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Hot Corrosion Behavior of a Cr-Modified Aluminide Coating on a Ni-Based Superalloy
Citation preview
Hot Corrosion Behavior of a Cr-Modified Aluminide Coatingon a Ni-Based Superalloy
Duoli Wu • Sumeng Jiang • Qixiang Fan • Jun Gong • Chao Sun
Received: 21 November 2013 / Revised: 9 January 2014 / Published online: 15 July 2014
� The Chinese Society for Metals and Springer-Verlag Berlin Heidelberg 2014
Abstract A Cr-modified aluminide coating is prepared on a Ni-based superalloy using arc ion plating and subsequent
pack cementation aluminizing. Hot corrosion behavior of the Cr-modified aluminide coating exposed to molten Na2SO4/
K2SO4 (3:1) or Na2SO4/NaCl (3:1) salts at 900 �C in static air are evaluated as well as the aluminide coating. The results
indicate that compared with the aluminide coating, the anti-corrosion properties of the Cr-modified aluminide coating in
the both salts are improved, which should be attributed to the beneficial effect of the Cr in the coating. The corrosion
mechanism of the Cr-modified aluminide coating, especially the role of Cr in the mixture salt corrosion, is discussed.
KEY WORDS: Arc ion plating; Pack cementation aluminizing; Superalloy; Cr-modified aluminide coating;
Hot corrosion
1 Introduction
Currently, Ni-based superalloys are widely applied to tur-
bine blades or other components of gas turbines for their
excellent high-temperature mechanical properties. These
components must exhibit a high level of resistance to the
oxidation and corrosion conditions generated by the com-
bustion environment and be resistant to any associated
erosion/corrosion conditions produced as a result of par-
ticulate ingestion or solids formed by incomplete com-
bustion [1]. The alloying requirements for these high
volume fraction gamma prime materials result in a reduc-
tion in corrosion resistance and therefore, surface coatings
are widely used. The general design philosophy is to select
a high-strength substrate alloy to withstand the stress and
apply a surface coating to give maximum protection
against the environment corrosion.
Ni-based superalloys turbine blades are commonly alu-
minized in a cementation pack for improving their oxida-
tion resistance. However, such aluminized coatings lack
adequate resistance to hot corrosion caused by deposits of
fused alkali sulfates. The corrosion resistance of aluminide
coatings intensively depends on their ability to form a
continuous, adherent and slow growing layer of a-Al2O3
[2]. Simple aluminide coating made of b-NiAl is brittle and
sensitive to sulfur which tends to segregate at grain
boundaries, weakening the oxide–metal interface, and thus
leading to exfoliation [3].
A Cr-modified aluminide coating would promise sub-
stantial improvements in the hot corrosion resistance of the
coating [4]. On one hand, the presence of Cr in the coating
can promote the establishment of an Al2O3 scale on the
coating with much lower Al concentration, which can be
called the third element effect. On the other hand, the
presence of Cr can prevent the martensitic phase transfor-
mation of the b-NiAl phase, which helps to reduce the
degradation of the coating.
Available online at http://link.springer.com/journal/40195
D. Wu � S. Jiang � Q. Fan � J. Gong � C. Sun (&)
State Key Laboratory of Corrosion and Protection, Institute of
Metal Research, Chinese Academy of Sciences,
Shenyang 110016, China
e-mail: [email protected]
123
Acta Metall. Sin. (Engl. Lett.), 2014, 27(4), 627–634
DOI 10.1007/s40195-014-0108-5
In this paper, a novel method is investigated to prepare
the Cr-modified aluminide coating. The Cr-modified alu-
minide coating is prepared on a Ni-based superalloy by arc
ion plating (AIP) Cr layer and subsequent pack cementa-
tion aluminizing. Compared with co-deposition of Cr and
Al by powder packing, the compositions of the Cr-modified
aluminide coating prepared by this two-step method can be
well distributed and the contents of Cr and Al can be easily
controlled. The hot corrosion behavior of the Cr-modified
aluminide coating and the aluminized coating in molten
Na2SO4/K2SO4 (3:1, in weight) or Na2SO4/NaCl (3:1, in
weight) salts at 900 �C are studied. The hot corrosion
degradation mechanism of the Cr-modified aluminide
coating and the beneficial effect of the Cr element in the
coating are also discussed.
