Combating Corrosion Degradation of Turbine Materials Using

8/15/2019 Combating Corrosion Degradation of Turbine Materials Using

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Hindawi Publishing CorporationInternational Journal o CorrosionVolume , Article ID , pageshttp://dx.doi.org/ . / /

Research ArticleCombating Corrosion Degradation of Turbine Materials Using HVOF Sprayed 25% (Cr 3C2-25(Ni20Cr)) + NiCrAlY Coating

N. Jegadeeswaran, 1 M. R. Ramesh, 2 and K. Udaya Bhat 1

Metallurgical & Materials Engineering Department, National Institute of echnology Karnataka, Surathkal , India Mechanical Engineering Department, National Institute of echnology Karnataka, Surathkal , India

Correspondence should be addressed to K. Udaya Bhat; [email protected]

Received July ; Revised September ; Accepted September

Academic Editor: Ramazan Solmaz

Copyright © N. Jegadeeswaran et al. Tis is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

High velocity oxy uel process (HVOF) is an advanced coating process or thermal spraying o coatings on to components usedin turbines. HVOF process is a thermal spray coating method and is widely used to apply wear, erosion, and corrosion protectivecoatingsto thecomponentsused in industrialturbines. %(Cr 3 C2 - (Ni Cr))+ NiCrAlY based coatingshave been sprayedon tothreeturbine materials,namely, i- , Superco- , andMDN- . Coated and uncoatedsubstrates weresubjected to hotcorrosionstudy under cyclic conditions. Each cycle consisted o hour heating at ∘C ollowed by minutes air cooling. Gravimetricmeasurements were done afer each cycle and a plot o weight gain as a unction o number o cycles is drawn. Parabolic rateconstants were estimated or the understanding o corrosion behaviour. It was observed that coated i- and MDN- were moreresistant compared to the uncoated ones. Uncoated superco- was undergoing sputtering during corrosion study and hencecomparison between coated and uncoated superco- was difficult. Te cross-sectional analysis o the corroded, coated samplesindicated the presenceo a thin layer o chromium oxide scale on the top o the coating and it imparted better corrosion resistance.Parabolic rate constants also indicated that coating is more bene cial to i- than to MDN- .

1. Introduction

Superalloys are extensively used in gas turbine engines,particularly in combustion zone which is subjected to hightemperature and long periodso working time [ – ]. Hot cor-rosionhas becomea majordegradationmechanism(solidsaltdeposit on the component ype I hot corrosion) in turbineswhich are operated with low grade ossil uels [– ]. o pro-tect the components rom degradation and to prolong theirli e, coatings are commonly used on the blades and vanes o gas turbines [ ].

Use o mechanical blend carbide alloypowderasa coatingmaterial is highly promising [ ]. A cermet system like Cr 3 C2 -NiCr is a strong contender or coating applications requiringhigher wear, abrasion, and wear resistance. Cr 3 C2 impartswear, abrasion, and wear resistance and NiCr acts as a metalbinder providing necessary coating toughness. It also hasgood thermal conductivity and hence helps in reducingcooling requirements. Alloy, NiCrAlY, is used in coatingapplications or imparting oxidation resistance and it reduces

thermal mismatch between coating and the substrate [ , ].Te alloy, NiCrAlY, is used as a bond coat in thermal barriersystems ( BCs) to minimize the mismatch in the coefficiento thermal expansion [ , ]. (Cr3 C2 -NiCr) + NiCrAlY coat-ings offers a combination o better wear, abrasion, corrosion,and oxidation resistance, a higher operating temperature up

to∘

C, and minimum stress at the inter ace due to thermalexpansion mismatch during thermal cycling [ , ]. Toughsome researchshowssatis actory results byusingCr 3 C2 -NiCrmixture as a coating system, much debate still exists overthe best hard phase/composition/matrix ratio or obtainingoptimum properties essential or a high temperature coatingsystem[ – ]. Tisinvestigation ison theuse o carbide alloy powder wt% (Cr3 C2 - (Ni Cr)) mechanically blendedwith NiCrAlY alloy powder ( % by weight) as a coatingmaterial or high temperature applications.

