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In order to define the mechanisms that control the corrosion of current collector materials in molten carbonate fuel cells, the corrosion of iron with Li2CO3‐K2CO3 melts at 650°C was investigated. Thermochemical calculations were used to identify equilibrium corrosion products and establish phase stability diagrams for the Fe‐Li‐K‐C‐O system. The corrosion of iron with a 2.0 mg/cm2 carbonate deposit of false(Li0.62K0.38)2CO3 under 20%O2‐false(0.01–50false)%CO2‐normalAr proceeded according to a parabolic rate law, where the corrosion rate was controlled by the diffusion of iron cations through normalFeO and Fe3O4 inner layers. The corrosion of iron with the carbonate deposit under Po2<102 Pa followed a linear rate, where the controlling step was the dissolution of oxygen from the gas phase into the carbonate. With the aid of the phase stability diagrams, the corrosion kinetics and scale morphologies were examined.

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Porous Ni/ZrO2-Y203 cermets are electronically con~ ducting at Ni contents greater than 30 v/o of total solids. Below this level, the conductivity falls to that of the ionically conducting zirconia matrix. The 30 v/o value was found to be independent of the zirconia particle size for the two sizes studied. Nevertheless, considerable improvement in the electronic conduction of the cermet (Ni content > 30 v/o) was made by increasing the zirconia particle size. This improvement probably results from better Ni particle-to-particle contact.

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The development of iron-aluminide weld overlay coatings for high-temperature oxidation and sulfidation resistance critically depends on the determination of the aluminum concentration range for which good corrosion behavior and weldability coexist. This study demonstrated that a sound weld overlay composition with about 30 at.% A1 has relatively low corrosion rates in a highly aggressive oxidizing/sulfidizing (H2S -H2 -H20 -Ar) environment at 800°C. Its corrosion resistance was superior to alloys with compositions like that of type 310 stainless steel and Fecralloy. The results with this overlay composition can be explained on the basis of what is known about the effects of variations in aluminum and chromium concentrations on high-temperature oxidationhlfidation from studies with bulk iron aluminides. While higher aluminum concentrations are even better for long-term corrosion resistance, the ability to reliably produce weld overlays of such compositions without hydrogen cracking is problematical and is the subject of continuing development.IRON ALUMINIDES have several attractive attributes that have prompted many alloy development studies of these systems (see, for example. refs. 1 and 2). Alloys based on Fe3A1 and FeAl have been shown to have good-to-exceptional resistance to degradation in high-temperature oxidizing, oxidizinglsulfidizing, sulfidizing, and oxidizing/chlorinating gases (1,3-9). Indeed, the oxidatiodsulfidation resistance of iron aluminides containing greater than about 18 -25% A1 (at.% unless noted otherwise) is maintained at temperatures that are well above those at which these alloys have adequate mechanical strength (for example, SOOOC). Because of this and possible cost considerations, it is anticipated that appropriate compositions of iron aluminides may find application as coatings or claddings on more conventional higher-strength materials which are generally less corrosion-resistant at high temperatures.In a previous paper (lo), initial results from 800°C exposures of weld-overlay iron-aluminide specimens to a highly aggressive (H2S -H, -H,O -Ar) environment were reported. The investigation involved the characterization of the thermodynamic stability and reaction kinetics of these coatings, but did not determine their ability to physically separate the corrosive species from susceptible substrate material. Under conditions that produced coatings with -35% AI, corrosion resistance equivalent to the best wrought iron aluminides was observed. A second set of weld deposits, with A1 concentrations e 2395, showed significantly less sulfidation resistance.A key issue in the development of appropriate weld deposit compositions is the determination of the optimal A1 concentration. Based solely on oxidation/sulfidation considerations, higher AI levels are preferable (8). However, the susceptibility to hydrogen-induced (cold) cracking of the weld overlays increases with the concentration of aluminum in the deposit (1 1,lZ). This type of cracking occurs due to the combined influence of st...