High-Temperature Behavior of a High-Velocity Oxy-FuelSprayed Cr3C2-NiCr Coating

MANPREET KAUR, HARPREET SINGH, and SATYA PRAKASH

High-velocity oxy-fuel (HVOF) sprayed coatings have the potential to enhance the high-tem-perature oxidation, corrosion, and erosion-corrosion resistance of boiler steels. In the currentwork, 75 pct chromium carbide-25 pct (nickel-20 pct chromium) [Cr3C2-NiCr] coating wasdeposited on ASTM SA213-T22 boiler steel using the HVOF thermal spray process. High-temperature oxidation, hot corrosion, and erosion-corrosion behavior of the coated and baresteel was evaluated in the air, molten salt [Na2SO4-82 pct Fe2(SO4)3], and actual boiler envi-ronments under cyclic conditions. Weight-change measurements were taken at the end of eachcycle. Efforts were made to formulate the kinetics of the oxidation, corrosion, and erosion-corrosion. X-ray diffraction (XRD) and field-emission scanning electron microscopy(FE-SEM)/energy dispersive spectroscopy (EDS) techniques were used to analyze the oxidationproducts. The coating was found to be intact and spallation free in all the environments of thestudy in general, whereas the bare steel suffered extensive spallation and a relatively higher rateof degradation. The coating was found to be useful to enhance the high-temperature resistanceof the steel in all the three environments in this study.

DOI: 10.1007/s11661-012-1118-4� The Minerals, Metals & Materials Society and ASM International 2012

I. INTRODUCTION

HIGH-TEMPERATURE oxidation, corrosion, anderosion-corrosion are serious problems in steam-gener-ation plants, gas turbines, internal combustion engines,fluidized bed combustors, industrial waste incinerators,and black liquor boilers. Erosive, high-temperature wearof heat-exchanger tubes and other structural materialsin coal-fired boilers has become a key material issue inthe design and operation of thermal power plants and isrecognized as one of the main causes of downtime inthese installations.[1] The maintenance costs for replac-ing broken tubes in these installations are also high.High-temperature oxidation and erosion caused by theimpact of fly ashes and unburnt carbon particles are themain problems in these applications, especially in thoseregions where the component surface temperature isabove 873 K (600 �C). The hot corrosion of a boilersteel/alloy usually occurs in the environments wheremolten salts such as sulfates (Na2SO4), chlorides (NaCl),or oxides (V2O5) are deposited onto the surfaces. TheNa2SO4-82 pct Fe2(SO4)3 environment is found usuallyin coal-fired boilers where the coal ash corrosion isinduced by the deposition of complex iron-alkali

sulfates, (Na, K)3Fe(SO4)3.[2] Therefore, the develop-

ment of wear-protection and high-temperature oxida-tion protection systems in industrial boilers is animportant topic from both engineering and an economicperspective.Protective coatings are used on the structural alloys in

energy-conversion and energy-utilization systems toprotect the surfaces from oxidation and erosion.[3,4]

Three methods are used widely for depositing coatings,which include chemical vapor deposition (CVD) from apack, physical vapor deposition (PVD), and thermalspraying (metal spraying). Illavsky et al.[5] reported thatoften, thermally sprayed deposits have superior proper-ties with potentially lower application costs or lessenvironmental issues as and when compared with otherindustrially used coatings such as CVD, PVD, and hardchromium plating. Thermal spray techniques such asplasma spray, high-velocity oxy-fuel (HVOF), anddetonation-gun spray processes are considered forcoating the boiler and gas turbine materials. Thecoatings enhance the life of these materials by makingthem resistant to erosion and corrosion. A much thickercoating thickness can be achieved by thermal spraying,which is another requisite for such applications. Amongthe various thermal spray techniques, HVOF spraying isa rapidly developing thermal spray technology fordepositing surface coatings to combat high-temperaturecorrosion, and it has challenged the vacuum plasmaspraying technique, which is comparatively expensive toset up.[6] Moreover, with the availability of portableHVOF spraying equipments, the process provides thepossibility of in situ applications in the actual boilerinstallations, which is not possible with other techniquessuch as CVD, PVD, carbonization, and laser cladding.The HVOF coatings have relatively low porosity, high

MANPREET KAUR, Assistant Professor, is with the Departmentof Mechanical Engineering, Baba Banda Singh Bahadur EngineeringCollege, Fatehgarh Sahib, 140 407 Punjab, India. Contact e-mail:mpk1610@gmail.com HARPREET SINGH, Assistant Professor, iswith the School of Mechanical, Material and Energy Engineering,Indian Institute of Technology Ropar, Roopnagar, 140001 Punjab,India. SATYA PRAKASH, Professor Emeritus, is with Metallurgicaland Materials Engineering Department, Indian Institute of Technol-ogy Roorkee, Roorkee, 247667 Uttarakhand, India.

Manuscript submitted August 15, 2011.Article published online March 9, 2012

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, AUGUST 2012—2979

hardness, high abrasive resistance, good wear resistance,and the ability to resist many high-temperature corro-sion environments.[7–11] The particles in the HVOFprocess attain supersonic velocities with a mach numberin the range of 1.5 to 2.0. The high kinetic energy ofparticles striking the substrate surface does not requirethe particles to be completely molten to form high-quality HVOF coatings.[12] This is certainly an advan-tage for the carbide cermet-type coatings (WC-Co andCr3C2-NiCr), which is where the process excels.

