high temperature corrosion of nickel in navo -v melts6 advancesinmaterialsscienceandengineering...

Research ArticleHigh Temperature Corrosion of Nickel in NaVO3-V2O5 Melts

J Porcayo-Calderon1 J J Ramos-Hernandez2 Jan Mayeacuten3 E Porcayo-Palafox1

G K Pedraza-Basulto4 J G Gonzalez-Rodriguez1 and L Martinez-Gomez25

1CIICAp Universidad Autonoma del Estado de Morelos Avenida Universidad 1001 62209 Cuernavaca MOR Mexico2Instituto de Ciencias Fısicas UniversidadNacional Autonoma deMexico AvenidaUniversidad sn 62210 CuernavacaMORMexico3CENIDET Tecnologico Nacional de Mexico Prolongacion Palmira sn Esq Apatzingan Col Palmira62490 Cuernavaca MOR Mexico4Facultad de Ingenierıa UNACAR 24180 Ciudad del Carmen CAM Mexico5Corrosion y Proteccion (CyP) Buffon 46 11590 Mexico City Mexico

Correspondence should be addressed to J Porcayo-Calderon jporcayocgmailcom

Received 23 February 2017 Revised 12 May 2017 Accepted 25 May 2017 Published 22 June 2017

Academic Editor Patrice Berthod

Copyright copy 2017 J Porcayo-Calderon et al This is an open access article distributed under the Creative Commons AttributionLicense which permits unrestricted use distribution and reproduction in any medium provided the original work is properlycited

Many alloys used at high temperature in industrial processes are Ni-based andmany others contain it in appreciable quantities so itis of interest to evaluate the performance of pure nickel in order to determine the behavior of its alloys once the elements responsiblefor their protection have been depleted due to accelerated corrosion processes in the presence of vanadium-rich molten salts Dueto this this work presents the study of Ni behavior in NaVO

3-V2O5mixtures at different temperatures The behavior of pure nickel

was determined by both electrochemical and mass loss measurements The results show that the aggressiveness of the vanadiumsalts is increased by increasing both the V

2O5content and temperature V

2O5addition considerably increases the current densities

of the anodic and cathodic reactions The corrosion process of Ni is modified due to the presence of its corrosion products and itspresence increases the activation energy by at least one order of magnitude Although nickel shows a high reactivity in vanadium-rich salts its reaction products are highly stable and protect it from the corrosive medium because the corrosion reactions trap thevanadium and block the migration of nickel ions

1 Introduction

Many alloys for engineering applications at high temperatureare Fe- or Ni-based Their excellent mechanical propertiesand corrosion resistancemake themuseful formanufacturingcomponents of industrial equipment such as industrial gasturbines and other energy conversion systems [1ndash6] Corro-sion resistance of alloys depends mainly on their chemicalcomposition In high temperature applications it is essentialthat the alloys be capable of developing protective oxides suchas Cr

2O3 Al2O3 or SiO

2 Chemical and physical charac-

teristics of this type of oxides such as good adhesion lowgas permeability high thermodynamic and low diffusioncoefficients provide excellent protection to alloys in gaseousenvironments mainly [7ndash9]

However the protection afforded by this type of oxidecan be limited when molten phases are involved in the

corrosion process In these cases the accelerated degradationof the materials may occur due to the so-called moltensalt corrosion hot corrosion or high temperature corrosionThis type of corrosion is characterized by the accelerateddegradation of the alloy when covered by a thin layer ofmolten salt at high temperature in an oxidant environmentIn these cases the integrity of any alloy will be compromisedif its protective oxides do not show low solubility in the melt[1 10 11]

The solubility of the protective oxides is greater in thepresence of vanadium-rich fused salts (sulfate-vanadate type)than in pure Na

2SO4 this is because the vanadate ions

increase the acidic fluxing of any oxide [10] On the otherhand in Na

2SO4-V2O5mixtures the maximum solubility

occurs in the interval 10 to 30 Na2SO4[12] In many indus-

trial systems where the use of heavy fossil fuels is commonthe species causing the catastrophic corrosion of thematerials

HindawiAdvances in Materials Science and EngineeringVolume 2017 Article ID 8929873 9 pageshttpsdoiorg10115520178929873

2 Advances in Materials Science and Engineering

are complex compounds of vanadium 120573 and 120574 sodium vana-dates (Na

2OsdotV2O4sdot5V2O5and 5Na

2OsdotV2O4sdot11V2O5 resp)

being the most corrosive vanadium species [6 12ndash14]When accelerated corrosion decreases the concentration

of the element responsible for forming the protective oxidecorrosion will proceed catastrophically if the rest of theelements of the alloy are unable to form stable corrosionproducts For example inmost alloys the element responsiblefor forming a protective oxide is Cr however in the presenceof acidic vanadium salts it is not capable of forming a stableprotective oxide [15]

Therefore because many Ni-based superalloys showexcellent corrosion resistance due to their ability to develop aprotective oxide Cr

2O3 this study evaluates the performance

of pure nickel in the presence of NaVO3-V2O5mixtures from

600 to 900∘CThis is a model system (few reaction productsNiO and vanadates) in order to determine if this elementwould be able to provide some protection to the alloy oncethe element responsible for protection has been depleted

2 Experimental Procedure

21 Corrosive Salts The corrosive medium employed con-sisted of different NaVO

3 V2O5ratios from pure NaVO

3

to pure V2O5 and two intermediates ratios that promote

the formation of the highly corrosive complex vanadatesnamely sodium 120573-vanadyl-vanadate (Na

2OsdotV2O4sdot5V2O5)

and sodium 120574-vanadyl-vanadate (5Na2OsdotV2O4sdot11V2O5) In

previous studies these complex vanadates have been syn-thesized from the following ratios 2NaVO

