Corrosion Behavior of Bare, Cr 3 C 2 -25%(NiCr), and Cr 3 C 2 -25%(NiCr)+0.4%CeO 2 -Coated Superni 600 Under Molten Salt at 900 °C

Corrosion Behavior of Bare, Cr3C2-25%(NiCr),and Cr3C2-25%(NiCr)+0.4%CeO2-Coated Superni 600

Under Molten Salt at 900 �CDeepa Mudgal, Sanjeet Kumar, Surendra Singh, and Satya Prakash

(Submitted October 28, 2013; in revised form July 7, 2014)

Cr3C2-25(NiCr) and Cr3C2-25(NiCr)+0.4%CeO2 coatings were deposited on nickel-based superalloy Su-perni 600 by Detonation-gun technique. Studies were conducted on bare and coated alloys in molten saltenvironment (Na2SO4-25%NaCl) at 900 �C under cyclic condition. Characterization of the corrosionproduct was done using field emission scanning electron microscopy/energy dispersive spectroscopy andx-ray diffraction techniques. The bare Superni 600 shows penetration of corrosion beneath the metal layerthereby indicating internal oxidation. The coating of Cr3C2-25(NiCr) with 0.4%CeO2 leads to the formationof adherent scale.

Keywords Cr3C2-25(NiCr) coating, detonation-gun, rare earthelements

1. Introduction

Rapid urbanization is creating a serious problem regardingwaste disposal. Utilizing this waste in incinerators to generateelectric power is found to be a viable solution. However, thewaste including both biological and domestic on burninggenerates a very corrosive environment. It is important to studythe behavior of various alloys and coatings for these aggressiveenvironments. Sulfates and chlorides are generally present inincinerator and biomass fuel fired boilers (Ref 1, 2). Metals andalloys may experience accelerated corrosion when their sur-faces are coated by a thin film of fused salt in an oxidizing gas.This mode of attack is called hot corrosion (Ref 3). Hotcorrosion is generally dependent on temperature, the maximumeffect lies in the temperature range of 800-1000 �C. Maximumattack occurs at around 900 �C. At lower temperatures, theoxidation rates are slower due to reduced ionic diffusion rates,while at higher temperatures the reduction may be due to thefact that the chlorides may become more volatile and evaporatefrom the surface (Ref 4).

High temperature thermal spray coatings are generally usedfor enhancing the life of materials used in corrosive environ-ments such as incinerators, bio-fuel fired boilers and turbines.Detonation-gun is one of the thermal spraying technique knownfor providing hard, wear resistant and dense microstructuredcoatings (Ref 5). It can be considered that D-gun coating is thebest technique from the viewpoint of coating density and its

porosity (Ref 6). Hence D-gun technique is widely used forthermal spray process to deposit coatings on heat facingcomponents of the large units such as turbine, power plant,chemical plants or incinerators.

Cr3C2-NiCr coating is widely used commercially because ofits high temperature erosion and corrosion resistance properties.Due to the high corrosion resistance of NiCr binder, Cr3C2-NiCr coatings can be used in corrosive environment (Ref 7) andpresence of carbide particles provide sufficient erosion andwear resistance. Much study was reported on friction, wear, anderosion behavior of D-gun-sprayed Cr3C2-25(NiCr) coating(Ref 8-16).

It was well established that the addition of rare earthelements refines microstructure, increases hardness, abrasivewear resistance as well as oxidation resistance of the coatings(Ref 17-23). Rare earth addition increases the adhesion betweenscale and alloy substrate which enhances the alloy�s resistanceto thermal cycling exposure. In some cases, the actual growthrate of the oxide found to be reduced (Ref 24). Addition ofreactive elements also alters the species that diffuse predom-inantly and leads to growth of Cr2O3 mainly by O diffusion(Ref 25). Yan et al. (Ref 26) investigated the protective role ofCeO2 dispersion strengthened chromium coating developed onlow carbon steel. It was concluded that oxidation resistance ofCeO2-dispersed coating was increased which related to thesufficient supply of chromium from the coating maintainedduring oxidation in atmosphere. Ogawa et al. (Ref 27) studiedthe effect of Ce and Si addition to CoNiCrAlY bond coatmaterials on its oxidation behavior and crack propagation ofthermal barrier coatings. They concluded that rare earthaddition drastically changed the morphology of coating andimproved the crack resistance. Wang et al. (Ref 28) discussedthe effect of rare earth addition in Ni-based powder which waslaser-clad on steel substrate. They found that the frictioncoefficient of coatings and wear resistance of the coatings wereenhanced significantly. Pillis et al. (Ref 29) also reported thatthe rare earth elements are often added to chromium dioxideforming alloys to improve its high temperature oxidationresistance. Results have been reported regarding the oxidation

Deepa Mudgal, Sanjeet Kumar, Surendra Singh, and SatyaPrakash, Department of Metallugical & Materials Engineering,Indian Institute of Technology, Roorkee, Uttrakhand, India. Contacte-mail: [email protected].