Thermally sprayed Cr3C2-NiCr coatings are used inapplications that demand protection against surfacedegradation because of oxidation, wear, and corrosionunder severe conditions of excessive heat and load.[13]

These coatings show good tribological properties inrigorous working conditions such as at high tempera-tures or in aggressive environments, for example, insteam turbine blades or in boiler tubes for powergeneration. These coatings maintain their high wear andcorrosion resistance up to 1253 K (980 �C) and can beused to improve the performance life of componentsworking at increased temperatures.[14,15] These coatingsexhibit high hardness with a high volume fraction ofcarbide being preserved during the spraying, providingdifferent wear behaviors.[16] Chromium carbide, Cr3C2

with NiCr, coatings are applied for heat-treatmentrolls[17] and coal burning boiler tubes because of theirsuperior heat resistance and erosion resistance againstfly ash.[18] Cr3C2-NiCr coatings offer good corrosionand oxidation resistance with a high melting point.Moreover, these coatings can maintain high hardness,strength, and wear resistance up to a maximum oper-ating temperature of 1173 K (900 �C).[19–21] In additionto these features, the coefficient of thermal expansion ofCr3C2 (10.3 9 10�6 �C�1) is nearly similar to that of theiron (11.4 9 10�6 �C�1) and nickel (12.8 9 10�6 �C�1)that constitute the base of most high-temperaturesteels.[22] This minimizes the stress generation occurringfrom the thermal expansion mismatch during thermalcycles. Several studies indicate that Cr2O3 (protectiveoxide) is formed preferentially at high temperatures onthe coating surface, and so it prevents oxidation of thewhole coating. This oxide has the ability to act as adiffusion barrier to the oxidizing species. Moreover, theCr2O3 scale usually has a good adherence with thematrix of the alloy and it grows at lower rates, whichmakes it work for longer hours. Because the Cr2O3 scaleis adherent and has good hardness properties, it usuallyprovides good erosion resistance as well. Based onweight-change studies, Berger et al.[23] concluded thatthe oxidation of Cr3C2-NiCr starts at 873 K (600 �C)and continues to be low up to 1073 K (800 �C).[23]According to Seong et al.,[2] Cr3C2-NiCr coatingssprayed by the HVOF process can be recommended asa promising coating for the heat-exchanger pipes.[2]

However, these coatings have not been studied in detailwith regard to their behavior in actual boiler environ-ments. Therefore, in the current investigation, theauthors studied the behavior of the Cr3C2-NiCr coatingin the low-temperature superheater of the stage II coal-fired boiler of a power plant. In this article, theoxidation, high-temperature corrosion (hot corrosion),

and erosion-corrosion behavior of ASTM SA213-T22(T22) boiler steel with and without the coating undercyclic conditions at 973 K (700 �C) has been investi-gated and reported. The high-temperature behavior ofdetonation-gun sprayed Cr3C2-NiCr coating on T22 hasbeen reported elsewhere by the authors.[24]

II. EXPERIMENTAL PROCEDURE

A. Substrate Material and Coating Formulation

The substrate material selected for the study was aboiler steel, namely, ASTM SA213-T22 (T22), whichwas procured from Guru Gobind Singh Super ThermalPower Plant (GGSSTPP) (Ropar, Punjab, India). Thecomposition of the steel is 0.15 C, 0.3-0.6 Mn, 0.03 maxP, 0.03 max S, 0.5 Si, 1.9 to 2.60 Cr, 0.87 to 1.13 Mo,and 94.66 Fe. Although T22 steel has adequate mechan-ical strength at increased temperatures, often it lacksresistance to oxidizing/corroding environments duringlonger periods of usage. Commercially available75Cr3C2-25(NiCr) [Cr3C2-NiCr] powder was depositedon this steel by the HVOF spray process. The coatingpowder has a particle size in the range of �45 to+15lm. The powder was found to be in the form of lumps ofirregular-sized particles. Most of the particles have aspherical morphology (Figure 1). The X-ray diffraction(XRD) analysis of the powder revealed the formation ofCr, CrNi, and Cr3C2 as strong-intensity phases and Nias a medium-intensity phase (Figure 2(b)). The speci-mens were grit blasted with Al2O3 (grit 60) before thedeposition of the coatings. The coatings were thendeposited on the steel specimens at Metallizing Equip-ment Company Private Limited (Jodhpur, India) withtheir commercial HVOF (HIPOJET-2100) apparatusoperating with oxygen and liquid petroleum gas (LPG)as input gases. The coatings were deposited on all sidesof the specimens. The spraying parameters adopted bythe manufacturer are given in Table I. The substratesteels were cooled by compressed air jets during andafter spraying. The thickness of the coating was kept in arange of 225 ± 25 lm. The coated samples were

Fig. 1—Morphology of the Cr3C2-NiCr powder.

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polished down to cloth wheel polishing and thensubjected to an XRD analysis by using a Bruker AXSD-8 Advance Diffractometer (Bruker AXS, Karlsruhe,Germany) with CuKa radiation and nickel filter at20 mA under a voltage of 35 kV. The surface and cross-sectional scanning electron microscope (SEM)/energy-dispersive spectroscopy (EDS) analysis of the sampleswas carried out with a field-emission scanning electronmicroscope (FE-SEM, FEI, Quanta 200F Company,Hillsboro, OR) attached with EDS Genesis software(made in the Czech Republic). The SEM micrographsalong with EDS spectrum were taken with an electronbeam energy of 20 keV. To study the cross-sectionaldetails, the samples were sectioned, mounted in trans-optic powder, and subjected to fine polishing usingemery papers of 220-, 400-, 600-grit sizes and subse-quently on 1/0, 2/0, 3/0, and 4/0 grades, successively.Cloth wheel polishing was performed to obtain amirror finish using diamond paste. The microhardness

of the Cr3C2-NiCr coating was measured using SHV-1000, Digital Micro Vikers Hardness Tester (ChennaiMetco Pvt. Ltd., Chennai, India). The microhardnessprofile along the cross section of the coating as afunction of distance from the coating–substrate interfacewas plotted.

B. High-Temperature Oxidation, Corrosion,and Erosion-Corrosion Tests

Cyclic oxidation studies were performed on thesamples in air and in a molten salt environmentresembling that of boilers [Na2SO4-82Fe2(SO4)3] for 50cycles. Each cycle consisted of 1 hour of heating at973 K (700 �C) in a silicon carbide tube furnacefollowed by 20 minutes of cooling at room temperature.The samples were examined physically after each cycleand weight-change measurements were made. The aimof cyclic loading is to create conditions for the acceler-ated oxidation/corrosion testing. Moreover, the cyclicconditions resemble the actual industrial conditionswhere the plants are operated and shut down frequentlyfor many reasons. These studies were performed for thebare and coated samples to obtain a comparativedatabase. All the specimens were polished down to1 lm alumina cloth wheel polishing before the oxida-tion/corrosion studies. In the case of molten saltcorrosion testing, a salt coating of uniform thicknessweighing 3 to 5 mg/cm2 of Na2SO4-82Fe2(SO4)3 paste

Diffraction Angle (2θ)

Inte

nsity

(A

rbitr

ary

Uni

ts)

10020 30 40 50 60 70 80 90

ββ

β

δ β β θ

β

δθμ

β

θδβ

μβ

β

θβ

θμ δβ

δβ β

θμ

θμ

α- Cr7C3; β- Cr3C2; ρ- Cr2C; δ-CrNi; θ-Cr; μ-Ni

β

βδαθ

βρ

ραβ

αθμδα

β

αβ α

β

αβμ

βρα

αβθ

μραβθδ

ραβδθ

ραβ

αθ

αμ μ

θμ θ

(a)

(b)

Fig. 2—XRD results of (a) HVOF spray Cr3C2-NiCr–coated T22 boiler steel (b) Cr3C2-NiCr powder.

