Page 208 - High Temperature Solid Oxide Fuel Cells Fundamentals, Design and Applications

Interconnects 18 5

In FeCr 18 steels containing 7-10 wt% tungsten [59], increasing tungsten
content results in thermal expansion coefficients close to that of YSZ electrolyte
and small additions of Ce, La or Zr lead to reduced corrosion rates. AIso,
in contact with lanthanum manganite, a Cr-Mn spinel/Cr203 double layer
is formed.
Honegger et al. [60] investigated powder metallurgically made model steels
containing 22-26% Cr and minor additions of Mo, Ti, Nb and Y203. After
oxidation in air, all materials showed a double-layered oxide scale composed of
chromia directly in contact with the alloy and Mn-Cr spinel at the outer surface.
Tn humidified hydrogen (20 vol% HzO), the corrosion experiments also revealed
a Cr203/Cr-Mn spinel double layer at 700°C, but at higher temperatures a
Fe-Cr spinel/Cr-Mn-Fe spinel system was formed. The contact resistance
measurements performed on steel/Lao.6Sro.&003 pa~te/La~.~Sr~.~Mn03
ceramic samples gave very low contact resistances at 800°C in air after 15,000 h
of exposure. With Mo-containing model alloys (FeCr 22 Mo 2 Ti Y203 and FeCr
26 Mo 2 Ti Y2O3), a contact resistance of 20 mQ cm2 for 40,000 h could be
deduced from the parabolic rate observed.
To achieve acceptable power densities in terms of W/kg and W/dm3, the
metallic interconnects are now made significantly thinner as listed in Table
7.5 and use of thin steel foils (0.1-1 mm) for interconnects becomes feasible
[61,62]. Such foil interconnects can easily be mass produced by pressing,
stamping, cutting and punching [63]. However, the corrosion behaviour of
thin foils of ferritic steels can be very different compared with that of thick
plates due to the effect of selective oxidation and depletion of alloyed
elements leading to composition changes in the foil. As long as the metallic
component is thick enough to serve as a quasi-infinite reservoir of chrcmium

Table 7.5 Stack developer and the interconnect materials used
Company Interconnect Interconnect Method to avoid Ref.
material thickness (mm) poisoning of the
cathode by volatile
chromium species
Sanyo Electric Inconel 600 5-6 Addition ofLazOs to cathode 35
Siemens Cr 5Fe lY203 ca. 3.5 Wet coating ofLaCo03 39
Siemens Cr 5Fe 1Y203 ca. 3.5 Plasma-sprayed coating 64,65
of (La,Sr)Cr03
Sulzer Hexis Cr 5Fe lY203 High-velocity oxygen 66
flame (HV0F)-sprayed
coating of (Y,Ca)Mn03
Sulzer Hexis Cr 5Fe 1Y203 HVOF-sprayed coating 47
of Lao,&o.2Mn03
Forschungszentrum Ferritic steel 6 LaCoO3 coating 49
Jiilich
Ceramic Fuel Ferritic steel 3 A1203 coating in gas 50
Cells Ltd channels, conductive
coating on the ribs