Hot Corrosion Behavior of Some Superalloysin a Simulated Incinerator Environment at 900 �C

Deepa Mudgal, Surendra Singh, and Satya Prakash

(Submitted January 19, 2013; in revised form August 13, 2013; published online October 10, 2013)

Incinerators are being used to burn solid waste of all types. This burning of waste creates a very aggressiveenvironment at extremely high temperature. This environment attacks the various components of theincinerators. Some studies have been reported regarding behavior of steels in simulated incinerator envi-ronment at 550 �C. In present work superalloys Superco 605, Superni 600, and Superni 718 have beensubjected to cyclic oxidation in 40 wt.% K2SO4 + 40 wt.% Na2SO4 + 10 wt.% KCl + 10 wt.% NaClenvironment at 900 �C under cyclic condition. Weight change measurements have been done and weightchange has been plotted against the numbers of cycles. The oxide scales formed on the surface of thecorroded superalloys have been characterize by FESEM, EDS, XRD, cross-sectional analysis, and x-raymapping. The nickel-based superalloys Superni 600 and Superni 718 indicated better resistance to cor-rosion in the above environment whereas Superco 605 lead to massive weight gain.

Keywords hot corrosion, incinerator, superalloys

1. Introduction

Utilization of waste incineration technology for disposal ofmunicipal solid waste, biomass, and biomedical waste havebecome more prevalent in many countries. In locations wherepopulation densities are high, the use of landfills for wastedisposal has become less feasible and waste incineration hasbeen more attractive option. An additional advantage ofincineration is the use of available heat for production ofelectricity. It is now known that chlorine-containing compoundsare the primary corrosive species in waste incinerators ascompared to coal-fired boilers, where sulfur-containing com-pounds intensify corrosion (Ref 1). During combustion ofbiomass, large amount of chlorine and sulfur together with thevolatile alkali metals such as potassium and sodium arereleased as vapors into the flue gases (Ref 2). Alkali chlorideparticles are formed during biomass combustion and trans-ported via aerosols or in the vapor phase within the combustiongas, subsequently depositing on the metallic surface or on thealready formed oxide layer which further accelerates theoxidation process leading to heavy metal consumption (Ref3). Investigations (Ref 4-7) confirmed that these failures weredue to corrosion, in which there was extensive penetration ofsulfur, chlorine, oxygen, carbon, etc., leading to the formationof metal sulfides, chlorides, oxides, carbides, etc., resulting inreduced mechanical properties of the components leading tocatastrophic failures. Most alloys that resist high temperature

corrosion protect themselves with a surface layer of Cr2O3.However, this Cr2O3 can be fluxed away by reactions that formalkali chromates or get volatilized as chromic acid (Ref 8).Non-destructive testing is one of the tools to assess theoxidation and corrosion behavior of high temperature materials.It can be used successfully for in-service inspection to ensuresafe and reliable operation of energy generation equipments.Raj et al. (Ref 9) suggested that NDT can be use to detect andcharacterize the anomalies that can adversely affect theperformance of the component under test without impairingits intended service. Some of the NDT techniques includevisual examination, liquid penetration testing, leak testing,vibration monitoring, magnetic particle inspection, ultrasonictesting, eddy current testing, gamma and x-radiography,acoustic emission, tomography, magnetic flux leakage methods,laser holograph, interferometry, infrared thermography, etc.Studies (Ref 10, 11) have been conducted to monitor the hightemperature oxidation behavior using NDT methods. Khannaet al. (Ref 12) investigate the oxidation behavior of Ni-10Cr-8Al-containing sulfur and/or yttrium addition. Acoustic emis-sion analysis has been used to know the scale adherence.Results confirm that addition of yttrium improves the oxidationbehavior, by increasing the scale adherence and reducing thescale growth. Jha et al. (Ref 13) studied the breakawayoxidation and in situ cracking of the oxide formed on 9Cr-1Mosteel in air at 900 and 950 �C using acoustic emissiontechnique. While Khanna et al. (Ref 14) studied the samemechanism on 21/4Cr-1Mo steel using acoustic emissiontechnique at different temperatures. It has been found thatvariations in AE parameters are marginal during heating at 600,700, and 800 �C, and a sudden rise in these parameters occursduring cooling. Increase in AE activity during cooling has beenrelated to spalling of the oxide layers. At 900 and 950 �C, aconsiderable increase in AE parameters (except voltage level)has been detected after certain times at the respective temper-atures. Raj et al. (Ref 15) use the AE technique to study theoxidation behavior of Cr-Mo steel in a wide range oftemperatures and reported that it has been possible to detect

Deepa Mudgal, Surendra Singh, and Satya Prakash, Department ofMetallurgical andMaterials Engineering, Indian Institute of TechnologyRoorkee, Roorkee 247667, India. Contact e-mail: deepamudgal01@gmail.com.

