ORI GIN AL PA PER

Microstructural Characterization and Cyclic HotCorrosion Behaviour of Sputtered Co–AlNanostructured Coatings on Superalloy

Atikur Rahman • R. Jayaganthan •

Ramesh Chandra • R. Ambardar

Received: 7 January 2011 / Revised: 14 March 2011 / Published online: 15 June 2011

� Springer Science+Business Media, LLC 2011

Abstract Nanostructured Co–Al coatings on Superni-718 superalloy substrate were

deposited by DC/RF magnetron sputtering in the present work. The microstructure

and cyclic hot-corrosion behavior of nanostructured Co–Al coatings on Superni-718

superalloy were investigated in molten salt of 40 wt% Na2SO4 ? 60 wt% V2O5 at

900 �C. The results showed that a dense scale formed on the coated samples exposed

to corrosive environment during thermal cycling. The spinel phases of CoCr2O4,

CoAl2O4 and NiCr2O4 were found in the corroded scale of the coatings, resulting in an

effective inhibition of O and S diffusion. The sputtered Co–Al coatings exhibited high

hot corrosion resistance due to the formation of b-CoAl phases in the coating. The

relevant corrosion mechanisms substantiating the role of coatings are discussed.

Keywords Nanostructured Co–Al coating � Magnetron sputtering �Molten salts � Hot corrosion

Introduction

MCrAlY coatings play a significant role in protection of hot section components in

gas turbine engine system, either as overlays or as bond coats for TBCs [1–4].

A. Rahman � R. Jayaganthan (&)

Department of Metallurgical and Materials Engineering, Indian Institute of Technology Roorkee,

Roorkee 247667, India

e-mail: rjayafmt@iitr.ernet.in

A. Rahman � R. Chandra

Institute Instrumentation Centre, Indian Institute of Technology Roorkee, Roorkee 247667, India

R. Ambardar

Department of Metallurgical Engineering, National Institute of Technology Hazratbal, Srinagar

190006, India

123

Oxid Met (2011) 76:307–330

DOI 10.1007/s11085-011-9257-3

Usually, the conventional MCrAlY coatings are designed to meet particular

operating conditions. In order to provide enhanced high temperature protection over

a wide range of operating conditions, the nanostructured coatings have been

designed based on MCrAlY coatings [5–7]. Nanostructured MCrAlY (M = Ni, Co,

or Fe) coatings have been identified as potential materials for providing good

resistance against high temperature oxidation and hot corrosion [8–16] as compared

to conventional coatings due to the formation of dense, continuous, non-porous and

adherent alumina scales over the coatings[17, 18]. The size effect of nanostructured

coatings facilitates the enhanced diffusivity of atoms to form the continuous

protective scales, which is normally discontinuous in the conventional coatings as

reported in the literature [19]. Wang et al. [20] investigated the oxidation and hot

corrosion behavior of sputtered nanocrystalline coating of superalloy K52 and result

showed that nanocrystallization of superalloy coating greatly reduced the oxidation

and hot corrosion rate due to formation of protective and adherent alumina scale.

Ren and Wang [21] have studied the high-temperature oxidation and hot-corrosion

behavior of a sputtered NiCrAlY coating with and without aluminizing. The results

indicated that the sputtered NiCrAlY coating possessed excellent oxidation

resistance at 1,000 �C due to the presence of extensive amount of chromium and

good amount of aluminum. The aluminized NiCrAlY coating showed a much better

hot-corrosion resistance in the presence of 75 wt% Na2SO4 ? 25 wt% K2SO4/NaCl

film as a result of the formation of a continuous and protective Al2O3 scale. The hot-

corrosion behavior of a nanocrystalline coating of K38G alloy, prepared by

magnetron sputtering and cast K38G in molten 75 wt% Na2SO4 ? K2SO4 at

900 �C was studied by Geng et al. [22]. The coating eliminated internal sulfidation

during the early stage of corrosion as a result of the formation of a continuous and

compact scale. The nanocrystallization of K38G alloy prolonged the incubation of

breakaway corrosion and improved the corrosion resistance of K38G.

It is well known that the content of Al is vital for MCrAlYs because the selective

oxidation of Al occurs only while the Al concentration is adequate [23–29]. Besides,

the life time of MCrAlY coatings depends greatly on the extent of Al depletion

occurring on ceaseless consumption from the repeated spallation/rehabilitation of

the alumina scale and the interdiffusion with substrates. An increase of the Al

content in MCrAlY overlays is deemed effective for both enhancing the

performance and extending the service life. However, the direct increase of the

Al content in the coating is hard to be accomplished due to the reduction of

mechanical properties and the deterioration of machinability.

The present work has been focused to study the microstructure and hot corrosion

behaviour of magnetron sputtered Co–Al coatings on the Superni-718 superalloy

substrate and compared with uncoated bare Superni-718 substrate in the environ-

ment of molten salt of 40% Na2SO4 ? 60% V2O5 at 900 �C for 100 cycles. The

formation of intermetallic b-CoAl phase (Al reservoir) in the Co–Al based

superalloy coatings exhibits high melting point (1,640 �C), which is beneficial for

providing high temperature oxidation and type I hot corrosion resistance at very

high temperature above 1,000 �C, in the actual service applications, especially in

gas turbines. The CoAl based nanostructured superalloy coatings could serve as

potential protective coatings for high temperature applications to combat high

308 Oxid Met (2011) 76:307–330

123

temperature corrosion. The improvement in oxide scale spallation resistance is

believed to be related to the fine-grained structure of the coating [30–33].

The weight changes of the coated samples exposed to corrosive environments

under cyclic conditions are continuously monitored to establish its corrosion rate,

The nanostructured Co–Al coatings and corroded products of the coatings were

characterized by using XRD, FE-SEM/EDS to understand the mechanisms of high

temperature corrosion in the present work.

