NICKEL
ALLOYS
Nickel and nickel alloys have useful resistance to a wide variety of corrosive
environments encountered in various
industrial processes. This report is
based on a paper presented at the
NACE Corrosion 2001 conference in
March in Houston, Texas.
Part II of this two-part series covers
Meet Corrosion Challenges
PART TWO
D.C.Agarwal*
Krupp VDM Technologies
Houston, Texas
T
he latter half of the 20th century saw a
phenomenal growth in the development
of nickel-base corrosion resistant alloys,
primarily due to the excellent metallurgical compatibility of nickel with alloying elements
such as chromium, molybdenum, cobalt, iron,
copper, tantalum, tungsten, and nitrogen. These elements impart unique and very specific corrosionresistant and high-temperature properties for han*Member of ASM International
alloy 825, the G and C families of
alloys, and several others.
dling the corrosive environments of chemical
process, petrochemical, marine, pulp and paper,
agrichemicals, oil and gas, heat treat, energy conversion, and many other industries. Improved
melting and thermo-mechanical process innovations, together with a better fundamental understanding of the role of these alloying elements and
their physical metallurgy, were key factors in the
development of binary, ternary, and other complex
nickel alloy systems.
This article continues the discussion of nickel alloys that was begun in the June article. It covers
alloy 825, the “G” family alloys, 6Mo alloys, Alloy
20, and the “C” family of alloys. It presents their
major characteristics, the effects of alloying
elements, and their strengths,
limitations, and applications.
Alloy 825
Alloy 825 is a modification of
alloy 800 with an addition of
molybdenum (3%), copper (2%),
and titanium (0.9%). These elements provide improved resistance to aqueous corrosion and
a wide variety of other corrosive
media. Its high nickel content of
about 42% provides excellent resistance to chloride-ion stress
corrosion cracking when tested
in boiling magnesium chloride
solutions. This alloy has been an
upgrade to the 300 series stainless steels, for which localized
corrosion and stress corrosion
cracking has been a problem.
The high nickel, in conjunction
with the higher molybdenum
and copper, provides good resistance to reducing environments such as those containing
sulfuric and phosphoric acids.
Laboratory test results and service experience have confirmed
This pressure vessel is made of alloy 59 to handle very severe corrosive media containing fluorides and the useful resistance of alloy 825
in boiling solutions of sulfuric
chlorides during production of various agrichemicals for Bayer AG, Germany.
48
ADVANCED MATERIALS & PROCESSES/AUGUST 2001
acid up to 40% by weight, and at all concentrations
up to a maximum temperature of 66°C. Corrosion
resistance is also higher in the presence of oxidizing
species other than oxidizing chlorides, which may
form HCl by hydrolysis.
Therefore, the alloy is suitable for acid mixtures
containing nitric acid, and cupric and ferric sulfates.
In pure phosphoric acid, the alloy is resistant at concentrations and temperatures up to and including
boiling 85% acid. High chromium content confers
resistance to a variety of oxidizing media such as
nitric acid, nitrates, and oxidizing salts. The titanium addition with an appropriate heat treatment
serves to stabilize the alloy against sensitivity to intergranular attack.
Alloy 825 is a versatile alloy capable of handling
a wide variety of corrosive media, but it has gradually begun to be replaced by other alloys that provide superior resistance to localized corrosion.
These include the “G” family alloys and the 6%
moly superaustenitic stainless steels, such as alloy
1925hMo (N08926) and alloy 31 (N08031). Some
typical applications include various components
in sulfuric acid pickling of steel and copper, components in petroleum-refineries and petrochemicals (tanks, agitators, valves, pumps), and equipment for production of ammonium sulfate
“G” family: G /G-3 / G30
• Alloy G was a development from alloy F, an
alloy of similar composition, but with addition of
about 2% copper. This addition of copper significantly improved the corrosion resistance in both sulfuric and phosphoric acid environments. Alloy G,
developed in the 1960s, had excellent corrosion resistance in the as-welded condition, and could handle
the corrosive effects of both oxidizing and reducing
agents. The alloy exhibited resistance to mixed acids,
fluorosilicic acid, sulfate compounds, concentrated
nitric acid, flue gases of coal-fired power plants, and
hydrofluoric acid. Its higher nickel and molybdenum
content (over alloy 825) render it essentially immune
to chloride stress corrosion cracking. It also has significantly superior localized corrosion resistance.
However, this alloy is now obsolete, and has been
replaced by alloy G-3.
• Alloy G-3 is an improved version of alloy G. It
has similar excellent resistance to corrosion, but
greater resistance to HAZ attack, and offers better
weldability. Its lower carbon content enables slower
kinetics of carbide precipitation, and its slightly
higher molybdenum content provides for superior
resistance to localized corrosion. Alloy G-3 has replaced alloy G in almost all industrial applications
to date, as well as alloy 825 in many applications
where better localized corrosion resistance is needed.