Table I. Spray Parameters as Employed Duringthe HVOF Spraying

Oxygen flow rate (SLPM) 250Fuel (LPG) flow rate (SLPM) 60–70Air flow rate (SLPM) 600Spray distance (in) 6–7Powder feed rate (g/min) 25

SLPM, standard liters per minute.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, AUGUST 2012—2981

was applied with a camel hairbrush on the preheatedsamples [523 K (250 �C)].

The erosion-corrosion studies were conducted in anactual boiler environment. The coated and uncoatedsamples were hung through soot-blower dummy pointsin the middle zone of the low-temperature superheaterof the stage II boiler of GGSSTPP for erosion-corrosionstudies under cyclic conditions. In this zone, thetemperature was about 700 �C ± 10 �C and volumetricflow of flue gases was approximately 700 tonnes/h. Theflue gases contain 16 pct CO2 and 3 pct O2 by volume.The gas stream contained the ash particles. The chemicalanalysis of ash and flue gases inside the boiler is given inTable II. The velocity of the gas stream was 13 m/s. Thesamples were hung with the help of nichrome wires inthe boiler. The angle of gas/fly ash stream impingementat the top edge of the surface was 90 deg, and on theother four edges (two major flat surfaces as well), thestream went parallel, and at the bottom, no gas streamwas found. The studies were conducted for 15 cycles;each cycle consisted of 100 hours of exposure, followedby 1 hour of cooling at ambient conditions.

The weight-change measurements were taken at theend of each cycle using an electronic balance with asensitivity of 1 mg. Any spalled scale was included alsoat the time of weighing to determine the total rate ofoxidation/corrosion; however, the same was not possibleduring erosion-corrosion testing. Efforts were made toformulate the kinetics of the oxidation, corrosion, anderosion-corrosion. However, in actual boiler environ-ments, weight-change data could not be of much use forpredicting the erosion-corrosion behavior because ofsuspected spalling and ash deposition on the samples.Hence, the extent of erosion-corrosion has also beenmonitored by measuring the thickness of the exposedsample after the total exposure of 1500 hours. After theend of an exposure time of 1500 hours, the samples weretaken out, cooled, cleaned with soft brush, washed withacetone, and dried. The thickness of the eroded-cor-roded specimens was recorded carefully with a digitalvernier calliper (Mitutoyo, Mizonokuchi, Japan) andthen converted into mils per year (mpy). The exposedsamples were analyzed using FE-SEM/EDS and XRDfor a surface analysis. To identify the cross-sectional

details, the samples were sectioned and prepared as perthe procedure already given in Section II–A. Thepolished samples were characterized to obtain theircross-sectional morphology and compositions by usingFE-SEM/EDS. The EDS Genesis software was used tocalculate the composition of the elements in the coatingsfrom their corresponding emitted X-ray peaks. The scalein the current work has been referred to as the materialabove the base steel, which includes coating plus someother oxide layers (if formed) on the outermost layer ofthe coating. The assumption was made because after theexposure, every coating might suffer some internaloxidation to different depths, which is usually not easyto measure.

III. RESULTS

A. Characterization of Boiler Steeland As-Sprayed Coating

1. Microstructure of boiler steelAn optical micrograph of the substrate steel is shown

in Figure 3(a). The microstructure consists mainly offerrite (light etching constituent) along with a smallamount of pearlite (dark etching constituent). The lighttan areas are most likely martensite.

2. Surface SEM/EDS and XRD analysis of coatingA SEM micrograph of as-sprayed steel is shown in

Figure 3(b). The coating is found to have a splat-likemorphology with a nearly uniform microstructure. Thesplats are surrounded by the white splat boundaries.This morphology is a typical characteristic feature ofthermal spray coatings. The EDS analysis of the coatingat the point 1 indicates the presence of mainly Cr and Niin the black matrix. The splats also consist of mainly Crand Ni, along with significant amounts of C and Si. Asmall amount of O at point 2 is present, which indicatesthat some oxides might have been formed also in thestructure. An XRD analysis of the coated steel iscompiled in Figure 2(a). The analysis revealed theformation of Cr7C3 and Cr3C2 as the strong phases;CrNi, Cr2C, and Cr as the medium-intensity phases; andNi as a weak-intensity phase.

Table II. Chemical Analysis of Ash and Flue Gases Inside the Boiler

Ash Flue Gases

Constituent Wt pct age Constituent Value relative to flue gases.

Silica 54.70 SOx 236 mg/m3

Fe2O3 5.18 NOx 1004 lg/m3

Al2O3-Fe2O3/Al2O3 29.56 CO2 14 to 16.5 pctCalcium oxide 1.48 O2 2.5 to 5 pctMagnesium oxide 1.45 40 pct excess air was supplied to the boiler for the

combustion of coalSO3 .23Na2O .34K2O 1.35Ignition loss 4.31

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3. Cross-sectional analysis of coatingAverage thickness of the coating was measured from

its backscattered electron image (BSEI) shown inFigure 4. In the micrograph, three regions (namely thesubstrate, coating, and epoxy) are visible. The averagethickness of the coating was measured as 200 lm. The

coating seems to have a continuous contact with thesubstrate steel. The coating has a laminar microstructurewith the presence of some microvoids.

