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Hot corrosion study of coated separator plates of molten carbonate fuel cells by slurry aluminides
Surface and Coatings Technology 161 (2002) 293–301
Hot corrosion study of coated separator plates of molten carbonate fuel cells by slurry aluminides a ´ a,*, D. Dudaya, M.P. Hierroa, C. Gomez ´ ´ b, R. Muelab, ¨ b, M.C. Garcıa F.J. Perez , A. Aguero A. Sanchez Pascualb, L. Martinezb a
´ Universidad Complutense de Madrid, Facultad de Ciencias Quımicas, Departamento de Ciencias de los Materiales, 28040 Madrid, Spain b ´ ´ de Ardoz, Madrid, Spain Instituto Nacional de Tecnica Aerospacial, 28850 Torrejon Received 15 May 2002; accepted in revised form 3 July 2002
Abstract The corrosion behavior of Al coated AISI 310S stainless steel by slurry and ion vapor deposition (IVD) was investigated as an electrolyte seal material in a mounted carbonate fuel cell (MCFC) at 650 8C. The results were compared with uncoated AISI 310S stainless steel and TA6V alloy. The characterization of the samples after exposure to the eutectic 62 mol.% Li2CO3 –38 mol.% K2CO3 mixture at 650 8C up to 1000 h has shown the presence of LiAlO2 (coated samples), LiFeO2 and LiCrO2 (stainless steels), and Li2TiO3 (TA6V) oxides at the scale–melt interface. The electrochemical impedance spectroscopy (EIS) technique has shown high values of polarization resistances for TA6V and lower values for coated steels. The uncoated AISI 310S stainless steel have shown the lowest polarization resistance. A mechanism for the corrosion of Al-coated stainless steels in molten carbonate is proposed taking into account thermodynamic simulations, X-ray diffraction (XRD), scanning electron microscopy (SEM) characterizations, and EIS results. This proposed mechanism confirms that a slurry aluminide coating is able to improve the stainless steel behavior in molten carbonate. However, the TA6V titanium alloy is the most resistant material in molten carbonate but the Al-coated stainless steels appear as the best lifetime-cost compromise. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Alloys; Coating; Molten carbonate; Hot corrosion; Fuel cell; Electrochemical impedance spectroscopy
1. Introduction The conventional molten carbonate fuel cell (MCFC) operates at 650 8C and consists of several cells composed of a porous, lithiated NiO cathode, a molten (Li,K)2CO3 electrolyte in a LiAlO2 ceramic matrix, and a porous Ni anode (Fig. 1) connected by stainless steel separator plates. The CO2y anions assume the ionic 3 conduction in the eutectic 62 mol.% Li2CO3 –38 mol.% K2CO3 electrolyte between the cathode and the anode. A part of the electrolyte extends beyond the electrode area and forms a molten seal against the stainless steel cell separator plates. The corrosion of the steel at the molten seal is one of the cell lifetime limiting parameters w1–23x. As a *Corresponding author. Tel.: q34-91-394-4215; fax: q34-913943-4357. ´ E-mail address: [email protected] (F.J. Perez).
result, the electrolyte can be wasted both by consumption in the corrosion process and by leaking due to loss of seal, contributing to overall cell efficiency loss. The separator plates commonly made of stainless steels (AISI 310 or 316) which are not sufficiently resistant to molten carbonate corrosion and therefore require coatings. The first studies on the aluminizing of the wet seal surfaces have shown an increase to their corrosion resistance by increasing the surface aluminum content w24,25x. The corrosion resistance of the coated alloys was shown to improve with increasing thickness of the coating. However, above a critical thickness the mechanical properties and the adhesion of the coating decreased rapidly w26x. Recently, the most commonly applied techniques have been the deposition of pure Al by ion vapor deposition (IVD) followed by heat treatment, electrodeposition of Ni followed by Al and heat treat-
0257-8972/02/$ - see front matter 䊚 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 2 . 0 0 4 1 7 - 6
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steel corrosion in molten carbonate with the EIS technique for long testing times, that can be extrapolated for real operational times of MCFC. 2. Experimental 2.1. Coatings preparation
Fig. 1. Schematic diagram of the molten carbonate fuel cell.
ment, and Al deposited by thermal spray or slurry also followed by heat treatment. IVD seems to be the technique mostly employed in large scale fuel cells but since it requires introducing the component in a vacuum chamber, it is too expensive for the large-scale production of MCFC. However, slurry application probably constitutes one of the most economical methods to produce coatings and therefore is an excellent candidate for this application. Although wet-seal corrosion and its prevention have received some attention in the literature w27–29x, few studies regarding the corrosion mechanism have been reported. No study has employed electrochemical impedance spectroscopy (EIS) to determine the corrosion mechanism of Al coatings deposited on MCFC separator plates in molten carbonate. This technique consists of applying a small electrical sinusoidal perturbation to the system studied and to analyze the sinusoidal electrical response after an appropriate mathematical treatment. It has the advantage of not significantly perturbing the studied system which allows a more precise analysis of the electrochemical processes carried out in molten carbonates. The EIS technique was recently used for the determination of alloy corrosion mechanisms in molten salts w6,14,17,18,30–32x. Recently in our laboratory, we have determined the corrosion mechanism of different stainless steels and TA6V alloy in molten carbonate by EIS w6,14,33x. In these studies, long exposure tests were made and the corrosion rates and evolution of the alloys were determined. It was shown that the TA6V alloy is much more resistant than coated stainless steels. In this work the corrosion resistance of pure Al slurry coatings on AISI 310S steel has been studied both by microstructural characterization and by EIS testing for long testing times. An IVD-coated sample as well as uncoated AISI 310S steel and TA6V were also tested for comparison purposes. This work is the first contribution to propose a precise determination of Al coated
The tested material was stainless steel AISI 310S (24.53% Cr, 19.28% Ni, 0.077% C) which is both a separator plate and current collector candidate. The specimen (6=15=2 mm) was ground with SiC噛1200 paper on all sides and washed with ethanol. Then, Al was deposited on the sample in two different ways. First, as liquid solution (Al-slurry) followed by a baking treatment during 3.5 h at 400 8C and diffusion treated 10 h at 820 8C under Ar atmosphere (COAT1). Second, as an IVD coating also diffusion treated 10 h at 820 8C under Ar (COAT2). The IVD method applied consisted of placing the sample on a vacuum chamber during 20 min at a pressure of 1.4=10y2 torr; the current to obtain the evaporation was 0.1 amperes at 250 V. After the deposition, specimens were spot-welded to a 0.5-mm conducting wire. A TA6V titanium alloy (6% Al, 4% V) as new candidate material was also tested for comparison with the coated steels. 2.2. Electrochemical tests and characterization of specimens Electrochemical tests were performed at 650 8C in an alumina crucible. The working electrode was composed of the test sample connected to a conducting wire, which was protected from the electrolyte by an alumina tube and a ceramic compound placed at the base of the tube. The electrochemical cell was composed of two identical electrodes formed of the tested material; this method worked at zero potential vs. open circuit. It permits work without the reference electrode which cannot be stable for long exposure times, and also, allows the possibilities of on-line monitoring to be explored. The carbonate mixture used as electrolyte was the eutectic mixture of lithium and potassium carbonate (62 mol.% Li2CO3 –38 mol.% K2CO3). It is used in an atmosphere that consists of the gases being formed during the melting process, and not introducing a gas mixture as CO2:O2 because this atmosphere induces a similar corrosion mechanism to the MCFC one. Indeed, the oxygen partial pressure is similar in both cases and the CO2 negative effect only appears for higher partial pressures for both MCFC and in our experimental systems w19x. The EIS measurements were started after the melting of the carbonate. Various measurements were made from 0 to 200 h. A 5 mV-perturbation amplitude was applied with a frequency scanning range of 10 mHz–30 kHz. All experiments were recorded with a frequency