Experimental Procedures

Ni-based superalloy namely Superni-718 has been chosen to deposit Co–Al coating

in the present work. The superalloy was procured from Mishra Dhatu Nigam

Limited, Hyderabad, India in annealed and cold rolled sheet form and its chemical

composition is shown in Table 1. Each specimen measuring approximately 18 mm

(length) 9 15 mm (width) 9 3 mm (thickness) were cut from the rolled sheet and

grinded by using SiC emery papers. Subsequently, it was polished on cloth

polishing disc by using alumina powder followed by diamond paste. Commercially

available Co Target (99.99% pure) with 2 inch diameter and 2.0 mm thickness and

similarly, commercially available Al Target (99.99% pure) with 2 inch diameter and

5.0 mm thickness were fixed at an angle of 45� to each other in the sputtering

chamber. With the use of rotator, the substrate heater was rotated between Co and

Al targets continuously to perform co-sputtering. The target to substrate distance

was fixed at 40 mm during sputter deposition of the coatings. Before starting the

deposition, the targets were pre-sputtered for 15 min with a shutter located in

between the targets and the substrate. The shutter was also used to control the

deposition time. Before deposition of Co–Al coatings, all samples were cleaned in

acetone, ethanol and deionized water. The detailed process parameters used in DC/

RF magnetron sputtering are shown in Table 2.

Characterization of Co–Al Coatings

XRD (Bruker AXS, D8 Advance) measurements were made using CuKa radiation to

characterize the as deposited Co–Al coatings at 500 and 700 �C on Superni-718

substrate. The scan rate and the scan range were used 0.1 s/step and from 10� to

110�, respectively. An average grain size of the Co–Al coatings is estimated using

its XRD peak broadening according to Scherrer formula [34], as given in Eq. 1. It

Table 1 Chemical composition of the superalloy used in study (Superni-718)

Chemical composition (wt%)

Midhani

grade

Fe Ni Cr Ti Al Mo Mn Si Co Nb P C S

Superni-718 19.8 Bal 17.6 0.96 0.53 3.23 0.02 0.03 0.01 4.91 0.005 0.02 0.007

Oxid Met (2011) 76:307–330 309

123

was found to be in the range of 20–35 nm and 21.8 nm and 25.7 nm at 500 and

700 �C respectively.

Bð2hÞ ¼ kkD cos h

ð1Þ

where D is the average size of crystallite, B is the broadening of the diffraction line

measured at half maximum intensity, k is constant (k = 1 is used in the present

work), k is the wavelength of the X-ray radiation (1.54052 A, CuKa) and h is the

Bragg angle. The instrumental broadening has been considered for the calculation of

grain size, and the value of 0.1 is subtracted (calculated using standard silicon

sample) from the full-width half maximum (FWHM) value (B value).

FE-SEM (FEI, Quanta 200F) is used to characterize the microstructures of the

Co–Al coatings at an acceleration voltage of 20 kV. The surface morphology of the

Co–Al coatings is characterized by AFM (NT-MDT, Ntegra) operated in a semi

contact (tapping) mode in order to calculate its surface roughness. The root-mean-

square (RMS) roughness of the coatings is calculated from the AFM scan at five

different spots chosen in each sample.

Hot Corrosion Studies

Hot corrosion studies under cyclic conditions were performed in molten salt of

(40% Na2SO4 ? 60% V2O5) environment up to 100 cycles for the bare, Co–Al

coated superalloy samples, and their corrosion behavior was compared. The samples

were mirror polished down to 1 lm alumina on a wheel cloth polishing machine.

Each cycle consisted of 1 h of heating at 900 �C, in silicon carbide tube furnace,

followed by 30 min of cooling at room temperature for up to 100 cycles. The

samples were cleaned using acetone and heated in an oven at about 250 �C for

10 min. Thereafter, a layer of 40% Na2SO4 ? 60% V2O5 mixture, prepared by

mixing it with distilled water, was applied uniformly on the warmed polished

samples with the help of camel hair brush. The amount of salt coating was

maintained within the range of 3.0–5.0 mg/cm2. The salt coated samples were kept

in the alumina boats and then inserted into SiC tube furnace for 3–4 h at 100 �C for

drying and proper adhesion of the salt. Subsequently, the dried salt coated samples

were again weighed and introduced into the silicon tube furnace at 900 �C. During

Table 2 Sputtering parameters for Co–Al coatings

Target Al (99.99% pure, 2 inch diameter & 5 mm thickness)

Co (99.99% pure, 2 inch diameter & 2 mm thickness)

Base pressure 4.0 9 10-6 Pa

Deposition gas pressure (Ar) 1.9 9 10-2 Pa

Deposition power 70 W/150 W for Co/Al target

Deposition time for Co–Al coating 90 min

Substrate Superni-718

Substrate temperature 500 & 700 �C

Total deposition time 90 min

310 Oxid Met (2011) 76:307–330

123

hot corrosion runs, the weight of boats and samples was measured together at the

end of each cycle with the help of an electronic balance Model CB-120 (Contech,

Mumbai, India) with a sensitivity of 1 mg. During each cycle, the weight changes

measured for bare and coated samples were used to calculate the corrosion rate. The

spalled scale (if any) was also included at the time of measurements of weight

change to determine total corrosion rate. The kinetics of hot corrosion was

determined from the weight change measurements. After hot corrosion studies, the

corroded samples were analyzed by using XRD and FE-SEM/EDS.

Results and Discussion

Microstructures Before Hot Corrosion

Figure 1a shows the XRD pattern of as deposited Co–Al coatings on Superni 718 at

different substrate temperatures. The intermetallic b-CoAl phase is observed at

different substrate temperatures, since the outer layer of coatings contains high Co

and Al as evident from the XRD results and the CoAl binary phase diagram. Co–Al

coating at substrate temperature of 500 �C, shows only one major peak of b-CoAl

phase; where as, the coating at substrate temperature of 700 �C shows one major

peak along with two small peak of b-CoAl phase with different orientation.

The 2D and 3D AFM images of the Co–Al coating at different substrate

temperatures are shown in Fig. 2a–d. 2D image of Fig. 2a indicates that coating has

some voids and spherical grains, where as 2D image of Fig. 2c shows that it has

dense coating and spherical grains. The surface roughness of the coatings is found to

be 12.2 and 22.5 nm for the coatings deposited at 500 and 700 �C, respectively. The

particle size of Co–Al coated samples are also confirmed by AFM images (Fig. 2a,

c) and are used for calculation of particle size, which are in tandem with the XRD

results.