JMEPEG (2014) 23:238–249 �ASM InternationalDOI: 10.1007/s11665-013-0721-x 1059-9495/$19.00

238—Volume 23(1) January 2014 Journal of Materials Engineering and Performance

and characterize various scale damage processes using thistechnique. Apart from NDT method, it is also necessary tocheck the corrosion performance by actual exposure of thespecimens in salt environment. Uusitalo et al. (Ref 6)performed the high temperature corrosion test on low alloyferritic steel and austenitic stainless steel, five high velocityoxyfuel coatings, a laser cladding, and diffusion-chromizedsteel. In the experimentation, samples were exposed tosynthetic salt (40 wt.% Na2SO4 + 40 wt.% K2SO4 + 10 wt.%KCl + 10 wt.% NaCl) in oxidizing and reducing atmospherefor 100 h at 550 �C under isothermal condition. They observedthat corrosion in oxidizing atmosphere in presence of depositswas more severe than in reducing conditions. A study has alsobeen reported on behavior of T91 and AC 66 steels in simulatedcoal-fired boiler and incinerator environment. Authors con-cluded that waste incinerator environment was much morecorrosive as compared to coal-fired boiler environment. Furtherconcluded that AC 66 performed better as compared to T91 andthis improvement has been attributed to higher chromium andnickel content (Ref 7). Some work has been done on Superco605 alloy in incinerator environment which indicated that thealloy has very good resistance to oxidation in air whereas inincinerator environment its performance was not satisfactory(Ref 16). Ishitsuko et al. (Ref 17) studied the stability of variousprotective oxide films in waste incinerator environment usingNaCl-KCl and NaCl-KCl-Na2SO4-K2SO4 salt mixture withthree different levels of basicity. As only few studies have beenreported regarding hot corrosion in this environment mainly forsteels under isothermal condition and at temperature below750 �C. Hence it is necessary to conduct the study on corrosionat higher temperature. Superalloys are widely used for hightemperature application but not studied in this incineratorenvironment. In the present study, (40 wt.% Na2SO4 +40 wt.% K2SO4 + 10 wt.% KCl + 10 wt.% NaCl) environment

has been chosen to simulate incinerator and biofuel-fired boilerconditions. The environment has been taken from the existingreported literature of Ishitsuko et al. (Ref 17, 18). Cycliccondition has been selected so as to create the severe condition asthis will give the maximum thermal shocks and create maximumstresses on the substrate. It was opined that if a material cansustain in such severe condition than it can sustain elsewherealso. In the previous studies, the behavior of the similarsuperalloys in air at 900 �C has been reported which confirmsits sustainability at high temperature in air (Ref 19). Hence thestudy will help in finding out the performance of varioussuperalloys in simulated incinerators environment for longerperiod under cyclic condition (more severe condition ascompared to isothermal) at higher temperature, i.e., 900 �C.

2. Experimental Procedure

Three superalloys were obtained from MIDHANI (Hyder-abad, India) named Superni 600, Superni 718, and Superco 605in form of hot-rolled and annealed sheets. Composition of thesuperalloys is shown in Table 1. The alloy sheets were cut intorectangular samples of size 20 mm9 15 mm9 5 mm. Speci-mens were mirror polished. The physical dimensions ofsamples were recorded carefully using digital vernier caliperof resolution 0.01 mm to evaluate their surface area. Weightmeasurement of samples as well as preheated alumina boats hasbeen carried out using digital weighing balance with 1 mgaccuracy. All the samples were subjected to hot corrosion testfor 100 cycles at 900 �C in a silicon carbide tubular furnace.Each cycle consist of 1 h heating in the furnace at 900 �Cfollowed by 20 min cooling in the ambient air. Weight of thesamples has been measured after every cycle along with the