3 5V2O5and

10NaVO3 7V2O5 respectively [15] where NaVO

3is the

vanadium salt with the highest basicity V2O5is the com-

pound with the highest acidity and 2NaVO3 5V2O5and

10NaVO3 7V2O5ratios are compounds with intermediate

acidity and high oxygen adsorption capacity Since theNaVO

3can be synthesized from the Na

2O V2O5ratio it is

considered that by increasing the molar ratio of V2O5the

acidity of the resulting compound increases For simplicityhereafter the above-mentioned compounds will be identifiedaccording to the following nomenclature NV is NaVO

3 NV3

is sodium 120574-vanadyl-vanadate NV6 is sodium 120573-vanadyl-vanadate andV is V

2O5 where N is Na

2O Its knownmelting

points are 630∘C 627∘C 535∘C and 691∘C respectively [14]

22 Electrochemical Measurements Electrochemical evalua-tions were performed using a three-electrode arrangementAs pseudo-reference electrode and counter electrode a plat-inum wire was used Nickel samples were spot-welded witha Ni20Cr wire in order to get electrical connection Ni20Crwire was isolated with a ceramic tube and sealed withrefractory cement The test temperatures selected were 700800 and 900∘C which are above the melting point of thesalts The test temperature was measured constantly using atype K thermocouple and was controlled to 2∘C with respectto the test temperature Before starting the electrochemicalmeasurements the electrode array is immersed in themoltensalt and the open-circuit potential of the working electrodeis measured until steady state (plusmn5mV) is reached in all cases

this was achieved in about 30minutes In all experiments theatmosphere above the melt was static air Potentiodynamicpolarization curves and linear polarization resistance (LPR)measurements were carried out Potentiodynamic polariza-tion curves were obtained polarizing the working electrodefrom minus300 to 300mV at scanning rate of 1mVs and theelectrochemical parameters (Tafel slopes 119864corr and 119868corr)were obtained from the active regions of the correspond-ing anodic and cathodic zones LPR measurements wereobtained applying a potential of plusmn10mV and the variationin current intensity associated with that polarization wasmeasured during 24 hr An ACM Instrument potentiostatmodel GillAC controlled by a personal computer was usedfor the electrochemical experiments

23 Mass Loss Corrosion Testing Before mass loss tests thespecimenswere cleanedwith acetone and dried subsequentlyweighed by means of an analytical digital scale with aprecision of 000001 g and then packed in themixture of saltsin porcelain crucibles with 500mgcm2 of vanadium saltsThe corrosion tests were carried out in electric furnaces instatic air during 100 hours at temperatures of 700 800 and900∘C Four specimens were exposed to each test conditionAfter corrosion tests the corrosion rate was measured asmass loss Three specimens of each condition test weredescaled and chemically cleaned according to ASTM G1standard and the other specimen was mounted in ther-mosetting resin in cross section and polished to analyzethe subsurface corrosive attack using a Karl Zeiss DSM-950scanning electronic microscope (SEM) with an X-ray energydispersive (EDS) analyzer In short the specimen descalingwas realizedmechanically with a cutter in order to remove thecoarse and soft layers of products of corrosion subsequentlythe heavily adhered corrosion products were removed bychemical cleaning in a solution of 15 hydrochloric acid (vv)at room temperature with immersion times of 3 minutes andcleaning by light brushing until the complete removal of thecorrosion products (constant weight of the metal)

In all cases high purityNi (999)was used Ni specimenswere obtained from rectangular bars and cut in samples withdimensions of 10 times 5 times 5mm In order to obtain a homo-geneous and defined reaction area specimens were polishedwith silicon carbide sandpaper to grade 600 Subsequentlythe specimens were washed with distilled water and acetonebefore performing the corrosion tests

3 Results and Discussion

31 Polarization Curves Figures 1ndash3 show the potentiody-namic polarization curves for Ni in the different vanadiumsalts at 700 800 and 900∘C respectively It is observed thatin the presence of NV3 NV6 and V at all test temperaturesthe Ni polarization curves were shifted to higher currentdensities and nobler potentials than those observed in NVNo significant variation in 119864corr values was observed in thepresence of NV3 NV6 and V at all test temperatures Inall cases by increasing the vanadium content in the salt anincrease in the current densities of the anodic and cathodic

Advances in Materials Science and Engineering 3

minus500

minus400

minus300

minus200

minus100

0

100

200

300

00001 0001 001 01 1 10 100 1000

NVNV3

NV6V

Current (mAcm2)

Pote

ntia

l (m

V v

s Pt R

E)

Figure 1 Potentiodynamic polarization curves for Ni in NV NV3NV6 and V at 700∘C

001 01 1 10 100 1000minus500

minus400

minus300

minus200

minus100

0

100

200

300

Pote

ntia

l (m

V v

s Pt R

E)

NVNV3

NV6V

Current (mAcm2)


branches was observed This difference is most noticeable asthe temperature increases

The observed behavior is associated with the conductivenature of the vanadium salts because in the molten statethe vanadium salts act as electronic conductors or ionicconductors [16] The conductive nature of the vanadiumsalts depends on the Na

2O concentration V5+V4+ ratio

and oxygen partial pressure such that by increasing theNa2O concentration the ionic conduction increases and

by increasing the V5+V4+ ratio and the partial pressureof oxygen the electronic conduction increases Becausevanadium can coexist in different oxidation states (the moststable oxidation states are +5 +4 and +3) the aggressive-ness of the vanadium salts increases by increasing the V5+proportion (higher concentration of V

2O5) and the partial

pressure of oxygen (higher oxidation state of vanadium isfavored) on the other hand its aggressiveness is reduced by

