The Effect of Minor Alloying Elements in Ferritic Steels for ...

P0804-Schuisky
The Effect of Minor Alloying Elements in Ferritic Steels
for Interconnects in SOFCs
Mikael Schuisky 1 , Andreas Rosberg 1 , Lars Mikkelsen 2 ,
Peter Vang Hendriksen 2 , Niels Christiansen 3 and Jørgen Gutzon Larsen 3
1 AB Sandvik Materials Technology
SE-81181 Sandviken / Sweden
Tel.: +46-26-264083
Fax: +46-26-264177
Mikael.schuisky@sandvik.com
2 Fuel Cell & Solid State Chemistry Department
Risø National Laboratory
DK-4000 Roskilde / Denmark
3 Topsoe Fuel Cells A/S, Nymølley 55
DK-2800 Lyngby / Denmark
Abstract
The influence on oxidation resistance as well as conductivity, by the addition of minor
alloying elements in ferritic chromium steels, have been investigated. This was done by
producing several different steel melts in which the chemical content of alloying elements
such as Si, Al, Mn, Ti, Nb, Zr, Ni, and Mo were varied. The addition of rare earth elements
such as Ce was also briefly studied. The steels investigated were produced in a
conventional steel making process. The materials were further processed by hot-rolling
and cold rolling including annealing, down to stainless steel strips with a thickness of 0,2
mm. The weight gains of coupons during oxidation in air of the different alloys were
studied by. The oxidation was also studied by means of a thermal balance to investigate
the initial oxidation mechanisms of the different steels. The oxide scales of the oxidized
samples were further investigated by means of X-ray diffraction, scanning electron
microscopy combined with energy dispersive spectroscopy. The electrical interface
resistance between the alloys and a LSM plate was measured in air at 750°C. Minor
addition of Al has a positive effect on the high temperature corrosion properties due to
formation of a well protected aluminium oxide scale. However, as expected this scale is
well electrical insulating and will therefore increase the area specific resistance at the
alloy-LSM interface. The effect of Al on the corrosion properties of the ferritic chromium
alloy as well as the effect of the other minor alloying elements will be presented.
Introduction
In the last decade ferritic chromium steels have become the strongest candidate materials
for long life time interconnects in solid oxide fuel cells (SOFC) [1,2]. In SOFC life times
over 40 000 operating hours are required for the components in the stack, including the
interconnects. The main features which make chromia forming ferritic steels the prime
candidate material for interconnects is: a good corrosion stability, alloys are cheap to
produce but are also more ductile and flexible in comparison to ceramic interconnects, and
a thermal expansion close to the thermal expansion of the ceramic materials used in
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SOFC’s [3]. Thus in the last decade, SOFC developers and steel producers have shown
an increased interest to develop new suitable steel grades for the application as
interconnect in SOFC [1-8]. The first generation of ferritic chromium steel for the use as
interconnect in SOFC developed at Sandvik in the 1990s was called 0YC44. The 0YC44
grade is in principle a simple iron-chromium steel with a small addition of the rare earth
metal cerium. The grade 0YC44 has previously been investigated for the application as
interconnect in SOFC [3, 7-11]. The grade 0YC44 was referred to as a Fe 0.78 Cr 0.22 alloy in
ref. [9]. The 0YC44 grade has successfully been tested in a 13 000 hours SOFC stack test
at Topsoe Fuel A/S [11]. The second generation of interconnect materials, i.e. grade
1C44Mo20, developed at Sandvik is a ferritic chromium steel alloyed with Mo and Nb as
well as some minor addition of Ti and Zr [10]. In the present paper the influence on
oxidation resistance in air as well as conductivity, by the addition of minor alloying
elements in the second generation ferritic chromium steels, have been compared with the
0YC44 grade. This was done by producing 0,2 mm steel strips from several steel melts in
which the chemical content of alloying elements such as Si, Al, Mn, Ni, Nb, N, and Mo
were varied. The addition of Ce was also briefly studied. The oxidation resistance of the
different ferritic chromium alloys in air was investigated by comparing the area specific
weight gain of each sample. The electrical interface resistances between the alloys and
La 0.85 Sr 0.15 MnO 3 (LSM) were measured during aging at 750°C.