Figure 3a, b shows the FE-SEM surface micrographs with EDS at different

points on the surface of as deposited Co–Al coatings at 500 and 700 �C. The

observed microstructural features of the coatings can be classified by using the

structure zone model proposed by Messier [35]. The homogeneous temperature,

(ratio of substrate temperature to melting point of the coating materials), is used to

classify the coating microstructures observed at different substrate temperature

during sputter deposition of the coatings. In the present work, the coating deposited

at 500 �C exhibits a homogeneous temperature of 0.41 and showed spherical shape

grains, which approaches zone 2 structures as reported in Messier’s work. The

homogeneous temperature (TS/Tm) is 0.51 for the coatings deposited at 700 �C and

the rod type elongated shape grains, but their edges are round and dense coating are

observed at this deposition temperature, which comes under zone 3 structure. With

increase in substrate temperature, the surface mobility of condensed atoms

increases, which could easily diffuse from island side to lower potential zone of

substrate, resulting in denser coating with reduced porosity or voids in coatings. The

thickness of coatings was calculated by its cross sectional FE-SEM images and it

was found to be approximately 4 lm. The cross-sectional FE-SEM micrographs of

Oxid Met (2011) 76:307–330 311

123

as deposited Co–Al coating on Superni-718 at two different substrate temperatures

are shown in Fig. 3c, d. The coating deposited at 500 �C substrate temperature has

fine columnar grains (Fig. 3c), where as coating deposited at 700 �C substrate has

slightly thicker columnar grain (Fig. 3d). Thus, as per AFM analysis and FE-SEM

surface and cross-sectional analysis, it is observed that coating deposited at 700 �C

substrate temperature is more dense as compared to coatings fabricated at 500 �C

substrate temperature. The detailed chemical compositions of the as deposited

Co–Al coating on Superni-718 substrate has been analyzed at the surface as well as

the depth profile as shown in cross-sectional FE-SEM/EDS micrographs (Fig. 3a–d).

(210

)

(111

)(1

10)

Co

Al

Co

Al

Co

Al

At 700OC

At 500OC

Inte

nsi

ty (

a.u

)

2θ (Degree)20 40 60 80 100

20 40 60 80 100

(1,2

,8)

(1,3

,4,5

,8)

(3,6

)(1

)(1,2

,3,8

)(1

,2,3

,4,6

,8)

(1,2

,3,6

,8)

(1,3

)

(1,5

,8)

(2,3

,4,8

)(1

,3,6

,8)

(3,8

)

(1,2

,6)

(3,4

,5,8

)

(1,4

)

(1,4

,5,6

)

(1,2

,5)

(1,2

,5)

(1)

(1,2

,6)

(2,5

)(1

,5)(1,2

,4,5

)(5

)

(1,2

,6)

(6,7

,8)

(6,8

)(2)

(2,6

,8)

(2,6

)(2

)(6,7

)(8

)(6

,8)

(2,7

)(8

)(2

,8)

(8)

Corroded Co-Al Coating at 700OC

Corroded Co-Al Coating at 500OC

Corroded Bare Substrate

Inte

nsi

ty (

a.u

)

2θ (Degree)

(1) Al2O

3

(2) Cr2O

3

(3)CoCr2O

4

(4) CoO

(5) CoAl2O

4

(6) Fe2O

3

(7) NiO(8) NiCr

2O

4

Fig. 1 XRD pattern of a as deposited Co–Al coatings and b corroded Co–Al coatings on Superni-718 atdifferent substrate temperatures and corroded bare Superni-718 substrate

312 Oxid Met (2011) 76:307–330

123

Microstructures of Corroded Samples

Figure 1b shows the XRD results for the corroded products of bare and Co–Al

coated samples on superni-718 superalloy at different substrate temperature exposed

to 40% Na2SO4 ? 60% V2O5 environment at 900 �C for 100 cycles. XRD analysis

was used to identify the phases present in the scale after hot corrosion and the

Hanawalt index method was used for indexing the XRD peaks. The indexing of

XRD peak was done by ASTM handbook and results were compared with pcpdf

win. During indexing, it was considered that a phase (d-spacing) must satisfy at least

three strongest peaks. If a phase does not satisfies this condition, it indicates an

absence of this phase in the scale. In this way, one by one, every phase (d-spacing)

was checked with the help of ASTM handbook and indexing of XRD peak was

done. Therefore, it is possible that on one peak, more than one phase is present. In

the present work, ex-situ XRD analysis was used after hot corrosion, hence, only

Fig. 2 AFM 2D and 3D images of Co–Al coatings on Superni-718 substrate at different substratetemperatures a–b at 500 �C and c–d at 700 �C

Oxid Met (2011) 76:307–330 313

123

Fig. 3 FE-SEM/EDS surface micrographs (a–b) of Co–Al coatings on Superni-718 substrate at differentsubstrate temperatures a at 500 �C, and b at 700 �C, and c–d FE-SEM/EDS cross-sectional micrographs ofCo–Al coatings on Superni-718 substrate at different substrate temperatures c at 500 �C and d at 700 �C

314 Oxid Met (2011) 76:307–330

123

stable phases (mainly oxides) were observed and indexed. It is observed that the

oxide scales such as Cr2O3, NiO, Fe2O3 and NiCr2O4 are formed on the bare

superalloy and Cr2O3, Al2O3, CoO, and CoAl2O4 are formed as a major phase and

Fe2O3 as a minor phase on corroded coating at 500 �C substrate temperature.

However, Cr2O3, Al2O3, Fe2O3, CoO and CoCr2O4, CoAl2O4 and NiCr2O4 scales

are found as major phases on the corroded Co–Al coatings deposited at 700 �C

substrate temperature. The presence of Fe2O3 and Cr2O3 phases on the surface of

hot corroded Co–Al coatings indicates the outward diffusion of Fe and Cr element

from the substrate to coating during hot corrosion of the samples exposed to 40%

Na2SO4 ? 60% V2O5 environment at 900 �C. Therefore, it is required that a third

element should be present in the coating which act as barrier for cation as well

anion. Hence, most of the researchers reported that Cr or Y as a third element in the

coating is beneficial for corrosion protection. Cr is more beneficial since its presence

in the coating may induce transient oxidation containing Cr-oxide patches, which

act as oxidation retardants and nucleation centers for Al-oxide scales. Thus, it favors

the formation of more alumina in scale. Once sufficient amount of alumina in scale

is formed, it enables to act as a barrier layer with corrosive environment. Thus, it

can retard the diffusivity of substrate element towards coating during hot corrosion.