2 Experimental
A Ni-based superalloy DSM11 with nominal composition
(wt%) of Al 2.9, Ti 4.9, Cr 13.5, Co 9.5, W 3.7, Mo 1.5, Ta
2.8, C 0.1 and balanced of Ni was used as the substrate.
Specimens with dimensions of 15 mm in diameter and
2 mm in thickness were ground with 800-mesh SiC paper,
peened in a wet atmosphere (200-mesh glass ball), and then
ultrasonically cleaned with acetone, ethanol, and deionised
water successively. The Cr-modified aluminide (for short
as Al–Cr) coating was prepared by depositing the Cr layer
firstly and then aluminizing by pack cementation. Before
deposition, bombardment cleaning was carried out after
base pressure of the chamber was pumped below
6 9 10-3 Pa. The working pressure was maintained at
0.2 Pa by flowing argon into the chamber. Detailed depo-
sition parameters are given in Table 1. Pack powder mix-
ture for aluminizing consisted of 96 wt% FeAl powders as
source of aluminum and 4 wt% NH4Cl as activator. The
specimens were buried in the powder mixture. The furnace
chamber was pumped to at least 1 9 102 Pa, then heated to
900 �C. The soaking time was 5 h. The aluminized coating
was also prepared by pack cementation. The contents of the
two coatings are listed in Table 2.
Hot corrosion behaviors of the DSM11 alloy, the alu-
minized coating, and the Al–Cr coating specimens were
performed in a muffle furnace in static air. Before hot
corrosion test, specimens were placed on a hot plate and
then brushed with Na2SO4/K2SO4 (3:1) or Na2SO4/NaCl
(3:1) mixtures. The content of deposited salt on each
sample was about 1 mg/cm2. Afterwards, the samples were
exposed in the muffle furnace at 900 �C to carry out the hot
corrosion testing. At regular intervals of 20 h, the speci-
mens were taken out, cooled down to room temperature
and then washed in boiling distilled water to obliterate the
remained salt. After being dried, the specimens were
weighed by an electronic balance with sensitivity of
0.01 mg. Subsequently, a fresh salt coating was brushed
again to continue the hot corrosion experiment.
Phase identifications of the coating and corrosion scales
were carried out using an X-ray diffractometer (XRD).
Microstructure and morphology were characterized by a
scanning electronic microscope (SEM) equipped with dis-
persive X-ray spectrometer (EDS). Electroless Ni-plating
was plated on the surface of the cross-section samples to
prevent spallation of scales from the surface in sample
preparation process.
3 Results and Discussion
3.1 Microstructures of the Two Coatings
Figure 1 shows the XRD patterns of the aluminized coating
and the Al–Cr coating. Both coatings are composed of b-
NiAl phase and Ni2Al3 phase. The a-Cr phase and AlCr2
phase could be also detected in the Al–Cr coating. But the
diffraction peaks of the AlCr2 phase are weak compara-
tively. In the Al–Cr coating, a-Cr particles are precipitated
near the original substrate surface and also grow inwardly
from the gas phase at the coating surface. Figure 2a and b
show the cross-sectional BSE images of the Al–Cr coating
and the aluminized coating. The thickness of the Al–Cr
coating is about 40 lm, and the aluminized coating is a
little thicker than the former. These two coatings are dense
and adhere tightly to the substrate. The microstructures of
the Al–Cr coating and aluminized coating are characterized
in two distinguished areas: the outer layer and the inter-
diffusion zone. The interdiffusion zone is comprised of b-
Table 1 Deposition parameters of Cr layer by AIP
Operation stage Arc voltage (V) Arc current (A) Bias voltage (V) Bias duty cycle (%) Temperature(�C)
Bombardment cleaning 20–30 60–70 -800 30 50–100
Chromium target 20–30 60–70 -200 30 100–150
Table 2 The compositions of the two coatings (wt%)
Coating Al Ti Cr Co Ni
Al–Cr coating 31.9 0.4 13.2 6.4 Bal.
aluminized coating 25.6 0.5 5.3 6.8 Bal.
628 D. Wu et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(4), 627–634
123
NiAl and Ni3Al, M23C6, MC-type carbides, and r-phase
[5, 6].
