Comparative High-Temperature Corrosion Behavior of Ni-20CrCoatings on T22 Boiler Steel Produced by HVOF, D-Gun,and Cold Spraying

GAGANDEEP KAUSHAL, NIRAJ BALA, NARINDER KAUR, HARPREET SINGH,and SATYA PRAKASH

To protect materials from surface degradations such as wear, corrosion, and thermal flux,a wide variety of materials can be deposited on the materials by several spraying processes. Thispaper examines and compares the microstructure and high-temperature corrosion of Ni-20Crcoatings deposited on T22 boiler steel by high velocity oxy-fuel (HVOF), detonation gun spray,and cold spraying techniques. The coatings’ microstructural features were characterized bymeans of XRD and FE-SEM/EDS analyses. Based upon the results of mass gain, XRD, andFE-SEM/EDS analyses it may be concluded that the Ni-20Cr coating sprayed by all the threetechniques was effective in reducing the corrosion rate of the steel. Among the three coatings,D-gun spray coating proved to be better than HVOF-spray and cold-spray coatings.

DOI: 10.1007/s11661-013-1984-4� The Minerals, Metals & Materials Society and ASM International 2013

I. INTRODUCTION

HOT corrosion is a serious problem in high-temper-ature applications such as boilers, gas turbines, wasteincinerations, diesel engines, coal gasification plants,chemical plants, and other energy generation systems.[1]

One method of hot corrosion prevention is to coat thealloy with a protective layer, which has been adopted inthe current investigation. This is the preferred approach,even when relatively hot corrosion-resistant alloys areused.[2] The concern to develop and investigate theperformance of various coating compositions hasbecome a major area of research. The numerousvariants of high-temperature coatings that are in usetoday may be categorized into three generic types:diffusion, overlay, and thermal barrier coatings.[2] In theservice environment, the coating forms an oxide surfacelayer which ideally inhibits corrosion. To a degree, theoxide surface layer can be selected according to theenvironment, and the coating is designed to serve as areservoir for the element forming the surface oxide.Performance of the surface coatings depends on com-position and characteristics of feed stock powders aswell as coating deposition process and its parameters.

Thermal spray processes represent an important andcost-effective technique for tailoring the surface proper-ties of engineering components with a view to enhancetheir durability and performance under a variety ofoperating conditions. Thermal spraying has developedfaster due to progress in the advancement of materials,and modern coating technology. Among the varioustechniques of thermal spraying, high velocity oxy-fuel(HVOF) thermal spray process has gained popularity asa viable technology for in situ applications, wear andcorrosion management and dimensional restoration,mainly because of its important benefits such as highbond strength, low porosity and low stress coatings.[3,4]

HVOF spray technique enables higher kinetic energy ofthe particulates and lower melting coating materials thatenable particle-flattening in the plastic state.[5–9] How-ever, its application for high-temperature corrosionprotection is a relatively recent concern, especially forthe steam-generating plants. Detonation gun spray(D-gun) coatings are known for their high density, highbond strength, moderate substrate heating, superiorsurface finish, better wear/corrosion resistance, and lowcost. Recently, another thermal spray process, known ascold gas dynamic spraying, commonly known as coldspray (CS), is just out of its infancy. The process hasbeen introduced to produce metal, alloy, and compositecoatings with superior qualities.[10,11] This process useshigh velocity rather than high temperature to producecoatings, and thereby avoid/minimize many deleterioushigh-temperature reactions, which are characteristics oftypical thermal-sprayed coatings.[12] Because cold-sprayed materials contain fewer oxide impuritiesand less porosity, these typically have much higherthermal conductivities than traditional thermallysprayed materials.Considerable work has been done to evaluate the

performance of HVOF, D-gun spray, and CS coatings

GAGANDEEP KAUSHAL, Assistant Professor, is with theMechanical Engineering Section, Yadavindra College of Engineering,Punjabi University, Guru Kashi Campus, Talwandi Sabo, Distt.,Bathinda, 151302 Punjab, India. Contact e-mail: gagankaushal@yahoo.com NIRAJ BALA, Assistant Professor, is with the MechanicalEngineering Department, Baba Banda Singh Bahadur EngineeringCollege, Fatehgarh Sahib, 140407 Punjab, India. NARINDERKAUR, Technical-Executive, and HARPREET SINGH, AssociateProfessor, are with the Indian Institute of Technology Ropar,Rupnagar, 140001 Punjab, India. SATYA PRAKASH, ProfessorEmeritus, is with the Indian Institute of Technology Roorkee,Roorkee, Uttarakhand, India.