Experimetal
In this studied the two commercial Sandvik alloys 0YC44 and 1C44Mo20 as well as
several model alloys were produced by ordinary steel manufacturing processes. The
materials were further processed to cold rolled strips by first hot rolling the alloys down to
intermediate sizes, followed by further cold rolling down to a final strip thickness of 0,2
mm. The nominal chemical compositions of all alloys referred to in this study are listed in
table 1 below, together with the designated names.
Cyclic oxidations studies of the material were done in air in a conventional furnace at spec.
temperature range. The samples were cooled to room temperature before measuring the
weight gains at 168 hours intervals. The samples were oxidized for a total of 1008 hours.
Initial oxidation mechanisms of some of the model alloys were also studied by thermal
balance using a Thermo Cahn Thermax 700 equipment. The investigations were done in
air at 850°C for 72 hours with a ramp up time of 5° /min.
For phase identification studies of the formed oxide scales, x-ray diffraction (XRD) was
performed using a Bruker D8 Discover diffractometer. The surface morphologies as well
as the cross sectional micrographs of the oxide scales were investigated utilizing a Leo
DSM 962 scanning electron microscope (SEM) equipped with Energy Dispersive
Spectroscopy (EDS) for elemental analysis.
The electrical interface resistances between LSM plates and the alloys were measured by
a DC four-point method in air at 750°C over a period of 1000 h. A contact layer of
(La,Sr)(Mn,Co)O 3 was applied between the alloys and LSM plates. Experimental details
are given in [12].
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Table 1. Chemical composition of the ferritic grades discussed in this paper (in wt. %) the
roman number of the 1C44Mo20 denoted different melts
Name Cr Mn Si Ni Mo Add. Fe
0YC44 21,6 0,3 0,3 0,2 0 Ce Bal.
1C44Mo20 - I 22,2 0,1 0,1 0,2 1,0 Nb, Ti, Zr Bal.
1C44Mo20 - II 22,3 0,1 0,2 0,3 1,0 Nb, Ti, Zr Bal.
1C44Mo20 - III 22,3 0,1 0,1 0,09 1,0 Nb, Ti, Zr Bal.
#514 21,9 0,3 0,1 0,07 0,6 Nb, Ti, Zr Bal.
#515 21,9 0,3 0,1 0,07 1,5 Nb, Ti, Zr Bal.
#517 21,9 0,3 0,2 0,06 0 Nb, Ti, Zr Bal.
#518 22,1 0.4 0,2 0,06 1,0 Nb,Ti,Zr, Ce Bal.
#521 22,1 0,4 0,1 0,06 1,0 Nb, Ti, Zr, N Bal.
#522 22,2 0,4 0,2 1,0 1,0 Nb, Ti, Zr Bal.
#525 22,1 0,3 0,2 0,06 1,0 Nb, Ti, Zr ,Al Bal.
#527 22,0 0,4 0,2 0,06 0 Ce Bal.
Results and Discussion
An oxidation study was done on coupons of all the model alloys at 850°C in air for 1008
hours. The weight gains of two coupons of each alloy were measured with intervals of 168
hours and the area specific weight is plotted versus time in Fig. 1. In the figure it can be
seen that 3 model alloys are behaving differently that the rest of the model alloys. The 3
alloys standing out are the model alloys #525, #521 and #527. The rest of the model alloys
are all having similar oxidation behaviour and end up with a weight gain of less than 1,0
mg/cm 2 after 1000 hours of oxidation at 850°C in air. The model alloy #525 with a small
addition of Al (0,3 wt-%) shows the by far best oxidation resistance. The SEM-EDS
analysis of the cross section shows that, despite the low Al-content, a thin well adherent
aluminium oxide scale has formed explaining the low growth rate. The second alloy to
stand out is #521, which shows a higher weight gain than the majority of the model alloys.
