Note: Descriptions are shown in the official language in which they were submitted.
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SPECIFICATION
HYDROGEN SULFIDE CORROSION RESISTANT
HIGH-Cr AND HIGH-Ni ALLOYS
TECHNICAL FIELD
This invention relates to austenitic high-Cr and high-Ni alloys and
more particularly, to high-Cr and high-Ni alloys which exhibit a good
corrosion resistance when placed in an aqueous solution having a
relatively low hydrogen sulfide concentration at a partial pressure of -
hydrogen sulfide gas of 1 atm., or below.
TECHNICAL BACKGROUND
Hydrogen sulfide contained in liquids, such as petroleum, is highly
corrosive against alloys. Accordingly, the alloys used in liquids
containing hydrogen sulfide should have a good corrosion resistance to
hydrogen sulfide. The examples of alloys which are employed in
environments where they are in contact with hydrogen sulfide-containing
liquids include drill pipes, pipes for flow lines from oil wells, oil country
tubular goods for oil and natural gas wells, plate members for natural
steam power stations, plate members for installation for desulfurization
from exhaust gases, and the like. Especially, drilling of oil wells,
exploitation and production of natural gas involve corrosive environments
which are severe. For an index indicating the corrosive environment of
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hydrogen sulfide, the usual practice is to use a partial pressure of
hydrogen sulfide in a gas phase. This is because the concentration of
hydrogen sulfide in an aqueous solution is substantially proportional to
the partial pressure of hydrogen sulfide in a gas phase, thus enabling one
to simply express the degree of influence on alloys. With production
pipes for oil wells, it has been frequently experienced that alloys are
exposed to severe environmental conditions such as a partial pressure of
hydrogen sulfide of about 10 atm., and a temperature of about 200 C.
The corrosion of an alloy ascribed to the hydrogen sulfide In such
an environment as mentioned above results mainly in the cracking of the
alloy under stress(stress corrosion cracking). Accordingly, alloys which
are to be employed in an environment containing hydrogen sulfide should
have a good resistance to stress corrosion cracking.
Known alloys which are used in an environment such as of a
hydrogen sulfide-containing oil well wherein a partial pressure of
hydrogen sulfide is as high as approximately 10 atm., include Ni-Cr-Mo-Fe
Ni-based alloys which contain Ni in amounts as great as 30 -50%
(Japanese Laid-open Patent Application Nos. 57-131340, 57-134544 and
57- 134545)
For instance, Japanese Laid-open Patent Application No. 57-
131340 proposes an alloy which comprises, aside from Ni, Cr, Mo and W,
Cu and Co, if necessary, in order to improve the resistance to stress
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corrosion cracking.
These alloys for oil wells are so designed as to improve the
corrosion resistance to hydrogen sulfide including a resistance to stress
corrosion cracking. More particularly, the corrosion resistance to
hydrogen sulfide is greatly influenced by the content of hydrogen sulfide
present in raw oil and the temperature of the raw oil. Accordingly, when
used in such an environment of hydrogen sulfide as having set out above,
the alloy is so designed that a corrosion-resistant film is formed on the
surfaces of the alloy. The corrosion-resistant film should have a two-
layer structure including an outer layer consisting of a Ni sulfide film and
an inner layer consisting of a Cr oxide film. In order to facilitate the
growth of the inner layer of the Cr oxide film, at least one of Mo and W is
incorporated in the alloy. The reason why the corrosion-resistant film is
designed to have a double-layer structure is so that hydrogen sulfide is
prevented from entering into the inner layer by means of the outer layer of
the Ni sulfide film thereby preventing the breakage of the Cr oxide inner
film with the hydrogen sulfide. The Cr oxide inner film is able to
suppress the dissolution of the alloy and thus, acts to improve the
corrosion resistance, ensuring a good corrosion resistance to hydrogen
sulfide.
However, it has been confirmed that in an environment where the
partial pressure of hydrogen sulfide is relatively low at a level of about 1
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atm., or below and the temperature is about 150~C, conventional alloys of
the type set out above are not satisfactorily resistant to corrosion. The
reason for this has now been found as follows: the Ni sulfide film serving
as the outer layer which has the corrosion resistance to hydrogen sulfide is
unlikely to grow under conditions where a partial pressure of hydrogen
sulfide gas is low.