Manuscript submitted December 26, 2012.Article published online September 17, 2013

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in air environment under laboratory conditions.[13]

However, it is learnt from the literature that furtherresearch is still needed to investigate the performance ofthese coatings in some newer and more aggressive boilerenvironments, which may be simulated in the laboratoryor in actual industrial conditions. The aim of the currentinvestigation is to explore the possibility of deposition ofNi-20Cr coatings on a boiler steel ASTM-SA213-T-22(T22) by HVOF, D-gun, and CS techniques, and tostudy comparative performance of these in laboratoryunder corrosive conditions. Cyclic conditions have beenchosen in the light of the fact that they resemble actualindustrial situations. The steel selected is used exten-sively for the power plant boiler components mainly inthe superheater zone where it has been observed that itsuffers from high-temperature erosion-corrosion by flyash particles. The outcome of the study will be useful toexplore the possibility of use of this coating-processsystem for the power plant boilers.

II. EXPERIMENTAL PROCEDURE

A. Substrate and Feedstock Powder

Substrate material selected for the study is 2.25Cr-1Mo steel designated as ASTM-SA213-T-22 (T22) withchemical composition C 0.15, Mn 0.3-0.6, P 0.03 max,

S 0.03 max, Si 0.5, Cr 1.9-2.6, Mo 0.87-1.13, and Fe94.66 (wt pct) was procured from Guru Gobind SinghSuper Thermal Power Plant, Ropar (India). The spec-imens each measuring 20 mm 9 15 mm 9 5 mmapproximately were cut from the fresh boiler tubes.The specimens were polished down to 180 grit SiC paper

Table I. Spray Parameters Employed for HVOF Coating[14]

Oxygen flow rate 200 SLPMFuel (LPG) flow rate 50 SLPMAir-flow rate 900 SLPMSpray distance 20 cmPowder feed rate 25 to 30 g/minFuel pressure 6.00 kg/cm2

Oxygen pressure 8.00 kg/cm2

Air pressure 6.00 kg/cm2

Table II. Spray Parameters Employed for the D-GunCoating[15]

Parameter Related Value

Oxygen flow rate (kg/cm2) 2640C2H2 flow rate (kg/cm2) 2240N2 flow rate (kg/cm2) 960Spray distance (mm) 140Frequency of shots (s) 3

Table III. Spray Parameters Employed for the CS Coating

Process Gas Helium

Gun temperature 673 K (400 �C)Gun pressure 20.5 barsProcess gas flow rate 150 m3/hPowder feed rate 40 g/minCarrier gas NitrogenFlow rate of gas 4 m3/hCoating thickness 250 lm

Fig. 1—FE-SEM BSEI showing cross-section morphologies of Ni-20Cr sprayed coating on T22 steel by HVOF (a), D-gun (b), and CS(c) deposition techniques.

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finish and were grit blasted with Al2O3 (grit 60) beforethe deposition of the coating. Commercially availablecoating powder Ni-20Cr procured from Sulzer MetcoInc. (Barboursville, WV) was used as coating materials.The particle size of the powders was �45+15 lm.

B. Deposition Techniques and Equipment

Three types of coating methods namely HVOFspraying, D-gun spraying, and CS were used fordepositing coatings. The HVOF coatings were sprayedat M/S Metallizing Equipment Co. Pvt. Ltd., Jodhpur(India) by using commercial HVOF thermal spraysystem. A HVOF-spray (HIPOJET-2100) thermal sprayprocess was used for the powder spraying. Liquefied

petroleum gas (LPG) was used as a fuel. Processparameters are reported in Table I.[14] The specimenswere cooled with the compressed air jets during andafter spraying. The D-gun spray coatings were depositedat M/S Sai Surface Coating Technologies, Hyderabad,India. The flame temperature was 4173 K (3900 �C) andmaximum spray rate was 3 kg/h. The gas flow rate was11 m3/h and particle impact velocity was 600 to 1200 m/s. The D-gun spraying process parameters have beenreproduced in Table II.[15] Cold spray coatings weredeposited at ASB Industries, Inc., Barbeton, OH, USA.The system used for the coating process was Kinetics3000 Cold-spray system (CGT Technologies, GmbH,Ampfing, Germany). The various parameters usedduring CS are given in Table III.