4. Microhardness of coatingThe microhardness values of the coating as a function

of distance from the coating–substrate interface areshown in Figure 5. The microhardness values lie in therange of 697 to 1205 Hv, with an average value of966 Hv, whereas the substrate steel has an averagemicrohardness of 223 Hv. The value of microhardnessat the interface was found to be 423 Hv.

B. High-Temperature Oxidation, Corrosion, andErosion-Corrosion Behavior

1. Visual observations and weight-change analysisPhotographs of the bare and coated steel after

oxidation in air at 973 K (700 �C) for 50 cycles areshown in Figure 6. The color of the oxide scale formedon the bare steel was dark gray with some brownishspots. The scale showed intensive spallation from the

(a)

(b)

Fig. 3—(a) Surface microstructure analysis of the uncoated boilersteel showing the presence of mainly ferrite along with smalleramounts of pearlite. (b) Surface morphology and EDS analysis forthe HVOF spray Cr3C2-NiCr–coated T22 boiler steel.

Fig. 4—BSEI micrographs showing cross-sectional morphology ofHVOF sprayed Cr3C2-NiCr coating on T22 boiler steel specimen.

Fig. 5—Cross-sectional microhardness profile of HVOF sprayCr3C2-NiCr–coated T22 boiler steel.

(a) (b)

Fig. 6—Photographs of (a) uncoated and (b) HVOF spray Cr3C2-NiCr–coated T22 boiler steel subjected to cyclic oxidation in air at973 K (700 �C) for 50 cycles.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, AUGUST 2012—2983

first cycle of the study, which increased with the passageof time. In contrast, the coated steel was initially lightgray in color. The color changed to dark gray after theinitial two cycles, which continued to become darkertoward a blackish-gray appearance with the increase inexposure. A few white spots were indicates on theblackish-gray scale after the 44th cycle, which vanishedtoward the end of cycles (Figure 6(b)). These white spotsare most likely MnO or MnCr2O4 as per the observa-tions of Liu[25] during his study on the oxidationbehavior of Fe-based and Ni-based alloys. The coating,in general, showed good adherence to the boiler steelwith no tendency for spalling.

Photographs of the bare and coated steel after hotcorrosion experimentation at 973 K (700 �C) for 50cycles are shown in Figure 7. On the bare steel, a gray-colored scale with brown patches appeared from the firstcycle. After the first cycle, a fragile scale could beobserved on the surfaces of the specimen. The fragilescale could not sustain and started detaching from thesurfaces in the form of tiny flakes by the end of the 3rd,8th, 16th, and 20th cycles. The spallation continueduntil the end of study. The oxide scale showed increasingdominance of the brownish spots on the gray back-ground with the progress of the study. In contrast, thecoated steel was initially light gray in color. The colorchanged to lustrous gray after the initial two cycles withthe appearance of salt crystals on one surface. No majorchange was noticed until the 29th cycle. Afterward,some small superficial cracks were observed on the flatsides (Figure 7(b)). No spallation was observed duringthe 50 cycles of experimentation.

Photographs of uncoated steel and coated steel after1500 hours of exposure in the low-temperature super-heater of the stage II area of the coal-fired boiler areshown in Figure 8. Reddish-brown colored scaleappeared on the surface of bare T22 steel after exposureto the first cycle of the study, which continued to becomedarker toward a blackish-brown appearance with theincrease in exposure. The scale became fragile by the endof the third cycle, and blisters could be observed on thesurfaces of the sample. Significant spallation of scale wasindicated after the end of 4th, 9th, and 14th cycles,although the minor spallation of the scale was observed

invariably throughout the exposure time once it startedby the end of the 4th cycle. The oxide scale looked shinygray and a lot of debris and spalled areas were observedafter the completion of 1500 hours of study (Figure 8(a)).On the contrary, the coated steel turned gray afterexposure to the first cycle, which continued to becomedarker gray with the passage of exposure. The coatingremained intact without any spallation until the end ofthe exposure time (Figure 8(b)).The weight change (mg/cm2) vs number of cycles plots

for the uncoated and coated steel oxidized at 973 K(700 �C) in air for 50 cycles are shown in Figure 9. It canbe inferred from Figure 9 that uncoated steel showedlesser air oxidation resistance compared with its coatedcounterpart. The total weight gain at the end of 50 cyclesfor the uncoated steel is 106.99 mg/cm2, whereas thecorresponding value for the coated steel is found to be11.86 mg/cm2. This shows that the overall weight gainfor the steel was decreased by 89 pct, which is signifi-cant. Moreover, the oxidation process followed the

(a) (b)

Fig. 7—Photographs of (a) uncoated and (b) HVOF spray Cr3C2-NiCr–coated T22 boiler steel subjected to cyclic oxidation inNa2SO4-82 pct Fe2(SO4)3 environments at 973 K (700 �C) for 50cycles.

Fig. 8—Photographs of (a) uncoated and (b) HVOF spray Cr3C2-NiCr–coated T22 boiler steel subjected to a low-temperature super-heater of the stage II boiler of GGSSTPP at 973 K (700 �C) ±10 �C after 1500 h.

Fig. 9—Weight gain vs number of cycles plots for the uncoated andHVOF spray Cr3C2-NiCr–coated T22 boiler steel subjected to cyclicoxidation in air and Na2SO4-82 pct Fe2(SO4)3 salt environments at973 K (700 �C) for 50 cycles.

2984—VOLUME 43A, AUGUST 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

parabolic rate law of oxidation for both the bare and thecoated case. The parabolic rate constants (Kp) werecalculated according to the parabolic rate law ofoxidation (DW/A)2 = Kpt, where DW/A is the massgain per unit area at the oxidation time (t). Kp is equalto the slope of (DW/A)2 vs time plot.[25] The Kp valuesare shown in Table III. It is clear that the value of Kp

for the steel has decreased significantly after the depo-sition of the coating.

The weight-change data for the uncoated and coatedsteel cyclically oxidized for 50 cycles at 973 K (700 �C)in Na2SO4-82 pct Fe2(SO4)3 environments are shown inFigure 9. It is obvious that the uncoated steel has shownlesser oxidation resistance compared with its coatedcounterpart. The oxide scales for bare steel showedintensive tendency toward cracking or spalling duringthe course of 50 cycles. The steel has shown an overallweight gain of 77.87 mg/cm2, which is substantiallyhigher than its coated counterpart. The overall weightloss value for the coated steel was found to be– 3.83 mg/cm2, which is only marginal. Therefore, itcan be inferred that the coating is useful in enhancingthe hot corrosion resistance of the steel.