JMEPEG �ASM InternationalDOI: 10.1007/s11665-014-1177-3 1059-9495/$19.00

Journal of Materials Engineering and Performance

behavior of 0.4 wt.%CeO2 incorporated NiCrAlY coatingsprayed using D-gun and HVOF process. Mahesh et al.(Ref 30) and Kamal et al. (Ref 31) reported that the incorporationof CeO2 in NiCrAlYpowder has contributed to the developmentof adherent oxide scale in the coating at elevated temperature andprovided the better oxidation resistance.

Studies have already been reported on the subject of the hotcorrosion behavior of Cr3C2-(NiCr) coating in Na2SO4-V2O5

(Ref 32-34), Na2SO4-25K2SO4 (Ref 35, 36), and Na2SO4-82Fe2(SO4)3 (Ref 37, 38) environments. As per open literature,no results have been found regarding the effect of CeO2 in

Cr3C2-25(NiCr) coating in Na2SO4-25%NaCl environment.The present paper will show the effect of CeO2 in chlorinecontaining environment at 900 �C. The environment chosensimulates the environment present in energy generating unitssuch as incinerators, boilers, and gas turbines (Ref 39-42). Thetest temperature preferred for the study is 900 �C as thistemperature is close to that which provokes the greatest attackwith Na2SO4 (melting point 884 �C) (Ref 43).

2. Experimental

2.1 Substrate Materials

Ni-based superalloy Superni 600 (SN600) (Cr-15.5, Fe-10(max), Mn-0.5, C-0.2, and Ni as balance) conforming tointernational specification ASTM B 166, B 168, DTD 328A hasbeen procured from Mishra Dhatu Nigam Ltd, Hyderabad,India in the form of rectangular sheets in hot rolled andannealed condition.

2.2 Coating Powder

Commercially available Cr3C2-25(75Ni 25Cr) (Praxair)coating powder of particle size 20-45 lm was deposited onthe substrate using D-gun technique. The coating powdermorphology is spherical and spongy as shown in Fig. 1(a). Theparticle shape of CeO2 is cylindrical with particle size less than5 lm as shown in Fig. 2. A mixture was prepared taking0.4 wt.%CeO2 (99.99% purity) and Cr3C2-25(NiCr) powderand was dry-ball milled in a conventional rotating ball mill withstainless steel balls as a milling/grinding medium for 8 h to

Fig. 1 FESEM micrograph of coating powders (a) Cr3C2-25(NiCr), (b)Cr3C2-25(NiCr) +0.4 wt.%CeO2, and (c) Cr3C2-25(NiCr)+0.4 wt.%CeO2

with EDS analysis

Fig. 2 FESEM of the Ceria powder


obtain the homogenous powder for the deposition of coatings.The morphology of the powder after milling is shown inFig. 1(b) and (c). The shape (spherical and spongy) and size(20 to 45 lm) of the powder were found to be unaffected aftermixing.

2.3 Formulation of Coating

The specimens of size 20 mm9 15 mm9 5 mm were cutfrom the sheet and polished using 220, 320, 400, 600, and 800grit size emery paper followed by cloth polishing using aluminapowder (0.3 lm) suspension. Specimens used for coating werealso polished so as to keep the condition of the specimen same.Polishing helps in removing any pre-oxidized film or scalepresence in as-received specimen. Then, the samples weredegreased with acetone and grit blasted to roughen the surfaceof the specimen using alumina powder just before coating thesubstrate. Detonation-gun process was used to deposit thecoating powder at SVX powder M Surface Pvt. Ltd, GreaterNoida, India. Parameters set by the firm for coating are given inTable 1. All the process parameters, including the spraydistance, were kept constant throughout the coating process.Around 150-200 lm approx. thick coating was deposited onthe six faces of the alloy sample with detonation-gun.