Te coating properties are in uenced not only by theproperties o the powders used but also signi cantly by thespray process and parameters [ , ]. In this investigation,


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International Journal o Corrosion

: Chemical compositions o the substrates used or HVOFcoatings.

Sl.no.

Alloy material Chemical composition, wt%

i- i-bal, Al- , V-

Superco- Co-bal, Fe- , Ni- , Cr- , Mn- . , W- ,

Si- . MDN- Fe-bal, Ni- . , Cr- , Mo- , Mn- . , C- .

high velocity oxy uel (HVOF) process is used or coatingspraying. Compared to other spray coatings, such as amespraying and plasma spraying, HVOF spraying produces acoating having a better bond strength with the substrate [ –

].Te present investigation is an attempt to evaluate

the molten salt corrosion o HVOF sprayed % (Cr3 C2 -(Ni Cr)) + NiCrAlY alloy coated i- , Superco- , and

MDN- at∘

C under cyclic conditions in the presence o the Na 2 SO4 - %V2 O5 salt mixture.

2. Experimental

. . Substrate Material and Coating Formulation. Te alloysi- , Superco- , and MDN- were used as the sub-

strate materials, in the present study. Te equivalent AS Mstandards o the substrate materials are AS M B Grade

, AS M F - , and AS M A Gr , respectively.Te materials were purchased rom MIDHANI, Hyderabad,India, in rolled orm. Te nominal chemical compositions o the substrate materials are given in able . Te specimens

with dimensions o the approximately mm × mm ×mm were cut, ground, and subsequently grit blasted withalumina powders (Grit ) be ore spraying o the coatings by HVOF process. Tis was done to enhance adhesion o thecoating to the substrate. Te specimen was prepared man-ually without any structural changes. Figure shows micro-graphs o the three substrates used or the HVOF coatings.

Commercially available cermet powder (chromiumbased) was used as the eedstock alloy or HVOF spraying.Te details o the eedstock alloy used and HVOF coating aregiven in able . Te powder was mechanical blend type andmanu actured by M/s Sultzer Metco Ltd. Te coatings weresprayed at Anod Plasma Industries, Kanpur, India. A MetcoDJ (India) gun was used or powder spraying. Te spray parameters used or HVOF spraying were oxygen ow rate:

LPM; LPG ow rate: LPM; air ow rate: LPM;spray distance: about cm; powder eed rate: g/min; uelpressure: kg/cm2 ; air pressure: . kg/cm2 ; oxygen pressure:

kg/cm2 ; nitrogen gas (powder carrying gas) pressure:kg/cm2 . During HVOF spraying, the specimen was cooled

with compressed air.

. . Molten Salt Hot Corrosion est. Hot corrosion studieswere conducted at ∘C in a laboratory silicon carbide tube

urnace with an accuracy o ± ∘C. Physical dimensions o the specimen were recorded care ully with a vernier caliper,

50 m

(a)

50 m

(b)

50 m

(c)

F : Optical micrograph o substrate materials. (a) i- , (b)Superco- , and (c) MDN- .

to evaluate their sur ace area. Subsequently, specimens werewashed with acetone and dried in hot air by means o heatingboth boat and specimen in anovenat ∘C or about min-utes, to remove the moisture. A coating o uni orm thicknesswas applied on to the dry sur ace, using a camel hairbrush,both on bare substrates andHVOF coated sur aces. Te coat-ingmixture wasNa 2 SO4 - %V2 O5 (ratiobyweight) andcov-erage was – mg/cm2 . Duringexperimentation the preparedspecimens werekept in analumina boat and the weight o theboat and specimen was measured. Te hot corrosion studies,under cyclic conditions, were conducted in molten salt envi-ronment o above composition. Te tests were conducted or

cycles o which each cycle consists o hour heating at∘C in silicon carbide tube urnace ollowed by min

cooling in air.Te weight change values were measured at the end o

each cycle with the aim to understand the kinetics o corro-sion. Visual observations were made afer the end o each


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: Composition o the eedstock alloys, coating thickness, and porosity.

