U.S. patent application number 14/431865 was filed with the patent office on 2015-08-27 for titanium alloy having high corrosion resistance in bromine-ion-containing environment.
This patent application is currently assigned to NIPPON STEEL & SUMITOMO METAL CORPORATION. The applicant listed for this patent is NIPPON STEEL & SUMITOMO METAL CORPORATION. Invention is credited to Masaru Abe, Hideya Kaminaka, Hiroshi Kamio, Kouichi Takeuchi.
Application Number | 20150240332 14/431865 |
Document ID | / |
Family ID | 51227634 |
Filed Date | 2015-08-27 |
United States Patent
Application |
20150240332 |
Kind Code |
A1 |
Kaminaka; Hideya ; et
al. |
August 27, 2015 |
TITANIUM ALLOY HAVING HIGH CORROSION RESISTANCE IN
BROMINE-ION-CONTAINING ENVIRONMENT
Abstract
To provide a titanium alloy having corrosion resistance that is
as high as or higher than before, and high corrosion resistance in
a bromine-ion-containing environment, the alloy being able to be
manufactured at low cost. [Solution] Provided is a titanium alloy
to be used in a bromine-ion-containing environment, the titanium
alloy consisting of, in mass %, a platinum group element: greater
than or equal to 0.01% and less than or equal to 0.10%, a rare
earth element: greater than or equal to 0.001% and less than 0.02%,
O: greater than or equal to 0% and less than 0.1%, and the balance:
Ti and impurities. The titanium alloy may contain, instead of part
of Ti, one or more selected from the group consisting of Ni, Co,
Mo, V, Cr, and W. The platinum group element is desirably contained
in, in mass %, greater than or equal to 0.01% and less than or
equal to 0.05%. The rare earth element is desirably contained in,
in mass %, greater than or equal to 0.001% and less than 0.02%.
Inventors: |
Kaminaka; Hideya; (Tokyo,
JP) ; Kamio; Hiroshi; (Tokyo, JP) ; Abe;
Masaru; (Tokyo, JP) ; Takeuchi; Kouichi;
(Tokyo, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL & SUMITOMO METAL CORPORATION |
Tokyo |
|
JP |
|
|
Assignee: |
NIPPON STEEL & SUMITOMO METAL
CORPORATION
Tokyo
JP
|
Family ID: |
51227634 |
Appl. No.: |
14/431865 |
Filed: |
January 24, 2014 |
PCT Filed: |
January 24, 2014 |
PCT NO: |
PCT/JP2014/051550 |
371 Date: |
March 27, 2015 |
Current U.S.
Class: |
420/421 |
Current CPC
Class: |
C22C 14/00 20130101;
C22F 1/183 20130101 |
International
Class: |
C22C 14/00 20060101
C22C014/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 25, 2013 |
JP |
2013-012115 |
Claims
1-7. (canceled)
8. A titanium alloy to be used in a bromine-ion-containing
environment, the titanium alloy consisting of, in mass %, a
platinum group element: greater than or equal to 0.01% and less
than or equal to 0.10%, a rare earth element: greater than or equal
to 0.001% and less than 0.02%, O: greater than or equal to 0% and
less than 0.1%, and the balance: Ti and impurities.
9. The titanium alloy according to claim 8, wherein the titanium
alloy contains, instead of part of Ti, one or more selected from
the group consisting of Ni, Co, Mo, V, Cr, and W, the content of Ni
is less than or equal to 1.0 mass %, the content of Co is less than
or equal to 1.0 mass %, the content of Mo is less than or equal to
0.5 mass %, the content of V is less than or equal to 0.5 mass %,
the content of Cr is less than or equal to 0.5 mass %, and the
content of W is less than or equal to 0.5 mass %.
10. The titanium alloy according to claim 8, consisting of, in mass
%, the platinum group element: greater than or equal to 0.01% and
less than or equal to
0. 05%.
11. The titanium alloy according to claim 9, consisting of, in mass
%, the platinum group element: greater than or equal to 0.01% and
less than or equal to 0.05%.
12. The titanium alloy according to claim 8, wherein the platinum
group element is Ru.
13. The titanium alloy according to claim 9, wherein the platinum
group element is Ru.
14. The titanium alloy according to claim 10, wherein the platinum
group element is Ru.
15. The titanium alloy according to claim 11, wherein the platinum
group element is Ru.
16. The titanium alloy according to claim 8, wherein the rare earth
element is Y.
17. The titanium alloy according to claim 9, wherein the rare earth
element is Y.
18. The titanium alloy according to claim 10, wherein the rare
earth element is Y.
19. The titanium alloy according to claim 11, wherein the rare
earth element is Y.
20. The titanium alloy according to claim 12, wherein the rare
earth element is Y.
21. The titanium alloy according to claim 13, wherein the rare
earth element is Y.
22. The titanium alloy according to claim 14, wherein the rare
earth element is Y.
23. The titanium alloy according to claim 15, wherein the rare
earth element is Y.
24. The titanium alloy according to claim 8, wherein the content of
O is less than 0.05 mass %.
25. The titanium alloy according to claim 8, wherein the titanium
alloy is used in a chemical plant apparatus.
Description
TECHNICAL FIELD
[0001] The present invention relates to a titanium alloy,
particularly to a titanium alloy having high corrosion resistance
(crevice corrosion resistance, acid resistance, and the like in a
bromine-ion-containing environment) and high economic
efficiency.
BACKGROUND ART
[0002] Titanium is being actively used in the aircraft field and
the like, utilizing its feature of lightness and strength. Further,
having high corrosion resistance, titanium is beginning to be used
in wide range of fields as a material for chemical industry
equipment, a material for thermal and nuclear power generation
equipment, and a material for seawater desalination equipment, and
the like.
[0003] However, the environment in which titanium can exhibit its
high corrosion resistance is limited to oxidizing acid (nitric
acid) environment and neutral chloride environment such as
seawater. Titanium does not have sufficient crevice corrosion
resistance in a high-temperature chloride environment, nor
sufficient corrosion resistance in a non-oxidizing acid solution
such as hydrochloric acid (hereinafter, the crevice corrosion
resistance and the corrosion resistance are simply referred to as
"corrosion resistance" unless otherwise specified). In order to
solve this problem, a titanium alloy in which titanium contains a
platinum group element (hereinafter referred to as
"platinum-group-element-containing titanium alloy") is proposed and
normalized to be used in various usages.
[0004] Specifically, in the alkali industry field, an anode
electrode used for electrolysis is used in a high-concentration,
20% to 30% brine containing hydrochloric acid at a high temperature
of 100.degree. C. or more. In this anode electrode, a
platinum-group-element-containing titanium alloy is used for a part
where crevice corrosion may occur.
[0005] In the Ni refining industry field, a reaction vessel is
subjected to a high-concentration sulfuric acid including slurry at
a high temperature, which is higher than 100.degree. C. The
reaction vessel is made of a platinum-group-element-containing
titanium alloy.
[0006] In the heat exchanger field, a heat exchanger tube used in
the salt manufacture field is subjected to high-temperature and
high-concentration salt water, and a heat exchanger tube used for
heat exchange of an exhaust gas of a furnace is subjected to an
exhaust gas containing chorine, NO.sub.x, and SO.sub.x. These heat
exchanger tubes are made of a platinum-group-element-containing
titanium alloy.
[0007] In the petrochemical industry field, a reaction vessel or
the like of desulfurization equipment used at a time of oil
refining is subjected to high-temperature hydrogen sulfide. Such a
reaction vessel is made of a platinum-group-element-containing
titanium alloy.
[0008] Further, a platinum-group-element-containing titanium alloy
is considered to be applied to a separator material for a fuel
cell, utilizing its high corrosion resistance.
[0009] Gr. 7 ("Gr." (Grade) complies with the ASTM standard. The
same holds true in the following description.), which is a Ti-0.15
Pd alloy, is a titanium alloy that has been developed to have
corrosion resistance in the above described usages. Pd contained in
this titanium alloy reduces hydrogen overvoltage to maintain a
natural potential within a passivation area. That is, Pd eluted
from the alloy by corrosion is precipitated again on the surface of
the alloy to be deposited, and thereby the hydrogen overvoltage of
the alloy is reduced and the natural potential is maintained within
the passivation area. Accordingly, this alloy has high corrosion
resistance.
