilitates achievement of
desirable surface conditions for improved corrosion resistance; and achieving corrosion
resistance comparable to or better than that of the conventional art with a reduced Pd
content as compared to that of the conventional art.
[0023]
In this regard, the present invention differs from the conventional art techniques
designed to achieve enhanced corrosion resistance of a titanium alloy by
supplementarily including additional elements that are effective in improving corrosion
resistance as described in Patent Literature 2 and Non-Patent Literature 1.
[0024]
FIG. 1 is a schematic diagram illustrating a mechanism for improvement of the
6
I is in the active sate in their initial condition. When immersed in an acid solution such
I as boiling hydrochloric acid, Ti and Pd, or Ti, Pd and Co in the surface are dissolved,
I and the dissolved Pd, or the dissolved Pd and Co are deposited onto the surface and
accumulated thereon to thereby lower the hydrogen overvoltage of the entire alloy.
This allows the alloy to be held in the passivation range potential and thus exhibit good
corrosion resistance.
[0025]
In order to ensure that Pd is deposited and accumulated quickly and uniformly
on the surface after the Ti-Pd alloy has been immersed in an acid solution, the present
inventors searched for elements that facilitate dissolution of the alloy matrix that occurs
at an early stage after the immersion in the solution.
100261
The following assumptions were made. If the presence of a non-conventional
element included in the alloy causes the alloy matrix to be dissolved at an early stage
after the immersion in the acid solution, an increase in Pd ion concentration in the
solution near the outermost surface may occur and therefore an adequate amount of Pd
deposition and accumulation may be achieved rapidly ("adequate amount" herein means
a greater amount of Pd than the case where the non-conventional element is not present).
If this Pd deposition and accumulation is achieved, the hydrogen overvoltage of the
Ti-Pd alloy may decrease rapidly even when the Pd content is low and thus allow a shift
to a more noble and stable potential (passivation range potential).
[0027]
In the case of a Ti-Pd alloy with a low Pd content, rapid dissolution of the alloy
matrix may be achieved in the early-stage active state by including such
non-conventional element. If this occurs, the Pd and Ti ion concentrations near the
surface should be increased as compared to the case where such element is not included
so that deposition and accumulation of Pd occurs. Because of this, the hydrogen
overvoltage of the alloy should decrease rapidly to thereby allow the alloy to be held in
the passivation range potential.
[0028]
On the other hand, if dissolution of the alloy matrix is not facilitated in an Ti-Pd
alloy with a low Pd content, the Pd and Ti ion concentrations near the surface may not
be increased and the leached Pd may be diffused. Thus, the deposition of Pd may be
less likely to occur, which may result in poor corrosion resistance.
[0029]
In the meantime, in the case of a Ti-Pd alloy with a high Pd content, even if
surface defects such as flaws occur in its service environment, the presence of the
non-conventional element may enable rapid deposition and accumulation of Pd on the
fresh surface resulting from the defects. This should allow the hydrogen overvoltage
of the alloy to shift to the passivation range potential, and therefore should result in the
healing of the defects. Thus, the advantage of less likelihood of corrosion growth
originating at defects should be achieved.
[0030]
Based on the above assumptions, the present inventors have camed out
experiments to search for elements that facilitate dissolution of the alloy matrix that
occurs at an early stage after immersion in the solution, i.e., elements that facilitate
deposition and accumulation of Pd on the Ti-Pd alloy surface. As a result, they have
found rare earth metals are the element that satisfies the need.
[003 11
The present invention has been accomplished based on this finding, and the
summaries thereof are set forth below in items (1) to (5) relating to titanium alloys.
[0032]
(1) A titanium alloy including by mass %, a platinum group metal: 0.01 to 0.15%
and a rare earth metal: 0.00 1 to 0.10%, with the balance being Ti and impurities.
[0033]
(2) The titanium alloy according to the above item (I), wherein Co is included,
as a partial replacement for Ti, in an amount of 0.05 to 1.00% by mass, and wherein the
rare earth metal is present in an amount of 0.001 to less than 0.02% by mass.
[0034]
(3) The titanium alloy according to the above item (1) or (2), wherein the
platinum group metal is present in an amount of 0.01 to 0.05% by mass.
[0035]
(4) The titanium alloy according to any one of the above items (1) to (3),
wherein the platinum group metal is Pd.
[0036]
(5) The titanium alloy according to any one of the above items (1) to (4),
wherein the rare earth metal is Y.
[0037]
In the description below, terms "% by mass" and "ppm by mass" used in relation
to the titanium alloy composition are simply referred to as " % and "ppm," respectively,
unless otherwise noted.
ADVANTAGEOUS EFFECTS OF INVENTION
[0038]
The titanium alloy of the present invention has high corrosion resistance and
good workability. Because of this, with the use of the titanium alloy of the present
invention, it is possible to enhance performance and reliability of equipment and
machinery that are used in corrosive environments (particularly in hot concentrated
chloride environments). When the platinum group metal is included in relatively small
amounts, it provides an advantage of more economical material costs for producing
such titanium alloys. When a platinum group metal is included in relatively large
amounts, it provides an advantage of less likelihood of corrosion growth originating at
defects such as flaws that occurred in the surface.
BRIEF DESCRIPTION OF DRAWINGS
[0039]
[FIG. 11 FIG. 1 is a schematic diagram illustrating a mechanism for improvement
of the corrosion resistance of a Ti-Pd (-Co) alloy.
[FIG. 21 FIG 2 is a schematic diagram of a specimen for a crevice corrosion
resistance test, with FIG. 2(a) being a plan view and FIG. 2(b) being a side view.
[FIG. 31 FIG 3 is a schematic diagram of the specimen when used for the crevice
corrosion test (ASTM G78).
[FIG. 41 FIG. 4 is a schematic diagram of a specimen for a hot (boiling)
hydrochloric acid test, with FIG 4(a) being a plan view and FIG 4(b) being a side view.
[FIG. 51 FIG 5 is a graph illustrating the variations with time in the corrosion
rates of Comparative Example 6 and Comparative Example 7 when immersed in a
boiling 3% hydrochloric acid solution.
[FIG. 61 FIG 6 is a graph illustrating the variations with time in the corrosion
rates of Inventive Example 8, Comparative Example 5 and Conventional Example 2
when immersed in a boiling 3% hydrochloric acid solution.
