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TRANSCRIPT
Corrosion Resistance of Ti3Al/BN Abradable Seal Coating
Feng Zhang • Hao Lan • Chuanbing Huang • Yang Zhou • Lingzhong Du • Weigang Zhang
Received: 25 February 2014 / Revised: 8 April 2014 / Published online: 17 September 2014
� The Chinese Society for Metals and Springer-Verlag Berlin Heidelberg 2014
Abstract Ti–Al mixed powder (Ti:Al = 3:1 in atomic ratio) and Ti3Al intermetallic alloy powder mechanically clad
hexagonal BN to fabricate TiAl/BN and Ti3Al/BN composite powders. The corresponding porous abradable seal coatings
(named as TAC-1 and TAC-2, respectively) were deposited using vacuum plasma spray (VPS) technology, and their
corrosion behavior was studied via salt spray corrosion and electrochemical tests. Phase compositions and microstructures
of these coatings before and after corrosion were characterized by X-ray diffraction (XRD) and scanning electron
microscopy (SEM) facilitated with energy dispersive X-ray spectrometer (EDS). The results showed that spontaneous
passivation of TAC-1 and TAC-2 granted the coatings excellent corrosion resistance than that of commercial Al/BN
coating. Additionally, TAC-2 exhibited higher corrosion potential (Ecorr) and breakdown potential (Ebp) but a lower
corrosion current density (icorr) than TAC-1. A small quantity of the corrosion product (Al(OH)3 and AlO) could be
detected on the surface of TAC-1, while no corrosion product appeared in TAC-2. The non-uniform elements distribution
in the metal matrix of TAC-1 resulted in localized corrosion and relatively poor corrosion resistance compared to TAC-2.
KEY WORDS: Coating; Galvanic corrosion; Thermal spraying; Titanium aluminide
1 Introduction
Abradable seal coatings are often deposited on the case to
minimize the clearance between the rotating and stationary
components of aircraft turbine engines [1–3]. Abradable
seal coating is a sacrificial material coating. During the
turbine engines operation, the blades scrape the coating and
tear off fine layers, creating a functional gap between the
blades and the coating seal. The combustion cannot escape
through the clearance and contribute to the power pro-
duction. The abradable seal coatings applied in a highly
harsh environment have to fulfill a great deal of require-
ments, such as abradability, corrosion resistance, high-
temperature stabilization, and sufficient bond strength. In
order to meet all these requirements, the abradable mate-
rials often consist of metal matrix, a second phase and a
controlled amount of pores [4, 5]. The metal matrix pro-
vides the coating enough mechanical strength and resists
corrosive action under elevated temperature and salt spray.
The second phase acted as a solid lubricant offering the
coating a good abradability.
The Al/BN [6] and Ni/graphite [7, 8] coatings with good
abradability have been widely applied in the fan section
and low pressure compressor of aero-engine. Their corro-
sion behavior has attracted a lot of attention while being
used in navy aircrafts. Since these aircrafts serve in an
Available online at http://link.springer.com/journal/40195
F. Zhang � H. Lan � C. Huang � Y. Zhou � L. Du �W. Zhang (&)
State Key Laboratory of Multi-phase Complex Systems, Institute
of Process Engineering, Chinese Academy of Sciences, Beijing
100190, China
e-mail: [email protected]
F. Zhang
University of Chinese Academy of Sciences, Beijing 100049,
China
123
Acta Metall. Sin. (Engl. Lett.), 2014, 27(6), 1114–1121
DOI 10.1007/s40195-014-0133-4
environment with high salt spray, high humidity, and high
temperature, the corrosive substances could reach the
abradable seal coatings directly on the fan and low pressure
compressor and thus cause the damage of coatings during
the parking time. Xu et al. [9, 10] studied the salt spray
corrosion resistance of Ni/graphite coating, and the corre-
sponding results indicated that the coating suffered from
the serious galvanic corrosion between the top coating and
bond coating, causing a rapid decrease in the bond strength.
For Al/BN, the coating suffered severe corrosion damages
due to the poor corrosion resistance of the Al matrix to the
aggressive Cl-. When the corrosive substances penetrated
into the coating and arrived at the substrate, serious gal-
vanic corrosion might occur between the coating and the
substrate because of the high corrosion potential of tita-
nium-alloyed substrate. This severe corrosion might lead to
the peeling of the abradable seal coating from the substrate
when the blade tip incurs into the shroud, which seriously
imperil the flight safety.
