Materials for Solid fuel oxide cell for performance at high temperatures

Solid fuel oxide cell serves at high temperature with fuel like hydrogen gas or reformed natural gas on the anode side and air on the cathode zone. Moisture could occur on both electrodes hence in contact with metallic interconnects. Sulfur contaminants present in the fuel gas contact with the metallic interconnects. Sulfur contaminants present in the fuel gas stream are also anticipated to occur, however upstream desulfurization has been applied to reduce the sulfur contamination level to sub ppm or ppb levels. Hence, besides oxidation, interconnect could also experience sulfidation, hot corrosion and carburization. 

Thermal stresses produced in the SOFC stack because of large temperature gradients across the current collector could also speed up the corrosion process due to premature cracking and spallation of the oxide layer. The availability of complex gaseous species in the fuel condition also cause establishment of grain boundary corrosion, internal oxidation and localized metal loss causing overall reduction of component service life. Sulfidation refers to vigorous corrosion resulting into combined effects of oxidation and reactions with sulfur that may present in the fuel gas streams.

The metallic interconnect is also needed to have sufficient strength to help maintain the structural integrity of the stack during Solid oxide fuel cell service at high temperatures and under thermal cycling. The high temperature alloys for interconnect should have thermal fatigue resistance against feasible structure fracture during thermal cycling, creep resistance to maintain the size stability at high service temperature and rupture resistance to withstand peak thermal stresses produced during SOFC operation. The above stated strengths can be more or less correlated to the yield strength. For stainless steels, the compositions with higher yield strength often possess high creep and fatigue strengths.

Many alloys except annealed low carbon steels do not have standard yield strains, the stress is referred as yield strength. When feasible, the yield strength from bar analyses at room temperature and high temperature was gathered.

Nickel based superalloys

Depending on the ratio of chromium and aluminum, nickel based super alloys are classified containing Cr and Al into these categories:

A NiO scale with Cr2O3 and Al2O3 internal oxides for low chromium and aluminum concentrations.

An Cr2O3 scale with Al2O3 internal oxides for high Cr above 15% however low Al below 3%. An exclusive alpha- Al2O3 scale for considerably high chromium above 15% and high aluminum above 3%. The presence of an inner layer drastically decreases the local oxygen activity at the metal –interface so that an enrichment of alumina particles occurs. Alumina layer develops below the Cr3O3 inner layer that significantly improves the oxidation resistance and also acts as an electrical insulating layer. So, an aluminum concentration of 3% was established to be critical maximum. But none of used nickel base alloys contained chromium content higher than 18% and aluminum higher than 3%. Recommended nickel superalloys Inconel bars for use are Inconel 625.

Stainless steels

Stainless steels are popular for their oxidation resistance. They contain the compositions to withstand all temperatures and hence do not strengthening by heat processing although a few grades need. Austenitic steels are featured by larger linear thermal expansion coefficients.