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Corrosion behaviour of high-chromium ferritic steels in molten carbonate in cathode environment
Electrochimica Acta 46 (2001) 2593– 2604 www.elsevier.nl/locate/electacta
Corrosion behaviour of high-chromium ferritic steels in molten carbonate in cathode environment Baohua Zhu, Go¨ran Lindbergh *,1 Department of Chemical Engineering and Technology, Applied Electrochemistry, Royal Institute of Technology, KTH, SE-100 44 Stockholm, Sweden Received 12 August 2000; received in revised form 14 February 2001
Abstract The corrosion behaviour of four ferritic steels with a high chromium content and AISI 310 was investigated in (Li0.60/Na0.40)2CO3 melt in three different cathode gas environments. The electrochemical techniques used were linear polarisation resistance and Tafel extrapolation. The corrosion layers formed on the surface during the tests were analysed by glow discharge optical emission spectroscopy (GDOES). The corrosion layer formed on the Thermax 4762 sample consists of an iron-rich outer layer and a protective aluminium- and chromium-rich inner layer. The corrosion potential increased to a more positive value as the corrosion layer grew on the surface. This supports the supposition that the cathodic reaction in the corrosion process changes gradually from water reduction to oxygen reduction. It was shown that higher temperatures and low concentrations of oxygen and carbon dioxide under so-called outlet cathode gas conditions result in higher corrosion rates. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Corrosion; Molten carbonate fuel cell (MCFC); Chromium-containing steels; Tafel extrapolation; GDOES
1. Introduction The development of molten carbonate fuel cell (MCFC) technology still requires the cost of stack components to be reduced and their lifetime increased. Corrosion of the current collectors is one of the main lifetime-limiting factors. Studies have been made of the electrochemical corrosion behaviour of pure metals, binary model alloys and commercial steels in molten carbonates [1–8]. Basic research and technology development has shown that high-chromium steels are applicable for cathode-side conditions [3,4]. The * Corresponding author. Tel.: + 46-8-7908143; fax: + 46-8108087. E-mail address: [email protected] (G. Lindbergh). 1 ISE member.
state-of-the-art material used as a current collector in MCFCs is AISI 310 austenitic stainless steel, the lifetime of which is about 20000 h [9]. An extension of the lifetime to 40000 h by using alternative stainless steels would improve economic feasibility. Vossen et al. [10] reported the corrosion behaviour of some commercially available alloys, including AISI 310 and AISI 316 stainless steels, the nickel-base alloy Inconel 601, ferritic steels Thermax 4762 and Kanthal A1. The materials were investigated in an MCFC anode environment. A ranking of the corrosion properties was undertaken and Thermax 4762 and Kanthal A1 were considered to have the best corrosion resistance. Lindbergh et al. [11] studied the corrosion behaviours of some aluminium-containing ferritic steels (including some Kanthal series steels, PM 2000 and MA 956) in three different MCFC anode environments, and found that Kanthal A1 had the best corrosion resistance.
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Table 1 Chemical compositions of the investigated materials (wt.%)
Thermax 4762 AL 29-4-2 AISI 310 7Mo+N Thermax 4742
Cr
Ni
23 29 25 26 18
2.3 20 4.8
Al
Mo
1.5 4 1.4 1
However, this high-aluminium steel is not suitable as a cathode current collector material due to the high contact resistance of the aluminium oxide layer formed on the surface. There are some reports on the corrosion behaviour of Al-containing steels in molten salts under cathode gas conditions [12–14]. With more than 5 wt.% Al, a very thin layer of LiAlO2 is formed on the surface. This oxide acts as a quite stable corrosion-protective layer [12]. Swarr [14] reported the characterisation of aluminium coatings on AISI 310, AISI 316 and INCO 825 separators in molten salts in the O2 –CO2 gas environment. The b-(Fe,Ni)Al phase forms an Al2O3 layer on the surface and this layer is converted to LiAlO2 during exposure in molten carbonate. The resistance of materials to corrosion in molten carbonate is dependent on a large number of factors, including moisture and impurity content, temperature, and the gas atmosphere above the melt. In general, the anode-side environment is more corrosive than the cathode-side. A bi-metallic design is expected to increase the corrosion resistance of the current collectors on both the anode and cathode sides. Some of the high-chromium ferritic steels, namely Thermax 4762, AL 29-4-2, 7Mo +N and Thermax 4742, are expected to have a lifetime of more than 40000 h, based on a small-scale laboratory exposure test [5]. In cathode gas environments, the oxidising agents of interest are dissolved water and oxygen in the carbonate melt. Therefore the cathodic reaction in the corrosion process can be water and/or oxygen reduction: Water reduction:
H2O+CO2 +2e− =H2 +CO23 − (1)
Oxygen reduction:
1/2O2 +CO2 +2e− =CO23 −
(2)
The general reaction for metal dissolution can be written as: Metal oxidation:
Me = Mez + + ze−
(3)
The equilibrium potentials of Eqs. (1) and (2) were reported in the literature [15] and depend on the composition of the gas dissolved in the melt. The equilibrium potential of water reduction under standard anode gas conditions is about − 1120 mV vs the gold reference electrode used in this study, and for oxygen reduction it is about −50 mV vs the gold reference electrode. The calculated equilibrium potentials for oxide formation of metal/metal oxide are also reported in the literature [15]. Under standard cathode gas conditions, it is about − 1070 mV for FeO/Fe, − 1560 mV for Cr2O3/Cr and − 610 mV for NiO/Ni. The aim of the present study is to investigate the corrosion behaviour of the high-chromium ferritic steels mentioned above and AISI 310 stainless steel by means of electrochemical methods, under three different cathode conditions of the MCFC. Post-test characterisation was undertaken to investigate the corrosion layer formed on the sample surface.
