Materials Transactions, Vol. 52, No. 3 (2011) pp. 433 to 438
#2011 The Japan Institute of Metals
High Temperature Corrosion of CoNiCrAlY-Si Alloys
in an Air-Na2 SO4 -NaCl Gas Atmosphere
Toto Sudiro1; * , Tomonori Sano1; *, Shoji Kyo2 , Osamu Ishibashi3 ,
Masaharu Nakamori4 and Kazuya Kurokawa5
1
Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Kansai Electric Co. Inc., Amagasaki 661-0974, Japan
3
Osaka Fuji Corporation, Takaishi 592-0001, Japan
4
High Temperature Corrosion & Protection Technosearch Co. Ltd., Takasago 676-0082, Japan
5
Center for Advanced Research of Energy and Materials, Faculty of Engineering, Hokkaido University, Sapporo 060-8628, Japan
2
The addition of Si to CoNiCrAlY was considered in order to develop corrosion resistant alloys for coating applications. The high
temperature corrosion behavior of spark plasma sintered CoNiCrAlY alloys of 0, 10, 20, and 30 mass% Si content was investigated in an air(Na2 SO4 +25.7 mass%NaCl) gas atmosphere at elevated temperatures of 923, 1073, and 1273 K. The results showed that CoNiCrAlY-Si alloys
formed mainly an Al2 O3 or Al2 O3 /SiO2 scale, depending on the alloy composition and the temperature. The addition of 30 mass% Si
significantly improved the high-temperature corrosion resistance of CoNiCrAlY alloy in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere at
these temperatures. The high temperature corrosion behavior of CoNiCrAlY-Si alloys was discussed in the present paper.
[doi:10.2320/matertrans.MBW201026]
(Received October 21, 2010; Accepted December 1, 2010; Published January 26, 2011)
Keywords: CoNiCrAlY-Si, spark plasma sintering, high temperature corrosion, Na2 SO4 +25.7 mass%NaCl
1.
Introduction
Future advanced boiler power plants will be operated at
much higher temperatures and pressures to achieve higher
thermal efficiency.1,2) As a consequence, the fireside of
metallic components will be continuously exposed to high
temperatures and complex combustion products coming
from ingested air and fuel such as oxygen, sulfur, chlorine,
sodium, and potassium.3) In this atmosphere, high temperature corrosion can potentially take place.
STBA21 steel, which has been widely used for boiler
power plant applications, may not meet the future needs of
advanced boiler applications. At 923 K, STBA21 steel suffers
from severe corrosion in a Na2 SO4 -NaCl atmosphere.4) On
the other hand, high temperature corrosion of alloys generally
becomes more severe with increasing temperatures. As an
alternative approach, a protective coating can be used to
protect the base material against rapid degradation caused by
complex combustion products.
In recent years, increased attention has been paid to
MCrAlY (M = Co and/or Ni) for high-temperature coating
applications because of their satisfactory performance, such
as good ductility, high oxidation and corrosion resistance
via formation of Al2 O3 scale. However, in NaCl-containing
atmospheres, the corrosion resistance of materials generally
decreases.5–7) Therefore, much effort has been devoted to Sicontaining alloys or SiO2 base coatings which generally have
excellent corrosion resistance in corrosive atmospheres.8–10)
Several works on the addition of small amounts of Si
( 2 mass%) have been carried out by some researchers
to improve the properties of MCrAlY coating.11–19) In this
study, the addition of Si of up to 30 mass% to CoNiCrAlY
was considered in order to form a SiO2 scale or a SiO2 based
*Graduate
Student, Hokkaido University
multi-system scale and to develop a corrosion resistant alloy
for coating applications. The high temperature corrosion
behavior of CoNiCrAlY-Si alloys was investigated and the
results are discussed here.
2.
Experimental Procedures
2.1 Specimen preparation
Commercial CoNiCrAlY powder and high-purity Si
powders were used in this investigation. The chemical
compositions of CoNiCrAlY-Si powders are given in
Table 1. These powders were ball-milled in air using a WC
milling media with a ball to powder weight ratio of 5 : 1. The
powder millings were conducted at 150 rpm for 2 1:8 ks for
revised direction.
