Chromia-forming alloys alloy development

Another issue for intercoimectors is matching the coefficient of thermal expansion (CTE) of the other fuel cell components (particularly the zirconia electrolyte). This subject was addressed by Ueda and Taimatsu (2000) for Fe-Cr-W alloys. The lowest CTE was observed for Fe-20Cr-3W which was studied in a later paper (Brady et al., 2006). Additional work examined the CTE as well as the oxidation behavior of Fe-Cr-W and Fe-Cr-Mo alloys (Pint et al, 2007a). This is another example of opportunities for development of chromia-forming alloys. However, alloy cost with 20Cr and 3W may be prohibitive. 

With the problems outlined above for chromia-forming alloys, oxidation-resistant alloy development tends to focus on alumina-forming alloys. A few examples will be given to illustrate some of the potential and a few of the problems with these alloys. 

The second stage in the carburisation process, that of carbon ingress through the protective oxide layer, is suppressed by the development of alumina or silica layers as already discussed and in some cases protective chromia scales can also form. Diffusion and solubility of carbon in the matrix has been shown by Schnaas et to be a minimum for binary Fe-Ni alloys with a nickel content of about 80<7o, and Hall has shown that increasing the nickel content for the nickel-iron-2S<7o-chromium system resulted in lower rates of carburisation (Fig. 7.54). 

With an emphasis on scale electrical conductivity (surface stability as well), a number of new alloys have been recently developed specifically for SOFC interconnect applications. The one that has received wide attention is Crofer 22 APU, an FSS developed by Quadakkers et al. [136, 137] at Julich and commercialized by Thyssen Krupp of Germany. Crofer 22 APU, which contains about 0.5% Mn, forms a unique scale, as shown in Figure 4.6, comprised of a (Mn,Cr)304 spinel top layer and a chromia sublayer [137-139], The electrical conductivity of (Mn,Cr)304 has been reported... 

Fe-25Cr alloy was relatively free from iron. For both alloys, however, breakdown of the pre-formed Cr203 scales eventually occurred. Stott etal. (1985) showed that breakdown of pre-formed chromia and alumina scales in high-Ps, H2-H2O-H2S atmospheres is associated with the development of sulfide channels through the scales. Pre-formed alumina scales were found to provide significantly longer protection than the chromia scales. 

During oxidation, binary alloys with certain alloying elements (i.e. Cr, A1 and Si) can form a continuous layer of chromia, alumina or silica, conferring substantial oxidation resistance. This has formed the basis of the development of common... 

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