2. Experimental The experimental set-up used has been described elsewhere [3]. The corrosion tests were performed in a (Li0.60/Na0.40)2CO3 melt. The reference electrode consisted of an alumina tube with a small hole in the bottom (about 0.1 mm diameter); the tube was partially filled with carbonate in which a gold wire was inserted. The reference gas was a mixture of 33.3% CO2 and 66.7% O2. The samples were cut to 7 × 7×1 mm, polished with c 600 to c 1200 SiC abrasive paper, washed with distilled water and ethanol, and then dried before testing. The samples were welded with 0.8 mm diameter stainless steel wires. The investigated materials are listed in Table 1. The samples were investigated under three different cathode gas conditions, Table 2. The so-called inlet and outlet gas compositions should resemble the inlet and outlet conditions in a stack under operating conditions in a natural-gas-fuelled system. The electrochemical measurements were performed using a model 273
Table 2 The different cathode gas conditions
Inlet Standard Outlet
O2 (%)
CO2 (%)
N2 (%)
H2O (%)
T (°C)
12.0 14.1 8.5
22.0 28.2 5.0
54.0 51.7 74.5
12.0 5.9 12.0
600 650 700
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Fig. 1. Anodic polarisation curves (sweep rate 1 mV s − 1) of five steels exposed in the carbonate melt under standard cathode gas conditions at 650°C for half an hour.
EG&G Princeton Applied Research potentiostat/galvanostat. The polarisation curves were recorded at a scan rate of 1 mV s − 1. The corrosion rate expressed as current density at the open circuit (corrosion) potential was determined by linear polarisation resistance and Tafel extrapolation methods. The compositions of the surface layers of the exposed samples were analysed by means of glow discharge optical emission spectroscopy (GDOES). GDOES was performed by cathodic sputtering of the sample surface in a Grimm-type glow discharge lamp. The sputtering process produces a crater with an almost flat bottom on the sample. The rate of sputtering depends on the voltage, current, pressure, plasma gas and sample composition. The sputtered material is atomised in a plasma, and the characteristic emission lines are monitored by an optical spectrometer as a function of burning time, yielding an in-depth profile of sample composition.
3. Results and discussion
3.1. Anodic polarisation cur6es The anodic polarisation curves of different samples under different cathode conditions were measured after exposure for half an hour at open circuit potential. The initial potential was the open circuit potential and the final potential was −50 mV vs the reference electrode. All these anodic polarisation curves show a region in which the corrosion currents drop with increasing potential. In an operating fuel cell, the electrode potential
at the cathode side is usually between − 50 and − 200 mV, depending on the conductivity of the cathode material, the corrosion layer formed between the cathode and the current collector, and operating conditions. In this potential region, passivations of the materials seem to occur. The comparison of ferritic steels under standard cathode conditions, Fig. 1, shows that in the potential region −50 to − 200 mV, AL 29-4-2 and 7Mo+ N had a higher passive current density, while AISI 310 and Thermax 4742 had the lowest passive current density. Fig. 2 shows the anodic polarisation curves obtained on Thermax 4762 samples in three different cathode gas environments. In the same potential region, a higher passive current density was found under outlet conditions.
3.2. Corrosion potential Figs. 3 and 4 show the corrosion potentials as a function of time during exposure in (Li0.60/Na0.40)2CO3 melt under three different cathode gas conditions. These results are very different from those obtained in the anode environments [11]. The corrosion potentials measured on these high-chromium ferritic steels during exposure, as a function of time, change from − 1.2 V to much more positive values, close to the operating potential of the MCFC cathode. Subsequently, the cathodic reaction in the corrosion process, under cathode gas conditions, changes from water reduction and oxygen reduction at the beginning, to only oxygen reduction when the corrosion potential increases above the equilibrium potential of the water reduction reaction.
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Fig. 2. Anodic polarisation curves (sweep rate 1 mV s − 1) of Thermax 4762 samples exposed in the carbonate melt under three different cathode gas conditions for half an hour.
The time needed for the corrosion potentials to reach stable values in molten carbonate depends on the material, temperature and gas composition, Table 3. Thermax 4762, AISI 310 and AL-29-4-2 were investigated under different cathode gas conditions. It was shown that the time needed to reach the final potential was shorter under outlet conditions than under inlet and standard conditions, Fig. 4.
3.3. Corrosion rate 3.3.1. Linear polarisation resistance The linear polarisation resistance method is an alternative method for determining the corrosion current density at the open circuit potential. The perturbation potential was 930 mV, which is much smaller than for the Tafel extrapolation experiments. Therefore, the
Fig. 3. Corrosion potentials of five high-chromium steels, as a function of time, in lithium – sodium carbonate melt under standard cathode gas conditions at 650°C.
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Fig. 4. Corrosion potentials of Thermax 4762, as a function of time, in lithium – sodium carbonate melt under three different cathode gas conditions.
growth of the corrosion layer was disturbed by the measurements to a lesser extent. The Stern – Geary equation was used to determine the corrosion current density [3]. The Tafel slopes used in the equation were the average values of each sample obtained from the Tafel extrapolation curves. The linear polarisation resistance was measured at various times during exposure. The polarisation resistance was low at the beginning, and became more difficult to measure during fast increase of the potential. When the final potential has been reached, the polarisation resistance is about ten times higher than at the beginning and can be measured again. The polarisation resistances of four ferritic steels and AISI 310 measured at the end of the exposure tests are summarised in Table 3. From the results obtained on Thermax 4762, AISI 310 and AL 29-4-2, lower polarisation resistance and higher corrosion current densities were obtained under outlet conditions. The linear polarisation resistance was measured for Thermax 4762 as a function of time, Fig. 5. During the first 48 h, the polarisation resistance was almost constant or decreased slowly. It then increased tenfold during the following 100 h. This will be discussed later in connection with the analysis of the corrosion layer.
3.3.2. Tafel extrapolation cur6es The Tafel extrapolation curves were recorded at the end of each exposure test. The experimental curves were recorded in the anodic direction at a scan rate of 1 mV s − 1. The complete Tafel equation for the anodic and cathodic reactions is:
!