The powder mixture was then compacted in a carbon die
and sintered using a spark plasma sintering (SPS) technique.
The sintering process was performed at the same heating rate
of 0.17 K/s and under compressive stress of 40 MPa in an
evacuated chamber of less than 4 Pa. The sintering temperatures of the CoNiCrAlY-Si alloys were about 1123–1223 K
for 1.98–2.58 ks.
The test specimens were cut from the sintered
CoNiCrAlY-Si alloys into dimensions of about 11:4 5:8 1:5 mm. The test specimens were then polished down
using various grades of SiC paper of up to #1500, and
diamond polished for mirror finishing prior to ultrasonic
cleaning in ethanol solution.
2.2 High temperature corrosion test
High temperature corrosion tests were performed in an air(Na2 SO4 +25.7 mass%NaCl) gas atmosphere at elevated
temperatures of 923 K and 1073 K for up to 720 ks and
1273 K for 72 ks. As shown in Fig. 1, the test specimens were
put in a lidded alumina crucible containing a salt mixture of
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T. Sudiro et al.
Chemical composition of CoNiCrAlY-Si powders.
30
Nominal composition
of respective elements (mass%)
Powder compositions
Co
Ni
Cr
Al
Y
Si
CoNiCrAlY
38.5
32.0
21.0
8.0
0.5
—
CoNiCrAlY-10 mass%Si
34.7
28.8
18.9
7.2
0.5
10.0
CoNiCrAlY-20 mass%Si
30.8
25.6
16.8
6.4
0.4
20.0
CoNiCrAlY-30 mass%Si
27.0
22.4
14.7
5.6
0.4
30.0
Consumed Thickness of Alloy (µm)
Table 1
CoNiCrAlY
CoNiCrAlY-10Si
CoNiCrAlY-20Si
CoNiCrAlY-30Si
20
10
0
Tested
specimen
Alumina
rod
Na2SO4+25.7mass%NaCl
Fig. 1 Schematic of high-temperature corrosion test of CoNiCrAlY-Si
alloys in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere.
Na2 SO4 +25.7 mass%NaCl which is close to the eutectic
composition (Na2 SO4 +30.8 mass%NaCl: m.p. = 901 K)20)
and heated in a muffle furnace at the desired temperatures.
The salt mixture was chosen in this investigation to simulate
the corrosive atmosphere that results from the combustion
products of fuel. After the test, the corroded specimens were
cooled down to room temperature and washed in water prior
to further investigation.
2.3 Evaluation and characterization
The corrosion behavior of the tested specimens was
evaluated by measuring the consumed thickness of alloy,
which is sum of the thickness loss of the alloy and the
thickness of an internal corrosion attack zone, based on the
cross-sectional observations of the corroded specimens. The
scale morphology and the alloy metallurgy of the tested
specimens before and after the corrosion tests were analyzed
using X-ray diffraction (XRD), scanning electron microscope
with energy dispersive X-ray analysis (SEM-EDX), and
electron-probe microanalysis (EPMA).
3.
Results and Discussion
3.1 Phases of CoNiCrAlY-Si alloys
Phase composition analysis before the corrosion test
revealed that the CoNiCrAlY alloy was composed of Ni3 Al,
(Co,Ni)Al, Cr, and (Co,Ni). The addition of Si changed the
microstructure and phase composition of the CoNiCrAlY
alloy. XRD results showed that new phases such as CrSi2 and
NiSi2 were formed in the CoNiCrAlY-Si alloys. With
increase in Si content, the peak intensities of the new phases
sharply increased. For the CoNiCrAlY alloy with 30 mass%
Si content, the alloy was composed of Cr3 Si, AlNi2 Si,
(Co,Cr)Si, NiSi2 , CrSi2 , and Si-precipitates.