2.4 Characterization of Coatings

Micrograph for all the specimens was seen under fieldemission scanning electron microscope (FESEM) FEI com-pany, Quanta 200F (Accelerating voltage 20 kV) attached withthe Oxford, X-max for EDS analysis. Secondary electron

images (SEI) were obtained for surface analysis while back-scattered electron images (BSEI) were recorded for cross-sectional surfaces. The elemental composition analysis ofcoatings were carried out using an energy dispersive x-rayanalysis (EDS) (OXFORD, X-Max) with variation of±0.3 at.% for heavy elements and ±2 at.% for light elements.EDS analysis is based on ASTM-E1508-12a standard. Toidentify the phases, XRD test was performed using BrukerAXS D-8 advance diffractometer (Germany) with Cu Karadiation and nickel filter at 30 mA under a voltage of 40 kV.The specimens were scanned with a scanning speed of 1 kcpsin 2h range of 10�-80� and the intensities were recorded with1�/min as Goniometer speed. The ‘‘d’’ values corresponding toeach peak along with its 2h values were indicated by defaultwith the XRD software DIFFRACplus on the diffractograms.The peaks were then identified using standard pattern X-pert Hiscore software which was confirmed from JCPDS (JointCommittee on Powder Diffraction Standards) cards. Forcross-sectional analysis, the coated samples were cut verticallyand mounted in transoptic resin. The mounted samples werethen polished using 220, 320, 600, and 800 emery paperfollowed by alumina cloth polishing. After that, the sampleswere gold coated and pasted on aluminum stub. Silver pastewas applied from the edge of the sample till the edge of the stubso as to make it conducting.

2.5 Hot Corrosion Experiment

Hot corrosion studies were carried out for bare and coatedspecimens for comparison. The dimensions of the sample weretaken using digital vernier caliper (resolution 0.01 mm) tocalculate the surface area of the substrate. The substrates werewashed with distilled water and cleaned with acetone. Aftercleaning the samples were kept in the furnace at 230 �C beforeapplying the salt solution. The salt slurry of Na2SO4-25%NaClwas prepared by thoroughly mixing it in distilled water and a layerof slurrywas applied (3-5 mg/cm2) uniformly on the samples. Thesamples were exposed to cyclic hot corrosion in a silicon carbidehorizontal tube furnace at 900 �C for 100 cycles. Each cycleconsisted of 1 h of heating followed by 20 min of cooling inambient air. During the experimentation, each sample was kept in

Table 1 Parameters of detonation-gun

Parameters Cr3C3-NiCr coating

Oxygen/acetylene flow rate 1:1.21Carrier gas flow rate N2, m

3/L 0.96Frequency, shots/s 3Diameter of spot (shot size), mm 20Spraying distance from Nozzle, mm 165Powder flow rate (gram shot) 1-2

Fig. 3 FESEM/EDS spectrum of as-sprayed Cr3C2-25(NiCr)+0.4 wt.%CeO2 coating


an alumina boat and the weight of the boat and specimen wasmeasured, thisweightwas taken as the initialweight for the sampleand weight change was noted down after every cycle usingweighing balance of 1 mg accuracy to determine the corrosionkinetics using weight change measurements.

3. Results

3.1 Surface Morphology of D-Gun As-Sprayed Coating

FESEM micrograph along with EDS analysis of as-sprayedD-gun-sprayed Cr3C2-25(NiCr)+0.4 wt.%CeO2 coating isshown in Fig. 3. It can be clearly observed that the coatingformed is dense and adherent with melted and semi-meltedparticles. FESEM/EDS micrograph of polished D-gun-sprayedCr3C2-25(NiCr)+0.4 wt.%CeO2 coating is shown in Fig. 4. Thelight gray phase corresponds to NiCr matrix and dark gray phaserepresents Cr3C2 particles as can be inferred fromEDS spectrum.CeO2 is found to be distributed along the particle boundaries.

3.2 Visual Analysis

Figure 5 shows the macrophoto of the specimens after 1st,50th, and 100th cycles subjected to hot corrosion underNa2SO4-25%NaCl environment at 900 �C. The bare alloyunderwent minor spallation upto 10th cycle whereas in case ofCr3C2-25(NiCr)-coated alloy microspallation in form of powderstarted after 10th cycle which got intensified till 50th cycle. Thespallation at the edges started after 27th cycle which continuedtill 50th cycle. After 50th cycle, no spallation was observed.The color of the oxide formed just after 1st cycle was dark gray

with green patches which remained till 100th cycle. While incase of Cr3C2-25(NiCr)+0.4 wt.%CeO2-coated Superni 600,green color oxide with dark gray background was formed onthe surface just after 1st cycle which got intensified till 10thcycle. After 10th cycle, the color of the oxide did not changedtill 100th cycle. Very little spallation occurred in form ofpowder after 20th cycle which continued upto 35th cycle. Butafter 35th cycle no spallation was observed.

