TECHNICAL PAPER TP 2739

Hot Corrosion Behaviour of HVOF Sprayed Stellite-6 Coatingson Gas Turbine Alloys

N. Jegadeeswaran M. R. Ramesh

S. Prakrathi K. Udaya Bhat

Received: 10 November 2012 / Accepted: 14 April 2013 / Published online: 23 August 2013

Indian Institute of Metals 2013

Abstract The coal burned natural gas in contact with gas

turbine can contain impurities of sodium, sulfur, vanadium,

silicon and possibly lead and phosphorous, induce acceler-

ated hot corrosion during long term operation. Coatings are

frequently applied on gas turbine components in order to

restrict surface degradation and to obtain accurate lifetime

expectancies. High velocity oxy-fuel thermal spraying has

been used to deposit Stellite-6 alloy coatings on turbine

alloys. Hot corrosion behavior of the coatings were inves-

tigated for 50 cycles of 1 h heating at 800 C followed by20 min cooling in presence of Na2SO4 ? 50 % V2O5measuring weight gain (or loss). X-ray diffraction and SEM/

EDAX techniques were used to characterize the oxide scale

formed. The superior performance of Stellite-6 coating can

be attributed to continuous and protective thin oxide scale of

CoO, Cr2O3 and SiO2 formed on the surface. The coating

region beneath this thin oxide scale was partially oxidized.

Uncoated SuperCo-605 and MDN-121 showed less weight

gain than Stellite-6 coated samples, but they showed spalling

or sputtering during cyclic oxidation. Stellite-6 coating was

dense and pore free even after 50 cycles, indicating that it

can resist the hot corrosion cycle.

Keywords Ti-31 SuperCo-605 MDN-121 Hot corrosion HVOF coatings Stellite-6

1 Introduction

Increasing global demand for energy is an immense chal-

lenge for power producers around the world. The global

consumption of energy is expected to double by 2020 [1].

Today, industrial gas turbines are used extensively to

produce power for satisfying the demands of electrical,

chemical, pharmaceutical, fertilizer sectors, etc. [2].

Though various alternatives exist for the fuel material, coal

is still a predominant fuel material for the power suppliers.

In developing countries, the fossil coal is generally of low

grade type and it contains sodium, vanadium and sulphur as

impurities. These impurities form compounds such as

Na2SO4 (m.p. 884 C) and V2O5 (m.p. 670 C), they reactwith the turbine components to induce hot corrosion [37].

Cobalt base alloys, Iron base alloys and Titanium base

alloys are some of the materials used in gas turbine

applications [8]. To improve the properties like oxidation

resistance, hot corrosion resistance, etc. these alloys will be

surface coated. The appropriate coating can add value to

products up to 10 times the cost of the coatings [9, 10].

High velocity oxy fuel (HVOF) spraying is an advanced

thermal spray process to produce dense and strong coatings

[11]. When appropriate coating material is selected, HVOF

coatings also have properties like high abrasion resistance,

good wear resistance and high temperature corrosion

resistance [12]. The possibilities of applying the HVOF

process to a much wider range of materials are now being

addressed [13, 14]. Literature has limited investigations on

HVOF based Stellite-6 coatings on different materials used

in turbine applications. The present article deals with the

N. Jegadeeswaran (&)Mechanical Engineering Department, Reva Institute of

Technology and Management, Bangalore, India

e-mail: jegupavi@yahoo.co.in

M. R. Ramesh

Mechanical Engineering Department, National Institute of

Technology Karnataka, Surathkal, India

S. Prakrathi K. U. BhatMetallurgical and Materials Engineering Department, National

Institute of Technology Karnataka, Surathkal, India

123

Trans Indian Inst Met (2014) 67(1):8793

DOI 10.1007/s12666-013-0317-z

high temperature hot corrosion behaviour of HVOF

sprayed satellite-6 coating in molten salt (Na2SO4

50 % V2O5) environment at 800 C. The experiments havebeen conducted under cyclic conditions as it constitutes a

more realistic approach towards solving the problem of

metal corrosion in actual applications, where conditions are

more or less cyclic rather than isothermal.

2 Experimental

2.1 Substrate Materials and Coating Formulation

The alloys Ti-31, SuperCo-605 and MDN-121 were used as

substrate materials and the respective equivalent ASTM

standards are ASTM B338 Grade 5, ASTM F90-09 and

ASTM A565 Grade 616. The specimen of 25 mm 9

25 mm 9 5 mm dimension were ground, grit blasted with

alumina powders (Grit 45) and were used for HVOF coating.

Commercially available Stellite-6 powder having particle size

distribution in the range of -45 ? 15 lm (spherical shape)was used as feedstock powder. The details of the Stellite-6

powder and substrate materials are given in Table 1.