3.2
High
temperature
corrosion
behavior
of
CoNiCrAlY-Si alloys
3.2.1 High temperature corrosion at 923 K
Figure 2 shows the corrosion kinetics with respect to the
0
150
300
450
Corrosion Time, t/ks
600
750
Fig. 2 Corrosion kinetics curve of CoNiCrAlY-Si alloys corroded at 923 K
for up to 720 ks in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere.
consumed thickness of CoNiCrAlY-Si alloys corroded at
923 K as a function of corrosion time for up to 720 ks in
an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere. Crosssectional morphologies of CoNiCrAlY alloys with 0, 10, 20,
and 30 mass% Si content after 720 ks exposure at 923 K are
shown in Fig. 3(a), (b), (c), and (d), respectively.
As can be seen in Fig. 2, the CoNiCrAlY alloy was
susceptible to corrosion compared to the other three alloys
due to the formation of a thick internal Al2 O3 -precipitate
zone. At 923 K, the CoNiCrAlY alloy formed a duplex oxide
layer; the outer layer was a mixture of CoAl2 O4 , NiCo2 O4 ,
and Al2 O3 , and the inner layer was Al2 O3 . EDX analysis
indicated that CrS formed at the interface. In addition, SEMEDX analysis confirmed that the (Co,Ni)Al phase was not
found down to a depth of 20 mm from the alloy surface. This
may be due to preferential oxidation of Al in the (Co,Ni)Al
phase.17)
In contrast, the addition of Si significantly improved the
high temperature corrosion resistance of CoNiCrAlY alloys
and this is attributable to the formation of dense Al2 O3 scale.
For CoNiCrAlY alloy with 30 mass% Si content, SiO2 scale
also formed. The formation of oxide scale consisting of
Al2 O3 and SiO2 provided excellent corrosion resistance.
Furthermore, the results also showed that the thickness of
the oxide scale decreased significantly with the addition of
30 mass% Si.
3.2.2 High temperature corrosion at 1073 K
Figure 4 shows the corrosion behavior of CoNiCrAlYSi alloys corroded at 1073 K for up to 720 ks in an air(Na2 SO4 +25.7 mass%NaCl) gas atmosphere. Figure 5
shows the cross-sectional morphologies of CoNiCrAlY
alloys with 0, 10, 20, and 30 mass% Si content after 720 ks
exposure at 1073 K.
The results indicate that at 1073 K, high temperature
corrosion of CoNiCrAlY-Si alloys becomes more severe
than at 923 K. The CoNiCrAlY alloy with 10 mass% Si
content showed the highest corrosion, followed by CoNiCrAlY alloy. Both alloys suffered from serious internal
corrosion. As can also be seen in Fig. 4, the consumed
thickness for both alloys increased catastrophically after
exposure of 360 ks. This was caused by insufficient Al supply
to sustain the formation of a protective Al2 O3 scale. As a
High Temperature Corrosion of CoNiCrAlY-Si Alloys in an Air-Na2 SO4 -NaCl Gas Atmosphere
CoAl2O4, NiCo2O4, Al 2O3
(a)
435
(b)
Al2O3
Al2O3
CrS
Internal
Al2O3
10µm
(c)
1µm
(d)
Al2O3
Al2O3 / SiO2
1µm
100nm
Fig. 3 Cross-sectional morphologies of CoNiCrAlY alloys with (a) 0, (b) 10, (c) 20, and (d) 30 mass% Si content after 720 ks exposure at
923 K in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere.
Consumed Thickness of Alloy (µm)
250
CoNiCrAlY
CoNiCrAlY-10Si
CoNiCrAlY-20Si
CoNiCrAlY-30Si
200
150
100
50
0
0
150
300
450
600
750
Corrosion Times, t/ks
Fig. 4 Corrosion kinetics curve of CoNiCrAlY-Si alloys corroded at
1073 K for up to 720 ks in an air-(Na2 SO4 +25.7 mass%NaCl) gas
atmosphere.
result, fused salt and oxygen were able to diffuse inward
through the oxide scale to the underlying alloy, leading to the
formation of serious internal corrosion. Internal precipitates
consisted of mainly Al2 O3 and probably Cr-sulfides. The
internal corrosion was more severe in the CoNiCrAlY10 mass% Si alloy than in the CoNiCrAlY alloy. This is due
to oxidation and sulfidation of islands of compounds such as
(Co,Ni)Al and Cr in the substrate of CoNiCrAlY-10 mass%
Si alloy.