[0010] Pd contained in Gr. 7 is, however, very expensive ( 1905/g
according to the morning edition of Nihon Keizai Shimbun on Dec.
13, 2012, for example); accordingly, the fields using Pd have been
limiting.
[0011] In order to solve this problem, as disclosed in Patent
Document 1 below, a titanium alloy (Gr. 17) having a lower content
of Pd, which is 0.03 to 0.1 mass %, than Gr. 7, and also having
high crevice corrosion resistance is proposed and put into
practical use.
[0012] Patent Document 2 below discloses, as a titanium alloy that
can be manufactured at low cost while preventing a reduction in
corrosion resistance, a titanium alloy containing one or more
platinum group elements in 0.01 to 0.12 mass % in total, and one or
more of Al, Cr, Zr, Nb, Si, Sn, and Mn in 5 mass % or less in
total. In a usage at a time of development of the titanium alloy,
sufficient corrosion resistance is obtained when Pd is in a range
of 0.01 to 0.12 mass %. In a usage of recent years, however, a
further increase in characteristics is required, so that the
corrosion resistance has become unsufficient particularly when the
content of Pd is less than 0.05 mass %. Also in the usage at the
time of development, a further reduction in cost is required.
[0013] According to Non-Patent Document 1 below, however, by adding
Co, Ni, or V, as a third element, to a Ti--Pd alloy, the crevice
corrosion resistance is increased, but the content of Pd needs to
be 0.05 mass % or more in order to obtain sufficient crevice
corrosion resistance.
[0014] As for a reduction in cost, Ru, which is the most
inexpensive element in the platinum group element, has been
actively used to develop a material. Patent Document 3 below
discloses a titanium alloy to which 0.005 to 0.2 mass % Ru is
added. As shown in an example in this document, in order to obtain
sufficient crevice corrosion resistance, the addition of Ru to this
titanium alloy needs to be 0.05 mass % or more.
[0015] Patent Document 4 below discloses a material of a system in
which Ru and Ni are added in order to further increase corrosion
resistance. This material has not only crevice corrosion
resistance, but also high corrosion resistance in an environment
containing non-oxidizing acid such as sulfuric acid or hydrochloric
acid. Ti-0.06Ru-0.5Ni is an alloy having a structure within the
range shown in Patent Document 4 below and is normalized as Gr. 13
to be used practically as a corrosion-resistant titanium alloy.
However, the addition of Ni results in a problem that a Ti.sub.2Ni
compound is precipitated in the titanium alloy. Further, due to
this compound precipitation, the processability of the titanium
alloy, such as stretch, becomes inferior to that of Gr. 17.
[0016] In addition to these problems, in some cases in which a
Ti--Pd alloy is applied to a usage as an anode for electrolysis and
an inexpensive raw material (brine) is used, crevice corrosion has
occurred by bromine (bromine ions) contained in the raw material,
although crevice corrosion has been considered not to occur in a
case of using normal brine. In addition, corrosion due to bromine
(bromine ions) has sometimes occurred also in a chemical plant, for
example. Accordingly, a titanium alloy having high corrosion
resistance even in a bromine-ion-containing environment has been
demanded.
[0017] Patent Document 5 and Patent Document 6 disclose materials
to which a platinum group element, a rare earth element, and a
transition element are added. Each of these materials is, however,
a titanium alloy for an ultra-high vacuum vessel. In Patent
Document 5 and Patent Document 6, the platinum group element and
the rare earth element are added in order to obtain an effect of
preventing a phenomenon in which a gas component dissolved in a
material is dispersed and released to a vacuum in the ultra-high
vacuum. It is known that the platinum group element has a function
of trapping hydrogen and that the rare earth element has a function
of trapping oxygen in the titanium alloy. Further, in Patent
Document 5 and Patent Document 6, in addition to the platinum group
element and the rare earth element, a transition element such as
Co, Fe, Cr, Ni, Mn, or Cu is given as a necessary element. It is
known that the transition element has a role of fixing atomic
hydrogen that is adsorbed on the surface of the vacuum vessel by
the platinum group element. However, none of Patent Document 5 and
Patent Document 6 is made considering corrosion resistance, and
refers to corrosion resistance in a bromine-ion-containing
environment.
PRIOR ART DOCUMENT(S)
Patent Document(s)
[0018] [Patent Document 1] JP H4-57735B [0019] [Patent Document 2]
WO 2007/077645 [0020] [Patent Document 3] JP S62-56219B [0021]
[Patent Document 4] JP S62-20269B [0022] [Patent Document 5] JP
H6-65661A [0023] [Patent Document 6] JP H6-64600A
Non-Patent Document(s)
[0023] [0024] [Non-Patent Document 1] Hideaki MIYUKI, and one
other, "Low alloy titanium SMI-ACE with high crevice corrosion
resistance", The Society of Materials Science, Japan, Committee on
Corrosion and Protection, Sep. 12, 2001. [0025] [Non-Patent
Document 2] Chihiro TAKI, "Characteristics of corrosion-resistant
titanium alloy TICOREX and usage examples thereof", Nippon Steel
Cooperation Technical Report, 2011, Vol. 375, pp. 73-77. [0026]
[Non-Patent Document 3] Tatsuhiro OKADA, "Pitting potential of
titanium in bromide solution", DENKI KAGAKU, 1981, Vol. 49, No. 9,
pp. 584-588.
SUMMARY OF THE INVENTION
Problem(s) to Be Solved by the Invention
[0027] The present invention has been made in view of the above
problems, and aims to provide a titanium alloy having high
corrosion resistance, particularly in a bromine-ion-containing
environment.
Means for Solving the Problem(s)
[0028] Further, by obtaining a titanium alloy by adding Ru, which
is less expensive than Pd, instead of adding Pd, which is an
expensive platinum group element, the price of the titanium alloy
can be lower than before.
[0029] In order to achieve the above object, the present inventors
have studied the following. [0030] (i) Revealing a mechanism by
which corrosion resistance is expressed in a Ti--Pd alloy, and
adding an element that promotes a preferable surface state to
increase corrosion resistance. Increasing corrosion resistance also
in a case of a Ti--Ru alloy. [0031] (ii) Obtaining corrosion
resistance that is as high as or higher than before and high
corrosion resistance in a bromine-ion-existing environment by using
a platinum group element at a low content.
[0032] FIG. 1 is a schematic diagram showing a mechanism by which
corrosion resistances of a Ti--Pd alloy and a Ti--Pd--Co alloy are
expressed. The surface of the Ti--Pd alloy and the Ti--Pd--Co alloy
is active in an initial state before being immersed in a solution.
When being immersed in an acid solution such as boiling
hydrochloric acid, Ti and Pd on the surface, or Ti, Pd, and Co on
the surface are melted, and the melted Pd, or Pd and Co is/are
precipitated on the surface to be condensed. Accordingly, the
hydrogen overvoltage of the entire titanium alloy is decreased.
Thus, the potential of the titanium alloy is maintained in a
passivation area, and the titanium alloy has high corrosion
resistance.
[0033] The present inventors have studied the Ti--Ru alloy and
confirmed that the corrosion resistance of the Ti--Ru alloy is
secured by the same mechanism as in Ti--Pd. However, when Pd and Ru
in the same addition amount are compared with each other under the
same conditions, Pd has a higher effect of increasing corrosion
resistance. Therefore, it is revealed that a greater amount of Ru
needs to be added in order to obtain the same level of corrosion
resistance.
[0034] Patent Document 4 and Non-Patent Document 2 above disclose
that high corrosion resistance can be obtained by precipitating a
large amount of Ti.sub.2Ni.sub.1-xRu.sub.x (a compound including Ru
instead of part of Ni in Ti.sub.2Ni) in a titanium base material by
adding Ru and Ni without adding a large amount of Ru. However,
there is a problem that the titanium alloy to which a large amount
of Ni is added has poor processability such as stretch.
