Oxidation behavior of (Co,Mn)₃O₄ coatings on preoxidized stainless steel

• Main Author: Hoyt, Kathryn Olivia
• Language: en
• Published: 2011
• Online Access: http://etd.lib.montana.edu/etd/2011/hoyt/HoytK0511.pdf

As global energy challenges grow, alternative energy technologies like fuel cells are being investigated. Solid Oxide Fuel Cells (SOFCs) provide the advantages of high energy conversion efficiency, low emissions, fuel flexibility and both portable and stationary application. High material cost and need for longer material lifespan still impede the wider use of SOFCs. To produce substantial voltage, planar SOFCs are joined into stacks using interconnects. Interconnects both separate and connect each individual fuel, separating gas flow and conducting current. For SOFCs that operate at less than 800°C, metal alloys are being considered for the interconnect, particularly ferritic stainless steel. Ceramic coatings are being explored to improve the surface conductivity over time and significantly reduce Cr volatility from the steel. In addition, the coating must have a matching coefficient of thermal expansion (CTE) and be compatible with electrode and seal materials. One promising coating is (Co,Mn) ₃O ₄ spinel, which is deposited using various techniques, resulting in different thicknesses, compositions and microstructures. In this study, stainless steel 441HP samples were subjected to three levels of preoxidation prior to coating with 2 micron CoMn alloy using magnetron sputtering. Samples were subsequently annealed to Co 1.5Mn 1.5O ₄ in 800°C air. Oxidation behaviors were evaluated as a function of exposure to laboratory air and dual atmospheres (3% H ₂O and H ₂ on one side, 3% H ₂O and air on the other) and area specific resistance (ASR) measurements in lab air, all at 800°C. In addition, chemical and phase composition, mass gain, and adhesion were investigated using a complimentary suite of analytical techniques. Preoxidation was found to inhibit Fe transport from the stainless steel into the coating and exhibited a substantially thinner surface oxide layer after oxidation. Preoxidized samples also maintained slightly lower ASR values after 1650 hours in 800°C air compared to non-preoxidized samples. Oxidation behaviors, their possible mechanisms, and implications for SOFC interconnects will be presented and discussed.