3.3 Weight Change Measurements

Weight gain per unit area versus number of cycles graph wasplotted for bare Superni 600, Cr3C2-25(NiCr)-coated Superni600, and Cr3C2-25(NiCr)+0.4 wt.%CeO2-coated Superni 600after hot corrosion at 900 �C under molten salt for 100 cycles(Fig. 6a). From the graph, it can be inferred that bare Superni600 underwent marginal weight gain till 10th cycle followed byminor gradual weight loss in the subsequent cycles. While incase of Cr3C2-25(NiCr)-coated Superni 600 and Cr3C2-25(NiCr)+0.4 wt.%CeO2 Superni 600, the rate of weight gainobserved was very high during initial cycles and became nearlyconstant after about 35th cycle. It can be inferred from weightchange (mg2/cm4) versus number of cycle plots that Cr3C2-25(NiCr)-coated Superni 600 and Cr3C2-25(NiCr)+0.4 wt.%CeO2 Superni 600 follow nearly a parabolic rate law. Theparabolic rate constants (Kp in 10�10 g2/cm4/s) for both thecorroded coated specimens are given in Table 2. R2 defines thegoodness of linear fit of trend line to the data and its range liesbetween 0 and 1. Cr3C2-(NiCr)-coated Superni 600 hasR2 = 0.62 (Fig. 6b) which means that it follows 62% linearrate law of oxidation and 38% parabolic rate law of oxidationwhereas CeO2-doped coating has R2 = 0.64, which indicates

Fig. 4 FESEM/EDS of polished detonation-gun-sprayed Cr3C2-25(NiCr) coating


that CeO2-doped coating follows 64% linear rate law and 36%parabolic rate law.

3.4 Surface Scale Analysis

FESEM micrographs along with EDS analysis at someselected sites of interest of the hot-corroded bare, D-gun-sprayedCr3C2-25(NiCr)-coated, and Cr3C2-25(NiCr)+0.4 wt.%CeO2-coated Superni 600 are shown in Fig. 7, 8, and 9, respectively.The scale formed on the Bare Superni 600 after 100 cycles showsthe presence of porous and fragile oxide consists of Cr, Ni, and Oin major amounts and minor amounts of Fe, Mn, and Na. On theother hand, the scale formed on D-gun-sprayed Cr3C2-25(NiCr)and 0.4 wt.%CeO2-incorporated coating on Superni 600 were

adherent and compact containing mainly Cr and O along withminor amount ofMn. In CeO2-incorporated coating, Ce was alsoappeared as an additional element.

3.5 Phase Identification

The x-ray diffractogram profiles for phase identification ofthe scales formed on bare, D-gun-sprayed Cr3C2-NiCr withoutand with 0.4 wt.%CeO2-coated Superni 600 after hot corrosionin Na2SO4+25%NaCl environment for 100 cycles at 900 �C areshown in Fig. 10. The identified major phases on the surface ofcorroded bare Superni 600 were Cr2O3 and NiCr2O4 along withthe minor phases of Ni, NiO, and NiS. On the other hand, incase of corroded Cr3C2-(NiCr)-coated Superni 600, the majorphases identified were Cr2O3 and NiCr2O4 along with someminor phases of Na2CrO4 and Cr23C6. Whereas in case ofCr3C2-(NiCr)+0.4 wt.%CeO2-coated Superni 600, additionalminor phases of CeO2 and CeS were detected along with themajor phases of Cr2O3, NiCr2O4, Na2CrO4, and Cr23C6.

3.6 Examination of Cross-Sectional Surface

Cross-sectional analyses for corroded bare Superni 600 andD-gun-sprayed Cr3C2-25(NiCr) without and with0.4 wt.%CeO2 are shown in Fig. 11. EDS has been taken atthe different points across the substrate, coating and scale. Incase of corroded Bare Superni 600, about 70 lm thick porousoxide scale was observed on the surface as shown in Fig. 11(a).

