Chromia-forming alloys

The most widely used high temperature alloys are Fe- or Ni-base alloys that form protective chromia-scales during exposure. Because of the long history of research in this class of materials (Kofstad, 1988), the possibility of 

One area where there is considerable current interest in chromia-forming alloys is for metallic intercoimectors for solid oxide fuel cells (Quadakkers et al., 2003 Brady et al, 2006). For this application, a key figure of merit is the electrical conductivity of the external scale. This criterion excludes alumina and silica scales which are too insulating. Two examples are provided which show some of the potential alloy design strategies for this application. 

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. 

1 Time to accelerated attack as a function of specimen thickness for alloy 230 (Ni-27Cr-4W + La) and alloy 956 (ODS Fe-20Cr-9AI) for exposure in 100 h cycles at 1100°C in laboratory air. For alloy 956, the time is for the onset of FeO formation and increases significantly with thickness. For alloy 230, the times are to total mass gains of 20-40 mg/cm that do not change significantly with thickness. The empirical n values relate the thickness to lifetime based on Equation 

In addition to the temperature limitation, chromia-forming alloys suffer problems at lower temperatures due to the presence of water vapor in the environment (i.e. steam or combustion gas) (Opila, 2004). This is a significant issue as many high temperature applications involve these environments. The study of water vapor effects has been ongoing for many years (e.g. Caplan and Cohen, 1959 Fujii and Meussner, 1964) and is a very active area of current research (Otsuka et al., 1989 Shen et al 1997 Nickel et al.. 

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Other categories of chromia forming alloys—including Ni(-Fe)-Cr base and Fe(-Ni)-Cr base alloys (e.g., austenitic stainless steels)—have a face-centered cubic (FCC) substrate structure. In comparison to the FSS, the FCC base alloys, in particular the Ni(-Fe)-Cr base alloys, are generally much stronger and potentially more oxidation resistant in the SOFC interconnect operating environment [6, 123-129], However, the FCC Ni(-Fe)-Cr base alloys with sufficient Cr for an appropriate... 

The electrical conductivity requirement for interconnect applications necessitates the use of chromia-forming (or Cr-rich spinel) oxidation-resistant alloys. One drawback of the chromia-forming alloys for this particular application, however, is the Cr volatility of the chromia or Cr-rich scale. As indicated by many studies [185-189], during high-temperature exposure Cr203 (s) reacts with 02 via the following reaction... 

Besides the glass seal interfaces, interactions have also been reported at the interfaces of the metallic interconnect with electrical contact layers, which are inserted between the cathode and the interconnect to minimize interfacial electrical resistance and facilitate stack assembly. For example, perovskites that are typically used for cathodes and considered as potential contact materials have been reported to react with interconnect alloys. Reaction between manganites- and chromia-forming alloys lead to formation of a manganese-containing spinel interlayer that appears to help minimize the contact ASR [219,220], Sr in the perovskite conductive oxides can react with the chromia scale on alloys to form SrCr04 [219,221],... 

FIGURE 4.10 Schematic of mass transport in a conductive oxide coating on a chromia-forming alloy. 

High-temperature stainless steels, most polycrystalline superalloys, and chromized coatings rely on the formation of a surface layer of chromia for oxidation protection. The effects of reactive element additions are often more dramatic in the case of chromia-forming alloys than alumina formers in that, in addition to improving adherence (Figure 5.41), they decrease the amount of transient oxidation, reduce... 

S. P. S. Badwal, R. Deller, K. Foger, Y. Ramprakash, and J. P. Zhang. Interaction between chromia forming alloy interconnects and air electrode of solid oxide fuel cells. Solid State Ionics 99, (1997) 297-310. 

High-nickel chromia-forming alloys, such as alloy 600, are particularly susceptible to rapid sulfidation attack at temperatures above 645 °C owing to the formation of a liquid corrosion product. Fig. 5-23 shows the influence of Ni-Fe-Cr alloy composition on the melting temperature of the product sulfide scale. It is apparent that the melting temperature of the sulfide scale generally increases with increasing Fe and... 

It has repeatedly been confirmed that in chromia-forming alloys, the mechanism of oxide-scale growth is changed in the presence of rare-earth elements from cation to anion control, and consequently the direction of growth also changes (Hussey et al. 1989, Graham 1991). 

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