Corrosion Resistance of Ti Al/BN Abradable Seal Coating … · Abradable seal coatings are often...

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

http://link.springer.com/journal/40195

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

123

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


123

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


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


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


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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