Fig. 5 Macrograph of (a) bare Superni 600 in boat, (b) Cr3C2-25(NiCr)-coated Superni 600 in boat, (c) Cr3C2-NiCr-coated Superni 600 in boatafter 1, 50th, and 100th cycle after hot corrosion in Na2SO4-25%NaCl at 900 �C

Fig. 6 (a) Weight change/surface area vs. number of cycles (mg/cm2). (b) Weight change/surface area (mg/cm2)2 graphs of uncoatedand coated Superni 600 subjected to cyclic hot corrosion inNa2SO4-25%NaCl at 900 �C after 100 cycles

Table 2 Parabolic rate constants (Kp) (10210 g2/cm4/S)

Superalloys

Hot corrosionin molten saltat 900 �C

Cr3C2-NiCr-coated Supemi 600 6.1Cr3C2-NiCr+0.4%CeO2-coated Supemi 600 3.41


The oxide scale mainly consists of Cr, Ni, and O which can beobserved from points 5 and 7 which lie in the oxide layer.Figure 11(b) shows the cross-sectional view of hot-corrodedD-gun-sprayed Cr3C2-25(NiCr) having a non-uniform scalearound 60-70 lm thick. It consists of Cr, Ni, and O as can benoticed from points 5, 6, and 7. Figure 11(c) shows the cross-section of corroded CeO2-blended coating. It can be clearlyseen from the figure that a thin oxide scale of around 35-40 lmwas formed on the surface of the coating. The oxide layerformed was dense and non-porous and consisted of Cr, Ni, andO. X-ray mappings of the cross-section for the corroded bareSuperni 600 after 100 cycles under Na2SO4-25%NaCl envi-ronment at 900 �C (Fig. 12) indicates the scale mainlyconsisting of Cr, Ni, and O along with some Mn and Fe.Presence of S was also observed along the particle boundariesof the substrate. In case of corroded Cr3C2-25(NiCr)-coatedSuperni 600, presence of Cr and O was identified in the scaleand also as a thin band along the coating/substrate interface.Below this band, there is a Cr-depleted region in the substrateas shown in Fig. 13. Similar features were again noticed in theCeO2-added coating as shown in Fig. 14. Ni-rich splats werealso present in the entire partially oxidized coating along withCr and O present at the inter splat boundaries.

4. Discussion

Ce-incorporated Cr3C2-25(NiCr)-coated Superni 600 shows25% less gain in weight as compared to conventional Cr3C2-

25(NiCr)-coated Superni 600 as can be inferred from histogramshown in Fig. 15. It can also be observed that both the coatingsfollowed parabolic rate law (Fig. 6b). Parabolic law is diffu-sion-controlled reaction of the coated specimens. Once nearlyconstant weight gain is obtained, it indicated the parabolic lawwhich slows down further oxidation/corrosion process (Ref 44).

In case of corroded bare Superni 600, there is a marginalincrease in weight upto 10th cycle, beyond which weight startsdecreasing. This decrease in weight may be due to reactionbetween oxide formed on the substrate alloy and its reactionwith chlorides in the presence of oxygen (Eq 1)

2NaClþ Cr2O3 þ 0:5O2 ! Na2Cr2O4 þ Cl2: ðEq 1Þ

The Cl2 which is generated due to the reaction betweenoxygen and alkali chlorides penetrates into the scale andgenerates volatile transition metal chlorides at scale-metalinterface as shown in Eq 2-3. These chlorides then diffuse backinto the scale-gas interface where they get oxidized by oxygenthereby precipitating metal oxide and releasing Cl2 gas whichenters the process again as shown in Eq 4 (Ref 45, 46)

Cr þ Cl2 ! CrCl2ðsÞ ðEq 2Þ

CrCl2ðsÞ ! CrCl2ðgÞ ðEq 3Þ

2CrCl2ðsÞ ! 1:5O2 ! Cr2O3 þ 2Cl2: ðEq 4Þ

In case of coated superalloys, the initial higher weight gainmay be because of the accelerated interfacial reaction andoxidation due to entrapped air in the coating (Ref 12). Parabolic