Diffraction angle 2θ

Inte

nsity

(ar

bitr

ary

units

)

Ni

Ni

Ni

NiNi

Ni

Ni

Ni Ni Ni

(c)

(b)

(a)

Fig. 2—X-ray diffraction profiles for Ni-20Cr sprayed coating on T22 steel by HVOF (a), D-gun (b), and CS (c) deposition techniques.

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C. As-Sprayed Coating Characterization

Apparent surface porosity of the as-sprayed Ni-20Crcoating was measured by Image analyzer system fromChennai Metco Pvt. Ltd (India) using Envision 3.0software based on ASTM 276. The images wereobtained through the attached inverted metallurgicalmicroscope. Image analysis of 10 fields on the surface ofeach coated sample produced the value of the averageporosity reported. X-ray diffraction (XRD) analysis ofthe coated samples was carried out using a Bruker AXSD-8 Advance Diffractometer (Germany) with CuKa

radiation and nickel filter at 20 mA under a voltage of35 kV. The specimens were scanned with a scanningspeed of 1kcps in the 2h range of 10 deg to 110 deg andthe intensities were recorded at a chart speed of 1cm/minwith 2 deg/min as Goniometer speed. The diffractome-ter interfaced with the Bruker DIFFRAC Plus X-Raydiffraction software provided the ‘d’ values directlyon the diffraction pattern. FE-SEM/EDS analysis(FE-SEM, FEI, Quanta 200F) was done to characterizethe surface morphology of the coating. After the surfacecharacterization, the samples were sectioned, mounted,and mirror polished as per the standard metallurgicalpractice. The polished samples were characterized toobtain their cross-sectional morphology and composi-tions by using the FE-SEM/EDS analysis. Compositionof the elements in the samples was evaluated from theircorresponding emitted X-ray peaks by Genesis EDSsoftware. X-ray mappings of the various elements werealso taken by the same FE-SEM/EDS machine.

D. Corrosion Experiments

All the specimens were polished down to 1 lmalumina wheel before being subjected to acceleratedhot corrosion runs. Coating of Na2SO4-60 pct V2O5

paste was applied on the preheated specimens 523 K(250 �C) with a camel hairbrush so as to have approx-imately 3 to 5 mg of the paste per cm2 of the specimensurface area. Accelerated hot corrosion studies wereperformed under cyclic conditions for 50 cycles. Eachcycle consisted of 1 hour heating at 1173 K (900 �C) inSilicon Carbide tube furnace followed by 20 minutescooling at room temperature. A cyclic study of 50 cycleswas performed as the duration of 50 hours is consideredto be adequate for attaining a steady-state oxidation fora material.[16] The temperature of study was deliberatelykept high 1173 K (900 �C) as this will also take intoconsideration the overheating effects in case of boilers,which has been identified as the major cause offailure.[17] Moreover, at 1173 K (900 �C), the rate ofhigh-temperature hot corrosion (HTHC) has beenreported to be the severest.[18] The specimens were keptin alumina boats and inserted in the furnace. The studieswere performed for the uncoated and coated specimensfor the purpose of comparison. The weight changemeasurements were taken at the end of each cycle withthe help of Electronic Balance Model 06120 (Contech)with a sensitivity of 1 mg. Weight change data wasanalyzed to approximate the kinetics of corrosion. Afterthe exposure, all the exposed samples were analyzed for

the characterization of oxide scales. The corrodedsamples were subjected to the XRD and FE-SEM/EDAX analyses for the surface as well as the cross-sectional analysis as per the procedure mentioned inSection II–C.

III. RESULTS

A. Coating Thickness and Porosity

The average coating thickness of the HVOF, D-gun,and cold-sprayed Ni-20Cr coated specimens was mea-sured from the Back-Scattered Electron Images (BSEI)(Figure 1) and was found to be 160 lm, 110 lm, and164 lm, respectively. The average apparent surfaceporosity of the coating was found to be 1.3 pct forD-Gun coating and 1.6 pct for both HVOF and cold-sprayed Ni-20Cr coating. The corresponding standarddeviation for the porosity values was 0.06, 0.075, and0.13, respectively.

