Preparation And High Temperature Properties Of Cu-Mn Spinel Coatings

With the mature application of medium temperature SOFC technology,it is possible to use low-cost stainless steel ferrite as a linker.However,the surface of stainless steel ferrite will continue to oxidize and the gaseous Cr will continue to evaporate from ferrite in the SOFC working environment,resulting in surface resistance increasing and single cell cathode poisoning.Increasing a layer of high temperature resistant and conductive coatings on the surface of the substrate is an effective way to solve these problems.Recently,some spinel coatings,particularly?Cu,Mn?₃O₄,have received wide attention of scientists due to their advantages of good electrical conductivity,excellent coefficient of thermal expansion?CTE?matching with the ferritic stainless steel substrate and other cell components such as anode and cathode,and high scale capability for absorbing Cr species that migrate from the Cr2O3-rich scale to the surface.In this paper,Cu-Mn alloy coting was prepared on 430SS stainless steel in electrodeposition method in chlorate system with EDTANa₂ as complesing agent.Morphology and composition variation of the coating were studied by SEM and EDS.Effects of parameters such as the current density,pH value,deposition time and n?Cu?:n?Mn?ratio of the electrolyte on microstructure and composition of the Cu-Mn layer were analyzed.Then,the copper-manganese spinel coating was obtained by pre-oxidation heat treatment of Cu-Mn alloy at 750? in Ar and at 800? in air for 2h.Finally,the high temperature oxidation behavior of copper-manganese spinel coating in the SOFC cathode environment of 800? air and 800 ? air+10%H₂O air was studied.Phase structure of the oxide film,surface and cross section of the microstructure and element distribution were characterized by SEM,EDS and mechanism was studied.In addition,the surface specific resistance?ASR?of the Cu-Mn spinel coating after high temperature oxidation was tested by using a four-point method.The main results are as follows.The Cu-Mn alloy coating was successfully obtained on the surface of 430SS by electrode-position.The results showed that the Mn content in the layer increased with the increasing current density,pH value and deposition time in a currenL density range of 200 to 700mA/cm²,a pH value range of 3 to 7and a deposition time of 10 to 30min,decreased with the increasing the ratio of n?Cu?:n?Mn?in the range of 1:20 to 1:10.We can get uniform,dense,edgeless effect,copper-manganese alloy coating with higher the Mn content and the ratio of copper to manganese is 1:2,1:1,2:1,respectively,when the current density is 450,500,550mA/cm²,pH is 5,time is 20min,n?Cu?:n?Mn?is 1:10.Pre-oxidation treatment can make the copper-manganese alloy completely transformed into copper-manganese spinel coating.The oxide coating is composed of two layers,which the outer layer is?Cu,Mn?₃O₄ and the inner layer is Cr-rich oxide layer.Both coating and substrate have a low oxidation rate after oxidizing at 800? after 300h.The coating is well bonded to the substrate.In the process of air oxidation with 10%H₂O,430SS suffers from breakway oxidation after a period of incubation.However,?Cu,Mn?₃O₄ can prevent breakway oxidation.After oxidizing in air and humid air,the coating remains doublelayer structure.The surface specific resistance of?Cu,Mn?₃O₄ coating and 430SS in air oxidation at 800 ? for 300h and humidified air at 800 ? for 100h are all decreased with the increasing of test temperature.When the Cu-Mn atomic ratio of?Cu,Mn?₃O₄ are 1:1,1:2,2:1,ASR of the three spinel coatings at 800? for 300h is 23.2mQ cm²,15.3m?·cm²,35.3m?·cm²,respectively,which are all lower than the value of 11mE cm² based on the substrate under the same conditions.When the Cu-Mn atomic ratio of?Cu,Mn?304 are 1:1,1:2,2:1,ASR of the three spinel coatings after oxidation in 800VC humidified air is 33.2m? cm²,25.6mG cm²,69.3m?·cm²,respectively,which are all lower than the ASR value of the substrate under the same conditions of 83m?·cm².