Figure 3a and b show the line scanning images on the
cross-sections of the aluminized coating and the Al–Cr
coating. According to the figures, the distribution and the
relative content of different elements can be seen in the two
coatings. In the aluminized coating, the content of Ni and
Al elements is the highest, and Al element in the outer
layer of the coating distributes uniformly. In the Al–Cr
coating, the content of Ni, Al, and Cr elements rise to the
top with a uniform distribution of Al and Cr element in the
coating. These figures also prove that the structures of the
two coatings are continuous and dense with the main ele-
ments evenly distributed.
3.2 Corrosion Kinetics
Figure 4a, b show the corrosion kinetic curves of the
DSM11 alloy, the aluminized coating and the Al–Cr
coating deposited with salts of Na2SO4/K2SO4 (3:1, and
Na2SO4/NaCl (3:1). The mass change consists of a mass
gain owing to the formation of the scales, and a mass loss
caused by scale spallation and dissolution. The net mass
change of the specimens in molten sulfate represents the
combined effects of these two processes.
The DSM11 alloy, the aluminized coating and the Al–Cr
coating show different corrosion behavior in the molten
Fig. 1 XRD patterns of the aluminized coating and the Al–Cr coating
Fig. 2 Cross-sectional BSE images of the two coatings: a Al–Cr coating; b aluminized coating
Fig. 3 Line scanning images on the cross-sections of the aluminized coating a, the Al–Cr coating b
D. Wu et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(4), 627–634 629
123
sulfate. According to Fig. 4a, the corrosion of the DSM11
alloy is catastrophic with a sharp increase in the kinetic
curve in the first 20 h. In the first 20 h, the DSM11 alloy
reaches its maximum mass gain of 1.25 mg/cm2. After
40 h, the kinetic curve goes through a sharp decline. It can
be attributed to the low-melting-point phases at the crystal
boundaries as fast diffusion channels. The molten salt can
diffuse through these channels and reacts with the alloy
rapidly. While for the aluminized coating and the Al–Cr
coating, the mass gain is much less, and no rapid mass gain
is observed. It demonstrates that both coatings enhance the
alloy’s anti-corrosion ability greatly. For the aluminized
coating, the mass gain increases in the first 80 h, with a
maximum mass gain of 0.65 mg/cm2 and then starts to fall.
However, the mass gain of the Al–Cr coating is much
lower than that of the aluminized coating throughout the
process. According to Fig. 4a, after corrosion for 40 h, the
mass gain of the Al–Cr coating reaches to 0.35 mg/cm2.
The mass gain data of the Al–Cr coating almost remain
unchanged at the stage of 40 h to 100 h. The kinetic curve
comes into a ‘‘platform’’ stage. It shows that in this hot
corrosion interval, the growth rate of the oxidation film
equaled to its ‘‘dissolved or peeling off’’ rate.
According to Fig. 4b, the DSM11 alloy demonstrates
rather poor anti-corrosion ability in the salt of Na2SO4/
NaCl (3:1). It could be seen that NaCl was more corrosive
than K2SO4. Furthermore, when NaCl is added into
Na2SO4 salt deposit, the exfoliation of corrosion products
happens more easily than in Na2SO4 and K2SO4 salts
deposit. Some studies show that the presence of NaCl can
destroy the continuity of the surface oxidation film [7, 8].
The addition of NaCl will trigger a reaction at high tem-
perature [9]: NaCl (l) ? O2 (g) = 1/2 Na2O (s) ? Cl2(g) to generate Cl2. The Cl2 will react with Cr and Al
elements in the alloy to generate volatile elements such as
chloride. After that, the chloride will spread along the
cracks and voids of the alloy. When reach to the outer
surface of the oxide film will react with O2 again to gen-
erate Cl2. The newly generated Cl2 can diffuse into the
oxide film, and again react with the elements such as Cr
and Al. With the continuous reaction, the whole process of
hot corrosion accelerated. As a result, the mass loss
quantity increased. The corrosion kinetic curve shows a
significant increase of mass gain at initial stage up to 20 h,
and follows an obvious drop after 20 h. The severe mass
losses are caused by significant scale spallation and O2
fluxing, confirming it is necessary to apply protective
coatings on the alloy. For the aluminized coating, a rapid
increase of mass gain occurs in the first 20 h, with a
maximum mass gain of 1.75 mg/cm2 and then starts to fall.
The protective Al2O3 scale starts to crack, and spallation
also occurs. However, for the Al–Cr coating, the mass gain
maintains sustained increasing throughout the corrosion
process. The mass gain owing to the formation of the scales
is much bigger than the mass loss caused by scale spalla-
tion and dissolution. Obviously, the Al–Cr coating provides
much better protection to the alloy than the aluminized
coating in this kind of mixed salt.