• Alloy G30 is a modification of G-3 in which
chromium has been significantly increased and
molybdenum has been diminished. The alloy
shows excellent resistance in commercial phosphoric acids as well as many complex and mixed
acid environments of nitric/hydrochloric and nitric/hydrofluoric acids. The alloy has good resistance in sulfuric acid also. Some typical applications of this alloy have been in phosphoric acid
service, mixed acid service, nuclear fuel reprocessing, components in pickling operations, petrochemicals, agrichemicals manufacture (fertilizers,
insecticides, pesticides, herbicides), and mining industries. However, the advanced 6Mo alloy 31,
UNS N08031, has shown equal to better performance than G30, and at a significantly lower cost.
6 Mo alloys
These alloys were developed when it was discovered that chromium and molybdenum improve resistance to localized corrosion, and that higher levels
of nickel and nitrogen enhance resistance to chloride
SCC. The 6Mo alloys are very cost-effective, and approach or equal the corrosion resistance of more expensive high Ni-Cr-Mo alloys in many environments,
thus bridging the cost/performance gap.
Alloy 1925hMo was derived from the alloy 904L,
and alloy 28 was derived from alloy 31 metallurgy,
by increasing the molybdenum content from 4.5%
to 6.5% and by fortifying with 0.2% nitrogen. The
addition of nitrogen provides the added benefits
of improved localized corrosion resistance, mechanical properties, and thermal stability.
Alloy 1925hMo is readily weldable with overalloyed filler metals such as alloy 625, C-276, or 59,
which compensate for the segregation of molybdenum in the inter-dendritic regions of weldments.
However, alloy 31 may be welded only with alloy
59. Alloy 31, known as the “advanced 6 Mo alloy,”
is the higher “chromium-nickel” version, and these
elements impart higher corrosion resistance in a
variety of media. Alloy 31’s localized corrosion resistance is superior to the Ni-Cr-Mo alloy 625 and
alloy G30, as shown in ASTM G-48 testing. Its uniform corrosion resistance in sulfuric acid at a
medium concentration range is also superior to that
of Alloy C-276 and alloy 20.
However, care must be taken when specifying
this alloy for higher concentrations and temperatures. At 80% concentration and temperatures
above 80°C, alloy 31 exhibits active behavior. At
lower concentrations (less than 80%) the alloy remains passive up to 100°C. Alloy 31 uniform corrosion resistance in wet phosphoric acid production is equal to alloy G30, with the added advantage
of lower density, higher allowable ASME stresses,
and better thermal conductivity. All these contribute
to a much lower cost of shell and tube heat exchangers in phosphoric acid with concentrations
from 42 to 54% and higher.
The 6Mo alloys are extensively applied in pulp
and paper, phosphoric acid, copper smelters, sulfuric acid production, and reclamation of spent acid.
It is widely preferred for equipment in pollution
control, rayon production, specialty chemicals production, marine and offshore applications, heat exchangers with seawater and brackish water as
coolant, and pickling baths.
6Mo
alloys were
developed
when it was
discovered
that
chromium
and
molybdenum
improve
resistance to
localized
corrosion.
Alloy 20
The first version of alloy 20 was introduced in
1951 for sulfuric acid service. Later it was modified
with columbium (niobium) additions, and was
known as 20Cb. This allowed its service in the as-
ADVANCED MATERIALS & PROCESSES/AUGUST 2001
49
Table 1 — Uniform corrosion rates
of some Ni-Cr-Mo alloys in mils per year*
alloys in a variety of standard laboratory environments (Table 1 ). Eliminating tungsten and copper
and reducing the iron content to very low levels reBoiling media
C-276
22
686
2000
59
sulted in an alloy with superior thermal stability
characteristics, as shown in Table 2. Not only is the
ASTM 28A
240
36
104
27
24
uniform corrosion behavior and the thermal stability
ASTM 28B
55
7
38
4
4
improved, but also its localized corrosion resistance
Green Death
26
4
8
—
5
is improved over alloy C-276, 22, and 2000 (Table 3).
