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HIGH-CHROMIUM
RESISTS AQUEOUS
ALLOY CORROSION
Improvements in alloy metallurgy,
melting technology, and thermomechanical processing led to
development of Alloy 31,
a relatively low-cost alloy with
high corrosion resistance.
D.C. Agarwal*
ThyssenKrupp VDM USA Inc.
Houston, Texas
A
n advanced super-austenitic, highchromium 6 Mo alloy called alloy 31
(UNS N08031) bridges the cost/performance gap between high-performance Ni-Cr-Mo alloys and the 300 series stainless
steels. It is especially suitable as a material of construction for the modern chemical process and
petrochemical industries, where materials must
not only resist uniform corrosion caused by a
range of corrodents, but also must have effective
resistance to localized corrosion and stress corrosion cracking in halide and other media.
This article describes the development of Alloy
31, its physical metallurgy, corrosion resistance,
and a few of its many applications in various industries over the past 15 years.
Effects of molybdenum and nitrogen
Engineers have long known that chromium
and molybdenum improve resistance to localized
corrosion. They also have long known that increased amounts of nickel and nitrogen enhance
resistance to chloride stress corrosion cracking.
This knowledge led to a class of alloys that were
very cost effective and provided corrosion resistance that in many cases approached or equaled
the corrosion resistance of more expensive high
Ni-Cr-Mo alloys in many environments. Cronifer
1925hMo (UNS N08926) was derived from the
904L alloy, and Nicrofer 3127hMo (UNS N08031)
was derived from alloy 28 metallurgy, by increasing the molybdenum content to 6.5% and
fortifying the composition with 0.2% nitrogen.
The addition of nitrogen provided the added benefits of improved localized corrosion resistance,
stronger mechanical properties, and higher
thermal stability.
Alloy 31, known as the “advanced 6 Mo alloy,”
is the higher-chromium-and-nickel version, and
imparts significantly improved corrosion resistance in a variety of media. Its localized corrosion
resistance is superior to many alloys, including the
Ni-Cr-Mo alloy 625, as shown
per ASTM G-48 testing. Its
uniform corrosion resistance
in sulfuric acid in the medium
concentration range is superior to even that of Alloy C276 and alloy 20. As a result,
the 6Mo alloys have been applied extensively in pulp and
paper production, phosphoric acid production,
copper smelters, and sulfuric
acid production/reclamation.
Weldability: The alloy
1925hMo, a “standard 6Mo
alloy,” is readily weldable
with over-alloyed filler metals
such as alloy 625, C-276, or 59.
These filler metals compensate for the segregation of
molybdenum into the interdendrite regions of weldments. On the other hand,
Alloy 31 is welded only with
alloy 59, except for applications such as chlorine-dioxide
Tank truck is constructed of alloy 31 for
bleach washers in the pulp
transporting extremely corrosive liquids.
and paper industry.
Alloy 31 is also frequently selected for equipment in pollution control and in the production
of phosphoric acid, rayon, and specialty chemicals. Its resistance to salt water corrosion has made
it successful in marine and offshore applications,
pickling baths, and heat exchangers for which
seawater and brackish water act as coolants.
One recent application of Alloy 31 has been
in the pressure acid leaching process of recovering nickel from the lateritic ore deposits in Australia. Many hundreds of tons of this alloy have
been used in this technology.
Alloy optimization
Table 1 shows the basic chemical composition
of Alloy 31 compared with other alloys that mitigate aqueous corrosion. It has half the nickel content of alloy 625, with lower amounts of molybdenum. However, its localized corrosion
resistance is superior to that of alloy 625. Nickel
has been kept as low as possible for reasons of
cost, while retaining a fully austenitic structure.
At the same time, nitrogen has been added to improve the stability of this austenitic structure, to
increase resistance to localized corrosion, and
to improve mechanical properties. The chromium
and molybdenum levels are the highest possible
to still be consistent with a fully austenitic alloy.
*Member of ASM International
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Continued
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• ASTM coverage.
• NACE MRO-175 coverage to level 6.
• Excellent fabricability with good thermal
stability.
Corrosion properties
Alloy 31 has excellent resistance to localized
corrosion in halide media, both pitting and crevice
corrosion. It also provides:
• Outstanding resistance to sulfuric acid.
• Excellent resistance to phosphoric acid.
• Good resistance to nitric and nitric/hydrofluoric acid mixtures.
• Excellent resistance to chlorine dioxide media
in the pulp and paper industry.
• Excellent resistance to both oxidizing and reducing media.
• Good resistance to stress corrosion cracking.
• ASME SCVIII Div 1 coverage to 430°C
(800°F).
