U.S. patent application number 11/131307 was filed with the patent office on 2005-12-01 for heat-resistant steel.
This patent application is currently assigned to SANDVIK INTELLECTUAL PROPERTY HB. Invention is credited to Goransson, Kenneth, Rosberg, Andreas, Schuisky, Mikael.
Application Number | 20050265885 11/131307 |
Document ID | / |
Family ID | 32501930 |
Filed Date | 2005-12-01 |
United States Patent
Application |
20050265885 |
Kind Code |
A1 |
Schuisky, Mikael ; et
al. |
December 1, 2005 |
Heat-resistant steel
Abstract
A high temperature corrosion resistant stainless steel having a
composition (by weight) of: C.ltoreq.0.2%, but more than zero,
N.ltoreq.0.1% but more than zero, O.ltoreq.0.1% but more than zero,
Si.ltoreq.0.4% but more than zero, Al<0.5% but more than zero,
Mn.ltoreq.0.5% but more than zero, Cr 20 to 25%, Ni.ltoreq.2.0% but
more than zero, Zr+Hf 0.01 to 0.1%, Ti.ltoreq.0.5% but more than
zero, Mo+W.ltoreq.2.5% but more than zero, Nb+Ta.ltoreq.1.25% but
more than zero, V.ltoreq.0.5% but more than zero, and balance of Fe
and naturally occurring impurities and not more than 0.010% of S
impurity. This steel is particularly suitable for the manufacturing
of interconnects in solid oxide fuel cells.
Inventors: |
Schuisky, Mikael;
(Sandviken, SE) ; Rosberg, Andreas; (Sandviken,
SE) ; Goransson, Kenneth; (Gavle, SE) |
Correspondence
Address: |
BUCHANAN INGERSOLL PC
(INCLUDING BURNS, DOANE, SWECKER & MATHIS)
POST OFFICE BOX 1404
ALEXANDRIA
VA
22313-1404
US
|
Assignee: |
SANDVIK INTELLECTUAL PROPERTY
HB
SANDVIKEN
SE
|
Family ID: |
32501930 |
Appl. No.: |
11/131307 |
Filed: |
May 18, 2005 |
Current U.S.
Class: |
420/68 |
Current CPC
Class: |
C22C 38/46 20130101;
B01J 21/063 20130101; Y02E 60/50 20130101; C22C 38/44 20130101;
C22C 38/50 20130101; C22C 38/48 20130101 |
Class at
Publication: |
420/068 |
International
Class: |
C22C 038/28 |
Foreign Application Data
Date |
Code |
Application Number |
May 19, 2004 |
SE |
0401292-8 |
Claims
What is claimed is:
1. A high temperature corrosion resistant stainless steel,
consisting essentially of (by weight): C.ltoreq.0.2%, but more than
zero; N.ltoreq.0.1% but more than zero; O.ltoreq.0.1% but more than
zero; Si.ltoreq.0.4% but more than zero; Al<0.5% but more than
zero; Mn.ltoreq.0.5% but more than zero; Cr 20 to 25%;
Ni.ltoreq.2.0% but more than zero; Zr+Hf 0.001 to 0.1%;
Ti.ltoreq.0.5% but more than zero; Mo+W.ltoreq.2.5% but more than
zero; Nb+Ta.ltoreq.1.25% but more than zero; V.ltoreq.0.5% but more
than zero; and balance of Fe and naturally occurring impurities and
not more than 0.010% of S impurity.
2. The stainless steel according to claim 1, wherein the stainless
steel has a weight gain of less than 1.5 mg/cm.sup.2 when oxidized
in air or air +1% H.sub.2O at 850.degree. C. after 1000 hours,
without any spallation of the oxide scale.
3. The stainless steel according to claim 2, wherein the stainless
steel has a linear thermal expansion coefficient of more than
11.5.times.10.sup.-6(.degree. C..sup.-1) but less than
13.0.times.10.sup.-6(.degree. C.sup.-1) in the temperature range of
30 to 900.degree. C.
4. The stainless steel according to claim 1, wherein the stainless
steel has a linear thermal expansion coefficient of more than
11.5.times.10.sup.-6(.degree. C..sup.-1) but less than
13.0.times.10.sup.-6(.degree. C..sup.-1) in the temperature range
of 30 to 900.degree. C.
5. The stainless steel according to claim 1, wherein the stainless
steel has a thermal expansion mismatch (TEM) with an electro-active
ceramic material in a solid oxide fuel cell of less than
.+-.15%.
6. The stainless steel according to claim 1, wherein the stainless
steel is suitable for application as an interconnect or a bipolar
plate material in Solid Oxide Fuel Cells.
7. The stainless steel according to claim 1, wherein the stainless
steel is suitable for application as a catalytic converter in
automobile applications.
8. The stainless steel according to claim 1, wherein the stainless
steel has an Area Specific Resistance of less than 50 m.OMEGA.
cm.sup.2 after 1000 hours on both an anode side and a cathode side
of a Solid Oxide Fuel Cell.
9. The alloy according to claim 8, wherein an increment of the Area
Specific Resistance is not greater than 10 m.OMEGA. cm.sup.2 per
1000 hours.
10. A solid oxide fuel cell comprising the stainless steel
according to claim 1.
11. A catalytic converter intended for the automobile industry,
comprising the stainless steel according to claim 1.