The weight-change data in mg/cm2 as a function oftime expressed in hours for the bare and coated steels areshown in Figure 10. In both cases, the samples showedoverall weight loss during the exposure in the actual

boiler environment. The bare T22 steel has shown abruptvariations in its weight-change rates by the end of 4th,9th, and 14th cycles. At these points, the fluctuations inweight-change values are depicted in Figure 10. Theseweight fluctuations may be attributed to the significantspallation of the oxide scale during these cycles, asnoticed visually. In the bare steel case, a weight loss of –4.85 mg/cm2 has been registered, whereas the overallweight loss in the coated case is –7.70 mg/cm2.

2. Thickness change in actual boiler environmentThe extent of erosion-corrosion loss was measured in

terms of the metal layer lost from erosion-corrosionafter the exposure of 1500 hours. The thickness of metallost for the bare steel was measured to be 0.046 mm.Based on this value, on the one hand, the thickness lossrate indicated by the steel is calculated as 10.576 mpy.On the other hand, the thickness loss value for thecoated steel was 0.030 mm. A corresponding thicknessloss rate for the coating was found as 6.89 mpy.

3. XRD analysisThe XRD results for the uncoated and coated steel

after exposure to three environments are reported inTable IV. After exposure to air for 50 cycles, the Fe2O3

and Cr2O3 phases are found to be present in the oxidizedbare steel. In contrast, coated steel is found to have the

Table III. Values of Parabolic Rate Constant Kp (1028

g2cm

24s

21) for Coated and Uncoated T22 Steel Exposed to Air

and Molten Salt Environment for 50 Cycles at 973 K (700 �C)

SampleAir, 973 K (700 �C) Parabolic

Rate Constant (Kp)Molten Salt Environment, 973 K (700 �C)

Parabolic Rate Constant (Kp)

Uncoated SA-213 T22 steel 3.17 for first 4 cycles,10.3 6th to 10th cycle,5.59 10th to 32nd cycleand 8.34 32nd to 50th cycle.

0.27 for first 6 cycles to 5.09 for 6th to 8th cycle,followed by 3.91 for 10th to 32nd cycleand 4.23 32nd to 50th cycle.

Cr3C2-NiCr coated steel 0.066 behavior was not parabolic

Fig. 10—Weight change vs number of cycles plots for the uncoated and HVOF spray Cr3C2-NiCr–coated T22 boiler steel subjected to low-tem-perature superheater of the stage II boiler of GGSSTPP at 973 K (700 �C) ± 10 �C after 1500 h.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, AUGUST 2012—2985

formation of Cr2O3 and Cr7C3 as strong-intensityphases. CrNi, Cr, Ni, Cr3Ni2, and Cr3O4 are observedas the medium-intensity phases and Cr3C2 and Cr2C arethe weak-intensity phases. Furthermore, the mainphases identified from the X-ray diffraction patterns ofthe hot-corroded bare steel in the presence of Na2SO4-82pct Fe2(SO4)3 molten salt are Fe, Fe2O3 and Cr2O3. Thecoated steel is found to have Cr2O3 as a strong-intensityphase, NiCr2O4 as a strong-intensity phase, Fe2O3 andFeSO4 as medium-intensity phases, and Cr2C as a weak-intensity phase. After 1500 hours of exposure to thecoal-fired boiler, Fe2O3, Al2O3, and SiO2 are identifiedas the strong phases in the eroded-corroded steel. Cr2O3

and FeS are formed as medium-intensity phases. Incontrast, the coated steel contained CrNi as the mostprominent phase. Cr3C2, Cr, and Fe2O3 are formed asmedium-intensity phases and Cr2O3 and Al2O3 areformed as the weak-intensity phases.

C. SEM/EDS Analysis

1. Surface analysis of exposed samplesThe FE-SEM micrographs shown in Figure 11 depict

the morphology of the uncoated and coated steelspecimen subjected to cyclic oxidation. The oxide scaleon uncoated steel showed the dominance of Fe withsmall amounts of O, C, Cr, and S. The scale looks tohave an amorphous upper layer on a black matrix. Theupper layer has nodular structure, with irregular-sizednodules dispersed in the structure (Figure 11(a)). Themicrostructure of exposed coating was found to beuniform consisting of spongy nodules, which are rich inCr and O (Figure 11(b)), thereby indicating the forma-tion of a Cr2O3 oxide scale.

The FE-SEM/EDS analyses of the coated anduncoated steels after exposure to Na2SO4-82 pct Fe2(SO4)3salt environments are shown in Figure 12. The SEMmicrograph of the top surface of the scale in the case ofbare steel indicated the presence of some intergranularcracks and the scale is rich in Fe and O, which indicatesthe probable formation of Fe2O3 in the scale(Figure 12(a)). The exposed coating has a granularappearance in general. The grain size distribution isdifferent in different zones. In some zones, the grains arefine, whereas at some other locations, comparativelylarger spongy granules are found. The coating surface,in general, is rich in Cr and O (Figure 12(b)), whichreveals the formation of Cr2O3-rich scale. Fe and Ni arealso present in significant amounts at points 1 and 2.

A corresponding analysis of the bare and coated steelafter 1500 hours of erosion-corrosion in actual boilerenvironments is shown in Figure 13(a). The eroded-corroded surface of T22 steel indicates a granular scaleconsisting mainly of Fe and O with small amounts of Al,Si, Zn and C. The composition of the scale at point 2 isnearly similar to that at 1. The presence of Al and Zn inthe oxide scale represents the possibility of deposition ofsome ash particles on the sample surfaces, as theseconstituents belong to the boiler environment. An SEMmicrograph of exposed Cr3C2-NiCr–coated steel showsa massive scale consisting mainly of Ni and O at point 1with dispersed ash, rich in silica and alumina as shownin Figure 13(b). The composition at point 2 representsthe presence of mostly ash constituents.