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Table 1 Simulation at the scale–atmosphere interface at 650 8C and 1 atm with an excess of oxygen
Table 2 Simulation at the scale–melt interface at 650 8C and 1 atm with an excess in O2, Li2O and K2O
Compound
Initial quantity (mol)
Equilibrium quantity (mol)
Compound
Initial quantity (mol)
Equilibrium quantity (mol)
O2 Al2O3 Al Cr Fe Ni Fe2O3 Cr2O3 Fe2NiO4 Al2NiO4 NiO
1000 0.5 65 6 22 7 – – – – –
924 62.1 – – – – 7.5 3 4 1.7 1.2
O2 Li2O K 2O Al2O3 Fe2O3 Fe2NiO4 Cr2O3 Al2NiO4 NiO LiAlO2 LiFeO2 K2CrO4 Li2O2
100.0 500.0 100.0 62.0 7.5 4.0 3.0 1.7 1.2 – – – –
82.4 423.0 88.0 – – – – – 6.9 127.4 23.0 6.0 2.2
response an analyzer Solartron 1255 and a potentiostat EG&G283. After each experiment, the corroded specimens were taken out of the melt, washed with distilled water and investigated by X-ray diffraction (XRD) wPhilips X’Pertx and in the scanning electron microscope (SEM) wJEOL JSM 6400x with energy-dispersive X-ray microprobe analysis (EDX). 3. Results 3.1. Thermodynamic calculations When the lithiation begins, an oxide layer is formed (Table 1). In the case of the samples with a diffusion treatment, the results of the Table 1 are used to simulate the reaction of materials in contact with the melt. Two possibilities can occur. First, the Li2O from the melt is in excess compared with the oxides present (Table 2) or the Li2O is not sufficient to react with all the oxides present because of its low activity (Table 3). In the first case, the oxide scale is transformed mainly into LiAlO2 and LiFeO2 with a possible ratio of approximately 5 to 1 (Table 2). The NiO is supposed to form in a small quantity but it is more probable that Ni atoms are replacing Fe atoms in the LiFeO2 structure without formation of NiO. The Cr2O3 oxide is transformed into K2CrO4, which dissolves in the melt. When the Li2O and K2O are available in lower quantity than the oxides present, the scale obtained is supposed to be more complex (Table 3). It is composed mainly of alumina (45%), LiAlO2 (20%) and LiFeO2 (15%). Thus, the outer scale should be composed of approximately 60% LiAlO2 and 40% LiFeO2 at the scale–melt interface if the kinetics of the formation reactions are similar. The case of Cr2O3 is more complicated because it depends on supply of K2O and oxygen. If these two compounds are present in enough quantity, K2CrO4 will form and dissolve in the melt. In the opposite case, only LiCrO2 will form. The real situation is probably between these two cases, i.e.
Cr2O3 will transform partly into K2CrO4 and partly into LiCrO2 as shown in Table 3. The quantity of chromium products does significantly not affect the scale because they are present in a small quantity (5%). The Ni should be present in the LiFeO2 structure and as NiO. Other oxides can form in small quantities as shown in Table 4. In the case of samples without diffusion treatment, the scale is only composed of LiAlO2 because the metallic ions of the steel do not have time to reach the scale–melt interface before the LiAlO2 has formed. 3.2. Microstructural characterization 3.2.1. X-Ray diffraction On the XRD spectrum of the Al-coated steels with a post-coating diffusion treatment (Fig. 2), the Al5FeNi peaks appear. Taking into account the composition of the outer layer by microprobe analysis, it is more probable that a Al5Fe2 layer with a part of the Fe atoms Table 3 Simulation at the scale–melt interface at 650 8C and 1 atm with few Li2O K2O and O2 in excess Compound
Initial quantity (mol)
Equilibrium quantity (mol)
O2 Li2O K 2O Al2O3 Fe2O3 Fe2NiO4 Cr2O3 Al2NiO4 NiO LiAlO2 LiFeO2 K2CrO4 LiCrO2 LiAl5O8
100.0 20.0 5.0 62.0 7.5 4.0 3.0 1.7 1.2 – – – – –
86.1 – – 47.2 2.0 1.2 – 3.0 2.7 22.0 16.1 5.0 1.0 0.9
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Table 4 Results of simulation for the resistance values from the EIS spectra semicircles Immersion time (h)
PEP Al coated steel R1 (Ohms)
PEP Al coated steel R2 (Ohms)
INTA Al coated steel R1 (Ohms)
INTA Al coated steel R2 (Ohms)
Non-coated steel R1 (Ohms)
5 25 100 200
30 35 38 40
250 200 120 120
25 40 60 80
200 -100 150 150
10 15 20 –
R1, charge transfer resistance of the AL or Fe oxidation reaction; R2, resistivity of the scale.
(approx. 25%) replaced by Ni atoms is present. Indeed the XRD peaks are very similar between these two compounds. So during 10 h at 820 8C under Ar, the diffusion of the metallic elements of the steel and of Al is rapid enough to transform all the Al coating (approx. 10 mm) into an intermetallic layer. The FeAl phase is also detected. The low intensity of the peaks of this suggest that FeAl is situated under the Al5Fe2 layer. After the immersion of all the samples in molten carbonate, only the LiAlO2 compound appears (Fig. 3). Both the a-phase and g-phase are present on the surface of the immersed samples. The LiFeO2 does not appear maybe because it spalled off or because the kinetics or formation are much lower than for LiAl2. After 150 h the alumina layer is totally transformed into LiAlO2. 3.2.2. Scanning electron microscopy The cross-section of slurry-coated steel sample (COAT1) after 1000 h of immersion in molten carbonate is shown in Fig. 4. Few big pits are observed through the scale down to the middle of the diffusion layer, and the thick Al Oxide scale in steel is still protective on most parts of the sample.
The Al oxide scale is very thin after 1000 h of immersion for IVD samples (COAT2) and the corrosion seems to be more important because of the lower content of aluminum present and the formation of carbides at the steel–diffusion layer interface (Fig. 5). The SEM characterization has shown the best corrosion resistance of the slurry coating for 1000 h exposure in molten carbonate. In both cases the original thickness of the coatings were approximately 30 mm. 3.3. Electrochemical impedance spectroscopy results All the EIS curves coming from the immersion of the Al coated steels in molten carbonate show a little semicircular area at higher frequencies and a bigger semicircular area at lower frequencies. However, the evolution of the diameter of the semicircular arcs is different with time for both steels and at the lowest frequencies; a third arc or a line is observed for some samples. Indicating more than a electrochemical process that can occur on the surface. On the Fig. 6, the EIS spectra of different coated samples immersed for approximately 6 h are superposed
Fig. 2. X-Ray diffraction spectra of Al coated 310S stainless steels after the diffusion treatment.