Chemical composition (wt%) Particle size ( m)ShapeAverage coatingthickness ( m) Porosity Microhardness (Hv)

% (CrC - (Ni Cr)) +% (Bal Ni- Cr- Al- . Y)

−45 + 15Spherical < . %

(a) (b) (c)

F : Optical macrograph o % (Cr3 C2 - (Ni Cr)) + NiCrAlY coated materials. (a) i- , (b) Superco- , and (c) MDN- .Dimension o the samples: mm × mm.

cycle with respect to colour, luster, or any other physicalaspects o the oxide scales being ormed. Te reproducibility in the experiment was established by repeating the experi-ments. Te corrosion products o the uncoated (bare) andHVOF coated materials are analyzed by using X-ray diffrac-tometer (XRD), scanning electron microscope (SEM), andenergy dispersive X-ray spectroscope (EDX) to reveal theirmicrostructural and compositional eatures and or elucidat-ing the corrosion mechanisms.

3. Results and Discussion

. . Analysis of As-Coated Specimen. Figure shows opticalmacrographs o the as-coated specimens, namely, coated i-

, coated Superco- , and coated MDN- . Te coatingsappeared as dark grey in colour. Te coatings were smoothand roughness measurementsusing a stylusbased pro leme-ter showed a Ra value in the range o – m. Te typicalmorphological eatureso as-coatedsur aceshown in Figureindicate that the coating is partially molten and resolidi ed.Similar morphological eatures are reported in the case o HVOFsprayed coatingsbySidhuet al.[ – ],Lot [ ],andYuan et al. [ ]. Te micrograph in Figure shows the pres-ence o chromium carbide particles distributed in the matrixwhich is rich with melted Ni. Tere are no pores and cavitieswhich are highly penetrating towards the substrate. Occa-sionalcrackswereobserved but theyare limited to topsur aceonly. Te EDX analysis o the coating indicates that the coat-ingisrichinCrandNiandthatsomeamounto oxidationhastaken place during HVOF spraying. By cross-sectioning thecoatings ollowed by metallographic polishing and observa-tion using scanning electron microscope, the coating thick-ness was measured and the thickness was in the range o

58.1% Cr

28.7% O

6.4% C

6.8% Ni

0.1% Al

0.02% Y

57.3% Cr

26.7% O

14.9% C

1.0% Ni

0.1% Al100 m

F : ypical morphology o the cermet coated sur ace.

– m. A typical cross-sectional micrograph is shownin Figure . Te coating is reasonably dense and the eaturesare typical o HVOF coatings. We see partially molten splatsimpinging the substrate sur ace and suddenly solidi ying.Splat ormation mainlydepends on thevelocity o theimpact-ing particles, their temperature, extent o oxidation, andchemical complexity o the particles [ , ]. Te substrate-coating inter ace is good. Only small cavities are observed.Good contact between coating and substrate also indicatesthat the cooling o the splat layer is ast [ ]. Tere are nodirect channels passing through the coating and allowing thesubstrate to be in contact with the atmosphere.

Figure also shows the laminar pattern in which thecoat-ing was deposited during HVOF spraying. Te coatings weredeposited on a stationary substrate by moving HVOF gun


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54.6% Ni

26.0% Cr

16.7% Al

44.0% Ni

19.9% Cr

11.9% Al

F : Cross-sectional image showing the nature o HVOFsprayed coatings.

(rastering) and the required thickness was achieved by vary-ing the number o passes. In the cross-sectional micrographshown in Figure dark areas corresponds to porosity and voids. Voids are visible, both in the coating and at the coating-substrate inter ace. Similar microstructural eatures in HVOFsprayedcoatings are reported bySidhuet al. [ , ].Te EDXanalysis at various locations across the thickness shows thatcoating is rich in elements like Ni, Cr, and Al. Clear peakscorresponding to chromium carbides are visible in the plot.

Figure shows results o XRD analysis on coatingssprayed over different substrates. Te re ections clearly indicate that the coating is rich in terms o oxides o Ni andCr. Comparingwith theXRDpro les o initially used powderwe conclude that during spraying the particles have under-gone oxidation along with partial melting. Tese oxides are

predominantly oxides o Ni and Cr. Neiser et al. [ ] havereported that the oxidation o the elements during thermalspraying is controlled by transport phenomena.