Table IV. XRD Results of Uncoated and HVOF Spray Cr3C2-NiCr–Coated T22 Boiler Steel Subjected to Cyclic Oxidation

in Air, in Molten Salt, and in an Actual Boiler Environment

Samples Environment Strong Phases Medium-Intensity Phases Weak-Intensity Phases

Uncoated steel air Fe2O3 and Cr2O3 — —Coated steel Cr2O3 and Cr7C3 CrNi, Cr, Ni, Cr3Ni2,

and Cr3O4

Cr3C2 and Cr2C

Uncoated steel molten saltenvironment

Fe, Fe2O3, and Cr2O3 — —Coated steel Cr2O3 NiCr2O4 Fe2O3 and FeSO4 Cr2CUncoated steel actual boiler

environmentFe2O3, Al2O3, and SiO2 Cr2O3 and FeS —

Coated steel CrNi Cr3C2, Cr, and Fe2O3 Cr2O3 and Al2O3

(a)

70% Fe 12% C 10% O 04% Cr 02% S

85% Fe 07% O 05% C 01% Cr

50% Cr 40% O 05% C 02% Ni 02% Mn 01% Si

53% Cr 23% O 12% C 05% Ni 02% Si 02% Mn 02% Ca 01% Al

(b)

Fig. 11—Surface scale morphology and EDS analysis for the T22boiler steel subjected to cyclic oxidation in air at 973 K (700 �C) for50 cycles (a) in uncoated condition and (b) with HVOF spray Cr3C2-NiCr coating.

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2. Cross-sectional analysis of exposed samplesThe cross-sectional SEM/EDS analysis of steel sub-

jected to cyclic oxidation is shown in Figures 14(a) and(c). The oxide scale formed on the uncoated steel seemsto be bounded to the matrix loosely. The scale consistsmainly of iron and carbon along with significantamounts of molybdenum. On the one hand, someamounts of chromium and manganese are observed inthe scale also. On the other hand, the cross section ofcoated steel seems to be dense and intact with thesubstrate steel (Figure 14(d)). The exposed coatingcontains mainly chromium along with significantamounts of carbon, nickel, and oxygen, thus indicatingthe possibility of formation of oxides and carbides ofchromium in the coating. The substrate steel seems to beunaffected from the oxidation as oxygen concentrationat point 1 is only marginal (Figure 14(b)).

After molten salt corrosion, the scale formed on thesurface of uncoated steel has a discontinuous and looseappearance (Figure 15(c)). The scale consists mainly ofiron and carbon along with significant amounts ofoxygen. Some amounts of chromium and manganese areobserved in the scale also (Figure 15(a)). In contrast, thecross section of exposed coating seems to be dense andintact with the substrate steel. The point EDS analysisacross the cross section indicates the presence of mainlychromium and nickel along with significant amounts ofcarbon and oxygen, thus indicating the possibility of theformation of oxides, spinels, and carbides of chromium

and nickel. The substrate steel seems to be unaffectedfrom the corrosion, as the oxygen concentration at point1 and 2 is only marginal (Figures 15(b) and (d)).When exposed to a boiler environment, a thick oxide

scale is visible at the location of the SEM analysis on thebare steel (Figure 16(c)). The scale consists mainly ofiron and oxygen along with significant amounts ofcarbon and molybdenum (Figure 16(a)). On the con-trary, the cross section of the exposed coating seems tobe dense and adherent, which has retained its continu-ous contact with the substrate steel even after theexposure for 1500 hours (Figure 16(d)). The eroded-corroded coating contains mainly nickel and chromiumalong with significant amounts of carbon and oxygen,which indicates the possibility of the formation of oxidesand carbides of chromium and nickel. When nickel ispresent in richer amounts, Cr is in lower amounts, andvice versa. Small concentrations of manganese areobserved also throughout the thickness of the coating.Fe is confined mainly to the base steel. The substratesteel seems to be unaffected from the erosion-corrosionas the oxygen concentration at point 1 is only marginal(Figure 16(b)).

IV. DISCUSSION

In the current investigation, Cr3C2-NiCr powder wasdeposited successfully on T22 boiler steel by the HVOF

(a)

38% Fe 36% O 12% Cr 09% C 02% Mn 02% Si

71% Fe 14% O 06% C 03% Cr 01% Si 01% S 01% Mn 01% Na

28% O 24% Cr 18% Fe 18%Ni 07% C 03% S 02% Na

36% Cr 29% O 15% Fe 14% Ni 05% C

(b)

Fig. 12—Surface scale morphology and EDS analysis for T22 steelsubjected to cyclic oxidation in a Na2SO4-82 pct Fe2(SO4)3 moltensalt environment at 973 K (700 �C) for 50 cycles (a) in the uncoatedcondition and (b) with HVOF spray Cr3C2-NiCr coating.

(a)

(b)

38% Fe 29% O 09% Si 08% C 08% Al 07% Zn

48% Fe 28% O 08% Si 08% Zn07% Al

36% O 22% Si 19% Al 12% Cr 04% Zn 03% Ni 02% Fe

44% Ni 16% O 16% Al 10% Zn 09% Si 02% Cr 02% Fe

Fig. 13—Surface scale morphology and EDS analysis for T22 steelsubjected to a low-temperature superheater of the stage II boiler ofGGSSTPP at 973 K (700 �C) after 1500 h (a) in uncoated conditionand (b) with HVOF spray Cr3C2-NiCr coating.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, AUGUST 2012—2987

spray process. Microhardness is an important property,which is used to characterize the performance ofcoatings and have strong influences, for example onthe coating delamination, erosion-corrosion perfor-mance and residual stress state within the coatings.[26–29]

The observed microhardness values of the coating are ingood agreement with those reported by several otherresearchers.[30–34]

The as-sprayed coating possessed a typical splat-likemorphology, which is similar to that reported byMatthews et al.[35] during their long-term carbidedevelopment studies on HVOF spray Cr3C2-NiCr coat-ings on mild steel substrates at 1173 K (900 �C). So faras the XRD results are concerned, Crawmer et al.[36] andRusso and Dorfmann[37] also observed Cr7C3 andCr23C6 carbides in HVOF-sprayed Cr3C2-NiCr coating,in addition to Cr3C2 carbide. They suggested that thesecarbides may have formed through decarburization ofCr3C2. Li et al.