[FIG. 71 FIG 7 is a graph illustrating concentration profiles, versus depth from
the surface, of Pd, Ti and 0 of the titanium alloy of Inventive Example 4.
[FIG. 81 FIG 8 is a graph illustrating concentration profiles, versus depth from
the surface, of Pd, Ti and 0 of the titanium alloy of Comparative Example 5.
[FIG. 91 FIG 9 is a graph illustrating the results of a hot (boiling) hydrochloric
acid test. In the figure, FIG. 9(a) is a graph illustrating the relationship between the
96-hour mean corrosion rate and the Y content; and FIG. 9(b) is a graph illustrating the
relationship between the surface Pd concentration after the test and the Y content.
DESCRIPTION OF EMBODIMENTS
[0040]
As described above, the titanium alloy of the present invention includes by
mass %, a platinum group metal: 0.01 to 0.15% and a rare earth metal: 0.001 to 0.10%,
with the balance being Ti and impurities. The details of the present invention are set
out below.
[004 11
1. Composition Range of Titanium Alloy and Reasons for Limitations
1 - 1. Platinum Group Metal
The platinum group metal as used herein refers to Ru, Rh, Pd, Os, Ir, and Pt.
Platinum group metals produce the advantageous effect of lowering the hydrogen
overvoltage of a titanium alloy and maintaining the spontaneous potential in the
passivation range potential, and therefore are an essential component for a titanium
alloy having corrosion resistance. The titanium alloy of the present invention includes
one or more of the platinum group metals. The total content of the one or more of the
platinum group metals (hereinafter simply referred to as "content of the platinum group
metals") is in the range of 0.01 to 0.15%. This is because if the content of platinum
group metals is less than 0.01%, the alloy exhibits inadequate corrosion resistance and
thus may suffer corrosion attack in a hot concentrated chloride solution. Meanwhile, a
content of platinum group metals exceeding 0.15% does not offer any further
improvement in corrosion resistance while requiring an enormous material cost.
[0042]
For use in conventional applications, the content of platinum group metals
preferably ranges from 0.01 to 0.05% in light of balance between the economic
advantage and corrosion resistance. This is because, even with this range of platinum
group metal content, the titanium alloy of the present invention exhibits corrosion
resistance comparable to that of conventional titanium alloys having a platinum group
metal content higher than 0.05%.
[0043]
In the meantime, when flaws or the like occur in a titanium alloy, the higher the
content of the platinum group metals, the more rapidly deposition and accumulation of
the platinum group metals progresses in the fresh surface resulting from the flaws or the
like as described above taking a Ti-Pd alloy as an example. That is, the higher the
content of the platinum group metals, the more rapidly the potential at the site of flaw
(or the like) initiation shifts to the passivation range potential to allow restoration of the
surface, which results in less likelihood of corrosion attack originating at the flaws or
the like. Thus, even when a platinum group metal is contained in the range of 0.05 to
0.15%, there is also a benefit in terms of suitability for use in severe service
environments.
[0044]
In the present invention, Pd is most preferred among the platinum group metals,
Ru, Rh, Pd, Os, Ir, and Pt because Pd is relatively inexpensive and capable of providing
high degree of improvement in corrosion resistance per amount. Rh and Pt are
economically disadvantageous because they are very expensive. Ru and Ir are
somewhat less expensive than Pd and may be used as substitutes for Pd. However,
their output is not as high as that of Pd, and therefore Pd, which is stably available, is
preferred.
[0045]
1-2. Rare Earth Metal
1-2- 1. Reasons for Inclusion of Rare Earth Metal
The present inventors have studied the possibility of forming a Ti-0.02Pd alloy
by including therein a trace amount of an element that is readily soluble in hot
concentrated chloride environments. To discover the effect produced by such element,
they conducted research by immersing a titanium alloy formed with a possibly effective
element in a chloride solution and having them dissolved in the activation potential, and
examined the effect of shifting the entire alloy to the passivation range potential by
facilitating deposition and accumulation of a platinum group metal on the surface. As
a result of research on a variety of elements, rare earth elements were found to be
capable of producing this effect.
[0046]
As described above, the content of a platinum group metal is preferably in the
range of 0.01 to 0.05%. After further research, they have found that the same effect
can be produced when the platinum group metal content is greater than 0.05%. That is,
if a rare earth metal is included in a platinum group metal-containing titanium alloy
having a platinum group metal content greater than 0.05% as with the case of the
platinum group metal-containing alloy having a platinum group metal content of 0.01 to
0.05%, rapid dissolution of Ti and the platinum group metal occurs at an early stage
after being exposed to a corrosive environment. Thus, the platinum group metal ion
concentration near the outermost surface of the titanium alloy is increased to thereby
allow rapid deposition and accumulation of the platinum group metal on the surface of
the titanium alloy. As such, a platinum group metal-containing titanium alloy formed
with a rare earth metal is capable of causing deposition of a platinum group metal on the
surface more efficiently than a platinum group metal-containing titanium alloy that does
not contain a rare earth metal. Therefore it exhibits high corrosion resistance by
allowing efficient deposition of a platinum group metal even if the amount of corrosion
of the entire titanium alloy is small. Furthermore, a platinum group metal-containing
titanium alloy formed with a rare earth metal is capable of maintaining its corrosion
resistance even in environments more severe than conventionally experienced. For
example, when used in a plant or the like that uses a hot concentrated chloride solution,
even if a platinum group metal deposited on the surface are removed due to wear or the
like, or even if surface defects such as flaws occur as described above, this titanium
alloy is capable of restoring the surface by allowing rapid deposition and accumulation
of the platinum group metal, and therefore maintaining its corrosion resistance.
[0047]
Rare earth metals include Sc, Y, light rare earth elements (La to Eu), and heavy
rare earth elements (Gd to Lu). According to the results of the studies by the present
inventors, all the rare earth metals were found to be effective. Furthermore, it is not
required that only one of the rare earth metals be included. Use of a mixture of rare
earth metals such as mixed rare earth metals before separation and refinement (misch
metal, hereinafter also referred to as "Mm") or a didymium (a mixture of Nd and Pr)
were also found to be effective. Therefore preferred rare earth metals from the
economic standpoint are La, Ce, Nd, Pr, Sm, Mm, didymium, Y, and the like for their
availability and relative inexpensiveness. As for the compositions of Mm and
didymium, any composition ratios are applicable as long as commercially available
materials are used.