001 01 1 10 100 1000minus500

minus400

minus300

minus200

minus100

0

100

200

300

Pote

ntia

l (m

V v

s Pt R

E)

NVNV3

NV6V

Current (mAcm2)

Figure 3 Potentiodynamic polarization curves for Ni NV NV3NV6 and V at 900∘C

increasing the concentration of Na2O due to an increase in

the proportion of V+4 and V+3 (increase in melt basicity)[11 17 18] Therefore the presence of these multivalent ionsenhanced the charge transport through the melt either bycounterdiffusion of multivalent cations or electron hopping[11]

From the current corrosion densities calculated fromFigures 1ndash3 the effect of the NV V ratio and temperature onthe nickel corrosion rate was determined Figure 4 shows theeffect of NV V ratio and temperature on Ni corrosion rateAlthough the corrosiveness of the salts NV3 and NV6 showslittle difference it is clear that the aggressiveness of the meltincreases by increasing its acidity (higher proportion of V+5)and temperature and the contrary happens by increasing itsbasicity (higher concentration of Na

2O) Similar trends have

been observed in studies using chromium [15] In particularthe NV3 andNV6 compounds have little difference in their Vcontent and they have been considered as the only vanadiumspecies with the highest oxygen absorbing capacity [12 19]The increase in the aggressiveness of the molten salts byincreasing the V concentration is congruent with previousreports indicating that the V

2O5catalyzes the oxidation

reaction of Ni [20]

32 LPR Measurements Figures 5ndash7 show the 119868corr variation(from LPR measurements) with time for Ni in the differentvanadium salts from 700 to 900∘C The observed trends aresimilar to all test temperatures namely there is a strongvariation in the 119868corr values in the first two hours of testand subsequently a pseudo-stationary state is reached Thisbehavior shows the high reactivity of Ni in the vanadiumsalts and that the corrosion products formed are highlyprotective In all cases the vanadium salts corrosivenessincreases with both the melt acidity and temperature andagain it is observed that the NV3 andNV6 species show littledifference in their degree of aggressiveness Figure 8 showsthe appearance of the corrosion front of Ni evaluated in NV3


0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80 90 100V2O5 ( molar)

700∘C800∘C900∘C

I corr

(mA

cm

2)

(a)

0

5

10

15

20

25

30

35

40

45

650 700 750 800 850 900 950

NVNV3

NV6V

Temperature (∘C)

I corr

(mA

cm

2)

(b)

Figure 4 Effect of NV V ratio (a) and temperature (b) on Ni corrosion rate

001

01

1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)

Figure 5 119868corr variation versus time for Ni in V-rich molten salts at700∘C

at 800∘C A similar behavior was observed at all temperaturesand with the different vanadium salts

In almost all cases a thin film rich in oxygen andnickel (NiO) onto Ni surface and a thick and dense layerof corrosion products rich in Ni-V-O onto the thin filmwere observed Corrosion products showed a defined formof crystals growing either from the Ni surface or from theNiO film towards the melt On the other hand the presenceof Ni was not observed in the vanadium-rich zone All this iscongruent with the tendency of the LPR values that is to sayafter the initial attack (first hours of immersion) the corrosionproducts formed a protective barrier that limited the access ofthe vanadium salts to the Ni surface

In order to evaluate if there was a change in the corrosionprocess kinetics between the information obtained from the

1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)


polarization curves and LPR measurements the activationenergies were determined based on the 119868corr values obtainedfrom both tests In the case of the 119868corr values obtainedfrom the LPR tests the average of the last 4 hours of testwas considered The dependence between corrosion rate andtemperature can be obtained from the Arrhenius equation

119868corr = 119896 exp (minusΔ119864119877119879)

119877 ln (119868corr) = 119877 ln 119896 minus 1119879Δ119864(1)

where Δ119864 (Jmolminus1) is the activation energy 119896 is a con-stant and 119877 is the gas constant (8314472 JK-mol) Fig-ure 9 shows the 119877 ln(119868corr) versus 1119879 relationship of the


1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)

Figure 7 119868corr variation versus time for Ni in V-rich molten salts at 900∘C

Image of secondary electrons

Vanadium

Ni

Oxygen

Figure 8 Cross sectional aspect of Ni and element mapping after LPR measurements in NV3 at 800∘C for 24 hours

119868corr values obtained from the potentiodynamic polarizationcurves and the LPR measurements In the first case theinformation derived from the potentiodynamic polarizationcurves corresponds to the interaction of the bare Ni surfacewith the molten salts and in the second case due to thecorrosion process of Ni with corrosion products adheredto its surface (NiO and nickel vanadates) In the case ofa bare Ni surface the activation energies determined were

193 23 22 and 215 kJmolminus1 for NV NV3 NV6 and Vrespectively These values show that the Ni corrosion rateincreases with the vanadium content of the molten salts andthat the corrosiveness of the NV3 NV6 and V is at least onemagnitude order greater than that of the NV In contrast theactivation energy values after 20 hours of corrosion variedbetween 181 and 202 kJmolminus1 for the different vanadium saltsThe corrosiveness of the NV did not change significantly


minus25minus20minus15minus10minus5

05

101520253035

00008 000085 00009 000095 0001 000105 00011

NVNV3

NV6V

1T (1K)

Rln

I corr

(a)

NVNV3

NV6V


05

101520253035

00008 000085 00009 000095 0001 000105 000111T (1K)

Rln

I corr

(b)

Figure 9 119877 ln(119868corr)-1119879 relationship for Ni evaluated in vanadium salts (a) Data from the potentiodynamic polarization curves (b) Datafrom LPR measurements

0102030405060708090

100

0 20 40 60 80 100

Cor

rosio

n ra

te (m

gcm

2)

V2O5 ( molar)

900∘C800∘C700∘C

(a)