. . Hot Corrosion Studies on Coated and Uncoated Substrates

. . . Visual Observations and Termogravimetric Analysis.Te macrographs o samples o uncoated i- , Superco-

, and MDN- and the HVOF sprayed % (Cr3 C2 -(Ni Cr)) + NiCrAlY coatings on the i- , Superco- ,

and MDN- alloys which are subjected to hot corrosion inNa2 SO4 - %V2 O5 environment or cycles at

∘C areillustrated in Figure . It is noted that there was ormation o oxide scales on the sur ace o i- rom the rst cycle itsel ;on the rd cycle, the sur ace appeared brownish. Severespalling o oxide scaleswasobserved rom st cycle onwards.Te sur ace o Superco- started to become grey in colourwhereas the sur ace o the MDN- started to change intoshiny grey in colour. Te colouro the as-sprayed coating wasgrey which turned into brownish colour during the rst cycleo exposure to salt environment.

Te plots o cumulative weight gain (mg/cm 2 ) as a unc-tion o time expressed in number o cycles or uncoated sub-strates are shown in Figure (a) and or cermet coated sub-strates in Figure (b). Inthe caseo uncoated i- and MDN-

, the weight gains atthe end o cyclesare oundto be .

Cr CrCNi

Al

NiOCr

2O

3

Cr3C2-25 (Ni20Cr) + NiCrAlY powderTi 31

Superco 605

MDN 121

I n t e n s i t y ( a

. u . )

20 30 40 50 60 70 80 90 100Diffraction angle (2 )

♣

♣

⧫

⧫

⧫

⧫

⧫

::

:♠

♠ ♠ ♠ ♠

As-sprayed

F : XRD pro les o mechanically blend % (Cr3 C2 -(Ni Cr)) + NiCrAlY powder and HVOF sprayed coatings on

three substrates are shown.

and . mg/cm2 , respectively. Uncoated Superco- exhib-ited weight loss during the complete cycles o hot corrosionstudies as a result o intense spalling and sputtering o theoxide scale. Spalling and sputtering also made it difficult tomeasure the overall weightgain. iwari and Prakash [ ] andSingh et al. [ ] have reported that scale on the superco-

exposed to Na 2 SO4 - %V2 O5 is highly prone to cracking.Also, Anuwar et al. [ ] have reported a weight gain o aboutmg/cm2 afer cycles o hot corrosion at ∘C in pres-

ence o Na2 SO4 - %V2 O5 or uncoated i- .Te values o overall weight gain afer cycles o hot

corrosion or % (Cr3 C2 - (Ni Cr)) + NiCrAlY coated i-, Superco- , and MDN- are ound to be . , . , and

. mg/cm2 , respectively. Tough a small difference existsin weight gain or different coated substrates, the net weightgain or all coated materials is less compared to weight gain

or uncoated substrates. In other words, cermet coating canprotect the substrates rom the attack o molten Na2 SO4 -

%V2 O5 salt mixture. Further, to explore the possibility

o parabolic relationship between weight gain and time o exposure, square o weight gain (mg2 /cm 4 ) data is plotted asa unction o time [ – ]. Tey are shown in Figure (a) oruncoated substrates and in Figure (b) or coated substrates.Te plot shows a clear deviation rom the parabolic rate law

or the i- . With increase in the number o cycles, the “axis” value increases rapidly. Tis indicates that the oxidescale is not much protective in molten salt environment. Itis evident rom the plot shown in Figure (a) that the MDN-

alloys nearly ollow parabolic behavior. Te parabolic rateconstants, p , or the i- and MDN- are 107.6 ∗ 10

−8

and 0.152 ∗ 10−8 g2 cm−4 s−1 , respectively. Te parabolic rateconstant is verysmall orSuperco- , but it must be stressed


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10 mm 10 mm 10 mm

(a)

10 mm 10 mm 10 mm

(b)

F : Macrographs o uncoated substrates (top row, in sequence or i- , Superco- , and MDN- ) and % (Cr3 C2 - (Ni Cr)) +NiCrAlY coating (bottom row, in sequence or i- , Superco- , and MDN- ) subjected to hot corrosion in Na2 SO4 - %V2 O5environment at ∘C or cycles.