12. A high temperature corrosion resistant stainless steel
consisting essentially of (by weight): C.ltoreq.0.1% but more than
zero; N.ltoreq.0.1% but more than zero; O.ltoreq.0.1% but more than
zero; Si.ltoreq.0.4% but more than zero; Al<0.4% but more than
zero; Mn.ltoreq.0.4% but more than zero; Cr 20 to 25%;
Ni.ltoreq.1.0% but more than zero; Zr 0.001 to 0.1%; Ti.ltoreq.0.4%
but more than zero; Mo.ltoreq.2.5% but more than zero;
Nb.ltoreq.1.25% but more than zero; V.ltoreq.0.1% but more than
zero; and balance of Fe and naturally occurring impurities and not
more than 0.010% of S impurity.
13. The stainless steel according to claim 12, wherein the
stainless steel has a weight gain of less than 1.5 mg/cm.sup.2 when
oxidized in air or air +1% H.sub.2O at 850.degree. C. after 1000
hours, without any spallation of the oxide scale.
14. The stainless steel according to claim 13, wherein the
stainless steel has a linear thermal expansion coefficient of more
than 11.5.times.10.sup.-6(.degree. C..sup.-1) but less than
13.0.times.10.sup.-6(.degree. C..sup.-1) in the temperature range
of 30 to 900.degree. C.
15. The stainless steel according to claim 12, wherein the
stainless steel has a linear thermal expansion coefficient of more
than 11.5.times.10.sup.-6(.degree. C..sup.-1) but less than
13.0.times.10.sup.-6(.degree. C..sup.-1) in the temperature range
of 30 to 900.degree. C.
16. The stainless steel according to claim 12, wherein the
stainless steel has a thermal expansion mismatch (TEM) with an
electro-active ceramic material in a solid oxide fuel cell of less
than .+-.15%.
17. The stainless steel according to claim 12, wherein the
stainless steel is suitable for application as an interconnect or a
bipolar plate material in Solid Oxide Fuel Cells.
18. The stainless steel according to claim 12, wherein the
stainless steel is suitable for application as a catalytic
converter in automobile applications.
19. The stainless steel according to claim 12, wherein the
stainless steel has an Area Specific Resistance of less than 50
m.OMEGA. cm.sup.2 after 1000 hours on both an anode side and a
cathode side of a Solid Oxide Fuel Cell.
20. The stainless steel according to claim 19, wherein an increment
of the Area Specific Resistance is not greater than 10 m.OMEGA.
cm.sup.2 per 1000 hours.
21. A solid oxide fuel cell comprising the stainless steel
according to claim 12.
22. A catalytic converter intended for the automobile industry,
comprising the stainless steel according to claim 12.
Description
RELATED APPLICATION DATA
[0001] This application is based on and claims priority under 35
U.S.C. .sctn.119 to Swedish Application No. 0401292-8, filed May
19, 2004, the entire contents of which are incorporated herein by
reference.
FIELD OF THE DISCLOSURE
[0002] The present disclosure relates generally to a steel product,
which at high temperatures forms an oxide scale with good surface
conductivity and an excellent adhesion to the underlying steel. In
particular, it relates to a ferritic chromium steel suitable for
the use as interconnects or bipolar plates in solid oxide fuel
cells or other high temperature applications such as catalytic
converters in cars and trucks.
BACKGROUND
[0003] In the discussion of the state of the art that follows,
reference is made to certain structures and/or methods. However,
the following references should not be construed as an admission
that these structures and/or methods constitute prior art.
Applicants expressly reserve the right to demonstrate that such
structures and/or methods do not qualify as prior art against the
present invention.
[0004] Ferritic chromium steels are used for applications with high
requirements on heat resistance, such as for example interconnect
materials in Solid Oxide Fuel Cells (SOFC) or if alloyed with Al,
as a material for catalytic converters. They are very suitable
materials for use in SOFC applications since the Thermal Expansion
Coefficients (TEC) of ferritic steels are close to the TECs of the
electro-active ceramic materials used in the SOFC stack, such as
yttrium-stabilized zirconia (YSZ) which is the common material used
as electrolyte in the fuel cell. This has, for instance, been
studied by Linderoth et al. in "Investigation of Fe--Cr ferritic
steels as SOFC interconnect material", Mat. Res. Soc. Symp. Proc.,
Vol. 575, (1999), pp. 325-330.
[0005] It is desired that the oxide scale formed on the steel
interconnect material does not spall off or crack due to thermal
cycling, i.e., the oxide scale should have good adhesion. The
formed oxide scale should also have good electrical conductivity
and not grow too thick during the life time of the fuel cell, since
thicker oxide scales will lead to an increased electrical
resistance. The formed oxide should also be chemically resistant to
the gases used as fuels in an SOFC, i.e., no volatile
metal-containing species such as chromium oxyhydroxides should be
formed. Volatile species such as chromium oxyhydroxide will
contaminate the electro-active ceramic materials in a SOFC stack,
which will lead to a decrease in the efficiency of the fuel
cell.
[0006] One disadvantage with the use of commercial ferritic
chromium steel is that they are usually alloyed with aluminum
and/or silicon, which form Al.sub.2O.sub.3 and/or SiO.sub.2 at the
working temperature of the SOFC. Both these oxides are good
electrical insulating oxides, which will increase the electrical
resistance of the cell and lower the fuel cell efficiency.