19
2
—
—
2
10% HNO3
65% HNO3
750
52
231
—
40
Localized corrosion has caused more failures in the
10% H2SO4
23
18
—
—
8
chemical process industries than any other single
50% H2SO4
240
308
—
—
176
corrosion phenomenon, and has led to many un1.5% HCl
11
14
5
2
3
scheduled shut downs, causing huge economic
10% HCl
239
392
—
—
179
losses. Uniform corrosion in these high alloys has
87
354
—
—
70
10% H2SO + 1% HCl
not generally caused any major problems or un10% H2SO4 + 1% HCl (90°C) 41
92
—
—
3
scheduled shut downs. A description of these alloys
* To convert to mm/y multiply by 0.0254
is presented below:
• Alloy C (1930s to 1965): The compatibility of
Table 2 — Corrosion rate in mils per year*
nickel with chromium and molybdenum, and opMedia
C-276
22
686
2000
59
timization between Ni-Cr and Ni-Mo alloys, led to
the first alloy of the “C” family, alloy C in the 1930s.
1
1
1
1
2
G 28A
>500
>500
872
116
40
(The development of this alloy was well described
G 28 B
>5001
3391
171
>5001
42
by McCurdy in 1939.) It was the most versatile cor*As shown in ASTM G-28A and G-28B after sensitization at 1600°F (871°C) for 1 hour.
rosion-resistant alloy available in the 1930s through
1. Alloy C-276, 22, 2000 and 686 - Heavy pitting attack with grains falling due to deep inter-granular attack. 2. Alloy 59 - No pitting attack
mid 1960s to handle the needs of the chemical
process industry. However,
Table 3 — Localized corrosion resistance in “Green Death” solution
it had a few severe drawbacks. For example, in the
Pitting resistance
Critical pitting
Critical crevice
as-welded condition, alloy C
Alloy
equivalent, PRE
temperature, CPT, °C
temperature, CCT, °C
was often susceptible to se22
65
120
105
rious intergranular corrosion
C-276
69
110
105
attack in the heat affected
2000
76
110
100
zone in many oxidizing, low
686
74
>120*
110
pH, halide-containing envi59
76
>120*
110
ronments. This meant that
* Above 120°C, the Green Death solution (11.5% H2SO4 + 1.2 % HCl + 1% FeCl3 + 1% CuCl2) chemically breaks down.
for many applications, veswelded condition without the need for a post-weld sels fabricated from alloy C had to be solution heatheat treatment. Further research led to the alloy treated to remove the detrimental weld HAZ pre20Cb3, known as alloy 20, by increasing the nickel cipitates. This put a serious limitation on its
content. This modern version of alloy 20 has been usefulness.
successful because of its superior corrosion resisDuring the late 1940s and 1950s, the chemical
tance in sulfuric acid media and its resistance to process industry was constantly coming up with
stress corrosion cracking.
new processes, which needed an alloy without the
Among its applications are the manufacture of limitation of “solution heat treating” after welding.
synthetic rubber, high octane gasoline, solvents, ex- Another drawback was that in severe oxidizing
plosives, plastics, synthetic fibers, chemicals, phar- media, this alloy did not have enough chromium
maceuticals, food processing, and many others. to maintain useful passive behavior, thus exhibiting
However, it contains insufficient molybdenum for high uniform corrosion rates.
localized corrosion resistance in low pH acidic chlo• Alloy C-276 (1965 to present day): To overcome
ride media.
one of the above serious limitations, the chemical
composition of alloy C was modified by a German
C-Family Ni-Cr-Mo alloys
company, BASF. The modification basically conAlloy C, the oldest alloy of this family (now ob- sisted of reducing both the carbon and silicon levels
solete), was superseded by Alloy C-276 in the early by more than ten-fold, to the very low levels of typ1960s as a direct result of improvements in melting ically 50 ppm carbon and 400 ppm silicon. This was
technology. Between 1983 and 1996, four new al- possible only because of a new melting technology
loys of this family were commercially introduced known as the argon-oxygen decarburization (AOD)
to the market place: alloy 22 in 1983, alloy 59 in 1989, process. This low carbon-and-silicon content alloy
alloy 686 in 1995, and alloy 2000 in 1996. As the came to be known as alloy C-276, which then was
chemical composition shows, alloy 2000 is in reality produced in the United States under a license from
alloy 59 with the addition of 1.6% copper to cir- BASF Company. (Their patent expired in 1982.)
cumvent the alloy 59 patent.
The corrosion resistance of both alloys was esAlloy 59, the purest ternary alloy of the Ni-Cr-Mo sentially similar in many corrosive environments,
family , has the highest PRE (pitting resistant equiv- but without the detrimental effects of continuous
alent ) number and the lowest iron content. This pro- grain boundary precipitates in the weld HAZ of
vides for improved corrosion resistance over other alloy C-276. Thus alloy C-276 would be suitable for
50
ADVANCED MATERIALS & PROCESSES/AUGUST 2001
most applications in the as-welded condition
without undergoing severe intergranular attack.
