Stepwise Depletion of Coating Elements as a Result of HotCorrosion of NiCrAlY Coatings

Nidhi Rana, R. Jayaganthan, and Satya Prakash

(Submitted July 6, 2013; in revised form October 7, 2013; published online November 9, 2013)

Present investigation deals with the hot corrosion behaviour of the NiCrAlY coatings deposited by HVOFtechnique on Superni76 under cyclic conditions at 900 �C in the presence of Na2SO4 + 60% V2O5 salt. Theweight change behaviour of the coatings was followed with time up to 200 cycles and Kp value wascalculated for the hot corrosion process. Surface and cross-section of the corroded samples were examinedby FESEM/EDS and XRD to follow the progress of corrosion up to 200 cycles. In earlier cycles, thecorrosive species oxidised top surface of the coatings. With increasing number of cycles, oxidation of thecoatings occurred up to 40-lm depth. A Cr-depleted band was seen below the oxide scale. Further increasein number of cycles led to migration and oxidation of Al to form Al2O3 sublayer at coating/scale interface,thereby leading to formation of Al-depleted zone in the coating below the Al2O3 sublayer. The corrosionresistance of the NiCrAlY coatings is attributed to the formation of the continuous and dense Al2O3

sublayer at the coating/scale interface, which acts as barrier to the migration of Cr to the surface. Theappearance of Al3Y after 100 and 200 cycles also contributes to the increased corrosion resistance ofcoatings after 100 and 200 cycles.

Keywords coatings, hot corrosion, oxidation, scale

1. Introduction

NiCrAlY coatings are widely used in gas turbines to protectthe base alloy from oxidation and corrosion. These coatings arespecially designed for protecting the base metal by forming theprotective oxides, which act as barrier for further migration ofthe corrosive species through the coating to alloy substrate.NiCrAlY coatings protect the alloy surface through formationof stable alumina layer upon oxidation. The oxidation behav-iour of the NiCrAlY coatings at high temperature has beenthoroughly investigated (Ref 1-6). The relative amount of Crand Al in the coating composition, temperature, and time areimportant factors to determine the nature of oxides formedduring oxidation as reported in the literature

S and Vare the common impurities present in the fuel whereasNaCl is ingested from air (Ref 7). At high temperature, they reactwith each other and form low melting eutectic mixtureNa2SO4 + 60% V2O5 (556 �C). These molten salts dissolve theprotective oxides formed over the coatings and increase theoxidation rates. Hence, understandingmechanisms of depletion ofprotective elements in the coatings due to hot corrosion is essentialto estimate the life time of the coating in a given environment.

The hot corrosion of different types of coatings have beenstudied in various environments (Ref 8-12).The hot corrosion

resistance of the Ni-based coatings has been reviewed by Sidhuet al. (Ref 13). They reported that the Ni-based coatings such asNiCr, Ni3Al, NiCrBSi, and NiCrAlY exhibit good corrosionresistance due to the formation of oxides and spinels of Ni, Cr,and Al.

Hot corrosion behaviour of the NiCrAlY coating exposedto different environments has been reported in literature(Ref 14-17). It prevents the formation of Al2O3 layer onsurface of the coatings and non-protective spinel of mixedoxides grows on the surface. Al gets depleted fast in thecoatings, and aggravates intrinsic chemical failure of thecoating (Ref 14). When the Al concentration in the coatingbecomes less than a critical value, the formation, and retentionof adherent protective scale is suppressed. Sidhu et al.(Ref 15) have investigated the cyclic oxidation of the NiCrAlYoverlay coatings in the presence of Na2SO4 + 60% V2O5 upto 50 cycles duration. They reported that these coatingsprotected the base superalloy by forming spinels, NiCr2O4 andNiO. In the presence of ceramic top coat (YSZ), thermallygrown oxide layer grows along the interface of bond coat andYSZ which is rich in Al2O3. The penetration of salt throughthe YSZ leads to the formation of YVO4 crystals, which leadsto spallation and ultimately delamination of the YSZ top coat(Ref 16). The dense nature of HVOF coatings resulted in theimproved hot corrosion resistance of the coatings as comparedto other techniques (Ref 14, 18, 19).

The stepwise depletion of alloying elements in the coatingsduring hot corrosion has not been reported in the literature sofar. Therefore, the mechanism of stepwise degradation ofHVOF-sprayed NiCrAlY coatings exposed to molten saltenvironment at high temperature has been investigated in thiswork. The compositional and morphological changes ofcoatings exposed to molten salt environment with differentexposure periods were characterized to understand the degra-dation mechanisms.