[0048]
1-2-2. Content of Rare Earth Metal
In the titanium alloy of the present invention, the content of rare earth metals
ranges from 0.001 to 0.10%. The reason for the lower limit of 0.001% of the rare earth
metal content is to sufficiently produce the advantageous effect of facilitating deposition
of Pd on the alloy surface by making sure that Ti, Pd, and a rare earth metal are
dissolved simultaneously in a chloride solution in the activation potential of the Ti-Pd
alloy.
[0049]
The reason for the upper limit of 0.10% of the rare earth metal content is that an
excessively high amount of rare earth metal in a Ti-Pd alloy can produce a new
compound within the Ti alloy. This new compound preferentially dissolves in a
chloride solution, and therefore leads to initiation of pitting corrosion in the Ti-Pd alloy.
Because of this, Ti-Pd alloys having this compound exhibit inferior corrosion resistance
as compared to Ti-Pd alloys containing no rare earth metals. Furthermore, it is
preferred that the rare earth metal content in a Ti-Pd alloy be not more than its solid
solubility limit in a-Ti as shown in a phase diagram or the like.
[0050]
For example, the solid solubility limit of Y in a-Ti of a Ti-0.02Pd alloy is 0.02%
by mass (0.01 at %). Therefore, when Y is included, its content is preferably less than
0.02% by mass.
[005 11
The Y content of less than 0.02% is sufficient in terms of facilitating
accumulation of a platinum group metal on the titanium alloy surface while greater
advantages are achieved if the Y content is limited to 0.01% or less.
[0052]
La has a very large solubility limit, in a-Ti of a Ti-0.02Pd alloy, at 2.84% by
mass (1 at %) (T.B.Massalski, "Binary Alloy Phase Diagrams Volume 3," the United
States, Second Edition, ASM International, 1990, pg. 2432). However, in terms of
ensuring economic advantages, La, when included, is contained in an amount of 0.10%
or less by mass.
[0053]
As is the case with Y, a sufficient content of La is less than 0.02% in terms of
facilitating accumulation of platinum group metals on the titanium alloy surface while
greater advantages are achieved if its content is limited to 0.0 1% or less.
[0054]
1-3. Addition of Co in combination with Rare Earth Metal
The titanium alloy of the present invention may include Co, as a partial
replacement for Ti, in an amount of 0.05 to 1%. Co is an element that enhances
crevice corrosion resistance of a titanium alloy. The present inventors have found that
including Co as a partial replacement for Ti, in a platinum group metal-containing
titanium alloy formed with a rare earth metal, results in higher corrosion resistance due
to the synergy with the rare earth metal.
[0055]
To produce the synergy, Co must be present in an amount of 0.05% or more. In
the meantime, if the Co content exceeds 1%, intermetallic compounds of AB5 type (A =
rare earth metal, B = Co) are produced by the rare earth metal and Co, which results in a
decrease in corrosion resistance of the titanium alloy. This is the reason for specifying
the Co content of 0.05 to 1%.
[0056]
1-4. Ni, Mo, V, Cr and W
The titanium alloy of the present invention may include Ni, Mo, V, Cr, and W as
partial replacements for Ti. Including these elements results in high crevice corrosion
resistance due to the synergy with the rare earth metal. When these elements are
included, their contents are, Ni: 1.0% or less, Mo; 0.5% or less, V: 0.5% or less, Cr:
0.5% or less, and W: 0.5% or less.
[0057]
1-5. Impurity Elements
Impurity elements in a titanium alloy include, by way of example, Fe, 0, C, H,
N, and the like entering from raw materials, a dissolving electrode and the environment
as well as Al, Cr, Zr, Nb, Si, Sn, Mn, Cu, and the like introduced when scraps or the like
are used as materials. Introduction of these impurity elements is of no matter as long
as it does not adversely affect the advantages of the present invention. Specifically, the
compositional range not adversely affecting the advantages of the present invention is as
follows, Fe: 0.3% or less, 0: 0.35% or less, C: 0.18% or less, H: 0.015% or less, N:
0.03% or less, Al: 0.3% or less, Cr: 0.2% or less, Zr: 0.2% or less, Nb: 0.2% or less, Si:
0.02% or less, Sn: 0.2% or less, Mn: 0.01% or less, and Cu: 0.1% or less, with the total
of these being 0.6% or less.
Example 1
[0058]
To confirm the crevice corrosion resistance and hot (boiling) hydrochloric acid
resistance of the titanium alloys of the present invention, the following tests were
conducted and the results were evaluated.
[0059]
1. Test Conditions
1 - 1. Samples
1 - 1 - 1. Titanium Alloys of Conventional Examples
The titanium alloys of Conventional Examples 1 to 3 were prepared from
commercially available 4 mm thick sheets of Ti-Pd alloy purchased from a market.
Types and analysis values of the elemental compositions of the purchased materials are
shown in Table 1. Conventional Example 1 is ASTM grade 7; Conventional Example
2 is ASTM grade 17; and Conventional Example3 is ASTM grade 19. Conventional
Examples 4 and 5 are Ti-Pd alloys having a Pd content close to the lower limit of the
range disclosed in Patent Literature 1. Conventional Examples 1 to 5 are all an
example of a Ti-Pd alloy containing no rare earth metal. Conventional Examples 1 and
2 serve as benchmarks for the inventive examples that are discussed later.
[0060]
[Table 11
TABLE 1
Remarks
Classification
Inventive
Example 1
Inventive
Example 2
Inventive
Example 3
Inventive
Example 4
Inventive
Example 5
Alloy Composition
(mass %, balance being Ti and impurities)
R,
Earth
Metal
Y0.02
Y0.02
Y0.02
Y0.02
Y0.02
0
-
-
-
-
-
A1
-
-
-
-
-
platinum
Group
Metal
Pd:O. 15
Pd:O. 1 1
Pd:0.05
Pd:0.02
Pd:O.Ol
Co
-
-
-
-
-
Fe
-
-
-
-
-
TABLE 1 -Continued
Classification
Inventive
Example 10
Inventive
Example 11
Inventive
Example 12
Inventive
Example 13
Inventive
Example 14
Remarks
Alloy Composition
(mass %, balance being Ti and impurities)
Co content: outside the
R~~~
Earth
Metal
Y:0.10
Dy:O. 10
La:0.08
Didymiu
m:0.04
Pr:0.03
Comparative
Example 5
Comparative
Example 6
Comparative
Example 7
platinum
Group
Metal
Pd:0.03
Pd:0.03
Pd:0.03
Pd:0.03
Pd:0.03
-
YO.01
-
Co
-
-
-
-
-
Pd:0.02
-
-
Fe
-
-
-
-
-
-
-
-
0
-
-
-
-
-
-
-
-
A l
-
-
-
-
-
-
-
-
-
-
-
NOREM
NoPGM
JIS Class 1 Ti
TABLE 1 -Continued
[0061]
1-1-2. Samples of Inventive Examples and Comparative Examples
The titanium alloys of the inventive examples and comparative examples were
prepared using sheet materials having elemental compositions as shown in Table 1.