Fig. 7 FESEM micrographs with EDS spectrum of the Bare Superni 600 specimen showing surface morphology after cyclic hot corrosion inNa2SO4-25%NaCl environment for 100 cycles at 900 �C


rate constants for corroded Cr2C3-25(NiCr) and Cr3C2-25(NiCr)+0.4 wt.%CeO2-coated Superni 600 have been calcu-lated and given in Table 2. This indicates that the scale formedis protective and is not allowing further transport of anion orcation through it. There was slight delamination of coating fromthe edges in the case of non-ceria containing coating. CeO2-added coating did not undergo any spalling or peeling off up to100 cycles (Fig. 5). Ecer et al. (Ref 47) suggested that thepresence of superficially applied CeO2 improves the scaleadherence during oxidation. This effect is presumably causeddue to the segregated oxides of rare earth metals at the particleboundaries of the Cr2O3 scale, which is facilitated by aninternal diffusion of oxygen (Ref 48). Sigler (Ref 49) studiedthe oxidation behavior of rare earth added Fe-20Cr-5A1 alloysin air and synthetic exhaust gas and concluded that addition ofrare earth (Ce and La) results in adherent oxide scales on thesealloys.

FESEM analysis shows that in case of corroded bareSuperni 600 (Fig. 7) the scale formed is porous and indicatesnon-protective behavior. An inlay shown in Fig. 7 also revealedthe porosity in the oxide scale in form of round pits indicatingrelease of volatile gases. Nielsen et al. (Ref 50) reported that inthe presence of chlorine, the volatile metal chlorides whichformed after reaction of chlorine and metal oxide diffused outthrough the scale forming a loose non-protective oxide layer.EDS analysis depicts the presence of Cr, Ni, Mn, and O. Incontrast, the Cr-3C2-25(NiCr) (Fig. 8) and Cr3C2-25(NiCr)+0.4 wt.%CeO2 (Fig. 9) coated Superni 600 aftercorrosion in the given environment developed a dense andcompact scale. An inlay shown in Fig. 8 shows the formation

of a spinel structure whose EDS analysis revealed the existenceof Ni, Cr, and O together. This may be attributed to theformation of NiCr2O4. NiCr2O4 spinels formed after the solidphase reaction between Cr2O3 and NiO as shown in Eq 5 (Ref51, 52)

Cr2O3 þ NiO! NiCr2O4: ðEq 5Þ

In case of corroded Cr3C2-25(NiCr)+0.4 wt.%CeO2-coatedSuperni 600, needle type and nodular morphology were seen.The needle/platelet type oxide shows basically the presence ofCr and O. Nodular type morphology indicates higher amount ofCr, Ce, and O which may be attributed to the formation ofchromia and cerium oxide, respectively. The oxide scale formedon the Cr3C2-(NiCr)+0.4 wt.%CeO2 coating was relativelycompact. Bright nodules containing CeO2 were found welldistributed on the surface of scale as shown in Fig. 9. A similarresult has been reported by Mingzeng et al. (Ref 53) for25Cr20Ni-0.3%Ce sample after 10 h oxidation in H2-H2Omixtures at 950 �C. They reported that a small amount of Ceaddition (0.02 or 0.05 wt.%) promoted oxidation resistance andinhibited the growth of the needle-like oxide. Platelet-likemorphology consisting of Cr and O shows the formation ofCr2O3. Sreedhar and Raja (Ref 54) also reported that whenNiCrAlY coating exposed to Na2SO4-10%NaCl environmentfor 51 cycles, similar platelet type oxide structure was observedcorresponding to that of Cr2O3 along with the presence ofnickel-chromium-rich spinels. Cr2O3 and NiCr2O4 are presentas major phases in case of corroded bare and coated Superni600 as shown in Fig. 10. Ding (Ref 55) summarized the resultsof high temperature oxidation of NiCr-25(Cr3C2) coatings

Fig. 8 FESEM micrographs with EDS spectrum of the D-gun-sprayed Cr3C2-25(NiCr) coated Superni 600 specimen showing surface morphol-ogy after cyclic hot corrosion in Na2SO4-25%NaCl environment for 100 cycles at 900 �C


Fig. 9 FESEM micrographs with EDS spectrum of the D-gun-sprayed Cr3C2-25(NiCr)+0.4 wt.%CeO2 coated Superni 600 specimen showingsurface morphology after cyclic hot corrosion in Na2SO4-25%NaCl environment for 100 cycles at 900 �C