Table 1 Composition of superalloys

Name of superalloys

Elements, wt.%

Fe Ni Mn Cr Cu W Mo Co Si Ti C Ta Al

Superni 600 10 max Bal. 0.5 15.5 0.3 .021Superco 605 3.0 10.0 1.5 20.0 15.0 Bal. 0.3 0.08Superni 718 18.5 Bal. 0.18 19.0 0.15 3.05 0.18 0.9 0.04 5.13 0.5

Fig. 1 Macrographs of (a) Superco 605, (b) Superni 600, and (c) Superni 718 specimens along with their oxide which has fallen during experi-mentation in alumina boat

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alumina boat. For hot corrosion, mixture of salts has beenapplied on the surface of samples varying from 3 to 5 mg/cm2.For applying salts, samples were preheated in oven at 250 �Cfor 1 h. 40% Na2SO4 + 40% K2SO4 + 10% NaCl + 10% KClsalt solution has been made in distilled water and applied on thepreheated samples using camel hair brush. Weight of the salt-coated samples together with the alumina boat was measuredagain using digital weighing balance and noted down.

3. Results

3.1 Visual Observation

Macrographs of Superco 605, Superni 600, and Superni 718along with their oxides which have fallen during experimen-tation in the alumina boat have been shown in Fig. 1. Surfacemacrographs of Superco 605, Superni 600, and Superni 718subjected to cyclic hot corrosion in molten salt environment at900 �C for 100 cycles have been shown in Fig. 2. In Superco605 initially light gray color scale was formed which withprogress of cycles showed dark patches with massive growth ofthe scale accompanied by continued spalling. Superni 600 andSuperni 718 alloys indicated formation of very thin and non-adherent scale with very little spalling in the form of finepowder. Color of scale in Superni 600 was dark gray with somegreenish color was observed during initial cycles and finallysome white patches were also observed till the end of 100th

cycle. In Superni 718 dark gray oxide was initially formedwhich changes to light gray background with some brownpatches after 100 cycles.

3.2 Weight Gain Measurements

Corrosion kinetics can be monitored using weight changeplots. Weight change versus number of cycles graph has beenplotted in Fig. 3(a) and (b) which shows that during hotcorrosion, Superco 605 undergoes substantial weight gain.While in Superni 600 and Superni 718 minor loss in weightwas observed in initial cycles which became nearly constantafter 55 cycles. The parabolic rate constant Kp is calculatedfrom the slope of the linear regression fitted line from (weightgain/area)2 versus number of cycles for Superco 605 and hasfound to be 121.7 x 10�10 g2 cm�4 sec�1.

3.3 FESEM/EDS

The FESEM/EDS micrographs of hot-corroded Superco605, Superni 600, and Superni 718 are shown in Fig. 4, 5, and6. FESEM shows the surface morphology while EDS havebeen taken at some selected sites to know the elementalvariation throughout the scale. In Superco 605 alloy tungsten(W), chromium (Cr), cobalt (Co), and oxygen (O) are present asthe major elements while in Superni 600 and Superni 718alloys O, Cr, nickel (Ni), and iron (Fe) are present. Apart fromthese elements some amount of manganese (Mn) was alsofound in Superni 600 and molybdenum (Mo) in Superni 718,

Fig. 2 Surface macrographs of surface texture (a) Superco 605, (b) Superni 600, and (c) Superni 718 subjected to hot corrosion in 40%Na2SO4 + 40% K2SO4 + 10% NaCl + 10% KCl environment for 100 cycles at 900 �C

Fig. 3 (a) (Weight gain/area) vs. number of cycles plot with molten salt at 900 �C. (b) (Weight gain/area)2 vs. number of cycles plot forcorroded sample under molten in salt at 900 �C for 100 cycles

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respectively. A quite different surface morphology of scales isobserved in all the three superalloys after 100 cycles of hotcorrosion at 900 �C. In case of Superco 605 the top surfaceshowed a fine-grained structure with pits and voids. Around thepits the scale is rich in oxygen whereas inside the pits there isdepletion of oxygen. In case of Superni 600 the final scale wasthin and contained microcracks whereas in Superni 718, thescale is relatively of smaller grain size and crack-free.

3.4 XRD Analysis

The x-ray diffraction (XRD) profiles of the scale for Superco605, Superni 600, and Superni 718 after hot corrosion in 40%Na2SO4 + 40% K2SO4 + 10% NaCl + 10% KCl environmentfor 100 cycles at 900 �C are shown in Fig. 7. In Superco 605,the major phases observed are NiWO4, CoCr2O4, CoO, andNiCr2O3. In Superni 600 and Superni 718 the major phasesobserved are Cr2O3, NiCr2O4, and K2CrO4 and the minorphases observed are Ni, NiS, and NiMnO3. In addition to theabove-mentioned phases the peaks of Na2CrO4 is also identi-fied in Superni 718 and KClO4 in Superni 600.