−1.0−0.50.0

0.51.01.52.02.5

W e i g h t g a i n / a r e a

( m g / c m

2 )

0 10 20 30 40 50Number of cycles

Uncoated Ti-31Uncoated superco-605Uncoated MDN-121

1020304050607080

(a)

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

W e i g h t g a i n / a r e a

( m g / c m

2 )

0 10 20 30 40 50Number of cycles

Cr3C2-NiCr + NiCrAlY

Coated Ti-31Coated superco-605Coated MDN-121

(b)

F : Plots o weight gain per unit area as a unction o number o hot corrosion cycles. (a) Uncoated samples, (b) %(Cr 3 C2 - (Ni Cr)) + NiCrAlY coated substrates.


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( W e i g h t g a i n / a r e a

) 2 ( m g 2 / c m

4 )

0 5 10 15 20 25 30 35 40 45 50 55Number of cycles

Equation y = a + b × x

ValueTi-31

MDN-121

Slope

Slope

107.68992

0.15251

−2−1

0123456

100020003000400050006000

Ti-31

MDN-121Superco-605

(a)

( W e i g h t g a i n / a r e a

) 2 ( m g 2 / c m

4 )

Number of cycles

MDN-121Superco-605

Cr3C2-NiCr + NiCrAlYCoated Ti-31

Coated Ti-31

Coated superco-605Coated MDN-121

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0 10 20 30 40 50

Coated (Kp)0.045080.038370.0335

(b)

F : Plot o (weight gain/area)2 versus number o cycles or coatedsamples subjected to hot corrosion or cycles in Na2 SO4 - %V2 O5environment at ∘C. (a) Uncoated samples, (b) % (Cr3 C2 - (Ni Cr)) + NiCrAlY coated substrates. Multiplication actor or p =10−8 g2 cm−4 s−1 ).

that weight measurement was very difficult due to the prob-lems o spalling and sputtering. Te parabolic rate constants,

p, or the cermet coated i- , Superco- , and MDN-are 0.045 ∗ 10−8 , 0.0335 ∗ 10−8 , and 0.038 ∗ 10−8 g2 cm−4 s−1 ,

respectively. Te reported values o p are upper limit valuesas they are calculated considering to cycles. Kamal et al.[ ] haveshown thatCr 3 C2 -NiCr coatingsonNi and Febasedsuperalloys produced by detonation gun spraying improveshot corrosion resistance against Na 2 SO4 - %V2 O5 at

∘Cand the estimated p values are o the same order. Te low value o p also indicates that thecoating is protective againstNa2 SO4 - %V2 O5 environment. Te coating becomes satu-rated afer about cycles or coated i- as well as coatedMDN- and afer cycles or coated superco- . Tisagain shows that the hot corrosion rate drops as number o cycle increases. Tis behavior is distinct compared to hotcorrosion o bare substrates.

. . . X-Ray Diffraction Analysis. Te X-ray diffraction pat-terns o the top scale, afer its exposure to salt environment at

∘C or cycles, are shown in Figure . In Figure (a), thephases present in the scales o uncoated substrates exposedto salt environment are given. Te scales are rich in oxideso Fe, Cr (in corroded MDN- ) oxides o elements like CrandCo (incorroded superco- ), andoxides o iandAl (incorroded i- ). Addedto that,many otheroxides(likeNa 2 O,

iVO4 , WO3 , FeVO4 , and FeS2 ) are observed as relatively minor phases. Investigation on hot corrosion resistance o

i- in Na2 SO4 - %V2 O5 at ∘C by Anuwar et al. [ ]

also states that oxides o i and Al are the major phases in

the corrosion product. Te scale on the cermet coated i- ,Superco- , andMDN- sur acesunderstudy consisted o Cr2 O3 , NiO, and Al2 O3 as major phases. Te coated sur acesalso consisted o Na2 O, SO3 , SiO2 , V2 O5 AlVO4 , NiCr2 O4 ,and NiV 2 O5 as minor phases. Hot corrosion study o plasmasprayed Al2 O3 dispersed NiCrAlY in Na 2 SO4 - %V2 O5 at

∘C, by Sreedhar et al. [ ] has shown Al2 O3 , NiO, Cr2 O3 ,and NiCr 2 O4 as major constituents in the corrosion product.