The reason for the dramatic increase of the weight gain curve of the #521 is due to
spallation and cracking of the oxide scale leading to breakaway oxidation. This behaviour
of the #521 alloy can be explained by the fact that #521 was alloyed with a high N content
i.e. 0,08 wt-%. The N will consume all the Ti and Zr which are added to give a well
adherent oxide scale. The last alloy to stick out is the model alloy #527, which was a new
melt of the first generation of Sandvik materials for interconnects in SOFC i.e. the 0YC44
grade. It is appears that a small addition of a rare earth metals such as Ce to a ferritic
chromium steel will not have as positive effect on the corrosion resistance as the addition
of group IV and V metals. The #527 alloy had the by far worst corrosion resistance of all
the model alloys with a weight gain of more than 1,6 mg/cm 2 after 1000 hours oxidation at
850°C in air.
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Figure 1
Weight gain of the different model alloys as a function of time while oxidized
in air at 850°C
For the new melt of the commercial alloy 1C44Mo20-III the oxidation was performed at 5
different temperatures from 700 up to 900°C on coupons with the size 30x40x0,2 mm. The
weight gain of the alloy at 850°C was 0.9 mg/cm 2 which are very similar to the weight
gains shown in figure 1 for the majority of the model alloys. At the highest temperature
breakaway corrosion occurred after 840 hours giving rise to a increase of the weight gain
for these coupons. For the lower oxidation temperatures very low weight gains were
recorded below 0,1 mg/cm 2 at 700°C after 1000 hours exposure to air. The thicknesses of
the formed oxide scales were also measured with cross sectional SEM after the oxidation
study and the measured thicknesses are given in table 2. The oxide scale thicknesses are
in good agreement with the obtained weight gains.
Figure 2
The recorded weight gain during oxidation of 0.2 mm thick coupons of the
alloy 1C44Mo20 at different temperatures in air
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From the weight gains recorded in Fig. 2 in can be seen that kinetics of the oxidation of
1C44Mo20 is fairly good following the parabolic law. To illustrate this parabolic behaviour
of 1C44Mo20 the square of the weight gain per unit area (∆W/A) was plotted as a function
of the time, t for the five different oxidation temperatures This should then follow the
parabolic law and give a straight line when plotted:
(∆W/A) 2 = Kp * t Equation 1
Where Kp is the parabolic rate constant equal to the slope of the line shown in Fig. 3 for
the sample oxidized at 750°C.
Figure 3
Parabolic plot of the oxidation of 1C44Mo20-III at 750°C in air
The calculated parabolic rate constants for the different temperatures are given in Table 2,
below.
Table 2
Calculated parabolic rate constants, oxide scale thickness and estimated life
times before breakaway corrosion for 1C44Mo20 alloy in the temperature
range 700 - 900°C
Temperature (°C) Kp (mg 2 cm -4 h -1 ) Scale thickness (µm) Est. life time (h)
700 6,6 x 10 -6 0,8 1 100 000
750 4,8 x 10 -5 2 150 000
800 2,4 x 10 -4 3 29 000
850 7,8 x 10 -4 6 9 000
900 8,4 x 10 -3 16 840
Estimated life times of the 0,2 mm thick 1C44Mo20 alloy can be calculated from equation
1 at the different temperatures assuming that the mass gain recorded after 840 hours,
when the breakaway corrosion was observed for the sample oxidized at 900°C would be
the same for other temperatures. These estimated life times of the 0,2 mm thick strip of
the 1C44Mo20 alloy are listed above in table 2. These estimated life times for the
1C44Mo20 are in good agreement on earlier presented results on similar commercial
alloys [14].
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The SEM cross section of the formed oxide scale on the model alloys after 1008 hours
oxidization were also analysed below the SEM pictures for the model alloys #522 and
#527 are shown in Fig. 4.