3.5 Cross-Sectional Analysis

The hot-corroded samples were cut and mounted intransoptic mounting resin, mirror polished and gold coated tofacilitate cross-sectional analysis. EDAX analysis is carried outat different points of interest along the cross section of corrodedSuperco 605, Superni 600, and Superni 718 and results areshown in Fig. 8. In case of Superco 605, Cr seems to havediffused from the alloy and is present at higher concentrationnear the substrate interface. It decreases gradually as one move

towards the outer surface of the scale. The oxygen present isnearly at the same concentration along the cross section of thescale. Top layer of the scale consist of Cobalt, Nickel,Chromium, and Oxygen. EDS analysis cross section ofcorroded Superni 600 showed that top most layer of oxidewas rich in nickel while in the middle Nickel, Chromium, andoxygen are co-existing, whereas near the substrate scaleinterface mainly chromium is present with oxygen. The oxidelayer formed after 100 cycles on the surface of Superco 605was around 40 lm thick while the thickness of the oxide scaleformed on the surface of Superni 600 and Superni 718 werearound 7-10 lm. In case of corroded Superni 718, bottom layerof oxide scale mainly contains chromium and oxygen whereastop layer of consist of nickel, chromium, iron, and molybde-num.

3.6 X-Ray Mapping of Cross-Section

X-ray mapping of cross section of corroded Superco 605indicated formation of thick oxide scale consisting of Cr, Co,W, Ni, and O (Fig. 9). Whereas the scale formed on corrodedSuperni 600 (Fig. 10) and Superni 718 (Fig. 11) were thin andhad a band rich in Cr and O. A Cr-depleted area was seen in thesubstrate just below the scale.

4. Discussion

In the 40% Na2SO4 + 40% K2SO4 + 10% KCl + 10%NaCl environment, the color of the scales formed on Superni

Fig. 4 FESEM surface morphology of oxide layer formed on Superco 605 subjected to hot corrosion in 40% Na2SO4 + 40% K2SO4 + 10%NaCl + 10% KCl environment for 100 cycles at 900 �C

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600 and Superco 718 was dark gray after 100 cycles at 900 �Cwhich may be attributed to the formation of chromium oxide(Ref 20). Presence of white dots on the surface of scale formedon Superni 600 and Superni 718 indicated the diffusion ofmanganese from the substrate towards the surface during thetime of corrosion.

Maximum weight gain was observed in case of Superco605 while Superni 600 and Superni 718 showed superiorresistance to this simulated incinerator environment as can beinferred from Fig. 12. It can be immediately revealed fromFig. 3(a) that Superco 605, Superni 600, and Superni 718 donot follow the parabolic law. It was suggested that the kineticscan be proved to be parabolic if there is a oxide scale growthdictated by the ionic diffusion through the oxide scale. Forthis Kp value is used to characterize the corrosion kinetics. Ifthe Kp value is low, the scale growth is slow and hence showprotective behavior while high Kp value represent higherreaction rate and unprotective behavior (Ref 21). TheParabolic rate constant for Superco 605 has been calculatedwhich comes out to be 121 mg/cm2 which is very high. Alsoit is showing linear oxide growth which proves that the oxideformed is unprotective in nature. Similar linear nature ofkinetics for Co-based superalloy Ultimet has been reported at750 �C under thermal cycling for 120h in chlorine-containingenvironment where it is reported that the scale formed issusceptible to cracking followed by healing (Ref 22). WhileSuperni 600 and Superni 718 show minor gradual decrease inweight up to 100 cycles as shown in Fig. 3(a), which may beascribed to the formation of volatile chlorides. The reason forminor cumulative weight loss in case of Superni 718 and

Superni 600 may be due to presence of chloride-containingaggressive environment. In this environment chlorides areformed most of them are in gaseous form at the temperatureof 900 �C leading to weight loss. At the same time oxides areformed which may be leading to increase in weight. Overalleffect is slight decrease in the weight per unit area. Thechlorine plays a catalyzing role and often is not easily founddue to its volatile nature. The presence of chlorides in thedeposits may result in the formation of low melting pointeutectics which may dissolve the oxide layer which wasprotecting the metal surface (Ref 23). Such materials degra-dation induced due to the presence chlorine is known asActive Oxidation.