. . SEM and EDX Analysis

. . . Surface Analysis. Te morphology o the corroded sur-aces o uncoated i- , Superco- , andMDN- is shown

in Figure . All sur aces were relatively smooth. Te cor-roded i- sur ace showed a nodular structure and the scalewas rich with oxides o i and Al. Te corroded sur ace o uncoated Superco- showedmudcracks on thesur ace and

the scale was rich with oxides o Na, S, Cr, and Co. Te pres-ence o cracks and high levels o S in the scale also indicatesthat thecoating isnotadherentand proneto spalling, which isalso observed in visual examinations and gravimetric analy-sis. Amount o Al is not sufficient to orm a continuous scaleon the sur ace. Here we have iO2 scale with local patches o Al2 O3 . During cyclic heating and cooling, the inter ace getsopened out and acilitates penetration o oxidizing species.Tis leads to a high value o weight gain. Te corrodedsur ace o uncoatedMDN- showed extensivebroken scaleswhich are rich in oxides o Cr and Fe. It is also noted thatpresence o % Cr in the substrate is not sufficient to orma complete corrosion resistant oxide scale, especially when


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. u . )

10 20 30 40 50 60 70 80 90 100 110Diffraction angle (2 )

♣♣

♣

♣

♣ ♣♣♣

♣

♣:

♣ ♣♣

⧫:

⧫

⧫

⧫ ⧫⧫ ⧫

⧫⧫

⧫ ⧫⧫

♠:

♠:

♠ ♠

♠♠

♠

♠

♠

♠♠ ♠ ♠

♠

♠ ♠

♠

♠ ♠ ♠

♥

♥

♥:

· · ·

Cr2O3

Δ

Δ

Δ

ΔΔ Δ

Δ

Δ ΔΔ

ΔΔ

Δ: SO3 :FeO, : V2O5P:

P

FeVO4 Q:

Q QQ Q Q

QQ

CrVO 4 R:

R

R FeS2

Cr2O3 NiO, CoO

H:

H

H WO2 I:

I ILI

I SiO2 J:

J J

SO2K:

K

Na2O, L:

L

FeVO4 M:

M

M

M

CrVO 4N:

N

N

NN

N NA4FeO3 O:

O

O OO

Ni3V2O3

TiO2 Al2O3 A:A

A

A

A V3Ti6O17B:

B

AlV2O4 C:

C C

AlVO4 D:D

D

D

TiVO4

MDN 121

Superco-605

Ti 31

,,

, ,,

,

,, ,

,

,,

(a)

♥

♥♥

♥A

A

A A

A

AB

B B B

B

B

CC

C

C

CD

D

D D

E

E EE

F

F

F

F F

G G G

♣

♣ ♣♣

♣

♣⧫

⧫

⧫ ⧫

⧫

⧫

MDN 121

Superco-605

Ti 31

♥: :: :Cr2O3 Al2O3 NiO, Cr 23C6 SO3 Na2O, A: CrVO4B:NiV2O5 C:AlVO4 D: AlV2O4 E: NiCr2O4 F: NiAl2O4 G: FeS2♣: ⧫:


. u . )

10 20 30 40 50 60 70 80 90 100 110Diffraction angle (2 )

,, , , , ,

, , ,

(b)

F : X-ray diffraction patterns. (a) Uncoated alloys, (b) coated% (Cr3 C2 - (Ni Cr))+NiCrAlY, coated i- , Superco- , and

MDN- subjected to hot corrosion in Na2 SO4 - %V2 O5 or cycles at ∘C.

carbon percentage is . %. Because o this Fe was also gettingoxidized to produce a nonadherent scale and the coating gotbroke off and opened out during its ormation. Tis lead to

ormation o resh oxide scales and this process continuedthroughout thecycliccorrosionstudy. Te weightgain exper-iments also showed that the coating growth is rapid and ol-low a nonparabolic kinetic relationship. Te morphologies o the corroded samples coated with cermet coatings are givenin Figure . In Figures (a), (b), and (c) are the sub-strates o i- , Superco- , and MDN- , respectively.Te morphology looks similar in all three cases. Similarmorphological eatures are also reported during hot cor-rosion o MCrAlYSiB (M=Ni or Co) coated superalloys[ , ]. Tough the coatings look like broken scales (platelike morphology), there was no spalling o the scales. EDX