Nidhi Rana, R. Jayaganthan, and Satya Prakash, Department ofMetallurgical and Materials Engineering, Indian Institute ofTechnology Roorkee, Roorkee 247667 Uttarakhand, India. Contacte-mail: nidhiranathakur@gmail.com.

JMEPEG (2014) 23:643–650 �ASM InternationalDOI: 10.1007/s11665-013-0774-x 1059-9495/$19.00

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2. Experimental

2.1 Coating Formulation

Superni76 (Ni-19.79Fe-21.49Cr-9.05Mo-1.61Co-0.6W)was procured from M/s Midhani, Hyderabad, India. The rolledsheet was chosen as substrate material for depositing NiCrAlYcoatings using HVOF technique. The substrate samples ofdesired dimensions (209 159 3 mm3) were cut from the rolledsheets and polished using SiC abrasive papers to get theuniform and flat surface. Before deposition, these substratesamples were grit blasted by using irregular-shaped aluminaparticles at an angle of 90� ± 10�. The grit particles wereremoved from the surface by blowing air at high pressure.Commercially available NiCrAlY powder (Ni-bal, Cr-21%,Al-10%, Y-1%) obtained from Praxair Inc., US was sprayed byusing HVOF gun (JP5000). The combustion of the fuel wascarried out in the presence of oxygen and air flow. The pressureof the oxygen, air, and fuel was 1.44, 0.34, and 1.17 MPa,respectively. The distance between gun and sample was kept0.38 m and the powder was injected at a feed rate of2.59 10�3 kg/s. The detailed parameters and the propertiesof the as sprayed coatings have been reported in our previousstudy (Ref 20).

2.2 Hot Corrosion Test

Cyclic hot corrosion tests on the coatings were carried outfor 200 cycles at temperature 900 �C. A layer of the salt(Na2SO4-60% V2O5) with amount equal to 3-5 mg/cm2 wasapplied uniformly on the sample surface by using camel paintbrush. The samples were preheated at 250 �C before coatingof salt for better adhesion on the surface. After applying salts,the samples were put into the SiC tubular furnace, which wasmaintained at 900 �C. The samples were maintained at900 �C for 1 h and subsequently taken out from the furnace,kept in air for 20 min to attain room temperature. Aftercooling, the weight change of the sample was measured andagain it was put back into the furnace at 900 �C for nextcycle. The weight change was measured for three samples andan average curve was obtained between number of cycles andspecific weight change. The spalled scale was also included inthe weight measurement. Similar experiment was also carriedout for a set of three samples. Each sample was subjectedto hot corrosion for 5, 50,100, 200 number of cycles so as toget the insight in to corrosion mechanism under differentexposure period.

2.3 Characterisation Techniques

The oxidation products of the coated samples wereanalysed by using X-ray diffraction (Bruker AXS D-8Ad-vance Diffractometer with CuKa radiation) and FESEM/EDS (FEI Company, Quanta 200F). The cross-section of thecoatings was prepared by cutting the samples in transversedirection and then polishing by using standard metallo-graphic techniques. The BSEI was used to analyse thecross-sections of the corroded samples. On the other hand,the surface morphology and composition of as corrodedcoatings was characterised without any damage to thesample

3. Results

3.1 Visual Observations

After first cycle, the surface colour of coatings haschanged from light yellow to brownish. Greenish oxidecolour was observed with some brownish areas afterexposure of five cycles. Spallation started after fifth cycleand it increased rapidly up to 25 cycles. The whole surfacewas covered with green scale and the spalled scale was alsogreen. The spallation continued till 80 cycles. The macro-graphs of the corroded samples after different cycles areshown in Fig. 1.

3.2 Weight Change Measurements

The weight change curve is shown in Fig. 2(a). The weightchange increases sharply up to 20 cycles, after which itdecreases. The region up to 20 cycles follows the linear curve,whereas after 20 cycles, the curve started following paraboliccurve. The weight change becomes almost constant after 100cycles. The parabolic rate constant (Kp) has been calculatedfrom 30 cycles to 100 cycles as initially the curve follows thelinear equation only and after 100 cycles, weight gain becomesalmost constant. The Kp has been calculated from the leastsquare fit method as shown in Fig. 2(b).