Figure 4a shows the weight gain per unit area versus number of cycles plots for

the bare substrate and Co–Al coatings deposited at different substrate temperatures,

500 and 700 �C on Superni-718 subjected to hot corrosion studies in molten salt

(40% Na2SO4 ? 60% V2O5) environment at 900 �C for 100 cycles. It is shown that

under cyclic conditions, the bare substrate is more prone to corrosion attack. In case

of bare Superni-718 substrate after 20th cycle, weight increases rapidly up to 60th

cycle, and then onwards shows very little weight gain and appears like a constant

weight variation. In Co–Al coated sample at 500 �C substrate temperature, the

weight increases up to 20th cycle, and subsequently, the weight variation is less and

follows a constant weight variation. On the other hand, in Fig. 4a, Co–Al coated

sample at deposition temperature of 700 �C shows abruptly weight loss from 40

cycles to 60 cycles, and then onwards, the weight variation is constant. The total

weight gain at the end of 100 cycles after hot corrosion in Co–Al coated sample at

700 �C was 20.12 mg/cm2, where as in Co–Al coated sample at 500 �C was

23.42 mg/cm2 and for the bare substrate it was 46.39 mg/cm2 (Fig. 4c). The hot

corrosion kinetics is determined based on weight change measurements made

during the corrosion studies. Overall, the weight gain in Co–Al coated sample at

700 �C has found to be lower than that of Co–Al coating at 500 �C and bare

substrate.

Mass gain results (Fig. 4a) of coated specimen shows that it follows nearly a

parabolic law, where as bare substrate showed a deviation from the parabolic law.

Parabolic law is diffusion controlled after the initial chemical controlled reaction of

the coated specimens. Once a nearly constant weight gain is obtained, it indicates

the parabolic law, which precludes further oxidation/corrosion process. Any

coating/alloy designed for high temperature application should follow the parabolic

law for the better corrosion protection. The weight changes curves reveals that the

Co–Al coated substrate are found to be more hot corrosion resistant than the bare

superalloy. It is evident that Co–Al coated substrate obeys parabolic rate law and

Oxid Met (2011) 76:307–330 315

123

0

5

10

15

20

25

30

35(b)Corroded Bare Substrate Corroded Co-Al Coating at 500O CCorroded Co-Al Coating at 700O C

(Wei

gh

t C

han

ges

/ A

rea)

2 (m

g2 c

m-4

s-1)

Number of Cycles

-10 0 10 20 30 40 50 60 70 80 90 100 1100

1

2

3

4

5

6(a)

Wei

gh

t C

han

ges

/ A

rea

(mg

/ cm

2 )

Number of Cycles

Corroded Bare Substrate Corroded Co-Al Coating at 500O CCorroded Co-Al Coating at 700O C

0

10

20

30

40

50(c)

Co-Al Coating At 700oC

Co-Al Coating At 500oC

BareSuperni-718

Cu

mm

ula

tive

Wei

gh

t g

ain

/Are

a(m

g/c

m2 )

-10 0 10 20 30 40 50 60 70 80 90 100 110

Fig. 4 a Weight changes/area(mg/cm2) versus number ofcycles and b parabolic rateconstant, Kp (weight change/area)2, mg2/cm4 versus numberof cycles, for bare and coatedsamples oxidized in molten saltenvironment at 900 �C, andc cumulative weight gain/area(mg/cm2) versus number ofcycles, for bare and coatedsamples oxidized in molten saltenvironment at 900 �C for 100cycles

316 Oxid Met (2011) 76:307–330

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exhibit the tendency to act likes diffusion barriers to the corroded species. The

weight changes square (mg2/cm4) verses time (number of cycles) plots are shown in

Fig. 4b to establish the rate law for the hot corrosion. It is observed from the graph

that Co–Al coated sample follow nearly parabolic rate law where as bare substrate

deviated from parabolic law due to cyclic scale growth. The higher weight gain of

the specimens during the first few cycles might be due to the rapid formation of

oxides at the grain boundaries and within the open pores due to the penetration of

the oxidizing species, further the subsequent increase in weight is gradual. The

weight changes of the samples reveal that Co–Al coated sample at 700 �C substrate

temperature are found to be more hot corrosion resistant than the bare Superni-718

substrate and Co–Al coated at 500 �C substrate temperature. The nature of corrosion

kinetics of coatings and bare substrate was determined by using the changes in their

weight measured as a function of different cycles. The parabolic rate (Fig. 4b)

constant Kp was calculated by a linear least-square algorithm to a function in the

form of (W/A)2 = Kpt, where W/A is the weight gain per unit surface area

(mg cm-2) and ‘t’ indicates the number of cycles represents the time of exposure.

The calculated Kp for the Co–Al coating at 500 and 700 �C on superalloy is found to

be 9.721 9 10-12 and 5.472 9 10-12 g2 cm-4 s-1 respectively. It is very less as

compared to the Kp value of 98.921 9 10-12 g2 cm-4 s-1 obtained for the bare

superalloy substrate. Figure 4c shows the cumulative weight gain per unit area for

Co–Al coated sample at different substrate temperature and bare Superni-718

substrate subjected to cyclic hot corrosion for 100 cycles in 40% Na2SO4 ? 60%

V2O5 environment at 900 �C. Figure 4c indicate that Co–Al coating at 700 �C

substrate temperature shows minimum cumulative weight gain per unit area as

compared to bare substrate and Co–Al coating at 500 �C substrate temperature,

exposed to molten salt environment at 900 �C for 100 cycles. Co–Al coating on

Superni-718 substrate at 700 �C has been found successful in reducing the overall

weight gain by 56.6% (Fig. 4c) with reference to bare superni-718 substrate. On the

other hand, Co–Al coating on Superni-718 substrate at 500 �C has been reduced the

overall weight gain by 49.5% (Fig. 4c) with reference to bare superni-718 substrate.

