Materials Science and Engineering A 399 (2005) 344–350
Molten salt corrosion resistance of FeAl alloy withadditions of Li, Ce and Ni
J.G. Gonzalez-Rodrıgueza, A. Luna-Ramırezb,e, M. Salazarb,c,J. Porcayo-Calderonb, G. Rosasd, A. Martınez-Villafanee,∗a Universidad Aut´onoma del Estado de Morelos, FCQI-CIICAP, 62210 Cuernavaca, Mor., M´exico
b Instituto de Investigaciones El´ectricas, Cuernavaca, Mor., M´exicoc Instituto Mexicano del Petroleo, Eje Central Lazaro Cardenas, M´exico, D.F., Mexico
d Universidad Michoacana de San Nicolas de Hidalgo, Morelia, Mich., M´exicoe CIMAV, Miguel de Cervantes 120, Complejo Industrial Chihuahua, 31109 Chihuahua, Chih., M´exico
Received in revised form 30 March 2005; accepted 5 April 2005
The corrosion performance of FeAl intermetallic alloys with additions of (1 at.%)Li, Ce, Ni and combinations (Ce + Li and Ceolten salts have been studied using the weight loss technique. Salts included Na2SO4 and NaVO3 and testing temperatures included 600,nd 700◦C for NaVO3, and 900, 950 and 1000◦C for Na2SO4 during 100 h. The corroded specimens were studied in the scanning eleicroscope (SEM) and the corrosion products analyzed with an X-ray energy dispersive analyzer (EDX) attached to it. The corrosion
n NaVO3 increases as the temperature increased, whereas in Na2SO4 decreased. The effect of the different alloying elements dependedhe salt used. In NaVO3, for instance, the FeAl + Ce + Li alloy was one with the highest corrosion rates but in Na2SO4 it had the lowesorrosion rate. The addition of these elements most of times increased the corrosion rate of the FeAl-base alloy, whereas in Na2SO4 most ofimes decreased the corrosion rate. The results are discussed in terms of the degree of protectiveness that the external Al2O3 layer gives to thlloys depending on the testing temperature.2005 Elsevier B.V. All rights reserved.
eywords: Iron aluminide; Hot corrosion; Weight lost test
. Introduction
Iron aluminides, based around the compositions Fe3Al andeAl, are candidate for high temperature materials becausef their excellent oxidation resistance, relatively low costnd material density[1–3]. However, the main disadvan-
age of iron aluminides is their poor ductility, and manyfforts have been done to improve their mechanical prop-rties through control of microstructure and alloy compo-ition [4–7]. Recently, Salazar et al.[8] studied the effectf additions of lithium (Li), cerium (Ce) and nickel (Ni) on
he structural and mechanical properties of FeAl alloys and
found that additions of 0.1 at.%Li or with the combinatof Li + Ce improve the ductility of this intermetallic. Somworks reported the behavior of FeAl alloys in molten sand condensed deposits, however, little has been pubon the high-temperature corrosion in molten salt mixtof V2O5–Na2SO4. Gesmundo et al.[9] studied the corrosion behavior of Fe3Al (27 at.%Al–2.2 at.%Cr–0.1 at.%Band FeAl (40 at.%Al–0.05 at.%Zr–0.06 at.%B–0.085 at.%alloys coated by a layer of molten salt which consiof Na2SO4–15 wt.%V2O5 and exposed in a N2, 1%O2,0.5%SO2 (vol.%) simulated combustion gases at 600◦C. Thesalt was applied as a slurry by a brush. They foundafter 96 h of testing, the FeAl alloy showed better cosion resistance than the Fe3Al. Tortorelli and Natesan[3],when working with Fe3Al2Cr intermetallic alloy in molteNa2SO4 at 650 and 700◦C during 800 h, found that al
J.G. Gonz´alez-Rodr´ıguez et al. / Materials Science and Engineering A 399 (2005) 344–350 345
minum oxide (Al2O3) layer plays a very important role inthe alloy corrosion resistance. Corrosion of iron aluminidesin molten NaNO3(–KNO3)–Na2O2 at 650◦C proceeds byoxidation and a low release from an aluminum-rich productlayer into the salt such that the compositions with higher alu-minum concentrations yielded significantly better resistance.Recently Deabashis et al.[10] studied the effect of carbon(3.7 at.%) on the hot corrosion of Fe3Al in molten Na2SO4,finding that the alloy with carbon had a higher corrosion ratethan the alloy without carbon because the carbides presentalong the grain boundaries in this alloy hinder the diffusionof sulfur into the material.
