Preparation And High Temperature Performances Of NiFe₂O₄ Spinel Coating For Ferritic Stainless Steel

To meet the requirement in high power,the unit cells of solid oxide fuel cells(SOFC)are connected with interconnects.As one of the important components of SOFC,interconnects provide electrical connects between individual cells and separate oxidant gases(cathode)from fuels(anode).So the cost,efficiency and working life of SOFC are related to interconnects closely.With the progress in fabrication of SOFC,its operating temperature has been reduced from 1000 ? to 600-800 "C,which makes it possible to some cheap metallic materials as interconnects.The coefficient of thermal expansion(CTE)of interconnects must match well with the other components of SOFC.Besides,interconnects have to be oxidation-resistant and exhibit excellent electrical conductivity at high temperatures.Among the commercial oxidation-resistant alloys,Cr2O3-forming ferritic stainless steels are promising candidates for interconnects.However,two main problems need to be solved for the application of ferritic stainless steels,involving the fast increase of area specific resistance(ASR)and the poison of cathode caused by evaporation of chromia.A simple and effective method to solve these issues is preparing an oxidation-resistant and electrically conductive coating on the metallic interconnects.NiFe2O4 spinel is a suitable coating for interconnects due to its excellent CTE match with ferritic stainless steels and much higher electrical conductivity than that of Cr2O3.In the present study,NiFe2O4 spinel coatings unmodified and modified by Nb and CeO2 have been prepared on a type 430 ferritic stainless steel(430SS)by electroplating Ni-Fe,NiFe-Nb and NiFe-Ce02 alloy coatings followed by oxidation treatment,respectively.In addition,a CrN diffusion barrier was also applied,in an attempt to inhibit the mutual diffusion bewteen the substrate and Ni-Fe coating.The oxidation performance and electrical property of NiFe2O4 spinel coatings were investigated and the main achievements are as follows:(1).The variation of chemical composition of the deposited Ni-Fe alloy coatings against some electroplating parameters including the concentration of FeSO4,current density and temperature has been examined.Based on the investigation of the effect of the composition and thickness of the as-deposited Ni-Fe alloy coatings and of the pre-oxidation temperature on the formation of NiFe2O4 spinel coatings,three suitable alloy coatings of 1 ?m thick Ni-15at.%Fe,2 ?m thick Ni-15at.%Fe and 4 ?m thick Ni-25at.%Fe have been optimally developed for further oxidation and ASR measurements.It is shown that the NiFe2O4 spinel coatings could retard the outward diffusion of Cr and effectively reduce the ASR,but with less effectiveness in inhibiting the growth of Cr2O3.Many pores are generated near the oxide scale/substrate interface during oxidation.The NiFe2O4 spinel coatings enhance the oxidation resistance of 430SS in humid air at high temperatures.(2).Three Nb-containing NiFe-Nb alloy coatings were prepared on 430SS by composite electroplating.The effects of NiFe-Nb alloy coatings on the oxidation performance and electrical conductivity of 430SS were studied.The addition of 4.2at.%Nb in the Ni-Fe alloy coating could improve oxidation resistance,and decrease the growth rate of Cr2O3 and thus ASR.However excessive Nb addition is detrimental to oxidation resistance and ASR,promoting the growth of Cr2O3.The addition of Nb decreases the pores near oxide scale/substrate interface.(3).A NiFe-Ce02 alloy coating is prepared on 430SS by composite electroplating.During oxidation at 800 ?,the alloy coating has been converted to a bi-layered oxide scale consisting of outer NiFe2O4 and inner Cr2O3.The outer NiFe2O4 retards the outward diffusion of Cr.The addition of CeO2 in Ni-Fe alloy coatings improves the oxidation resistance of NiFe2O4 effectively,and also reduces the number and size of pores near the scale/substrate interface.As a result,the electrical conductivity is improved.(4).A bi-layered coating composed of an inner 1.7? m thick CrN diffusion barrier and outer 1.6 ?m thick Ni-32at%Fe alloy layer has been prepared on 430SS by multi-arc ion plating and then electroplating.The NiFe alloy coating could be oxidized to form a dense and adherent NiFe2O4 coating at 800? in air.CrN services as a barrier to stop the mutual diffusion between the electroplated alloy coating and substrate and provides Cr for the growth of Cr2O3.The application of CrN diffusion barrier could decrease the defects near the scale/substrate interface,reduce the growth rate of Cr2O3,and thus improve the oxidation resistance and electrical conductivity.