Figure 5a–d shows FE-SEM/EDS surface micrographs at the selected points of

interest of corroded bare substrate Superni-718. The scale formed on bare Superni-

718 has spalled, and the cracks were formed as shown in Fig. 5b. The large numbers

of small pores on the surface of the specimen are observed from Fig. 5b. The low

magnification (2,5009) micrograph shown in Fig. 5a, revels that the scale is non-

uniform. High magnification (20,0009) micrograph shown in Fig. 5c indicates the

formation of faceted oxides. The over all EDS result showed (Fig. 5d) that scale on

the surface of corroded bare substrate has chromium and nickel rich oxide and also

iron oxide is present. The EDS analysis (Fig. 5c) was carried out such as the one

marked as (1) correspond to the Fe2O3 phase i.e. iron-rich oxide (O = 43.36 at%,

Fe = 43.36 at%, Cr = 10.40 at% and Ni = 10.33 at%), the location ‘2’ corre-

sponds to the Cr-rich and Ni-rich oxide (O = 49.99 at%, Cr = 26.31 at%,

Ni = 17.96 at% and Fe = 10.76 at%). These results are also confirmed by XRD

results (Fig. 1b).

Figure 6a, b shows the FE-SEM surface micrographs of corroded coatings at the

selected points of interest along with its corresponding EDS results. The low

Oxid Met (2011) 76:307–330 317

123

magnification (2,5009) micrograph shown in Fig. 6a, b reveals that scale formed on

both corroded Co–Al coated substrate has uniform, continuous and defect free scale.

There is no spallation and cracks in scale are observed on the both corroded Co–Al

coated sample. At high magnification (15,0009), surface micrograph of corroded

Co–Al coating at 500 �C substrate temperature shown in corner Fig. 6a indicates

three dimensional hexagonal morphology of scale. The surface micrograph of

corroded Co–Al coating at 700 �C substrate temperature at high magnification

(15,0009) shown in corner Fig. 6b indicates the pyramidal morphology of scale.

EDAX Genesis 32 software was used to calculate the composition of the corroded

products, mainly, oxides using elemental composition of the coatings obtained from

EDS analysis. The formation of oxide scale such as Al2O3, Cr2O3, Fe2O3 and CoO

on the surface of the coating is evident from this figure. The coating deposited at

500 �C substrate temperature has Al-rich oxide (37.85 at%) and also chromium

oxide (09.58 at%), iron oxide (01.12 at%) and cobalt oxide (08.51 at%) scale are

Fig. 5 a–b FE-SEM surface micrographs of corroded bare Superni-718 substrate at lower magnification(92,500) and c at higher magnification (920,000), and d surface EDS analysis result at the differentlocations of corroded bare Superni-718 substrate

318 Oxid Met (2011) 76:307–330

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present, which is shown in Fig. 6a. The coatings deposited at 700 �C substrate

temperature has Al-rich oxide scale (42.47 at%) and Co-rich oxide (11.29 at%) and

also chromium oxide (02.92 at%), and iron oxide (01.08 at%) scale are present,

which is shown in Fig. 6b. The nanosized grains in the Co–Al coating, observed

using XRD and AFM analysis, facilitates the enhanced reactivity of Al and Co with

oxygen for the formation of continuous layer of their corresponding oxide, which is

essential for providing an adequate protection to the Superni-718 in the high

temperature environment. Thus, both corroded Co–Al coatings show uniform,

adherent, continuous, and dense scale on the surface.

Figure 7a–c shows the FE-SEM cross sectional micrographs of corroded bare

substrate and it reveals that the scale is non-uniform (Fig. 7a), crack (Fig. 7b) and

porous (Fig. 7c). The higher weight gain of bare alloy after 20th cycles is due to

spallation of initial oxide scales and formation of cracks in scales (Figs. 5b, 7b), the

Fig. 6 FE-SEM surface micrographs (at 92,500 magnification) with EDS of corroded Co–Al coatingson Superni-718 deposited at different substrate temperatures a at 500 �C, and b at 700 �C. The cornersurface micrograph is at higher magnification (915,000) of the same coating

Oxid Met (2011) 76:307–330 319

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continuous exposure of fresh surface of the materials takes place during hot

corrosion. The evaporation of volatile impurities in the bare sample, during high

temperature exposure, result in the formation of pores (Figs. 5b, 7c), which might

also contribute to the enhanced reaction between the bare sample and oxygen. After

60th cycles, weight variation in bare substrate is less and it seems nearly a constant.

XRD result revealed that there is NiCr2O4 spinel phase is present on the surface of

corroded bare substrate. The presence of NiCr2O4 spinel phase at the surface of

corroded bare substrate might be inhibit the diffusivity of O and S [36] and thus

weight variation is less in bare substrate after 60th cycle, but still it is higher than

corroded coating. Thus, the surface as well as cross-sectional FE-SEM micrograph

of corroded bare substrate revealed that it has poor hot corrosion resistance in

molten salt of 40% Na2SO4 ? 60% V2O5 environment at 900 �C for 100 cycles.

Typical cross-sectional images of the corroded Co–Al coating deposited on

Superni-718 at different substrate temperature after exposure to 40%

Fig. 7 FE-SEM cross-section micrographs of corroded bare Superni-718 substrate

320 Oxid Met (2011) 76:307–330

123

Na2SO4 ? 60% V2O5 environment at 900 �C for 100 cycle are compared in Fig. 8a,

b at lower magnifications. A loss in coating thickness due to hot corrosion was

found in all the cases. It can be seen that the loss of coating thickness due to hot

corrosion is reduced in corroded Co–Al coating on Superni-718 at 500 �C substrate

temperature (Fig. 8a) as compared to corroded Co–Al coating deposited on Superni-

718 at 700 �C substrate temperature. Micrograph (Fig. 8b) reveals that the Co–Al

coating on Superni-718 at 700 �C substrate temperature as a bond coat should have

higher hot corrosion attack in 40% Na2SO4 ? 60% V2O5 environment at 900 �C.

At higher magnification (10,0009), it is observed that the cross-sectional

micrograph of corroded Co–Al coating at 700 �C substrate temperature (Fig. 8d)

shows dense scale and appears like a solid mass, where as, the corroded Co–Al

coating at 500 �C substrate temperature (Fig. 8c) shows a less dense morphology of

the coating with an occurrence of swelling. Co–Al coating at 500 �C substrate

temperature has fine surface morphology (Fig. 3a), but it belongs to Structure zone

model 2, which has some voids. It has been confirmed by AFM images in Fig. 2a.