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Fig. 3. X-Ray diffraction spectra of Al coated 310S stainless steels after the diffusion treatment and 200 h of immersion in molten carbonate.
with the EIS spectra of a 310S steel without coating and a TA6V alloy. The first semicircle is bigger for the COAT2 sample. The COAT1 sample and the TA6V show a first circle diameter situated between the COAT2 sample and the non-protected steel. The first circle of the steel without coating is much lower. From those results, the corrosion rate at the scale–melt interface seems slower for the COAT2 sample after approximately 6 h of immersion, at the beginning of the corrosion
process. All the coated samples show a much better behavior than the non-protected steel (Fig. 6). The second semicircle arc is bigger for the TA6V alloy and the COAT2 sample which means the oxidation rate is lower at the intermetallic-scale layer or that the scale is less conductive for these samples. A COAT1 sample immersed just after the melting of the electrolyte (Fig. 6) shows a second semicircle bigger than the same sample immersed in the melt before the melting. A
Fig. 4. SEM cross-section micrograph of a COAT1 (Al-Slurry coating) sample after the diffusion treatment and 1000 h of immersion in molten carbonate. Some crevices appear but most parts of the sample is still protected by the LiALO2.
298
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Fig. 5. SEM cross-section micrograph of a COAT2 (Al-IVD coating) sample after the diffusion treatment and 1000 h of immersion in molten carbonate. The scale is very thin and does not more protect the steel.
study of the conductivity of the scales will be useful for a more precise comparison. In Fig. 7, the EIS spectra of the same samples are represented after 25 h of immersion. The first little circle diameter is a little bigger for COAT2 sample and the immersed after the melting COAT1 sample compared with the COAT1 sample immersed before melting. The first circle is much smaller for the non-protected. Thus, the oxidation rate at the scale melt interface is slower,
after 25 h, if the immersion of the samples takes place before the melting of the electrolyte. After 25 h, the corrosion rate of the COAT1 and COAT2 samples is similar. The second semicircle is bigger for the COAT2 sample after 25 h. The COAT1 sample shows a smaller second semicircle. The uncoated steel shows the smallest second semicircle. So the electronic conductivity of the COAT2 sample seems lower and the conductivity of the
Fig. 6. Nyquist impedance diagram in the frequency range of 10 mHz–30 kHz for COAT1, COAT2 and non-coated steel and the TA6V titanium alloy samples.
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299
Fig. 7. Nyquist impedance diagram in the frequency range of 10 mHz–30 kHz for COAT1, COAT2 and non-coated steel samples. Time of immersion: 25 to 27 h.
uncoated sample seems higher because of the iron contained in the scale.
Other reactions can occur but they do not affect the EIS spectra because of the low quantity at the reaction sites, compared with Al3q and Fe3q, some of the
4. Discussion: Nishina et al. w34x have shown that first stage of the reduction of O2 in the melt resulted in a CO2 production with the reactions. 2y 2CO2y q2CO2 3 ™2O
(1)
2y O2q2CO2y 3 ™2O2 q2CO2
(2)
These reactions are governed by the acidity of the melt and, in the less acidic melts which correspond to our conditions, the reaction in Eq. (2) prevails, including a more important CO2 formation in the melt. As the CO2 is not consumed in the absence of an anode in the corrosion cell, the CO2 partial pressure is higher than that of the initial atmosphere. The high number of the reactions and the presence of different oxide layers in the studied materials and coatings, may generally hinder the elaboration of an equivalent circuit model in the case of the metallic corrosion in molten salt. Nevertheless, with a good knowledge of the system, it is possible to select the main electrochemical reactions, which take place at the surface of the samples, to propose a simplified model. In the case of the immersion of Al coated steels in molten carbonate, three electrochemical reactions would take place at one interface which play an important role in the corrosion mechanism as shown on the Fig. 8: Al™Al3qq3ey to form Al2O3 and LiAlO2
(3)
Fe™Fe3qq3ey to form Fe2O3 and LiFeO2
(4)
O22yq2ey™2O2y form the reactions (2)-(5)
(5)
Fig. 8. Schematic diagram of the Al coated samples diffusion treatment and immersion in molten carbonate. The scale consists only of Al2O3 at the beginning and LiAlO2 after a few hours of immersion. Also, equivalent circuit corresponding to the impedance spectra of the Al-coated steels.
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following reactions could taken into account: Fe™Fe
2q
q2e
y
to form ŽFe,Ni.Cr2O4
(6)
Cr™Cr3qq3ey to form Cr2O3 and LiCrO2
(7)
Ni™Ni2qq2ey to form NiOŽFe,Ni.Cr2 O4
(8)
Cr
3q
™Al
6q
q3e
y
to form K2CrO4
(9)
As the LiAlO2 and the Li2TiO3 outer layers are formed, after immersion, and taking into account that they are not good electronic conductors, it is possible that they modify the EIS spectra by the addition of another time constant. It has to be noted that the Alcoated steel system is less complex to model than the non-protected steel system in molten carbonate. Indeed, the number of main reactions is lower (mainly Al3q and Fe3q) and only one main oxide layer would be formed. From the Fig. 8, the thermodynamic calculations and the microstructure characterizations, the equivalent electrical circuit of attachment is proposed for the EIS analysis of the Al-coated steel system studied in this work. This circuit is composed of two time constant elements corresponding to the oxidation reaction of the Al (and Fe) at the intermetallic–scale interface and to the scale–electrical response, respectively. The high frequency circle is thought to be due to the oxidation reaction of Al to Al3q (and, at a lower extent, to the oxidation of Fe) because of the value of the frequencies and of the evolution of semicircle diameter with time. The intermediary circle is thought to correspond to the scale resistance and capacity, and the semicircle arc at the lower frequencies is thought to be due to charge transport phenomena in the case of the COAT1 samples. The values of the electrical elements, corresponding to the proposed equivalent circuit (Fig. 8) obtained for different samples and times of immersion are shown in Table 4. The differences of behavior at the beginning of the immersion for the coated samples can be due to defects of outer layer, among other mechanisms. Indeed if the outer layer has little porosity, the alumina scale has a problem to reach a sufficient thickness without breaking because of its brittleness. The increase of the corrosion rate decreases which means this latter oxide is less brittle and can suffer more stresses without spall off. After 25 h, the thickening of the scales with time allows a decrease of the corrosion rate. In the case of the COAT1 samples, the second circle, which is thought to represent the scale electrical characteristics, shows first an increase of the scale electronic conductivity due to the possible transformation of the Al2O3 into LiAlO2 between 0 and 25 h of immersion (decrease of the semicircle diameter). For longer times, the electronic conductivity is decreasing maybe because