[0035] The present inventors have examined for a novel additive
element that promotes an alloy base material to be melted at an
initial stage after a Ti--Ru alloy is immersed in an acid solution
in order to enable Ru to be precipitated immediately and uniformly
on a surface to be condensed. It is considered that the addition of
such a novel additive element to the Ti--Ru alloy causes the alloy
base material to be melted in the initial stage in an active state
area after being immersed in the acid solution. Accordingly, the Ru
ion concentration is increased in the solution near the surface of
the alloy, and a sufficient amount of Ru is immediately
precipitated and condensed on the surface of the alloy so as to
make the alloy have a potential in a passivation area. Hereinafter,
such precipitation of such an amount of Ru on the surface of the
alloy is referred to as "Ru precipitation condensation". Even if
the content of Ru is low in the alloy, when the Ru precipitation
condensation occurs, it becomes possible to decrease the hydrogen
overvoltage of the
[0036] Ti--Ru alloy immediately so as to make the Ti--Ru alloy have
a potential that is more noble and stable (a potential in the
passivation area).
[0037] In the Ti--Ru alloy having a low content of Ru, when the
alloy base material is immediately melted in the initial active
state by such a novel additive element being added, the Ru ion
concentration and Ti ion concentration near the surface become
higher than in a case in which the additive element is not added.
Accordingly, the Ru precipitation condensation occurs. Thus, it can
be considered that the hydrogen overvoltage of the alloy is
immediately decreased and that the potential can be kept in the
passivation area.
[0038] On the other hand, in the Ti--Ru alloy having a high content
of Ru and containing this novel additive element, in a case in
which a damage such as a scratch is generated on the surface of the
alloy in a usage environment, the Ru precipitation condensation
propagates more immediately on a fresh surface generated by the
damage than in a case of a conventional titanium alloy.
Accordingly, it can be considered that the hydrogen overvoltage of
the alloy reaches the passivation area, and the damage will be
repaired. Therefore, corrosion starting from the damage is unlikely
to propagate.
[0039] As shown in Non-Patent Document 3, in a bromine-containing
environment, pitting or crevice corrosion is generated on pure
titanium. It has been considered that crevice corrosion may not
occur in a Ti--Pd-based titanium alloy, but crevice corrosion may
sometimes be generated in an environment of a chloride containing
bromine ions. The present inventors have intensively studied this
problem, and have found out that the resistance to corrosion caused
by bromine is increased by condensing Ru on the surface.
[0040] On the basis of such assumption and knowledge, the present
inventors have performed experiment to examine for an element that
promotes an alloy base material to be melted in an initial stage
after the alloy is immersed in a solution, that is, an element that
promotes Ru precipitation condensation on the Ti--Ru alloy surface
(the above described "novel additive element).
[0041] Accordingly, the present inventors have found out that rare
earth elements correspond to such an element, and that the
resistance to corrosion caused by bromine is further increased by a
synergetic effect of adding, in addition to Ru and a rare earth
element, one or more selected from the group consisting of Ni, Co,
Mo, Cr, V, and W Note that the description is made on the basis of
Ru; however, another platinum group element such as Pd is also
considered to increase the resistance to corrosion caused by
bromine in a similar manner.
[0042] The rare earth element itself does not have an effect of
increasing corrosion resistance of an alloy. In this light, the
rare earth element has a different function from the additive
element disclosed in each of Patent Documents 2 to 4 and Non-Patent
Document 1 above.
[0043] The usage of the alloy and the function of the element in
Patent Documents 5 and 6 are different from those in the present
invention. That is, the function of the rare earth element in
Patent Documents 5 and 6 are compared with that in the present
invention as below (the content is weight %).
[0044] Patent Documents 5 and 6: The titanium alloy has high solid
solubility of oxygen. The rare earth element is added in order to
fix oxygen as an oxide so as to prevent dissolved oxygen from being
dispersed in the alloy and to prevent its release to a vacuum
atmosphere in a gas state when being used in a high vacuum usage.
To obtain this effect, the lower limit of the rare earth element is
set to 0.02%. When the added amount exceeds 0.5%, ductility is
decreased by the precipitated oxide. Accordingly, the upper limit
of the rare earth element is set to 0.5%.
[0045] The present invention: When being immersed in an environment
of a chloride aqueous solution, the titanium alloy containing a
platinum group element is melted in an active state area, and the
platinum group element is precipitated to be condensed on the
surface, so that the potential of the alloy as a whole is shifted
to be in a passivation area (the potential becomes noble). The rare
earth element has a function of shortening the time for the
potential to become noble and a function of increasing the
condensing degree of the platinum group element on the surface. To
obtain this effect, the rare earth element is desirably in a
dissolving range of the titanium alloy. The lower limit is 0.001%
and the upper limit is 0.1% in order to obtain this effect. When
the amount exceeds 0.1%, a compound of titanium and the rare earth
element is produced, which may degrade corrosion resistance.
[0046] The role of the rare earth element in Patent Documents 5 and
6 is to react with oxygen dissolved in the titanium alloy to
produce an oxide. In contrast, in the present invention, the rare
earth element has a largely different role of promoting the
platinum group element to be condensed on the surface of the
titanium alloy in a wet corrosion environment. Further, in the
present invention, a desirable rare earth element component is
within the dissolving range, which is a content lower than that in
Patent Documents 5 and 6.
[0047] The present invention has been made on the basis of this
knowledge and provides titanium alloys as described in (1) to (7)
below.
(1)
[0048] A titanium alloy to be used in a bromine-ion-containing
environment, the titanium alloy consisting of, in mass %, a
platinum group element: greater than or equal to 0.01% and less
than or equal to 0.10%, a rare earth element: greater than or equal
to 0.001% and less than 0.02%, 0: greater than or equal to 0% and
less than 0.1%, and the balance: Ti and impurities.
(2)
[0049] The titanium alloy according to (1), wherein the titanium
alloy contains, instead of part of Ti, one or more selected from
the group consisting of Ni, Co, Mo, V, Cr, and W, the content of Ni
is less than or equal to 1.0 mass %, the content of Co is less than
or equal to 1.0 mass %, the content of Mo is less than or equal to
0.5 mass %, the content of V is less than or equal to 0.5 mass %,
the content of Cr is less than or equal to 0.5 mass %, and the
content of W is less than or equal to 0.5 mass %.
(3)
[0050] The titanium alloy according to (1) or (2), consisting of,
in mass %, the platinum group element: greater than or equal to
0.01% and less than or equal to 0.05%.
(4)
[0051] The titanium alloy according to any one of (1) to (3),
wherein the platinum group element is Ru.
(5)
[0052] The titanium alloy according to any one of (1) to (4),
wherein the rare earth element is Y.
(6)
[0053] The titanium alloy according to any one of (1) to (5),
wherein the content of O is less than 0.05 mass %.
(7)
[0054] The titanium alloy according to any one of (1) to (6),
wherein the titanium alloy is used in a chemical plant
apparatus.
Effect(s) of the Invention
[0055] The titanium alloy according to the present invention has
high corrosion resistance, the corrosion resistance particularly in
a bromine-ion-containing environment. Further, in a case of using
Ru, which is an inexpensive platinum group element, the raw
material cost of the titanium alloy becomes low. In a case in which
the content of the platinum group element is high (for example,
higher than 0.05 mass %), when a damage such as a removal of a
passivation film is generated on the surface by a scratch or the
like, corrosion starting from the damage is unlikely to
propagate.
[0056] In a case in which the titanium alloy contains, instead of
part of Ti, one or more selected from the group consisting of Ni,
Co, Mo, Cr, V, and W, the resistance to a high-concentration
chloride environment containing bromine is also obtained.
[0057] In a case in which the content of O is less than 0.05 mass
%, favorable processability is obtained.
[0058] Y is inexpensive among rare earth elements. In a case in
which Y is contained as the rare earth element, the raw material
cost becomes low.
Brief Description of the Drawing(s)
[0059] FIG. 1 is a schematic diagram showing a mechanism by which
corrosion resistance of a Ti--Pd(--Co) alloy is expressed.
[0060] FIG. 2 is schematic diagrams showing a test piece for a
crevice corrosion resistance test, and (a) shows a plan view and
(b) shows a side view.
[0061] FIG. 3 is a schematic diagram showing a state of a test
piece used in a crevice corrosion resistance test (ASTM G78).
[0062] FIG. 4 is a graph showing a relation between a Y content of
a Ti alloy containing 0.02% Pd in Example 2 and a corrosion speed
(96 hours average).
[0063] FIG. 5 is a graph showing a change in a surface Pd
concentration after a boiling hydrochloric acid test of a Ti alloy
containing 0.02% Pd in Example 2.