The HVOF coatings were sprayed using a Metco DJ2600

gun (Spraymet India Ltd, Bangalore India). The spray

parameters were: Oxygen flow rate-270 l/min; fuel (LPG)

flow rate-70 l/min; air flow rate-700 l/min; spray distance-

20 cm; powder feed rate-50 g/min; fuel pressure-7 kg/cm2;

air pressure-5.5 kg/cm2; oxygen pressure-10 kg/cm2; nitro-

gen gas (powder carrying gas) pressure-5 kg/cm2.

2.2 Molten Salt Hot Corrosion Test

Hot corrosion studies were conducted on uncoated and

Stellite-6 coated samples at 800 C in a laboratory siliconcarbide tube furnace with a temperature accuracy of

5 C. Physical dimension of the specimen before hotcorrosion were recorded carefully with a vernier caliper, to

evaluate their surface areas. The salt mixture of Na2SO4

50 % V2O5 applied uniformly (35 mg/cm2) using camel

hairbrush on the preheated samples (250 C). The speci-men was kept in a dried alumina boat and the weight of

boat and specimen was measured. Hot corrosion studies,

under cyclic conditions, were conducted in molten salt

environment. The tests were conducted for 50 cycles of

which each cycle consists of 1 h heating at 800 C fol-lowed by 20 min cooling in air.

At the end of each cycle, the weight change values were

measured. Visual observations were made after the end of

each cycle with respect to colour, luster or physical aspect

of the oxide scales being formed. The corrosion products of

Table 1 Composition of the feedstock powder, substrates and theirproperties

Material Chemical composition (wt%) Average

microhardness

(Hv)

Stellite-6 Co (bal)-28.8Cr2.6Ni4.5

W2.5Fe1.2C1.3Si

550

Ti-31 Ti(bal)-0.013C6Al4 V 375

SuperCo-605 Co(bal)-3Fe10Ni20Cr

1.5Mn15 W0.3Si

305

MDN-121 Fe(bal)-0.8Ni12Cr1Mo

0.6Mn0.2C

407

Fig. 1 Uncoated and Stellite-6 coated alloys subjected to hot corrosion. a Graph of weight gain/area versus number of cycles. b Graph of(weight gain/area)2 versus number of cycles

88 Trans Indian Inst Met (2014) 67(1):8793

123

the uncoated and HVOF coated materials were analyzed

using XRD, SEM and EDX to reveal their microstructural

and composition features.

3 Results

3.1 Thermogravimetric Analysis and Visual

Observations

It is to be recorded that there was indication of oxide for-

mation from the first cycle itself, both for uncoated and

Stellite-6 coated samples. In the case of uncoated Ti-31, the

surface became brownish in 23rd cycle and severe spalling

was observed from 31st cycle onwards. The surface of

uncoated SuperCo-605 and MDN-121 became grey in col-

our. Uncoated SuperCo-605 showed sputtering throughout

the experiment. The surface of the Stellite-6 coating was

dark grey in colour and it turned into black in the first cycle

and it remained as black throughout. Cracks were visible in

the Stellite-6 coated Ti-31 whereas coating on other two

substrates was free of cracks.

The plots of cumulative weight gain (mg/cm2) as a func-

tion of time expressed in number of cycles are shown in

Fig. 1a. The weight gain for Ti-31 and MDN-121 at the end

of 50 cycles was found to be 75.8 and 2.7 mg/cm2 respec-

tively. This clearly shows that during hot corrosion studies,

weight gain for Ti-31 is much higher compared to that for

MDN-121. Further, weight gain study on SuperCo-605 was

difficult due to spalling and sputtering and this resulted in

zero or negative weight gain during most of the cycles.

Further the weight gain square (mg2/cm4) data is drawn as a

function of time and shown in Fig. 1b to investigate the

possibility of parabolic relationship. Though, the plot does

not fit perfectly, the difference is well within the limit to

assume parabolic growth behavior. The parabolic rate con-

stants (kp) for the Ti-31, SuperCo-605 and MDN-121 are

1.412 9 10-8, 0.006 9 10-8 and 0.052 9 10-8 g2/cm4 s

respectively. The values of overall weight gain after

50 cycles of hot corrosion study for Stellite-6 coated Ti-31,

SuperCo-605 and MDN-121 are found to be 8.11, 2.48 and

6.37 mg/cm2 respectively. The kp values for the coated Ti-

31, SuperCo-605 and MDN-121 are 0.149 9 10-8,

0.052 9 10-8 and 0.119 9 10-8 g2/cm4 s, respectively.