of the increasing content of Li in the LiAlO2 structure andyor of increasing defects concentration in the scale layer after 25 h. In conclusion, the model used to analyze the EIS results of the Al coated steel samples allowed us to determine the oxidation rate, the conductivity of the scale and the influence of the charge transport phenomena with time. The degradation of the coated samples is generally due to the formation of pore and the interdifussion of Al into the substrate w29x. As the scale has difficulty in resisting the spallation because of the brittleness of the Al oxides and of the very irregular shape of the outer layer, the consumption of Al from the bulk is increased which leads to the growth of the pores. After 1000 h, a big part of the Al has been consumed and only a very thin and unprotective scale is present (Fig. 5). The pores can form at the metal–scale interface as for the COAT1 samples. In this case, the growth of the pores leads to the formation of very low crevices and the thick scale of Al oxides is still present and protective in most parts of the surface. For longer times of immersion (t)5000 h), the formation of pores and the interdifussion at the steel– intermetallic layer is also a cause of failure of the coated and heat-treated samples. In order to have an idea of the lifetime of the coated samples, some longer time experiments should be made. 5. Conclusions The original part of this work is the use of a new and economic Al coating process to protect MCFC separator plates and the use of the EIS technique for long exposition times to study the degradation of these coatings in molten carbonate. The TA6V titanium alloy is more resistant than the Al-coated steels in molten carbonate. Nevertheless, the Al-coated steels allowed an important increase of the steel lifetime in molten carbonate in an economical way. The method of deposition of the Al coating is very important because it influences the porosity of the outer layer after the diffusion treatment. The presence of big pores, which grow during the corrosion process in the outer layer seems to be the cause of the failure of the protective scale for the IVD samples. The slurry samples show a better corrosion resistance to molten carbonate because of the lower porosity of the outer scale after the diffusion treatment. Another degradation method of the Al-coated separator plates generally observed, which is the formation and growth of pores at the steel-diffusion layer w29x, needs longer time corrosion tests (t)5000–10000 h) to be characterized. The formation of LiFeO2 at the scale– melt interface was not observed as the thermodynamic calculations supposed. This may be due to kinetic reasons or to the spalling of the LiFeO2 formed. In the
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case of the slurry sample, pores are growing at the scale–intermetallic interface but it does not decrease the protective properties of the scale after 1000 h of exposure. Three mechanisms of coating degradation exist. The growth of the pores already presents after the diffusion treatment in the outer layer (IVD samples). The formation and growth of pores at the intermetallic layer– scale interface (slurry aluminides); and for longer times of immersion the formation and growth of pores at the steel–intermetallic layer interface. The first one seems to be the most dangerous for the lifetime of the separator plates. The second one can limit the lifetime of the scale between 1000 and 10 000 h of immersion. The optimization of the coating deposition and diffusion treatment should still be developed in order to increase the lifetime of the separator plates. The codeposition of another metallic element with Al (Ni, Cr, Ti«) should also significantly increase the lifetime of the separator plates in the future. Acknowledgments The authors want to express their gratitude for the financial support of this work to the Oficina de Ciencia ´ (Proyecto PETRI No. 95-0221-OP-03-03). y Tecnologıa References w1x K. Ota, K.T. Oda, N. Motohira, N. Kamiya, Mater. Res. Soc. Proc. 496 (1998) 231. w2x P. Biedenkopf, M. Spiegel, H.J. Grabke, Mater. Corros. 48 (1997) 477. w3x M. Cassir, V. Chauvaut, J. Deynck, V. Blanchot-Courtois, L9Actualite´ Chimique 10 (1997) 11. w4x G.J. Janz, A. Conte, E. Neuenschwander, Corrosion 19 (1963) 292. w5x G.J. Janz, A. Conte, Corrosion 20 (1964) 237. w6x F.J. Perez, ´ ´ M.P. Hierro, C. Gomez, D. Duday, M. Romero, Bol. Soc. Esp. Ceram. Vidrio 39 (2000) 323. w7x T.I. Manukhina, V.I. Elkina, Rasplavy 6 (1992) 65. w8x T.I. Manukhina, V.I. Elkina, Rasplavy 4 (1996) 58. w9x D.A. Shores, P. Singh, Proc. Electrochem. Soc. 84 (13) (1984) 271. w10x C. Yuh, R. Johnson, M. Farooque, H. Maru, in: D. Shores, H. Maru, I. Uchida, J.R. Selman (Eds.), Molten Carbonate Fuel Cell Technology. The Electrochemical Society Proceeding Series PV93-3, The Electrochemical Society, Pennington, NJ, 1993, p. 158.
301
w11x C. Yuh, in: D. Shores, H. Maru, I. Uchida, J.R. Selamn (Eds.), Molten Carbonate Fuel Cell Technology. The Electrochemical Society Proceeding Series PV90-6, The Electrochemical Society, Pennington, NJ, 1990, p. 368. w12x W.A. Ghanem, Steel Res. 64 (1993) 317. w13x M. Spiegel, P. Biedenkopf, H.J. Grabke, Corrosion Sci. 39 (1997) 1193. w14x F.J. Perez, ´ ´ M.P. Hierro, C. Gomez, D. Duday, M. Romero, L. Daza, Oxid. Met. 54 (3y4) (2000) 375. w15x K. Masamura, M. Takemura, M. Nishida, 6th Grove Fuel Cell Conference Proceedings, London, 1999. w16x B.Y. Yang, K.Y. Kim, in: D. Shores, H. Maru, I. Uchida, J.R. Selman (Eds.), Molten Carbonate Fuel Cell Technology. The Electrochemical Society Proceedings Series PV97-4, The Electrochemical Society, Pennington, NJ, 1997, pp. 286–295. w17x J.P.T. Vossen, R.C. Makkus, J.H.W. De Witt, J. Electrochem. Soc. 143 (1996) 66. w18x J.P.T. Vossen, Ph.D. Thesis, Delft University of Technology, The Netherlands, The Netherlands Energy Research Foundation ECN, Petten, 1994. w19x S. Frangini, Oxid. Met. 53 (1y2) (2000) 139. w20x H.S. Hsu, Proceedings of Corrosion 86, Houston, TX, Paper no. 85, 1986. w21x H.S. Hsu, J.H. Devan, M. Howell, J. Electrochem. Soc. 134 (1987) 3038. w22x S. Mitsushima, N. Kamiya, K. Ota, J. Electrochem. Soc. 137 (1990) 2713. w23x R.A. Donado, L.G. Marianowski, H.C. Maru, J.R. Selman, J. Electrochem. Soc. 131 (1984) 2535. w24x C. Yuh, P. Singh, L. Paetsch, H. Maru, Corrosion 87, San Francisco, March 17–21, paper 276, 1986. w25x M. Yamamoto, Y. Kato, Fuel Cell Seminar Program and Abstracts, Nov. 28–Dec. 1, San Diego, 1994, p. 144. w26x W. Betz, H. Huff, W. Track, Z. Werkstoff. 7 (1976) 161. w27x R.B. Swaroop, J.W. Sim, K. Kinoshita, J. Electrochem. Soc. 125 (1978) 1799. w28x J.E. Indacochea, I. Bloom, M. Krumpelt, J. Mater. Res 13 (7) (1998) 1834. w29x N. Fujimoto, M. Yamamoto, T. Nagoya, J. Power Sources 71 (1998) 231. w30x P. Spiteri, P. Saint-Paul, G. Picard, H. Lefebvre, B. Tremillon, Mater. Sci. Eng. 87 (1987) 369. w31x Y. Fujita, T. Nishimura, J. Hosokawa, H. Urushibata, A. Sasaki, in: D. Shores, H. Maru, I. Uchida, J.R. Selman (Eds.), Molten Carbonate Fuel Cell Technology, The Electrochemical Society Proceedings Series PV97-4, The Electrochemical Society, Pennington, NJ, 1997, p. 305. w32x B. Zhu, G Lindbergh, D. Simonsson, Corrosion Sci. 41 (1999) 1497. w33x F.J. Perez, ´ ´ D. Duday, M.P. Hierro, C. Gomez, Oxid. Met., in press. w34x T. Nishina, I. Uchida, J.R. Selman, J. Electrochem. Soc. 141 (1994) 1191.