In order to improve the corrosion resistance of the
abradable seal coating and eliminate the galvanic corrosion
between the coating and the substrate, coating materials with
good corrosion resistance and well compatibility with the
substrate material should be considered. Ti3Al intermetallic
has a D019 structure combined with excellent electro-
chemical corrosion resistance [11, 12] and high specific
strength, which makes it a good candidate for application in
anti-corrosion abradable seal coating. Thermal spray tech-
nique is the most popular manner to manufacture abradable
seal coating with porous structure [13, 14]. However, air
plasma-sprayed coating of titanium alloy is predominantly
comprised of oxides and nitrides of titanium, as the titanium
alloys are prone to react with ambient gases containing
oxygen and nitrogen at elevated temperatures [15, 16]. Thus,
a preferred method for the production of these porous coat-
ings is vacuum plasma spraying [17, 18].
In the present research, TiAl/BN and Ti3Al/BN com-
posite powders were prepared, and their derived anti-cor-
rosion abradable seal coatings (named as TAC-1 and TAC-2,
respectively) were deposited on the TC4 alloy [Ti–6.2Al–
4.1V–0.3Fe–0.1C–0.05N–0.015H–0.2O (in wt%)] substrate
by VPS technique. The corrosion resistance behavior of the
coatings was investigated via salt spray corrosion and elec-
trochemical tests. For comparison, some studies of Al/BN
abradable seal coating are also given in this article.
2 Experimental
2.1 Materials Preparation
TiAl/BN and Ti3Al/BN composite powders were prepared
by the mechanically clad technology using sodium silicate
as a binder. The agglomerated BN particles produced from
the reconstitution of commercially nanosized BN via spray
granulating and drying method were used as core materials.
High purity Ti and Al powders (Ti:Al = 3:1 in atomic
ratio) with size distribution of 3–20 lm were used as the
clad materials to produce the TiAl/BN composite powder.
Ti3Al intermetallic particles with average size of 5–30 lm
prepared by sintering and crushing methods were used to
fabricate the Ti3Al/BN composite powder.
TAC-1 and TAC-2 were prepared by vacuum plasma
spray technique (F4-VB, Sulzer Metco). The optimized
thermal spray parameters are given in Table 1. TC4 alloy
plate with a dimension of U25 mm 9 5 mm was used as
the substrate material.
2.2 Electrochemical Corrosion Test
Small round electrode samples with a diameter of 10 mm
were prepared, as shown in Fig. 1. The detailed process of
the preparation had been given in the previous study [9].
Before the electrochemical experiment, the samples were
ground using 1000 grit SiC paper, polished by 1 lm dia-
mond suspension, and cleaned by distilled water and ace-
tone. The electrical connection was identified by a
multimeter.
The open circuit potential (OCP) and potentiodynamic
polarization of the samples were tested in a 1-L glass cell.
The electrolyte was 5 wt% NaCl solution, and one fresh
solution was used for each electrochemical test. The
Table 1 Vacuum plasma spray process parameters
Spray parameters Value
Arc current 550–650 A
Primary plasma gas Ar 45–55 L/min
Secondary plasma gas H2 12–15 L/min
Carrier gas Ar 2.0–2.5 L/min
Pressure of spraying atmosphere 12.5–15.0 kPa
Spray distance 150 mm
Fig. 1 Schematic diagram of the tested specimen
F. Zhang et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(6), 1114–1121 1115
123
measurements were performed on a CHI-660D electro-
chemical workstation. A three-electrode cell was used for
measurements: the coating samples were used as the
working electrode, a platinum sheet with a size of
1.5 cm 9 1.5 cm was used as the counter electrode, and a
saturated calomel electrode (SCE) was used as the refer-
ence electrode. All the potentials quoted in this work had
been converted to the SCE scale. The OCP was monitored
for 180 min, and the potentiodynamic polarization was
measured at a scan rate of 0.16 mV/s. Three polarization
tests were carried out for each sample in order to ensure the
reproducibility of the results.
2.3 Salt Spray Corrosion Test
The salt spray corrosion (SSC) tests were performed fol-
lowing the GB/T10125-1997 (ISO9227) national standard
procedure. The round samples with a diameter of 25 mm
were placed in a chamber at (35 ± 0.5) �C under an
aqueous spray (5 wt% NaCl solution, pH value between
6.5 and 7.2) at a spray rate of 125–250 mL h-1 m-2. The
electrolyte can penetrate through the open pores on the
edges of the coatings. In order to avoid the impact of the
edges, the sample edges were sealed by epoxy resin before
placed in the chamber. The samples were subjected to
960 h salt spray corrosion and observed at regular
intervals.