In the case of CoNiCrAlY alloy with 20 mass% Si content,
the addition of Si led to the formation of mainly Al2 O3 and
showed much better corrosion resistance compared to
CoNiCrAlY and CoNiCrAlY-10 mass% Si alloys. However,
an Al-depleted zone of around 10 mm was also observed.
A significant difference can be seen in the CoNiCrAlY30 mass% Si alloy. The addition of 30 mass% Si led to the
formation of oxide scales containing Al2 O3 and SiO2 ,
providing an excellent corrosion resistance. The thickness
of the oxide scale also decreased significantly with the
addition of 30 mass% Si.
3.2.3 High temperature corrosion at 1273 K
Figure 6 shows the consumed thickness of CoNiCrAlY
alloys as a function of Si content corroded at 1273 K for
72 ks in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere.
Cross-sectional morphologies of CoNiCrAlY alloys of 0, 10,
20, and 30 mass% Si content after exposure of 72 ks at
1273 K are shown in Fig. 7(a), (b), (c), and (d), respectively.
At 1273 K, CoNiCrAlY and CoNiCrAlY-10 mass% Si
alloys experienced serious attack due to internal corrosion.
Differences in the scale morphologies were seen between
1073 K and 1273 K from the internal corrosion products.
Larger islands of internal corrosion products could be found
at 1273 K. This could have resulted from the growth of
internal corrosion precipitates at higher temperatures. EDX
analysis indicated that the oxide scale was composed mainly
of Al2 O3 , and the internal corrosion products were speculated
to be Al2 O3 and Cr-sulfides.
436
T. Sudiro et al.
(a)
Mainly Al 2O3
(b)
Mainly Al 2O3
Gap
Internal
oxides + sulfides
Internal
sulfides + oxides
10µm
10µm
(d)
(c)
Mainly Al 2O3
Al2O3 / SiO2
Al depleted zone
1µm
100nm
Consumed Thickness of Alloy (µm)
Fig. 5 Cross-sectional morphologies of CoNiCrAlY alloys with (a) 0, (b) 10, (c) 20, and (d) 30 mass% Si content after 720 ks exposure at
1073 K in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere.
Al2 O3 /SiO2 in the inner layer, resulting in an excellent
corrosion resistance.
40
30
20
10
0
0
10
20
Si content (mass%)
30
Fig. 6 Consumed thickness of CoNiCrAlY-Si alloys corroded at 1273 K
for 72 ks in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere.
Differences in the oxide scale structure can also be found
in CoNiCrAlY alloys with 20 and 30 mass% Si content. The
CoNiCrAlY alloy with 20 mass% Si content formed an oxide
scale consisting mainly of Al2 O3 and Al5 Y3 O12 and also
demonstrated much better corrosion resistance than the
CoNiCrAlY and the CoNiCrAlY alloy with 10 mass% Si
content. However, as shown in Fig. 7(c), an Al-depleted
zone of around 3 mm was also observed. On the other hand,
the addition of 30 mass% Si led to the formation of a double
layer scale consisting of Al5 Y3 O12 in the outer layer and
3.3 Effects of the addition of Si
Figure 8 summarizes the scale structures formed on
CoNiCrAlY-Si alloys in an air-(Na2 SO4 +25.7 mass%NaCl)
gas atmosphere. The beneficial effect of the addition of Si
on the corrosion resistance of CoNiCrAlY alloys depends
on the alloy constituents and the corrosion temperature.
This research demonstrated that the CoNiCrAlY alloy with
30 mass% Si content showed the lowest consumed thickness
of alloy and exhibited an excellent corrosion resistance at
923–1273 K due to the formation of oxide scale consisting of
Al2 O3 and SiO2 (according to XRD results, this phase is
probably quartz). Francis et al.21) concluded that when a thin
oxide layer containing silicon is formed at the oxide/metal
interface, it plays a significant role in maintaining the
protective oxide scale at high temperatures.