Fig. 10 X-ray diffraction patterns for the bare and coated Superni 600 subjected to cyclic hot corrosion in Na2SO4-25%NaCl at 900 �C after100 cycles


indicating Cr2O3-rich upper region and subscale containingoxides of chromium nickel spinels. In the present study, Cr23C6

and Na2CrO4 oxides were the other minor phases identified incase Cr3C2-25(NiCr)-coated Superni 600. Corrosion of CeO2-added coating resulted in the formation of CeO2 and CeS asnew extra phases. Gibbard (Ref 56) also reported the presenceof CeS in the corroded samples which have been ascribed tohigh volatilization temperature of CeS (2450 �C) and accordingto them it is a stable phase up to 900 �C. Amadeh et al. (Ref 57)studied the effect of rare earth metals addition on surfacemorphology and corrosion resistance of hot-dip-galvanizedsteel. They suggested that the rare earth metals react easily withO and S to form stable oxides and sulfides which act as aheterogeneous nucleation sites, hinder the grain growth duringsolidification process, and refine the coating microstructure(Ref 58, 59). Cr23C6 was detected in the hot-corroded scale incase of both with and without CeO2-doped coatings. From this,it can be inferred that Cr3C2 breaks up into Cr23C6 or Cr7C3

which are the stable phases at higher temperature. Cr23C6 is themost stable compound while Cr3C2 is the least stable at high

temperature (Ref 60). The stability of Cr23C6 can be determinedby the enthalpy and temperature graph in which Gibbs freeenergy is shown at different temperatures (Fig. 16). Clearly, itcan be seen from the graph that at high temperature Cr3C2

phase has positive Gibbs free energy and Cr23C6 has negativefree energy which prove that at high temperature, Cr23C6 phaseis stable (Ref 61). The formation of Cr2O3 may be contributedby reaction of chromium present in coating and oxidation ofchromium carbide in a stepwise reaction leading to formationof Cr2O3 (Ref 62)

Cr3C2�!þO2

Cr7C3�!þO2

Cr23C6�!þO2 ½CrMET� �!

þO2Cr2O3: ðEq 6Þ

Formation of sodium chromate also helps in protecting thematerial from degrading. It was reported that chromia is the bestoxide to resist hot corrosion in molten sulfates, as it preferen-tially reacts with O2� to form the chromate. The chromatestabilizes the melt chemistry, and consequently prevents thefurther dissolution of the protective oxide scale (Ref 63).

Fig. 11 Variations of elemental composition across the cross-section of bare and coated Superni 600 subjected to cyclic hot corrosion inNa2SO4-25%NaCl at 900 �C after 100 cycles


From cross-sectional analysis of corroded bare Superni 600shown in Fig. 11(a), it can be inferred that corroded bareSuperni 600 has suffered internal oxidation where there is alayer of unreacted alloy below which the oxidized scale ispresent. The overall weight loss in 100 cycles is marginal(�0.85 mg/cm2). X-ray mapping of corroded bare Superni 600(Fig. 12) confirmed that Cr, Mn, and O coexist which may beattributed to the formation of Cr2O3 and spinel of Cr and Mn.Ni remains unreacted with O. But a layer of Cr and O can beobserved below the layer of Ni. Point 5 lies on the layer of Niwhere no O is present while point 4 lies just below point 5where O and Cr are present. This might be due to the presenceof some crack or inclusion on the substrate through which theaggressive species may have penetrated and started reactingwith the substrate alloy. Presence of S ingress can also beobserved at point 3 which lies on the substrate around 50 lmaway from the substrate scale interface. X-ray mapping alsoindicates the similar feature. Johnson et al. (Ref 4) reported the

presence of internal sulfides in the form of CrS in cross sectionsof Co-25Cr-7.5W oxidized at 900 �C for 22 h under Na2SO4-40%NaCl salt. Gurrappa (Ref 64) also suggested that S is theonly element diffusing into the substrate along the particleboundaries. Kamal et al. (Ref 36) also observed the penetrationof S inside the substrate. X-ray mapping of corroded Cr3C2-25(NiCr) and Cr3C2-25(NiCr)+0.4 wt.%CeO2-coated Superni600 are shown in Fig. 13 and 14 which show the presence of Crand O in the oxide layer as well at the substrate coatinginterface from this it can be inferred that the protective nature ofthe coating may be due to the presence of Cr2O3 at thisinterface and also at the top of scale. The continuous thick bandof Cr2O3 in the scale will not allow any further transport of theoxidizing species and the metallic ions. In case of corrodedCr3C2-25(NiCr)+0.4 wt.%CeO2-coated Superni 600, a verycompact and adherent oxide can be observed on the surface ofthe coating. It was reported that the rare earth metals are surface

Fig. 12 X-ray mapping of bare Superni 600 subjected to hot corro-sion in Na2SO4-25%NaCl environment at 900 �C after 100 cycles Fig. 13 X-ray mapping of D-gun-sprayed Cr3C2-(NiCr) coated on