Hot corrosion study of coated separator plates of molten carbonate fuel cells by slurry aluminides a ´ a,*, D. Dudaya, M.P. Hierroa, C. Gomez ´ ´ b, R. Muelab, ¨ b, M.C. Garcıa F.J. Perez , A. Aguero A. Sanchez Pascualb, L. Martinezb a
´ Universidad Complutense de Madrid, Facultad de Ciencias Quımicas, Departamento de Ciencias de los Materiales, 28040 Madrid, Spain b ´ ´ de Ardoz, Madrid, Spain Instituto Nacional de Tecnica Aerospacial, 28850 Torrejon Received 15 May 2002; accepted in revised form 3 July 2002
Abstract The corrosion behavior of Al coated AISI 310S stainless steel by slurry and ion vapor deposition (IVD) was investigated as an electrolyte seal material in a mounted carbonate fuel cell (MCFC) at 650 8C. The results were compared with uncoated AISI 310S stainless steel and TA6V alloy. The characterization of the samples after exposure to the eutectic 62 mol.% Li2CO3 –38 mol.% K2CO3 mixture at 650 8C up to 1000 h has shown the presence of LiAlO2 (coated samples), LiFeO2 and LiCrO2 (stainless steels), and Li2TiO3 (TA6V) oxides at the scale–melt interface. The electrochemical impedance spectroscopy (EIS) technique has shown high values of polarization resistances for TA6V and lower values for coated steels. The uncoated AISI 310S stainless steel have shown the lowest polarization resistance. A mechanism for the corrosion of Al-coated stainless steels in molten carbonate is proposed taking into account thermodynamic simulations, X-ray diffraction (XRD), scanning electron microscopy (SEM) characterizations, and EIS results. This proposed mechanism confirms that a slurry aluminide coating is able to improve the stainless steel behavior in molten carbonate. However, the TA6V titanium alloy is the most resistant material in molten carbonate but the Al-coated stainless steels appear as the best lifetime-cost compromise. 䊚 2002 Elsevier Science B.V. All rights reserved. Keywords: Alloys; Coating; Molten carbonate; Hot corrosion; Fuel cell; Electrochemical impedance spectroscopy
1. Introduction The conventional molten carbonate fuel cell (MCFC) operates at 650 8C and consists of several cells composed of a porous, lithiated NiO cathode, a molten (Li,K)2CO3 electrolyte in a LiAlO2 ceramic matrix, and a porous Ni anode (Fig. 1) connected by stainless steel separator plates. The CO2y anions assume the ionic 3 conduction in the eutectic 62 mol.% Li2CO3 –38 mol.% K2CO3 electrolyte between the cathode and the anode. A part of the electrolyte extends beyond the electrode area and forms a molten seal against the stainless steel cell separator plates. The corrosion of the steel at the molten seal is one of the cell lifetime limiting parameters w1–23x. As a *Corresponding author. Tel.: q34-91-394-4215; fax: q34-913943-4357. ´ E-mail address: [email protected] (F.J. Perez).
result, the electrolyte can be wasted both by consumption in the corrosion process and by leaking due to loss of seal, contributing to overall cell efficiency loss. The separator plates commonly made of stainless steels (AISI 310 or 316) which are not sufficiently resistant to molten carbonate corrosion and therefore require coatings. The first studies on the aluminizing of the wet seal surfaces have shown an increase to their corrosion resistance by increasing the surface aluminum content w24,25x. The corrosion resistance of the coated alloys was shown to improve with increasing thickness of the coating. However, above a critical thickness the mechanical properties and the adhesion of the coating decreased rapidly w26x. Recently, the most commonly applied techniques have been the deposition of pure Al by ion vapor deposition (IVD) followed by heat treatment, electrodeposition of Ni followed by Al and heat treat-
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steel corrosion in molten carbonate with the EIS technique for long testing times, that can be extrapolated for real operational times of MCFC. 2. Experimental 2.1. Coatings preparation
Fig. 1. Schematic diagram of the molten carbonate fuel cell.
ment, and Al deposited by thermal spray or slurry also followed by heat treatment. IVD seems to be the technique mostly employed in large scale fuel cells but since it requires introducing the component in a vacuum chamber, it is too expensive for the large-scale production of MCFC. However, slurry application probably constitutes one of the most economical methods to produce coatings and therefore is an excellent candidate for this application. Although wet-seal corrosion and its prevention have received some attention in the literature w27–29x, few studies regarding the corrosion mechanism have been reported. No study has employed electrochemical impedance spectroscopy (EIS) to determine the corrosion mechanism of Al coatings deposited on MCFC separator plates in molten carbonate. This technique consists of applying a small electrical sinusoidal perturbation to the system studied and to analyze the sinusoidal electrical response after an appropriate mathematical treatment. It has the advantage of not significantly perturbing the studied system which allows a more precise analysis of the electrochemical processes carried out in molten carbonates. The EIS technique was recently used for the determination of alloy corrosion mechanisms in molten salts w6,14,17,18,30–32x. Recently in our laboratory, we have determined the corrosion mechanism of different stainless steels and TA6V alloy in molten carbonate by EIS w6,14,33x. In these studies, long exposure tests were made and the corrosion rates and evolution of the alloys were determined. It was shown that the TA6V alloy is much more resistant than coated stainless steels. In this work the corrosion resistance of pure Al slurry coatings on AISI 310S steel has been studied both by microstructural characterization and by EIS testing for long testing times. An IVD-coated sample as well as uncoated AISI 310S steel and TA6V were also tested for comparison purposes. This work is the first contribution to propose a precise determination of Al coated
The tested material was stainless steel AISI 310S (24.53% Cr, 19.28% Ni, 0.077% C) which is both a separator plate and current collector candidate. The specimen (6=15=2 mm) was ground with SiC噛1200 paper on all sides and washed with ethanol. Then, Al was deposited on the sample in two different ways. First, as liquid solution (Al-slurry) followed by a baking treatment during 3.5 h at 400 8C and diffusion treated 10 h at 820 8C under Ar atmosphere (COAT1). Second, as an IVD coating also diffusion treated 10 h at 820 8C under Ar (COAT2). The IVD method applied consisted of placing the sample on a vacuum chamber during 20 min at a pressure of 1.4=10y2 torr; the current to obtain the evaporation was 0.1 amperes at 250 V. After the deposition, specimens were spot-welded to a 0.5-mm conducting wire. A TA6V titanium alloy (6% Al, 4% V) as new candidate material was also tested for comparison with the coated steels. 2.2. Electrochemical tests and characterization of specimens Electrochemical tests were performed at 650 8C in an alumina crucible. The working electrode was composed of the test sample connected to a conducting wire, which was protected from the electrolyte by an alumina tube and a ceramic compound placed at the base of the tube. The electrochemical cell was composed of two identical electrodes formed of the tested material; this method worked at zero potential vs. open circuit. It permits work without the reference electrode which cannot be stable for long exposure times, and also, allows the possibilities of on-line monitoring to be explored. The carbonate mixture used as electrolyte was the eutectic mixture of lithium and potassium carbonate (62 mol.% Li2CO3 –38 mol.% K2CO3). It is used in an atmosphere that consists of the gases being formed during the melting process, and not introducing a gas mixture as CO2:O2 because this atmosphere induces a similar corrosion mechanism to the MCFC one. Indeed, the oxygen partial pressure is similar in both cases and the CO2 negative effect only appears for higher partial pressures for both MCFC and in our experimental systems w19x. The EIS measurements were started after the melting of the carbonate. Various measurements were made from 0 to 200 h. A 5 mV-perturbation amplitude was applied with a frequency scanning range of 10 mHz–30 kHz. All experiments were recorded with a frequency