[0062]
1 - 1-2- 1. Materials of the Samples
Titanium alloys of the inventive examples and comparative examples were
prepared using, as materials, commercially available industrial pure titanium sponge
(JIS class I), a palladium (Pd) powder manufactured by KISHIDA CHEMICAL Co.,
Ltd. (99.9% pure), a ruthenium (Ru) powder manufactured by KISHIDA CHEMICAL
Co., Ltd. (99.9% pure), yttrium (Y) chips manufactured by KISHIDA CHEMICAL Co.,
Ltd. (99.9% pure), a rare earth metal ingot, and an electrolytic cobalt (Co) ingot (99.8%
pure). The rare earth metals used were Mm, La, Nd, Ce, Dy, Pr, Sm and didymium, all
of which, except Mm and didymium, were 99% pure. Mm is composed of La: 28.6%,
Ce: 48.8%, Pr: 6.4%, and Nd: 16.2%, and didymium is composed of Nd: 70.1% and Pr:
29.9%.
[0063]
The titanium alloys of Inventive Examples 1 to 18 all have a composition
Classification
Comparative
Example 8
Conventional
Example 1
Conventional
Example 2
Conventional
Example 3
Conventional
Example 4
Conventional
Example 5
Remarks
PGM: RU
ASTM grade 7
ASTM grade 17
ASTM grade 19
Patent Literature 1
Patent Literature 2
(Example 4)
Alloy Composition
(mass %, balance being Ti and impurities)
R~~~
Earth
Metal
-
-
-
-
-
-
platinum
Group
Metal
Ru:0.04
Pd:0.14
Pd:0.06
Pd:0.06
Pd:0.03
Pd:0.02
Co
-
-
<0.0,1
0.3 1
-
-
Fe
-
0.073
0.036
0.042
0.08
0
-
0.109
0.07
0.103
0.07
0'102
A l
-
-
-
-
-
specified by the present invention. Among these, Inventive Examples 6, 7, 17 and 18
contain a rare earth metal, Pd and Co, Inventive Example 19 contains Y and Ru without
containing a platinum group metal, and the other inventive examples contain a rare earth
metal and Pd with no further compositional elements. In Table 1, the symbol "-"
indicates that the element was below detection limits.
[0064]
The titanium alloys of Comparative Examples 1 to 8 all have a composition
outside the range specified by the present invention. Comparative Examples 1 and 2
each contain Y and Pd. Comparative Example 1 has a Y content higher than the range
specified by the present invention, and Comparative Example 2 has a Y content lower
than the range of the present invention. Comparative Example 3 contains Y and Pd,
and its Pd content is lower than the range specified by the present invention.
Comparative Example 4 contains La, Pd, and Co, and its Co content is higher than the
range specified by the present invention. Comparative Examples 5 to 8 each contain
only one of a rare earth metal and a platinum group metal, or contain neither of them.
Among these, Comparative Example 7 is made of JIS Class 1 titanium.
[0065]
In Table 1, Inventive Example 4, Comparative Example 3, Comparative
Example 5, and Comparative Example 8 are listed in duplicate for ease of comparison.
[0066]
1-1 -2-2. Process for Preparation of Sample
Using an arc melting furnace under argon atmosphere, five ingots, made of the
above-mentioned materials, 80 grams each, were melted. Then all the five ingots were
combined and remelted to prepare a square ingot with a thickness of 15 mm. The
finished square ingot was remelted for homogenization and again formed into a square
ingot with a thickness of 15 mm. That is, three stages of melting were performed in
total.
[0067]
Since the square ingots of all examples contain trace quantities of Pd andlor a
rare earth metal, a heat treatment for homogenization was applied to reduce segregation
of the elements under the following conditions:
Atmosphere: vacuum ( 4o -t~or r);
Temperature: 1 1 OO°C; and
Time: 24 hours.
[0068]
The square ingots subjected to homogenization heat treatment were rolled under
the following conditions and formed into sheet materials with a thickness of 4 mm:
p phase hot rolling: at 1 OOO°C, thickness reduced from 15 mm to 9 mm; and
a + p phase hot rolling: at 875OC, thickness reduced from 9 mm to 4 mm.
[0069]
The sheet materials obtained from the rolling were stress relief annealed in a
vacuum at 750°C for 30 minutes.
[0070]
1-2. Test Conditions
Crevice corrosion resistance tests and hot (boiling) hydrochloric acid tests were
conducted using specimens taken from the sheet materials purchased from a market or
prepared by the above described process.
1-2-1. Crevice Corrosion Resistance Test
FIG. 2 is a schematic diagram of a specimen for a crevice corrosion resistance
test, with FIG. 2(a) being a plan view and FIG. 2(b) being a side view. A specimen
having a thickness of 3mm, a width of 30 mm, and a length of 30 mm, as shown in the
figure, was cut from the sheet material, and provided with a bore having a diameter of 7
mm in its center. This specimen was polished by 600 grit emery paper.
[007 11
FIG. 3 is a schematic diagram of the specimen when used for the crevice
corrosion test. The specimen polished with emery paper as shown in the figure was
used for a crevice corrosion test in accordance with the multiple crevice test of the
ASTM G78 specification. The specimen 1 was held, at both sides thereof, by multiple
crevice assemblies 2 pressed thereto and tightened to a torque of 10 kgf-cm using a bolt
3 and a nut 4 made of pure titanium. The multiple crevice assemblies 2 were made of
polytrifluoroethylene. They were placed such that their grooved surfaces were in
contact with the specimen 1.
[0072]
The crevice corrosion test was conducted under the following conditions:
Test Environment: 250glL NaC1, pH = 2 (pH adjusted with HCl), 150°C,
saturated atmosphere; and
Test Time: 240 hours.
