Developement of Ceramic Concentration-Graded Materials as a Corrosion-Resistant part for MCFC with Plasma Spray

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A plasma-spray method was applied to producing an alumina coating layr on an iron substrate in order to use it as a heat- and corrision-resistant wet seal part in MCFC.It was found that ductile chromium powder was required to metallize the iron surfce to keep good contact between an alumina layr and the iron substrate and simultaneously good corrosion resistance to molten alkali carbonate at 700ﾟC.In order to improve heat resistivity, concentration-graded chromium and alumina layrs were made attempt to be produced and their cheracteristics were evaluated. As a result, a tripple layr type coating (an alumina layr as the outermost layr, a chromium-alumina mixing layr as the middle layr and a chromium layr as the bottom layr) was recognized more suitable. Moreover, the concentration of chromium in the middle layr was optimum at 5 atmomic % which corresponded to 8 % in volume. This value agreed with that of the porosity in alumina sprayd layrs. This means that pores were almost filled with fused chromium. The corrosion current density was around 2 A・m^<-2>, which was not satisfactory. To reduce corrosion current density, sealing of pores in the outermost alumina layr was investigated. Mixture of aluminium and alumina powders were sprayd on the chromium-metallized iron substrate and oxidized under oxygen atmosphere at 700ﾟC.The effect of change in ratio of mixing of the powders on heat and corrosion rsistivity was studied. It was also found that there existed an optimum ratio at 5 to 10 volume % which also corresponded to the porosity. This led to the conclusion that the pores in the alumina layr were sealed with the oxidized aluminium. The corrosion current density was estimated to be about 1 A・m^<-2>. This value seems to correspond to the life time of one year in a running MCFC.