i= icorr exp − exp
n n"
haF ( E − Ecorr ) RT
− hc F ( E − Ecorr ) RT
(4)
where ha and hc are the anodic and cathodic transfer coefficients, respectively, icorr is the corrosion current density and Ecorr the corrosion potential. Eq. (4) was used to calculate corrosion rates, expressed as current densities. The corrosion current densities at the end of the exposure time, obtained by fitting experimental data to Eq. (4), are listed in Table 3. Fig. 6 shows one example of the experimental and fitting curves for the Thermax 4762 sample. The icorr values obtained here under the outlet cathode conditions are tenfold greater than under the standard and inlet cathode conditions for Thermax 4762 and AL 29-4-2. For AISI 310 steel, the corrosion current density under outlet conditions is only slightly higher than under standard and inlet conditions. Two more materials, Thermax 4742 and 7Mo+ N, were investigated using the same method under standard cathode conditions. The corrosion current densities are shown in Table 3. The icorr value for Thermax 4742 is 0.14 A m − 2, which is higher than for the other four materials. Fig. 7 shows the Tafel extrapolation curves for AISI 310 samples after different exposure times in standard cathode environments. A Tafel curve obtained for the same material in a standard anode environment and taken from Ref. [3] is also shown here, in order to compare with the results obtained in cathode environments. The corrosion rate during the first 24 h was
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more than tenfold after 120 h. From the anodic polarisation curves we know that the initial potential is in the active region, while the end potentials of the alloy are in the passive region. This also indicates a higher corrosion rate at the initial stage when water reduction is the main cathodic reaction in the corrosion process. The corrosion current densities, obtained by Tafel extrapolation methods at the end of the exposure period, are much lower than the passive current densities. This is probably because the anodic polarisation curves were recorded after a very short exposure time and therefore only a thin protective layer had formed on the surface.
3.4. Analysis of the corrosion layer In an earlier study [3,4], corrosion layers on AISI 310 and some iron –chromium binary alloys were investigated. It was shown that two layers are formed, a
chromium-rich inner layer close to the metal and a porous outer layer consisting mainly of lithium –iron oxides. Thermax 4762, containing 1.5% aluminium, is a very interesting material for use as a cathode current collector. In the present study, GDOES experiments were performed on Thermax 4762 samples exposed in lithium –sodium carbonate melt under standard cathode conditions for different exposure times, namely, 4, 24, 48, 72, 96 and 120 h, Fig. 8a – f. During the first 4 h, a corrosion layer of about 1.8 mm was formed, Fig. 8a. This layer was aluminium rich and contained lithium. The amount of iron and chromium decreased towards the surface. After 24 h, Fig. 8b, the thickness of the corrosion layer was 8 mm, while after 48 h, it had grown to 21 mm, Fig. 8c. The layer formed later consists of two parts. The outer part mainly contains lithium, iron and oxygen, while the inner part is chromium and aluminium rich, and con-
Table 3 Corrosion current densities, icorr, polarisation resistances, Rp, at the end of exposure and the exposure time needed for the corrosion potential to reach the final constant value
Thermax 4762 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) AL 29-4-2 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) AISI 310 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) Thermax 4742 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) 7Mo+N Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h)
Inlet cathode gas 600°C
Standard cathode gas 650°C
Outlet cathode gas 700°C
0.03
0.06
0.37
0.41 0.04 250
0.28 0.05 120
0.10 0.15 45
0.04
0.05
0.47
0.33 0.04 50
0.19 0.07 70
0.13 0.11 50
0.03
0.04
0.05
0.22 0.06 460
0.18 0.08 120
0.10 0.15 90
0.14 0.17 0.08 235 0.05 0.26 0.05 180
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Fig. 5. Linear polarisation resistance and thickness of the corrosion layer formed on Thermax 4762 as a function of time, under standard cathode conditions at 650°C.
tains lower levels of lithium, iron and oxygen. Also in the corrosion layer formed after 24 h, chromium enrichment may be found halfway through the layer. After the first 48 h, the growth of the corrosion layer became relatively slow. After 72 and 96 h of exposure, Fig. 8d,e, the total corrosion layer thickness increased to about 24 and 30 mm, respectively. After another 24
h, the corrosion layer was about 33 mm thick, Fig. 8f. This means that during the first 48 h the increase in thickness was 21 mm, while the following 72 h only led to an increase of 12 mm. All four figures, Fig. 8c –f, show that there were two layers in the corrosion products. The inner layer was chromium and aluminium rich while the outer layer mainly consisted of lithium, iron and oxygen.
Fig. 6. Tafel extrapolation curves (sweep rate 1 mV s − 1) obtained on Thermax 4762 samples exposed in the carbonate melt under three different cathode gas conditions when the open circuit potentials had reached their final constant value.
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4. Discussion The different methods used for estimating the corrosion current density agree fairly well under the standard and inlet conditions except for the Thermax 4742 sample, Table 3. Under outlet conditions, the difference between the methods is greater. In any case, the corrosion current densities under outlet cathode conditions are much higher than under standard and inlet conditions at the operating potential. Corrosion in molten salts can be characterised by two steps: the first is oxidation of the metal and the second is dissolution of the oxide scales [10]. The reactant, oxygen or water, has to diffuse through the melt to the metal surface. Therefore, the permeability of the oxygen/water in the melt is an important factor for the oxidation rate. The higher the temperature is in the melt, the more oxygen or water is transported in it. Therefore a higher corrosion rate can be expected under outlet conditions when the temperature is high. The corrosion current densities were determined at the end of exposure when the corrosion potential had reached a stable value. The time needed for the corrosion potential to reach this value depended on the environment and material. The corrosion current densities under outlet conditions were determined after a shorter exposure time than under standard and inlet conditions. The polarisation resistance of Thermax 4762 under outlet conditions after 45 h of exposure, Table 3, was higher than that under
standard conditions after 48 h of exposure, Fig. 5. This means that after approximately the same exposure time (but different corrosion potentials), a lower corrosion rate was obtained under outlet conditions. However, the corrosion potential of the sample under outlet conditions had already reached a stable value. GDOES results after more than 48 h of exposure, Fig. 8c –f, show that aluminium was not detected in the outer layer, but did exist in the inner layer. This means that no lithium aluminate protection layer is formed on the surface. This is in accordance with results presented by Matsuyama et al. [12]. They pointed out that a thin LiAlO2 layer was formed on the surface only when the aluminium content exceeded 5 wt.%. The aluminium content in Thermax 4762 is only 1.5 wt.% and is therefore not sufficient to form LiAlO2 on the surface. However, a protective Al-rich oxide inner layer is formed. Thermax 4762 could be expected to have a lower contact resistance than high aluminium containing steels due to the absence of a LiAlO2 layer on the surface. In determining the corrosion layer thickness from the GDOES results, the material in which the oxygen content exceeds 2% was defined as the corrosion layer. By using these criteria, the thickness of corrosion products was obtained as a function of exposure time, Fig. 5. The linear polarisation resistance (Rp) is also shown in this figure as a function of time. The thickness of the corrosion layer formed on Thermax 4762 as a function
Fig. 7. Tafel polarisation curves (sweep rate 1 mV s − 1) obtained on AISI 310 samples under standard cathode conditions at 650°C and standard anode conditions (56% H2, 8% CO2, 8% CO and 28% H2O) at 650°C after different exposure times.