Moreover, since the known alloys contain both or either of Mo and
W which are expensive, the resultant alloys become undesirably high in
cost.
Accordingly, there is a strong demand for the development of
high-Cr and high-Ni alloys which are excellent in corrosion resistance to
hydrogen sulfide including a resistance to stress corrosion cracking when
placed in an environment where the partial pressure of hydrogen sulfide is
as low as about 1 atm., or below and the temperature is about 150~C, and
which are available inexpensively.
The invention has for its object the provision of a high-Cr and
high-Ni alloy which overcomes the problems involved in the prior art,
which is imparted with good corrosion resistance to hydrogen sulfide
under environmental conditions of a partial pressure of hydrogen sulfide
of 1 atm., or below and a temperature of about 150~C, and which is low in
cost.
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DISCLOSURE OF THE INVENTION
The high-Cr and high-Ni alloy of the invention has a good corrosion
resistance to hydrogen sulfide under environmental conditions of a partial
pressure of hydrogen sulfide of 1 atm., or below and a temperature of
about 150 ~C. The alloy is free of Mo and W which are expensive
elements and is thus low in cost, and is mass-producible. The alloy of the
invention has the following chemical composition on the basis of percent
by weight:
Si: 0.05- 1.0%
Mn: 0.1- 1.5%
Cr: 20 - 30%
Ni: 20 - 40%
sol. Al: 0.01 - 0.3%
Cu: 0.5- 5.0%
REM: 0 - 0.10%
Y: 0 - 0.20%
Mg: 0 - 0.10%
Ca: 0 - 0.10%
Balance: Fe and incidental impurities
In the composition, the contents of C, P and S in the incidental
impurities should, respectively, be 0.05% or below, 0.03% or below and
0.01% or below.
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As defined above, REM, Y, Mg and Ca do not have to be added at all,
however, if these elements are used, it is sufficient to add at least one of
REM, Y, Mg and Ca. Preferable contents of these elements are as follows:
REM: 0.001 - 0.10%
Y: 0.00 1 - 0.20%
Mg: 0.001 - 0.10%
Ca: 0.001 - 0.10%
BEST MODE FOR CARRYING OUT THE INVENTION
In order to solve the problems in the prior art, we made extensive
experiments and studies from different angles. As a result, we obtained
information as set out in (1) and (2) below.
(1) In an environment where a partial pressure of hydrogen
sulfide is as relatively low as about 1 atm., or below, the resistance to
stress corrosion cracking of high-Cr and high-Ni alloys can be remarkably
improved by incorporating an appropriate amount of Cu and optimi~ing
the contents of Cr and Ni. In this connection, it is not necessary to use
any expensive Mo and W which have been conventionally considered
essential for ensuring good resistance to stress corrosion cracking.
According to the knowledge obtained by us, the improvement of the
resistance to stress corrosion cracking is not satisfactorily attained by
merely increasing the content of Ni. The mere increase in amount of Ni
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does not result in a dense outer layer made of a Ni sulfide film which is one
of the protective layers having a double-layer structure and formed on the
surfaces of the alloy. This permits hydrogen sulfide to enter into the alloy.
If at least one of Mo and W is added to an alloy in order to promote the
growth of a Cr oxide film as an inner layer, this does not lead to an
improved resistance to stress corrosion cracking.
When Cu, Cr and Ni are, respectively, added in amounts defined in
the present invention, the resultant alloy is improved in the resistance to
stress corrosion cracking when placed in an environment where a partial
pressure of hydrogen sulfide is low. The mechanism for this is assumed
as follows:
Cu is more likely to form its sulfide than Ni. Under
environmental conditions where a partial pressure of hydrogen sulfide is
low, the Cu sulfide serves to densify the Ni sulfide film formed as the outer
layer on the alloy surfaces. This is why the resistance to permeation of
hydrogen sulfide is enhanced. Like Mo and W, Cu is an element which is
likely to form oxides in an acidic environment. The thus formed Cu oxide
densifies the inner Cr oxide film. More particularly, Cu densifies both
the outer Ni sulfide film and the inner Cr oxide film thereby improving the
corrosion resistance of the films. Thus, it is considered that the resultant
alloy is remarkably improved in the corrosion resistance to hydrogen
sulfide.