[0007] This has led to the development of ferritic steels with low
Al and Si contents, to ensure good conductivity of the formed oxide
scales. These newly developed steels are usually also alloyed with
manganese. The addition of Mn in the steel will induce the
formation of chromium oxide based spinell structures in the formed
oxide scale. However, Mn in general has a poor effect on the
corrosion resistance of the steel; it is therefore desired that the
Mn content in the steel be monitored carefully at low levels. Too
high a concentration of Mn in the steel will lead to the growth of
thick oxide scales due to severe high temperature corrosion.
[0008] In addition to Mn, several of these new developed steels are
alloyed with group III elements, i.e., Sc, La and Y and/or other
rare earth elements (REM). The addition of La, Y or REM is made to
increase the lifetime of the material at high temperatures. Strong
oxide formers such as La, Y and REM are said to decreases the
oxygen ion mobility in the formed Cr.sub.2O.sub.3 scale, which will
lead to a decrease in the growth rate of the oxide scale. The
amount of added REM to the steel has to be carefully monitored,
since too high a concentration of REM will lead to production
process difficulties, as well as undesired corrosion properties of
the steel.
[0009] In patent application US 2003/0059335, the steel is alloyed
with a small amount of La (0.01-0.4% by weight) and optionally also
with small amounts of Y and Ce (0.1 to 0.4% by weight).
[0010] In patent application EP 1 298 228 A2, the steel is also
alloyed with either Y (.ltoreq.0.5% by weight) or REM (.ltoreq.0.2%
by weight) or La (0.005-0.1% by weight).
[0011] In U.S. Pat. No. 6,294,131 B1, the steel is also alloyed
with REM (0.005 to 0.5% by weight) and in patent application US
2002/0192468 A1 the final steel is alloyed with 0.01-1.5% yttrium,
rare earth metals, and oxides thereof.
[0012] In addition to these above-mentioned patents, there are some
commercially available ferritic steel for interconnects in SOFC.
Two of these are the steel sorts A and B (see further details of A
and B in Example 3 below), A being alloyed with 0.04% La, and B
with a La content of max 0.2% by weight. All the above mentioned
patents and commercially available steels are alloyed with small
amounts of rare earth metals such as Y, La and Ce. The addition of
reactive rare earth metals will lead to a decrease in corrosion
resistance compared with the steel alloy of this invention.
SUMMARY
[0013] It is an object of the present invention to provide a steel
alloy with excellent high temperature corrosion resistance. Another
object of the present invention is that the oxide scale on said
steel alloy has a good adherence and a low surface resistivity. A
further object of the present invention is that the above mentioned
properties are so good that said alloy does not need alloying with
any REM or group III metals, which in turn will lead to a simpler
and more cost-effective steel production process. Yet another
object of the present invention is to provide a steel alloy for the
manufacturing of interconnects and/or bi-polar plates to Solid
Oxide Fuel Cells. A further object of the present invention is to
provide a steel alloy for the manufacturing of catalytic converters
in automobile applications.
[0014] The above objects and further advantages are achieved by
carefully monitoring the contents of different alloying elements of
the steel alloy. This is done by alloying the steel with 20 to 25%
by weight of chromium and monitoring the content of oxide formers
such as silicon, aluminum and manganese at low levels. In addition
to this, elements, such as Ni, Mo and group IV (titanium group) and
group V elements (vanadium group) of the periodic table of
elements, are added to the alloy. Said alloy is produced in a
conventional steel production process. The final product of said
alloy can have the form of a strip, foil, wire, tube, bar or even
as a powder, preferably as strip or a foil.
[0015] One factor is that said alloy is heat resistant at
temperatures up to 900.degree. C. and that the oxide scale formed
does not grow too thick. Therefore, the mass gain per unit area of
said alloy is less than 1.5 mg/cm.sup.2, when the steel alloy has
been oxidized in air or in air +1% H.sub.2O mixture for 1000 hours
at 850.degree. C. or any environment similar to the gases used in a
Solid Oxide Fuel Cell. A further aspect is that the grown oxide
scale does not spall off, i.e., has a good adhesion to the
underlying alloy.
[0016] To be able to use the steel alloy as interconnect or bipolar
plate in SOFC, the thermal expansion of said alloy should not
deviate considerably from the thermal expansions of the anode
material or the electrolyte material used in the fuel cell. In one
exemplary embodiment, said alloy has a thermal expansion
coefficient of 10 to 15.multidot.10.sup.-6.degree. C..sup.-1 in the
temperature range 0 to 900.degree. C., or even more preferably 11
to 14.multidot.10.sup.-6C..sup- .-1, and most preferably 11.5 to
13.multidot.10.sup.-6.degree. C..sup.-1. As a consequence thereof,
the thermal expansion mismatch (TEM) between the electro-active
ceramic materials in the fuel cell and the thermal expansion of
said alloy is not greater than .+-.25%, or preferably less than
=20%, or most preferred lower than .+-.15%. Here the thermal
expansion mismatch (TEM) is defined as
(TEC.sub.ss-TE.sub.ce)/TEC.sub.ss, where the TEC.sub.ss is the
thermal expansion of the steel alloy and TEC.sub.ce is the thermal
expansion of the electro-active ceramic materials used in
anode-supported fuel cells.
[0017] Yet another important object is that said alloy has a good
conductivity. The bipolar plate works as a current collector in a
fuel cell. To avoid degradation of the fuel cell efficiency, the
contact resistance of the steel alloy is kept as low as possible
throughout the lifetime of the fuel cell. The area specific
resistance (ASR) of said alloy in a SOFC setup should be kept low
but also the increment of ASR with time is kept as low as possible.