3.2 X-ray Diffraction Analysis

The X-ray diffraction patterns of the top scale formed on the

uncoated and Stellite-6 coated substrates due to cyclic expo-

sure to salt environment at 800 C are shown in Fig. 2 and

Fig. 3 respectively. The scale on the uncoated Ti-31 contains

TiO2, Al2O3, TiVO4 and V3Ti6O17, scale on the uncoated

SuperCo-605 contains CoO, Cr2O3, CrVO4, Ni3V2O8 and

Na4FeO3 and the scale on the MDN-121 contains FeO, Cr2O3,

CrVO4, and FeS2 as major phases. The scale on the Stellite-6

coated substrates consists of CoO, Cr2O3 and CoCr2O4 as

major phases. The scale also shows the presence of NiCr2O4,

Co3V2O8 and FeVO4 as minor phases.

3.3 SEM/EDX Analysis

The SEM micrograph of a hot corroded Stellite-6 coated

MDN 121 is shown in Fig. 4. The microstructure (Fig 4a)

shows two distinct features: A type feature is rich in

coating forming oxides like CoO, Cr2O3 and CoCr2O4 and

B type feature is rich in coating damaging oxides like

Fig. 2 X-ray diffraction patterns of uncoated Ti-31, SuperCo-605and MDN-121 subjected to hot corrosion

Fig. 3 X-ray diffraction patterns of Stellite 6-coated Ti-31, Super-Co-605 and MDN-121 subjected to hot corrosion

Trans Indian Inst Met (2014) 67(1):8793 89

123

Co3V2O8 and FeVO4. It can be observed in Fig. 4b, that the

oxide scale has a glassy matrix with some coarse nodules

dispersed uniformly. The EDAX compositional analysis of

the glassy scale shows the dominant presence of Si and O

peaks along with the minor peaks of Co and Cr. This can be

attributed to the formation of a SiO2 glass scale.

Figure 5a shows a cross sectional micrograph of the

Stellite-6 coating on Ti-31 after hot corrosion. Delamina-

tion type cracks are restricted to top surface only. EDX

analysis was done at various locations in the coating and

the result is shown in Fig. 5b. Figure 5b indicates that, the

oxide layer formed on the surface is rich in CoO and

Cr2O3, remaining coating has almost uniform composition.

Small gradient exists for O concentration along the depth,

which indicates the partial oxidation of the coating beneath

the top layer.

3.4 Element X-ray Mapping

BSEI and elemental X-ray mappings for the cross-section

of Stellite-6 coated MDN 121 corroded in an environment

of Na2SO450 % V2O5 is shown in Fig. 6. The uppermost

scale consists mainly of oxides of Co and Cr. The com-

bined mapping for Si and O reveals the presence of patches

of silicon oxide in this oxide layer. The underlying region

just below oxide layer consists of partially oxidized Co rich

splats. The observation into the coating-substrate interface

reveals the diffusion of iron into the coating along with the

diffusion of coating elements of Co and Cr into the sub-

strate which helps in better bonding.

4 Discussion

By comparing the weight gain/area versus number of

cycles graphs of both the uncoated and the Stellite-6 coated

Ti-31 substrate, it is observed that the uncoated samples

showed a higher weight gain in comparison to the coated

samples (Fig. 1). In uncoated Ti-31 samples Al is a coating

former and the literature says that minimum amount of Al

for continuous coating is 50 at.% [16, 17]. Since Ti-31 has

much less Al, it cannot form a dense continuous protective

A

B

(a)

(b)

O-38%, Si-17.19%,Cr-13.73% Co-18.94%, V-2.91%, Na-2.9%

00 1.00 2.00 3.00 4.00 5.00 6.00 7.00 8.00

keV

CKa

OK

a

NaK

a

SiK

aPK

aSK

aSK

b

TiLl

TiLa

TiK

aTi

KbV

LlV

La

VK

a

VK

b

CrLl

CrLa

CrK

a

CrK

b

MnL

lM

nLa

MnK

a

MnK

b

FeLl

FeLa

FeK

esc

FeK

a

FeK

b

CoLl

CoLa

CoK

a

CoK

bN

iKa

NiK

b

MoL

l MoL

a

WM

z

WM

aW

Mr

WLl

WLa

Fig. 4 Surface morphology of a Stellite-6 coated MDN 121 subjected to hot corrosion

90 Trans Indian Inst Met (2014) 67(1):8793

123

alumina scale upon exposure to elevated temperature. Once

the cracks form in the scale, penetration of oxygen

becomes much faster, leading to faster degradation [9]. The

protection imparted by the Stellite-6 coating may be due to

the formation of oxides of Cr and Co along with the spinels

of Co, Cr and Ni. It is proposed that the spinel might reduce

the diffusion through the cobalt oxide which in turn sup-

presses the formation of the oxide. The EPMA analysis

(Fig. 6) and the compositional EDAX analysis on surface

(Fig. 4b) corroborates that there is the formation of patches

of SiO2. Further, there is no peak corresponding to the Si-

related oxide phase in the XRD analysis of the surface

oxide layer (Fig. 3), probably because it is amorphous

silica. Yu et al. [19], Douglass et al. [20] and Wu and Niu

[21] have reported about the possibility of formation of

amorphous SiO2 oxide scale using the energy dispersive

X-ray micro analysis results during the oxidation of Ni-Si

coatings and alloys. Lowell [22] has identified the silicon

as cristobalite at 1,200 C and an amorphous discontinuouslayer at 1,100 C while studying the ternary system Ni20Cr3Si under cyclic and isothermal conditions. These