It has also been reported (Ref 17) that when chlorinepenetrates deposit/scale, it forms several volatile solids andgaseous chloride as follow:

Cr þ Cl2 ! CrCl2 sð Þ ðEq 1Þ

As these solid chlorides have equilibrium vapor pressure,hence evaporate continuously and diffuses outwards towardsthe gas/oxide interface and further reacts with oxygen releasinggaseous chlorides.

CrCl2 sð Þ ! CrCl2 gð Þ ðEq 2Þ

2CrCl2 gð Þ þ 0:5O2 ! Cr2O3 þ 2Cl2: ðEq 3Þ

Stott and Shih (Ref 24) reported that main metal lossprocesses were evaporation of FeCl2 and CrCl2 in case ofreaction between 0.1-1% HCl with Fe-Cr, Fe-Cr-Y alloys at600 �C. It has also been reported that the reaction products of

Fig. 5 FESEM surface morphology of oxide layer formed on Superni 600 subjected to hot corrosion in 40% Na2SO4 + 40% K2SO4 + 10%NaCl + 10% KCl environment for 100 cycles at 900 �C

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Fig. 6 FESEM surface morphology of oxide layer formed on Superni 718 subjected to hot corrosion in 40% Na2SO4 + 40% K2SO4 + 10%NaCl + 10% KCl environment for 100 cycles at 900 �C

Fig. 7 X-ray diffraction patterns of Superco 605, Superni 600, and Superni 718 subjected to hot corrosion in 40% Na2SO4 + 40%K2SO4 + 10% NaCl + 10% KCl environment for 100 cycles at 900 �C

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chromium oxide with chlorine could be CrCl2, CrCl3, CrCl4,and CrO2Cl2 depending on temperature (Ref 25). According toliterature (Ref 24, 26, 27) it was also suggested that between627 and 977 �C the product formed after chlorination of Cr2O3

was CrO2Cl2 (g). The standard free energy of the reactions(Eq 4-9) between chlorine and chromium at temperature rangetill 1000 �C has been reported by Gaballah et al. (Ref 25) whichhas been shown in Fig. 13.

1=2Cr2O3 sð Þ þ Cl2ðgÞ þ 1=4O2 gð Þ ! CrO2Cl2 gð Þ ðEq 4Þ

1=3Cr2O3 sð Þ þ Cl2 gð Þ ! 2=3CrCl3 sð Þ þ 1=2O2 gð Þ ðEq 5Þ

1=4Cr2O3 sð Þ þ Cl2 gð Þ ! 1=2CrCl4 gð Þ þ 3=8O2 gð Þ ðEq 6Þ

4=9Cr2O3 sð Þ þ Cl2 gð Þ ! 2=9CrCl4 gð Þ þ 2=3CrO2Cl2 gð ÞðEq 7Þ

2=5Cr2O3 sð Þ þ Cl2 gð Þ ! 1=5CrCl4 gð Þ þ 3=5CrO2Cl2 gð ÞðEq 8Þ

Similarly with Mn, the reaction will be (Ref 28):

2MnCl2 gð Þ þ 3=2O2 ¼ Mn2O3 þ 2Cl2 ðEq 9Þ

Superco 605 showed accelerated gain in weight up to 80thcycle after which it leveled off as shown in Fig. 3(a). This wasaccompanied by continuous spalling up to 80th cycle after whichthe spalling got reduced. Spalling may be attributed to thermalexpansion mismatch between the oxide and the metal during thetime of cooling cycle. It may also be contributed by presence ofrefractory material such as tungsten which forms oxides that leadsNa2SO4 to form acidic fluxing. Hence when these elements getoxidized in presence of Na2SO4 deposits, they led to catastrophicself-sustaining hot corrosion via acidic fluxing which has alsobeen reported by Pettit et al. (Ref 29). It has also been reported

Fig. 8 Oxide scale morphology and variation of elemental composition across the cross section of (a) Superco 605, (b) Superni 600, and (c)Superni 718 subjected to hot corrosion in 40% Na2SO4 + 40% K2SO4 + 10% NaCl + 10% KCl environment for 100 cycles at 900 �C

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that potassium chloride reacts with sulfur dioxide to releasechlorine which reacts with the metallic surface (Ref 30).