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(2) The Cu-containing high-Cr and high-Ni alloy is provided with
adequate hot workability. The reason for this is that since the content of
Ni is so high that the austenitic phase becomes stable, resulting in the
reduction in amount of intermetallic compounds which cause hot
brittleness. The incorporation of appropriate amounts of one or more of
REM (rare earth elements), Y, Mg and Ca can further improve the hot
workability.
The function and appropriate content of each of the elements
comprising the alloy of the invention are described below, in which percent
for each element is by weight.
Si: 0.05- 1.0%
Si is an element necessary for deoxidation of molten steel at the
time of refining. In order to ensure a satisfactory effect of deoxidation,
the content should be 0.05% or above. However, if the content exceeds
1.0%, the hot workability worsens. Accordingly, the content of Si is in the
range of 0.05 - 1.0%, preferably 0.2 - 0.5%.
Mn: 0.1- 1.5%
Mn, like Si, is an element necessary for deoxidation of molten steel.
In order to achieve a good deoxidation effect, the content of Mn should be
0.1% or above. However, if the content of Mn exceeds 1.5%, the hot
workability becomes poor. Accordingly, the content of Mn is in the range
of 0.1 - 1.5%, preferably 0.5 - 0.75%.
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Cr: 20 - 30%
Cr is an element which is effective for improving the corrosion
resistance to hydrogen sulfide (especially, resistance to stress corrosion
cracking) in co-existence with other major components of Ni and N. If the
content is less than 20%, such an effect cannot be obtained satisfactorily.
Cr tends to worsen hot workability. If the content is reduced to a range of
less than 20%, no significant effect of improving the hot workability may
be obtained. On the other hand, where the content of Cr exceeds 30%,
any further improvement of the corrosion resistance to hydrogen sulfide
cannot be attained using a higher content of Cr within the above range.
Moreover, in the case where the content of Cr exceeds 30%, good hot
workability cannot be expected even if the content of S is reduced.
Accordingly, the content of Cr is in the range of 20 - 30%, preferably 22 -
27%.
Ni: 20 - 40%
Ni is effective in improving the corrosion resistance to hydrogen
sulfide. This effect is shown when the content of Ni is 20% or above.
However, when the content exceeds 40%, any further effect is not expected
using a higher content within the above range. Where Ni (which is
expensive) is contained in amounts higher than required, the resultant
alloy becomes expensive, thus being economically poor. Accordingly, the
content of Ni is in the range of 20 - 40%, preferably 22 - 30%.
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Sol. Al: 0.01- 0.3%
Al, like Si and Mn, is an element necessary for the deoxidation of
molten steel. The deoxidation effect is shown when the content of sol. Al
(i.e.. Al contained in alloy and soluble in hydrochloric acid) is 001% or
above. However, when the content of sol. Al exceeds 0.3%, hot
workability is impeded. Accordingly, the content of sol. Al is in the range
of 0.01 - 0.3%, preferably 0.1 - 0.15%.
Cu: 0.5 - 5.0%
Cu is the most important element for the invention which
constitutes a characteristic feature of the high-Cr and high-Ni alloy of the
invention. Cu serves to remarkably improve the corrosion resistance to
hydrogen sulfide in an environment of hydrogen sulfide gas whose partial
pressure is as low as 1 atm., or below. In order to achieve this
improvement, 0.5% or above of Cu should be present. However, if Cu is
added in excess of 5.0%, no further improvement is expected.
Additionally, when the content exceeds 5.0%, hot workability is degraded.
Accordingly, the content of Cu is in the range of 0.5 - 5.0%, preferably 1.0 -
3.0%.
REM, Y, Mg and Ca:
Aside from the above-stated alloying elements, the alloy of the
invention may contain one or more of REM (rare earth elements), Y, Mg
and Ca in order to improve hot workability. These elements are effective
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in improving hot workability as in the case where the alloy is hot-worked
under severe conditions.