B. XRD Analysis

The XRD diffraction patterns of the HVOF, D-gun,and cold-sprayed Ni-20Cr coated specimens inas-sprayed condition are reported in Figures 2(a) through(c). The analysis indicates almost similar results with the

(Point 1)74% Ni 24% Cr

(Point 2)77% Ni 21% Cr 01% O

1

2

1

2

(Point 2)79% Ni17% Cr03% O 01% Si

(Point 1)74% Ni20% Cr03% O 01% Si

(a)

(b)

(Point 1)69% Ni 18% Cr 08% Mo 04% O 01% Si

1

2

(Point 2)69% Ni 14% Cr 16% Mo 01% O

(c)

Fig. 3—FE-SEM along with EDS analysis of Ni-20Cr sprayed coat-ing on T22 steel by HVOF (a), D-gun (b), and CS (c) depositiontechniques.

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presence of Ni as the principal phase in all coatings,thereby predicting the formation of c-Ni solid solution.

C. SEM/EDS Analysis of As-Sprayed Coatings

The SEM micrograph showing the surface morphol-ogy of HVOF-sprayed Ni-20Cr coated T22 steel isshown in Figure 3(a). The microstructure consists ofirregular-sized splats with flattened appearance which isa typical characteristic of thermal spray coatings. Thereis the presence of some superficial microvoids in themicrostructure. The EDS measurements taken at thepoints 1 and 2 on the surface of the coating indicate a

nearly uniform composition with Ni and Cr as mainelements. This composition is nearly similar to that ofthe feedstock powder. A corresponding SEM micro-graph for the D-gun spray Ni-20Cr coated T22 steel isshown in Figure 3(b). The microstructure of the coatingconsists of irregular-sized splats. There is a presence ofsome low-lying channel which is circumscribing thesplats. It appears as if they are exposing the inner layerof the coating. The EDS analysis of the coating surfaceshows that the composition of the scale at points 1 and 2is nearly similar and is also approaching to that of thefeedstock powder. A noticeable presence of O at boththe points of the D-gun coating predicts the possible

Fig. 4—(a) Composition image (SEI) and X-ray mappings of the cross-section of the HVOF-sprayed Ni-20Cr coating on T22 boiler steel. (b)Composition image (SEI) and X-ray mappings of the cross-section of the D-gun-sprayed Ni-20Cr coating on T22 boiler steel. (c) Compositionimage (SEI) and X-ray mappings of the cross-section of the cold-sprayed Ni-20Cr coating on T22 boiler steel.

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formation of some oxides in the coating microstructure,whereas in the case of HVOF-sprayed coating, nooxygen has been found. The microstructure of cold-sprayed Ni-20Cr coating in as-sprayed condition isshown in Figure 3(c).[19] The surface appears to beuneven; however, in majority of the microstructure, aproper coalescence of the particles has taken place. Mostof the particles have deformed significantly. Some tinybroken particles are seen which might have beenproduced due to the excessive deformation of thepowder particles during impact on the substrate/coatedlayers. Some voids are also seen at few places, however,the structure by and large, seems dense. The EDSanalysis of the coating at two different points 1and 2 indicates that the coating has a nearly uniform

composition. As expected, Ni has been found as themain constituent (69 pct) with significant amounts of Crand Mo.

D. X-Ray Mapping of As-Sprayed Coatings

Composition image (SEI) and X-ray mappings of thecross-section of the as-sprayed specimens coated by theHVOF, D-gun, and CS techniques are shown inFigures 4(a) through (c), respectively. It is evident thatFe is mainly confined to the base metal, in general, andthe coating is mainly composed of Ni and Cr, as isanticipated from the composition of the feed stockpowder. The concentration of oxygen is higher in thecase of D-gun-sprayed coating. The elemental mappings

Fig. 4—continued.

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for the cold-sprayed coating show that there is presenceof some tiny spots which seem to be enriched with O,Mn shows minor diffusion in the coating region.

E. Molten Salt Corrosion Tests

1. KineticsMass-change data in mg/cm2 as a function of number

of cycles for the uncoated, HVOF and D-gun, and CS-coated T22 steel specimens subjected to Na2SO4-60 pctV2O5 environment at 1173 K (900 �C) for 50 cycles havebeen compiled in Figure 5. This mass-change data serveas a good index to compare the corrosion rates undersimilar conditions of exposure. It is obvious from the

figure that the uncoated steel has shown much highercorrosion rates in comparison with its coated counter-parts. The uncoated steel has shown the tendency toconceive mass gain continuously without showing anyindication of steady-state corrosion rate, whereas afterthe deposition of the coatings, the mass gains havereduced significantly. Moreover, the D-gun-sprayedsteel showed a comparatively better corrosion resistancein comparison with HVOF- and cold-sprayed steel.Talking quantitatively, the mass gains for the steel gotreduced by 61 pct and 64 pct after the application ofHVOF and CS coatings, whereas its appreciable reduc-tion was noted in case of D-gun-sprayed coating wheremass loss was reduced by 88 pct.