FESEM shows (Fig. 4-6) that the oxide layer formed onSuperco 605 is loose and porous. Physical voids can be clearlyobserved on the surface scale of corroded Superco 605. Theoxide formed on Superni 600 contains cracks whereas scale onSuperni 718 was free of cracks. The oxide formed on thesurface of Superni 600 is porous which may be due to thevolatile action of metal chlorides which when formed diffusesoutward from the metal/scale interface, leaving behind smalllocal cavities some of which coalesce to from detectable voids.Similar results have been reported by Zhang et al. (Ref 31)where corrosion behavior of Fe and four commercial steels withdifferent Cr contents have been investigated in oxidizingatmosphere containing HCl at 500-600 �C. EDS analysis ofscale surface of corroded Superni 600 and Superni 718 showedthe presence of Ni, Cr, and O together. Presence of theseelements may lead to the formation of Cr2O3 and NiCr2O4.

Following are the reactions between the Cr, Ni, and O(Ref 29, 32)

2Cr cð Þ þ O3 gð Þ ! Cr2O3 cð Þ ðEq 10Þ

NiOþ Cr2O3 ! NiCr2O4 ðEq 11Þ

These oxides may act as diffusion barriers for the inwardpenetration of oxidizing species as proposed by Goebel et al.(Ref 33) and Pettit et al. (Ref 29). However, it may be notedthat at temperature above 1000 �C, Cr2O3 have tendency tovolatilize to CrO3 and thus diminished the protective capability

of the alloy to the corrosion attack (Ref 34). Na and K were alsoseen in EDS analysis. Whereas EDS analysis of Superco 605(Fig. 4) showed presence of Co and W whose oxides are lessprotective in presence of molten salt environment at 900 �C.Hence Superni 600 and Superni 718 show good corrosionresistance as compared to Superco 605.

XRD of corroded Superco 605 clearly showed (Fig. 7.) thepresence of CoO, NiCr2O4, NiWO4, and CoCr2O4. Similar peaksof NiWO4 and CoCr2O4 in Superco 605 after 50 cycles at 900 �Cin presence of Na2SO4-60% V2O5 environment has been reportedby Goyal et al (Ref 35).McNallan et al. (Ref 36) also observed theformation of CoO as the major oxide formed in the presence ofchlorine containing environment. In case of corroded Superni 600,Cr2O3 and NiCr2O3 were identified which is supported by thefindings of Harpreet Singh et al. (Ref 37). In Superni 718, Cr2O3 isidentified as amajor oxide inXRD analysis. Formation of K2CrO4

has also been observed inXRD analysis of Superni 600 as given inFig. 7. Similar formation of chromates has been reported (Ref 38)in presence of KCl and NaCl at high temperature on 304 Lstainless steel in 5%O2 + 40%H2O environment at 600 �C. Theyfurther suggested that the chromate formation act as a sink forchromium in the oxide form and leads to a loss of its protectiveproperties. XRD analysis of corroded Superni 718 reveals thepresence of Na2CrO4 and K2CrO4. Formation of these chromateshas also been highlighted byAlbina (Ref 39) suggesting followingreactions of Cr2O3 in presence of NaCl and KCl.

4NaCl s; lð Þ þ Cr2O3 sð Þ þ 2:5O2 ! 2Na2CrO4 sð Þ þ 2Cl2 gð ÞðEq 12Þ

Fig. 9 Composition image and x-ray mappings along the cross section of Superco 605 subjected to hot corrosion in 40% Na2SO4 + 40%K2SO4 + 10% NaCl + 10% KCl environment for 100 cycles at 900 �C

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4KCl s; lð Þ þ Cr2O3 sð Þ þ 2:5O2 ! 2K2CrO4 sð Þ þ 2Cl2 gð ÞðEq 13Þ

The superior hot corrosion resistance of Superni 600superalloy may be attributed to formation of oxides of nickel,chromium and their spinel NiCr2O4, as indicated from XRDanalysis shown in Fig. 7. The presence of spinel phases on thesurface scale as indicated by XRD analysis may furtherenhance the corrosion resistance due to much lower diffusioncoefficients of the cations and anions in the spinel phases thanthose in the parent oxide phases (Ref 4, 40). It has also beenreported by Uusitalo et al. (Ref 6) that Ni-based alloys is moreresistant in hot corrosion environment than steels because thepartial pressure of nickel chloride is significantly lower thanpartial pressure of iron chloride. Another factor governing thestability of nickel chloride is its Gibbs free energy which is lessnegative as compared to Gibbs free energy of other oxides. Itwas reported by Grabke et al. (Ref 28) that due to the morenegative gibbs free energy of Cr, Fe, and Mn are stronglyattacked by chlorine leading to the formation of chlorideswhich results in the development of a unprotective oxide scale.

