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Copyright © 1991 by ASME
A Burner Rig Investigation of the Hot Corrosion
Behavior of Several Wrought Superalloys
and Intermetallics
G. Y. LAI, J. J. BARNES and J. E. BARNES
Haynes International, Inc.
Kokomo, Indiana
ABSTRACT
The hot corrosion behavior of wrought Ni- and Co-base
superalloys and nickel aluminide intermetallics was
studied in a burner rig at 900 ° C (1650 ° F) injected
with 5 and 50 ppm sea salt. Nickel aluminides (IC-50
and IC-218) were found to be extremely susceptible to
hot corrosion attack. Some state - of-the-art wrought
superalloys, such as HAYNES alloys 230 and 188, on
the other hand, were extremely resistant to hot corrosion. The behavior of different alloy systems,
breakaway corrosion and corrosion morphology are
discussed.
INTRODUCTION
Hot corrosion, a form of accelerated oxidation due to
sodium sulfate-rich deposits, has been an important
corrosion mode in gas turbines since the 1960's.
Sulfur from fuel reacts with sodium chloride from
ingested air during combustion in the combustor to
form sodium sulfate which deposits on the hotsection turbine components and results in accelerated
oxidation of those components. High Temperature alloys that suffered hot corrosion attack were generally found to exhibit both oxidation and sulfidation.
Various test methods have been used to study hot corrosion. The immersion testing is not considered a reliable method for simulating the gas turbine environment.( 1,2 ) The salt-coated method is quite popular
in academia for studying hot corrosion mechanisms.
However, little engineering data for gas turbine
alloys have been generated using this method. Engine
manufacturers, however, use the burner rig test system to determine the relative alloy performance ranking. This type of test system probably represents the
best laboratory apparatus for simulating the gas tur bine environment. The present paper discusses the
hot corrosion behavior of several Ni- and Co-base
superalloys based on the data generated by a burner
rig. Also discussed is the behavior of nickel aluminide intermetallics. Nickel aluminides along with
other intermetallics have recently generated a signif -
icant interest in academia and national laboratories.
The comparative performance of nickel aluminides and
and the state-of-the-art wrought superalloys is discussed. No attempts are made to correlate the burner
rig data with those generated by the salt-coated
method and immersion testing.
EXPERIMENTAL PROCEDURES
The burner rig uses fuel oil and air to generate the
test environment. A schematic of the rig used in the
present study is shown in Fig. 1. The description of
this type of the burner rig is available elsewhere.( 3 )
In the present study, No. 2 fuel oil was used, with
the sulfur content in the fuel being maintained at
about 0.4% (by weight). The combustion was generated
with an air-to-fuel ratio of 35 to 1. In order to ensure that a constant temperature is maintained during
the test, the test chamber is also heated by electrical resistance. The test specimens (typically 1.5 mm
thick x 9.0 mm width x 50 mm length) were loaded in a
carousel which rotated at about 30 rpm during testing
to ensure that all the specimens were subjected to the
same exposure condition (Fig. 2). In addition, the
specimens were cycled out of the combustion gas stream
once every hour for two minutes, during which time the
specimens were cooled by forced air (fan cool) to less
than 400 ° F.
During testing, a synthetic sea salt solution was continuously injected into the combustion zone to provide
a certain concentration of salt in the combustion gas.
The synthetic sea salt solution was prepared according
to ASTM D1141-52. The principal constituents of the
solution are NaCl, MgCl2, Na2SO4, CaCl2 and
KC1.
The materials tested in the present study are given
in Table 1. Two nickel aluminide intermetallics,
IC-50 and IC-218, were developed by Oak Ridge National Laboratory, and the samples were kindly provided
by the national laboratory.
Presented at the International Gas Turbine and Aeroengine Congress and Exposition
Orlando, FL June 3-6, 1991
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S.I,
Atomizing Air
Rot •1Inp
SMit
Fig. 1: Schematic of the hot corrosion test system
at Haynes International, Inc.