Fig. 4—continued.

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2. XRD analysis of the exposed specimensThe XRD phases of the uncoated, HVOF, D-gun,

and cold-sprayed Ni-20Cr coated T22 steel specimenssubjected to corrosion testing molten salt (Na2SO4-60pct V2O5) at 1173 K (900 �C) for 50 cycles have beenreported in Figures 6(a) through (d). Oxide scale of theuncoated steel (Figure 6(a)) mainly comprises Fe2O3

phase. Some weak peaks corresponding to Cr2O3 havebeen also observed. On the other hand, in the analysisfor HVOF-sprayed steel (Figure 6(b)), NiO wasobserved as major phase Fe2O3 along with Cr2O3 asmedium intensity phases and NiCr2O4 as weak intensityphase. For D-gun-sprayed steel (Figure 6(c)), NiO wasobserved as strong phase, NiCr2O4 along with Cr2O3 asmedium intensity and Fe2O3 as weak intensity phase. Incase of cold-sprayed steel (Figure 6(d)), NiO wasobserved as major phase after exposure to corrosiveenvironment.

3. SEM/EDS of exposed samplesThe surface FE-SEM/EDS analysis of the uncoated,

HVOF, D-gun, and cold-sprayed Ni-20Cr coated spec-imens subjected to corrosion testing in the Na2SO4-60pct V2O5 environment at 1173 K (900 �C) for 50 cycleshas been depicted in Figures 7(a) through (d), respec-tively. In the case of uncoated steel (Figure 7(a)) themicrograph shows badly damaged oxide scale withsignificant spalling of its top layers. The left over upperscale has wafer type appearance with deep cracks. Theexposed areas of the scale seem to be dense andadherent. At some other locations, the scale has anamorphous appearance. The EDS analysis reveals thatthe scale in general is found to be rich in Fe and O,which indicates the possibility of formation of Fe2O3.Small amounts of Cr as indicated by point 1 (up to5 pct) has also been observed, thereby predicting thepossibility of formation of Cr2O3. The HVOF-coated

specimen (Figure 7(b))[15] indicates the formation ofcrystalline oxide scale. These crystals appear to bemainly of NiO as evidenced from the EDS analysis atpoints 1 and 2. The crystals of various sizes and shapesseem uniformly distributed in the whole matrix. TheEDS analysis at point 1 indicates 79 pct Ni, 14 pct O,4 pct Cr, 1 pct Fe, and 1 pct S and at point 2 EDS analysisindicated 72 pct Ni, 17 pct O, 4 pct Cr, 2 pct Fe, 2 pctMn,and 2 pct S. As represented by the EDS analysis, therichness ofNi andO indicates the formation ofNiO.Afterthe deposition of the D-gun-sprayedNi-20Cr coating, theoxide scale is mainly composed of crystals of various sizesand shapes distributed in the matrix (Figure 7(c)).[15]

These crystals appear to be mainly of NiO as evidencedfrom the EDS analysis at points 1 and 2. The FE-SEMmicrograph indicating morphology of cold-sprayed Ni-20Cr coated T22 steel after being subjected to the hotcorrosion tests have been shown inFigure 7(d). The oxidescale has been found to be crystalline, which is rich in Niwith significant amounts ofO, indicating the possibility offormationofNiO. Small amounts ofCr can also be seen inthe scale.

4. X-Ray mappings of exposed specimensComposition image (SEI) and X-ray mappings of the

cross-section of the uncoated, HVOF, D-gun, and coldgas spray-coated T22 steel specimens subjected toNa2SO4-60 pct V2O5 environment at 1173 K (900 �C)for 50 cycles have been compiled in Figures 8(a) through(d), respectively. The analysis of the uncoated steel(Figure 8(a)) revealed that the scale is rich in O. Cr canalso be observed in the scale. Large irregular streaks ofV are also indicated. Mn is uniformly present through-out the scale. Fe is scattered throughout the scale. In thecase of HVOF-sprayed steel (Figure 8(b))[15] the basemetal is mainly composed of Fe. The scale is composedmainly of Ni and Cr. There is the presence of some pure

0

20

40

60

80

100

120

140

0 10 20 30 40 50

Mas

s ch

ange

/are

a (m

g/cm

2 )

Number of cycles

T22

HVOF Ni-20Cr

CS Ni-20Cr

DS Ni-20Cr

T22 Steel; Molten Salt; 900° C

Fig. 5—Mass change vs number of cycle’s plots for the uncoated, HVOF, D-gun, and CS Ni-20Cr coated T22 steel subjected to Na2SO4-60 pctV2O5 environment at 1173 K (900 �C) for 50 cycles.