Figure 4 SEM cross section of the formed oxide scale on the two model alloys #522
(right) and #527 (left) after 1008 hours oxidation at 850°C in air
From the SEM cross section micrographs above it can be seen that the thickness of the
#527 model alloy i.e. approx. 13 µm is more than twice as thick as the oxide scale formed
on the model alloy #522 i.e. approx. 5 µm. The EDS mapping of all alloys show an
increasing Mn content in the outer part of the oxide scale indicating the formation of
MnCr 2 O 4 spinel which was verified by X-ray diffraction. At the chromia scale - alloy
interface small particles of silica was formed. Under the chromia scale internal oxides of
titanium and aluminium can be observed in both alloys. The initial oxidation mechanism of
the model alloys were also studied by thermal balance and below in Fig. 5. the recorded
plots of weight vs. time for the alloys #527 and #524 are shown.
Figure 5
Thermal balance studies of alloys #527 and 1C44Mo20 - I
The area specific weight gains were calculated for the samples analysed with thermal
balance from the weight recorded at t = 3 hours until t = 72 hours. The time t = 3 hours
was chosen since at this time the temperature had stabilized at 850°C for the samples.
The weight difference was denoted ∆m and found to be 1,297 mg/cm 2 /69 hours or 0,019
mg/cm 2 /h for the #527 and 0,485 mg/cm 2 / 69 hours or 0,007 mg/cm 2 /h for the 1C44Mo20.
The much lower ∆m weight gain of the 1C44Mo20 alloy compared to the #527 alloy once a
gain shows a much better oxidation resistance of the 1C44Mo20 type of alloys which is
explained by the addition of Mo and Nb to the alloy.
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The area specific resistance (ASR) values of the alloy-LSM plate interfaces were
measured during aging at 750°C. The ASR-values for some of the alloys are shown in Fig.
6. The ASR-values for the model alloy #525, with a small addition of Al showed the by far
highest degradation rate well over 100 mΩcm 2 /10 3 h of all the model alloys. The second
largest degradation rate was found for the #527 alloy with a degradation of more than 7
mΩcm 2 /10 3 h. The much thicker oxide scale of the #527 alloy is likely the cause of this
increased ASR degradation rate. However, this is still lower than the earlier reported ASR
degradation rate for the similar 0YC44 alloy of 45 mΩcm 2 /10 3 h [10]. This could be
explained by the difference in silicon content of the two 0YC44 type alloys. The
commercial alloy had a twice as high Si content as the model alloy #527. The rest of the
model alloys all had similar degradation rates, of below 3 mΩcm 2 /10 3 h. The new melt of
the commercial alloy 1C44Mo20 had a very low degradation rate of

Conclusions
The first general conclusion to be drawn for this study is that the aluminium content in the
alloy used for interconnects in SOFC should be kept low preferably below 0,3 wt-%.Minor
additions of Al to the alloy increases the corrosion resistance, but it also increases the
degradation rate of the ASR by a factor of 100.
The second conclusion to be made is that the second generation of steel i.e. the
1C44Mo20 type of alloys have much better corrosion resistance as well as conductivty of
the oxide scale compared to the 0YC44 type of alloys. Especially, the new melt which has
a very low degradation rate of the ASR < 1 mΩcm 2 /10 3 h and 2,5 times lower weight gain
that the 0YC44 type of alloys.
Furthermore, the additon of small amounts of rare earth metals such a Ce to the
1C44Mo20 alloy did not greatly increase the oxidation resistance of the alloy nor the
conductivity of the formed oxide scale.
The Si content seems to have a very important role in the electrical resistance and should
therefore be kept as low as possible preferably below 0,1 %.
Nitrogen should be kept low to avoid spallation of the formed oxide scale.
The parabolic rate constant for the 1C44Mo20 alloy was calculated to be 6,6 x 10 -6 mg 2
cm -4 h -1 at 700°C, 2,4 x 10 -4 mg 2 cm -4 h -1 at 800°C and 8,4 x 10 -3 mg 2 cm -4 h -1 at 900°C,
respectively.
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