3.3 XRD Analysis

The XRD patterns for the as received and corroded coatingsare shown in Fig. 3. As received coatings shows the peakscorresponding to the Ni3Al and NiAl phases. The patternsobtained after five cycles of exposure shows the presence of thephases such as Ni3V2O8, YVO4, NiCr2O4, and Cr2O3. Theintensity of the peaks corresponding to the coatings also gotdiminished after five cycles. After 50 cycles, the peakscorresponding to NiCr2O4 and Cr2O3 become sharp butNi3V2O8 peak got diminished. After 100 cycles of exposure,the peaks pertaining to spinels and Cr2O3 became prominentwith other phases such as YVO4, Al2O3, and Al3Y. Peakscorresponding to Al2O3 an Al3Y further grow in intensity after200 cycles of exposure.

3.4 Morphology and Composition of the Oxide Scales

The morphology and composition of the surface scale isshown in Fig. 4. After five cycles, the main elements observedon the surface are V, O, and Ni. Hence, the surface is composedof the vanadium compounds like Ni3V2O8, which is inaccordance with XRD results. The smaller amount of Cr, Al,and Y are also present, which correspond to the spinels, Cr2O3,and YVO4. Two types of crystals such as elongated and globulecrystals were observed as shown in Fig. 4(b). The elongatedcrystals contain V, O, and Ni as indicated by EDS results.Hence, it corresponds to Ni3V2O8 and the other one is oxide ofCr and Al. However, the elongated crystals have almostdisappeared after 50 cycles and whole surface is covered withthe Cr- and Al-rich oxides (Fig. 4c). These oxides correspondto spinels and Cr2O3 as indicated by XRD results. The Y is alsopresent on the surface as YVO4. The crystals of YVO4 can beobserved at higher magnification as shown in Fig. 4d. Such rodtype crystals of YVO4 have also been reported by Sreedhar

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et al. (Ref 21) in their study on NiCrAlY + YSZ coatings. Thesurface morphology and composition after 100 and 200 cyclesis almost same and is slightly different from after 50 cycles ofexposure (Fig. 5a, b).

3.5 Cross-Section Analysis of Scale

The cross-sectional images of the corroded coatings areshown in Fig. 6. After five cycles, as shown in Fig. 6(a),corrosion of the coatings has occurred along the splatboundaries. There are some corroded regions, where theconcentration of V is high. The x-ray mapping of the scaleafter five cycles (Fig. 7a) and the EDX (Table 1) indicates thatthe V is present along the splat boundaries inside the coating.Hence, it can be inferred that V is reacting mostly along thegrain boundaries and forming the vanadates of Ni (Ni3V2O8) asconfirmed by XRD results also. However, the corrosive saltsare found to be highly reactive and corroded the rest of theaffected coating in the subsequent cycles. The cross-sectionimages after 50 cycles indicate the formation of complete oxidescale up to affected region. The scale is almost 50-lm thick andis porous too.

A distinct region can be observed inside the coating after 50cycles where the Cr has depleted as shown in x-ray map andEDS results (Fig. 7b; Table 1). This Cr-depleted band is almost40 lm in depth. The composition of the Cr-depleted zone wasnoted at five points and the average value of the Cr was foundto be 10 wt.% instead of starting value of 20 wt.%. The Alconcentration in this zone has also lowered from 10 to 8 wt.%.

After 100 cycles of exposure, the Al-depleted zone can beobserved just below the oxide/coating interface (Fig. 6c). TheAl-depleted region can also be observed in x-ray mapping(Fig. 8a).This Al-depleted zone lies within the Cr-depletedregion. However, the Cr concentration increases where Al is

Fig. 1 Macrographs of the samples after oxidation in the presence of Na2SO4-60% V2O5 at 900 �C for (a) 5 cycles (b) 10 cycles (c) 25 cycles(d) 50 cycles, and (e) 200 cycles

Fig. 2 (a) Weight change curve for NiCrAlY coating oxidized in the presence of Na2SO4-60% V2O5 at 900 �C. (b) Square of weight change curve

Fig. 3 XRD pattern of NiCrAlY-coated samples oxidized after dif-ferent number of cycles in the presence of Na2SO4-60% V2O5 at900 �C

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Fig. 4 Surface morphology and composition of NiCrAlY coatings after oxidation in the presence of Na2SO4 + 60% V2O5: (a) after 5 cycleswith area composition of the shown surface, at 9500 (b) after 5 cycles with point composition of the arrowed regions, at 92000, (c) after 50cycles with area composition of the whole surface, at 9500 (b) after 50 cycles with point composition of the arrowed regions, at 92000

Fig. 5 Surface morphology and area composition of given area of NiCrAlY coatings after oxidation in the presence of Na2SO4 + 60% V2O5

(a) after 100 cycles (b) after 200 cycles

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depleted (Table 1). Hence, the Cr-depleted region gets reducedfrom 40 to 30 lm and shifted to just below the Al-depletedzone. The concentration of Al gets lowered from 10 to 3 wt.%(Table 1). The Al is getting oxidized continuously and Al2O3

layer can be seen just below the mixed oxide layer. This type ofduplex layer has also been reported by earlier study on theMCrAlYSiB coatings at 900 �C in NaCl salt (Ref 22).Withfurther increase in number of cycles, the Al-depleted zoneincreases as Al is now getting continuously oxidised at theoxide boundary. The Cr-depleted zone shifted just below theAl-depleted zone as can be seen from x-ray maps (Fig. 8b) andEDS results (Table 1).