The goal of this work was to evaluate the effect of theadditions of minor elements such as Li, Ce and Ni on thehot corrosion performance of the FeAl intermetallic alloy intwo commonly found salts in power stations, Na2SO4 andNaVO3.
2. Experimental procedure
The FeAl alloys, with chemical compositions as given inTable 1, were obtained under conventional gravity castingtechniques with pure Fe, Al, Li, Ce using SiC crucibles. Toreveal the microstructure of the alloys, they were mechani-cally polished to 1.0�m grade diamond paste and etched withaH
diummA lg anedw acedit elec-t 600,6 gp -f nti atew ondi-t ng toA inb bsur-f copy( ) toc
TC
A n
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Fig. 1. SEM micrograph of the FeAl alloy showing the presence of bothAl2O3 and AlFe3C precipitates.
3. Experimental results
3.1. Microstructures
The microstructures obtained for some FeAl alloys areshown inFigs. 1–4. In Fig. 1 it is shown the matrix, FeAl,which has some needle-type precipitates which turn out to bealumina (Al2O3) and AlFe3C according to the X-ray exper-iments as published elsewhere[8]. The addition of 1 at.%Ce(Fig. 2) the alloy had a high density of Al2O3 precipitation,without the presence of the AlFe3C phase. When 1 at.%Liwas added (Fig. 3), both the Al2O3 and AlFe3C phaseswere drastically lowered, i.e. there was a lack of precipi-tates. Finally, with the addition of 1Ce + 1Ni,Fig. 4, a strongformation of the AlFe3C phase covering large regions alongthe grain boundaries.
F ceo
solution consisting of 50% CH3COOH + 33% HNO3 + 17%Cl.The corrosive agent used in the tests was synthetic so
etavanadate (NaVO3) and pure sodium sulfate (Na2SO4).ll the reagents, i.e. Na2SO4 and NaVO3 were analyticarade. Before corrosion tests the specimens were cleith acetone and dried. After this, the specimens were pl
n a 15 porcelain crucibles and covered with 500 mg/cm2 ofhe synthetic salt. The corrosion tests were carried out inric furnaces in a static air during 100 h at temperatures of50 and 700◦C for the case of NaVO3 because its meltinoint is close to 625◦C. Finally, the tests in Na2SO4 were per
ormed at 900, 950 and 1000◦C, because its melting pois close to 890◦C. After corrosion tests the corrosion ras measured as weight loss. Two specimens of each c
ion test were decaled and chemically cleaned accordiSTM G1 81 standard[11]. One of each heat was mountedakelite in cross section and polished to analyze the su
ace corrosive attack using a scanning electron microsSEM) aided with energy dispersive spectroscopy (EDSarry out micro chemical analysis.
able 1hemical composition of used alloys
lloy designation Chemical compositio
eAl (base alloy) 50 at.%Fe–50AleAl + Ni FeAl + 1 at.%NieAl + Li FeAl + 1LieAl + Ce FeAl + 1CeeAl + Ce + Li FeAl + 1Ce + 1LieAl + Ce + Ni FeAl + 1Ce + 1Ni
ig. 2. SEM micrograph of the FeAl + 1 at.%Ce alloy showing the presenf Al2O3 and precipitates without the AlFe3C precipitates.
346 J.G. Gonz´alez-Rodr´ıguez et al. / Materials Science and Engineering A 399 (2005) 344–350
Fig. 3. SEM micrograph of the FeAl + 1 at.%Li alloy, showing the apprecia-ble reduction in precipitates.
3.2. Hot corrosion results
The effect of Ni, Ce and Li on the hot corrosion resistanceof the FeAl intermetallic alloy in NaVO3 as a function of tem-perature is shown inFig. 5. In all cases it is evident that thecorrosion rate increases as the temperature increases, and thecorrosion rate of the FeAl, base material, has an intermediatevalue with respect to the other alloys. At 600◦C, when thesalt is still unmelted, for example, the lowest corrosion ratewere shown by the FeAl + Ce and the FeAl + Ce + Ni alloys,whereas the highest values were exhibited by the FeAl + Niand FeAl + Ce + Li alloys. At 650 and 700◦C, however, thelowest corrosion rates were for the FeAl + Ce + Ni alloy, likeat 600◦C, whereas the highest weight lost was shown by theFeAl + Ce + Li, so, it seem that in all cases this alloy had a veryrapid deterioration, whereas the FeAl + Ce + Ni resisted themost the attack of the NaVO3 melt. In Na2SO4, on the otherhand, with the exception of the FeAl + Li alloy, in all cases thecorrosion rates decreased as temperature increases, as shownin Fig. 6. The FeAl + Li alloy had the highest weight lost in
F gl
Fig. 5. Effect of Ce, Li and Ni on the corrosion rate of FeAl as a function oftemperature in NaVO3.