Resistance to acids
Sulfuric acid: Table 2 gives the uniform corrosion data at various concentrations and temperatures. As is evident , Alloy 31 exhibits excellent corrosion resistance in dilute and medium
concentration up to 100°C (212°F). At concentrations greater than 80% and at 100°C (212°F) and
over, the alloy becomes active. However, in the
presence of oxidizing species such as ferric ions,
the range of Alloy 31 passivity can be expanded.
Table 1 — Metallurgical optimization of Alloy 31 vs. some other alloys
Alloy
Ni
Cr
Mo
Fe
Others
PRE*
316L
904L
1925hMo
20
825
28
31
G-3
G30
625
C-276
59
12
25
25
38
40
31
31
48
45
62
57
59
17
21
21
20
22
27
27
23
29
23
16
23
2.3
4.8
6.5
2.4
3.2
3.5
6.5
7
5
9
16
16
66
48
46
34
31
36
32
20
15
3
5
<1
—
Cu
Cu, N
Cu, Cb
Cu
Cu
Cu, N
Cu, Cb
Cu,W,Cb, Ta
Cb
W
—
24
37
48
29
32
38
54
45
45.5
52
69
76
*PRE = Pitting Resistance Equivalent = % Cr + % (3.3 Mo) + 30N
Table 2 — Corrosion rate in sulfuric acid at various temperatures in mils/year
H2SO4
60°C (140°F)
Percent, %
80°C (175°F)
100°C (210°F)
Alloy
20
Alloy
C276
Alloy
31
Alloy
20
Alloy
C276
Alloy
31
Alloy
20
Alloy
C276
Alloy
31
<5
<5
>5
5
<1
<2
<2
<1
<0.1
<0.1
<0.1
0.2
10
10
11
18
4
3
4
15
<0.1
<0.2
0.4
0.8
>25
>25
>50
>50
>1
10
11
240
0.3
0.6
1
240
20
40
60
80
Table 3 — Corrosion rate of various alloys in phosphoric acid in mm/year
Test media
52% P2O5 + Impurities
52% P2O5
30% P2O5
44% P2O5
54% P2O5
Very pure
+ Impurities
+ Impurities
+ Impurities
Temp., ° C
Alloy 31
Alloy 926
Alloy 28
Alloy G-3
Alloy 30
80
120
116
80
116
116
0.02
0.78
0.08 (1)
0.015
—
0.05 (1)
0.06
—
—
0.03
—
—
0.08
—
1.2
—
—
1.4
—
—
0.28
—
0.55
0.40
—
—
0.10
—
0.18
0.20
Table 4 — Critical pitting and crevice corrosion temperature
Alloy
316
904L
20
825
G-3
1925hMo
625
33
30
31
Critical pitting
corrosion temp., °C (°F)
15 (60)
45 (113)
15 (59)
30 (86)
70 (158)
70 (158)
77.5 (171.5)
85 (1850
75 (167)
85* (185)
Critical crevice
corrosion temp., °C (°F)
<0 (32)
25 (77)
<10 (50)
<5 (41)
40 (104)
40 (104)
57.5 (135.5)
40 (104)
50 (122)
65 (149)
Pitting resistance
equivalent (PRE)**
24
37
29
32
45
48
52
50
46
54
Per ASTM G-48 (10% FeCl3 ) * Above 85° C, the 10% FeCl3 solution chemically breaks down ** PRE = 5%Cr + 3.3(%Mo) + 30N
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Hydrochloric acid: In hydrochloric acid, Alloy
31 is resistant at room temperature up to a concentration of 8%, with corrosion rates below the
0.13 mm/y (5 mpy).
From 8% HCl to 40% HCl, the corrosion rate at
room temperature is between 0.13 and 0.5 mm/y
(5 mpy to 20 mpy).
At a higher temperature of 80°C (175°F), the
alloy is resistant up to 3% concentration. Hence,
in many chemical processes where only traces of
HCl are present, Alloy 31 presents a good alternative over more expensive nickel alloys of the
“B” or “C” family that contain higher amounts of
molybdenum.
Nitric acid: Tests were conducted in 67% boiling
HNO3 (Huey test). Even after ten boiling periods
of 48 hours each, Alloy 31 exhibited a very low corrosion rate of only 0.1 mm/y (4 mpy).
On welded samples (filler metal was alloy 625),
the corrosion rate increased to 0.2 mm/y (8 mpy),
still an acceptable rate. Despite the high molybdenum content of 6.5%, it exhibits excellent resistance in strongly oxidizing media.