measurement on the scales shows that they are rich in oxideso Cr and Ni. Te weight gain measurement presented earlierindicates that the weight gain is smaller compared to that inthe case o uncoated substrates. Tis indicates compactnesso the scales on the coated sur ace. Sreedhar et al. [ ] havereported that the plate like morphology is primarily Cr 2 O3

and in between the plates NiO and Al 2 O3 . Teir coatingis NiCrAlY based and the hot corrosion environment isNa2 SO4 - %V2 O5 . Formation o oxides on the sur ace o MCrAlY (M=Ni or Co) based coating during hot corrosioninpresenceo sulphate lm isalso reportedbyJianget al. [ ].

. . . Cross-Sectional Analysis. Figure shows a cross-sectional micrograph o one o the cermet coated specimenexposed to hot corrosion environment. Te micrographshows a clear oxide layer at the top o the coating (markedarrow). Te oxide layer is thin compared to total thickness o thecoating andit indicatesthat a major raction o thecoatingis unaffectedby the hotcorrosionenvironment. Formation o a thin oxide rich layer on the sur ace o Cr3 C2 -NiCr coatingand NiCrAl coating on Ni and Fe based superalloys duringhot corrosion is reported by Kamal et al. [ ], Jiang et al.[ ], and Mahesh et al. [ ]. Te thickness o the oxide layeris nonuni orm and it is attributed to heterogeneity in themicrostructure which is due to lack o complete melting andnonhomogeneity o the initial powder. Also, HVOF sprayingproduces a coating, which is lamellar in nature with distinctsplat boundaries oriented parallel to the coating-substrateinter ace [ , , ]. Te EDX analysis (Figure (a)) doneat various points across the cross-section (Figure (b)) alsoindicates that the top layer (point ) is rich in oxides. Tisis marked using an arrow in the “O” mapping shown in

Figure . InFigure mapping orelements Al,Cr,andNi arealso shown. Te mapping clearly indicates nonhomogeneity in the chemical composition. Te nonhomogeneity in thecomposition across the coating is attributed to the reasonsexplained in previous lines.

. . Degradation of HVOF Sprayed Protective Coatings. Cer-met, % (Cr3 C2 - (Ni Cr)) + NiCrAlY based mechan-ically blend coatings have been applied on to types o substrates used in gas turbines using high velocity oxy uel(HVOF) process and their hot corrosion resistance was stud-ied. Te results clearly indicate that compared to uncoatedsubstrates the coating provides excellent hot corrosion resis-tance. It is a promising coating developed or the protectiono components used in ossil based turbines.

Te gravimetric analysis under cyclic hot corrosionstudy indicates that, the samples continuously gain the weight(Figures (a) and (b)). Te weight gain is more in theinitial cycles o corrosion study and the extent o weight gainreduces with successive cycles. Te higher weight gain inthe rst ew cycles o corrosion study might be due to therapid ormation o oxides at the grain boundaries and withinthe pores present at the top layer, due to the penetration o the oxidizing species. Aferwards, the ormation o oxidesdepends on the penetration o oxygen through the existingoxide layers.


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74.1% TiO2

21.8% Al2O3

3.8% V2O5

(a)

44.6% Na2O

47.3% SO30.7% CoO

2.2% WO3

(b)

69.4% FeO19.9% Cr2O3

5.7% SO32.6% V2O51.3% Na2O

1.8% MnO

(c)

F : Morphology andEDX analysiso corrodedsamples.Micrographscorresponding to (a), (b), and(c)are or uncoated i- ,Superco-, and MDN- samples, respectively.

30 m

21.22% NiO

47.46% Cr2O3

4.98% Na2O7.98% SO3

0.8% Al2O3

3.94% V2O5

(a)

30 m

14.8% NiO

67.37% Cr2O3

2.88% Na2O5.96% SO3

0.76% Al2O36.69% V2O5

1.53% C

(b)

2.00% NiO

77.10% Cr2O30.26% Na2O

0.25% SO30.6% Al2O35.54% V2O5

4.26% C

15.94% NiO

70.87% Cr2O3

0.03% Na2O0.35% SO3

0.10% Al2O3

7.51% V2O5

1.38% C

3.17% Y2O3

(c)

F : SEM and EDX analysis o % (Cr3 C2 - (Ni Cr)) + NiCrAlY coated materials subjected to hot corrosion or cycles inNa2 SO4 + %V2 O5 at

∘C. (a) i- , (b) Superco- , and (c) MDN- .