MODE(S) FOR CARRYING OUT THE INVENTION
[0064] As described above, the titanium alloy according to the
present invention consists of, in mass %, a platinum group element:
greater than or equal to 0.01% and less than or equal to 0.10%, a
rare earth element: greater than or equal to 0.001% and less than
0.02%, 0: greater than or equal to 0% and less than 0.1%, and the
balance: Ti and impurities. The present invention will be described
below in detail.
1. Platinum Group Element
[0065] Platinum group elements have an effect of decreasing a
hydrogen overvoltage of a titanium alloy and of maintaining a
natural potential in a passivation area. Among the platinum group
elements, the titanium alloy according to the present invention
contains Ru, for example. Ru is less expensive than other platinum
group elements and is preferable to secure economic efficiency. The
market price of Ru was about 1/6 of that of Pd as of January in
2012.
[0066] According to the present inventors' study, by adding the
platinum group element and the rare earth element to the titanium
alloy, an effect of preventing corrosion of the titanium alloy can
be obtained even in a bromine-ion-containing environment, although
the mechanism thereof has not been revealed. In the present
invention, the content of the platinum group element is 0.01 to
0.10 mass %. In a case in which the content of the platinum group
element is less than 0.01 mass %, corrosion resistance of the
titanium alloy may be insufficient and corrosion may occur in a
high-temperature and high-concentration chloride aqueous solution.
On the other hand, even if the content of the platinum group
element is higher than 0.10 mass %, an increase in corrosion
resistance is not expected, and in addition, the raw material cost
becomes high and processability becomes poor.
[0067] Considering the balance between processability and corrosion
resistance, the content of the platinum group element having a
.beta.-stabilizing function, such as Ru, is preferably set to 0.01
to 0.05 mass %, for example. This is because the titanium alloy
according to the present invention, in which the content of the
platinum group element is in this range, has corrosion resistance
as high as a conventional titanium alloy in which the content of
the platinum group element is higher than 0.05 mass %. Note that,
in a case in which a scratch or the like generates a damage on the
titanium alloy, such as a removal of a passivation film, the Ru
precipitation condensation propagates more immediately on a fresh
surface generated by the scratch or the like as the content of Ru
is higher in the titanium alloy, as described above by taking the
Ti--Ru alloy as an example. Accordingly, since a potential of a
portion where a scratch or the like is generated reaches the
passivation area immediately and the surface is repaired (the
passivation film is repaired), as the Ru content is higher,
corrosion starting from the damage is more unlikely to occur. In a
case in which the Ru content is higher than 0.05 mass %, the
titanium alloy according to the present invention is suitable for a
usage in a harsh environment where a damage can be generated in the
passivation film.
2. Rare Earth Element
2-1. Reason of Containing Rare Earth Element
[0068] The present inventors have considered to add, to a Ti-0.04
mass % Ru alloy, a minute amount of various elements that are
likely to be melted in an environment of a high-temperature and
high-concentration chloride aqueous solution. A titanium alloy
containing such elements was immersed in a chloride aqueous
solution to be melted in the active state area. Then, the present
inventors have investigated whether an effect of shifting the
potential of the entire alloy to the passivation area is obtained
by promoting the Ru precipitation condensation on the surface of
the titanium alloy. As a result, rare earth elements are confirmed
as elements having this effect.
[0069] A further investigation has revealed that the same effect
can be obtained not only in a case in which the Ru content is 0.04
mass %, but also in a case in which the Ru content is in a range of
0.01 to 0.05 mass % or greater than 0.05 mass % in the
Ru-containing titanium alloy. That is, it is found out that the
addition of the rare earth element to the titanium alloy, in which
the Ru content is in a range of 0.01 to 0.10 mass %, enables Ti and
Ru to be melted immediately after the titanium alloy is subjected
to a corrosion environment. In other words, it is found out that
the Ru ion concentration can be increased (the Ru precipitation
condensation can be generated) immediately in a solution near the
surface of the titanium alloy. As compared with a titanium alloy
containing Ru and not containing the rare earth element, the
titanium alloy containing Ru and the rare earth element can
efficiently precipitate Ru on the surface. Even when a melted
amount (corrosion amount) of the entire titanium alloy is small,
and the titanium alloy containing Ru and the rare earth element can
efficiently precipitate Ru and has high corrosion resistance. Note
that although the description is made on Ru, another platinum group
element such as Pd is similarly considered to have an effect of
increasing the resistance to corrosion caused by bromine
[0070] Rare earth elements include Sc, Y, light rare earth elements
(La to Eu), and heavy rare earth elements (Gd to Lu). According to
the present inventors' study, any of the rare earth elements has
the above described effect. Further, it is not necessary to add
only one element as the rare earth element. The above described
effect has been confirmed also in a case of using a mixture or
compound of rear earth elements, such as a mixture of rare earth
elements (also referred to as mischmetal or "Mm" below) before
separation purification or a didymium alloy (alloy including Nd and
Pr).
[0071] Considering the above description, in terms of economic
efficiency, it is preferable to use La, Ce, Nd, Pr, Sm, Mm, a
didymium alloy, Y (Y is particularly preferable), which are easily
obtained and relatively inexpensive among rare earth elements. Any
Mm and didymium alloy that are commercially available can be used
for the present invention regardless of the component ratio of rare
earth element(s).
2-2. Content of Rare Earth Element(s)
[0072] The range of the content of the rare earth element(s) in the
titanium alloy according to the present invention is greater than
or equal to 0.001 and less than 0.02 mass %. When the content of
the rare earth element(s) is 0.001 mass % or more, in a passivation
area of a Ti-alloy, it is possible to melt Ti, the platinum group
element, and the rare earth element(s) simultaneously in a chloride
aqueous solution, and to obtain a sufficient effect of promoting
the precipitation of the platinum group element on the surface of
the alloy.
[0073] The upper limit of the content of the rare earth element(s)
is set to less than 0.02 mass % because the content of the rare
earth element(s) being more than this limit does not increase the
above effect, and in addition, a compound that is not produced in a
case of not adding the rare earth element(s) may be produced in the
Ti alloy. This compound is melted preferentially in a chloride
aqueous solution, and generates pit-like corrosion in the
Ti-platinum group element. Accordingly, the Ti-platinum group
element alloy in which this compound is produced has lower
corrosion resistance than in a case of not adding the rare earth
element(s).
[0074] The content of the rare earth element(s) in the Ti-platinum
group element alloy is preferably set to the solid solubility limit
or less in .alpha.-Ti, the solid solubility limit being shown in a
phase diagram or the like. For example, the solid solubility limit
of Y in .alpha.-Ti is 0.02 mass % (0.01 at %). Accordingly, it is
preferable that the content of Y is less than 0.02 mass % in a case
of adding Y. Further, the solid solubility of La in .alpha.-Ti is
extremely high, which is 2.84 mass % (1 at %) according to
Non-Patent Document 4 above. However, also in a case of adding La,
in view of securing economic efficiency, the content of La is set
to less than 0.02 mass %.
3. O (Oxygen)
[0075] The titanium alloy according to the present invention
contains O in less than 0.1 mass %. The content of O is set to less
than 0.1 mass % because corrosion resistance and favorable
processability are secured. Ti has high solid solubility of oxygen,
so that Ti having high solid solubility of oxygen (JIS type-2 to
type-4 titanium) is intentionally used for a usage for which high
strength is required. Indeed the solid solution of oxygen is
effective in increasing the strength, but it may degrade
processability. Accordingly, considering processability in addition
to corrosion resistance and economic efficiently, the upper limit
of the content of O is set to 0.1 mass %. For a usage that does not
need high strength or a usage that puts much value on
processability, the content of O is preferably set to less than
0.05 mass %.
4. Ni, Co, Mo, V, Cr, and W
[0076] The titanium alloy according to the present invention may
contain, instead of part of Ti, one or more of Ni, Co, Mo, V, Cr,
and W. In this case, in combination with the effects of the
platinum group element and the rare earth element(s), the titanium
alloy can have higher corrosion resistance in a
bromine-ion-containing environment.
[0077] In a case of the titanium alloy containing one or more of
these elements, the contents thereof are as follows: Ni: 1.0 mass %
or less, Co: 1.0 mass % or less, Mo: 0.5 mass % or less, V: 0.5
mass % or less, Cr: 0.5 mass % or less, and W: 0.5 mass % or
less.