In addition, at 923 and 1073 K, the thickness of the oxide
scale significantly decreased with the addition of 30 mass%
Si. A possible mechanism for this phenomenon could be as
follows: Since the Si content is high, preferential oxidation
of Si may take place in the early stage of corrosion, leading
to the formation of a thin SiO2 scale on the surface of the
CoNiCrAlY-30 mass% Si alloy. A previous study reported
that a vitreous SiO2 scale has low defect density.8) As
a result, outward diffusion of Al was suppressed by the
existence of SiO2 scale. Only a small amount of Al ions
High Temperature Corrosion of CoNiCrAlY-Si Alloys in an Air-Na2 SO4 -NaCl Gas Atmosphere
(a)
(b)
Mainly Al 2O3
437
Mainly Al 2O3
Internal
sulfides + oxides
Internal
oxides + sulfides
10µm
10µm
(c)
Al5Y3O12
(d)
Al2O3 / SiO2
Al2O3 + Al 5Y3O12
Al depleted zone
1µm
1µm
Fig. 7 Cross-sectional morphologies of CoNiCrAlY alloys with (a) 0, (b) 10, (c) 20, and (d) 30 mass% Si content after 72 ks exposure at
1273 K in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere.
CrS
CoAl2O4, NiCo2O4, Al 2O3
CoNiCrAlY
Al2O3
Al2O3
internal
Al2O3
Mainly Al 2O3
CoNiCrAlY
Mainly Al 2O3
internal
corrosion
Mainly Al 2O3
CoNiCrAlY-10Si
CoNiCrAlY-10Si
Al2O3
Mainly Al 2O3
CoNiCrAlY-20Si
CoNiCrAlY-20Si
Al2O3/SiO2
Al2O3/SiO2
CoNiCrAlY
Mainly Al 2O3
internal
corrosion
CoNiCrAlY-10Si
Al2O3+Al5Y3O12
Al depleted
zone
CoNiCrAlY-20Si
Al5Y3O12
Al2O3 /SiO2
CoNiCrAlY-30Si
CoNiCrAlY-30Si
CoNiCrAlY-30Si
(a)
(b)
(c)
Fig. 8 Schematic of scale structures formed on CoNiCrAlY-Si alloys in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere at elevated
temperatures of (a) 923, (b) 1073, and (c) 1273 K.
438
T. Sudiro et al.
diffused through the SiO2 scale and oxidized on the surface
of SiO2 scale to form a thin Al2 O3 layer. For this reason, the
thickness of the oxide scale decreased significantly with the
addition of 30 mass% Si. A further advantage of the addition
of Si is the extreme suppression of inward diffusion of S and
Cl. The formation of a protective scale consisting of Al2 O3
and SiO2 resulted in outstanding suppression of internal
corrosion.
4.
Summary
The high temperature corrosion behavior of CoNiCrAlYSi alloys in an air-(Na2 SO4 +25.7 mass%NaCl) gas atmosphere at elevated temperatures of 923, 1073 and 1273 K was
examined in this paper. The results of this study can be
summarized as follows:
(1) CoNiCrAlY-Si alloys form mainly Al2 O3 or Al2 O3 /
SiO2 scale, depending on the alloy composition and the
temperature;
(2) CoNiCrAlY alloy with 30 mass% Si content shows an
excellent corrosion resistance at 923–1273 K due to the
formation of oxide scale containing mainly of Al2 O3
and SiO2 ;
(3) Formation of SiO2 has an outstanding advantage for
the suppression of the growth of external scale and
formation of internal corrosion products.
From these results, it is found that CoNiCrAlY-30 mass% Si
coating on STBA21 alloy is very effective for improving the
high temperature corrosion resistance of STBA21 steel.
Acknowledgement
Kind assistance and help rendered by Dr. A. Yamauchi
during the period of this work is grateful acknowledged.
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