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Fig. 8. Compositions of the surface layers, obtained by GDOES, of Thermax 4762 after different exposure times in carbonate melt under standard cathode conditions at 650°C: (a) 4 h, (b) 24 h, (c) 48 h, (d) 72 h, (e) 96 h and (f) 120 h.
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Fig. 8. (Continued).
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of time displays two linear regions. A higher corrosion rate was recorded during the first 48 h. Then oxygen reduction becomes the dominant reaction, and the corrosion rate decreases. The constant corrosion resistance (Rp) in the two stages indicates that the corrosion current densities are constant, resulting in a linear growth of the corrosion layer. When comparing the experimental results in the anode and cathode environments, Fig. 7, we can see that the corrosion potentials are almost the same in both environments during the first 24 h of exposure. This indicates that the corrosion process is very similar in both environments. In the anode gas environment, the corrosion process proceeds with water reduction as the cathodic reaction [3], and it is reasonable to assume that the cathodic reaction is the same in the cathode environment. The corrosion potential as a function of time over a long time period is a useful indicator of corrosion layer formation. The corrosion potential is determined by the mixed potential of metal oxidation and reduction of the oxidising agents. As shown in Figs. 3 and 4, the initial corrosion potential indicated a mixed potential of water reduction and metal dissolution. At the end of the exposure, the corrosion potential was close to the oxygen reduction potential, which indicates that the main cathodic reaction had changed from water reduction to oxygen reduction and the corrosion rate decreased due to the corrosion layer formed on the surface. The first potential shift describes the initial formation of the oxides. The chromium oxide then dissolves into the carbonate melt in the cathode environment, and a layer of iron oxide (usually porous) remains on the surface. The iron oxide is lithiated in the lithium carbonate to lithium ferrite. This outer layer separates the inner oxide from the aggressive molten carbonate and prevents the inner oxide from further dissolution reaction. The corrosion potential increases due to the anodic polarisation caused by the corrosion layer. After about 48 h, an inner Cr- and Al-rich layer was formed, Fig. 8c, and passivation begins. The inner Cr- and Al-rich oxide reduces the growth of the whole corrosion scale, because Cr- and Al-rich oxide exhibit very small diffusion coefficients for iron ions. The growth of the ironrich oxide dominates the growth of the whole corrosion scale [16,17]. The inner Cr- and Al-rich layer provides an effective protection against further growth of the outer iron-rich oxide layer. 5. Conclusions The corrosion behaviour of four ferritic steels and AISI 310 stainless steel with a high chromium content in cathode gas environments was investigated. The corrosion rate is higher under so-called outlet conditions at 700°C compared to standard conditions at 650°C and so-called inlet conditions at 600°C.
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The corrosion potential increases with time when a protective corrosion layer is formed. Eventually it reaches a more positive potential, which is close to the operating potential of the fuel cell cathode. On Thermax 4762 steel, the corrosion layer grows relatively fast in the first 48 h, then slows down when a protective layer is formed. The corrosion layer consists of an Fe-rich outer layer and a protective Al- and Cr-rich inner layer.
Acknowledgements This work was funded by the Commission of the European Communities, Contract JOU3-CT95-0024 and the Swedish National Energy Administration (STEM). The authors thank Dr Arne Bengtsson, Swedish Institute for Metals Research, for the GDOES measurements and Dr Jinshan Pan, Department of Materials Science and Engineering, KTH, for helpful discussion of the manuscript. The authors thank Dr Carina Lagergren, KTH, for useful comments on the manuscript.
References [1] H.S. Hsu, J.H. Devan, M. Howell, J. Electrochem. Soc. 134 (1987) 3038. [2] J.P.T. Vossen, A.H.H. Janssen, J.H.W. de Wit, J. Electrochem. Soc. 143 (1996) 58. [3] B. Zhu, G. Lindbergh, D. Simonsson, Corros. Sci. 41 (1999) 1497. [4] B. Zhu, G. Lindbergh, D. Simonsson, Corros. Sci. 41 (1999) 1515. [5] R.C. Makkus, G. Lindbergh, B. Zhu, Y. Denos, V. Blanchot-Courtois, V. Chauvaut, M. Cassir, E. Bullock, Selection and Development of Materials for High-Endurance MCFC Bipolar Plate, Final report, Contract JOE3-CT 95-0024, 1998. [6] M. Spiegel, P. Biedenkopf, H.J. Grabke, Corros. Sci. 39 (1997) 1193. [7] P. Biedenkopf, M. Spiegel, H.J. Grabke, Electrochim. Acta 44 (1998) 683. [8] M. Keijzer, G. Lindbergh, K. Hemmes, P.J.J.M. van der Put, J. Schoonman, J.H.W. de Wit, J. Electrochem. Soc. 146 (1999) 2508. [9] J.R. Selman, Fuel Cell System, Plenum Press, New York, 1993. [10] J.P.T. Vossen, L. Plomp, J.H.W. de Wit, G. Rietveld, J. Electrochem. Soc. 142 (1995) 3327. [11] G. Lindbergh, B. Zhu, Electrochim. Acta 46 (2001) 1131. [12] H. Matsuyama, T. Nishina, I. Uchida, in M.-L. Saboungi, H. Kojima, J. Duruz, D. Shores (Eds.), Molten Salts Chemistry and Technology III, PV 93-9, The Electrochemical Society Proceedings Series, Pennington, NJ, 1993. [13] V.I. Sannikov, I.N. Ozeryanaya, T.I. Manukhina, V.E. Khavin, Elektrokhimiya 23 (1987) 797 translated.
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[14] T.E. Swarr, Microstructural Science, Proceedings of the Annual Technical Meeting of the International Metallographic Society, vol. 19, 1992, p. 83. [15] R.A. Donado, L.G. Marianowski, H.C. Maru, J.R. Selman, J. Electrochem. Soc. 131 (1984) 2535.
[16] C. Yuh, R. Johnsen, M. Farooque, H. Maru, J. Power Sources 56 (1995) 1. [17] P. Biedenkopf, M. Bischoff, T. Wochner, Mater. Corros. 51 (2000) 287.