0102030405060708090

100

650 700 750 800 850 900 950Temperature

NVNV3

NV6V

Cor

rosio

n ra

te (m

gcm

2)

(b)

Figure 10 (a) Effect of the NV V ratio and (b) temperature on Ni corrosion rate after 100 hours

but it is clear that the reactivity of the other vanadium saltsdecreased by one order of magnitude The drastic increasein the activation energy values shows that the corrosionproducts adhering to the Ni surface provide high protection

33Mass Loss Tests Figure 10 shows the effect of the differentvanadium salts on the corrosion rate of Ni expressed in termsof weight loss (mgcm2) at different temperatures after 100hours of immersion From the figures it is observed thatat higher exposure times the Ni behavior in the differentvanadium salts shows the same tendency as observed inthe previous tests namely the aggressiveness of the moltensalts increases with the vanadium content and temperature

Corrosiveness of NV3 and NV6 was similar being slightlyhigher with NV6

Figure 11 shows the cross sectional images of the Nisamples after being immersed in vanadium salts at 700 800and 900∘C for 100 hours In all cases similar characteristics areobserved At all temperatures onto Ni surface a nickel oxidelayer was observed (in NV3 NV6 and V at 900∘C it is notvery evident) and on it a thick layer of corrosion productsrich inVNi andO It is clear that nickel was rapidly corrodedby the vanadium salts however the thick layer of corrosionproducts formed can act as a barrier against the penetrationof the corrosive agent

Regardless of the molten salt the X-ray diffractionanalysis of the corrosion products showed that these were


NV

NV3

NV6

V

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

Figure 11 Micrographs of the cross sections of Ni exposed to NV3 at 600 700 800 and 900∘C during 100 hours

mainly composed of nickel oxide (NiO) and nickel vanadate(Ni3V2O8) similar observations have been made in other

studies [21 22] This indicates the high corrosiveness of thevanadium salts because they increase the acidic solubility of

the metallic oxides [10] According to the chemical composi-tion of the vanadium salts used the Ni degradation occurredaccording to the following reactions

2Ni +O2997888rarr 2NiO (2)


0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25

minuslog a (Na2O(c))

Ni (cr)V2O5 (L)NaVO3 (cr)

NiO (cr-c)Ni3V2O8 (s)

Figure 12 Thermodynamic stability diagrams for Ni-Na-V-O at900∘C

3Ni + 2NaVO3+ 12O2 997888rarr Ni

3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)

3NiO + 2NaVO3997888rarr Ni

3V2O8+Na2O (5)

3NiO + V2O5997888rarr Ni

3V2O8 (6)

In the corrosion process by vanadium salts in molten statethe melt acts as n-type semiconductors where it is possiblethat oxygen diffuses as Ominus2 or VO

3

minus and the transfer ofcharge probably involves the redox reaction V+5 harr V+4[16 20 23] Therefore any modification to the equilibriumof these processes will affect the corrosiveness of the meltThe performance of a material in a highly corrosive mediumdepends on the chemical stability of both the metallicelements and their compounds such as oxides and vanadates[22] Therefore the observed decrease in the Ni corrosionrate as time elapses (LPR measurements) may be due to thehigh melting point of the nickel vanadate (1220∘C) whichis considered a refractory compound [24] The presence ofthis compound may have limited the mobility of aggressivespecies towards the metal surface

Because the Na2O reduces the acidity of the molten salts

the increase in the corrosion rate observed by increasing theV concentration is consistent with the reactions mentionedabove since at higher V content the proportion of Na

2O

formed is reduced Based on the above reactions in additionto the equilibrium 2NaVO

3= V2O5+ Na2O the phase sta-

bility diagram shown in Figure 12 was constructed accordingto the procedure indicated elsewhere [25]

According to the diagram it is observed that in awide range of partial oxygen pressures equivalent to themelt-atmosphere interface (asymp021 atm) and to the melt-metal

interface (asymp1 times 10minus4 to 1 times 10minus12 atm) if acid vanadium salts(NV3 NV6 andV) are present the only reaction product willbeNi3V2O8and on the contrary in low acidity salts (NV) the

additional presence of NiO is possibleThis is congruent withthe micrographs shown in Figure 11 In these it is observedthat only in the presence of NV at 900∘C a NiO thick layer isevident onto Ni surface According to other studies the lowaggressiveness in NV melts is due to the fact that during itsreaction with Ni or NiO in addition to forming the metallicvanadate it also generates Na

2O as a byproduct this further

reduces the acidity of themelt and affects the charge transportbecause the V5+ V4+ ratio is modified [11] In contrast thereaction of Ni or NiO with V only generates the metallicvanadate without any modification in the acidity of the meltIn the case of the NV3 and NV6 species the decrease intheir corrosiveness compared to that of the V is due to thepresence of NV which reduces the acidity of the mixtureHowever its aggressiveness is much greater than that shownby NV because these complex vanadates in molten state alsohave the ability to absorb a large amount of oxygen whichprovides a rapid transport of oxygen to the metal surface[12 19]

The high initial reactivity of Ni in the presence ofvanadium salts has a beneficial effect because it forms ahighly stable refractory phase Ni

3V2O8 which reduces the

corrosiveness of the melt by trapping the vanadium andblocking the migration of nickel ions [3 23 26] and thestability of the refractory phase still prevails in the presenceof Na2SO4[2] The results obtained are congruent with other

studies whereNiO has been proposed as a corrosion inhibitorin vanadium-rich molten salts [2 3 24] or Ni-derivativesto remove vanadium compounds [26] Notwithstanding theabove in the case of nickel-based alloys with a multielementchemical composition the results may be different due tosynergistic dissolution reactions between the different oxidesdeveloped [11 27ndash29]