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1

2

3

4

(a)

C o m p o s i t i o n

( w t % )

0

20

40

60

1 2 3 4

Point of analysis

CrOAl

Ti

CNiY

(b)

F : Cross-sectional micrograph o the HVOF coated and hot corroded i- sample. (a) Micrograph and points o EDX analysis, (b)plots o EDX analysis at various points.

200 m 200 m

200 m 200 m

O K Al K

Cr K Ni K

F : X-ray mapping or the elements o O, Al, Cr, and Ni over the region given in Figure (a).

Te parabolic rate constants or the hot corroded alloys( i- and MDN- ) are higher compared to cermet coatedalloys (Figures (a) and (b)). Te uncoated i- alloy undergoes very rapid weight gain compared to HVOFsprayed alloy. It is attributed to nonadherent oxide layer

orming at the sur ace. At the top o i- , a mixture o iO2and Al2 O3 orms, as the raction o Al2 O3 is not sufficient to

orm a continuous protective oxide layer on the sur ace. Teinter acebetween theoxidesis an easy path oroxygenmigra-tion and inter ace gets opened out during cyclic heating and


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cooling. End result is a high level o weight gain or uncoatedi- . Even in the case o mechanical blend carbide coated

samples, the coating reduces weight gain by more than hal .Observed low parabolic rate constant or uncoated superco-

is an arti act introduced due to severe spalling andsputtering. It is reported that ormation o Fe2 O3 during

hot corrosion introduces severe strain into the top layer andpromotes spalling [ ]. In Ni based superalloys, sputtering isreported byKamal et al. [ ]. Te superco- alloy hasabout

wt% Fe and it will oxidize and introduce strain in the topcontinuous layer. When strain exceeds the top layer strength,the top layer bursts and gets sputtered out. A large drop inthe value o p o coated MDN- substrate compared touncoated MDN- indicates that the cermet coating is moreuse ul or MDN- alloy, compared to i- alloy.

During hot corrosion, the corroding species, mainly O 2− ,ormeddue to the actthat thedissolution o sodium sulphate

reacts with the top sur ace o thecoating and starts to migratethrough the inter ace. As the reaction proceeds, the elementslike Fe and Ni were oxidized at the top sur ace but moreimportantly, Cr gets oxidized into Cr 2 O3 . Cr2 O3 orms acontinuous layer and imparts corrosion resistance property [ ]. Continuous Cr 2 O3 layeratthe top o the coating will notallow urther transport o the oxidizing species and metallicions across it, easily. Tis reduces availability o oxygen in theunderlying layer. Due to this with the passage o time, urthercorrosion will be reduced. During this process, some o theoxides will undergo reaction within them orming mixedcompounds. Compounds like NiCrO 4 , CrVO4 , and NiAl2 O4are ormed because o such reactions.

4. ConclusionsMechanical blend cermet alloy, % (Cr3 C2 - (Ni Cr)) +NiCrAlY alloy powder, was success ully coated on alloysused in turbine applications, namely, i- , Superco- , andMDN- alloys, using high velocity oxy uel process. Tecoated samples were characterized or microstructural ea-tures. Te coated and uncoated specimens were subjected tocyclic hot corrosion study at ∘C under the conditions o Na2 SO4 - wt%V2 O5 . Weight gain measurements were con-ducted afer each cycle and it showed that coated specimens( i- and MDN- ) showed minimum weight gain com-pared to uncoated specimens. Parabolic rate constants were

estimated and the value is much lower or coated specimensthan or uncoated specimens. It also indicates that or i- ,thecoating wasmore bene cial than orMDN- . Te bettercorrosionprotection o coated substrates wasattributed to the

ormation o a top layer consisting o oxides o Cr and Ni. Inthecase o uncoated Superco- , there wassevere spalling atthesur ace during cycliccorrosionstudyanddue to thiscom-parison o corrosion resistance between coated and uncoatedsubstrate was difficult.

Conflict of Interests

Te authors declare that they have no con ict o interests.

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