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Table 1 Simulation at the scale–atmosphere interface at 650 8C and 1 atm with an excess of oxygen
Table 2 Simulation at the scale–melt interface at 650 8C and 1 atm with an excess in O2, Li2O and K2O
Compound
Initial quantity (mol)
Equilibrium quantity (mol)
Compound
Initial quantity (mol)
Equilibrium quantity (mol)
O2 Al2O3 Al Cr Fe Ni Fe2O3 Cr2O3 Fe2NiO4 Al2NiO4 NiO
1000 0.5 65 6 22 7 – – – – –
924 62.1 – – – – 7.5 3 4 1.7 1.2
O2 Li2O K 2O Al2O3 Fe2O3 Fe2NiO4 Cr2O3 Al2NiO4 NiO LiAlO2 LiFeO2 K2CrO4 Li2O2
100.0 500.0 100.0 62.0 7.5 4.0 3.0 1.7 1.2 – – – –
82.4 423.0 88.0 – – – – – 6.9 127.4 23.0 6.0 2.2
response an analyzer Solartron 1255 and a potentiostat EG&G283. After each experiment, the corroded specimens were taken out of the melt, washed with distilled water and investigated by X-ray diffraction (XRD) wPhilips X’Pertx and in the scanning electron microscope (SEM) wJEOL JSM 6400x with energy-dispersive X-ray microprobe analysis (EDX). 3. Results 3.1. Thermodynamic calculations When the lithiation begins, an oxide layer is formed (Table 1). In the case of the samples with a diffusion treatment, the results of the Table 1 are used to simulate the reaction of materials in contact with the melt. Two possibilities can occur. First, the Li2O from the melt is in excess compared with the oxides present (Table 2) or the Li2O is not sufficient to react with all the oxides present because of its low activity (Table 3). In the first case, the oxide scale is transformed mainly into LiAlO2 and LiFeO2 with a possible ratio of approximately 5 to 1 (Table 2). The NiO is supposed to form in a small quantity but it is more probable that Ni atoms are replacing Fe atoms in the LiFeO2 structure without formation of NiO. The Cr2O3 oxide is transformed into K2CrO4, which dissolves in the melt. When the Li2O and K2O are available in lower quantity than the oxides present, the scale obtained is supposed to be more complex (Table 3). It is composed mainly of alumina (45%), LiAlO2 (20%) and LiFeO2 (15%). Thus, the outer scale should be composed of approximately 60% LiAlO2 and 40% LiFeO2 at the scale–melt interface if the kinetics of the formation reactions are similar. The case of Cr2O3 is more complicated because it depends on supply of K2O and oxygen. If these two compounds are present in enough quantity, K2CrO4 will form and dissolve in the melt. In the opposite case, only LiCrO2 will form. The real situation is probably between these two cases, i.e.
Cr2O3 will transform partly into K2CrO4 and partly into LiCrO2 as shown in Table 3. The quantity of chromium products does significantly not affect the scale because they are present in a small quantity (5%). The Ni should be present in the LiFeO2 structure and as NiO. Other oxides can form in small quantities as shown in Table 4. In the case of samples without diffusion treatment, the scale is only composed of LiAlO2 because the metallic ions of the steel do not have time to reach the scale–melt interface before the LiAlO2 has formed. 3.2. Microstructural characterization 3.2.1. X-Ray diffraction On the XRD spectrum of the Al-coated steels with a post-coating diffusion treatment (Fig. 2), the Al5FeNi peaks appear. Taking into account the composition of the outer layer by microprobe analysis, it is more probable that a Al5Fe2 layer with a part of the Fe atoms Table 3 Simulation at the scale–melt interface at 650 8C and 1 atm with few Li2O K2O and O2 in excess Compound
Initial quantity (mol)
Equilibrium quantity (mol)
O2 Li2O K 2O Al2O3 Fe2O3 Fe2NiO4 Cr2O3 Al2NiO4 NiO LiAlO2 LiFeO2 K2CrO4 LiCrO2 LiAl5O8
100.0 20.0 5.0 62.0 7.5 4.0 3.0 1.7 1.2 – – – – –
86.1 – – 47.2 2.0 1.2 – 3.0 2.7 22.0 16.1 5.0 1.0 0.9
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Table 4 Results of simulation for the resistance values from the EIS spectra semicircles Immersion time (h)
PEP Al coated steel R1 (Ohms)
PEP Al coated steel R2 (Ohms)
INTA Al coated steel R1 (Ohms)
INTA Al coated steel R2 (Ohms)
Non-coated steel R1 (Ohms)
5 25 100 200
30 35 38 40
250 200 120 120
25 40 60 80
200 -100 150 150
10 15 20 –
R1, charge transfer resistance of the AL or Fe oxidation reaction; R2, resistivity of the scale.
(approx. 25%) replaced by Ni atoms is present. Indeed the XRD peaks are very similar between these two compounds. So during 10 h at 820 8C under Ar, the diffusion of the metallic elements of the steel and of Al is rapid enough to transform all the Al coating (approx. 10 mm) into an intermetallic layer. The FeAl phase is also detected. The low intensity of the peaks of this suggest that FeAl is situated under the Al5Fe2 layer. After the immersion of all the samples in molten carbonate, only the LiAlO2 compound appears (Fig. 3). Both the a-phase and g-phase are present on the surface of the immersed samples. The LiFeO2 does not appear maybe because it spalled off or because the kinetics or formation are much lower than for LiAl2. After 150 h the alumina layer is totally transformed into LiAlO2. 3.2.2. Scanning electron microscopy The cross-section of slurry-coated steel sample (COAT1) after 1000 h of immersion in molten carbonate is shown in Fig. 4. Few big pits are observed through the scale down to the middle of the diffusion layer, and the thick Al Oxide scale in steel is still protective on most parts of the sample.
The Al oxide scale is very thin after 1000 h of immersion for IVD samples (COAT2) and the corrosion seems to be more important because of the lower content of aluminum present and the formation of carbides at the steel–diffusion layer interface (Fig. 5). The SEM characterization has shown the best corrosion resistance of the slurry coating for 1000 h exposure in molten carbonate. In both cases the original thickness of the coatings were approximately 30 mm. 3.3. Electrochemical impedance spectroscopy results All the EIS curves coming from the immersion of the Al coated steels in molten carbonate show a little semicircular area at higher frequencies and a bigger semicircular area at lower frequencies. However, the evolution of the diameter of the semicircular arcs is different with time for both steels and at the lowest frequencies; a third arc or a line is observed for some samples. Indicating more than a electrochemical process that can occur on the surface. On the Fig. 6, the EIS spectra of different coated samples immersed for approximately 6 h are superposed
Fig. 2. X-Ray diffraction spectra of Al coated 310S stainless steels after the diffusion treatment.