4 Conclusions

From the results of this work it can be concluded that Nishows a high reactivity in vanadiummolten salts An increasein both the acidity and temperature of the melt causes anoticeable increase in the current densities of the anodicand cathodic reactions due to an increase in the transport ofcharge by the presence of vanadium inmultivalent state (V5+V+4) Corrosion products form a dense and protective barrieronto Ni surface which limits the access of the vanadium saltssince no presence of Ni into vanadium melt was observedThe corrosion products increase the activation energy of thecorrosive process by at least one order of magnitude Theinitial reactivity of the nickel forms a highly stable refractoryphase that reduces the corrosiveness of the melt by trappingthe vanadium and blocking the migration of nickel ions Thecorrosiveness of the vanadium salts increased in the orderNV lt NV3 lt NV6 lt V Low corrosiveness of the NV isdue to the fact that during the corrosion process the Na

2O

is released as a reaction byproduct and the melt basicity isincreased affecting the charge transport and decreasing the


V5+ V4+ ratio On the other hand the high corrosiveness ofV is due to the fact that its corrosion reactions only formmetallic vanadates which do not affect the melt acidity

Conflicts of Interest

The authors declare that there are no conflicts of interestregarding the publication of this paper

Acknowledgments

Financial support from Consejo Nacional de Ciencia y Tec-nologıa (CONACYT Mexico) (Project 159898) is gratefullyacknowledged

References

[1] X Montero and M C Galetz ldquoInhibitors and coatings againstvanadate-containing oil ash corrosion of boilersrdquo Surface andCoatings Technology vol 304 pp 211ndash221 2016

[2] E Rocca L Aranda and M Moliere ldquoChemistry of ash-deposits on gas turbines hot parts Reactivity of nickel zinc andiron oxides in (Na V S) molten saltsrdquoMaterials Science Forumvol 595-598 pp 169ndash176 2008

[3] E Rocca P Steinmetz and M Moliere ldquoRevisiting the inhi-bition of vanadium-induced hot corrosion in gas turbinesrdquoJournal of Engineering for Gas Turbines and Power vol 125 no3 pp 664ndash669 2003

[4] C Cuevas-Arteaga J Uruchurtu-Chavarın J Gonzalez GIzquierdo-Montalvo J Porcayo-Calderon and U Cano-Castillo ldquoCorrosion evaluation of Alloy 800 in sulfatevanadatemolten saltsrdquo Corrosion vol 60 no 6 pp 548ndash560 2004

[5] H Singh D Puri and S Prakash ldquoAn overview of Na2SO4

andor V2O5induced hot corrosion of Fe-and Ni-based super-

alloysrdquo Reviews on Advanced Materials Science vol 16 pp 27ndash50 2007

[6] J G Gonzalez-Rodriguez S Haro A Martinez-Villafane VM Salinas-Bravo and J Porcayo-Calderon ldquoCorrosion perfor-mance of heat resistant alloys in Na

2SO4-V2O5molten saltsrdquo

Materials Science and Engineering A vol 435-436 pp 258ndash2652006

[7] W J Quadakkers andM J Bennet ldquoOxidation induced lifetimelimits of thin walled iron based alumina forming oxidedispersion strengthened alloy componentsrdquo Materials Scienceand Technology vol 10 no 2 pp 126ndash131 1994

[8] IWolfH J Grabke andP Schmidt ldquoCarbon transport throughoxide scales on Fe-Cr alloysrdquoOxidation of Metals vol 29 no 3-4 pp 289ndash306 1988

[9] H W Grunling and R Bauer ldquoThe role of silicon in corrosion-resistant high temperature coatingsrdquo Thin Solids Films vol 95no 1 pp 3ndash20 1982

[10] Y S Zhang and R A Rapp ldquoSolubilities of CeO2 HfO

2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure

Na2SO4at 900∘Crdquo Corrosion vol 43 no 6 pp 348ndash352 1987

[11] Y-S Hwang andR A Rapp ldquoThermochemistry and solubilitiesof oxides in sodium sulfate-vanadate solutionsrdquo Corrosion vol45 no 11 pp 933ndash937 1989

[12] G W Cunningham and A d S Brasunas ldquoThe effects ofcontamination by vanadium and sodium compounds on theair-corrosion of stainless steelrdquo Corrosion vol 12 pp 389ndash4051956

[13] AWong R IMarchan and LMartinez ldquoMolten salt corrosionof heat resistant alloysrdquo in Proceedings of the CORROSION 95Paper 465 NACE International Houston TX USA 1995

[14] A Wong-Moreno Y Mujica and L Martinez ldquoHigh tempera-ture corrosion enhanced by fuel oil ash sepositsrdquo in Proceedingsof the CORROSION 94 paper 185 NACE International Hous-ton TX USA 1994

[15] J Porcayo-Calderon VM Salinas Bravo R A Rodrıguez-Dıazand L Martinez-Gomez ldquoEffect of the NaVO

3-V2O5ratio on

the high temperature corrosion of Chromiumrdquo InternationalJournal of Electrochemical Science vol 10 pp 4928ndash4945 2015

[16] R B Dooley and J R Wilson ldquoThe Corrosion of a 50Cr-50Ni alloy in liquid vanadate systems in the temperature range750ndash950∘Crdquo Journal of Engineering for Power vol 97 no 3 pp422ndash428 1975

[17] R Mittelstadt and K Schwerdtfeger ldquoThe dependence of theoxidation state of vanadium on the oxygen pressure in melts ofVOx Na

2O-VOx and CaO-SiO

2-VOxrdquo Metallurgical Transac-

tions B vol 21 no 1 pp 111ndash120 1990[18] I B Singh ldquoCorrosion and sulphate ion reduction studies on

Ni and Pt surfaces in with and without V2O5in (LiNaK)