If the increment of the ASR is large, this can lead to a decrease
of the fuel cell efficiency.
[0018] In one exemplary embodiment, a high temperature corrosion
resistant stainless steel, consists essentially of (by weight):
[0019] C.ltoreq.0.2%, but more than zero;
[0020] N.ltoreq.0.1% but more than zero;
[0021] O.ltoreq.0.1% but more than zero;
[0022] Si.ltoreq.0.4% but more than zero;
[0023] Al<0.5% but more than zero;
[0024] Mn.ltoreq.0.5% but more than zero;
[0025] Cr 20 to 25%;
[0026] Ni.ltoreq.2.0% but more than zero;
[0027] Zr+Hf 0.001 to 0.1%;
[0028] Ti.ltoreq.0.5% but more than zero;
[0029] Mo+W.ltoreq.2.5% but more than zero;
[0030] Nb+Ta.ltoreq.1.25% but more than zero;
[0031] V.ltoreq.0.5% but more than zero; and
[0032] balance of Fe and naturally occurring impurities and not
more than 0.010% of S impurity
[0033] In another exemplary embodiment, a high temperature
corrosion resistant stainless steel consists essentially of (by
weight):
[0034] C.ltoreq.0.1% but more than zero;
[0035] N.ltoreq.0.1% but more than zero;
[0036] O.ltoreq.0.1% but more than zero;
[0037] Si.ltoreq.0.4% but more than zero;
[0038] Al<0.4% but more than zero;
[0039] Mn.ltoreq.0.4% but more than zero;
[0040] Cr 20 to 25%;
[0041] Ni.ltoreq.1.0% but more than zero;
[0042] Zr 0.001 to 0.1%;
[0043] Ti.ltoreq.0.4% but more than zero;
[0044] Mo.ltoreq.2.5% but more than zero;
[0045] Nb.ltoreq.1.25% but more than zero;
[0046] V.ltoreq.0.1% but more than zero; and
[0047] balance of Fe and naturally occurring impurities and not
more than 0.010% of S impurity.
BRIEF DESCRIPTION OF THE FIGURES
[0048] FIG. 1 shows the weight gain per unit area plotted vs. time
of an exemplary embodiment of a disclosed steel alloy, together
with the four steel alloys (Sandvik ID numbers 433, 434, 436 and
437) produced for comparison, oxidized in air for 336, 672 and 1008
hours, respectively.
[0049] FIG. 2 shows a SEM cross-section micrograph of the oxide
scale formed on an exemplary embodiment of a disclosed steel alloy
oxidized in air for 336 hours at 850.degree. C.
[0050] FIG. 3 shows a Glow Discharge Optical Emission Spectroscopy
(GDOES) depth profile of the oxide scale formed on an exemplary
embodiment of a disclosed steel alloy oxidized in air for 336 hours
at 850.degree. C.
[0051] FIG. 4 shows the weight gain per unit area for eight
different steel grades including an exemplary embodiment of a
disclosed steel alloy and the four steel alloys (Sandvik ID numbers
433, 434, 436 and 437) produced for comparison after oxidization in
air+1% H.sub.2O at 850.degree. C. after 500 hours.
DETAILED DESCRIPTION
[0052] Chemical Composition: The chemical composition of the steel
alloy according to the present disclosure comprises the following
elements (by weight-%):
[0053] C.ltoreq.0.2% but more than zero, preferably
0.001<C.ltoreq.0.2%
[0054] N.ltoreq.0.1% but more than zero, preferably
0.001<N.ltoreq.0.1%
[0055] O.ltoreq.0.1% but more than zero, preferably
0.001<O.ltoreq.0.1%
[0056] Si.ltoreq.0.4% but more than zero, preferably
0.01<Si.ltoreq.0.4%
[0057] Al<0.5% but more than zero, preferably
0.001<Al<0.5%
[0058] Mn.ltoreq.0.5% but more than zero, preferably
0.01<Mn.ltoreq.0.5%
[0059] Cr 20 to 25%
[0060] Ni.ltoreq.2.0% but more than zero, preferably
0.01.ltoreq.Ni.ltoreq.2.0%
[0061] Zr+Hf.ltoreq.0.1% but more than zero, preferably
0.001.ltoreq.Zr+Hf.ltoreq.0.1%
[0062] Ti.ltoreq.0.5% but more than zero, preferably
0.01.ltoreq.Ti.ltoreq.0.5%
[0063] Mo+W.ltoreq.2.5% but more than zero, preferably
0.01.ltoreq.Mo+W.ltoreq.2.5% or even more preferably
0.1.ltoreq.Mo+W.ltoreq.2.0%
[0064] Nb+Ta.ltoreq.1.25% but more than zero, preferably
0.01.ltoreq.Nb+Ta.ltoreq.1.25%
[0065] V.ltoreq.0.5% but more than zero, preferably
0.01.ltoreq.V.ltoreq.0.5%
[0066] and balance of Fe and naturally occurring impurities, but
not more than 0.010% of S impurity. Said alloy is produced in an
ordinary steel production process.