Fig. 2: Test specimens in a carousel
Table 1: Nominal Compositions (wt%) of Materials Under the Current Study
Material
Cr
Ni
HASTELLOY® alloy X
22.0
Bal
HASTELLOY alloy S
15.5
Bal
Co
1.5
Fe
W
18.5
0.6
3.0*
-
-
HAYNES@ alloy 230
22.0
Bal
-
3.0*
Alloy 625
21.5
Bal
-
2.5
HAYNES alloy 188
22.0
22.0
Bal
3.0*
HAYNES alloy 25
20.0
10.0
Bal
3.0
HAYNES alloy 150
27.0
Bal
18.0
IC-50
IC-218
-
7.8
-
14.0
Mo
Others
9.0
14.5
La 0.05, B 0.015*
2.0
La 0.02, B 0.015*
9.0
Cb 3.6
14.0
-
La 0.04
15.0
-
-
-
-
Bal
-
-
-
-
Bal
-
-
-
-
Al 11.3, Zr 0.6, B 0.02
Al 8.5, Zr 0.8, B 0.02
* maximum
HASTELLOY and HAYNES are registered trademarks of Haynes International, Inc.; 230 is a trademark of
Haynes International, Inc. IC-50 and IC-218 were developed by Oak Ridge National Laboratory.
RESULTS AND DISCUSSION
Figures 3 and 4 show the remaining metal in the center
of the sample for IC-50 and IC-218, respectively, after the hot corrosion test. The SEM/EDX analysis revealed pure nickel and nickel sulfides in the remaining material for both intermetallics. Apparently, the
aluminum or the level of aluminum in the aluminides
was not capable of forming a protective oxide scale.
Natesan( 4 ) found only Ni-rich oxides formed on
IC-50 when tested in air. Barnes et al.( 5 ) observed
the formation of NiO on IC-50 during the early stage
of the dynamic oxidation test at 1090 ° C (2000 ° F).
The results of the test conducted at 900 ° C (1650 ° F)
for 200 hours with 50 ppm salt are summarized in Table 2. Both Ni- and Co-base superalloys (i.e., alloy
X, 230, 25 and 188) suffered little corrosion attack.
On the other hand, the nickel aluminides were severely
corroded. In fact, the samples were completely corroded. Most of the metal was oxidized to form a thick,
porous Ni- or Ni-rich oxide for both intermetallics.
The surface analysis by scanning electron microscopy
with an energy dispersive x-ray analyzer (SEM/EDX) revealed mainly nickel in the surface oxides for both
IC-50 and IC-218. For IC-218, some areas enriched in
both nickel and aluminum in the surface oxides were
also detected.
2
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Table 2: Results of the Hot Corrosion Test at 900 ° C (1650 ° F)
for 200 Hours with 50 ppm Salt
Metal Loss
Weight Change
Material
mg/cm2
X
- 0.76
Average Metal Affected*
mm (mils)
mm (mils)
0.07 (2.8)
0.02 (0.6)
230
- 1.35
0.02 (0.8)
0.06 (2.4)
25
- 1.62
0.02 (0.9)
0.07 (2.8)
0.02 (0.7)
0.04 (1.6)
0.93
188
IC-50
72
Completely corroded (28.3 mils or 0.72mm thick)
IC-218
83
Completely corroded (29.5 mils or 0.75mm thick)
* metal loss + internal penetration
µ
.T
T
Semi-Quantitative Analysis (%)
Area
Ni
Al
1
96.4
3.6
2
74.7
-
3
100.0
-
4
74.9
-
Semi-Quantitative Analysis (%)
S
Area
Ni
Cr
Al
S
1
100
-
-
-
74
-
-
26
100
-
-
-
88
8
4
-
98
-
2
-
2
-
25.3
3
4
-
25.1
5
Fig. 4: SEM/EDX analysis on the cross-section of
the IC-218 intermetallic sample after the
hot corrosion test at 900 ° C (1650 ° F) for
200 hours with 50 ppm salt.