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Corrosion behaviour of high-chromium ferritic steels in molten carbonate in cathode environment Baohua Zhu, Go¨ran Lindbergh *,1 Department of Chemical Engineering and Technology, Applied Electrochemistry, Royal Institute of Technology, KTH, SE-100 44 Stockholm, Sweden Received 12 August 2000; received in revised form 14 February 2001
Abstract The corrosion behaviour of four ferritic steels with a high chromium content and AISI 310 was investigated in (Li0.60/Na0.40)2CO3 melt in three different cathode gas environments. The electrochemical techniques used were linear polarisation resistance and Tafel extrapolation. The corrosion layers formed on the surface during the tests were analysed by glow discharge optical emission spectroscopy (GDOES). The corrosion layer formed on the Thermax 4762 sample consists of an iron-rich outer layer and a protective aluminium- and chromium-rich inner layer. The corrosion potential increased to a more positive value as the corrosion layer grew on the surface. This supports the supposition that the cathodic reaction in the corrosion process changes gradually from water reduction to oxygen reduction. It was shown that higher temperatures and low concentrations of oxygen and carbon dioxide under so-called outlet cathode gas conditions result in higher corrosion rates. © 2001 Elsevier Science Ltd. All rights reserved. Keywords: Corrosion; Molten carbonate fuel cell (MCFC); Chromium-containing steels; Tafel extrapolation; GDOES
1. Introduction The development of molten carbonate fuel cell (MCFC) technology still requires the cost of stack components to be reduced and their lifetime increased. Corrosion of the current collectors is one of the main lifetime-limiting factors. Studies have been made of the electrochemical corrosion behaviour of pure metals, binary model alloys and commercial steels in molten carbonates [1–8]. Basic research and technology development has shown that high-chromium steels are applicable for cathode-side conditions [3,4]. The * Corresponding author. Tel.: + 46-8-7908143; fax: + 46-8108087. E-mail address: [email protected] (G. Lindbergh). 1 ISE member.
state-of-the-art material used as a current collector in MCFCs is AISI 310 austenitic stainless steel, the lifetime of which is about 20000 h [9]. An extension of the lifetime to 40000 h by using alternative stainless steels would improve economic feasibility. Vossen et al. [10] reported the corrosion behaviour of some commercially available alloys, including AISI 310 and AISI 316 stainless steels, the nickel-base alloy Inconel 601, ferritic steels Thermax 4762 and Kanthal A1. The materials were investigated in an MCFC anode environment. A ranking of the corrosion properties was undertaken and Thermax 4762 and Kanthal A1 were considered to have the best corrosion resistance. Lindbergh et al. [11] studied the corrosion behaviours of some aluminium-containing ferritic steels (including some Kanthal series steels, PM 2000 and MA 956) in three different MCFC anode environments, and found that Kanthal A1 had the best corrosion resistance.
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Table 1 Chemical compositions of the investigated materials (wt.%)
Thermax 4762 AL 29-4-2 AISI 310 7Mo+N Thermax 4742
Cr
Ni
23 29 25 26 18
2.3 20 4.8
Al
Mo
1.5 4 1.4 1
However, this high-aluminium steel is not suitable as a cathode current collector material due to the high contact resistance of the aluminium oxide layer formed on the surface. There are some reports on the corrosion behaviour of Al-containing steels in molten salts under cathode gas conditions [12–14]. With more than 5 wt.% Al, a very thin layer of LiAlO2 is formed on the surface. This oxide acts as a quite stable corrosion-protective layer [12]. Swarr [14] reported the characterisation of aluminium coatings on AISI 310, AISI 316 and INCO 825 separators in molten salts in the O2 –CO2 gas environment. The b-(Fe,Ni)Al phase forms an Al2O3 layer on the surface and this layer is converted to LiAlO2 during exposure in molten carbonate. The resistance of materials to corrosion in molten carbonate is dependent on a large number of factors, including moisture and impurity content, temperature, and the gas atmosphere above the melt. In general, the anode-side environment is more corrosive than the cathode-side. A bi-metallic design is expected to increase the corrosion resistance of the current collectors on both the anode and cathode sides. Some of the high-chromium ferritic steels, namely Thermax 4762, AL 29-4-2, 7Mo +N and Thermax 4742, are expected to have a lifetime of more than 40000 h, based on a small-scale laboratory exposure test [5]. In cathode gas environments, the oxidising agents of interest are dissolved water and oxygen in the carbonate melt. Therefore the cathodic reaction in the corrosion process can be water and/or oxygen reduction: Water reduction:
H2O+CO2 +2e− =H2 +CO23 − (1)
Oxygen reduction:
1/2O2 +CO2 +2e− =CO23 −
(2)
The general reaction for metal dissolution can be written as: Metal oxidation:
Me = Mez + + ze−
(3)
The equilibrium potentials of Eqs. (1) and (2) were reported in the literature [15] and depend on the composition of the gas dissolved in the melt. The equilibrium potential of water reduction under standard anode gas conditions is about − 1120 mV vs the gold reference electrode used in this study, and for oxygen reduction it is about −50 mV vs the gold reference electrode. The calculated equilibrium potentials for oxide formation of metal/metal oxide are also reported in the literature [15]. Under standard cathode gas conditions, it is about − 1070 mV for FeO/Fe, − 1560 mV for Cr2O3/Cr and − 610 mV for NiO/Ni. The aim of the present study is to investigate the corrosion behaviour of the high-chromium ferritic steels mentioned above and AISI 310 stainless steel by means of electrochemical methods, under three different cathode conditions of the MCFC. Post-test characterisation was undertaken to investigate the corrosion layer formed on the sample surface.
2. Experimental The experimental set-up used has been described elsewhere [3]. The corrosion tests were performed in a (Li0.60/Na0.40)2CO3 melt. The reference electrode consisted of an alumina tube with a small hole in the bottom (about 0.1 mm diameter); the tube was partially filled with carbonate in which a gold wire was inserted. The reference gas was a mixture of 33.3% CO2 and 66.7% O2. The samples were cut to 7 × 7×1 mm, polished with c 600 to c 1200 SiC abrasive paper, washed with distilled water and ethanol, and then dried before testing. The samples were welded with 0.8 mm diameter stainless steel wires. The investigated materials are listed in Table 1. The samples were investigated under three different cathode gas conditions, Table 2. The so-called inlet and outlet gas compositions should resemble the inlet and outlet conditions in a stack under operating conditions in a natural-gas-fuelled system. The electrochemical measurements were performed using a model 273
Table 2 The different cathode gas conditions
Inlet Standard Outlet
O2 (%)
CO2 (%)
N2 (%)
H2O (%)
T (°C)
12.0 14.1 8.5
22.0 28.2 5.0
54.0 51.7 74.5
12.0 5.9 12.0
600 650 700
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Fig. 1. Anodic polarisation curves (sweep rate 1 mV s − 1) of five steels exposed in the carbonate melt under standard cathode gas conditions at 650°C for half an hour.