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Fig. 3. X-Ray diffraction spectra of Al coated 310S stainless steels after the diffusion treatment and 200 h of immersion in molten carbonate.
with the EIS spectra of a 310S steel without coating and a TA6V alloy. The first semicircle is bigger for the COAT2 sample. The COAT1 sample and the TA6V show a first circle diameter situated between the COAT2 sample and the non-protected steel. The first circle of the steel without coating is much lower. From those results, the corrosion rate at the scale–melt interface seems slower for the COAT2 sample after approximately 6 h of immersion, at the beginning of the corrosion
process. All the coated samples show a much better behavior than the non-protected steel (Fig. 6). The second semicircle arc is bigger for the TA6V alloy and the COAT2 sample which means the oxidation rate is lower at the intermetallic-scale layer or that the scale is less conductive for these samples. A COAT1 sample immersed just after the melting of the electrolyte (Fig. 6) shows a second semicircle bigger than the same sample immersed in the melt before the melting. A
Fig. 4. SEM cross-section micrograph of a COAT1 (Al-Slurry coating) sample after the diffusion treatment and 1000 h of immersion in molten carbonate. Some crevices appear but most parts of the sample is still protected by the LiALO2.
298
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Fig. 5. SEM cross-section micrograph of a COAT2 (Al-IVD coating) sample after the diffusion treatment and 1000 h of immersion in molten carbonate. The scale is very thin and does not more protect the steel.
study of the conductivity of the scales will be useful for a more precise comparison. In Fig. 7, the EIS spectra of the same samples are represented after 25 h of immersion. The first little circle diameter is a little bigger for COAT2 sample and the immersed after the melting COAT1 sample compared with the COAT1 sample immersed before melting. The first circle is much smaller for the non-protected. Thus, the oxidation rate at the scale melt interface is slower,
after 25 h, if the immersion of the samples takes place before the melting of the electrolyte. After 25 h, the corrosion rate of the COAT1 and COAT2 samples is similar. The second semicircle is bigger for the COAT2 sample after 25 h. The COAT1 sample shows a smaller second semicircle. The uncoated steel shows the smallest second semicircle. So the electronic conductivity of the COAT2 sample seems lower and the conductivity of the
Fig. 6. Nyquist impedance diagram in the frequency range of 10 mHz–30 kHz for COAT1, COAT2 and non-coated steel and the TA6V titanium alloy samples.
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299
Fig. 7. Nyquist impedance diagram in the frequency range of 10 mHz–30 kHz for COAT1, COAT2 and non-coated steel samples. Time of immersion: 25 to 27 h.
uncoated sample seems higher because of the iron contained in the scale.
Other reactions can occur but they do not affect the EIS spectra because of the low quantity at the reaction sites, compared with Al3q and Fe3q, some of the
4. Discussion: Nishina et al. w34x have shown that first stage of the reduction of O2 in the melt resulted in a CO2 production with the reactions. 2y 2CO2y q2CO2 3 ™2O
(1)
2y O2q2CO2y 3 ™2O2 q2CO2
(2)
These reactions are governed by the acidity of the melt and, in the less acidic melts which correspond to our conditions, the reaction in Eq. (2) prevails, including a more important CO2 formation in the melt. As the CO2 is not consumed in the absence of an anode in the corrosion cell, the CO2 partial pressure is higher than that of the initial atmosphere. The high number of the reactions and the presence of different oxide layers in the studied materials and coatings, may generally hinder the elaboration of an equivalent circuit model in the case of the metallic corrosion in molten salt. Nevertheless, with a good knowledge of the system, it is possible to select the main electrochemical reactions, which take place at the surface of the samples, to propose a simplified model. In the case of the immersion of Al coated steels in molten carbonate, three electrochemical reactions would take place at one interface which play an important role in the corrosion mechanism as shown on the Fig. 8: Al™Al3qq3ey to form Al2O3 and LiAlO2
(3)
Fe™Fe3qq3ey to form Fe2O3 and LiFeO2
(4)
O22yq2ey™2O2y form the reactions (2)-(5)
(5)
Fig. 8. Schematic diagram of the Al coated samples diffusion treatment and immersion in molten carbonate. The scale consists only of Al2O3 at the beginning and LiAlO2 after a few hours of immersion. Also, equivalent circuit corresponding to the impedance spectra of the Al-coated steels.
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following reactions could taken into account: Fe™Fe
2q
q2e
y
to form ŽFe,Ni.Cr2O4
(6)
Cr™Cr3qq3ey to form Cr2O3 and LiCrO2
(7)
Ni™Ni2qq2ey to form NiOŽFe,Ni.Cr2 O4
(8)
Cr
3q
™Al
6q
q3e
y
to form K2CrO4
(9)
As the LiAlO2 and the Li2TiO3 outer layers are formed, after immersion, and taking into account that they are not good electronic conductors, it is possible that they modify the EIS spectra by the addition of another time constant. It has to be noted that the Alcoated steel system is less complex to model than the non-protected steel system in molten carbonate. Indeed, the number of main reactions is lower (mainly Al3q and Fe3q) and only one main oxide layer would be formed. From the Fig. 8, the thermodynamic calculations and the microstructure characterizations, the equivalent electrical circuit of attachment is proposed for the EIS analysis of the Al-coated steel system studied in this work. This circuit is composed of two time constant elements corresponding to the oxidation reaction of the Al (and Fe) at the intermetallic–scale interface and to the scale–electrical response, respectively. The high frequency circle is thought to be due to the oxidation reaction of Al to Al3q (and, at a lower extent, to the oxidation of Fe) because of the value of the frequencies and of the evolution of semicircle diameter with time. The intermediary circle is thought to correspond to the scale resistance and capacity, and the semicircle arc at the lower frequencies is thought to be due to charge transport phenomena in the case of the COAT1 samples. The values of the electrical elements, corresponding to the proposed equivalent circuit (Fig. 8) obtained for different samples and times of immersion are shown in Table 4. The differences of behavior at the beginning of the immersion for the coated samples can be due to defects of outer layer, among other mechanisms. Indeed if the outer layer has little porosity, the alumina scale has a problem to reach a sufficient thickness without breaking because of its brittleness. The increase of the corrosion rate decreases which means this latter oxide is less brittle and can suffer more stresses without spall off. After 25 h, the thickening of the scales with time allows a decrease of the corrosion rate. In the case of the COAT1 samples, the second circle, which is thought to represent the scale electrical characteristics, shows first an increase of the scale electronic conductivity due to the possible transformation of the Al2O3 into LiAlO2 between 0 and 25 h of immersion (decrease of the semicircle diameter). For longer times, the electronic conductivity is decreasing maybe because