If these elements are added, it is preferred to use 0.001 - 0.10% of
REM, 0.001 - 0.20% of Y, 0.001 - 0.10% of Mg and 0.001 - 0.10% of Ca.
Where the lower limit in content of each element is less than
0.001%, any significant effect of improving the hot workability is not
obtained. On the other hand, when the contents of the respective
elements exceed the defined upper limits, coarse oxides are formed, thus
impeding hot workability. Thus, when these elements are used, it is
preferred to use the contents defined above.
Incidental impurities:
Major incidental impurities include C, P and S. Among these
elements, the content of C should preferably be not more than 0.05%.
If the content of C exceeds 0.05% and Nb or V co-exist as an
impurity, coarse carbide is formed along with the co-existing element, and
Cr carbide is formed at grain boundaries in a contiguous state. The
formation of these carbides causes Cr depletion zones, so that stress
corrosion cracking is liable to occur along the grain boundaries. For this
reason, the upper limit of the content of C is detern~ined at 0.05%, and the
content of C is preferably 0.03% or below.
When the content of P exceeds 0.03, the susceptibility to stress
corrosion cracking in an environment of hydrogen sulfide increases.
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Accordingly, the upper limit is 0.03%. The content is preferably 0.02% or
below.
When the content of S exceeds 0.01%, hot workability is
considerably impeded. Accordingly, the content of S is defined to be not
more than 0.01. If the content of S is so great, hot workability is
considerably impeded as set out above. In this connection, if the content
of S is as low as about 0.0007% or below, hot workability is improved.
Accordingly, if good hot workability under severe conditions is essential, it
is preferred to reduce the content of S to a level of 0.0007% or below.
In the alloy of the invention, the incidental impurity elements may
further comprise, aside from the above-stated C, P and S, 0.10% or below
of B, Sn, As, Sb, Bi, Pb and Zn. The impurities present in such amounts
as set out above exerts little influence on the characteristics of the alloy of
the invention.
The alloy of the invention and articles such as alloy pipes made of
the alloy of the invention as a base metal can be made using
manufacturing apparatus and methods which are employed for ordinary
commercial manufacture. For instance, melting of the alloy may be
conducted by utilizing electric furnaces, argon-oxygen decarburization
furnaces (AOD furnaces), vacuum-oxygen decarburization furnaces (VOD
furnaces) and the like. The molten metal may be cast into ingots or may
be cast into rod-shaped billets according to a continuous casting technique.
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When alloy pipes are manufactured, for example, from these billets, it is
preferred to use an extrusion pipe maklng processes such as the Ugine
Sejournet process, or the Mannesman pipe making process. The pipe
making conditions such as a heating temperature of billets prior to pipe
making may be those of the case using conventional high-Cr and high-Ni
alloys.
EXAMPLES
In order to confirm the characteristic properties of the alloys of the
invention, 19 alloys indicated in Table 1 were made by melting. These
alloys were each made according to a procedure wherein the respective
alloys were molten in an electric furnace and, after substantial adjustment
in chemical composition, were subjected to decarburization and
desulfurization treatments by use of an argon-oxygen decarburization
furnace (AOD furnace). The resultant molten metal was cast into an
ingot having a weight of 1500 kg and a diameter of 500 mm. Among the
19 alloys indicated in Table 1, alloy Nos. 1 - 12 are for the invention and
alloy Nos. 13 - 19 are for comparison.
Individual ingots having such chemical compositions as indicated
in Table 1 were treated in the following manner:
Initially, each ingot was heated to 1250~C and subjected to hot
forging at 1200~C to obtain a rod with a diameter of 150 mm. The rod
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was cut into pieces having a length of 1000 mm to obtain billets for
extrusion pipe making. The billet was shaped into a pipe having a
diameter of 60 mm, a thickness of 5 mm and a length of about 20 m
according to the Ugine Sejournet hot extrusion pipe making process.
Only one pipe was made for each alloy indicated in Table 1.