[0067] The chemical composition of the steel alloy according to the
present disclosure can also comprise the following elements (by
weight-%):
[0068] C.ltoreq.0.1% but more than zero, preferably
0.001<C.ltoreq.0.1%
[0069] N.ltoreq.0.1% but more than zero, preferably
0.001<N.ltoreq.0.1%
[0070] O.ltoreq.0.1% but more than zero, preferably
0.001<O.ltoreq.0.1%
[0071] Si.ltoreq.0.4% but more than zero, preferably
0.01<Si.ltoreq.0.4%
[0072] Al<0.4% but more than zero, preferably
0.001<Al.ltoreq.0.4%
[0073] Mn.ltoreq.0.4% but more than zero, preferably
0.01<Mn.ltoreq.0.4%
[0074] Cr 20 to 25%
[0075] Ni.ltoreq.1.0% but more than zero, preferably
0.01.ltoreq.Ni.ltoreq.1.0%
[0076] Zr.ltoreq.0.1% but more than zero, preferably
0.001.ltoreq.Zr.ltoreq.0.1%
[0077] Ti.ltoreq.0.5% but more than zero, preferably
0.01.ltoreq.Ti.ltoreq.0.4%
[0078] Mo.ltoreq.2.5% but more than zero, preferably
0.01.ltoreq.Mo.ltoreq.2.5% or even more preferably
0.1.ltoreq.Mo.ltoreq.2.0%
[0079] Nb.ltoreq.1.25% but more than zero, preferably
0.01.ltoreq.Nb.ltoreq.1.25%
[0080] V.ltoreq.0.1% but more than zero, preferably
0.01.ltoreq.V.ltoreq.0.1%
[0081] and balance of Fe and naturally occurring impurities, but
not more than 0.010% of S impurity. Said alloy is produced in an
ordinary steel production process.
[0082] High Temperature Corrosion Resistance: The disclosed alloy
is heat resistant at temperatures up to 900.degree. C. and the
oxide scale formed upon oxidization does not grow too thick. In
Table 1, the theoretical mass gain per area unit for a chromium
oxide scale with different thicknesses is calculated. The
calculations assume that a dense and pure Cr.sub.2O.sub.3 scale is
formed on the surface of the steel. The Cr.sub.2O.sub.3 has a
density of 5300 mg/cm.sup.3 and the mass percent of oxygen in the
Cr.sub.2O.sub.3 is 31.6%. This will give a mass gain per unit area
of 0.16 mg/cm.sup.2 for a 1 .mu.m thick pure and dense chromium
oxide scale, and 0.82 mg/cm.sup.2 for 5 .mu.m thick oxide
scale.
[0083] Noted here should be that these values of mass gain of the
formation of pure Cr.sub.2O.sub.3 are theoretical. When a ferritic
chromium steel alloy is oxidized, usually mixed oxides are formed
and the weight gain depends on the added alloying elements.
However, a low weight gain is important since higher weight gains
will lead to thicker oxide scale formations, which in turn will
increase the resistance of the steel. Exemplary embodiments of the
disclosed steel alloy therefore have a weight gain of less than 1.5
mg/cm.sup.2 after 1000 hours of exposure to air and/or air +1%
H.sub.2O at 850.degree. C.
[0084] A further feature of the disclosed alloy is that the grown
oxide scale does not spall off, i.e., has a good adhesion to the
underlying alloy.
[0085] Thermal Expansion: To be able to use steel as interconnects
or bipolar plates in SOFC, the thermal expansion of the alloy
should not deviate greatly from the thermal expansion of the anode
material or the electrolyte material used in the fuel cell.
Therefore, the disclosed steel alloy has a thermal expansion
coefficient of 10.times.10.sup.-6 to 15.times.10.sup.-6.degree.
C..sup.-1 in the temperature range 0 to 900.degree. C., or even
more preferably 11.times.10.sup.-6 to 14.times.10.sup.-6.degree.
C..sup.-1, and most preferably 11.5.times.10.sup.-6 to
13.times.10.sup.-6.degree. C..sup.-1. Further, the thermal
expansion mismatch (TEM) between the electro-active ceramic
materials in the fuel cell and the thermal expansion of said alloy
is not greater than .+-.25%, preferably less than .+-.20%, and most
preferably lower than .+-.15%.
[0086] Here the thermal expansion mismatch (TEM) is defined as
(TEC.sub.ss-TEC.sub.ce)/TEC.sub.ss, where the TEC.sub.ss is the
thermal expansion of the alloy and TEC.sub.ce is the thermal
expansion of the electro-active ceramic materials. The thermal
expansion of said steel alloy can be tuned to match the thermal
expansion of the electro-active ceramic materials in the fuel cell
by carefully monitoring the amount of alloying elements, such as
nickel, in the steel alloy.
[0087] Conductivity: Embodiments of the disclosed steel alloy have
a good conductivity and, in a SOFC setup, have an ASR of less than
50 m.OMEGA. cm.sup.2 after 1000 hours, preferably an ASR even lower
that 25 m.OMEGA. cm.sup.2 after 1000 hours, on both anode and
cathode side of the interconnect. Moreover, the increment of the
ASR is not greater than 10 m.OMEGA. cm.sup.2 per 1000 hours,
preferably even lower than 5 m.OMEGA. cm.sup.2 per 1000 hours on
both the anode and the cathode side of the interconnect. This
factor promotes a good efficiency of the fuel cell throughout the
life time of the fuel cell, which might be as long as 40,000
hours.