EG&G Princeton Applied Research potentiostat/galvanostat. The polarisation curves were recorded at a scan rate of 1 mV s − 1. The corrosion rate expressed as current density at the open circuit (corrosion) potential was determined by linear polarisation resistance and Tafel extrapolation methods. The compositions of the surface layers of the exposed samples were analysed by means of glow discharge optical emission spectroscopy (GDOES). GDOES was performed by cathodic sputtering of the sample surface in a Grimm-type glow discharge lamp. The sputtering process produces a crater with an almost flat bottom on the sample. The rate of sputtering depends on the voltage, current, pressure, plasma gas and sample composition. The sputtered material is atomised in a plasma, and the characteristic emission lines are monitored by an optical spectrometer as a function of burning time, yielding an in-depth profile of sample composition.
3. Results and discussion
3.1. Anodic polarisation cur6es The anodic polarisation curves of different samples under different cathode conditions were measured after exposure for half an hour at open circuit potential. The initial potential was the open circuit potential and the final potential was −50 mV vs the reference electrode. All these anodic polarisation curves show a region in which the corrosion currents drop with increasing potential. In an operating fuel cell, the electrode potential
at the cathode side is usually between − 50 and − 200 mV, depending on the conductivity of the cathode material, the corrosion layer formed between the cathode and the current collector, and operating conditions. In this potential region, passivations of the materials seem to occur. The comparison of ferritic steels under standard cathode conditions, Fig. 1, shows that in the potential region −50 to − 200 mV, AL 29-4-2 and 7Mo+ N had a higher passive current density, while AISI 310 and Thermax 4742 had the lowest passive current density. Fig. 2 shows the anodic polarisation curves obtained on Thermax 4762 samples in three different cathode gas environments. In the same potential region, a higher passive current density was found under outlet conditions.
3.2. Corrosion potential Figs. 3 and 4 show the corrosion potentials as a function of time during exposure in (Li0.60/Na0.40)2CO3 melt under three different cathode gas conditions. These results are very different from those obtained in the anode environments [11]. The corrosion potentials measured on these high-chromium ferritic steels during exposure, as a function of time, change from − 1.2 V to much more positive values, close to the operating potential of the MCFC cathode. Subsequently, the cathodic reaction in the corrosion process, under cathode gas conditions, changes from water reduction and oxygen reduction at the beginning, to only oxygen reduction when the corrosion potential increases above the equilibrium potential of the water reduction reaction.
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Fig. 2. Anodic polarisation curves (sweep rate 1 mV s − 1) of Thermax 4762 samples exposed in the carbonate melt under three different cathode gas conditions for half an hour.
The time needed for the corrosion potentials to reach stable values in molten carbonate depends on the material, temperature and gas composition, Table 3. Thermax 4762, AISI 310 and AL-29-4-2 were investigated under different cathode gas conditions. It was shown that the time needed to reach the final potential was shorter under outlet conditions than under inlet and standard conditions, Fig. 4.
3.3. Corrosion rate 3.3.1. Linear polarisation resistance The linear polarisation resistance method is an alternative method for determining the corrosion current density at the open circuit potential. The perturbation potential was 930 mV, which is much smaller than for the Tafel extrapolation experiments. Therefore, the
Fig. 3. Corrosion potentials of five high-chromium steels, as a function of time, in lithium – sodium carbonate melt under standard cathode gas conditions at 650°C.
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Fig. 4. Corrosion potentials of Thermax 4762, as a function of time, in lithium – sodium carbonate melt under three different cathode gas conditions.
growth of the corrosion layer was disturbed by the measurements to a lesser extent. The Stern – Geary equation was used to determine the corrosion current density [3]. The Tafel slopes used in the equation were the average values of each sample obtained from the Tafel extrapolation curves. The linear polarisation resistance was measured at various times during exposure. The polarisation resistance was low at the beginning, and became more difficult to measure during fast increase of the potential. When the final potential has been reached, the polarisation resistance is about ten times higher than at the beginning and can be measured again. The polarisation resistances of four ferritic steels and AISI 310 measured at the end of the exposure tests are summarised in Table 3. From the results obtained on Thermax 4762, AISI 310 and AL 29-4-2, lower polarisation resistance and higher corrosion current densities were obtained under outlet conditions. The linear polarisation resistance was measured for Thermax 4762 as a function of time, Fig. 5. During the first 48 h, the polarisation resistance was almost constant or decreased slowly. It then increased tenfold during the following 100 h. This will be discussed later in connection with the analysis of the corrosion layer.
3.3.2. Tafel extrapolation cur6es The Tafel extrapolation curves were recorded at the end of each exposure test. The experimental curves were recorded in the anodic direction at a scan rate of 1 mV s − 1. The complete Tafel equation for the anodic and cathodic reactions is:
!
i= icorr exp − exp
n n"
haF ( E − Ecorr ) RT
− hc F ( E − Ecorr ) RT
(4)
where ha and hc are the anodic and cathodic transfer coefficients, respectively, icorr is the corrosion current density and Ecorr the corrosion potential. Eq. (4) was used to calculate corrosion rates, expressed as current densities. The corrosion current densities at the end of the exposure time, obtained by fitting experimental data to Eq. (4), are listed in Table 3. Fig. 6 shows one example of the experimental and fitting curves for the Thermax 4762 sample. The icorr values obtained here under the outlet cathode conditions are tenfold greater than under the standard and inlet cathode conditions for Thermax 4762 and AL 29-4-2. For AISI 310 steel, the corrosion current density under outlet conditions is only slightly higher than under standard and inlet conditions. Two more materials, Thermax 4742 and 7Mo+ N, were investigated using the same method under standard cathode conditions. The corrosion current densities are shown in Table 3. The icorr value for Thermax 4742 is 0.14 A m − 2, which is higher than for the other four materials. Fig. 7 shows the Tafel extrapolation curves for AISI 310 samples after different exposure times in standard cathode environments. A Tafel curve obtained for the same material in a standard anode environment and taken from Ref. [3] is also shown here, in order to compare with the results obtained in cathode environments. The corrosion rate during the first 24 h was
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more than tenfold after 120 h. From the anodic polarisation curves we know that the initial potential is in the active region, while the end potentials of the alloy are in the passive region. This also indicates a higher corrosion rate at the initial stage when water reduction is the main cathodic reaction in the corrosion process. The corrosion current densities, obtained by Tafel extrapolation methods at the end of the exposure period, are much lower than the passive current densities. This is probably because the anodic polarisation curves were recorded after a very short exposure time and therefore only a thin protective layer had formed on the surface.