Gibbs free energy of some of the compounds are given inTable 2. The Nickel may prove to be a better base element inthis environment because the protective property of NiO ishigher as compared to other oxides of base metals, Ishitsukoet al. (Ref 11).

Cross-sectional EDS analysis of corroded Superco 605,Superni 600, and Superni 718 have been shown in Fig. 8. InSuperco 605 some unreacted tungsten was found in thesubscale. Stott et al. (Ref 41) reported that presence of tungstenleads to rapid thickening of the scale thereby resulting in fastdegradation of substrate alloys. Oh et al. (Ref 42) and Elliott

Fig. 10 Composition image and x-ray mappings along the cross section of Superni 600 subjected to hot corrosion in 40% Na2SO4 + 40%K2SO4 + 10% NaCl + 10% KCl environment for 100 cycles at 900 �C

Table 2 The Gibbs free energy are as follows (Ref 28)

Compounds Gibbs free energy

MnCl2 (s) �366.7CrCl2 (s) �286.0FeCl2 (s) �232.1NiCl2 (s) �174.2Mo-chlorides �148.0

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(Ref 43) have also suggested that alloy with higher content ofrefractory metals suffers higher rate of corrosion. Cross-sectional analysis of Superni 600 showed the presence ofchromium oxide in entire scale. Decrease in concentration ofchromium in the substrate just below the scale indicatesdiffusion of chromium from the substrate towards the oxidescale to form the chromium oxide layer. The oxide layer formedon Superni 600 and Superni 718 were 20 and 10 lm,

respectively, whereas the oxide layer on corroded Superco605 was 40 lm thick as presented.

X-ray mappings of Superco 605, Superni 600, and Superni718 have been shown in Fig. 9, 10, and 11, respectively.Elemental x-ray mapping of Superco 605 indicates that toplayer consist of Nickel, Cobalt, Oxygen along with someChromium. This shows presence of CoO and NiCr2O4 which isfurther confirmed from the XRD (Fig. 7). Cr and O coexist just

Fig. 11 Composition image and x-ray mappings along the cross section of Superni 718 subjected to hot corrosion in 40% Na2SO4 + 40%K2SO4 + 10% NaCl + 10% KCl environment for 100 cycles at 900 �C

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above the substrate scale interface indicating presence ofCr2O3. Above this Cr2O3 layer, unreacted tungsten has beenobserved. Ni, Mn, and O are present in the subscale aboveunreacted tungsten. Formation of Cr2O3 below the unreactedtungsten and cobalt shows the probability of inward penetrationof corrosion species through the gaps in between the metallayer. X-ray mapping of corroded Superni 600 showed thepresence of Cr, Mn and O throughout the scale. There is apresence of Fe and O in the top surface of the scale. X-raymapping of corroded Superni 718 showed that Cr, Ti, and Ocoexist in the oxide layer while Fe and Ni are absent where Crwas present.

5. Conclusion

1. In hot corrosion under the given simulated incineratorenvironment, Superco 605 shows massive weight gain ascompared to Superni 600 and Superni 718. Lot of spall-ing was observed in case of Superco 605.

Corrosion resistance of the superalloys is in the followingorder:

Superni 718> Superni 600> Superco 605

2. Parabolic rate constant during hot corrosion in incineratorenvironment for Superco 605 is very high showingunprotective behavior of oxide while Superni 600 andSuperni 718 follow active oxidation mechanism leadingto slight decrease in weight.

3. The scale was found to be thin in case of both Nickel-based alloys while in Cobalt-based alloy the oxide scaleformed on the surface was thick. In corroded Superco605 oxides were found to be CoO, NiWO4, NiCr2O4,and CoCr2O4 while in Superni 600 and Superni 718,Cr2O3 and NiCr2O4 were formed.

4. From the study it can be concluded that bare Cobalt-based alloy may not be suitable for incinerator environ-ment whereas the nickel-based alloys may be suitable forthe given environment.

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Fig. 13 Standard free energy change of Cr2O3 chlorination reac-tions as a function of temperature (Ref 25)

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