[0088] A preferred embodiment of the disclosed steel alloy will now
be described in more detail. First, the steel alloy is produced by
ordinary metallurgical steel making routines to the chemical
composition as described, for example, in the following Examples.
Then said steel alloy is hot-rolled down to an intermediate size,
and thereafter cold-rolled in several steps with a number of
recrystallization steps, until a final specific thickness of
normally less than 3 mm, and a width of maximally 400 mm. The
linear thermal expansion of said steel alloy was determined by
dilatometer measurement and was found to be
12.3.times.10.sup.-6.degre- e. C..sup.-1 for the temperature range
30 to 900.degree. C.
EXAMPLE 1
[0089] A 0.2 mm thick steel alloy strip with a nominal composition
(by weight) of max 0.2% C, max 0.1% N, max 0.1% 0, max 0.4% Si, max
0.5% Al, max 0.5% Mn, 20 to 25% Cr, max 2.0% Ni, 0,001 to 0.1%
Zr+Hf, max 0.5% Ti, max 2.5% Mo+W, max 0.5% V, max 1.25% Nb+Ta and
balance of Fe (with naturally occurring impurities) was produced by
an ordinary steel making process, followed by hot-rolling down to a
thickness of less than 4 mm. Thereafter, it was cold-rolled in
several steps with a number of recrystallization steps down to a
final thickness of 0.2 mm. Strips of four other steel alloys were
produced in the same way for comparison with the steel alloy of
Example 1. The compositions of these additional steel alloys and
their Sandvik identity numbers are given in Table 2.
[0090] Coupons of the five steel alloy strips with the size
70.times.30.times.0.2 mm were oxidized in air at 850.degree. C. for
336, 672 and 1008 hours, respectively. In FIG. 1, the mass gain per
unit area is plotted as a function of time for the five steel
alloys. According to FIG. 1, a mass gain of less than 1.1
mg/cm.sup.2 per 1000 hours is obtained for the steel alloy of
Example 1, insuring a good high temperature corrosion resistance
and a lower growth rate of the formed oxide scale. However, for the
steel alloys made for comparison (Sandvik ID numbers 433, 436 and
437) with a Mn content of 0.5% (by weight) and the addition of rare
earth metals in the form of Ce, all showed a mass gain of more than
1.8 mg/cm.sup.2 per 1000 hours, and the steel alloy with a Mn
content of 5% (by weight) had a mass gain of almost 5 mg/cm.sup.2
per 1000 hours. The extreme large weight gain for steel alloy with
5% Mn (Sandvik ID number 434) shows the importance of good
monitoring of the Mn content in the alloy to avoid high temperature
corrosion. A conclusion to be drawn from this is that the Mn
content in the steel alloy should be carefully monitored and low
amounts of Mn as alloying element is preferred if good high
temperature corrosion resistance is to be obtained.
[0091] In FIG. 2, a cross sectional Scanning Electron Microscopy
(SEM) micrograph of the formed oxide scale after 336 hours at
850.degree. C. in air on the steel alloy of Example 1 is shown. In
FIG. 2, it can be seen that the formed oxide scale is also well
adherent to the underlying steel alloy and that the oxide scale
thickness is less than 3 .mu.m.
[0092] The chemical composition of the formed oxide scale after
oxidization in air for 336 hours at 850.degree. C. was determined
by Glow Discharge Optical Emission Spectroscopy (GDOES). In FIG. 3,
the GDOES depth profile for the formed oxide scale is shown. The
different scales for different elements should be noted. In FIG. 3,
it can be seen that the manganese content in the formed oxide scale
increases at the surface to about 12% by weight. The thickness of
this manganese-rich oxide scale is about 0.5 .mu.m, followed by a
more chromium-rich oxide scale of approximately less than 2.4
.mu.m. The formation of a manganese-rich oxide scale at an
outermost layer at the surface is of importance since ternary
chromium oxides such as MnCr.sub.2O.sub.3 are believed to lower the
formation of volatile chromium species such as chromium
oxyhydroxides. It can also be seen that the titanium content is
approximately 0.4% by weight in the oxide scale. Finally, it can be
noted that at the interface of the steel alloy and the oxide scale,
a region of silicon oxide is obtained. The formation of silicon
oxide should be kept as low as possible but is unavoidable if the
steel is alloyed or has small residuals of silicon in the matrix.
However, as long as the formation of insulating silicon oxide at
the steel interface is only as small islands of particles, and not
as a continuous layer, it is acceptable for the performance of the
fuel cell. X-ray diffraction of the oxidized coupon showed that the
oxides formed in the scale had both spinell (MCr.sub.2O.sub.3) and
corundum (M.sub.2O.sub.3) types of structures.
EXAMPLE 2
[0093] As an additional example of an exemplary embodiment of the
disclosed steel alloy, coupons of the final steel alloy strip with
the sizes of approximately 30.times.40.times.0.057 mm were oxidized
in air at both 750.degree. C. and 850.degree. C. for 500 and 1000
hours, respectively. In Table 3, a summary of oxidization results
of the four samples together with the exact coupon sizes of the
initial samples is given. The result at 750.degree. C. showed a
very low mass gain per unit area, lower than 0.2 mg/cm.sup.2 after
500 hours of oxidization and the mass gain did not increase greatly
after 1000 hours. Instead, it was still lower than 0.3 mg/cm.sup.2
after 1000 hours. For the two sample oxidized at 850.degree. C.,
the mass gain per unit area was larger but still low, less than 1.1
mg/cm.sup.2, which was also the result for the thicker strip (0.2
mm) samples, oxidized in Example 1.