protective oxides of Co, Cr and Si formed on the surface

restrict the corrosive species to enter into the coating there

by result in lower weight gain as compared to the uncoated

samples. Such observations are reported earlier in the case

of Stellite-6 coating on Fe and Ni based superalloys when

subjected to cyclic oxidation in Na2SO460 % V2O5 at

900 C [15]. The finite value of weight gain (per unit areaand unit time) indicates that the higher corrosion rate in the

case of coated Ti-31 is due to the propagation of the crack,

formed as a result of thermal mismatch at the coating-

substrate interface during early cycles. Further, the Stellite-

6 coated.

MDN-121 the have conceived more overall weight gains

as compared to uncoated steel. The major portion of the

overall weight gain has been conceived in the early cycles

of study and the weight has become nearly steady as the

exposure time is increased. This indicates that the corrosion

behaviour is governed by parabolic rate law. The possible

reasons for this initial high weight gains might partially be

attributed to the rapid formation of oxides at the coating

splat boundaries and within open pores due to the pene-

tration of the oxidizing species along the splat boundaries/

open pores in the early cycles of the study. Once the oxides

are formed at places of porosity and splat boundaries, the

coating becomes dense and the diffusion of oxidizing

species to the internal portions of the coatings gets slowed

down and the growth of the oxides becomes limited mainly

to the surface of the specimens. This, in turn, will make the

weight gain and hence the corrosion rate steady with the

further progress of exposure time. The uncoated MDN has

12 % Cr as oxide forming element. Considering it has

0.2 % C, the amount of Cr is far less for continuous

chromium oxide coating.

Chromium oxide also forms gaseous species during

interaction as following.

Na2SO4 V2O5 2NaVO3 SO2 1=2O2Melting point of sodium vanadate (NaVO3) is 610 C andit is in liquid state at 800 C. It acts as a catalyst andpromotes oxygen diffusion [8, 18]. Sodium vanadate

formed undergoes following reaction.

Cr2O3 4NaVO3 3=2O2 Na2CrO4 2V2O5Na2CrO4 is in gaseous state and it escapes out causing

some degree of sputtering. Lower weight gain in the bare

SuperCo-605 and MDN-121 is partially due to simulta-

neous growth and dissolution of oxide scale.

Absence of cracks/porosities, uniformity in the composi-

tion in the bulk of the coating indicates that the coating has

Fig. 5 Back scattered image across the cross section a and EDX analysis (wt%) at various points. b The Stellite-6 coated Ti-31 subjected to hotcorrosion

Trans Indian Inst Met (2014) 67(1):8793 91

123

retained its dense structure and is able to provide protection

against hot corrosion by forming a Cr2O3 and CoO rich scale

on the uppermost part of the coating. Still a small gradient

exists for O concentration along the depth, which indicates

that a small level of inward diffusion of oxygen which is

responsible for increase in weight as observed in thermo-

gravimetric studies. Also partially oxidized coating has suc-

cessfully maintained its integrity on all the substrate alloys.

Fig. 6 X-ray mapping alongthe cross-section of the Stellite 6

coated MDN 121 material

subjected to hot corrosion for 50

cycles in

Na2SO4 ? 50 % V2O5environment at 8008 C

92 Trans Indian Inst Met (2014) 67(1):8793

123

5 Conclusions

Hot corrosion behaviour of uncoated and Stellite-6 coated

Ti-31, SuperCo-605 and MDN-121 was studied at 800 Ctemperature and under cyclic conditions in presence of

Na2SO4 ? 50 % V2O5. The Stellite-6 coating gave a

noticeable improvement in corrosion resistance and the

corrosion behaviour was parabolic in nature. The protec-

tion imparted by the Stellite-6 coating is due to the for-

mation of oxides of Cr, Co and Si. The Stellite-6 coating

was dense and maintained its integrity, even after 50 cycles

of heating and cooling in presence of salt mixture.

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Trans Indian Inst Met (2014) 67(1):8793 93

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Hot Corrosion Behaviour of HVOF Sprayed Stellite-6 Coatings on Gas Turbine AlloysAbstractIntroductionExperimentalSubstrate Materials and Coating FormulationMolten Salt Hot Corrosion Test

ResultsThermogravimetric Analysis and Visual ObservationsX-ray Diffraction AnalysisSEM/EDX AnalysisElement X-ray Mapping

DiscussionConclusionsReferences