[0073]
1-2-2. Hot (Boiling) Hydrochloric Acid Test
FIG. 4 is a schematic diagram of a specimen for a hot (boiling) hydrochloric acid
test, with FIG. 4(a) being a plan view and FIG. 4(b) being a side view. A specimen
having a coin shape, with a thickness of 2 mm and a diameter of 15 mm, as shown in
the figure, was cut from the sheet material. This specimen was polished by use of 600
grit emery paper. After the specimen was immersed in hot hydrochloric acid under the
following conditions, the amount of corrosion (corrosion rate) per unit time was
calculated from the reduced mass resulting from corrosion.
[0074]
The hot (boiling) hydrochloric acid test, which is a corrosion test that simulates
the crevice internal environment in crevice corrosion, was conducted under the
following conditions. The boiling test vessel was provided with a coiled condenser for
cooling and condensing hot vapor back into a liquid to make sure that the concentration
of the solution does not change:
Concentration and temperature of the solution: 3% hydrochloric acid (boiling);
pH of the solution: pH = 0 (normal temperature); and
Immersion time: 96 hours.
[0075]
1-2-3. Investigation into Variation in Pd Concentration near Titanium Alloy
Surface
As described above, a rare earth metal included in a Ti-Pd alloy facilitates
dissolution of the alloy matrix in a hot concentrated chloride solution environment.
This facilitates deposition of Pd on the titanium alloy surface to produce the
advantageous effect of shifting the entire alloy to the passivation range potential. Thus,
it is assumed that, after the crevice corrosion test, the titanium alloy containing a rare
earth metal has a higher Pd concentration on its surface than a titanium alloy containing
no rare earth metal. To verify this assumption, the specimens after the 96 hour hot
(boiling) hydrochloric acid test were examined as to the variation in Pd concentration
versus depth from the outermost surface.
[0076]
The examination of the Pd concentration was carried out under the following
conditions:
Analysis Method: Marcus type RF Glow Discharge Optical Emission
Spectroscopy (hereinafter referred to as "GDOES");
Analyzer: HORIBA GD-Profiler 2;
Site Analyzed: 4 rnm diameter specimen surface area that was in contact with
boiling hydrochloric acid; and
Depth: Region up to 250 nm depth from the outermost surface.
[0077]
2. Test Results
Evaluation was made on the number of crevice sites attacked by corrosion, the
mean corrosion rate, and the economic advantage as well as evaluation based on all
these factors together. The results are shown in Table 2.
[0078]
[Table 21
TABLE 2
TABLE 2-Continued
Economic
Advantage
(Material
Cost
Considered)
* 2
0
0
0
0
0
0
0
Classification
Inventive
Example 6
Inventive
Example 7
(Comparative
Example 1)
(Inventive
Example 4)
Inventive
Example 8
Inventive
Example 9
(Comparative
Example 2)
Crevice Corrosion
Resistance
Number of crevice
Sites Attacked by
Corrosion * 1
0
0
8
0
0
0
15
Hot (Boiling) Hydrochloric Acid
Resistance
Inventive
Example 14
Inventive
Example 15
Inventive
Example 16
Inventive
Example 17
Inventive
Example 18
I
First 7-hour
mean corrosion
rate [mmlyear]
2.22
2.38
6.12
3.98
4.40
4.78
15.39
96-hour mean
C O ~ O S r~atOe ~
[mmlyear]
0.13
0.17
1.74
0.19
0.27
0.29
1.90
0
0
0
0
0
3.49
3.8 1
3.91
2.9 1
3.09
0.2 1
0.22
0.24
0.18
0.19
P
0
0
0
0
0
TABLE 2-Continued
*I Crevice corrosion resistance: evaluated based on the number of crevice sites attacked by
corrosion (the number of crevice corrosion sites of all 40 crevice sites)
*2 Economic advantage: symbol "0" is assigned for Pd content of less than 0.05% or Ru content of
0.04%, and symbol "A" is assigned for Pd content of 0.05 to 0.1 5%
Classification
Inventive
Example 19
(Comparative
Example 8)
Comparative
Example 1
Comparative
Example 2
Comparative
Example 3
Comparative
Example 4
Comparative
Example 5
Comparative
Example 6
Comparative
Example 7
Comparative
Example 8
Conventional
Example 1
Conventional
Example 2
Conventional
Example 3
Conventional
Example 4
Conventional
Example 5
Crevice Corrosion
Resistance
Number of Crevice
Sites Attacked by
Corrosion * 1
0
11
8
15
28
1
20
40
40
11
0
0
0
7
3
Economic
Advantage
(Material
Cost
Considered)
* 2
0
0
0
0
0
0
0
0
0
0
A
A
A
0
0
Hot (Boiling) Hydrochloric Acid
Resistance
First 7-hour
mean corrosion
rate [mmlyear]
4.12
8.35
6.12
15.39
9.14
4.82
9.54
16.20
4.10
8.35
0.21
4.17
3.02
5.38
6.86
96-hour mean
corrosion rate
[mm/year]
0.28
1.82
1.74
1.90
3.87
1.11
0.70
16.60
4.12
1.82
0.04
0.37
0.20
1.68
1.93
2- 1. Crevice Corrosion Resistance
Table 2 includes evaluation of the crevice corrosion resistance indicated by the
number of sites attacked by corrosion among 40 crevice sites formed by the multiple
crevice assemblies. After the tests conducted under the above conditions, none of the
inventive examples (Inventive Examples 1 to 19) and none of Conventional Examples 1
to 3 suffered corrosion attack in any of the 40 crevice sites. Among these examples,
Inventive Examples 4 to 18, with a Pd content of less than 0.05%, and Inventive
Example 19, with a Ru content of 0.04%, have an economic advantage.
[0080]
Meanwhile, all the comparative examples (Comparative Examples 1 to 8) and
Conventional Examples 4 and 5 suffered corrosion attack. From the results of
Conventional Examples 1 to 5, it is seen that if a rare earth metal is not included, a Pd
content of about 0.06% is necessary to ensure the crevice corrosion resistance.
[008 11
2-2. Hot (Boiling) Hydrochloric Acid Test
Since the corrosion rate of T-.i -Pd. alloys decreases over time, evaluation in the
hot (boiling) hydrochloric acid test under the above conditions was made by the use of
two indices: the mean corrosion rate for the first 7 hours and the mean corrosion rate
during 96 hours after the start of the immersion.