2.4 Characterization
The morphology of the composite powders, the as-sprayed
coatings and the corrosion products were characterized
using a FEI Quanta 200 FEG scanning electron microscope
(SEM), equipped with an energy dispersive X-ray spec-
trometer (EDS) and a backscattered electron detector. The
crystalline structures of the powders, coatings, and corro-
sion products were analyzed by X-ray diffraction (XRD,
X’pert, PAnalytic) with CuKa radiation over an angular
range of 10� B 2h B 90�.
3 Results and Discussion
3.1 Characterization of the Spray Powder
The cross-sectional micrographs of the TiAl/BN and Ti3Al/
BN composite powders are shown in Fig. 2. It was found
that the composite powders have a nearly spherical shape,
indicating a good flowability. The metal matrixes clad on
the exterior of the BN core particles. During the spray
process, the metal matrix was in melting state, which
covered the surface of the core particles and reduced the
ablation of BN.
The XRD patterns of TiAl/BN and Ti3Al/BN powders
are shown in Fig. 3a. The main phases in the TiAl/BN
powder are pure Ti, Al, and hexagonal BN. For the Ti3Al/
BN powder, the phase constituent contains Ti3Al, hexag-
onal BN, and a very small amount of Al, which indicates
some Al left during the preparation of Ti3Al intermetallic
powder. The diffraction patterns of TAC-1 and TAC-2
coatings have similar phase constituents, as shown in
Fig. 3b. The Ti3Al and hexagonal BN are the dominant
phases in these two coatings. For TAC-1, the Ti3Al metal
matrix was synthesized during the flight of TiAl/BN
powder particles in the plasma jet. The exothermic reaction
between Ti and Al occurred during the plasma-sprayed
process, resulting in a large amount of heat and the
improvement of the deposition efficiency for powder par-
ticles. The chemical equation of Ti3Al intermetallic for-
mation is given below [19]:
3Ti þ Al ¼ Ti3Al ðDGf ¼ �29633:6þ 6:70801 TÞ;
where DGf is the Gibbs energy of Ti3Al formation
(J mol-1) and T is the temperature (K).
Fig. 2 Cross-section SEM images of the composite powders: a TiAl/BN; b Ti3Al/BN
1116 F. Zhang et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(6), 1114–1121
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The intensities and the widths of the XRD peaks of the
coatings are much weaker and wider as compared to those of the
powders. This is expected in case of thermal-sprayed coating,
which involves rapid cooling of the molten particles onto the
relatively cool substrate and results in the formation of non-
equilibrium and metastable phases in the coating [20]. Some
TiN peaks appear in the diffraction patterns of the coatings,
which probably caused by the reaction between Ti and BN
under the high-temperature process of thermal spray [21].
The cross-sectional morphologies of the as-sprayed
coatings can be observed from Fig. 4. Obviously, TAC-1 is
thicker than TAC-2, which indicates a higher deposition
efficiency of the TiAl/BN composite powders. For both the
TAC-1 and TAC-2, the interfaces between coatings and
substrates reveal good mechanical bonding, guaranteeing
high bonding strength of the coatings. The bonding
strengths of the coatings were evaluated based on ASTM
standard C633-01 using the cylindrical specimen with a
diameter of 25 mm. The bonding strengths of TAC-1 and
TAC-2 are (19.5 ± 0.5) and (18.1 ± 0.7) MPa, respec-
tively. The coatings exhibited typical lamellar abradable
seal coating’s micro-structure features. The metal matrix
(light gray contrast) and the BN particles (deep gray con-
trast) can be clearly distinguished as two major structure
components. The hexagonal BN with low strength and a
graphite-like crystal lattice as the dislocator phase was
trapped in an open network of lamellar metallic splats.
Another major characteristic of the coatings is the presence
of a large amount of pores. These pores also act as dislo-
cator phase, aiding the abradability of the coating.
3.2 Salt Spray Corrosion Test
Al/BN coating, TAC-1, and TAC-2 were tested in the salt spray
chamber for 960 h. Figure 5 presents the free surfaces of the
samples after salt spray test. Obviously, the extents of salt spray
corrosion damage among the three samples are totally distinct.
It can be observed that a large amount of corrosion products
Fig. 3 XRD patterns of the powders a and as-sprayed coatings b
Fig. 4 Cross-sectional micrographs of TAC-1 a, b TAC-2 coatings
F. Zhang et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(6), 1114–1121 1117
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accumulated on the surface of the Al/BN coating. By contrast,
just a small quantity of white corrosion product formed on the
surface of TAC-1. No corrosion product was detected on the
surface of TAC-2. The results indicate that both TAC-1 and
TAC-2 have improved salt spray corrosion resistance com-
pared with Al/BN coating, and the TAC-2 exhibits the most
excellent corrosion resistance.