• Alloy C-4 (1970s to present): In addition to the
ten-fold decrease in carbon and silicon of alloy C,
alloy C-4 had three other major modifications: omission of tungsten from its basic chemical composition, reduction in the iron level, and the addition
of some titanium. The above changes resulted in
significant improvement in the precipitation kinetics of intermetallic phases. When exposed in the
sensitizing range of 550°C to 1090°C for extended
periods of time, the intermetallic and grain
boundary precipitation of the “mu” phase is practically eliminated. The mu phase has a (Ni, Fe, Co)3
(W, Mo, Cr)2 type structure and various other
phases. These phases are detrimental to ductility,
toughness, and corrosion resistance.
The general corrosion resistance of alloy C-276
and alloy C-4 was essentially the same in many corrosive environments, except that in strongly reducing media such as hydrochloric acid, alloy C276 was better. In highly oxidizing media, the
opposite was true, and alloy C-4 was better. Alloy
C-4 offers good corrosion resistance to a wide variety of media, including organic acids and acid
chloride solutions.
• Alloy 22 (1982 - Present): After the alloy C-276
patent expired in the United States in 1982, a newer
development in the “C” family, was introduced:
alloy 22. Inventors claimed that the “mu” phase
was controlled in alloy C-4 by controlling the “electron vacancy” number by means of deleting tungsten and reducing iron. However, the result was
less resistance to corrosion in reducing chloride solutions, where tungsten is a beneficial element. In
addition, both alloys C-276 and C-4 had high corrosion rates in oxidizing, non-halide solutions, due
to their relatively low chromium levels of 16%.
Therefore, an alloy with higher chromium levels
and an optimized balance of Cr, Mo, and W levels
was needed for oxidizing environments, thus
yielding superior corrosion properties and good
thermal stability.
This led to the alloy 22 composition, with approximately 21% Cr, 13% Mo, 3% W, 3% Fe with balance nickel. Even though its corrosion resistance was
superior to alloy C-276 and C-4 in highly oxidizing
environments, and showed slightly better pitting
corrosion resistance in “Green Death” solution, its
behavior in highly reducing environments and in
severe localized crevice corrosion conditions was
still inferior to the 16% molybdenum alloy C-276.
• Alloy 59 (1990 - present): Research efforts during
the 1980s in Germany led to the another alloy development within the Ni-Cr-Mo family: alloy 59.
This overcame the shortcomings of both alloy 22
and C-276. It also provided solutions to the most
severe and critical corrosion problems of the CPI,
petrochemical, pollution control, and other industries. As the composition of the various members
of the “C” family shows, alloy 59 has the highest
chromium-plus-molybdenum content with the
lowest iron content, typically less than 1%. It is one
of the highest nickel-containing alloy of this family,
and is the purest form of a “true” Ni-Cr-Mo alloy
Table 4 — Hazardous waste incineration
scrubber corrosion data*
Alloy
Mils per year1
Remarks
59
686
22
31
622
C-276
625
825
1.1
5.4
6.7
7.1
12.1
35.1
58.6
117
Clean
Clean
Clean
Clean
Weld attack
Clean
Rough
Pitting attack
*3M study. 1. To convert to mm/y multiply by 0.0254.
without the addition of any other alloying elements,
such as tungsten, copper, or titanium. This purity
and balance of alloy 59 in the ternary Ni-Cr-Mo
system, is mainly responsible for its superior
thermal stability behavior.
• Alloy 686 (1993 - present): This is another recent
development in the “C” family of Ni-Cr-Mo alloys,
and is very similar in composition to alloy C-276.
The difference is that the chromium level has been
increased from 16% to 21%, while maintaining the
molybdenum and tungsten at similar levels. This
composition is highly over-alloyed, with the combined chromium, molybdenum, and tungsten content of around 41%. To maintain its single-phase
austenitic structure, it must be solution annealed at a
very high temperature of around 1200°C, followed
by very rapid cooling to prevent precipitation of intermetallic phases. Its thermal stability behavior is
significantly inferior to alloy 59 (Table 2) and its performance in field tests in a hazardous waste incinerator at 3M Co., St. Paul, Minn., showed five times
lower corrosion resistance than alloy 59. Table 4 presents this data comparing various alloys.
• Alloy 2000 (1995 - present): This is the another
recent introduction in the “C” alloy family, in which
basically 1.6% copper has been added to the alloy 59
composition. Addition of copper has resulted in
significantly lower thermal stability behavior in
comparison to alloy 59 (Table 2) and lower local■
ized corrosion resistance (Table 3).
For more information: D.C. Agarwal, Krupp VDM Technologies, 11210 Steeplecrest Drive # 120, Houston, TX
77065-4939; tel: 281/955-6683; fax: 281/955-9809; e-mail:
[email protected].
ADVANCED MATERIALS & PROCESSES/AUGUST 2001
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