2SO4

meltrdquo Corrosion Science vol 45 no 10 pp 2285ndash2292 2003[19] N D Phillips and C L Wagoner ldquoUse of differential thermal

analysis in exploring minimum temperature limits of oil ashcorrosionrdquo Corrosion vol 17 no 8 pp 396tndash400t 1961

[20] E Chassagneux and G Thomas ldquoStudies on V2O5induced hot

corrosionrdquo Materials Chemistry and Physics vol 17 no 3 pp273ndash284 1987

[21] S-I Muroi M Someno and M Kobayashi ldquoCorrosion prod-ucts of pure metals in V

205Meltrdquo Tetsu-To-Hagane vol 58 no

6 pp 773ndash786 1972[22] O Sotelo-Mazon J Porcayo-Calderon C Cuevas-Arteaga J J

Ramos-Hernandez J A Ascencio-Gutierrez and L Martinez-Gomez ldquoEIS Evaluation of Fe Cr and Ni in NaVO

3at 700∘Crdquo

Journal of Spectroscopy vol 2014 10 pages 2014[23] E Chassagneux G Thomas and M Soustelle ldquoCorrosion of

Nickel by V2O5 Critical phenomenon due to O

2pressurerdquo

Materials Science and Engineering vol 87 pp 379ndash382 1987[24] E Rocca L Aranda M Moliere and P Steinmetz ldquoNickel

oxide as a new inhibitor of vanadium-induced hot corrosion ofsuperalloysmdashcomparison to MgO-based inhibitorrdquo Journal ofMaterials Chemistry vol 12 no 12 pp 3766ndash3772 2002

[25] J J Ramos-Hernandez J Porcayo-Calderon V M Salinas-Bravo C D Arrieta-Gonzalez J G Gonzalez-Rodriguezand L Martinez-Gomez ldquoPhase stability diagrams for hightemperature corrosion processesrdquo Mathematical Problems inEngineering vol 2013 Article ID 542061 7 pages 2013

[26] N Di Salvia M P Malavasi G Di Salvia and A VaccarildquoFormation of Ni and Mg vanadates during the flameless oxy-combustion of heavy fuelsrdquo Fuel Processing Technology vol 138pp 534ndash539 2015

[27] R A Rapp ldquoHot corrosion of materialsrdquo Pure and AppliedChemistry vol 62 pp 113ndash122 1990

[28] R A Rapp ldquoHot corrosion ofmaterials A fluxingmechanismrdquoCorrosion Science vol 44 no 2 pp 209ndash221 2002

[29] Y Longa-Nava Y S Zhang M Takemoto and R A RappldquoHot Corrosion of Nickel-Chromium and Nickel-Chromium-AluminumThermal-SprayCoatings by SodiumSulfate-SodiumMetavanadate Saltrdquo Corrosion vol 52 no 9 pp 680ndash689 1996

Submit your manuscripts athttpswwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of



are complex compounds of vanadium 120573 and 120574 sodium vana-dates (Na

2OsdotV2O4sdot5V2O5and 5Na

2OsdotV2O4sdot11V2O5 resp)

being the most corrosive vanadium species [6 12ndash14]When accelerated corrosion decreases the concentration

of the element responsible for forming the protective oxidecorrosion will proceed catastrophically if the rest of theelements of the alloy are unable to form stable corrosionproducts For example inmost alloys the element responsiblefor forming a protective oxide is Cr however in the presenceof acidic vanadium salts it is not capable of forming a stableprotective oxide [15]

Therefore because many Ni-based superalloys showexcellent corrosion resistance due to their ability to develop aprotective oxide Cr

2O3 this study evaluates the performance

of pure nickel in the presence of NaVO3-V2O5mixtures from

600 to 900∘CThis is a model system (few reaction productsNiO and vanadates) in order to determine if this elementwould be able to provide some protection to the alloy oncethe element responsible for protection has been depleted

2 Experimental Procedure

21 Corrosive Salts The corrosive medium employed con-sisted of different NaVO

3 V2O5ratios from pure NaVO

3

to pure V2O5 and two intermediates ratios that promote

the formation of the highly corrosive complex vanadatesnamely sodium 120573-vanadyl-vanadate (Na

2OsdotV2O4sdot5V2O5)

and sodium 120574-vanadyl-vanadate (5Na2OsdotV2O4sdot11V2O5) In

previous studies these complex vanadates have been syn-thesized from the following ratios 2NaVO

3 5V2O5and

10NaVO3 7V2O5 respectively [15] where NaVO

3is the

vanadium salt with the highest basicity V2O5is the com-

pound with the highest acidity and 2NaVO3 5V2O5and

10NaVO3 7V2O5ratios are compounds with intermediate

acidity and high oxygen adsorption capacity Since theNaVO

3can be synthesized from the Na

2O V2O5ratio it is

considered that by increasing the molar ratio of V2O5the

acidity of the resulting compound increases For simplicityhereafter the above-mentioned compounds will be identifiedaccording to the following nomenclature NV is NaVO

3 NV3

is sodium 120574-vanadyl-vanadate NV6 is sodium 120573-vanadyl-vanadate andV is V

2O5 where N is Na

2O Its knownmelting

points are 630∘C 627∘C 535∘C and 691∘C respectively [14]