[0094] The low mass gains observed in both Examples 1 and 2 above
were compared with published values of mass gain attained on other
commercially available steels, and other test melts. In Table 4,
values as obtained from the literature on other steel grades
together with the values obtained in the present study have been
summarized for comparison with an embodiment of the present
invention. In Table 4, it can be seen that the weight gain of the
steel alloy of exemplary embodiments of the disclosed steel alloy
is low compared to other commercially available steel grades. For
instance, it has been reported that the commercially available
steel ZMG232 has a weight gain of approximately 0.5 mg/cm.sup.2,
already after 100 hours exposure of air at 850.degree. C. The same
alloy when exposed to air +1% H.sub.2O mixture for only 670 hours
at 850.degree. C. has an even larger weight gain of 1.54
mg/cm.sup.2.
EXAMPLE 3
[0095] As a third example, coupons of exemplary embodiments of the
disclosed steel alloy and the four test melts (Sandvik ID numbers
433, 434, 436 and 437) described in Example 1 and the Sandvik 0C44
alloy, were oxidized together with coupons of two commercially
available steel grades designed for the use as interconnects in
SOFC, alloy A and alloy B at 850.degree. C. in air +1% H.sub.2O for
500 hours. In FIG. 4, the weight gain for the different steel
grades after oxidation at 850.degree. C. in air +1% H.sub.2O is
shown. In FIG. 4, it can be seen that the exemplary embodiments of
the disclosed steel alloy have a much lower weight gain compared
with the four Sandvik test melts, and also have a much lower weight
gain than the two commercially available steel grades. In this
context, a low weight gain is equal to a good high temperature
corrosion resistance. The second lowest weight gain is obtained by
the Sandvik 0C44 alloy with the nominal composition (by weight) of
max 0.018% C, max 0.025% N, max 0.5% Si, max 0.35% Mn, 21.1 to
21.8% Cr, max 0.3% Ni, max 0.02% P, max 0.007% S, max 0.15% Mo, max
0.010% Ti, max 0.01% Nb, max 0.03% Ce, max 0.015% Mg and balance of
Fe (with naturally occurring impurities). As seen in Example 1, the
steel alloy Sandvik ID number 434 with a high Mn content has the
largest weight gain almost 3 mg/cm.sup.2 after 500 hours of
exposure. The commercial available alloy B with a composition of
(by weight) C=0.02%, Si=0.40%, Mn=0.50%, Ni=0.26%, Cr=21.97%
Al=0.21%, Zr=0.22%, La=0.04% and balance of Fe according to
reference "Development of Ferritic Fe--Cr Alloy for SOFC
separator", T. Uehara, T. Ohno & A. Toji, Proceedings Fifth
European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, edited
by J. Huijsmans (2002) p. 281, has the second largest weight gain
of almost 2.5 mg/cm.sup.2 after 500 hours of exposure. The three
other Sandvik steel alloys ID#433, 436 and 437 with only 0.5% by
weight Mn and the commercial available alloy A with a nominal
composition of (by weight) Cr 21.0 to 24.0%, C max 0.03%, Mn max
0.8%, Si max 0.5%, Cu max 0.5%, Ti max 0.25, P max 0.05, La max
0.2% and balance of Fe, have weight gains of less than 1
mg/cm.sup.2, but still much higher than exemplary embodiments of
the disclosed steel alloy and the Sandvik 0C44 alloy.
EXAMPLE 4
[0096] All the three previous examples have described the excellent
high temperature corrosion resistance of exemplary embodiments of
disclosed steel alloy. In this fourth example, the low electrical
resistivity of the exemplary steel alloys will be exemplified. The
contact resistance was measured in dry air for 2900 hours at
750.degree. C. with a temperature peak of 850.degree. C. for 10
hours in the beginning. The load of the contact was 1 kg/cm.sup.2
at the start and the contact area was 0.5 cm.sup.2. The measured
area specific resistance (ASR) was initially, i.e., after the
850.degree. C. temperature peak lower than 15 m.OMEGA. cm.sup.2 and
had after 2900 hours, including 6 thermal cycles, increased to
below 25 m.OMEGA. cm.sup.2. The increment of the ASR with time was
lower than 5 m.OMEGA. cm.sup.2 per 1000 hours. If extrapolated
linearly, the ASR of the contact would be less than 200 m.OMEGA.
cm.sup.2 after 40,000 hours of exposure. It is important for the
fuel cell efficiency that the ASR is low throughout the lifetime of
the fuel cell. Furthermore, when the contact resistance was tested
under anode gas environment, i.e., Ar+9% H.sub.2 at 750.degree. C.,
the ASR was even lower, well under 10 m.OMEGA. cm.sup.2 after 600
hours of exposure. The increment of the ASR was very low, under 2
m.OMEGA. cm.sup.2 per 1000 hours. If extrapolated linearly the ASR
of the contact on the anode side would be much lower than 200
m.OMEGA. cm.sup.2 after 40,000 hours of exposure, even lower than
100 m.OMEGA. cm.sup.2 after 40,000 hours of exposure. These values
can be compared with the contact resistance of approximately 26
m.OMEGA. cm.sup.2 after exposure of air at 750.degree. C. for 1000
hours for the commercially available steel ZMG 232 which have been
reported in "Development of Ferritic Fe--Cr Alloy for SOFC
Separator", T. Uehara, T. Ohno & A. Toji, Proceedings Fifth
European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, edited
by J. Huijsmans (2002) p. 281.