On the other hand Co–Al coating at 700 �C substrate temperature has rod type

surface morphology (Fig. 3b) and it belongs to zone 3 as per the structure model

proposed by Messier. It has dense coating which is confirmed by AFM images

(Fig. 2c) and also by cross-sectional micrograph (Fig. 3d) which has thick columnar

structure. During hot corrosion, due to some voids in as deposited coating, corrosion

rate is slightly high as compared to Co–Al coating at 700 �C. Besides, it forms only

one spinel phase (CoAl2O4) in the corroded coating at 500 �C. Thus, they are not

able to heal up the all possible defects in scale. Therefore, cross-section micrograph

(Fig. 8c) indicated that Co–Al coated sample at 500 �C has less dense scale as

compared to Co–Al coating at 700 �C (Fig. 8d). Due to this reason, at lower

magnification (2,5009), the thickness of corroded Co–Al coated sample at 500 �C

substrate temperature (Fig. 8a) observed to be thicker as compared to corroded

Co–Al coated sample at 700 �C substrate temperature. Now, it is clear that the

Co–Al coating on Superni-718 at 700 �C substrate temperature as a bond coat

exhibit lower hot corrosion attack in 40% Na2SO4 ? 60% V2O5 environment at

900 �C for 100 cycles. Figure 8c, d shows.

EDS analysis of the cross-section micrographs of the corroded Co–Al coatings at

the selected points of interest. EDAX Genesis 32 software was used to calculate the

composition of the corroded products, mainly, oxides using elemental composition

of the coatings obtained from EDS analysis. The presence of oxygen at the coating–

substrate interface indicates that there is diffusion of oxygen into the substrate

material. EDS analysis of corroded Co–Al coating at 500 �C substrate temperature

(Fig. 8c) shows the top surface of the scale at point (1) contains mainly oxides of Co

(23.97 at%), Cr (08.01 at%), Al (05.59 at%), Fe (03.02 at%) and Ni (03.70 at%)

with subsequent point (2) shows a second oxide layer rich with Co (32.52 wt%), Al

(18.15 at%) and also Ni (02.32 at%),Cr (03.35 at%) and Fe (03.02 at%) oxide

scale. The presence of Ni, Cr, and Fe in oxide scale indicates the outward diffusion

of Ni, Cr, and Fe element from substrate to coating has occurred during hot

corrosion. At point (3), oxide scale rich with Co (26.11 at%), and Al (19.62 at%)

and also Ni (02.16 at%), Cr (03.24 at%) oxide are present in scale. Similarly, at

coating–substrate interface-1 (point 4), the main oxides are composed of Co

Oxid Met (2011) 76:307–330 321

123

(b)(a)

1 2 3 4 50

10

20

30

40

50

Ato

m %

of

each

ele

men

ts

Point of analysis

1 2 3 4 50

5

10

15

20

25

30

35

40

45

50

55

60

Ato

m %

of

each

ele

men

ts

Point of analysis

1

2

3

4

5

(c)

1

2

3

4

5

(d)

O Fe Co Cr Ni Al

O Ni Al Co Cr Fe

322 Oxid Met (2011) 76:307–330

123

(26.99 at%), Al (15.04 at%) and Cr (05.67 at%). Further, very close to coating–

substrate interface-2 at points (5), oxide scale is rich with Co (54.23 at%), Cr

(10.66 at%) and Al (05.44 at%) and also Ni (04.03 at%) scale are present. Oxygen

content in the scale at the top surface of corroded coating is found to be 55.12 at%

and decreases rapidly up to point (2); where as, it is 40.64 at% at point (4) and then

increases up to coating–substrate interface-1 (49 at%). Subsequently, it further

decreases rapidly at point (5) at the coating–substrate interface-2 (22.91 at%).

EDS analysis of corroded Co–Al coating at 700 �C substrate temperature

(Fig. 8d) shows the top surface of the scale (point (1)) exhibiting a dense and

adherent oxide scale. At point (1), the main oxide scale is rich with Al (29.53 at%),

Co (08.25 at%) and Ni (05.44 at%) and also Fe (02.43 at%), Cr (02.72 at%) scales

are found. The presence of Ni, Cr and Fe on top of the scale indicates the outward

diffusion of substrate elements to coating has occurred during hot corrosion. In

second layer of oxide scale, at point (2), the oxide scale is rich with Al (29.01 at%),

Co (10.07 at%) and Ni (06.07 at%) and also Fe (02.88 at%) and Cr (03.34 at%)

oxide are observed. At point (3), the oxide scale is rich with Al (22.79 at%), Co

(13.72 at%), and Ni (09.73 at%) and also Fe (03.21 at%) and Cr (03.27 at%) are

present. At point (4), coating–substrate interface, the oxide scale is rich with Al

(18.02 at%), Ni (16.38 at%) and Co (10.38 at%) and also Cr (04.56 at%), Fe

(04.22 at%) oxide are present. At point (5), the scale has fallen during cutting of the

corroded sample. The oxygen content in scale at the top of surface of corroded

coating is 51.63 at% and it decreases rapidly to 46.48 at% at the coating–substrate

interface (point 4).

The following observations are made from Fig. 8c, d: (i) The over all content of

Al in the corroded Co–Al coating at 700 �C substrate temperature when it is

exposed to molten salt environment at 900 �C is high as compared to Co–Al coating

at 500 �C substrate temperature. It reveals that a loss of Al2O3 scale during fluxing

is less in corroded Co–Al at 700 �C temperature. (ii) Distribution of Co element in

corroded Co–Al coating at 700 �C is nearly same throughout the coating, from top

surface of corroded Co–Al Coating to coating–substrate interface. It suggests that

the role of Co is not only to the formation of CoO but also to formation of CoAl2O4

and CoCr2O4 spinel phase in coating. (iii) In the case of corroded Co–Al coating at

700 �C substrate temperature, Ni and Cr contents increase from top of the coating to

coating–substrate interface during hot corrosion, which indicates the formation of

second spinel phase NiCr2O4. (iv) Just after point (1). From point (2) to point (4),

Co and Cr contents in the corroded Co–Al coatings at 700 �C substrate temperature

is sufficient for the formation of third spinel phase (CoCr2O4) as observed in the

corroded coating. Due to this reason, during hot corrosion, the voids and defects

formed in Co–Al coating at 700 �C, are healed by these spinel phases. Hence, dense

scale morphology is observed in the corroded Co–Al coating at 700 �C substrate

temperature (Fig. 8d). The presence of spinel phase in the corroded Co–Al coating

Fig. 8 a, b FE-SEM cross-section micrograph at lower magnification (92,500) of corroded Co–Alcoatings on Superni-718 deposited at different substrate temperatures a at 500 �C, b at 700 �C and c,d FE-SEM cross-section micrographs (at higher magnification 910,000) with EDS profile of corrodedCo–Al coatings on Superni-718 deposited at different substrate temperatures c at 500 �C and d at 700 �C

b

Oxid Met (2011) 76:307–330 323

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at 700 �C substrate temperature is also confirmed by XRD result (Fig. 1b). The

presence of oxygen content at coating–substrate interface (point (4)) is less, which

indicates a decrease in the diffusivity of O.