[0082]
FIG. 5 and FIG. 6 are graphs illustrating the variations with time in the corrosion
rates of Comparative Examples 6 and 7, and of Inventive Example 8, Comparative
Example 5 and Conventional Example 2, respectively, when immersed in a boiling 3%
hydrochloric acid solution. From the figures and the results shown in Table 2, the
following findings (1) to (8) were obtained.
[0083]
(1) The titanium alloys of Comparative Examples 6 and 7, which do not contain
Pd, experienced corrosion growth with no decrease in the corrosion rate as shown in
FIG. 5. It is assumed that the greater mean corrosion rate of Comparative Example 6
than that of Comparative Example 7 results from the presence of Y which facilitated
dissolution of the alloy matrix.
[0084]
(2) Inventive Examples 1 to 18 had a mean corrosion rate lower than or
comparable to that of Conventional Example 2 that serves as a benchmark, both for the
first 7 hours and for the 96 hours. Specifically, Conventional Example 2 had mean
corrosion rates of 4.17 mmlyear and 0.37 mdyear for the first 7 hours and the 96 hours,
respectively, whereas Inventive Examples had mean corrosion rates of 5 mm or
lesslyear and 0.3 mm or lesslyear, respectively. Furthermore, as shown in FIG. 6,
Inventive Example 8 with a Y content of 0.01% and a Pd content of 0.02% had a mean
corrosion rate comparable to or lower than that of Conventional Example 2 with a Pd
content of 0.06%. From FIG. 6, it is also seen that when Y is not included, a higher Pd
content leads to a smaller corrosion rate.
[0085]
(3) A comparison between the results of Inventive Example 1 with a high Pd
content of 0.15% and Conventional Example 1, as a benchmark, also with a high Pd
content of 0.14%, shows that the presence of Y results in smaller mean corrosion rates
both for the first 7 hours and for the 96 hours as well as in better hot (boiling)
hydrochloric acid resistance.
[0086]
(4) A comparison between the results of Inventive Examples 1 to 5 and
Comparative Example 3, all having the same Y content of 0.02%, shows that the higher
the Pd content, the smaller the mean corrosion rates both for the first 7 hours and for the
96 hours, and the better the hot (boiling) hydrochloric acid resistance.
[0087]
(5) A comparison between the results of Inventive Example 4, Inventive
Example 8, Inventive Example 9, Comparative Example 1, Comparative Example 2,
and Comparative Example 5, all having the same Pd content of 0.02%, shows that the
higher the Y content, the smaller the mean corrosion rates both for the first 7 hours and
for the 96 hours, and the better the hot (boiling) hydrochloric acid resistance. However,
a Y content exceeding 0.1% (Comparative Example 1) results in poorer hot (boiling)
hydrochloric acid resistance for the reason stated above. In addition, in Comparative
Example 5, the mean corrosion rate significantly decreased from 9.54 mmlyear for the
first 7 hours to 0.70 mdyear for the 96 hours. This indicates that in the absence of a
rare earth metal, deposition and accumulation of Pd requires a long time and thus its
efficiency is low.
[0088]
(6) A comparison between the results of Inventive Example 4, Inventive
Example 6, and Inventive Example 7, all having the same Y content of 0.02% and Pd
content of 0.02%, shows that the higher the Co content, the smaller the mean corrosion
rates both for the first 7 hours and for the 96 hours, and the better the hot (boiling)
hydrochloric acid resistance.
[0089]
(7) Inventive Examples 10 to 16 have a Pd content of 0.03% or less and a rare
earth metal content of 0.03 to 0.10%, with each example containing a different rare
earth metal. It is seen from these results that the presence of any rare earth metal
results in smaller mean corrosion rates both for the first 7 hours and for the 96 hours and
in better hot (boiling) hydrochloric acid resistance than Conventional Example 2. This
means that the presence of a rare earth metal facilitated dissolution of the alloy matrix
and thus increased the efficiency of deposition and accumulation of Pd. It is also
found that including Y, rather than the other rare earth metals, contributes to better hot
(boiling) hydrochloric acid resistance.
[0090]
(8) A comparison between the results of Inventive Example 19 and Comparative
Example 8, both having the same content of Ru, which is a platinum group metal, of
0.04%, shows that Inventive Example 19, which contains Y, exhibits better hot (boiling)
hydrochloric acid resistance than Comparative Example 8, which contains no rare earth
metal.
[009 11
2-3. Economic Advantage
The economic advantage shown in Table 2 is evaluation made with
consideration of raw material costs, in which the Pd contents of less than 0.05% and the
Ru content of 0.04% are assigned the symbol "0" (good) and the Pd contents of 0.05 to
0.1 5% are assigned the symbol "A" (fair).
[0092]
As shown in Table 2, Inventive Examples 4 to 19 provide an economic
a
advantage, and exhibit high crevice corrosion resistance and hot (boiling) hydrochloric
acid resistance. Inventive Examples 1 to 3 were subjected to the hot (boiling)
hydrochloric acid test under the above conditions after being provided with flaws on the
surface. The results of the test confirm that they suffered no corrosion growth
originating at the flaws and thus exhibit very high corrosion resistance. It is also
confirmed that the titanium alloys of the inventive examples all have workability
comparable to that of pure titanium of Comparative Example 7.
[0093]
2-4. Investigation into Variation in Pd Concentration near Titanium Alloy
Surface
Investigation into variation in Pd concentration near the titanium alloy surface
was conducted for Inventive Example 8 and Comparative Example 5. Inventive
Example 8 and Comparative Example 5 have the same Pd content of 0.02% while
Inventive Example 8 contains Y and Comparative Example 5 does not. As described
above, using the specimens after the hot (boiling) hydrochloric acid test as the samples,
the surfaces of the specimens were examined as to the concentration profiles, versus
depth from the surface, of Pd, Ti and 0 using the GDOES method.
[0094]
FIG. 7 and FIG. 8 are graphs illustrating concentration profiles, versus depth
from the surface, of Pd, Ti and 0 of the titanium alloys of Inventive Example 8 and
Comparative Example 5, respectively. In the figures, the concentration of each
element is indicated by the intensity measured by the GDOES method.
[0095]
As seen from FIG. 7, in the titanium alloy of Inventive Example 8 containing Y,
a peak was observed indicating an accumulation of Pd near the surface. On the other
hand, as seen from FIG. 8, no peak of Pd was observed for the titanium alloy of
Comparative Example 5 that does not contain Y. From these observations, the
following findings (1) and (2) were obtained.