3.3 Corrosion Scales
Figure 5a shows the XRD patterns of the surfaces of the
DSM11 alloys with the aluminized coating and the Al–Cr
coating after hot corrosion at 900 �C in the mixture salt of
Na2SO4/K2SO4 (3:1). Besides the Cr2O3 scale, a large
amount of un-protective corrosion products, identified as
Cr3S4 and TiO2 are formed on the surfaces of the samples.
The existence of S in the corrosion products proves that
S2- can diffuse from the surface of the oxidation film into
the substrate through the rapid diffusion channel, which is
formed by the low-melting phase. As for the aluminized
coating and the Al–Cr coating, the corrosion scales formed
on both coatings are mainly composed of a-Al2O3. A large
amount of b-NiAl phase is also detected, which
(a) (b)
Fig. 4 Corrosion kinetic curves of DSM11 substrate, aluminized coating, and Al–Cr coating in the mixed salts of Na2SO4/K2SO4 a, Na2SO4/
NaCl b at 900 �C
630 D. Wu et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(4), 627–634
123
demonstrates that the Al reservoir in the two coatings is
still abundant. It can promise subsequent formation and
growth of Al2O3 in the later hot corrosion process.
Figure 5b shows the XRD patterns of the corrosion
scales formed on the surfaces of the DSM11 alloys with the
aluminized coating and the Al–Cr coating after hot corro-
sion at 900 �C in the mixture salt of Na2SO4/NaCl (3:1). A
mixed oxidation scale of NiO and Cr2O3 is formed on the
surface of the samples. Meanwhile, NiCr2O4 spinel forms
via solid-phase reaction between NiO and Cr2O3, resulting
in the formation of a mixture layer of NiO, Cr2O3, and
NiCr2O4 spinel. The major phases on the surfaces of the
aluminized coating and the Al–Cr coating are a-Al2O3 and
b-NiAl, while some peaks of Ni3Al are also identified in
the Al–Cr coating.
Figure 6 shows the SEM images of the DSM11 alloy,
the aluminized coating and the Al–Cr coating exposed in
salts of Na2SO4/K2SO4 (3:1) at 900 �C for 100 h, sep-
arately. As it can be seen in Fig. 6a, different structures
of oxide scales are formed on the surface of the DSM11
substrate. These oxidate scales were mixed uniformly
rather than hierarchical, with distinct characteristics of
the rod-like TiO2 oxide. According to Fig. 6b and c,
similar Al2O3 scales are formed on the surface of the
aluminized coating and the Al–Cr coating. Although the
morphologies of Al2O3 scales are similar, however, the
grains sizes of the oxide films are a little different. The
grains of the oxide film on the surface of the Al–Cr
coating are much smaller, which illustrates the growth
rate of the oxide is also much slower. Obviously, the Al–
Cr coating has much better corrosion resistance in this
kind of salt.
Figure 7 shows the cross-sectional BSE morphologies of
the DSM11 alloys with aluminized coating and the Al–Cr
coating exposed in salts of Na2SO4/K2SO4 (3:1) at 900 �C
for 100 h. According to Fig. 7a, the DSM11 alloy suffers
serious hot corrosion with cracks and spallation arise on the
surface. The DSM11 alloy does not form a protective
Al2O3 scale due to its low Al content. By the cross-sec-
tional BSE morphologies (Fig. 7b, c) of the aluminized
coating and the Al–Cr coating show the similar hot cor-
rosion behaviors in this kind of salt. It is found that a dense
and intact layer of a-Al2O3 is formed on the surface of both
coatings. Besides, the Al reservoir phase b-NiAl is extre-
mely abundant in both coatings, which agrees well with the
XRD patterns shown in Fig. 5a.
(b)(a)
Fig. 5 XRD patterns of the DSM11 substrate, the aluminized coating and the Al–Cr coating after corrosion at 900 �C for 100 h in mixed salts of
Na2SO4/K2SO4 a, Na2SO4/NaCl b
Fig. 6 SEM images showing the surfaces of DSM11 substrate a, aluminized coating b, Al–Cr coating c after corrosion at 900 �C for 100 h in the
salt of Na2SO4/K2SO4
D. Wu et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(4), 627–634 631
123
Figure 8 shows the SEM images of the DSM11 alloy
with aluminized coating and the Al–Cr coating after cor-
rosion in the mixed salt of Na2SO4/NaCl (3:1) at 900 �C
for 100 h. According to Fig. 8, the mixture oxide scales
including NiO, Cr2O3, and NiCr2O4 spinel are formed on
the surfaces of the samples. The oxide formed on the sur-
face of the aluminized coating, and the Al–Cr coating is sill
a-Al2O3, while the density of the oxide declines.