4. Discussion

It is confirmed from the XRD and EDS results that theoxides grown on the surface, after five cycles, are mainlyNiV2O8 and YVO4 and lesser amount of the spinels and Cr2O3.It implies that initially the salt mixture has reacted chemically

with Ni and formed the corresponding vanadate, which iscorrosive in nature. The formation of the NiV2O8 in the earlystage of hot corrosion of the NiCrAl alloy in the presence ofNa2SO4 + V2O5 has also been reported in the earlier study(Ref 23).

The presence of YVO4 has also been reported in thecoatings containing Y2O3 addition or the top coat of the YSZ(Ref 16, 21). The Y is added to the NiCrAlY coating to increasethe spallation resistance of the scale, by the formation of theY2O3, which act as barrier for migration of the coating elements(Ref 24). In this wok, the formation of the YVO4 in early stagesof corrosion indicates that the Y is oxidised rapidly to formY2O3 and subsequently reacting to form YVO4.

The formation of the YVO4 resulted in the deleterious effectof Yon the corrosion resistance. The rod-shaped YVO4 crystalsin the oxide scale may act as stress raiser due to which the scalebecome prone to the cracking and spallation (Ref 16, 21).However, after 50 cycles, the concentrations of Cr and Al onthe surface increases and surface is covered with the spinels andCr2O3 as also indicated by XRD results. The Cr-depleted zoneobserved after 50 cycles confirms that Cr is diffusing from

Fig. 6 BSEI of cross-section of NiCrAlY coating (a) 5 cycles, penetration of V along grain boundaries of the coating (b) 50 cycles appearanceof Cr-depleted region inside the coating (c) 100 cycles, formation of Al-rich layer at the coating/scale interface due to migration of Al and (d)200 cycles, growth of Al2O3 layer with depletion of more Al from the unoxidized coating

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coating and getting depleted in the hot corrosion process. Thereaction through which the Cr2O3 layer dissolves inNa2SO4 + V2O5 salt has been already reported by Swamina-than et al. (Ref 25).

As number of cycles increases, the Al2O3 layer has startedforming at the coating/scale interface below the porous layer ofthe oxides. Al forms a stable and slow growing oxide at loweroxygen pressure. The porous scale formed in initial cyclesbecame the path of the migration of the O to form Al2O3

sublayer. The formation of this Al2O3 layer resulted in theformation of Al-depleted zone just below the oxide layer insidethe coating. With further increase in exposure time, the width ofAl-depleted zone increases and the Cr-depleted zone getsshifted below the Al-depleted zone. It implies that as Al isgetting consumed in the formation of the Al2O3; Cr is getting

diffused from the bulk towards the oxide layer. However, thismigration of Cr is being hindered by the Al2O3 layer at theinterface and hence the Cr-rich region existed just below theoxide scale. Hence, it can be inferred that the Al2O3 sublayer isacting as a diffusion barrier for the outside migration of the Crto the surface as also observed in EDS results, indicating thedepletion of Cr decreased after 100 cycles. The weight changecurve also indicates that the rate of corrosion has decreasedafter 100 cycles. The formation of slow growing alumina scaleand its role against hot corrosion has been also reported in theliterature (Ref 22, 26). However, in this study, the differentsteps were observed prior to the formation of Al2O3 sublayer atinterface. The cross-sectional images along with x-ray mapsobtained after 5 and 50 cycles indicates that the Al2O3 sublayerdoes not exist upto 50 cycles.