The resultant pipes were each subjected to solution treatment
under conditions where the pipe was maintained at 1100~C for 0.5 hours
and quenched with water. Further, the pipe was cold-worked so that the
yield strength (0.2% offset) was adjusted to 125 ksi grade (125 - 140 ksi =
87.75 - 98.228 kgf/mm~), thereby providing a product pipe.
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T a b 1 e
Chemical Composition~l (Weight %) Corrosion Hot Work-
No Resistance ability
C Si Mn P S Cr Ni sol.AI Cu Other Elements
1 0.04 0.08 0.75 0.015 0.002 22 35 0.05 0.6 - O O
2 0.03 0.25 0.86 0.012 0.001 20 30 0.12 1.5 - O O
3 0.03 0.05 0.25 0.027 0.003 29 27 0.28 3.0 - O O
4 0.02 0.56 0.43 0.015 0.005 21 39 0.23 2.0 - O O
0.01 0.28 0.55 0.009 0.001 25 35 0.06 4.7 Zn:0.05 0 0
Exam- 6 0.02 0.05 0.85 0.015 0.001 21 30 0.08 0.8 Y:0.08 0 0
of the 7 0.03 0.95 0.12 0.025 0.008 25 25 0.02 1.7 Mg:0.075 0 0 o
tion 8 0.02 0.32 0.56 0.023 0.002 25 21 0.09 2.5 Ca:0.03 0 0
9 0.02 0.25 0.95 0.018 0.001 20 35 0.08 4.3 Y:0.04~ Mg:0.05 0 0 . r
0.01 0.36 0.53 0.012 0.002 25 28 0.15 1.3 Mg:0.03~ Ca:0.05 0 0
11 0.02 0.41 0.41 0.010 0.001 23 28 0.12 0.8 REM O Ol~Y:0.02 0 0 0
12 0.01 0.52 0.30 0.009 0.001 27 37 0.10 2.5 REMoOol03CY o 02 0 0
13 0.03 0.25 0.56 0.015 0.002 18 35 0.10 1.5 - x x
14 0.05 0.69 0.75 0.025 0.001 25 18 0.08 2.0 - x x
Exam- 15 0.03 0.56 0.25 0.020 0.001 20 30 0.15 0.3 - x O
ples
of the 16 0.03 0.75 0.60 0.015 0.003 25 32 0.11 0.3 Mo:3.0~Mg:0.075 x O
Compa-
rison 17 0.02 0.09 0.85 0.009 0.002 25 27 0.05 0.2 Mo 3 O~W:l.O x O
18 0.01 0.56 0.52 0.012 0.001 23 30 0.08 0.3 W:6.0 x x
19 0.01 0.45 0.55 0.011 0.001 25 38 0.18 8.0 - O x
* 1 : The balance is Fe and incidental impurities.
* 2 : The values with underlines show that they are outside the range of the invention.
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A sample piece was taken out from each pipe and subjected to a
corrosion test in a hydrogen sulfide environment to check corrosion
resistance to hydrogen sulfide. The corrosion test in the hydrogen sulfide
environment was conducted in the following manner. It should be noted
that the hot workability was evaluated in terms of the presence (symbol
"x" in Table 1) or the absence (symbol "o" in Table 1) of defects through
visual observation of defects on the inner surfaces of a pipe obtained by
extrusion pipe making. Two sample pieces for each alloy were used for
the corrosion test in the hydrogen sulfide environment.
Corrosion test method in the hydrogen sulfide environment:
Device used: autoclave
Test piece: width of 10 mm, thickness of 2 mm and length of 75 mm,
with a 0.25 mm U-shaped notch (made at the center of a
test piece)
Test solution: 0.5% CH3COOH-20% NaCl aqueous solution
Testing atmosphere: 0.1 atm. H2S-30 atm. CO2
Testing temperature: 150 C
Test piece immersion time: 720 hours
Added stress: 125 ksi (87.75 kgf/mm~)
(the stress was added by 10 mm at the central portion of the test
piece according to a four-point supporting method.)