[0096]
(1) It is presumed that the presence of Y allows rapid dissolution of Ti and Pd at
an early stage after exposure to a corrosive environment compared to the case where Y
is not included, which results in an increased Pd ion concentration in hot hydrochloric
acid near the outermost surface of the titanium alloy. Thus, deposition and
accumulation of Pd on the surface of the titanium alloy progresses rapidly to thereby
allow the titanium alloy as a whole to shift to the passivation potential within a short
period of time. Accordingly, a titanium alloy formed with a platinum group metal and
a rare earth metal is believed to exhibit better hot (boiling) hydrochloric acid resistance
than a titanium alloy formed with a platinum group metal but not containing a rare earth
metal.
[0097]
(2) A comparison of the depth profiles of the Ti concentrations reveals the
following. In the titanium alloy of Inventive Example 4, the alloy matrix composition
(nearly 100% titanium) substantially resides immediately under the 0 and Pd
accumulation layer of the surface throughout the entire alloy, except for a region up to a
depth of 120 nm from the surface. This indicates that accumulation of Pd near the
surface causes the titanium alloy as a whole to shift to a noble potential where the
passivation of the surface is stably maintained. In contrast, in the titanium alloy of
Comparative Example 5, the alloy matrix composition (nearly 100% titanium)
substantially resides throughout the entire alloy except for a region up to a depth of 250
nm from the surface. This indicates that corrosion has developed inward from the
surface in the depth direction.
Example 2
[0098]
In Example 2, regarding the rare earth metal content of less than 0.02%, hrther
detailed examinations were conducted for the crevice corrosion resistance and hot
(boiling) hydrochloric acid resistance.
[0099]
1. Test Conditions
1 - 1. Samples
The elemental compositions of the titanium alloys of the inventive examples and
the comparative examples, used in Example 2, are listed in Table 3. Among these, the
alloys of Inventive Example 8, Comparative Example 2, and Comparative Example 5
were also used in Example 1.
[O 1 001
[Table 31
TABLE 3
[Ol 011
The titanium alloys of Inventive Examples 8, and 20 to 27 all have a
composition specified by the present invention. Among these, Inventive Example 25
Remarks
No REM
REM: outside the
specified range
Classification
Comparative
Example 5
Comparative
Example 2
Inventive
Example 20
Inventive
Example 2 1
Inventive
Example 22
Inventive
Example 8
Inventive
Example 23
Inventive
Example 24
Inventive
Example 25
Inventive
Example 26
Inventive
Example 27
contains Mm and Pd with no further compositional elements, Inventive Example 26
contains Y, Pd, and Co, Inventive Example 27 contains Y, Pd, and Ru, and the other
inventive examples contain Y and Pd with no further compositional elements.
[O 1021
The titanium alloys of Comparative Examples 2 and 5 both have a composition
specified by the present invention. Comparative Example 2 contains Y and Pd with no
Alloy Composition
impurities)
Co
-
-
-
-
-
-
-
-
-
0.5
-
(mass %,
'
Rare Earth
Metal
-
Y:4ppm
Y:l lppm
Y:2 1 ppm
Y40ppm
Y: 1 OOppm
Y: 190ppm
Y:290ppm
Mm:100ppm
Y5Oppm
Y:40ppm
balance being Ti and
Platinum Group
Metal
Pd:0.02
Pd:0.02
Pd:0.02
Pd:0.02
Pd:0.02
Pd:0.02
Pd:0.02
Pd:0.02
Pd:0.02
Pd:0.02
Pd:O.O I ,
Ru:0.03
further compositional elements, and Comparative Example 5 contains Pd without
containing Y. In Table 3, the symbol "-" indicates that the element was below
detection limits.
[0 1031
Comparative Examples 5 and 2 as well as Inventive Examples 20 to 22, 8,23,
and 24 are materials used for investigation into the effects of the content of a rare earth
metal (Y). Inventive Example 26 is a material used for investigation into the effects
produced when a transition metal is included, and Inventive Example 27 is a material
used for investigation into the effects produced by platinum group metals.
[0 1 041
All the titanium alloys used in Example 2 were prepared using the same
materials and by the same method as in Example 1.
[0 1051
1-2. Test Conditions
1-2- 1. Crevice Corrosion Resistance Test and Hot (Boiling) Hydrochloric Acid
Test
In Example 2, the crevice corrosion resistance test and the hot (boiling)
hydrochloric acid test were conducted under the same conditions as in Example 1.
[0 1 061
1-2-2. Investigation into Variation in Pd Concentration near Titanium Alloy
Surface
For the investigation into variation in Pd concentration near the titanium alloy
surface, intensities measured by the GDOES method were used in Example 1. On the
other hand, in Example 2, calibration curves of intensity versus concentration were
generated through analysis of pure Ti, ASTM grade 17 (Ti-0.06 Pd), ASTM grade 7
(Ti-0.14 Pd), and pure Pd by the GDOES method so that approximate Pd concentrations
on the titanium alloy surface can be computed. Since Ti and 0 are detected in addition
to Pd on the titanium alloy surface, in Example 2, Pd concentrations corrected such that
the total content of Ti, 0 , and Pd is 100% were used.
[0 1 071
For Comparative Example 5, Inventive Examples 20 to 22, 8,23 and 24,
GDOES analysis was performed on each of them under the same conditions as those
used in the generation of the calibration curves, and Pd concentrations on the titanium
alloy surface were computed from the newly obtained calibration curves.
[0 1081
2. Test Results
Evaluation was made on the number of crevice sites attacked by corrosion, the
mean corrosion rate, and the economic advantage as well as evaluation based on all
these factors together. The results are shown in Table 4. The alloys of the inventive
examples and comparative examples used in Example 2 were all rated as good (0)
regarding the economic advantage.