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white regions in the scale, which are rich in Ni and aredepleted of O and Cr. These are basically Ni-rich un-oxidized splats. The gray splats are containing both Nias well as Cr. O, by and large, seems to be present alongthe splat boundaries. Fe is confined mainly to thesubstrate steel. Corresponding X-ray mappings of theexposed D-gun-coated steel is shown in Figure 8(c).[15] Itis clearly revealed that the coating zone is mainly

composed of Ni. Similar analysis for the cold-sprayedsteel is shown in Figure 8(d). It is clearly observed thatNi is uniformly present in the scale along with Cr. A Cr-rich layer is present in the topmost region of the scale,where O is also seen co-existing with the formerindicating the formation of a Cr2O3 layer. There aresome Cr-rich clusters which are depleted of Ni. Fe ismainly confined to the substrate region.

Diffraction angle 2θ

Inte

nsity

(ar

bitr

ary

units

)

µ

γ

α

α

γµ γα β α

α

β

µ

α

ααα

γ

γ γβ

βµ

µ

µ

α NiO β Cr2O3

γ NiCr2O4

µ Fe2O3α

α

α

αα α

µ

µ

µ

µββ

β

γ

(d)

(c)

(b)

(a)

Fig. 6—X-ray diffraction profiles for Ni-20Cr sprayed coating on (a) uncoated, (b) HVOF-sprayed Ni-20Cr coated, (c) D-gun-sprayed Ni-20Crcoated, and (d) cold gas-sprayed Ni-20Cr coated T22 steel subjected to Na2SO4-60 pct V2O5 environment at 1173 K (900 �C) for 50 cycles.

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IV. DISCUSSION

Ni-20Cr powder was successfully deposited on thechosen T22 steel by the HVOF, D-gun, and CStechniques using standard process parameters. Theaverage thickness of the HVOF-spray coating and CScoating was found to be nearly same. The measuredvalue of porosity was found to be less than 2.0 pct in allthe coatings. For the D-gun spray coating the averagethickness and porosity value was slightly lesser than theother two coatings. Porosity of the coatings has asignificant role to play as far as the oxidation or hotcorrosion resistance of thermal sprayed spray coatings isconcerned. Dense coatings usually provide better cor-rosion resistance than the porous coatings. The coatingsshould have minimum possible porosities because theycan do harm to the persistent corrosion resistance ofthermal spray coatings.[20,21] The measured porosity

value was in good agreement with earlier studies onHVOF spray coatings,[22–24] D-gun spray coatings,[25,26]

and CS coating.[27]

The XRD results for the coating types are in goodagreement with those reported earlier.[28] Ni as the mainphase for as-sprayed Ni-20Cr has also been observed bySingh[29,30] for plasma-sprayed Ni-20Cr coatings. Thiswas further confirmed by EDS analysis along surface(Figures 3(a) through (c)) which showed the predomi-nance of Ni in the coating together with Cr as asecondary element. The FE-SEM analysis indicated atypical splat-like morphology for HVOF and D-gunspray coatings. The surface FE-SEM analysis of thecold-sprayed coating represented that most of thedeformed particles were interlocked to each other inthe coatings. Lee et al.[31] suggested that the voidreduction of interparticles is enhanced mainly bymechanical interlocking among the fine-fragmentedparticles with highly irregular shapes, therefore theparticles are well packed and interlocked with each otherin CS coatings. The mappings indicated rich presence ofNi and Cr in all the as-sprayed coatings.The uncoated steel showed higher rates of hot

corrosion in comparison with its coated counterparts(Figure 5). Kolta et al.[32] suggested that in the temper-ature range of 1173 K (900 �C), the Na2SO4 and V2O5