all temperatures tested, and the FeAl + Ce + Li was amongthe alloys with the lowest corrosion rate. If we compareFigs. 5 and 6, it is clear that the effect of the alloying elementson the corrosion behavior of the FeAl intermetallic alloy isnot unique, since whereas in NaVO3 the FeAl + Ce + Li alloyhad one of the highest corrosion rates, in Na2SO4 was theopposite, i.e. it had one of the lowest corrosion rates. Simi-larly, additions of Ce + Ni induced one of the lowest weightlost in NaVO3 but one of the highest in Na2SO4.
3.3. Micrographs
A cross section image in the SEM of the FeAl alloy cor-roded in NaVO3 at 700◦C together with X-ray mappingsof Al, Fe and V is shown inFig. 7, whereasFig. 8 showsthe FeAl + Li alloy corroded in NaVO3 at 650◦C. It can beseen that, in both cases, on the external layer there is thepresence of Al, Fe and V; oxygen was found also but, evenmore important, the fact that the external layer is poorer inFe than the underlying alloy. Deabashis et al.[10] found that
F tiono
ig. 4. SEM micrograph of the FeAl + 1 at.%Ce + 1 at.%Ni alloy, showinarge regions of the AlFe3C phase.
ig. 6. Effect of Ce, Li and Ni on the corrosion rate of FeAl + Li as a funcf temperature in Na2SO4.
J.G. Gonz´alez-Rodr´ıguez et al. / Materials Science and Engineering A 399 (2005) 344–350 347
Fig. 7. SEM micrograph of the cross section of the FeAl corroded in NaVO3 at 700◦C and X-ray mappings of Al, Fe and V.
Fig. 8. SEM micrograph of the cross section of the FeAl + Li alloy corroded in NaVO3 at 650◦C and X-ray mappings of Al, Fe and V.
348 J.G. Gonz´alez-Rodr´ıguez et al. / Materials Science and Engineering A 399 (2005) 344–350
Fig. 9. SEM micrograph of the cross section of the FeAl corroded in Na2SO4 at 950◦C and X-ray mappings of Al, Fe and S.
the corrosion products on the external layer of his alloy con-sisted mainly of alumina (Al2O3) and iron oxide (�-Fe2O3)as minor compound, so, it is very likely that aluminum ispresent as Al2O3 whereas iron is present as Fe2O3. Vana-dium seems to be distributed in the same places as the Al is,which means that the molten salt is corroding the externalAl2O3 layer. Similarly,Fig. 9shows a cross section of FeAlalloy corroded in Na2SO4 at 950◦C together with the X-raymappings of Al, Fe and S. Again, the presence of Al and Feon the external layer is clear, showing maybe the presence ofAl2O3 and Fe2O3. There is some evidence of internal sulfi-dation, since some sulfur was detected inside the alloy. Also,some voids can be seen near the metal–scale interface, asshown inFig. 10, which is a magnification ofFig. 9a.
It is well known that the corrosion rate of an alloy dependsupon the degree of protectiveness that the external layerformed by the corrosion products on its surface gives to it.If this external layer is protective enough, the corrosion rateshould be low, but if this layer is not protective at all, then thecorrosion rate will be high. The Al-based alloys base theircorrosion resistance due to the establishment of a protective,compact, adherent, aluminum oxide, alumina (Al2O3) layer.With the establishment of the Al2O3 layer, the corrosion rateshould decrease. However, there are different forms of Al2O3,and they give different degrees of protection to the alloy.The predominant surface product that forms between 600 and8p rer ore
voluminous, more porous and less protective than�-Al2O3[14]. The temperature at which there is a transition from othertypes of alumina to the slower growing�-Al2O3 appears tobe 900◦C [14]. This could explain the observed fact of theincrease in the corrosion rate with increasing the temperaturein NaVO3 (Fig. 5) where the testing temperatures were lowerthan 900◦C, where the predominant is non-protective, fastgrowing Al2O3, and, on the other side, the decrease in corro-sion rate in Na2SO4 (Fig. 6) where the testing temperatureswere those for the formation of Al2O3 protective. On the otherhand, the formation of Al2O3 consumes certain quantity of
00◦C has been reported to be�-Al2O3, [12] but it is quiteossible for�-Al2O3 or �-Al2O3 to exist in this temperatuange[13]. These forms of alumina are the fast growing, m
Fig. 10. A magnification of image shown inFig. 9a.