of the increasing content of Li in the LiAlO2 structure andyor of increasing defects concentration in the scale layer after 25 h. In conclusion, the model used to analyze the EIS results of the Al coated steel samples allowed us to determine the oxidation rate, the conductivity of the scale and the influence of the charge transport phenomena with time. The degradation of the coated samples is generally due to the formation of pore and the interdifussion of Al into the substrate w29x. As the scale has difficulty in resisting the spallation because of the brittleness of the Al oxides and of the very irregular shape of the outer layer, the consumption of Al from the bulk is increased which leads to the growth of the pores. After 1000 h, a big part of the Al has been consumed and only a very thin and unprotective scale is present (Fig. 5). The pores can form at the metal–scale interface as for the COAT1 samples. In this case, the growth of the pores leads to the formation of very low crevices and the thick scale of Al oxides is still present and protective in most parts of the surface. For longer times of immersion (t)5000 h), the formation of pores and the interdifussion at the steel– intermetallic layer is also a cause of failure of the coated and heat-treated samples. In order to have an idea of the lifetime of the coated samples, some longer time experiments should be made. 5. Conclusions The original part of this work is the use of a new and economic Al coating process to protect MCFC separator plates and the use of the EIS technique for long exposition times to study the degradation of these coatings in molten carbonate. The TA6V titanium alloy is more resistant than the Al-coated steels in molten carbonate. Nevertheless, the Al-coated steels allowed an important increase of the steel lifetime in molten carbonate in an economical way. The method of deposition of the Al coating is very important because it influences the porosity of the outer layer after the diffusion treatment. The presence of big pores, which grow during the corrosion process in the outer layer seems to be the cause of the failure of the protective scale for the IVD samples. The slurry samples show a better corrosion resistance to molten carbonate because of the lower porosity of the outer scale after the diffusion treatment. Another degradation method of the Al-coated separator plates generally observed, which is the formation and growth of pores at the steel-diffusion layer w29x, needs longer time corrosion tests (t)5000–10000 h) to be characterized. The formation of LiFeO2 at the scale– melt interface was not observed as the thermodynamic calculations supposed. This may be due to kinetic reasons or to the spalling of the LiFeO2 formed. In the
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case of the slurry sample, pores are growing at the scale–intermetallic interface but it does not decrease the protective properties of the scale after 1000 h of exposure. Three mechanisms of coating degradation exist. The growth of the pores already presents after the diffusion treatment in the outer layer (IVD samples). The formation and growth of pores at the intermetallic layer– scale interface (slurry aluminides); and for longer times of immersion the formation and growth of pores at the steel–intermetallic layer interface. The first one seems to be the most dangerous for the lifetime of the separator plates. The second one can limit the lifetime of the scale between 1000 and 10 000 h of immersion. The optimization of the coating deposition and diffusion treatment should still be developed in order to increase the lifetime of the separator plates. The codeposition of another metallic element with Al (Ni, Cr, Ti«) should also significantly increase the lifetime of the separator plates in the future. Acknowledgments The authors want to express their gratitude for the financial support of this work to the Oficina de Ciencia ´ (Proyecto PETRI No. 95-0221-OP-03-03). y Tecnologıa References w1x K. Ota, K.T. Oda, N. Motohira, N. Kamiya, Mater. Res. Soc. Proc. 496 (1998) 231. w2x P. Biedenkopf, M. Spiegel, H.J. Grabke, Mater. Corros. 48 (1997) 477. w3x M. Cassir, V. Chauvaut, J. Deynck, V. Blanchot-Courtois, L9Actualite´ Chimique 10 (1997) 11. w4x G.J. Janz, A. Conte, E. Neuenschwander, Corrosion 19 (1963) 292. w5x G.J. Janz, A. Conte, Corrosion 20 (1964) 237. w6x F.J. Perez, ´ ´ M.P. Hierro, C. Gomez, D. Duday, M. Romero, Bol. Soc. Esp. Ceram. Vidrio 39 (2000) 323. w7x T.I. Manukhina, V.I. Elkina, Rasplavy 6 (1992) 65. w8x T.I. Manukhina, V.I. Elkina, Rasplavy 4 (1996) 58. w9x D.A. Shores, P. Singh, Proc. Electrochem. Soc. 84 (13) (1984) 271. w10x C. Yuh, R. Johnson, M. Farooque, H. Maru, in: D. Shores, H. Maru, I. Uchida, J.R. Selman (Eds.), Molten Carbonate Fuel Cell Technology. The Electrochemical Society Proceeding Series PV93-3, The Electrochemical Society, Pennington, NJ, 1993, p. 158.
301
w11x C. Yuh, in: D. Shores, H. Maru, I. Uchida, J.R. Selamn (Eds.), Molten Carbonate Fuel Cell Technology. The Electrochemical Society Proceeding Series PV90-6, The Electrochemical Society, Pennington, NJ, 1990, p. 368. w12x W.A. Ghanem, Steel Res. 64 (1993) 317. w13x M. Spiegel, P. Biedenkopf, H.J. Grabke, Corrosion Sci. 39 (1997) 1193. w14x F.J. Perez, ´ ´ M.P. Hierro, C. Gomez, D. Duday, M. Romero, L. Daza, Oxid. Met. 54 (3y4) (2000) 375. w15x K. Masamura, M. Takemura, M. Nishida, 6th Grove Fuel Cell Conference Proceedings, London, 1999. w16x B.Y. Yang, K.Y. Kim, in: D. Shores, H. Maru, I. Uchida, J.R. Selman (Eds.), Molten Carbonate Fuel Cell Technology. The Electrochemical Society Proceedings Series PV97-4, The Electrochemical Society, Pennington, NJ, 1997, pp. 286–295. w17x J.P.T. Vossen, R.C. Makkus, J.H.W. De Witt, J. Electrochem. Soc. 143 (1996) 66. w18x J.P.T. Vossen, Ph.D. Thesis, Delft University of Technology, The Netherlands, The Netherlands Energy Research Foundation ECN, Petten, 1994. w19x S. Frangini, Oxid. Met. 53 (1y2) (2000) 139. w20x H.S. Hsu, Proceedings of Corrosion 86, Houston, TX, Paper no. 85, 1986. w21x H.S. Hsu, J.H. Devan, M. Howell, J. Electrochem. Soc. 134 (1987) 3038. w22x S. Mitsushima, N. Kamiya, K. Ota, J. Electrochem. Soc. 137 (1990) 2713. w23x R.A. Donado, L.G. Marianowski, H.C. Maru, J.R. Selman, J. Electrochem. Soc. 131 (1984) 2535. w24x C. Yuh, P. Singh, L. Paetsch, H. Maru, Corrosion 87, San Francisco, March 17–21, paper 276, 1986. w25x M. Yamamoto, Y. Kato, Fuel Cell Seminar Program and Abstracts, Nov. 28–Dec. 1, San Diego, 1994, p. 144. w26x W. Betz, H. Huff, W. Track, Z. Werkstoff. 7 (1976) 161. w27x R.B. Swaroop, J.W. Sim, K. Kinoshita, J. Electrochem. Soc. 125 (1978) 1799. w28x J.E. Indacochea, I. Bloom, M. Krumpelt, J. Mater. Res 13 (7) (1998) 1834. w29x N. Fujimoto, M. Yamamoto, T. Nagoya, J. Power Sources 71 (1998) 231. w30x P. Spiteri, P. Saint-Paul, G. Picard, H. Lefebvre, B. Tremillon, Mater. Sci. Eng. 87 (1987) 369. w31x Y. Fujita, T. Nishimura, J. Hosokawa, H. Urushibata, A. Sasaki, in: D. Shores, H. Maru, I. Uchida, J.R. Selman (Eds.), Molten Carbonate Fuel Cell Technology, The Electrochemical Society Proceedings Series PV97-4, The Electrochemical Society, Pennington, NJ, 1997, p. 305. w32x B. Zhu, G Lindbergh, D. Simonsson, Corrosion Sci. 41 (1999) 1497. w33x F.J. Perez, ´ ´ D. Duday, M.P. Hierro, C. Gomez, Oxid. Met., in press. w34x T. Nishina, I. Uchida, J.R. Selman, J. Electrochem. Soc. 141 (1994) 1191.