ASR evaluation of different kinds of coatings on a ferritic stainless steel as SOFC interconnects

The demanding technical requirements for solid oxide fuel cell (SOFC) interconnect materials pose development challenges for commerciallyviable systems. SOFC interconnects physically separate and manifold fuel (reducing) and oxidant (oxidizing) gases, while electrically connecting adjacent cell electrodes (anode to cathode) into series, and also permitting SOFC stacking and sealing. Due to the challenging multifunctional requirements, SOFC interconnect material investigations are intensifying to enable deployment of highly-efficient and inexpensive SOFC systems. Commercial ferritic stainless steels are used as interconnects in SOFC operating at ~800 °C. They contain elements such as Rare Earths, Mn, Al, and Ti as microalloying elements, designed to produce a thin surface oxide layer with specific characteristics: high mechanical strength; excellent adhesion to the metallic substrate; low growth rate (interconnect durability must be around 40,000 h); high chemical resistance (to avoid Cr depletion and electrode poisoning) and, low area specific resistance (ASR<10(-1) Ohm cm(2)). However, long-term performance of commercial and even speciality ferritic stainless steels remains insufficient, and therefore surface treatments and/or coatings are required. These treatments have several important tasks, with the most important being to stabilize condensed-phase Cr(III) compounds in order to limit formation of volatile Cr (VI) compounds, which are responsible of SOFC cathode poisoning. This paper deals with the efficacy of two different kind of promising coatings: reactive element oxide nanolayers, produced by metal-organic chemical vapour deposition (MOCVD); and, dual segment coatings (CrAlYO+CoMnO), obtained by filtered arc deposition for the CrAlYO bottom segment and filtered arc-assisted electron beam physical vapour deposition for the CoMnO top segment. The coatings were applied on Crofer 22 APU (a commercial ferritic stainless steel from Thyssenn Krupp VDM) and tested under conditions simulating the aggressive service environment of a SOFC interconnect.