Page 209 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

186 High Temperature Solid Oxide Fuel Cells: Fundurnentals, Design and Applications

the enrichment of chromium at the surface (i.e. the formation of the chromia
scale) has no significant influence on the composition of the thick steel plate.
However, when thin foils are used, the amount of chromium is limited and
the chromia formation can lead to compositional changes within the thin foil
leading to very different corrosion behaviour. Tests over several thousand
hours are required to demonstrate the reliability of these lightweight
interconnect designs.

7.3.3 Other Metallic Materials

Sanyo and Fuji Electric started their SOFC stack development with metallic
interconnects using nickel-based alloys such as Inconel 600 and Ni 22Cr,
respectively [35,67]. In a long-term exposure experiment of 12,000 h duration,
the electrical resistance of a Ni 20Cr alloy coated with Lao.6Sro.4C003 did not
change significantly and remained below 10 mS2 cm2, although SrCr04 formed
at the interface [68]. However, thermal cycling with these alloys led to voltage
drops after each thermal cycle due to the mismatch in thermal expansion with
the other cell components (Figure 7.5) leading to cracks at the interconnect/
electrode interfaces [ 6 71. Nevertheless, properties of austenitic steels and
Ni-based superalloys for use in SOFC stacks continued to be explored. Linderoth
et nl. [69] investigated the oxidation resistance and the corrosion products of
Fe-Cr-Ni steels (Haynes 230, Inconel 601), Ni-Cr steel (Inconel 657), Fe-Cr-A1
steel (APM-Kanthal), and the Plansee Ducrolloy. Among the Ni-containing
steels, the Haynes 230 showed the best oxidation resistance and the oxide scale
composed of Cr203 and spinel might have better electronic conductivity than a
pure chromia scale.
England and Virkar [70, 711 investigated thin foils of Ni-based superalloys
(Inconel 625, Inconel 718, Hastelloy X, and Haynes 230) as possible
interconnect materials. They also observed the slowest oxidation in air for
Haynes 2 30 and the formation of a Cr-Mn spinel at the outer surface leading to a
complete depletion of Mn in the inner part of the thin foil. Hastelloy X also formed
a spinel layer at the beginning and both alloys exhibited the lowest electronic
resistance of the oxide scale formed. In wet hydrogen, the oxidation resistance
was also the best for Haynes 230 but the oxide scale growth was much faster
than in air [71], chromia was the dominating phase in the oxide scale and hence
the electronic resistance of the oxide scale was 1-2 orders of magnitude higher
than after oxidation in air.
Another concept for interconnecting SOFCs is the use of FeCrAIY steels in
combination with silver pins [62]. The FeCrAlY steel is used as a thin foil and
quickly forms an alumina scale inhibiting the release of Cr from the steel. To
avoid high resistances of the alumina scales. the steel foil is perforated with Ag
pins acting as contacts between the anode of one cell to the cathode of the next
cell in the stack. The use of silver is very attractive due to the low contact
resistances [62,72,73]. However, problems regarding silver evaporation at
operation temperatures > 700°C [62] and during thermal cycling [72] need to
be addressed.