[38] also opined that the rebounding off ofthe Cr3C2 particles during coating formation is respon-sible mainly for the carbon loss and the change of

carbide content. During their studies on HVOF sprayedCr3C2-NiCr coating on 42CrMo6 steel, Guilemanyet al.[39] suggested that the Cr7C3 and Cr23C6 phasescorrespond to the decomposition process of the initialCr3C2 powder during its spray deposition. Degradationof the cermet composition during thermal sprayingoccurs through oxidation, decarburization, and carbidedissolution, the effects of which are retained in theas-sprayed coating as a result of the rapid solidificationof the material on impact.[40]

The uncoated steel conceived enormous weight gainsduring the course of oxidation and hot corrosion studiesalong with a substantial amount of spalling of its scales.The rate of oxidation was higher during initial cycles ofstudy, which might be attributed to the rapid oxygenpick up by diffusion of oxygen through the molten saltlayer. In contrast, the coating showed good adherence tothe boiler steel with no tendency for spallation duringthe cyclic oxidation in air. This indicates the effective-ness of the coating as the cycle-oxidation behavior of analloy is dictated mainly by scale spallation resistance as

Fig. 14—Cross-sectional SEM/EDS analysis for T22 boiler steel subjected to cyclic oxidation in air at 973 K (700 �C) for 50 cycles: (a) and (c) inuncoated condition, and (b) and (d) with HVOF spray Cr3C2-NiCr coating.

2988—VOLUME 43A, AUGUST 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

per the opinion of Stott.[41] The weight-change plotsshowed that the coating was successful to decrease theair oxidation rate of T22 steel. Moreover, the coatedsteel followed a parabolic rate law of oxidation. Thisindicates that the coating has shown the tendency to actas diffusion barrier to the oxidizing species (Singhet al.[42]), thus predicting the oxidation-resistive natureof the coating. When exposed to the salt environment,the coated steel showed the presence of some superficialcracks with no spallation at all. The weight change inthis case was relatively marginal; therefore, it can beinferred that the coating under investigation was suc-cessful in decreasing the hot corrosion rate of the steel.

During 1500 hours of the exposure in the boiler, theuncoated steel exhibited a fragile scale with intensecracking and spallation. Identical results have also beenreported by Wang,[43] where he observed severe scalingand spalling for 2.25Cr-1Mo steel during 1000 hours ofcyclic study at 1013 K (740 �C) in a medium-BTU coal-fired boiler. Singh[44] reported exceptionally high

erosion-corrosion rates for this steel. In contrast, nospallation was observed for the coated steel. The coatingwas successful to retain its continuous contact with thesubstrate steel, as shown in Figure 16(d). The overallweight change for the coating was slightly higher com-pared with the bare steel. However, it is possible that thecoating shows higher weight changes for several reasons,such as because of pores, voids, and splat boundariespresent in the coating microstructure, especially duringthe transient period of exposure. Moreover, duringexposure in the boiler environments, the probability ofattachment/detachment/embedment of ash particles ex-ists also, which can be a random process during cyclicstudies. In the erosion-corrosion environment, it isbelieved that the rate of erosion-corrosion (weight loss)might be higher during the initial cycles of the studybecause erosion is also happening in the micro-hillspresent on the surfaces of the samples, along withcorrosion. However, once these hills and valleys aresmoothened, only negligible erosion takes place because

Fig. 15—Cross-sectional SEM/EDS analysis for T22 boiler steel subjected to cyclic oxidation in Na2SO4-82 pctFe2(SO4)3 molten salt environ-ments at 973 K (700 �C) for 50 cycles: (a) and (c) in uncoated condition, and (b) and (d) with HVOF spray Cr3C2-NiCr coating.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, AUGUST 2012—2989

the harder scales were formed on the surface, which arenot easy to be eroded. Once the oxides are formed atthese places, the growth of scale becomes limited to thesurface of the coatings; hence, the weight change mightbecome nearly steady. However, it does not mean thatthe coating makes the situation worse. A coating can becalled useful if it can achieve a steady state of weightchange after the initial random weight changes, whichhas happened in this case. Second, it should be able tostop the depletion of the basic elements of the substratesteels so that the steels do not lose their mechanicalstrength. These facts show that the coating could beuseful to control the erosion-corrosion rate of the steelduring longer hours of exposure.

The XRD data (Table IV) revealed the presence ofmainly Fe2O3 and Cr2O3 in the oxidized steel. The EDSanalysis also confirmed the presence of an upper layer ofFe2O3. The formation of Fe2O3 has also been reportedby Lai,[45] where he observed an upper layer of Fe2O3

for oxidized iron-chromium alloys. Lai reported thatalloys with 2 pct chromium could form only the oxidesof chromium along with iron oxide in the innermostlayer, which is probably why only marginal amounts ofCr have been observed in the top scale of the steel by thesurface SEM/EDS analysis. The presence of Cr2O3 asrevealed by the XRD analysis for the steel mightperhaps be caused by spalling the top Fe2O3 layer ofthe scale. The presence of Cr7C3 in the coated steel afteroxidation might be to the result of the high affinity ofchromium toward carbon, which is well docu-mented.[46,47] The strong presence of Cr2O3 oxide inthe oxidized coating may inhibit oxidation of thesubstrate steel by blocking the diffusion of reactingspecies toward the substrate alloy, as has been suggestedby Nicoll and Wahl.[48]

During the exposure to a molten salt environment, thesteel was found to have Fe2O3 in its oxide scale. Thepresence of Fe2O3 indicates the presence of nonprotective

Fig. 16—Cross-sectional SEM/EDS analysis for T22 boiler steel subjected to cyclic erosion-corrosion in a low-temperature superheater of thestage II boiler of GGSSTPP at 973 K (700 �C) after 1500 h: (a) and (c) in uncoated condition, and (b) and (d) with HVOF spray Cr3C2-NiCrcoating.