22 Electrochemical Measurements Electrochemical evalua-tions were performed using a three-electrode arrangementAs pseudo-reference electrode and counter electrode a plat-inum wire was used Nickel samples were spot-welded witha Ni20Cr wire in order to get electrical connection Ni20Crwire was isolated with a ceramic tube and sealed withrefractory cement The test temperatures selected were 700800 and 900∘C which are above the melting point of thesalts The test temperature was measured constantly using atype K thermocouple and was controlled to 2∘C with respectto the test temperature Before starting the electrochemicalmeasurements the electrode array is immersed in themoltensalt and the open-circuit potential of the working electrodeis measured until steady state (plusmn5mV) is reached in all cases

this was achieved in about 30minutes In all experiments theatmosphere above the melt was static air Potentiodynamicpolarization curves and linear polarization resistance (LPR)measurements were carried out Potentiodynamic polariza-tion curves were obtained polarizing the working electrodefrom minus300 to 300mV at scanning rate of 1mVs and theelectrochemical parameters (Tafel slopes 119864corr and 119868corr)were obtained from the active regions of the correspond-ing anodic and cathodic zones LPR measurements wereobtained applying a potential of plusmn10mV and the variationin current intensity associated with that polarization wasmeasured during 24 hr An ACM Instrument potentiostatmodel GillAC controlled by a personal computer was usedfor the electrochemical experiments

23 Mass Loss Corrosion Testing Before mass loss tests thespecimenswere cleanedwith acetone and dried subsequentlyweighed by means of an analytical digital scale with aprecision of 000001 g and then packed in themixture of saltsin porcelain crucibles with 500mgcm2 of vanadium saltsThe corrosion tests were carried out in electric furnaces instatic air during 100 hours at temperatures of 700 800 and900∘C Four specimens were exposed to each test conditionAfter corrosion tests the corrosion rate was measured asmass loss Three specimens of each condition test weredescaled and chemically cleaned according to ASTM G1standard and the other specimen was mounted in ther-mosetting resin in cross section and polished to analyzethe subsurface corrosive attack using a Karl Zeiss DSM-950scanning electronic microscope (SEM) with an X-ray energydispersive (EDS) analyzer In short the specimen descalingwas realizedmechanically with a cutter in order to remove thecoarse and soft layers of products of corrosion subsequentlythe heavily adhered corrosion products were removed bychemical cleaning in a solution of 15 hydrochloric acid (vv)at room temperature with immersion times of 3 minutes andcleaning by light brushing until the complete removal of thecorrosion products (constant weight of the metal)

In all cases high purityNi (999)was used Ni specimenswere obtained from rectangular bars and cut in samples withdimensions of 10 times 5 times 5mm In order to obtain a homo-geneous and defined reaction area specimens were polishedwith silicon carbide sandpaper to grade 600 Subsequentlythe specimens were washed with distilled water and acetonebefore performing the corrosion tests

3 Results and Discussion

31 Polarization Curves Figures 1ndash3 show the potentiody-namic polarization curves for Ni in the different vanadiumsalts at 700 800 and 900∘C respectively It is observed thatin the presence of NV3 NV6 and V at all test temperaturesthe Ni polarization curves were shifted to higher currentdensities and nobler potentials than those observed in NVNo significant variation in 119864corr values was observed in thepresence of NV3 NV6 and V at all test temperatures Inall cases by increasing the vanadium content in the salt anincrease in the current densities of the anodic and cathodic


minus500

minus400

minus300

minus200

minus100

0

100

200

300

00001 0001 001 01 1 10 100 1000

NVNV3

NV6V

Current (mAcm2)

Pote

ntia

l (m

V v

s Pt R

E)


001 01 1 10 100 1000minus500

minus400

minus300

minus200

minus100

0

100

200

300

Pote

ntia

l (m

V v

s Pt R

E)

NVNV3

NV6V

Current (mAcm2)









001 01 1 10 100 1000minus500

minus400

minus300

minus200

minus100

0

100

200

300

Pote

ntia

l (m

V v

s Pt R

E)

NVNV3

NV6V

Current (mAcm2)








reaction of Ni [20]



0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80 90 100V2O5 ( molar)

700∘C800∘C900∘C

I corr

(mA

cm

2)

(a)

0

5

10

15

20

25

30

35

40

45

650 700 750 800 850 900 950

NVNV3

NV6V

Temperature (∘C)

I corr

(mA

cm

2)

(b)


001

01

1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)





1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)



119868corr = 119896 exp (minusΔ119864119877119879)

119877 ln (119868corr) = 119877 ln 119896 minus 1119879Δ119864(1)



1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)



Vanadium

Ni

Oxygen






05

101520253035

00008 000085 00009 000095 0001 000105 00011

NVNV3

NV6V

1T (1K)

Rln

I corr

(a)

NVNV3

NV6V


05

101520253035

00008 000085 00009 000095 0001 000105 000111T (1K)

Rln

I corr

(b)


0102030405060708090

100

0 20 40 60 80 100

Cor

rosio

n ra

te (m

gcm

2)

V2O5 ( molar)

900∘C800∘C700∘C

(a)

0102030405060708090

100

650 700 750 800 850 900 950Temperature

NVNV3

NV6V

Cor

rosio

n ra

te (m

gcm

2)

(b)








NV

NV3

NV6

V

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C





2Ni +O2997888rarr 2NiO (2)


0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25






3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)


3V2O8+Na2O (5)


3V2O8 (6)


3




2O













4 Conclusions


2O






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of



minus500

minus400

minus300

minus200

minus100

0

100

200

300

00001 0001 001 01 1 10 100 1000

NVNV3

NV6V

Current (mAcm2)

Pote

ntia

l (m

V v

s Pt R

E)


001 01 1 10 100 1000minus500

minus400

minus300

minus200

minus100

0

100

200

300

Pote

ntia

l (m

V v

s Pt R

E)

NVNV3

NV6V

Current (mAcm2)









001 01 1 10 100 1000minus500

minus400

minus300

minus200

minus100

0

100

200

300

Pote

ntia

l (m

V v

s Pt R

E)

NVNV3

NV6V

Current (mAcm2)








reaction of Ni [20]



0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80 90 100V2O5 ( molar)