Fig. 7 x-ray mapping of NiCrAlY coatings after (a) 5 cycles, indicating penetration of V along the splat boundaries of the coatings (b) 50cycles, indicating Cr-depleted band in the coating and Cr-, Al-rich oxide layer

Table 1 Elemental composition (EDS) of the various regions in the cross-section of the corroded samples (average of fivepoints)

Elements

Composition in wt.%

After 5 cycles(Point 1) Fig. 5(a)

After 50 cycles(Point 1) Fig. 5(b)

After 100 cycles Fig. 5(c) After 200 cycles Fig. 5(d)

Point 1 Point 2 Point 3 Point 1 Point 2 Point 3

O 7.10 0.71 1.44 0.96 19.45 2.98 21.15 1.87Al 9.28 8.12 2.67 8.45 45.67 3.02 47.37 6.74Cr 17.36 10.13 25.23 11.23 15.23 14.56 13.23 8.97Ni 26.15 71.89 68.98 76.78 10.15 78.32 8.15 79.56Y 3.13 1.14 0.97 1.03 4.13 1.13 3.13 0.87V 34.89 0.35 0.46 0.66 1.34 0.18 1.34 0.13

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It has been reported by Bouaeshi et al. (Ref 27) that theAl3Y refines the Al microstructure and consequently increasesthe corrosion resistance of the metal. Hence, the lowercorrosion rates after 100 cycles, observed in this work, canalso be attributed to the formation of Al3Y along with the Al2O3

as evident from the XRD results.From 100 to 200 cycles, the Al is getting consumed as the

Al-depleted zone in the unaffected coating increases from 10 to30 lm. The Cr starts accumulating in the Al-depleted zone andthe Cr-depleted zone again moves further away from thecoating/scale interface. Finally, only two distinct regions exist,one depleted in Al and other in Cr. This analysis support the

fact that even after 200 cycles, the coating has sufficient Al leftfor the formation of the protective Al2O3 layer. This sublayer isformed by the migration of O through the Al2O3 layer, which isthe slowest step in whole corrosion process. Hence, the rate ofcorrosion further decreased from 100 to 200 cycles as observedin weight change curve. The various steps involved in the hotcorrosion of the NiCrAlY coatings in the presence ofNa2SO4 + V2O5 salt at 900 �C are shown schematically inFig. 9.

The first step up to five cycles corresponds to the penetrationof the V inside the coating and formation of the Ni-basedcorrosive vanadate. The oxides like Cr2O3 and spinels also

Fig. 8 x-ray mapping of NiCrAlY coatings after (a) 100 cycles, narrower Cr-depleted band and Al migrating to coating/scale interface to formAl-rich layer at the coating/scale interface (b) 200 cycles, showing Al-rich layer at the coating/scale interface with Al-depleted region below theinterface

Fig. 9 Schematic diagram showing the various steps involved in the hot corrosion of the NiCrAlY coatings at 900 �C

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form on the surface of the corroded coatings. The second stepinvolves the appearance of the Cr-depleted zone inside thecoating just below the coating/scale interface. The Cr getsdepleted due to the continuous dissolution of Cr2O3 in thecorrosive salt. The thickness of the mixed oxide layer increasesdue to the rapid oxidation of the coating elements specially Crup to 50 cycles. In next step, the Al2O3 layer started forming atthe coating/scale interface leading to the formation ofAl-depleted zone just below the coating/interface. The last stepinvolves the thickening of Al2O3 layer at the interface due toinward migration of the oxygen through the Al2O3 sublayer.

5. Conclusion

1. The NiCrAlY coatings were oxidised at 900 �C in thepresence of Na2SO4 + V2O5 for different exposure peri-ods up to 200 cycles. The observed corrosion resistanceof the NiCrAlY coatings was due to the formation ofAl2O3 sublayer at the coating/scale interface.

2. The different steps were observed during the hot corro-sion process of the NiCrAlY coatings due to the forma-tion of different oxides at the different depth of the oxidescale.

3. During initial cycles (up to five cycles), the salt reactwith coatings and forms corrosive compound, Ni3V2O8.Y also reacts with vanadium to form YVO4.

4. In subsequent cycles, Cr2O3 forms and dissolves in thecorrosive salts and spallation of sale was observed. Con-tinuous dissolution of Cr2O3 resulted in Cr-depletedregion in the coating.

5. The Al started oxidizing at the interface due to migrationof O through the porous oxide layer. Hence, Al-rich layerwas observed at the interface. This oxidation of Al resultedin Al-depleted region just below the interface after 100cycles. The Al2O3 sublayer provided the desired protectionby inhibiting the outward migration of the Cr to the sur-face. The appearance of Al3Y phase after 100 cycles alsoincreased the resistance to the hot corrosion.

6. From 100 to 200 cycles, minimum rate of the corrosionwas observed as O migration through the Al2O3 sublayerwas only mechanism by which the scale was growing.The diffusion of O through the Al2O3 sublayer corre-sponds to the slowest step during hot corrosion processas observed in this work.

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