J.G. Gonz´alez-Rodr´ıguez et al. / Materials Science and Engineering A 399 (2005) 344–350 349
Al, reducing its activity and partial pressure of the oxygen.This causes a relative increase in the activities of the Fe andthe S driving them to react with the molten mixture and toobtain compound such as the FeS.
The corrosive activity of a mixture depends on its elec-tric conductivity. As it is expected, as the temperature isincreased, the mobility of the ions of the salt is increased, andin that way also the conductivity. Zheng et al.[15] found thatthe conductivity of a mixture of 0–30 wt.%Na2SO4–NaVO3was increased with the temperature in a range from 1150 to1220 K. Another important characteristic of the molten saltswas its character acid-basic, where the SO3(g) it is the sourcomponent in the sulphates. These characteristics based onthe concept of Lewis and Lux–Flud that speak of the abil-ity of to donate load or to accept it electronically, considerthat, for example for the molten Na2SO4, it has the followingbalance:
Na2SO4 → Na2O + SO3(g) (1)
and the activity parameters and partial pressure are:
logaNa2O + logPSO3
= �G◦2/2.303RT = −16.7 at 927◦C (2)
where the parameter− logaNa2O can be taken as a measure oft sionsifab tens tallici
2
w
S
a tionf
A
lec-t tiono ofc sub-s aread tivelym pos-s ef h Crc oresa
ivesa thus,
the corrosion rate is increased. This can explain the incre-ment of the corrosion rates values at 700◦C at the end ofthe tests for the six materials in NaVO3. Additionally to thedissolution of the protective scales, the dissociation of theNa2SO4 increased the concentrations of Na2O in the melt,and later on, with the diffusion of Na and S, an increase ofthese elements in the metal/scale interface. This caused thedetachment and cracking of the protective scale, allowing thecorrosion of the material. This phenomenon was observed onthe FeAl + Li alloy at 1000◦C, which presented high corro-sion rates and an important metal degradation. This explainsthe values of the corrosion rates that were observed inFig. 6for the FeAl + Li alloy.
4. Conclusions
The effect of Li, Ce and Ni as minor elements on the hotcorrosion performance of FeAl alloys has been evaluated inNaVO3 and Na2SO4 salts using the weight loss technique.From these tests, the following conclusions can be drawn:
1. In NaVO3 the corrosion rate increases as the temperatureincreases, whereas in Na2SO4 the corrosion rate decreaseswith the temperature.
2. The addition of Ce, Li or Ni did not present a clear effectr in
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he salt basicity. In accordance with the previous express obvious to consider to (basicity) and logPSO3 (acidity)or Na2SO4, besides the partial pressure of the SO3 in bal-nce with Na2SO4 fixed the activity of Na2O or the meltingasicity[16,17]. Keeping in mind that balance in the molalt, the following reactions can be presented in the menterface:
FeAl + SO3(g) → Al2O3 + S + 2Fe (3)
hich generates the conditions for reaction carried out:
+ Fe → FeS (4)
nd in the oxide/molten salt interface the basic dissoluor reaction occurs[18]:
l 2O3 + Na2O → 2NaAlO2 (5)
The hot corrosion in molten salts usually exhibits seive attack and internal oxidation. For example, the deplef Cr in Fe–Cr–Ni alloys can happen by the formationhromium compounds in the metal surface and for theequent removal of chromium from the alloy, leaving anepleted in Cr. Therefore the species removed selecove outwards, when the vacancies move inwards and
ibly they leave pores behind them (Fig. 9). The pores arormed in the grain boundaries in most of the metals witontents, but in some alloys of high nickel content, the pre formed in the grains[19].
The scale cracking, spaling or dissolution in a melt, gs a result that the film becomes less protector and,
on the corrosion rates of the FeAl-base alloy neitheNaVO3 nor in Na2SO4.
. In NaVO3, the alloy with the lowest corrosion rate wFeAl + Ce + Ni, whereas the one that showed the higweight loss was the FeAl + Ce + Li one.
. In Na2SO4, there was not a single alloy with the lowcorrosion rate since most of them showed similar cosion rate, instead, the one with the highest weight lossthe FeAl + Ce + Ni.
. These results are discussed in terms of the abilitthe Al2O3 to protect the alloys: at temperatures lowthan 900◦C, the type of Al2O3 formed is no protectiveso the corrosion rate should increase as the tempeincreases. For temperatures higher than 900◦C, the formof Al2O3 formed on the external surface is more protive, and the corrosion rate decreases with increasintemperature.
cknowledgments
We kindly acknowledge to Mrs. H. Esparza-Ponce forssistance in the SEM work.
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