3.4. Analysis of the corrosion layer In an earlier study [3,4], corrosion layers on AISI 310 and some iron –chromium binary alloys were investigated. It was shown that two layers are formed, a
chromium-rich inner layer close to the metal and a porous outer layer consisting mainly of lithium –iron oxides. Thermax 4762, containing 1.5% aluminium, is a very interesting material for use as a cathode current collector. In the present study, GDOES experiments were performed on Thermax 4762 samples exposed in lithium –sodium carbonate melt under standard cathode conditions for different exposure times, namely, 4, 24, 48, 72, 96 and 120 h, Fig. 8a – f. During the first 4 h, a corrosion layer of about 1.8 mm was formed, Fig. 8a. This layer was aluminium rich and contained lithium. The amount of iron and chromium decreased towards the surface. After 24 h, Fig. 8b, the thickness of the corrosion layer was 8 mm, while after 48 h, it had grown to 21 mm, Fig. 8c. The layer formed later consists of two parts. The outer part mainly contains lithium, iron and oxygen, while the inner part is chromium and aluminium rich, and con-
Table 3 Corrosion current densities, icorr, polarisation resistances, Rp, at the end of exposure and the exposure time needed for the corrosion potential to reach the final constant value
Thermax 4762 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) AL 29-4-2 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) AISI 310 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) Thermax 4742 Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h) 7Mo+N Tafel method icorr (A m−2) Linear polarisation resistance Rp (V m2) icorr (A m−2) Time needed for potential to reach constant value (h)
Inlet cathode gas 600°C
Standard cathode gas 650°C
Outlet cathode gas 700°C
0.03
0.06
0.37
0.41 0.04 250
0.28 0.05 120
0.10 0.15 45
0.04
0.05
0.47
0.33 0.04 50
0.19 0.07 70
0.13 0.11 50
0.03
0.04
0.05
0.22 0.06 460
0.18 0.08 120
0.10 0.15 90
0.14 0.17 0.08 235 0.05 0.26 0.05 180
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Fig. 5. Linear polarisation resistance and thickness of the corrosion layer formed on Thermax 4762 as a function of time, under standard cathode conditions at 650°C.
tains lower levels of lithium, iron and oxygen. Also in the corrosion layer formed after 24 h, chromium enrichment may be found halfway through the layer. After the first 48 h, the growth of the corrosion layer became relatively slow. After 72 and 96 h of exposure, Fig. 8d,e, the total corrosion layer thickness increased to about 24 and 30 mm, respectively. After another 24
h, the corrosion layer was about 33 mm thick, Fig. 8f. This means that during the first 48 h the increase in thickness was 21 mm, while the following 72 h only led to an increase of 12 mm. All four figures, Fig. 8c –f, show that there were two layers in the corrosion products. The inner layer was chromium and aluminium rich while the outer layer mainly consisted of lithium, iron and oxygen.
Fig. 6. Tafel extrapolation curves (sweep rate 1 mV s − 1) obtained on Thermax 4762 samples exposed in the carbonate melt under three different cathode gas conditions when the open circuit potentials had reached their final constant value.
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4. Discussion The different methods used for estimating the corrosion current density agree fairly well under the standard and inlet conditions except for the Thermax 4742 sample, Table 3. Under outlet conditions, the difference between the methods is greater. In any case, the corrosion current densities under outlet cathode conditions are much higher than under standard and inlet conditions at the operating potential. Corrosion in molten salts can be characterised by two steps: the first is oxidation of the metal and the second is dissolution of the oxide scales [10]. The reactant, oxygen or water, has to diffuse through the melt to the metal surface. Therefore, the permeability of the oxygen/water in the melt is an important factor for the oxidation rate. The higher the temperature is in the melt, the more oxygen or water is transported in it. Therefore a higher corrosion rate can be expected under outlet conditions when the temperature is high. The corrosion current densities were determined at the end of exposure when the corrosion potential had reached a stable value. The time needed for the corrosion potential to reach this value depended on the environment and material. The corrosion current densities under outlet conditions were determined after a shorter exposure time than under standard and inlet conditions. The polarisation resistance of Thermax 4762 under outlet conditions after 45 h of exposure, Table 3, was higher than that under
standard conditions after 48 h of exposure, Fig. 5. This means that after approximately the same exposure time (but different corrosion potentials), a lower corrosion rate was obtained under outlet conditions. However, the corrosion potential of the sample under outlet conditions had already reached a stable value. GDOES results after more than 48 h of exposure, Fig. 8c –f, show that aluminium was not detected in the outer layer, but did exist in the inner layer. This means that no lithium aluminate protection layer is formed on the surface. This is in accordance with results presented by Matsuyama et al. [12]. They pointed out that a thin LiAlO2 layer was formed on the surface only when the aluminium content exceeded 5 wt.%. The aluminium content in Thermax 4762 is only 1.5 wt.% and is therefore not sufficient to form LiAlO2 on the surface. However, a protective Al-rich oxide inner layer is formed. Thermax 4762 could be expected to have a lower contact resistance than high aluminium containing steels due to the absence of a LiAlO2 layer on the surface. In determining the corrosion layer thickness from the GDOES results, the material in which the oxygen content exceeds 2% was defined as the corrosion layer. By using these criteria, the thickness of corrosion products was obtained as a function of exposure time, Fig. 5. The linear polarisation resistance (Rp) is also shown in this figure as a function of time. The thickness of the corrosion layer formed on Thermax 4762 as a function
Fig. 7. Tafel polarisation curves (sweep rate 1 mV s − 1) obtained on AISI 310 samples under standard cathode conditions at 650°C and standard anode conditions (56% H2, 8% CO2, 8% CO and 28% H2O) at 650°C after different exposure times.
