| Solid oxide fuel cell(SOFC)is an electrochemical device that produces electricity directly from fossil fuel.It exhibits high efficiency and low emission,as well as excellent fuel flexibility.Interconnect is one of the key components of SOFC,which provides electrical connection between individual cells and acts as a physical barrier to separate fuel from oxidant gases.With the reduction of SOFCs operating temperature to the range of 600?800 ?,metallic alloys with high oxidation resistance are used to replace traditional ceramic interconnects.Ferritic stainless steel(FSS)is a promising metallic interconnects candidate due to its acceptable oxidation resistance and electrical conductivity of the oxide scale,coefficient of thermal expansion(CTE)matching with other components and low cost.However,during the long term operation FSS interconnect suffers from the growing chromia and volatile Cr poisoning cathode,which causes cell degradation.Protective coatings have been developed to address these limitations.NiFe2O4 spinel coating is promising for the steel interconnect application for its CTE matching with the steel and acceptable electrical conductivity,as well as capability of restraining Cr out-migration.In this work,NiFe2O4 spinel coating was thermally converted from the sputtered NiFe2 coating on the SUS 430 steel after oxidation in SOFC cathode environment.The oxidation performance of the coated steel and the electrical properties of the oxide scale were investigated.The influences of the exposure temperature,the thickness of the NiFe2 coating and steel preoxidation treatment were discussed.In addition,a dual-layer coating of Ni/NiFe2 was also prepared by electroplating a layer of metallic Ni between the steel and the sputtered NiFe2 layer to further enhance the Cr-blocking effectiveness.The main research results are as follows:The initial and long-term oxidation behaviors of the steel with sputtered NiFe2 coating were investigated in air at 800?,and the influence of reducing the oxidation temperature to 700? was also discussed.The NiFe2 alloy coating initially converted to a multilayer structure oxide of NiO/NiFe2O4/Fe2O3.The mid-layer of NiFe2O4 grew via the solid-state reaction of NiO inner layer and Fe2O3 outer layer.Meanwhile,a Cr2O3 layer formed at the steel/scale interface.After long-term exposure,surface scale on the NiFe2 coated steel was thermally converted to a double layer structure with a NiFe2O4 spinel layer atop chromia sub-layer.The top NiFe2O4 spinel layer not only suppressed chromia layer growth,but also reduced the out-migration of Cr species.An area specific resistance(ASR)of 49.25 m?·cm 2 was achieved for NiFe2 coated steel after 15-week thermal exposure.When the exposure temperature decreased to 700?,the oxdation rate of the NiFe2 coated steel,the out-migggation of Cr and the growth rate of NiFe2O4 spinel layer were significantly reduced.NiFe2 alloy coatings with different thickness of(3.5,6.5 and 9.5 ?m)were sputtered on steel substrate by controlling the sputtering time(2,4 and 6 h),followed by exposing in air at 800? to investigate the effect of coating thickness on the oxidation behavior.The NiFe2 coatings were converted to multilayer structure oxide of NiO/NiFe2O4/Fe2O3.Compared to the 2 h sample,4 h and 6 h samples showed better oxidation resistance and Cr-blocking effectiveness,but at the same time they took longer time to form outer spinel layers.Moreover,localized spallation of the Fe2O3 layer occurred for the 6 h sample during oxidation.After 15-week exposure,the 4 h sample formed an outer NiFe2O4 layer(containing some Fe2O3)atop the inner Cr2O3 layer basically,while the 6 h sample still presented an obvious outer Fe2O3 layer.The 4 h sample exhibit a lower scale ASR(44.71 m?·cm2)than those of 2 h and 6 h samples.NiFe2 alloy coating was sputtered on bare and preoxidized steel(100 h in air at 800 ?),respectively,followed by exposing in air at 800 ? to investigate the influence of steel preoxidation on high temperature behaviors of the coated steels.It was found that steel preoxidation accelerated the transformation of NiFe2 alloy coating to NiFe2O4 spine.The preformed chromia layer restrained interdiffusion of steel substrate and coating at the initial stage of oxidation.During the long-term exposure,the steel preoxidation enhanced the oxidation resistance of the coated sample and reduced Cr out-migration to NiFe2O4 spinel layer by reducing the interfacial reaction between the Cr2O3 and NiFe2O4.After 15 weeks,scale ASR for the coated preoxidized steel(37.15 m?·cm2)was much lower than that for the coated bare steel.A dual-layer coating composed of an inner Ni layer and an outer NiFe2 layer was prepared on steel substrate by electroplating and then sputtering,followed by exposing in air at 800?.The results indicated that an outer NiFe2O4 spinel layer atop an inner Cr2O3 layer formed on the coated samples after oxidation.However,the breakaway oxidation occurred after 5 weeks exposure.The NiO/NiFe2 sample(Ni coated steel was preoxidized in air at 900? for 10 h and then sputtered with NiFe2 alloy layer)showed a similar oxide scale after oxidation in air at 800? while exhibited a good oxidation resistance during 15-week exposure.The outer spinel layer of NiO/NiFe2 sample was more effective in blocking Cr out-migration than that of NiFe2 coated steel.The scale ASR for the NiO/NiFe2 sample was 73.22 m?·cm2 after 15-week oxidation. |
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