2990—VOLUME 43A, AUGUST 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

conditions as per the suggestions of Das et al.[49] Thepresence of Cr2O3 in the scale along with Fe2O3 isconsistent with the findings of Sadique et al.,[50] whoreported that the Fe-Cr alloys in oxygen at hightemperature form spinel (FeCr2O4) and Cr2O3 on theinner side and Fe2O3 on the outside of the scale.Intensive spalling of the scale as observed can beattributed to severe strain developed because of Fe2O3

precipitation from the liquid phase and inter-diffusion ofintermediate layers of iron oxide.[51] A cross-sectionalview showing the probable hot corrosion mode for theuncoated T22 boiler steel exposed to a molten saltenvironment at 973 K (700 �C) for 50 cycles is shown inFigure 17.

In contrast, the XRD analysis (Table IV) of thecoated steel exposed to molten salt corrosion indicatedthe formation of mainly Cr2O3 and NiCr2O4. The high-temperature, corrosion-resistant nature of Cr2O3 oxideprevents oxidation of the whole coating. Liu[25] alsostudied the isothermal and cyclic oxidation kinetics ofseveral alloys and found that initially, the Cr2O3 layerwas formed selectively, and it functioned as a blockingbarrier to decrease the attack to the inner matrix of thealloys. Wang et al.[52] suggested that chromia is the bestoxide to resist hot corrosion in molten sulfates, as itreacts preferentially with O2- in molten sulfates to formchromate. The chromate will stabilize the melt chemistryand, consequently, will prevent the dissolution of theprotective oxide scale. Cr has the tendency to come up atthe surface of the alloys preferentially and form itsoxide, which grows by nucleation and growth, and ittends to come up as an outermost layer in the oxide

layers. A cross-sectional EDS analysis of the coatedsample confirms that the oxygen, by and large, has notreached the substrate steel (Figure 15(b)). The presenceof the spinel NiCr2O4 in the oxide scales also helps todevelop oxidation resistance as these spinel phasesusually have much smaller diffusion coefficients of thecations and anions than those in their parent oxides.[53]

The presence of Fe2O3 has been revealed in the coating,which indicates the probable diffusion of iron from thesubstrate steel to the coating, as the iron moves outwardalong the inter splat space at the coating substrateinterface. The presence of iron may be attributed topolishing effects as some iron particles are entrapped inthe space between the sample and the epoxy. It ispossible also that the Fe from the molten salt environ-ment can diffuse inside the coating and form Fe2O3

because the coatings generally have porosities andintersplat boundaries.When subjected to the actual boiler environment, the

formation of hematite (Fe2O3) as revealed by the X-rayanalysis of the steel might be caused by the reaction ofiron with oxygen because iron is the main constituent ofthe steel. The formation of oxides of iron and chromiumindicates that oxidation of the steel also takes place witherosion. The observed spallation in this case might beattributed partially to the erosion by the fly ashparticles. Brasunas[54] reported that even mildly abrasiveconditions may remove a corrosion film from a surface,which is protective to a substrate, thus exposing a freshmetal to corrode and thereby accelerate damage. Theformation of Al2O3 might be caused by the deposition ofash on the eroded-corroded tubes. A schematic diagram

Interface badly damaged. O present

in the higher concentrations in the

inner layers of the scale

Fe,

Cr,

Si a

nd

Mn

pre

sen

t

in t

he

sub

stra

te

Loose oxide scale

consisting mainly of

Fe and Cr Fe, Cr2O3 and Fe2O3

phases present in

the scale

Air, 700°C

Na

2 SO

4 -82%F

e2 (S

O4 )3

T22

ste

el

Fig. 17—Cross-sectional view showing the probable hot-corrosion mode for the uncoated T22 steel exposed to Na2SO4-82 pct Fe2(SO4)3 envi-ronments at 973 K (700 �C) for 50 cycles.

METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 43A, AUGUST 2012—2991

showing probable erosion-corrosion mode for theuncoated T22 boiler steel exposed to the actual boilerenvironment at 973 K (700 �C) for 1500 hours has beenreported elsewhere.[24]

A lesser thickness loss rate in the case of Cr3C2-NiCr–coated steel exposed to the actual boiler environmentmight be attributed to the compact and less porousstructure of its oxide scale. Fukuda and Kumonn[55]

suggested that the melting behavior of the coatingpowder is important to form a poreless and high-bond-strength coating layer. The more compact and lessporous the coating, the higher is its erosion-corrosionresistance.[56] The cross-sectional EDS analysis (Fig-ure 16(b)) showed that the substrate steel is unaffectedfrom the oxidation as the oxygen concentration at point1 is only marginal. Fe is observed to be confined mainlyto the base steel, which is a positive sign. It is wellknown that Cr3C2 particles impart the high erosionresistance and CrNi provides the necessary oxidationresistance, whereas the Cr2O3 oxide prevents subsequentoxidation of the coating. Therefore, from the ongoingdiscussion, it may be concluded that the HVOF-sprayedCr3C2-NiCr coating showed protective behavior in allthe three environments of the study.

V. CONCLUSIONS

1. The Cr3C2-NiCr coating, in general, was successfulto act as a reservoir for the formation of protectiveoxides/spinels and consequently may increase theservice life of the steel.

2. The oxide scale of T22 boiler steel showed a severetendency of cracking and spalling during all theenvironments of the study. On the contrary, theHVOF spray Cr3C2-NiCr–coated steel showed nosuch spallation. The coating was found to be intactduring the exposures.

3. The cross-sectional FE-SEM/EDS analysis of thecoating exposed to all the environments showsclearly that the oxygen penetration into the coatingwas only marginal, thereby indicating the effective-ness of the same to control the inflow of oxidizing/corroding species.

4. During the molten salt environment, a simultaneousformation of protective phases such as Cr2O3 andNiCr2O4 as strong phases may have contributed abetter high-temperature corrosion resistance to thecoating compared with that in the air environment.

5. The erosion-corrosion rate of the steel was found todecrease by 35 pct after the application of the coating.

ACKNOWLEDGMENTS

Harpreet Singh et al. thankfully acknowledge theresearch grant from Department of Science and Tech-nology, New Delhi (India) under SERC FAST Scheme(File No. SR/FTP/ETA-06/06, dated March 16, 2006)

to carry out this research and development work,titled ‘‘Development of Erosion-Corrosion ResistantThermal Spray Coatings for Power Plant Boilers.’’

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