Figure 9a shows that the spallation of scale has occurred at corner in one of the

corroded Co–Al coated sample at 700 �C substrate temperature where it is clamped

during coating deposition, deposition of coating was less at this location. It is

observed that there are four layers of scales in the corroded Co–Al coated sample

(Fig. 9a) and EDS analysis was carried out at four different locations which

correspond to different layer of scale. In each layer, the oxide of Al, Co, Cr, Fe and

the spinel phase are observed. On the top layer (points 1 & 2) of scale, CoCr2O4,

CoAl2O4 and NiCr2O4 scales are observed. In the second layer of scale (point 3),

NiCr2O4 and CoCr2O4 are observed. The third layer (points 4 & 5) of scale contains

CoCr2O4 and in the fourth layer (point 6) of scale, NiCr2O4 spinel is observed. In

hot corrosion of Co–Al coated sample, Al2O3 scale is fluxed by NaVO3 and it forms

NaAlO2 phase which has a melting point around 1,785 �C, Thus, it increases the hot

corrosion resistance of coating. During cyclic hot corrosion, some defects in the

scale might form due to mismatch in coefficient of thermal expansion. Hence,

corrosive species might have diffused through these defects, which causes the

oxidation of the alloying elements in the coating as well as in the substrate. When

Cr2O3 scale is fluxed, it forms Na2CrO4 phase in the scale whose melting point is

around 765 �C, it evaporates and creates another defects in the scale. This defect is

healed by the spinel phases like CoCr2O4, CoAl2O4 and NiCr2O4 [36]. Due to this

reason, the cross-section FE-SEM micrograph (Fig. 8a–d) of corroded Co–Al

coated sample has dense and uniform scale as compared to corroded bare substrate

(Fig. 7a–c). Corroded Co–Al coated sample at 700 �C (Fig. 8d) has dense scale and

look like solid mass as compared to the corroded Co–Al coating at 500 �C (Fig. 8c)

since it has three spinel phase i.e. CoAl2O4, CoCr2O4 and NiCr2O4, beside Fe2O3

scale present as a major phase, which is confirmed by XRD (Fig. 1b) results. In case

of corroded bare substrate, dense and uniform scale is not observed on surface

(Fig. 5a, b) as well as in the cross-sectional micrograph (Fig. 7a–c) due to the

following reasons.

(i) Substrate does not have sufficient Al content (Table 1) and

(ii) There is only one spinel phase present in the scale (Fig. 1b), and it is not able

to rectify the all defects in scale, which has formed during hot corrosion.

It is evident from Fig. 9a that the spinel phase heals the defects developed during

hot corrosion and retards the diffusivity of O and S, which in turns increases the hot

corrosion resistance.

Hot Corrosion Mechanisms

The reactivity of bare and coated samples in molten salt environment can be

explained as follows:

The salts Na2SO4 and V2O5 react at 900 �C and form a low melting compound of

NaVO3 (M.P. 610 �C) as per the reaction given below [37].

324 Oxid Met (2011) 76:307–330

123

Fig. 9 FE-SEM surface micrograph of damage portion of corroded Co–Al coating on Superni-718 at700 �C substrate temperature with EDS analysis results at different location

Oxid Met (2011) 76:307–330 325

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Na2SO4 þ V2O5 ¼ 2NaVO3 1ð Þ þ SO2 þ 1=2ð ÞO2 ð2ÞNaVO3 acts as a catalyst and also serve as an oxygen carrier to the bare

superalloy through the open pores present on the surface, which leads to the rapid

oxidation of the substrate elements and form non protective scales [38–42]. Hence,

an increase in the mass gain of the bare superalloy occurs during hot corrosion.

Also, a simultaneous dissolution of Cr2O3 could occur in the molten salt due to the

reaction [43–45] as given below;

Cr2O3 þ 4NaVO3 þ 3=2ð ÞO2 ¼ 2Na2CrO4 þ 2V2O5 ð3ÞIn case of Co–Al coatings deposited on Superni-718 substrate, diffusion of Al and

Co from the coatings, and Cr, Fe and Ni from the substrate, respectively, may occur

and subsequently get oxidized. Once these oxides of Al, Cr, Fe and spinel phases of

CoCr2O4, CoAl2O4, and NiCr2O4 are formed, it might obstruct the diffusion of

corrosive species across the coatings and stops further corrosion of coated samples.

Fig. 9 continued

326 Oxid Met (2011) 76:307–330

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The coating is compact and adherent up to 100 cycles. The parabolic rate constant for

the bare superalloy is found to be greater than that of Co–Al coated superalloy. It can

be inferred that the nanostructured Co–Al coating provides necessary protection to

the superalloy. Particularly, Co–Al coating at 700 �C substrate temperature exhibits

a high hot corrosion resistance and provided the best protection. During the

subsequent cycles, cracks are developed in corrode bare scale as shown in Fig. 5b.

The crack has developed due to the stress build up at the scale–base metal interface,

during cooling as a result of the thermal expansion mismatch. Through these cracks,

the corrosion environment can quickly reach to the base metal and cut its way under

the scale to result in adhesion loss and spalling of scale. Similar type of cracks in base

metal (ASTM-SA213-T22 Steel) was also reported by Wang [43].