Figure 9 shows the cross-sectional BSE morphologies of
the DSM11 alloys with aluminized coating and the Al–Cr
coating after corrosion in the mixed salt of Na2SO4/NaCl
(3:1) at 900 �C for 100 h. As seen in Fig. 9a, the DSM11
alloy suffers more severe hot corrosion than that in the salt of
Na2SO4/K2SO4 (3:1). When NaCl is introduced into the salts
film, the corrosion is more serious since internal oxide and
chromium sulfide emerged in the two coatings. The major
role of NaCl in hot corrosion induced by Na2SO4–NaCl
mixtures is to cause cracking of the protective oxide scale as
well as to produce the internal voids by means of oxychlo-
rination and chlorination/oxidation cyclic reactions,
Fig. 7 Cross-sectional BSE images of DSM11 substrate a, aluminized coating b, Al–Cr coating c after corrosion at 900 �C for 100 h in the salt
of Na2SO4/K2SO4
Fig. 8 SEM images showing the surfaces of DSM11 substrate a, aluminized coating b, Al–Cr coating c after corrosion at 900 �C for 100 h in the
salt of Na2SO4/NaCl
Fig. 9 Cross-sectional BSE images of DSM11 substrate a, aluminized coating b, Al–Cr coating c after corrosion at 900 �C for 100 h in the salt
of Na2SO4/NaCl
632 D. Wu et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(4), 627–634
123
additionally resulting in the dissolution of the protective
oxide scale [10–13]. According to Fig. 9b, the protective a-
Al2O3 scale on the surface of the aluminized coating is
depleted and a continuous a-Al2O3 scale does not exist after
corrosion for 100 h. The interdiffusion zone disappears
completely and heavy internal oxidation and sulfidation are
found beneath the surface. The inward diffusion of S would
increase the basicity of the molten sulfates, leading to the
dissolution of Al2O3, since alumina can dissolve in basic
sulfate, but be stable in neutral salt. Obviously, the alumi-
nized coating almost loses its protective effect against this
kind of hot corrosion. As seen in Fig. 9c, a very dense Al2O3
scale also exists on the surface of the Al–Cr coating after
corrosion for 100 h, which is mainly related to the NiAl
phase and Cr element in the coating. The addition of Cr
element in the coating can promote to form the Al2O3 scale
on the coating with much lower Al concentration, which can
be called the third element effect [14]. Besides, slight
internal oxidation is found under the Al2O3 scale. According
to the above analyses, a certain amount of b-NiAl phase still
remains in the coating and the scale is also continuous and
compact, which testifies the Al–Cr coating can still protect
the substrate from corrosion effectively.
3.4 Hot Corrosion Mechanism of the Al–Cr Coating
According to Ref. [7] and Ref. [8], hot corrosion is an
accelerated form of oxidation that occurs when metals are
heated in the temperature range 700–900 �C. Hot corrosion
problems are a direct result of the combination of salt
contaminants such as Na2SO4, NaCl, and V2O5, which
produces low-melting point deposits that can dissolve the
protective surface oxides [15]. A number of fluxing
mechanisms has been proposed to account for the different
corrosion morphologies that are observed [16–18] and this
has resulted in the general classification of high tempera-
ture (type I, 800–950 �C) hot corrosion and low tempera-
ture (type II, 600–800 �C) hot corrosion. The degradation
sequence usually consists of an initiation stage, during
which the attack is virtually the same as for the alloy in the
absence of deposit, and a propagation stage, during which
the attack is substantially increased.
The Al–Cr coating presents excellent corrosion resis-
tance in the molten salt with low corrosion rate mainly due
to the beneficial effect of the Cr element in the out layer of
the coating. The molten Na2SO4, which deposits on the
surface of the superalloy, can cause hot corrosion of the
alloy and flux the normally protective oxides such as Cr2O3
and Al2O3 [19, 20]. In a liquid deposit, there are the fol-
lowing thermodynamic equilibriums:
Na2SO4 ¼ Na2Oþ SO3: ð1Þ
SO3 ¼1
2S2 þ
3
2O2: ð2Þ
The experimental results presented in this paper are
consistent with the observations proposed by other
researchers [21–24] on the beneficial effect of chromium
in inhibiting the hot corrosion of an aluminide coating.