Fig. 3: SEM/EDX analysis on the cross section of
the IC-50 intermetallic sample after the
hot corrosion test at 900 ° C (1650 ° F) for
200 hours with 50 ppm salt.
"blisters" of oxides, consisting of Co- and Co-Cr
rich oxides (Fig. 9). Also observed was Co - rich
sulfides, as shown in Fig. 9.
As shown in Table 2, alloys X, 230 and 188 showed
little attack after the test. These three alloys
were found to exhibit a compact oxide scale, as shown
in Figs. 5, 6 and 7. These scales were Cr-rich oxides. Alloy 25, although exhibiting little corrosion
attack after 200 hours with 50 ppm salt, showed evidence of initial breakdown of the oxide scale (Fig.
8). The SEM/EDX analysis revealed a Co-rich oxide
scale (Fig. 8). The surface of the specimen showed
A long term test (1000-h) was performed under the
same test conditions (i.e., 900 ° C and 50 ppm salt) on
four Ni -base alloys (alloys S, X, 625 and 230) and
three Co-base alloys (alloys 25, 150 and 188). The
test results are summarized in Table 3. The samples
3
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the localized attack for the alloy 25 samples (two
samples) after 400 hours and those samples after 1000
hours.
of three Ni-base alloys (alloys S, X and 625) and one
Co-base alloy (alloy 25) were completely corroded
prior to the termination of the test. Alloys 230,
150 and 188 showed little attack after 1000 hours of
exposure.
It is not clear why alloy 25 was significantly more
susceptible to hot corrosion attack than other Ni and Co-base alloys with a similar level of chromium
(20-22%). Chromium has been found to be most effective in im p roving the hot corrosion resistance of
the alloy.( 6,7 ) These authors found that decreases
in chromium in the alloy resulted in increases in hot
corrosion attack. The poor hot corrosion resistance
of HASTELLOY alloy S (Table 3 and Fig. 11) may be attributed to the low chromium content (15.5%) in the
alloy. It may also be due to the alloy's high Mo content (14.5%). Molybdenum has been found to be det rimental.( 7 ) In the 1000-h test with 50 ppm salt,
three nickel-base alloys that did not perform well
contained molybdenum. More studies are needed in order to better understand the role of molybdenum.
Based on the excellent performance of HAYNES alloys
230 and 188 (14%W for both alloys), tungsten appears
to play no detrimental role in hot corrosion resistance for Ni or Co-base alloys containing about 22%
chromium.
The gravimetric data for alloys 25, 188 and 230 are
summarized in Fig. 10. Alloy 25 (Co-Ni-Cr-W alloy)
suffered breakaway corrosion after probably 200 hours
of exposure, and the sample was consumed after about
470 hours. Alloy 188, with a similar system but higher Ni plus a rare earth element (La), exhibited good
corrosion resistance. The 230 alloy is a Ni-Cr-W
system with La and was found to behave similarly to
alloy 188. In fact, both alloys have very similar
compositions except one is a Ni-base while the other
is a Co-base. Both utilize La for improving the alloy oxidation resistance. Comparison of alloy 188
with 230 alloy suggests that, contrary to common be lief, cobalt does not provide a better alloy base
than nickel in hot corrosion resistance, based on the
current burner rig data. Figure 11 shows appearances
of several samples after the test.
When the level of salt was reduced to 5 ppm, alloy 25
again did not survive the 1000-h test. Both alloys X
and 625, which failed to survive the 1000-h test with
50 ppm salt, survived the 1000-h test with 5 ppm
salt. These results are summarized in Table 4. The
gravimetric data for alloy 25 in comparison with alloy 188 are summarized in Fig. 12. The initiation of
breakaway corrosion for alloy 25 took place between
300 and 600 hours. Figure 13 shows the initiation of
1 S
-
::
^ Nm
Semi-Quantitative Analysis (%)
Ni
Cr
Fe
Co
W
Mo
1
9.5
75.1
13.3
2.1
-
-
-
2
7.2
85.8
4.5
-
0.7
-
1.8
3
32.9
41.7
19.9
-
-
5.5
-
4
44.6
26.5
23.3
-
0.2
5.5
-
5
42.0
31.1
21.2
-
-
5.7
-
Area
S
Fig. 5: SEM/EDX analysis on the cross-section of the HASTELLOY alloy X
sample after the hot corrosion test at 900 ° C (1650 ° F) for 200
hours with 50ppm salt.