Figure 6 shows the surface morphology of the TAC-1
after 960 h corrosion test. The surface of the TAC-1 was
occupied by the corrosion crack, which possibly caused by
the dehydration of the surface corrosion product in the
subsequent heat treatment [22]. The EDS analysis shows
that the main elements of the corrosion product are Al and
O, indicating the preferential corrosion of Al during the salt
spray corrosion tests. Some Cl and Na from the NaCl
residue were detected as well. The corrosion product of
TAC-1 was separated from the surface of the coating,
washed, and analyzed by XRD. The results in Fig. 7 show
that the major constituents of the corrosion products are
Al(OH)3 and AlO.
Figure 8 shows the EDS results which were used to
analyze the TAC-2 after 960 h salt spray corrosion test. As
it is shown in Fig. 8, little oxygen was detected, indicating
Fig. 5 Free surface of the as-sprayed coatings after 960 h salt spray tests: a Al/BN coating; b TAC-1; c TAC-2
Fig. 6 Surface morphology of the TAC-1 after 960 h salt spray tests a, EDS result of the marked site in Fig. 6a b
Fig. 7 XRD pattern of the surface corrosion product of the TAC-1
1118 F. Zhang et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(6), 1114–1121
123
no corrosion products like oxides or hydroxides formed
during the process of salt spray corrosion test. This result
further confirmed that the TAC-2 possesses excellent anti-
corrosion property in the test.
3.3 Electrochemical Tests
Figure 9 shows the open circuit potential (EOCP) of Al/BN,
TAC-1, and TAC-2 in 5 wt% NaCl solution over 180 min.
The results show that, after a slight increase in the poten-
tials, OCP of the three coatings reached stable values at
about -1.13, -0.61, and -0.22 V, respectively. The EOCP
of TAC-2 is much higher than that of the other two coat-
ings, and the potential moves smoothly toward the positive
direction. The results suggest that a protective passive film
formed on the surface of TAC-2 sample, which explains
the better corrosion resistance of TAC-2 comparing to the
other two coatings. By contrast, the corrosion behaviors of
Al/BN and TAC-1 were characterized by frequent potential
fluctuations, which indicate the localized corrosion or the
dissolution of the oxides [23].
Figure 10 shows the potentiodynamic polarization
curves of Al/BN coating, TAC-1, and TAC-2 in 5 wt%
NaCl solution. For Al/BN coating, with the potential rais-
ing from the corrosion potential (Ecorr, -1.10 V) to
-0.65 V, the curve exhibits typical activation polarization,
revealing a parabolic relationship between the potential and
the current densities. When the potential value surpassed -
0.65 V, the anodic current increased sharply and the anodic
Tafel slope was almost zero, indicating serious corrosion
damage occurred on the surface of the coating. However,
the potentiodynamic polarization curves of both TAC-1
and TAC-2 exhibit a typical active–passive characteriza-
tion and translate directly into the passive region from the
Tafel region. The passivity is a primary reason for the more
excellent corrosion resistance of TAC-1 and TAC-2 than
Al/BN coating. The high corrosion resistance of Ti and its
alloy can be ascribed to the formation of a three-layer
protective passive film on their surface, which consist of
TiO, Ti2O3, and TiO2 [24, 25]. Marino et al. [26] found that
the transformation of TiO or Ti2O3 to TiO2 (more stable)
initiated at the electrode/electrolyte interface, and then the
electrode eventually reached a relatively stable state.
The open circuit potential (EOCP), corrosion potential
(Ecorr), passivation potential (Epp), breakdown potential
(Ebp), and the corrosion current densities (icorr) were
acquired from the potentiodynamic polarization curves,
and the corresponding corrosion parameters are given in
Table 2. The values of Ecorr of TAC-1 and TAC-2 deter-
mined from the polarization are lower than the values of
EOCP. Considering the polarization tests started at the
Fig. 8 SEM image and the EDS results of the TAC-2 after 960 h salt spray corrosion test
Fig. 9 Open circuit potentials (EOCP) of the as-sprayed coatingsFig. 10 Potentiodynamic polarization curves of the as-sprayed
coatings
F. Zhang et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(6), 1114–1121 1119
123
cathodic potential, the partially passive film at the surface
was removed due to the high initial reducing potentials
[27]. The Ebp of TAC-2 is higher than that of TAC-1,
indicating the passive film of TAC-2 is more stable. The
icorr of TAC-2 is lower than that of TAC-1, indicating a
lower corrosion rate of TAC-2. Additionally, while raising
the potential of TAC-1 from -0.49 V to Ebp, a current
fluctuation is observed, which suggests that the passive film
undergoes a continuous process of breakdown/repair.