5. Impurities
[0078] Examples of impurities in the titanium alloy include Fe, O,
C, H, N, Al, Zr, Nb, Si, Sn, Mn, and Cu. Fe, O, C, H, and N are
mixed from a raw material, a melting electrode, and an environment,
and Al, Zr, Nb, Si, Sn, Mn, and Cu are mixed in a case of using
scrap as a raw material. These impurities may be mixed without any
problem as long as the amount thereof is as small as not to impede
the effects of the present invention seriously. Specifically, the
amounts of the impurities being as small as not to impede the
effects of the present invention seriously are as follows: Fe: 0.3
mass % or less, 0: less than 0.1 mass %, C: 0.18 mass % or less, H:
0.015 mass % or less, N: 0.03 mass % or less, Al: 0.3 mass % or
less, Zr: 0.2 mass % or less, Nb: 0.2 mass % or less, Si: 0.02 mass
% or less, Sn: 0.2 mass % or less, Mn: 0.01 mass % or less, and Cu:
0.1 mass % or less. The total amount of these elements is 0.6 mass
% or less.
EXAMPLE 1
[0079] To confirm crevice corrosion resistance and processability
(bendability and stretch) of the titanium alloy according to the
present invention, the following tests were performed and the
results were evaluated.
1. Test Methods
1-1. Samples
[0080] Table 1 shows samples that were used for the tests and
composition thereof (analytical values are shown for elements other
than Ti, and Ti is the balance (bal.)).
TABLE-US-00001 TABLE 1 Sam- La + ple Ce + num- Nd + Pr ber Pd Ru Ni
Cr Co Mo W V O C H N Fe Y (Mm) Ti Comparative 1 0.14 <0.01
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.109 0.006
0.0018 0.0077 0.073 -- -- Bal. material 1 Comparative 2 0.06
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.07
0.006 0.0042 0.005 0.036 -- -- Bal. material 2 Comparative 3 --
0.054 0.52 0.01 <0.01 <0.01 <0.01 <0.01 0.04 0.004
0.001 0.003 0.03 -- -- Bal. material 3 Comparative 4 -- 0.052 0.01
0.01 <0.01 <0.01 <0.01 <0.01 0.06 0.006 0.0022 0.005
0.04 -- -- Bal. material 4 Present 5 -- 0.042 <0.01 <0.01
<0.01 <0.01 <0.01 <0.01 0.04 0.005 0.0021 0.006 0.03
0.005 -- Bal. invention example 1 (claim 1) Present 6 -- 0.049
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.08 0.006
0.0032 0.004 0.03 0.004 -- Bal. invention example 2 (claim 1)
Present 7 -- 0.087 <0.01 <0.01 <0.01 <0.01 <0.01
<0.01 0.04 0.005 0.0022 0.006 0.04 -- 0.01 Bal. invention
example 3 (claim 1) Present 8 -- 0.013 <0.01 <0.01 <0.01
<0.01 <0.01 <0.01 0.04 0.007 0.0028 0.006 0.03 0.01 --
Bal. invention example 4 (claim 1) Example beyond 9 -- 0.048
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.12* 0.007
0.0033 0.004 0.03 0.01 -- Bal. invention range 1 Example beyond 10
-- 0.004* <0.01 <0.01 <0.01 <0.01 <0.01 <0.01
0.04 0.006 0.0034 0.006 0.04 0.01 -- Bal. invention range 2 Example
beyond 11 -- 0.043 <0.01 <0.01 <0.01 <0.01 <0.01
<0.01 0.04 0.007 0.0044 0.005 0.04 -- 0.12* Bal. invention range
3 Present 12 -- 0.025 0.12 <0.01 <0.01 <0.01 <0.01
<0.01 0.03 0.008 0.002 0.005 0.02 0.01 -- Bal. invention example
5 (claim 2) Present 13 -- 0.027 <0.01 <0.01 0.18 <0.01
<0.01 <0.01 0.04 0.007 0.003 0.006 0.03 0.01 -- Bal.
invention example 6 (claim 2) Present 14 -- 0.028 <0.01 0.18
<0.01 <0.01 <0.01 <0.01 0.04 0.008 0.003 0.006 0.04 --
0.01 Bal. invention example 7 (claim 2) Present 15 -- 0.029
<0.01 <0.01 <0.01 0.11 <0.01 <0.01 0.03 0.006 0.004
0.005 0.03 0.01 -- Bal. invention example 8 (claim 2) Present 16 --
0.028 <0.01 <0.01 <0.01 <0.01 0.14 <0.01 0.04 0.006
0.005 0.006 0.03 0.015 -- Bal. invention example 9 (claim 2)
Present 17 -- 0.027 <0.01 <0.01 <0.01 <0.01 <0.01
0.19 0.03 0.007 0.004 0.005 0.03 0.01 -- Bal. invention example 10
(claim 2) Present 18 -- 0.029 <0.01 0.04 0.01 0.01 0.02 0.03
0.04 0.006 0.003 0.004 0.03 0.015 -- Bal. invention example 11
(claim 2) In sample numbers 9 to 11, numerical values with * means
being beyond the range of the present invention.
[0081] The following titanium alloy sheet materials were prepared
as the samples used for tests: Comparative materials which are
conventional materials (sample numbers 1 to 4); Present invention
examples (sample numbers 5 to 8 each corresponding to claim 1, and
sample numbers 12 to 18 each corresponding to claim 2); Examples
beyond the range of the present invention, which are not
conventional materials (hereinafter simply referred to as "Examples
beyond invention range", sample numbers 9 to 12). Comparative
materials 1 to 3 were obtained from the market, and the other
samples (including Comparative material 4) were fabricated in a
laboratory. Comparative material 4 was obtained by employing the
composition of the Ti--Ru alloy disclosed in Patent Document 3
above, which is described as "having high crevice corrosion
resistance and bendability".
1-1-1. Compositions of Samples
[0082] Comparative material 1 is Gr. 7, Comparative material 2 is
Gr. 17, and Comparative material 3 is Gr. 13. Each of Comparative
materials 1 to 4 is an alloy that does not contain a rare earth
element.
[0083] The samples as Present invention examples and Examples
beyond invention range have the following characteristics.
[0084] Present invention example 1, 4: the Ru content is less than
0.05 mass % and the oxygen content is less than 0.05 mass %.
[0085] Present invention example 2: the Ru content is less than
0.05 mass % and the oxygen content is 0.05 mass % or more.
[0086] Present invention example 3: the Ru content is 0.05 mass %
or more and the oxygen content is less than 0.05 mass %.
[0087] Present invention example 5: containing Ni.
[0088] Present invention example 6: containing Co.
[0089] Present invention example 7: containing Cr.
[0090] Present invention example 8: containing Mo.
[0091] Present invention example 9: containing W.
[0092] Present invention example 10: containing V.
[0093] Present invention example 11: containing Cr, Co, Mo, W, and
V.
[0094] In Present invention examples 1 to 11, the content of the
rare earth element(s) is less than 0.02%.
[0095] Example beyond invention range 1: being beyond the range of
the present invention in that the O content exceeds 0.10 mass
%.
[0096] Example beyond invention range 2: being beyond the range of
the present invention in that the Ru content is less than 0.01 mass
%.
[0097] Example beyond invention range 3: being beyond the range of
the present invention in that the content of the rare earth
element(s) is 0.02 mass % or more.
1-1-2. Raw Materials used for Fabrication of Samples
[0098] Raw materials used for the fabrication of the titanium
alloys were commercially available pure Ti sponge (JIS type-1) for
industrial use, ruthenium (Ru) powder (purity 99.9 mass %) produced
by Kishida chemical Co., Ltd., turning yttrium (Y) (purity 99.9
mass %) produced by Kishida chemical Co., Ltd., and a massive form
Mm (mixed rare earth elements). The ratio of rare earth elements in
Mm was as follows: La: 28.6 mass %, Ce: 48.8 mass %, Pr: 6.4 mass
%, and Nd: 16.2 mass %.
1-1-3. Method for Fabricating Samples
[0099] The raw materials were measured to be in the predetermined
ratio for each sample to be fabricated, and were melted (molten) in
an argon atmosphere by an arc melting furnace to fabricate five
ingots (each of which weighs 80 g). Then, all the five ingots were
re-melted together to fabricate square ingots each having a
thickness of 15 mm. Each of the square ingots was re-melted for
homogenization to fabricate square ingots each having a thickness
of 15 mm again. That is, melting was performed three times in
total.