Observation of Corrosion
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(1) Pitting corrosion: visual observation of the test piece
(2) Cracking: observation of the section of the U-notched
portion through an optical microscope at 100X magnifications
In Table 1, the results of the corrosion test in the hydrogen sulfide
environment and the results of evaluation of the hot workability are
shown in Table 1. With respect to the corrosion resistance to hydrogen
sulfide, where either pitting corrosion or cracking was not observed at all,
this resistance is indicated as "o". Where either of pitting corrosion or
cracking was observed, this is indicated as "x".
As shown in Table 1, the alloys of the inventive example (alloy Nos.
1 - 12), wherein the chemical compositions are within the range of the
invention, were not recognized with respect to the pitting corrosion and
the cracking in the corrosion test in the hydrogen sulfide environment.
Moreover, no defect on the inner surface of the pipes obtained after pipe
making was found.
These results reveal that the alloys of the invention are excellent
in the corrosion resistance to hydrogen sulfide and hot workability.
Especially, where at least one element selected from REM, Y, Mg and Ca is
present within a range defined in the invention, the hot workability is
better than that of the case where such an element is not added at all.
Although not shown in Table 1, those alloys of the invention
wherein Cu was in the range of 1.0 - 3.0%, Cr was in the range of 22 - 27%
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and Ni was in the range of 22 - 30% had a good corrosion resistance to
hydrogen sulfide and good hot workability. Thus, it was found that these
ranges were more preferred.
On the other hand, with the comparative alloys (alloy Nos. 13 - 18),
wherein the chemical composition was outside the scope of the invention,
either cracking or pitting corrosion took place during in the corrosion test
in the hydrogen sulfide environment. Alloy No. 19 has a high Cu content,
the corrosion resistance to hydrogen sulfide was not so poor. Although
the Cu content was within the range of the invention, Alloy Nos. 13 and 14
were not good with respect to the corrosion resistance to hydrogen sulfide.
This was because the content of Cr in alloy No. 13 and the content of Ni in
alloy No. 14 were, respectively, lower than those defined in the present
invention.
With alloy Nos. 13, 14, 18 and 19, defects were found on the inner
surfaces of the pipes, suggesting poor hot workability. The reason why
the hot workability was poor was that the balance in content between Cr
and Ni was not good for alloy No. 13, Ni was so low in content that the
resultant austenitic phase was unstable for alloy No. 14, the content of W
was so high that an intermetallic compound was formed for alloy No. 18,
and the content of Cu was outside the range for alloy No. 19.
Comparative alloy Nos. 16 - 18 contain either or both of Mo and W
and those alloys have been hitherto accepted as showing good corrosion
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resistance to hydrogen sulfide in an environment where a partial pressure
of hydrogen sulfide gas is high. Nevertheless, as will be apparent from
this example, these alloys were poor in corrosion resistance to hydrogen
sulfide. These results revealed that where the partial pressure of
hydrogen sulfide gas was low, like those conditions of this example, Mo
and W did not serve to improve the corrosion resistance to hydrogen
sulfide .
Aside from the above example, alloy No. 2 of the invention was
further subjected to another corrosion test in the hydrogen sulfide
environment where a partial pressure of hydrogen gas in the testing
atmosphere was set at 0.8 atm., with the other conditions being the same
as in Example 1. As a result, it was found that where the partial pressure
of hydrogen sulfide gas was 0.8 atm., the inventive alloy was excellent in
corrosion resistance to hydrogen sulfide.
POSSIBILITY OF INDUSTRIAL UTILIZATION
The alloys of the invention exhibit an excellent corrosion
resistance to hydrogen sulfide in an environment where a partial pressure
of hydrogen sulfide gas is as low as about 1 atm., or below, along with good
hot workability. Since it is not necessary to add Mo and W(which are
expensive), the cost of raw materials for alloys making decreases.
Moreover, the alloys of the invention can be made by use of manufacturing
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apparatus and method which have been conventionally used for the
manufacture, thus making it possible to mass-produce the alloy
nexpenslvely.
Accordingly, where the alloy of the invention is employed, for
example as a material for pipes which are in contact with a hydrogen
sulfide-containing corrosive fluid produced from oil wells, good corrosion
resistance is ensured. The alloy of the invention thus has very high
practical value for use as a material which is used in an environment
where a partial pressure of hydrogen sulfide gas is relatively low.
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