The results of electrochemical tests are consistent with
that obtained from salt spray corrosion tests. Spontaneous
passivation of TAC-1 and TAC-2 granted them better salt
spray corrosion resistance than that of Al/BN coating.
Furthermore, the higher Ebp, the lower icorr, and more
stable passive process enable the TAC-2 better salt spray
corrosion resistance than that of TAC-1.
3.4 Localized Corrosion of TAC-1
The BSE micrographs and the element maps of TAC-1 and
TAC-2 are shown in Figs. 11 and 12, respectively. Com-
paring the metal matrix of the two coatings, it is found that
the distributions of elements in the two samples are dif-
ferent. For TAC-1, some Al-enrichment areas and many
lamellar boundaries exist in the metal matrix. By contrast,
the elements are well distributed and continuous in TAC-2.
Several typical areas (marked in the micrographs) were
analyzed by EDS, the results are listed in Tables 3 and 4.
The sample after 120 h salt spray corrosion test was
analyzed by the SEM/EDS, and the micrograph is shown in
Fig. 13. It visually confirms that some darker spots with a
diameter of up to 20 lm are severely attacked. The EDS
analysis shows that the dark spot site (marked by 1) is rich
in O and Al, suggesting that the micro area was corroded.
By comparing the Ti/Al mass ratio of site 1 and site 2, it is
clear that the area with higher Al content was preferentially
corroded. The corrosion behavior of the coatings during the
Table 2 Corrosion resistance parameters of Al/BN coating, TAC-1,
and TAC-2
Sample EOCP (V) Ecorr (V) Epp (V) Ebp (V) icorr (lA/cm2)
Al/BN -1.13 -1.10 – – 20.2
TAC-1 -0.61 -0.91 -0.75 -0.15 10.5
TAC-2 -0.22 -0.67 -0.52 0.02 5.1
Fig. 11 SEM image and EDS results of TAC-1
Fig. 12 SEM image and EDS results of TAC-2
Table 3 Chemical compositions of the marked areas shown in
Fig. 11 analyzed by EDS (in wt%)
Element Site 1 Site 2 Site 3 Site 4
Ti 94.5 ± 0.3 79.6 ± 0.9 75.5 ± 0.4 71.2 ± 1.1
Al 5.5 ± 0.3 20.4 ± 0.9 24.5 ± 0.4 28.8 ± 1.1
Table 4 Chemical compositions of the marked areas shown in
Fig. 12 analyzed by EDS (in wt%)
Element Site 1 Site 2
Ti 83.3 ± 0.4 85.8 ± 0.3
Al 16.7 ± 0.4 14.2 ± 0.3
1120 F. Zhang et al.: Acta Metall. Sin. (Engl. Lett.), 2014, 27(6), 1114–1121
123
salt spray was significantly influenced by the compositions
and micro-structure features [28, 29], especially by the Al
content and its distribution. The enrichment of elements
may lead to the non-uniform surface potential distribution
and localized corrosion [30, 31].
4 Conclusions
(1) The dominant phases in both TAC-1 and TAC-2 are
Ti3Al and hexagonal BN. The two coatings exhibit
the typical abradable seal coating’s micro structure
features. The BN particles and the pores were trapped
in an open network of lamellar metallic splats.
(2) Salt spray corrosion tests confirm that TAC-1 and
TAC-2 exhibit improved salt spray corrosion resis-
tance compared with Al/BN coating. After 960 h salt
spray tests, just very few white corrosion products
(Al(OH)3 and AlO) can be found on the surface of
TAC-1, but no corrosion products can be detected on
the surface of TAC-2.
(3) TAC-2 exhibited higher corrosion potential (Ecorr)
and breakdown potential (Ebp) but a lower corrosion
current density (icorr) than that of TAC-1. The higher
Ebp and lower icorr grant the TAC-2 better salt spray
corrosion resistance than TAC-1.
(4) Non-uniform elements distribution in metal matrix of
TAC-1 leads to localized corrosion.
Acknowledgments This work was financially supported by the
Fund of State Key Laboratory of Multiphase Complex Systems, IPE,
CAS (No. MPCS-2012-A-06) and the Natural Science Foundation of
Jiangsu Province, China (No. BK2011452).
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Fig. 13 Surface morphology of the TAC-1 after 120 h salt spray tests a, EDS results of the sites marked by 1 b, 2 c in Fig. 13a
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