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Fig. 8. Compositions of the surface layers, obtained by GDOES, of Thermax 4762 after different exposure times in carbonate melt under standard cathode conditions at 650°C: (a) 4 h, (b) 24 h, (c) 48 h, (d) 72 h, (e) 96 h and (f) 120 h.
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Fig. 8. (Continued).
B. Zhu, G. Lindbergh / Electrochimica Acta 46 (2001) 2593–2604
of time displays two linear regions. A higher corrosion rate was recorded during the first 48 h. Then oxygen reduction becomes the dominant reaction, and the corrosion rate decreases. The constant corrosion resistance (Rp) in the two stages indicates that the corrosion current densities are constant, resulting in a linear growth of the corrosion layer. When comparing the experimental results in the anode and cathode environments, Fig. 7, we can see that the corrosion potentials are almost the same in both environments during the first 24 h of exposure. This indicates that the corrosion process is very similar in both environments. In the anode gas environment, the corrosion process proceeds with water reduction as the cathodic reaction [3], and it is reasonable to assume that the cathodic reaction is the same in the cathode environment. The corrosion potential as a function of time over a long time period is a useful indicator of corrosion layer formation. The corrosion potential is determined by the mixed potential of metal oxidation and reduction of the oxidising agents. As shown in Figs. 3 and 4, the initial corrosion potential indicated a mixed potential of water reduction and metal dissolution. At the end of the exposure, the corrosion potential was close to the oxygen reduction potential, which indicates that the main cathodic reaction had changed from water reduction to oxygen reduction and the corrosion rate decreased due to the corrosion layer formed on the surface. The first potential shift describes the initial formation of the oxides. The chromium oxide then dissolves into the carbonate melt in the cathode environment, and a layer of iron oxide (usually porous) remains on the surface. The iron oxide is lithiated in the lithium carbonate to lithium ferrite. This outer layer separates the inner oxide from the aggressive molten carbonate and prevents the inner oxide from further dissolution reaction. The corrosion potential increases due to the anodic polarisation caused by the corrosion layer. After about 48 h, an inner Cr- and Al-rich layer was formed, Fig. 8c, and passivation begins. The inner Cr- and Al-rich oxide reduces the growth of the whole corrosion scale, because Cr- and Al-rich oxide exhibit very small diffusion coefficients for iron ions. The growth of the ironrich oxide dominates the growth of the whole corrosion scale [16,17]. The inner Cr- and Al-rich layer provides an effective protection against further growth of the outer iron-rich oxide layer. 5. Conclusions The corrosion behaviour of four ferritic steels and AISI 310 stainless steel with a high chromium content in cathode gas environments was investigated. The corrosion rate is higher under so-called outlet conditions at 700°C compared to standard conditions at 650°C and so-called inlet conditions at 600°C.
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The corrosion potential increases with time when a protective corrosion layer is formed. Eventually it reaches a more positive potential, which is close to the operating potential of the fuel cell cathode. On Thermax 4762 steel, the corrosion layer grows relatively fast in the first 48 h, then slows down when a protective layer is formed. The corrosion layer consists of an Fe-rich outer layer and a protective Al- and Cr-rich inner layer.
Acknowledgements This work was funded by the Commission of the European Communities, Contract JOU3-CT95-0024 and the Swedish National Energy Administration (STEM). The authors thank Dr Arne Bengtsson, Swedish Institute for Metals Research, for the GDOES measurements and Dr Jinshan Pan, Department of Materials Science and Engineering, KTH, for helpful discussion of the manuscript. The authors thank Dr Carina Lagergren, KTH, for useful comments on the manuscript.
References [1] H.S. Hsu, J.H. Devan, M. Howell, J. Electrochem. Soc. 134 (1987) 3038. [2] J.P.T. Vossen, A.H.H. Janssen, J.H.W. de Wit, J. Electrochem. Soc. 143 (1996) 58. [3] B. Zhu, G. Lindbergh, D. Simonsson, Corros. Sci. 41 (1999) 1497. [4] B. Zhu, G. Lindbergh, D. Simonsson, Corros. Sci. 41 (1999) 1515. [5] R.C. Makkus, G. Lindbergh, B. Zhu, Y. Denos, V. Blanchot-Courtois, V. Chauvaut, M. Cassir, E. Bullock, Selection and Development of Materials for High-Endurance MCFC Bipolar Plate, Final report, Contract JOE3-CT 95-0024, 1998. [6] M. Spiegel, P. Biedenkopf, H.J. Grabke, Corros. Sci. 39 (1997) 1193. [7] P. Biedenkopf, M. Spiegel, H.J. Grabke, Electrochim. Acta 44 (1998) 683. [8] M. Keijzer, G. Lindbergh, K. Hemmes, P.J.J.M. van der Put, J. Schoonman, J.H.W. de Wit, J. Electrochem. Soc. 146 (1999) 2508. [9] J.R. Selman, Fuel Cell System, Plenum Press, New York, 1993. [10] J.P.T. Vossen, L. Plomp, J.H.W. de Wit, G. Rietveld, J. Electrochem. Soc. 142 (1995) 3327. [11] G. Lindbergh, B. Zhu, Electrochim. Acta 46 (2001) 1131. [12] H. Matsuyama, T. Nishina, I. Uchida, in M.-L. Saboungi, H. Kojima, J. Duruz, D. Shores (Eds.), Molten Salts Chemistry and Technology III, PV 93-9, The Electrochemical Society Proceedings Series, Pennington, NJ, 1993. [13] V.I. Sannikov, I.N. Ozeryanaya, T.I. Manukhina, V.E. Khavin, Elektrokhimiya 23 (1987) 797 translated.
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[14] T.E. Swarr, Microstructural Science, Proceedings of the Annual Technical Meeting of the International Metallographic Society, vol. 19, 1992, p. 83. [15] R.A. Donado, L.G. Marianowski, H.C. Maru, J.R. Selman, J. Electrochem. Soc. 131 (1984) 2535.
[16] C. Yuh, R. Johnsen, M. Farooque, H. Maru, J. Power Sources 56 (1995) 1. [17] P. Biedenkopf, M. Bischoff, T. Wochner, Mater. Corros. 51 (2000) 287.
.