The bare substrate upon exposure to molten salt environment shows small pores,

indicating the possible escape of vaporized phases. These pores can help corroding

species to easily penetrate into the substrate and cause damage as shown in Fig. 5a,

b. The rapid increase in the mass gain was also reported for the superalloy coatings

subjected to hot corrosion studies as reported in the literature [46–49]. Further, the

formation of spinel phase could be explained due to the dissolution of NiO and

Cr2O3 in the molten salt of NaVO3, and thus facilitating the formation of mixed

oxide through the reaction given below.

NiOþ Cr2O3 ¼ NiCr2O4 spinel phaseð Þ ð4ÞDue to this spinel phase observed on the surface of corroded bare substrate, the

weight variation in bare substrate has slightly decreased after 60th cycles (Fig. 4a).

The similar observations were reported for Fe–Cr alloys oxidized in the molten salt

environment at 1,000 �C by Sadique et al. [36].

Figure 6a, b shows the surface micrographs of corroded Co–Al coatings on

superni-718 at different substrate temperature, which indicates the defect free scale

on the surface. The FE-SEM/EDS analysis of the scale on the Co–Al coated

superalloy specimens shows the presence of Cr2O3, Al2O3, Fe2O3, NiCr2O4 and

CoO. Further, CoCr2O4, CoAl2O4 (spinel phase) are formed due to the reaction

between CoO and Cr2O3, Al2O3. The presence of these oxides at the surface of

coatings decreases oxygen availability in the underlying superalloy and favor the

formation of thermodynamically stable oxide, Al2O3. The EDS results are further

supplemented by XRD analysis.

Figure 8c, d shows the Al content in the corroded Co–Al coating at 500 �C

dropped from 45.37 to 5.59 at% on surface (located by point 1 in Fig. 8c),where as

in corroded Co–Al coating at 700 �C, it dropped to 29.53 at% on surface (located by

point 1 in Fig. 8d). Similarly, in the corroded Co–Al coating at 500 �C, it dropped to

15.04 at% at coating–substrate interface (located by point 4 in Fig. 8c), where as in

corroded Co–Al coating at 700 �C, it decreased to 18.02 at% at coating–substrate

interface (located by point 4 in Fig. 8d) after 100 thermal cycles due to the inward

and outward diffusion of Al. Thus, loss of Al in corroded Co–Al coating at 700 �C

is less as compared to corroded Co–Al coating at 500 �C. Further, no spallation of

scale has occurred in both the corroded coatings. The improvement in spallation

resistance of oxide scale is believed to be related to the fine grain structure of the as

deposited coating [50].

Oxid Met (2011) 76:307–330 327

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Co–Al coating deposited at 700 �C substrate temperature on Superni-718

exhibits higher hot corrosion resistance properties as compared to that of the

coatings deposited at 500 �C substrate temperature due to the following reasons:

(i) It is evident from the FE-SEM surface as well as cross-sectional micrographs

(Fig. 3b, d) of the Co–Al coatings deposited at 700 �C that it exhibits dense

coating, as per the zone-3 structure model. The surface and cross-sectional

morphology of the coating showed rod type and columnar structure,

respectively. It may be mentioned that the diffusivity of atoms such as Al

and Co in the coatings through columnar structure is fast, which results in the

formation of dense and defect free scale as confirmed by surface as well as

cross-sectional FE-SEM micrograph shown in Figs. 6b and 8d. Therefore, the

coatings deposited at substrate temperature of 700 �C show Al-rich and Co-

rich oxide scale shown in Fig. 6b, which inhibits the movement of anions as

well as cations.

(ii) XRD analysis (Fig. 1b) revealed that the corroded Co–Al coating deposited at

700 �C substrate temperature has three types of spinel phase on the surface of

corroded coating i.e. CoCr2O4, CoAl2O and NiCr2O4, which might have

blocked the diffusivity of corrosive reagent.

In brief, the nanostructured Co–Al coating is designed in such a way that outer

layer is rich with high aluminum and high cobalt so that it forms b-CoAl

intermetallic phases (pool of Al) and are effectively utilized to combat hot

corrosion. The resistance to hot corrosion fluxing of the Co–Al coating is greatly

enhanced at the cost of sacrificing its beneficial element (especially Al), while the

bare substrate Superni-718 exhibits poor hot corrosion resistance in molten salt of

(Na2SO4 ? 60% V2O5) environment at 900 �C, due to the shortage of aluminum

reservoir. In other words, a coating with an insufficient amount of aluminum

reservoir cannot survive for a longer duration in chemically aggressive environ-

ments at high temperature [51–54]. The low corrosion rate of nanostructured Co–Al

coatings observed in the present work is in accordance with the results of

nanocrystalline NiCrAlY coatings reported in the literature [21].

Conclusion

Microstructure and cyclic hot corrosion of nanostructured Co–Al coatings on the

superalloy substrate exposed to molten salt of (40% Na2SO4–60% V2O5)

environment at 900 �C have been investigated in the present work. The hot

corrosion kinetics of Co–Al coatings has been compared with that of the bare

Superni-718 and found that the parabolic rate constant is very less for the former,

which is due to the formation of continuous, nonporous, adherent and protective

oxide scale over the surface of the coating. The continuous mass gain of Superni-

718 is due to the spallation of initial oxide scales, renewal of fresh surface of the

materials, and the pores resulting from the evaporation of volatile impurities in the

alloy at high temperature. The XRD analysis of the corroded products of the Co–Al

coatings indicated the formation of protective oxide scale such as Cr2O3, Al2O3,

328 Oxid Met (2011) 76:307–330

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Fe2O3, CoCr2O4, CoAl2O4, CoO and NiCr2O4. The morphological features of the

corroded coatings and bare samples were characterized by FE-SEM and it was

evident that the scales formed on the surface of coatings are adherent, continuous

and dense as compared to bare samples.

Co–Al coatings, deposited at 700 �C, showed a lower corrosion rate as compared

to that of the coatings deposited at 500 �C and bare substrate. It is due to the denser

morphology, nanosized grains (20–35 nm) of the as deposited coatings, and

pyramidal type grain morphology of scales/oxides with higher amount of alumina

(42.27 wt%). The nanostructured coatings have improved their oxidation resistance

as well as the scale-metal adherence as observed in the present work.

Acknowledgment One of the authors, Dr. R. Jayaganthan would like to thank CSIR, New Delhi, India

for their financial support to this work.

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