According to Otsuka and Rapp [25], an oxide scale that
contains Cr2O3 will dissolve partially in a Na2SO4 melt by
a basic dissolution reaction to form chromate anions:
Cr2O3 þ3
2O2 þ 2SO2�
4 ¼ 2CrO2�4 þ 2SO3: ð3Þ
Similarly, if Al2O3 is present in the scale it can also
undergo basic dissolution:
Al2O3 þ SO2�4 ¼ 2AlO�2 þ SO3: ð4Þ
These two reactions result in the consumption of sulfate
ions. However, SO42- consumption by the reaction of
forming CrO42- is much more effective than by AlO2
-
formation. This is primarily a consequence of the greater
solubility of the chromate anion in the basic salt melt [26].
From Eq. (4), if the Cr2O3 reaction consumes a sufficient
amount of the sulfate ions, the basic dissolution of the
Al2O3 can be suppressed. As a result, the Al2O3-rich scale
is able to act as an effective protective barrier between the
coating and the salt medium.
Later, as the hot corrosion process proceeds, when most
of the Al reservoirs are sacrificed and the internal layer
enriching of Cr is exposed, the Cr will supersede the Al to
form a protective Cr2O3 scale [27]. Since the fluxing of
Cr2O3 primarily obeys the equation:
Cr2O3 þ O2� þ 3
2O2ðgÞ ! Cr2O2�
7 : ð5Þ
The dissolution of chromia needs the assistance of oxygen
[28]. Thus, the solubility of Cr2O3 at the salt/gas interface
is higher than that at the oxide/salt, and there will exist as a
‘‘positive solubility gradient’’ which is not self-sustainable
for Cr2O3 to be fluxed. Moreover, the Cr can sacrifice itself
to entrap the element sulfur to form a relatively stable
Cr2S3 [29] as long as the chromium sulfide is effectively
insulated from further oxidation.
As reported by Deb et al. [13], chlorides can cause the
formation of volatile species, which form voids and pits at
grain boundaries, thus forming an easy path for flowing
corrodents and oxygen. The deleterious feature of the
sodium chloride should be attributed to the extremely
small ion of Cl-. Since the melting point (801 �C) of
NaCl is lower than that of Na2SO4 (884 �C), the molten
mixed salt at the current exposure temperature establishes
an easier corrosive electrolyte where the Cl- can travel
freely.
D. Wu et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(4), 627–634 633
123
As the protective feature of alumina scale has been
deteriorated, oxygen and sulfur could easily penetrate into
the coatings and cause the formation of internal oxide and
chromium sulfide [30, 31]. These internal oxides and
chromium sulfides would accelerate the consumption of
benefit element, and consequently speed up the corrosion
process. Compared with the aluminized coating, the Al–Cr
coating has sufficient aluminum to maintain a self-healing
ability and prevent the inward diffusion of oxygen and
sulfur, accompanying with inhibiting the invasion of NaCl.
In addition, it can be seen that the Cr-rich inner layer of the
Al–Cr coating is not attacked yet. So, it can be predicable
that the Al–Cr coating would resist the corrosion attack
much longer than the aluminized in its service life.
4 Conclusions
(1) The Cr-modified aluminide coating is prepared by a
novel method with two steps of deposition of AIP Cr
and then Al pack cementation. The coating is com-
prised essentially of an outward-grown b-NiAl matrix
with a-Cr precipitates.
(2) The Cr-modified aluminide coating and the alumi-
nized coating both show excellent corrosion resis-
tance in the mixture salt of Na2SO4/K2SO4 (3:1). The
Cr-modified aluminide coating presents much better
corrosion resistance than the aluminized coating.
(3) The Cr-modified aluminide coating still possesses
good corrosion resistance in the mixture salt of
Na2SO4/NaCl (3:1) due to the beneficial effect of Cr
in the coating. However, the aluminized coating has
been damaged seriously and lost its protective effect
after corrosion.
Acknowledgments This work was financially supported by the
National Natural Science Foundation of China (No. 51001106) and
National Basic Research Program of China (No. 2012CB625100).
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