4
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5
1QP_
1
Semi-Quantitative Analysis (%)
Area
Ni
Cr
W
Mo
S
1
39.2
56.3
4.5
-
-
2
22.0
74.2
3.8
-
-
3
69.6
21.7
6.3
-
2.4
4
69.0
25.0
5.3
-
0.7
5
58.0
34.2
5.6
2.2
-
Fig. 6: SEM/EDX analysis on the cross - section of the HAYNES
alloy 230 sample after the hot corrosion test at 900 ° C
(1650 ° F) for 200 hours with 50 ppm salt.
3
2
4
io NmI
Semi-Quantitative Analysis (Y)
Area
Co
Cr
Ni
W
S
1
18.7
73.2
4.9
1.5
1.7
2
31.2
39.2
22.1
6.5
1.0
3
39.2
27.9
27.4
5.5
-
4
33.0
37.8
22.4
6.8
-
Fig. 7: SEM/EDX analysis on the cross - section of the
HAYNES alloy 188 sample after the hot corrosion
test at 900 ° C (1650 ° F) for 200 hours with 50 ppm salt.
5
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i1 0psi'
Semi-Quantitative Analysis (%)
Area
Co
Ni
16.3
1
76.0
2
8.7
3
48.5
Cr
-
11.2
W
S
7.0
0.7
-
78.5
12.8
-
20.8
7.5
12.5
Fig. 8: SEM/EDX analysis on the cross-section of the
HAYNES alloy 25 sample after the hot corrosion
test at 900 ° C (1650 ° F) for 200 hours with 50ppm
salt.
wFez
w
Semi Quantitative Analysis (%)
-
Area
Co
Cr
Ni
S
Na
Ca
1
56.1
41.3
2.5
0.1
-
-
-
2
46.5
51.4
2.1
-
-
-
-
W
3
47.6
42.1
2.2
6.6
1.5
-
-
4
31.3
64.1
2.1
2.1
0.4
-
-
5
91.1
6.8
2.1
6
36.5
10.5
3.8
-
40.2
-
5.0
-
2.4
-
1.6
Fig. 9: SEM/EDX analysis on the surface of the HAYNES alloy 25 sample after
the hot corrosion test at 900 ° C (1650 ° F) for 200 hours with 50ppm salt.
6
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Table 3: Results of the Hot Corrosion Test at 900 ° C (1650°F)
for 1000 hours with 50 ppm Salt
Weight Change
Material
Metal Loss
mg/cm2
Average Metal Affected*
mm (mils)
mm (mils)
S
-
X
-
Localized attack through thickness in 500h (45.9 mils or 1.2mm thick)
625
-
Completely corroded in 940h (63.7 mils or 1.6mm thick)
230
11.5
25
-
150
10.0
0.03 (1.2)
0.13 (5.2)
188
9.9
0.03 (1.0)
0.08 (3.2)
Completely corroded in 350h (49.2 mils or 1.3mm thick)
0.02 (0.9)
0.11 (4.4)
Completely corroded in 476h (37.8 mils or 1.0mm thick)
* metal loss + internal penetration
+20
HAYNES alloy 230
+10
0
HAYNES alloy 188
-10
-20
-30
-40
-50
N 60
E
E
e
Ce
-70
-80
-90
-100
-110
-120
-130
-140
-150
-160
-170
-180
0
100 200 300 400 500 600 700 800 900 1000
Exposure Time, hr
Fig. 10: Gravimetric data for HAYNES alloy 230, 188 and 25 from the
hot corrosion test at 900°C (1650°F) with 50ppm salt.