1TABLE 1 Area Oxide Scale Volume Weight of the Weight gain
(cm.sup.2) Thickness (.mu.m) (cm.sup.3) scale (mg) (mg/cm.sup.2) 1
1 0.0001 0.52 0.16 1 2 0.0002 1.04 0.33 1 5 0.0005 2.60 0.82 1 10
0.0010 5.20 1.64 1 1.4 0.00014 0.73 0.23
[0097]
2TABLE 2 Sandvik ID # C Si Mn P S Cr Ni Mo Nb V Al Ce N 433 0.008
0.16 0.55 0.005 <0.001 22.37 0.11 <0.01 <0.01 0.013 0.040
0.11 0.029 434 0.009 0.14 5.06 0.004 0.001 22.23 0.11 <0.01
<0.01 0.012 0.014 0.081 0.028 436 0.007 0.15 0.52 0.005
<0.003 22.27 1.04 <0.01 <0.01 0.013 0.039 0.12 0.029 437
0.009 0.14 0.50 0.004 0.001 22.39 2.96 <0.01 <0.01 0.014
0.020 0.05 0.032
[0098]
3TABLE 3 Size Thickness Mass Time Temp. Mass after Gain Sample
mm.sup.2 mm grams hours .degree. C. grams mg/cm.sup.2 1 29.5
.times. 40.0 0.058 0.5278 500 750 0.5311 0.14 2 30.0 .times. 40.0
0.057 0.5256 1000 750 0.5322 0.27 3 30.0 .times. 39.0 0.057 0.5096
500 850 0.5290 0.83 4 30.0 .times. 40.0 0.056 0.5219 1000 850
0.5466 1.03
[0099]
4TABLE 4 Temperature Time Weight gain Steel/Supplier .degree. C.
(hr) Atmosphere mg .multidot. cm.sup.-2 Reference ZMG 232 - Hitachi
750 100/1000 Air 0.18/0.36 1 850 100 Air 0.5 1 1000 100 Air 2.4 1
JS-3 Julich-KTN 800 500/1000 Air 0.25/0.32 2 JS-1 Julich-KTN 800
500/1000 Air 0.78/1.03 2 ZMG 232 - Hitachi 750 670 Air + 1%
H.sub.2O 0.27 3 800 670 Air + 1% H.sub.2O 0.73 3 850 670 Air + 1%
H.sub.2O 1.54 3 900 670 Air + 1% H.sub.2O 1.95 3 JS-3 Julich-KTN
750 670 Air + 1% H.sub.2O 0.38 3 800 670 Air + 1% H.sub.2O 0.67 3
850 670 Air + 1% H.sub.2O 2.0 3 900 670 Air + 1% H.sub.2O 2.6 3
Steel of the 850 1008 Air 1.05 Example 1 invention Sandvik ID# 433
850 1008 Air 1.87 Example 1 Sandvik ID# 434 850 1008 Air 4.94
Example 1 Sandvik ID# 436 850 1008 Air 1.98 Example 1 Sandvik ID#
437 850 1008 Air 2.25 Example 1 Steel of the 750 1000 Air 0.27
Example 2 invention Steel of the 850 1000 Air 1.03 Example 2
invention Steel of the 850 500 Air + 1% H.sub.2O 0.22 Example 3
invention Sandvik ID# 433 850 500 Air + 1% H.sub.2O 0.72 Example 3
Sandvik ID# 434 850 500 Air + 1% H.sub.2O 3.0 Example 3 Sandvik ID#
436 850 500 Air + 1% H.sub.2O 0.80 Example 3 Sandvik ID# 437 850
500 Air + 1% H.sub.2O 0.92 Example 3 Alloy A 850 500 Air + 1%
H.sub.2O 0.90 Example 3 Sandvik OC44 850 500 Air + 1% H.sub.2O 0.3
Example 3 Alloy B 850 500 Air + 1% H.sub.2O 2.4 Example 3 Notes to
Table 4: 1. "Long term oxidation behaviour and compatibility with
contact materials of newly developed ferritic interconnect steel",
J. Pirn-Abelln, F. Tietz, V. Shemet, A. Gil, T. Ladwein, L.
Singheiser & W. J Quadakkers, Proceedings Fifth European Solid
Oxide Fuel Cell Forum, Lucerne, Switzerland, edited by J. Huijsmans
(2002) p. 248. 2. "Development of Ferritic Fe--Cr Alloy for SOFC
separator", T. Uehara, T. Ohno & A. Toji, Proceedings Fifth
European Solid Oxide Fuel Cell Forum, Lucerne, Switzerland, edited
by J. Huijsmans (2002) p. 281. 3. "Corrosion Behaviour of Chromium
Steels for Interconnects in Solid Oxide Fuel Cells" T. Fich
Pedersen, P. B. Friehlin, J. B. Bilde-Sorensen, S. Linderoth,
presented at the conference "Corrosion Science in the 21.sup.st
Century" held at UMIST in July 2003.
[0100] Although the present invention has been described in
connection with preferred embodiments thereof, it will be
appreciated by those skilled in the art that additions, deletions,
modifications, and substitutions not specifically described may be
made without department from the spirit and scope of the invention
as defined in the appended claims.
* * * * *