[0100] Since each square ingot contained a minute amount of Pd and
rare earth element(s), in order to reduce segregation of the
elements and homogenize the elements in the alloy, heat treatment
was performed under the following conditions.
[0101] Atmosphere: vacuum (<10.sup.-3 Torr)
[0102] Temperature: 1100.degree. C.
[0103] Time: 24 hours
[0104] The square ingot subjected to heat treatment was rolled
under the following conditions to obtain a sheet material having a
thickness of 2.5 mm
[0105] .beta.-phase region hot rolling: rolling was performed with
a heating temperature of 1000.degree. C. to reduce the thickness
from 15 mm to 9 mm.
[0106] .alpha.+.beta.-phase region hot rolling: rolling was
performed on the sheet material, subjected to the (3-phase region
hot rolling, with a heating temperature of 875.degree. C. to reduce
the thickness from 9 mm to 2.5 mm.
[0107] The sheet material obtained by rolling was annealed in
vacuum at 750.degree. C. for 30 minutes to remove strain.
[0108] From the thus obtained hot-rolled sheet, test pieces to be
used for the following tests were obtained by machining.
1-2. Crevice Corrosion Resistance Tests
[0109] Crevice corrosion resistance tests were performed by using
the thus obtained test pieces.
1-2-1. Test Pieces for Crevice Corrosion Resistance Tests
[0110] FIG. 2 is schematic diagrams showing a test piece for a
crevice corrosion resistance tests, and (a) shows a plan view and
(b) shows a side view. As shown in the figures, this test piece haf
a thickness of 2 mm, a width of 30 mm, and a length of 30 mm. A
hole having a diameter of 7 mm was formed in the center of the test
piece. Further, on one surface (front surface) of the test piece
was polished using an emery paper with a grit of 600.
[0111] FIG. 3 is a schematic diagram showing a state of a test
piece used in a crevice corrosion resistance test. A test piece 1
was interposed between devises (spacers) 2 formed of
poly-trifluoroethylene. A hole was formed in the center of each of
the devises so as to correspond to the hole in the test piece 1. On
one surface of the clevis 2, a plurality of trenches were formed,
and the surface including the trenches were made to be in contact
with the test piece 1. The trench formed a crevice between the test
piece 1 and the clevis 2.
[0112] A bolt 3 was inserted into the hole of the test piece 1 and
the clevis 2, and a nut 4 was attached to the bolt 3, so that the
test piece 1 and the devises 2 were tightened. The bolt 3 and the
nut 4 were obtained by oxidizing the surface of a bolt and a nut
which were made of pure titanium, by heat treatment in air. The
torque at the time of tightening was 40 kgfcm.
1-2-2. Crevice Corrosion Resistance Tests in Environment not
Containing Bromine Ions Substantially
[0113] By use of the test piece in the above state shown in FIG. 3,
crevice corrosion resistance tests based on a multi-clevis test
regulated in ASTM G78 were performed. Specifically, the test piece
was immersed in a 250 g/L NaCl aqueous solution (pH=2, pH was
adjusted by hydrochloric acid), and the test was performed in a
manner that the aqueous solution maintained an air-saturated liquid
state at 150.degree. C. by using an autoclave apparatus. The test
time was 500 hours.
[0114] After the test, the number of portions in which crevice
corrosion occurred in the test piece was counted, and an increase
or decrease in the weight of the test piece due to the test (the
value obtained by subtracting the weight of the test piece before
the test from the weight of the test piece after the test) was
measured. The weight per test piece before the test was about 7
g.
1-2-3. Crevice Corrosion Resistance Tests in Bromine-Ion-Containing
Environment
[0115] The same tests and evaluation were performed as in "Crevice
corrosion resistance tests in environment not containing bromine
ions substantially" except that, instead of the NaCl aqueous
solution used in the above "Crevice corrosion resistance tests in
environment not containing bromine ions substantially", an aqueous
solution obtained by adding sodium bromide reagent to set the
bromine ion concentration to 0.01 mol/L was used for corrosion
tests.
1-3. Evaluation of Processability
[0116] The processability of a material was evaluated by bending
tests and tension tests. The test conditions were as follows.
1-3-1. Bending Tests
[0117] The test piece was obtained in the following manner. A sheet
material having a thickness of 2.0 mm to 2.5 mm was extended to a
thickness of 0.5 mm by rolling and then was annealed. From this
sheet material, a fragment having a size and a shape which are
based on JIS Z 2204 (a width of 20 mm and a length of 60 mm) was
cut out, and a surface of the fragment was polished by an emery
paper with a grit of 600 in the rolling direction and the
perpendicular direction.
[0118] The bending tests were performed by a method based on JIS Z
2248, and T-direction adhesion bendability was evaluated.
1-3-2. Tension Tests
[0119] From each of the test pieces 1 for the above crevice
corrosion resistance tests, which were not used in the crevice
corrosion resistance tests, two test pieces each having half the
size of ASTM with a thickness of 2 mm were cut out in a direction
parallel to the rolling longitudinal direction. The cut out test
pieces were subjected to tension tests by using an autograph
tension tester manufactured by Shimadzu Cooperation. The tension
rate was 0.5%/min up to the bearing force, and was 5 mm/min
thereafter. An average value of breaking extension measured for the
two test pieces was set as a stretch of that test piece in an L
direction.
2. Test Results
2-1. Crevice Corrosion Resistance
[0120] Table 2 shows results of the crevice corrosion resistance
tests. In Table 2, results of the crevice corrosion resistance
tests in an environment not containing bromine ions substantially
are shown in cells with "250 g/L-NaCl, pH=2, 150.degree. C".
Results of the crevice corrosion resistance tests in a
bromine-ion-containing environment are shown in cells with "250
g/L-NaCl, Br 0.01 mol/L, pH=2, 150.degree. C".
TABLE-US-00002 TABLE 2 250 g/L-NaCl, pH = 2, 150.degree. C. 250
g/L-NaCl, Br 0.01 mol/L, pH = 2, 150.degree. C. Corrosion Increase/
Corrosion Increase/ Sample occurrence decrease in occurrence
decrease in number rate weight Note rate weight Note Comparative 1
0/40 3.5 mg increase No corrosion 2/40 36 mg decrease Crevice
corrosion material 1 Comparative 2 0/40 2.4 mg increase No
corrosion 2/40 41 mg decrease Crevice corrosion material 2
Comparative 3 0/40 2.6 mg increase No corrosion 1/40 .sup. 3.6 mg
decrease Crevice corrosion material 3 Comparative 4 3/40 44 mg
decrease Crevice corrosion 12/40 325 mg decrease Crevice corrosion
material 4 Present invention 5 0/40 2.8 mg increase No corrosion
0/40 1.5 mg increase No corrosion example 1 Present invention 6
0/40 2.9 mg increase No corrosion 0/40 3.1 mg increase No corrosion
example 2 Present invention 7 0/40 3.4 mg increase No corrosion
0/40 2.7 mg increase No corrosion example 3 Present invention 8
0/40 2.6 mg increase No corrosion 0/40 1.9 mg increase No corrosion
example 4 Example beyond 9 0/40 3.3 mg increase No corrosioa 0/40
4.3 mg increase No corrosion invention range 1 Example beyond 10
5/40 108 mg decrease Crevice corrosion 6/40 469 mg decrease Crevice
corrosion invention range 2 Example beyond 11 0/40 1.8 mg increase
No corrosion 0/40 2.3 mg increase No corrosion invention range 3
Present invention 12 0/40 1.1 mg increase No corrosion 0/40 0.9 mg
increase No corrosion example 5 (claim 2) Present invention 13 0/40
1.6 mg increase No corrosion 0/40 1.1 mg increase No corrosion
example 6 (claim 2) Present invention 14 0/40 2.4 mg increase No
corrosion 0/40 2.1 mg increase No corrosion example 7 (claim 2)
Present invention 15 0/40 2.9 mg increase No corrosion 0/40 2.7 mg
increase No corrosion example 8 (claim 2) Present invention 16 0/40
2.7 mg increase No corrosion 0/40 1.8 mg increase No corrosion
example 9 (claim 2) Present invention 17 0/40 3.1 mg increase No
corrosion 0/40 1.9 mg increase No corrosion example 10 (claim 2)
Present invention 18 0/40 2.2 mg increase No corrosion 0/40 2.1 mg
increase No corrosion example 11 (claim 2)
[0121] As for "Corrosion occurrence rate" in Table 2, "40" as
denominators is the number of crevices formed between the test
piece 1 and the clevis 2 due to the trenches of the clevis 2. The
numbers as numerators are the number of portions where corrosion
occurred among portions corresponding to the crevices on the
surface of the test piece 1.