700∘C800∘C900∘C

I corr

(mA

cm

2)

(a)

0

5

10

15

20

25

30

35

40

45

650 700 750 800 850 900 950

NVNV3

NV6V

Temperature (∘C)

I corr

(mA

cm

2)

(b)


001

01

1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)





1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)



119868corr = 119896 exp (minusΔ119864119877119879)

119877 ln (119868corr) = 119877 ln 119896 minus 1119879Δ119864(1)



1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)



Vanadium

Ni

Oxygen






05

101520253035

00008 000085 00009 000095 0001 000105 00011

NVNV3

NV6V

1T (1K)

Rln

I corr

(a)

NVNV3

NV6V


05

101520253035

00008 000085 00009 000095 0001 000105 000111T (1K)

Rln

I corr

(b)


0102030405060708090

100

0 20 40 60 80 100

Cor

rosio

n ra

te (m

gcm

2)

V2O5 ( molar)

900∘C800∘C700∘C

(a)

0102030405060708090

100

650 700 750 800 850 900 950Temperature

NVNV3

NV6V

Cor

rosio

n ra

te (m

gcm

2)

(b)








NV

NV3

NV6

V

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C





2Ni +O2997888rarr 2NiO (2)


0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25






3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)


3V2O8+Na2O (5)


3V2O8 (6)


3




2O













4 Conclusions


2O






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of



0

5

10

15

20

25

30

35

40

0 10 20 30 40 50 60 70 80 90 100V2O5 ( molar)

700∘C800∘C900∘C

I corr

(mA

cm

2)

(a)

0

5

10

15

20

25

30

35

40

45

650 700 750 800 850 900 950

NVNV3

NV6V

Temperature (∘C)

I corr

(mA

cm

2)

(b)


001

01

1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)





1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)



119868corr = 119896 exp (minusΔ119864119877119879)

119877 ln (119868corr) = 119877 ln 119896 minus 1119879Δ119864(1)



1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)



Vanadium

Ni

Oxygen






05

101520253035

00008 000085 00009 000095 0001 000105 00011

NVNV3

NV6V

1T (1K)

Rln

I corr

(a)

NVNV3

NV6V


05

101520253035

00008 000085 00009 000095 0001 000105 000111T (1K)

Rln

I corr

(b)


0102030405060708090

100

0 20 40 60 80 100

Cor

rosio

n ra

te (m

gcm

2)

V2O5 ( molar)

900∘C800∘C700∘C

(a)

0102030405060708090

100

650 700 750 800 850 900 950Temperature

NVNV3

NV6V

Cor

rosio

n ra

te (m

gcm

2)

(b)








NV

NV3

NV6

V

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C





2Ni +O2997888rarr 2NiO (2)


0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25






3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)


3V2O8+Na2O (5)


3V2O8 (6)


3




2O













4 Conclusions


2O






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of



1

10

100

0 2 4 6 8 10 12 14 16 18 20 22 24Time (hours)

NVNV3

NV6V

I corr

(mA

cm

2)



Vanadium

Ni

Oxygen






05

101520253035

00008 000085 00009 000095 0001 000105 00011

NVNV3

NV6V

1T (1K)

Rln

I corr

(a)

NVNV3

NV6V


05

101520253035

00008 000085 00009 000095 0001 000105 000111T (1K)

Rln

I corr

(b)


0102030405060708090

100

0 20 40 60 80 100

Cor

rosio

n ra

te (m

gcm

2)

V2O5 ( molar)

900∘C800∘C700∘C

(a)

0102030405060708090

100

650 700 750 800 850 900 950Temperature

NVNV3

NV6V

Cor

rosio

n ra

te (m

gcm

2)

(b)








NV

NV3

NV6

V

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C





2Ni +O2997888rarr 2NiO (2)


0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25






3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)


3V2O8+Na2O (5)


3V2O8 (6)


3




2O













4 Conclusions


2O






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of




05

101520253035

00008 000085 00009 000095 0001 000105 00011

NVNV3

NV6V

1T (1K)

Rln

I corr

(a)

NVNV3

NV6V


05

101520253035

00008 000085 00009 000095 0001 000105 000111T (1K)

Rln

I corr

(b)


0102030405060708090

100

0 20 40 60 80 100

Cor

rosio

n ra

te (m

gcm

2)

V2O5 ( molar)

900∘C800∘C700∘C

(a)

0102030405060708090

100

650 700 750 800 850 900 950Temperature

NVNV3

NV6V

Cor

rosio

n ra

te (m

gcm

2)

(b)








NV

NV3

NV6

V

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C





2Ni +O2997888rarr 2NiO (2)


0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25






3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)


3V2O8+Na2O (5)


3V2O8 (6)


3




2O













4 Conclusions


2O






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of



NV

NV3

NV6

V

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C

700∘C 800∘C 900∘C





2Ni +O2997888rarr 2NiO (2)


0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25






3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)


3V2O8+Na2O (5)


3V2O8 (6)


3




2O













4 Conclusions


2O






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of



0

minus2

minus4

minus6

minus8

minus10

minus12

minus14

minus16

minus18

minus20

logP

(O2)

5 9 13 17 21 25






3V2O8+Na2O (3)

3Ni + V2O5+ 2O2997888rarr Ni

3V2O8+ 12O2 (4)


3V2O8+Na2O (5)


3V2O8 (6)


3




2O













4 Conclusions


2O






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of






Acknowledgments


References















2and

Y2O3in fused Na

2SO4-30 molNaVO

3and CeO

2in Pure







3-V2O5ratio on




2O-VOx and CaO-SiO




2SO4









3at 700∘Crdquo



2pressurerdquo















CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of









CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


high temperature corrosion of nickel in navo -v melts6 advancesinmaterialsscienceandengineering...

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