will react to form NaVO3 which acts as a catalyst andalso serves as an oxygen carrier to the base alloy throughthe open pores present on the surface, which in turnleads to rapid oxidation of the base elements of thesubstrate. Fe2O3 was observed as a main phase in theoxide scale of the uncoated steel. This was furthersupported by FE-SEM/EDS analysis (Figure 3(a)),which reveals higher amounts of Fe and O in thesurface of scales of the uncoated substrate. The presenceof Fe2O3 in the scale has been reported to be non-protective by Das et al.[33] The mass gains weresubstantially reduced after the applications of thecoatings. The D-gun coating reduced the mass gain by87 pct followed by HVOF-spray and CS coatings whichreduced the mass gain by 61 pct and 64 pct, respectively.In other words, the D-gun coating looked to be a betterchoice in comparison with the HVOF-spray and CScoating from the standpoint of molten salt corrosionresistance.The XRD analysis of the D-gun and HVOF-spray-

coated T22 substrates (Figure 6) after corrosion studiesrevealed the presence of NiO phase along with Cr2O3

and NiCr2O4 phases. The surface FE-SEM/EDS ana-lysis of the coated steel further supported the formationof these oxides (Figures 6(b) and (c)). The results wereconfirmed by X-ray mapping analysis, which showedthat the scale was mainly rich in Ni metal constituents.It has been observed from the X-ray mappings that thecoatings, by and large, have retained its continuouscontact with the substrate steel and the scale is mainlyrich in Ni and Cr. Fe has diffused from the base alloyinto the coating area in the case of HVOF, as well as,cold-sprayed cases (Figures 8(b) and (d)), however,same has not been observed in the D-gun-sprayed case,which is a desirable feature expected from any idealcoating. No other element showed the tendency for

80% Ni13% O 04% Cr 01% Al 01% Fe

73% Ni17% O 04% C 04% Cr 01% Al

1

2

1 2

3

61% Fe28% O 07% C 02% Mn 02% Mo

57% Fe27% O 06% Mn 03% C 03% Si

59% Fe28% O 05% Cr02% C 02% Si 02% Mn 01% V

3

2

1

75% Ni12% O 08% C 03% Cr 01% V59% Ni

29% O 07% C 02% Cr 01% Si

67% Ni21% O 07% C 03% Cr 01% S

77% Ni13% O 08% Cr 01% Fe 01% Mn

83% Ni08% Cr 05% O 02% Mn01% Fe

2

1

(a)

(b)

(c)

(d)

Fig. 7—Surface scale morphology and FE-SEM/EDS analysis forthe (a) uncoated, (b) HVOF-sprayed Ni-20Cr coated,[15] (c) D-gun-sprayed Ni-20Cr coated,[15] and (d) cold gas-sprayed Ni-20Crcoated[19] T22 steel subjected to Na2SO4-60 pct V2O5 environment at1173 K (900 �C) for 50 cycles.

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Fig. 8—(a) Composition image (SEI) and X-ray mappings of the cross-section of the uncoated T22 boiler steel subjected to hot corrosion inNa2SO4-60 pct V2O5 environment at 1173 K (900 �C) after 50 cycles. (b) Composition image (SEI) and X-ray mappings of the cross-section ofthe HVOF-spray Ni-20Cr coated T22 boiler steel subjected to cyclic hot corrosion in Na2SO4-60 pct V2O5 at 1173 K (900 �C) for 50 cycles.[15]

(c) Composition image (SEI) and X-ray mappings of the cross-section of the D-gun spray Ni-20Cr coated T22 boiler steel subjected to cyclic hotcorrosion in Na2SO4-60 pct V2O5 at 1173 K (900 �C) for 50 cycles.[15] (d) Composition image (SEI) and X-ray mappings of the cross-section ofthe CS Ni-20Cr coated T22 boiler steel subjected to cyclic hot corrosion in Na2SO4-60 pct V2O5 at 1173 K (900 �C) for 50 cycles.

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inter-diffusion between coating and substrate steel. Thepenetration of oxygen along the coating depth wasrelatively higher in the case of CS-coated steel incomparison with that in HVOF- and D-gun-spray-coated steel. Moreover, the formation of Cr2O3 scale inthe case of cold-sprayed coating was discontinuous asindicated by the x-ray mappings.

It has been reported that chromium has a higheraffinity for oxygen than Ni and forms more stable oxide.NiO is a less stoichiometric oxide than Cr2O3.

[34] Duringthe initial stages of oxidation of Ni-Cr system, the nucleiof all stable oxide phases, NiO and Cr2O3 form on thesurface. The fast growing NiO overgrows on Cr2O3, anda layer of NiO develops on the surface. Since NiO is less

Fig. 8—continued.