[0 1 091
[Table 41
TABLE 4
* 1 Crevice corrosion resistance: evaluated based on the number of crevice sites attacked by
C1assification
Comparative
Example 5
Comparative
Example 2
Inventive
Example 20
Inventive
Example 2 1
Inventive
Example 22
Inventive
Example 8
Inventive
Example 23
Inventive
Example 24
Inventive
Example 25
Inventive
Example 26
Inventive
Example 27
corrosion (the number of crevice corrosion sites of all 40 crevice sites)
*2 Economic advantage: symbol "0" is assigned for Pd content of less than 0.05% or Ru content of
0.04%, and symbol "A" is assigned for Pd content of 0.05 to 0.1 5%
[OllO]
2-1. Crevice Corrosion Resistance
Table 2 includes evaluation of the crevice corrosion resistance indicated by the
number of sites attacked by corrosion among 40 crevice sites formed by the multiple
crevice assemblies. After the tests conducted under the above conditions, none of the
inventive examples (Inventive Examples 8, and 20 to 27) suffered corrosion attack in
Crevice
Corrosion
Resistance
Number of
Crevice Sites
Attacked by
Corrosion * l
20
15
0
0
0
0
0
0
0
0
0
Hot (Boiling) Hydrochloric
Acid Resistance Economic
Advantage
(Material Cost
considered) *2
0
0
0
0
0
0
0
0
0
0
0
First 7-hour
mean
corrosion rate
[mmlyear]
9.54
15.39
3.21
2.14
2.0 1
4.40
3.61
3.84
4.22
2.2 1
1.02
Surface Pd
concentration
[%I
0.4
-
1.2
1.9
3.4
2
1.5
-
-
-
-
9(j-hour mean
corrosion rate
[mmlyear]
0.70
1.90
0.29
0.22
0.13
0.27
0.28
0.30
0.25
0.15
0.09
any of the 40 crevice sites. Comparative Examples 2 and 5 both suffered corrosion
attack. It is seen from these results that Y must be present in an amount of about 10
pprn in order to achieve high crevice corrosion resistance when the Pd content is 0.02%.
[Olll]
2-2. Hot (Boiling) Hydrochloric Acid Test
In Example 1, the inventive examples exhibited a low corrosion rate, with mean
corrosion rates of 5 rnmlyear for the first 7 hours and of 0.3 mmlyear for the 96 hours,
respectively. In Example 2, investigation was made into the influence of the rare earth
metal content on the 96-hour mean corrosion rate. Hot (boiling) hydrochloric acid
resistance is closely related to crevice corrosion resistance.
[0112]
FIG. 9 is a graph illustrating the results of a hot (boiling) hydrochloric acid test.
In the figure, FIG. 9(a) is a graph illustrating the relationship between the 96-hour mean
corrosion rate and the Y content; and FIG. 9(b) is a graph illustrating the relationship
between the surface Pd concentration after the test and the Y content. FIG. 9 shows
compiled results of the cases in which the Y content is varied while the Pd content is
constant at 0.02%.
[0113]
2-3. Summary of Test results
After studies of the test results of Example 2, the following findings (1) to (7)
were obtained.
[0114]
(1) The cases that satisfy the Y content of 0.001 to 0.10% specified by the
present invention exhibited a good hot (boiling) hydrochloric acid resistance of 0.30
rnmlyear, as evaluated by the 96-hour mean corrosion rate (FIG. 9(a)).
(2) It is found that a preferred Y content is in the range of 10 pprn to 200 ppm, in
which the mean corrosion rate is further decreased, and a more preferred Y content is in
the range of 20 pprn to 100 ppm.
(3) In the concentration range of the Y content of 20 pprn to 100 ppm, the
surface Pd concentration after the test was high (FIG. 9 (b)).
(4) Inventive Example 24 is a material having a Y content of 290 ppm, which is
greater than the limit of the solid solubility of Y in Ti of about 200 ppm. Inventive
Example 24 exhibited a hot (boiling) hydrochloric acid resistance of 0.30 mdyear in
terms of the 96-hour mean corrosion rate. Although this is within the range of the
present invention as shown in Example 1, it is the upper limit of the range. Inventive
Example 23 having a Y content not exceeding the solid solubility limit exhibited a
96-hour mean corrosion rate of 0.28 mrnlyear. From these results, it is preferred that
the Y content be no greater than the solid solubility limit of 200 ppm.
(5) In the case that includes a transition metal, which is an essential element of
Patent Literatures 3 and 4, a small mean corrosion rate and thus high hot (boiling)
hydrochloric acid resistance are achieved by the Y content of 50 ppm, which is no
greater than the solid solubility limit (Inventive Example 26).
(6) In the case that includes a platinum group metal other than Pd, a small mean
corrosion rate and thus high hot (boiling) hydrochloric acid resistance are also achieved
by the Y content of no greater than 200 ppm (Inventive Example 27).
(7) In the case that includes a rare earth metal other than Y (Inventive Example
25 with 100 ppm Mm), a small mean corrosion rate and thus high hot (boiling)
hydrochloric acid resistance are also achieved by the rare earth metal content of no
greater than 200 ppm.
From the facts obtained in the above experiments, it is found that titanium alloys
exhibit high corrosion resistance with the Y content of 0.001 to 0.10% as specified by
the present invention, and even higher corrosion resistance if the Y content is limited to
less than 0.02%.
INDUSTRIAL APPLICABILITY
[0116]
The titanium alloy of the present invention has high corrosion resistance and
good workability. Because of this, with the use of the titanium alloy of the present
invention, it is possible to enhance performance and reliability of equipment and
machinery that are used in corrosive environments (particularly in hot concentrated
chloride environments). When the platinum group metal is included in relatively small
amounts, the invention provides an advantage of more economical material costs for
producing such titanium alloys. When the platinum group metal is included in
relatively large amounts, the invention provides an advantage of less likelihood of
corrosion growth originating at defects such as flaws that occurred in the surface.
REFERENCE SIGNS LIST
[0117]
1: specimen, 2: multiple crevice assembly, 3: bolt, 4: nut

I
1 B6 JAN 2014
We claim:
1. A titanium alloy characterized by consisting of, by mass %, a platinum group
metal: 0.01 to 0.15% and a rare earth metal: 0.001 to 0.10%, with the balance being Ti
and impurities.
2. The titanium alloy according to claim 1, characterized in that Co is included, as
a partial replacement for Ti, in an amount of 0.05 to 1.00% by mass, and wherein the
rare earth metal is present in an amount of 0.00 1 to less than 0.02% by mass.
3. The titanium alloy according to claim 1 or 2, characterized in that the platinum
group metal is present in an amount of 0.01 to 0.05% by mass.
4. The titanium alloy according to any one of claims 1 to 3, characterized in that
the platinum group metal is Pd.
5. The titanium alloy according to any one of claims 1 to 4, characterized in that
the rare earth metal is Y.
Dated this day of January, 2014.
Sumitomo Metal Corporation
Suresh A. Shroff & Co.
Attorneys for the Applicant