[0122] Results of the tests in the above "environment not
containing bromine ions substantially" are as follows.
[0123] Corrosion did not occur at all in the 40 crevices in all of
Present invention examples (Present invention examples 1 to 4 and 5
to 11), Comparative materials 1 to 3, and Examples beyond invention
range 1 and 3. In these samples, oxidation coloring was found in a
portion other than the portions corresponding to the crevices, and
a minute increase in weight due to the oxidation was found.
[0124] Crevice corrosion occurred in Comparative material 4
(material described in Patent Document 3) and Example beyond
invention range 2 (material having a content of Ru lower than the
range of the present invention). As for these samples, white
corrosion products were found in a portion corresponding to the
crevices, and the weight was decreased by more than 40 mg by the
corrosion.
[0125] Results of tests in the above "bromine-ion-containing
environment" were as follows.
[0126] Corrosion did not occur at all in the 40 crevices in all of
Present invention examples (Present invention examples 1 to 4 and 5
to 11), and Examples beyond invention range 1 and 3. In these
samples, oxidation coloring was found in a portion other than the
portions corresponding to the crevices, and a minute increase in
weight due to the oxidation was found.
[0127] Crevice corrosion occurred in Comparative materials 1 to 4
and Example beyond invention range 2. Among these samples, the
weight was decreased particularly largely by corrosion in
Comparative material 4 and Example beyond invention range 2.
[0128] It is found out that Present invention examples have high
corrosion resistance (crevice corrosion resistance) both in a
chloride environment that does not contain bromine ions
substantially and a chloride environment containing bromine
ions.
2-2. Processability
[0129] Table 3 shows results of bendability tests (sealing-bending)
and tension tests.
TABLE-US-00003 TABLE 3 Sam- Sealing- Stretch in ple bending in
L-direc- num- T-direc- tion ber tion (%) Comparative material 1 1 C
29 Comparative material 2 2 A 52 Comparative material 3 3 C 35
Comparative material 4 4 B 49 Present invention example 1 5 A 54
Present invention example 2 6 B 46 Present invention example 3 7 A
53 Present invention example 4 8 A 54 Example beyond invention
range 1 9 C 43 Example beyond invention range 2 10 A 56 Example
beyond invention range 3 11 C 49 Present invention example 5 (claim
2) 12 B 44 Present invention example 6 (claim 2) 13 B 45 Present
invention example 7 (claim 2) 14 B 47 Present invention example 8
(claim 2) 15 A 50 Present invention example 9 (claim 2) 16 B 45
Present invention example 10 (claim 2) 17 B 47 Present invention
example 11 (claim 2) 18 B 46
[0130] In Table 3, alphabetical characters in the cells of
"sealing-bending in T-direction" denote the following.
[0131] A: A break was not generated.
[0132] B: A fine break was generated in any of the test pieces.
[0133] C: A break was generated in any of the test pieces.
[0134] As for Comparative materials 1 and 3, breaks were generated
by sealing-bending in the T-direction and stretches in the
L-direction were small. That is, the bendability of Comparative
materials 1 and 3 was low. As for Comparative material 2, a break
was not seen in sealing-bending in the T-direction and the stretch
in the L-direction was as large as that of a JIS type-1 material.
As for Comparative material 4, although the stretch in the
L-direction was as high as that of a JIS type-1 material, a fine
break was seen on the surface of the test piece in sealing-bending
in the T-direction.
[0135] As for each of Present invention examples 1, 3, 4, and 8, a
break was not seen in sealing-bending in the T-direction, and the
stretch in the L-direction was as high as that of JIS type-1
titanium, which is 50% or more. In contrast, as for Present
invention examples 2, 5, 6, 7, 9, 10, and 11, the stretch in the
L-direction was lower than that of the other Present invention
examples, which is lower than 50%, and fine breaks were generated
on the surface in sealing-bending in the T-direction. In this
manner, Present invention examples 2, 5, 6, 7, 9, 10, and 11 have
lower processability than Present invention examples 1, 3, 4, and
8. Present invention example 8 has a relatively smaller stretch in
the L-direction than Present invention examples 1, 3, and 4.
[0136] As for Example beyond invention range 1, the stretch in the
L-direction was poor and a break was generated in sealing-bending
in the T-direction. As for Example beyond invention range 2, both
results of sealing-bending in the T-direction and the stretch in
the L-direction were favorable. As for Example beyond invention
range 3, although the stretch in the L-direction was large, a break
was generated in sealing-bending in the T-direction.
[0137] In general, the processability tends to increase as the O
content is lower and the contents of Ni, Cr, Co, Mo, W, and V are
lower. Each of Present invention examples 1, 3, and 4 has more
favorable processability than Present invention example 2 possibly
because the O content of Present invention examples 1, 3, and 4 was
less than 0.05 mass % whereas the O content of Present invention
example 2 was 0.05 mass % or more (however, the O content is less
than 0.1 mass % and is within the region of the present invention).
Each of Present invention examples 5 to 11 has lower processability
than Present invention examples 1, 3, and 4 possibly because each
of Present invention examples 5 to 11 contains any of Ni, Cr, Co,
Mo, W, and V.
[0138] The content of the rare earth element(s) of Example beyond
invention range 3 exceeded the range of the content of the rare
earth element(s) in the present invention (0.01 to 0.10 mass %),
and a compound containing a rare earth element was produced in this
sample. The break generated by sealing-bending in the T-direction
of Example beyond invention range 3 was assumed to have started
from this compound.
3. Overall Evaluation
[0139] The overall evaluation of each sample was performed by
taking into consideration the above test results and economic
efficiency.
[0140] Table 4 shows calculation results of cost of a platinum
group element in the price of row materials, based on the ratio of
the platinum group element contained in the samples. In
calculation, the price of the bare metal of the platinum group
element was set to 1905/g for Pd and 300/g for Ru.
TABLE-US-00004 TABLE 4 Cost of platinum Relative cost Sample Pd Ru
group element of platinum number (mass %) (mass %) ( /kg) group
element Comparative material 1 1 0.14 -- 2667 100.00 Comparative
material 2 2 0.06 -- 1143 42.86 Comparative material 3 3 -- 0.054
162 6.07 Comparative material 4 4 -- 0.052 156 5.85 Present
invention example 1 5 -- 0.042 126 4.72 Present invention example 2
6 -- 0.049 147 5.51 Present invention example 3 7 -- 0.087 261 9.79
Present invention example 4 8 -- 0.013 39 1.46 Example beyond
invention range 1 9 -- 0.048 144 5.40 Example beyond invention
range 2 10 -- 0.004 12 0.45 Example beyond invention range 3 11 --
0.043 129 4.84 Present invention example 5 (claim 2) 12 -- 0.025 75
2.81 Present invention example 6 (claim 2) 13 -- 0.027 81 3.04
Present invention example 7 (claim 2) 14 -- 0.028 84 3.15 Present
invention example 8 (claim 2) 15 -- 0.029 87 3.26 Present invention
example 9 (claim 2) 16 -- 0.028 84 3.15 Present invention example
10 (claim 2) 17 -- 0.027 81 3.04 Present invention example 11
(claim 2) 18 -- 0.029 87 3.26
[0141] In Table 4, "Cost of platinum group element" means the cost
( ) of the platinum group element in 1 kg of the titanium alloy,
and "Relative cost of platinum group element" means the cost ratio
of the platinum group element in each sample when the cost of the
platinum group element of Comparative material 1 is set to 100. On
the assumption of the above price of the bare metal, the cost of
the platinum group element of each Present invention example is
1/10 or less of the cost of the platinum group element of
Comparative material 1, and is 1/4 or less of the cost of the
platinum group element of Comparative material 2.
[0142] Table 5 shows results of the overall evaluation of
Comparative materials and Present invention examples.
