Page 206 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

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weldability, and thermal expansion match with the anode substrate (Figure
7.5). Several SOFC stacks with ferritic steel interconnects have been fabricated
and tested [49-5 11. However, long-term stack tests showed large degradation in
power output (a degradation rate between 2 and 25%/1000 h of operation
[50,52] is typical), and the corrosion, for instance of the ferritic steel X10 CrAl
18, was not sufficient for the targeted 40,000 h operation of SOFC systems: after
only 3000 h of operation, the growth of nodular corrosion products led to a
partial detachment of the cathode contact layer from the cathode (Figure 7.6).
Therefore, it became evident that new steel compositions having better corrosion
resistance than the commercially available ferritic steels needed to be developed.

Figure 7.6 Corrosion products (circles) at the edges ofan interconnect coated with LaCoOl after 3000 h of
operation at 800"Candaconstantpowerdensityof0.22 W/cm2.

Malkow et al. systematically investigated the thermal expansion and corrosion
behaviour of commercial steels and model alloys [53]. The thermal expansion
coefficient of ferritic steels decreases with increasing Cr content up to 20 wt%
and increases with increasing A1 content. By such alloying, the thermal
expansion of ferritic steels can be adjusted and matched to the Ni/YSZ anode
substrate, but not to the YSZ electrolyte. The oxidation of steels depends not only
on the Cr content, but also on small amounts of alloyed elements, especially A1
and Si. Once a compositional threshold is reached, alumina and silica layers
are formed instead of a chromia layer. This leads to a reduction in oxidation
rate. However, such alumina and silica layers are insulating and have to be
avoided when the steel interconnect is in contact with a contact or electrode
material. In a comparative study [54] of commercially available ferritic steels
with chromium contents between 12 and 28%, the contact resistance against