406—VOLUME 45A, JANUARY 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

stable than Cr2O3, it supplies oxygen, to react withchromium to produce Cr2O3 and results in the completehealing layer.[35] The chromium oxide phase (Cr2O3) isthermodynamically stable[36] up to very high tempera-tures due to its high melting point and forms dense,continuous, and adherent layers that grow relativelyslowly.[37] The scale of this type forms a solid diffusionbarrier that inhibits the interaction of oxygen ofunderlying coating. Moreover, the additional presenceof a spinel NiCr2O4 phase in the oxide scale might alsohave helped to develop better oxidation resistance in theNi-20Cr coating as the spinel phases usually havesmaller diffusion coefficients of the cations and anionsthan those in their parent oxides.[37] NiCr2O4 has been

observed in both the D-gun, as well as, HVOF-sprayedcoatings. The spinel phase is expected to form when NiOreact with Cr2O3 as per the following reactions,[38]

NiO+Cr2O3 = NiCr2O4. These mixed oxides areknown for providing superior corrosion resistance, ashas been reported by several investigators.[39–42] Thisphase is of very weak intensity in the case of HVOF-sprayed coating, whereas it is found to be absent in thecase of cold-sprayed coating. This might be the reasonfor better protection offered by the D-gun-sprayedcoating in comparison to the other two coatingprocesses.Based upon the results and observations of the study,

consolidated diagrams showing oxide scale details of

Fig. 8—continued.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 45A, JANUARY 2014—407

HVOF, D-gun, and CS coatings (Figures 9(a) through(c)) subjected to Na2SO4-60 pct V2O5 environment at1173 K (900 �C) for 50 cycles have been proposed toillustrate the hot corrosion mode after exposure to thecorrosive environment. As is clear from Figure 9(b),presence of a protective scale consisting of protectivephases such as NiO, Cr2O3, and NiCr2O4 may haveprovided a best corrosion resistance to the D-gun-coatedsteel, in comparison with the other investigated cases.

V. CONCLUSIONS

1. Ni-20Cr coatings were successfully deposited byHVOF, D-gun, and CS techniques on T22 boiler steel.

2. Results of the corrosion studies in molten saltenvironment demonstrated that thermal-sprayedNi-20Cr coatings by all the three techniques werehighly beneficial.

3. Overall mass gain was minimal for the D-gun spraycoating, followed by HVOF and CS coatings.

4. The D-gun spray coating was found to be bestamong all the studied coatings. The formation of aprotective Cr2O3 phase along with NiCr2O4 in itsoxide scale might have imparted a better corrosionresistance to the coating.

5. D-gun-sprayed Ni-20Cr coating may be recom-mended as a suitable process-coating combinationfor the said environment.

Fig. 8—continued.

408—VOLUME 45A, JANUARY 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

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Molten Salt EnvironmentNa2SO4-60%V2O5 at 1173K (900°C)

Corrosive Species

SUBSTRATEFe3+ Fe3+

Ni

Cr3+

Scale rich in Ni and Cr

Top scale having

(a)

(b)

(c)

XRD phases like NiO, Cr2O3, NiCr2O4

Top scale having XRD phases like NiO, NiCr2O4

Cr2O3,O, S

Ni Ni

Molten Salt EnvironmentNa2SO4-60%V2O5 at 1173K(900°C)

Corrosive Species

SUBSTRATEFe3+ Fe3+

Ni Cr3+

Scale rich in Ni and Cr

Top scale having XRD phases like NiO

Molten Salt EnvironmentNa2SO4-60%V2O5 at 1173K (900°C)

Corrosive Species

SUBSTRATEFe3+ Fe3+

Ni

Cr3+

Scale rich in Ni and Cr

Microvoids

Fig. 9—(a) Consolidated diagram showing probable hot corrosionmode for the HVOF-sprayed Ni-20Cr coating on T22 boiler steelsubjected to Na2SO4-60 pct V2O5 at 1173 K (900 �C) for 50 cycles.(b) Consolidated diagram showing probable hot corrosion mode forthe D-gun-sprayed Ni-20Cr coating on T22 boiler steel subjected toNa2SO4-60 pct V2O5 at 1173 K (900 �C) for 50 cycles. (c) Consoli-dated diagram showing probable hot corrosion mode for the cold-sprayed Ni-20Cr coating on T22 boiler steel subjected to Na2SO4-60pct V2O5 at 1173 K (900 �C) for 50 cycles.

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