Superni 600 subjected to hot corrosion in Na2SO4-25%NaCl envi-ronment at 900 �C after 100 cycles


active elements having tendency to concentrate on the surfaceand form dense and uniform oxide layer there. The oxide layermay act as a diffusion barrier and thus inhibits the oxidation

and corrosion processes. Various mechanisms have beenproposed to explain the effect of rare earth elements onimproving oxidation resistance. The mechanism most widelyaccepted explains the improvements observed in high temper-ature oxidation of alloy containing rare earth, as being due tothe diffusion of the rare earth ions to oxide scale grainboundaries and blocking of alloy cation diffusion along thesepaths (Ref 26). Based on the FESEM/EDS, XRD and x-raymapping analysis, the corrosion mechanism for bare Superni600, with and without CeO2-doped Cr3C2-(NiCr)-coated Su-perni 600 oxidized under molten salt at 900 �C are proposedand shown in Fig. 17.

In Fig. 17(a), an attempt has been made to represent themechanism of bare Superni 600, hot corroded in Na2SO4-25%NaCl environment at 900 �C for 100 cycles. There is theformation of oxides in the initial stages and these oxides thenreact with the salt environment. There is a penetration of thereacting species through the substrate alloy layer which maybe along the grain-boundary. A scale is formed below theunreacted metal layer. But overall weight change is verysmall. The species from the alloy are diffusing toward thescale-metal interface and reacting with the chloride andforming volatile chlorides which while passing through theoxide scale get reduced thereby precipitating oxides andreleasing chlorine which further reacts with the elementsdiffusing from the substrate. In case of Cr3C2-25(NiCr)-coated Superni 600 (Fig. 17b), alloy exposed under similarconditions showed development of relatively thick scalehaving unoxidized Ni surrounded by Cr2O3 and spinels. Theextent of internal oxidation of the substrate is marginal butthere is the formation of Cr2O3 layer at the substrate coatinginterface and just below this there is a Cr-depleted layer.Initially formed oxides of Cr are fluxed by the molten saltmixture. The chlorides formed get volatilized and whilepassing through the oxide scale there is precipitation ofoxides. Chlorine thus released is available for further reactionwith the coating. In case of CeO2-added coating (Fig. 17c),other than Cr2O3 and spinels, CeS is detected on the surfacescale and CeO2 along the splat boundaries. During thecorrosion run, the CeO2 presents at the splat boundaries willhinder the transport of reacting species thereby decreasingthe extent of corrosion.

Fig. 14 X-ray mapping of D-gun-sprayed Cr3C2-(NiCr)+0.4 wt.%CeO2 coated on Superni 600 subjected to hot corrosion in Na2SO4-25%NaCl environment at 900 �C after 100 cycles

Fig. 15 Histogram for the overall weight gain for bare Superni600, D-gun-sprayed Cr3C2-(NiCr), and Cr3C2-(NiCr)+0.4 wt.%CeO2-coated Superni 600 subjected to hot corrosion in Na2SO4-25%NaClenvironment at 900 �C after 100 cycles

Fig. 16 Evolution of the standard-free enthalpy of formation forvarious chromium compounds and of the standard free enthalpy ofreaction between chromium and methane to form various chromiumcarbides (Ref 60)


5. Conclusion

(1) Ceria is embedded in the NiCr nodule in the blendedpowder and also in as-sprayed Cr3C2-25(NiCr) coating.

(2) Bare Superni 600 corroded in Na2SO4-25%NaCl at900 �C for 100 cycles had porous scale and showedweight loss which was accompanied with minor internaloxidation whereas corroded Cr3C2-25(NiCr) and Cr3C2-25(NiCr)+0.4 wt.%CeO2 coating in the similar conditionshowed a dense scale and overall weight gain.

(3) Parabolic rate constant for ceria-doped coating wasfound to be 60% of the coating without ceria.

(4) Cr2O3 and NiCr2O4 were found to be the commonphases in the corrosion product in all the three cases.Cr23C6 and Na2CrO4 were present in both the corrodedcoatings whereas CeO2 and CeS also appeared in ceria-doped coating.

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Corrosion Behavior of Bare, Cr 3 C 2 -25%(NiCr), and Cr 3 C 2 -25%(NiCr)+0.4%CeO 2 -Coated Superni 600 Under Molten Salt at 900 °C

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