Characteristics Of Co-based Spinel On SOFC Interconnect For Ship Application

In order to promote the development of green transportation,solid oxide fuel cell(SOFC),as a clean and efficient energy conversion technology,has attracted enough attention in the field of ship transportation.With the reduction of the SOFC operation temperature into an intermediate range of 650-800?,the typically contained chromium ferritic stainless steels(AISI 430 SS)have recently been adopted as hopeful interconnect candidates due to excellent electric and mechanical properties compared with ceramics.However,the bare steel will be oxidized when operating in SOFC stacks directly,which leads to the rapid growth of oxide film,increase of contact resistance and Cr poisoning cathode.In this paper,we innovatively propose the preparation of Co-M-O(M=Ni?Cu?Mn)spinel protective layer on the surface of AISI 430 SS by Plasma Surface Alloying-Preoxidation Process(PASA-PP)to improve oxidation resistance,prevent Cr outward diffusion and maintain appropriate electrical conductivity.The conversion mechanism of Co-M alloy layer to Co-M-O spinel layer and the conduction mechanism of spinel layer are explored.The main work and results are as follows:Co-Ni-O spinel oxide protective layer is prepared on the surface of AISI 430 SS by electroplating-preoxidation treatment.Firstly,the Co-Ni alloy layer is prepared by electrodeposition process,and then the alloy layer is converted into Co-Ni-O spinel coating by pre-oxidation process and its conversion mechanism is explored.The results show that:the Co-Ni-O spinel coating consists of NiCo2O4 and NiO phases;the Co-Ni-O spinel coating can effectively improve the oxidation resistance of the substrate and reduce the growth rate of the oxide film;this coating can significantly prevent Cr outward diffusion,but Cr can still be detected in the spinel layer,indicating that the coating does not completely inhibit Cr outward diffusion;Co-Ni-O spinel coating improves the conductivity of the substrate,but in the long-term oxidation(>100 h)its conductivity does not meet the objectives of DOE(Department of Energy).In order to improve the problem of insufficient conductivity and inability to completely inhibit Cr outward diffusion in the long-term oxidation,a Co-Ni-O spinel oxide protective layer is prepared by PAS A-PP onto the surface of 430 SS.An alloy layer is prepared by plasma surface alloying process,and the alloy layer includes:an outer Co-Ni alloy layer and an inner diffusion layer.After the preoxidation process,the Co-Ni alloy layer is converted into a spinel coating consisting of NiCo2O4 single phase;the inner diffusion layer is converted into an inner diffusion oxide layer containing a mixed spinel phase.The results show that this coating is superior to the Co-Ni-O 430 SS specimen by electrodeposition-preoxidation process in oxidation resistance,inhibition of Cr outward diffusion and conductivity.In view of the fact that Co and a variety of transition metal atoms can form spinel oxides,the Co-Cu-0 spinel coating is prepared on the AISI 430 SS surface by PASA-PP.The results show that:the microstructure of the coating is dense and metallurgically bound to the substrate;the conductivity of the Co-Cu-O 430 SS specimen has been greatly improved,with an ASR value of 0.089 ? cm2@408 h;the coating can effectively inhibit Cr outward diffusion;however,the Co-Cu-O 430 SS specimen has a lower oxidation resistance than that of Co-Ni-O 430 SS specimen by the same process,which results from the Co-Cu-O spinel coating being decomposed at high temperatures.A Co-Mn-O spinel coating is prepared onto the AISI 430 SS surface by PASA-PP.The results show that:the microstructure of the coating is dense and metallurgically binding to the substrate;compared to above spinel coatings,the Co-Mn-O 430 SS specimen has the highest oxidation resistance with the lowest oxidation dynamics constant(9.0929×10-4 mg2 cm-4h-1),and the highest conductivity with the lowest ASR(0.029 ? cm2@408 h);the Co-Mn-O spinel coating can effectively inhibit Cr outward diffusion;after the cyclic oxidation measurements,the Co-Mn-O 430 SS specimen shows a uniform and dense surface and shows a good adhesion to the substrate without cracks and spallation.Nb-modified layer onto the AISI 430 SS surface with different diffusion alloying times(0.5 h,1 h,1.5 h,2 h and 2.5 h)is prepared by plasma surface alloying process.The effects of the alloying time on the surface morphology and microstructure of the coating are investigated.The corrosion resistance and conductivity of the Nb-modified specimens are also investigated.The results suggest that the 1.5 h Nb 430 SS specimen has better performances in the hydrophobicity,conductivity and corrosion resistance.