| Solid oxide fuel cell(SOFC)is a new type of fuel cell in the forefront of scientific research and interconnect is one of the key components in SOFC.With the development of intermediate temperature SOFC technology,it is possible to apply ferritic stainless steel for interconnect materials.However,the surface of ferritic stainless steel is oxidized continuously in the working environment of SOFC.With the extension of oxidation time,the Cr2O3 gradually thickens and the surface specific resistance continuously increases.Meanwhile,the Cr2O3 is transformed into volatilized oxides such as CrO3(g)or CrO2(OH)2(g),resulting in the poisoning on the cathode.At present,it is an effective way to solve this problem by applying a protective coating with high temperature oxidation resistance and conduction on the surface of stainless steel.In this paper,a layer of Cu was pre-plated on the surface of ferrite stainless steel,and then uniformly distributed Mn3O4 particles were deposited on the sample surface by electrophoresis.Finally,metal Cu was electroplated in the pores of the particles to obtain a Cu-Mn3O4 composite coating with Mn3O4 particles embedded Cu.The amount of Mn3O4 particles is same for all cases,and the effect of electroplating deposition process parameters(plating bath pH,bath temperature,cathode current densities and plating time)on the content of Mn3O4 particles in the composite coating was investigated to obtain the optimal process conditions.Based on the optimal conditions,the short-term and long-term high-temperature oxidation behavior of Cu-Mn3O4 coated steel were carried out in air at 800℃.Moreover,the phase structure,surface morphology and elements of the coated steels were characterized by SEM,EDS and XRD.In addition,the four-point method was employed to test the area specific resistance(ASR)of the coated steels after oxidation.The main conclusions are as follows:The optimal process conditions for electrophoresis-electroplating deposition of Cu-Mn3O4 composite coating:bath temperature 50℃,cathode current density 15 mA·cm-2,bath pH 9 and plating time 10 min.The coating prepared under this condition is well bonded with the steel,with a thickness of about 5.5 μm.The content of Mn3O4 in the coating is the highest and the distribution is the most uniform.The coated samples were subjected to short-term oxidation.The coated samples showed a significant mass gain in the initial stage of oxidation,followed by a slight increase.After 1 hour of oxidation,a three-layered oxide scale was formed on the surface.The outer layer was CuO,the middle layer was(Cu,Mn,Fe)3O4,and the inner layer was Cr2O3.Increasing the oxidation time to 100 h,the thickness of the Cr2O3 layer is basically unchanged,indicating that the CuO/(Cu,Mn,Fe)3O4 effectively inhibits the outward diffusion of Cr during the oxidation process.The ASR value of the coated sample oxidized for 100 h was 22.9 mΩ·cm2,which indicates that it has good electrical properties.The coated samples obtained by the optimal process were oxidized for a long time.The coated steel experienced a large specific mass gain in the first week,followed by a transition to slower oxidation kinetics.During the 5 weeks oxidation,the scale surface was dense and uniform,and no voids or cracks were observed.Simultaneously,the oxide scale with three layers was formed.The middle layer was composed of(Cu,Mn,Fe)3O4 and(Cu,Mn,Fe,Cr)3O4 spinels.The other structures were consistent with the short-term oxidation.After 10 weeks exposure,the surface scale was converted into(Cu,Mn,Fe,Cr)3O4 spinel,which effectively prevented the outward growth of the inner layer Cr2O3.The ASR value(25.7 mΩ·cm2)of the coated steels oxidized for 10 weeks was less than the normal working standard of 100 mΩ·cm2,thus the conductivity of the coated steel met the requirements. |
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