Phosphoric acid: Laboratory tests in synthetic
phosphoric acid media with no solids present
(Table 3) did indeed confirm that the corrosion
resistance of Alloy 31 was superior to those of alternative alloy 926 (UNS N08926, a standard 6Mo
alloy) and alloy 28 (UNS N08028). It was comparable or even better than alloy G-30 (UNS
N06030). These tests also confirmed that corrosion of metals in phosphoric acid could increase
rapidly at or above a certain threshold temperature, which appears to be around 120°C (250°F).
Actual plant coupon tests in wet phosphoric
acid also confirmed the superior behavior of Alloy
31 over alloy G-30. A current application with approximately 780 tubes in concentrated 46% to 54%
P2O5 is demonstrating excellent performance. The
plant saved over $100K by selecting Alloy 31 as
the heat exchanger material over alloy G30.
Pitting/crevice corrosion
One of the key service criteria after uniform
corrosion is the ability to resist pitting and crevice
corrosion in the process side, which is primarily
acidic low pH with chlorides. ASTM G48 is one
of the standard laboratory tests established to
evaluate this type of corrosion, and it is being increasingly applied in specifications for sea/offshore and sour gas applications. The higher the
critical pitting temperature (CPT) and crevice corrosion temperature (CCT), the better is the resistance to localized corrosion. Table 4 lists the CCT
and CPT for a variety of alloys. Alloy 31 in this
group had the highest localized resistance, even
better than the Ni-Cr-Mo alloy 625.
Many tests over the years have been conducted
on Alloy 31 at Laque Center for Corrosion Testing,
Wrightsville Beach, N.C. For example, up to 60
days of multiple crevice assembly tests in filtered
seawater at 30°C (86°F) showed that the alloy has
good resistance to crevice corrosion attack.
Of the various test conditions, the real world
is most closely approximated when chlorinated
sea-water is pumped through pipe loops. Such
tests have been carried out with water from both
the Baltic Sea and the North Sea. In the Baltic Sea
test, Alloy 31 was resistant at temperatures up to
40°C (100°F) and chlorine levels up to 2 ppm.
Only at a temperature of 50°C (120°F) and a chlorine level of 2 ppm did alloy 31 show slight crevice
corrosion on two out of the ten flange joints.
In contrast, when placed in North Sea water,
which has much greater salinity, the alloy was resistant up to 45°C (110°F) at a 1.5 ppm level. However, at a temperature of 50°C (120°F) and a chlorine level of only 0.5 ppm, it showed crevice attack
on three out of the ten flange joints.
Intergranular corrosion resistance
Resistance to intergranular attack has been determined by the TTS (time-temperature-sensitization) plot when tested per the ASTM G28A test
method. Corrosion depth of greater than 0.002 in.
(50 microns) only develops after a few hours (between two and three hours) at the nose of the plot
(between 650 and 700°C, 1200 and 1290°F). To complement this result, welded samples of 6 mm plate
were tested in the ASTM G28A test. The weld
metal, heat affected zone, and the base metal were
totally free from intergranular attack. Hence the
alloy can be and is used in the welded condition
without the need for any post-weld heat treatment.
In the
Baltic Sea
test,
Alloy 31
was
resistant at
temperatures
up to 40°C
(100°F)
and chlorine
levels up
to 2 ppm.
Stress corrosion cracking
Austenitic stainless steels are susceptible to
stress corrosion cracking in chloride media. Alloy
31 will also crack in boiling 45% MgCl2 solution.
However, this environment is not encountered in
real world situations. Alloy 31 was tested in
boiling 62% CaCl2 solution, an environment more
closely related to the real world, for more than
2000 hours and exhibited total resistance to stress
corrosion cracking.
Alloy 31 was also tested under sour gas conditions (10 bar H2S at 232°C, 450°F). After 35 days,
the corrosion rate was 0.01 mm/y with no signs of
any pitting or stress corrosion cracking at a stress
of 95% of 0.2% yield strength. This alloy also complies with the NACE MRO-175 up to level 6.
Fabrication and specifications
Alloy 31 is readily fabricated by the standard
methods for stainless steels. It is weldable by all
conventional processes, including GTAW, GMAW,
SWAW, and plasma arc welding. The recommended filler metal is alloy 59, which has an AWS
A5.14 designation ErNiCrMo-13 and AWS 5.11
designation ENiCrMo-13. Greater details on fabrication are listed in the Alloy 31 data sheet. This
alloy is covered in all the standard ASTM specifications, NACE MRO-175 specification, and ASME
SC II, Part D for pressure vessel applications. For more information: D.C. Agarwal, ThyssenKrupp
VDM USA, 11210 Steeplecrest Drive #120, Houston, TX 77065-4939; tel: 281/955-6683; fax: 281/9559809; e-mail: dcagarwal@pdq.net; Web site: www.
thyssenkrupp.com.
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