7
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Fig. 11: Samples of (a) HASTELLOY alloy S after 350h, (b) HAYNES
alloy 230 after 1000h, (c) HAYNES alloy 25 after 476h and
(d) HAYNES alloy 188 after 1000h in the hot corrosion test
at 900 ° C (1650 ° F) with 50 ppm salt.
Table 4: Results of the Hot Corrosion Test at 900 ° C (1650 ° C)
for 1000 Hours with 5 ppm Salt
Weight Change
Material
mg/cm2
Metal Loss
mm (mils)
Average Metal Affected*
mm (mils)
X
- 0.24
0.04 (1.6)
0.14 (5.5)
230
- 0.79
0.03 (1.2)
0.13 (5.1)
625
5.87
0.05 (1.9)
0.14 (5.3)
25
188
-
1.09
Completely corroded in 1000ti (43 mils or 1.09mm thick)
0.02 (0.8)
0.07 (2.8)
* metal loss + internal penetration
8
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.10
HAYNES alloy 188
0
-10
-20
.30
-40
-50
-60
E -70
N
E
-80
ui
N
-90
U -100
0
-110
-120
-130
-140
HAYNES alloy 25
150
-160
-170
-180
-1901
1
1
1
1
1
1
1
1
1
1
0 100 200 300 400 500 600 700 600 900 1000
Exposure Time, hr
Fig. 12: Gravimetric data for HAYNES alloy 188 and 25 from the hot
corrosion test at 900°C (1650°F) with 5ppm salt.
b
Fig. 13: Photographs showing (a) the localized attack for alloy 25 samples after 400 hours
at 900 ° C (1650 ° F) with 5 ppm salt, and (b) the same alloy 25 samples (two samples)
after 1000 hours. The samples were completely corroded after 1000 hours.
9
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CONCLUSIONS
Nickel aluminide intermetallics (IC-50 and IC-218)
were found to be extremely susceptible to hot corrosion attack. Both intermetallics, which failed to
produce a protective scale, suffered severe oxidation
and sulfidation. On the other hand, some state-ofthe-art wrought superalloys, such as HAYNES alloys 230
and 188, were extremely resistant to hot corrosion attack. These alloys formed a protective Cr-rich oxide
scale with little or no sulfidation attack.
REFERENCES
1.
Conde, J. F. G and Booth, G. C., in Deposition
and Corrosion in Gas Turbines, Edited b
Hart and A. J. B. Cutler, John Wiley & Sons, New
York (1973), p. 278.
2.
Donachie, M. J., Sprague, R. A., Russel, R. N.,
Boll, K. G., and Bradley, E. F., in Hot
Corrosion Problems Associated with Gas Turbines,
STP 421, ASTM, Philadelphia, PA (1967), p. 85.
3.
Doering, H. Von E., and Bergman, P., Materials
Research and Standards, Vol. 9, No. 9, Sept.
1969, p. 35.
4.
Natesan, K., Met. Res. Soc. Symp. Proceedings,
Vol. 81 (1987), p. 459.
5.
Barnes, J. J., Lai, G. Y. and Barnes, J. E.,
Paper No. 67, CORROSION/91, NACE, Houston, TX.
6.
Bergman, P. A., Beltran, A. M. and Sims, C. T.,
"Development of Hot Corrosion-Resistant Alloys
for Marine Gas Turbine Service", Final Summary
Report to Marine Engineering Lab, Navy Ship Res.
and Dev. Center, Annapolis, MD, Contract N600
(61533) 65661, Oct. 1. 1967.
7.
Bergman, P. A., Sims, C. T. and Beltran, A. M.
in Hot Corrosion Problems Associated with Gas
Turbines, STP 421, ASTM, Philadelphia, PA
(1967), p. 38.
10
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