TABLE-US-00005 TABLE 5 Crevice corrosion resistance Environment not
containing Environment Overall bromine ions containing Process-
Economic evalua- substantially bromine ions ability efficiency tion
Compar- A C C C -- ative material 1 Compar- A C A C -- ative
material 2 Compar- A C C A -- ative material 3 Compar- C C B A --
ative material 4 Present A A A to B A A invention examples
[0143] In Table 5, the evaluation is made for each evaluation item
in three grades: A (excellent), B (relatively poor), and C
(poor).
[0144] As described above, the processability of the present
invention may become relatively poor in some cases (such as in
Present invention example 2). Further, the processability is
considered to become poor in a case in which the content of O is
0.05 mass % or more or in a case in which Ni, Cr, Co, Mo, W, or V
is contained. Accordingly, in a case of being used in a usage that
puts much value on processability, the titanium alloy according to
the present invention has the O content of less than 0.05 mass %
and does not contain Ni, Cr, Co, Mo, W, and V substantially.
[0145] Other than processability, the present invention is
excellent in all the items.
[0146] In contrast, Comparative materials are poor in any of the
evaluation items. In particular, none of Comparative materials has
crevice corrosion resistance that is high enough to be used
substantially in a bromine-ion-containing environment.
EXAMPLE 2
[0147] 2.1 Composition of Titanium Alloy used in Example 2
[0148] In order to clarify an optimal content of the rare earth
element(s) and to confirm that Ru has high corrosion resistance to
bromine among platinum group elements, the following experiment was
performed. Table 6 shows compositions of titanium alloys used in
Example 2. In accordance with the method for manufacturing the
samples shown in Example 1, alloys having the compositions shown in
Table 6 were obtained.
TABLE-US-00006 TABLE 6 Rare earth Platinum Type element group
element Ni Cr Co Mo W V O C H N Fe Bal. Comparative -- Ru: 0.02
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.05 0.004
0.0024 0.006 0.05 Ti + Impurities material 5 Comparative Y: 4 ppm
Ru: 0.02 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.06
0.006 0.0032 0.005 0.04 Ti + Impurities material 6 Present
invention Y: 11 ppm Ru: 0.02 <0.01 <0.01 <0.01 <0.01
<0.01 <0.01 0.05 0.007 0.0027 0.005 0.04 Ti + Impurities
example 12 Present invention Y: 21 ppm Ru: 0.02 <0.01 <0.01
<0.01 <0.01 <0.01 <0.01 0.06 0.005 0.0035 0.004 0.05 Ti
+ Impurities example 13 Present invention Y: 40 ppm Ru: 0.02
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.08 0.006
0.0038 0.006 0.05 Ti + Impurities example 14 Present invention Y:
190 ppm Ru: 0.02 <0.01 <0.01 <0.01 <0.01 <0.01
<0.01 0.07 0.005 0.0024 0.007 0.06 Ti + Impurities example 15
Present invention Mm: 21 ppm Ru: 0.02 <0.01 <0.01 <0.01
<0.01 <0.01 <0.01 0.06 0.007 0.0033 0.006 0.04 Ti +
Impurities example 16 Present invention Mm: 23 ppm Ru: 0.04
<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 0.07 0.006
0.0028 0.007 0.04 Ti + Impurities example 17 Present invention Mm:
21 ppm Ru: 0.05 <0.01 <0.01 <0.01 <0.01 <0.01
<0.01 0.06 0.007 0.0024 0.005 0.05 Ti + Impurities example 18
Present invention Mm: 25 ppm Ru: 0.06 <0.01 <0.01 <0.01
<0.01 <0.01 <0.01 0.07 0.005 0.0032 0.006 0.06 Ti +
Impurities example 19
[0149] Comparative materials 5 and 6 contain a rare earth element
in less than 0.001%, and is beyond the range of the present
invention. From the materials shown in Table 6, a titanium alloy
sheet for crevice corrosion tests in FIG. 2 was obtained by
machining, and using the test piece, the crevice corrosion test
piece shown in FIG. 3 was formed. Note that the torque at the time
of tightening was 40 kgfcm. This crevice corrosion test piece was
used in each crevice corrosion test in a bromine-ion-containing
environment described in 1-2-3.
[0150] Table 7 shows results obtained by performing 500-hour
crevice corrosion tests. In Comparative material 5 which does not
contain a rare earth element, crevice corrosion was seen in a large
number of portions, and the decrease due to corrosion was 325 mg.
In Comparative material 6 which does not contain a sufficient rare
earth element, crevice corrosion was also seen, and the decrease
due to corrosion was 32 mg. It is considered that a desirable
content of rare earth element(s) is 200 ppm or less in a
bromide-ion-containing environment. The content of the rare earth
element in each of Present invention examples 12 to 15 was within
the range of the present invention, and accordingly, crevice
corrosion did not occur, and the decrease in weight thereof due to
corrosion was small.
TABLE-US-00007 TABLE 7 250 g/L-NaCl, Br 0.01 mol/L pH = 2,
150.degree. C. Rare earth Platinum group Corrosion Increase/ Type
element element occurrence rate decrease in weight Note Comparative
-- Ru: 0.02 15/40 325 mg decrease Crevice corrosion material 5
Comparative Y: 4 ppm Ru: 0.02 3/40 32 mg decrease Crevice corrosion
material 6 Present Y: 11 ppm Ru: 0.02 0/40 2.1 mg increase No
crevice corrosion invention example 12 Present Y: 21 ppm Ru: 0.02
0/40 1.8 mg increase No crevice corrosion invention example 13
Present Y: 40 ppm Ru: 0.02 0/40 2.2 mg increase No crevice
corrosion invention example 14 Present Y: 190 ppm Ru: 0.02 0/40 1.9
mg increase No crevice corrosion invention example 15
[0151] Next, crevice corrosion test pieces of materials of Present
invention examples 16 to 19 having different contents of Ru were
used in crevice corrosion tests in the bromine-ion-containing
environment shown in 1-2-3. Further, Eriksen tests based on JIS Z
2247 were performed to investigate the press formability of the
materials.
[0152] For the tests, a sheet material having a thickness of 2 mm
and a size of 90 mm.times.90 mm was prepared, and a steel ball
having diameter of 20 mm was pressed into the sheet material. When
a break reaches the rear surface, the stroke of the punch at that
time was set as the Eriksen value. Formation tests were performed
using graphite grease for lubricating at a speed of 5 mm/min. The
results are shown in Table 8.
TABLE-US-00008 TABLE 8 250 g/L-NaCl, Br 0.01 mol/L pH = 2,
150.degree. C. Eriksen value Rare earth Platinum Corrosion
Increase/ Based on Type element group element occurrence rate
decrease in weight Note JIS Z 2247 Present Mm: 21 ppm Ru: 0.02 0/40
4.1 mg increase No crevice 11.4 mm invention corrosion example 16
Present Mm: 23 ppm Ru: 0.04 0/40 2.8 mg increase No crevice 10.6 mm
invention corrosion example 17 Present Mm: 21 ppm Ru: 0.05 0/40 3.2
mg increase No crevice 10.1 mm invention corrosion example 18
Present Mm: 25 ppm Ru: 0.06 0/40 2.9 mg increase No crevice 9.6 mm
invention corrosion example 19
[0153] In Present invention examples 16 to 18, crevice corrosion
did not occur, and accordingly, Present invention examples 16 to 18
have high corrosion resistance in an environment of a
bromide-ion-containing solution. Note that the Eriksen value
representing formability was decreased slightly when the content of
Ru exceeded 0.05%. In contrast, as the content of Ru was increased,
the decrease in weight due to corrosion tended to be decreased. In
order to achieve both high corrosion resistance and formability, Ru
is preferably set to a range of 0.01 to 0.05%.
[0154] The thus obtained experimental facts have revealed that
particularly high corrosion resistance can be obtained when the
content of the rare earth element(s) is greater than or equal to
0.001 and less than 0.02% in the present invention range. Further,
when the Ru content is 0.01 to 0.05%, high formability can also be
secured.
INDUSTRIAL APPLICABILITY
[0155] The titanium alloy according to the present invention can be
applied to equipment, apparatuses, and the like that are to be used
in an environment that requires corrosion resistance in a
bromine-ion-containing environment (in particular, a
high-temperature and high-concentration chloride environment).
* * * * *