高能微弧火花沉积导电Mn(Re)-Co尖晶石涂层及其高温腐蚀性能研究

Ferritic stainless steels are considered as the most promising interconnect materials in intermediate temperature solid oxide fuel cells (SOFCs) due to their electrically conducting oxide scale (in comparison to alumina- and silica-forming steels),appropriate thermal expansion behavior and low cost. However, the Cr2O3 scale is known to release volatile oxides in the oxidizing environments of the SOFC cathode side, which poisons the cathode's electrochemical activity and drastically deteriorates SOFC performance. In this study a novel high-energy micro-arc alloying (HEMAA) process has been introduced to prepare (Mn,Re,Co)3O4 conductive coatings on the type 430 stainless steel. To overcome the disadvantages of low room temperature electrical conductivity and high inherent brittleness of pure (Mn,Re,Co)3O4 spinel electrode, mixed oxide scale and metallic Co powder are introduced to prepare deposition electrode. Moreover, a pretreatment on 430 SS by HEMAA using a Co deposition electrode is conducted to improve the bonding between the substrate alloy and spinel coating. The thickness homogeneity, microstructure, elemental composition and area specific resistance of the coatings are investigated. Analyses of the structural evolution were carried out when samples were oxidized in order to observe reactions and mechanical stability between the alloys and the applied coatings. The presence of the dense, Cr-free (Mn,Re,Co)3O4 spinel layer will contribute to the improvement in cell performance stability.

铁素体不锈钢是最有应用前景的固体氧化物燃料电池连接体材料，但在电池环境中面临氧化与铬的挥发问题。氧化会导致接触电阻增加，而铬挥发产物会污染电池阴极，因此必须发展耐蚀、导电涂层。本课题首次提出用高能微弧合金化技术在430铁素体不锈钢表面制备Mn(Re)-Co尖晶石导电涂层，采用金属氧化物、稀土氧化物与金属Co组成的混合电极取代纯尖晶石电极，实现涂层的良好沉积，并通过涂层的梯度设计改进涂层与基体的结合。研究金属/稀土尖晶石复合涂层的界面、相组成及微观结构，阐明复合涂层在模拟SOFC阴极环境中的高温腐蚀行为与机制，揭示复合涂层的面比电阻随氧化时间的变化规律。最终发展结合牢固、致密、具有低的离子传导率、有效抑制Cr气态介质挥发的高温耐蚀导电涂层，以推动金属连接体的商业化。

固体氧化物燃料电池金属连接体材料在工作环境中面临氧化与铬的挥发问题，氧化会导致接触电阻增加，而铬挥发会污染电池阴极，本项目采用高能微弧合金化技术在金属连接体表面制备Mn(Re)-Co尖晶石导电涂层。以Co-Mn-Dy和Co-Mn-La金属电极制备冶金结合合金涂层热生长成尖晶石复合层，得到各电极沉积工艺与涂层热生长机制，高Mn电极单位面积热消耗量更小，熔滴沉积量大于基体溶蚀损失率，而低Mn电极熔滴沉积量是小于基体溶蚀损失率的。添加纯Co梯度涂层，增加厚度利于减少基体元素熔入涂层，且减缓高温下基体元素向涂层的扩散。制备(MnO,Co3O4)-Co和MnCo2O4-Co高性能复合电极，并沉积对应氧化物涂层，涂层结合致密，但是氧化后涂层性能欠佳。低功率制备Co-Mn、Co-Mn-Dy和Co-Mn-La合金涂层，空气中和模拟SOFC阴极环境中的腐蚀结果显示高Mn浓度及添加元素Dy的合金具有更好的高温氧化性能，并且氧化层导电性更好。高功率制备的Co-40Mn合金涂层腐蚀产物主要为(Mn,Co)3O4尖晶石，界面处为Cr2O3层，而Co-10Mn涂层腐蚀产物内层有富Fe氧化物生成，加入Co梯度层，内层氧化物则转换为CoFe2O4。活性元素Dy的添加促进(Mn,Co)3O4尖晶石的生成，且氧化物粘附性明显改善，测得所有氧化层面比电阻值均低于50mΩcm2。Co-25Mn和Co-23Mn-2Dy涂层腐蚀产物主要为Mn-Co-Fe复合氧化物层。对Co-Mn涂层在高氧压下扩散机制研究表明，在750℃时，涂层中的Mn同时向外层氧化物和内氧化物前端扩散富集，而涂层中的部分Co与向内扩散的氧结合生成氧化物。在850℃时Mn无明显扩散富集层，而涂层中Co内扩散形成金属扩散层。Co-Mn氧化物层对Cr的外扩散抑制明显，因此Cr的氧化物只在内层生成。长期的模拟环境中腐蚀结果显示，Co-40Mn和Co-38Mn-2Dy展示高的抗氧化性和低的面比电阻值。设计温度和高氧压，可促进高Mn合金涂层（Co-40Mn和Co-38Mn-2Dy）生成(Mn,Co)3O4尖晶石层，减少Mn的内氧化，氧化层结合力增强，预氧化后涂层在模拟腐蚀中抑制了Cr的外扩散，且腐蚀层厚度增加缓慢，面比电阻值低于30mΩ cm2。对本研究对于发展新型高温涂层制备理论与技术，推动多功能高温涂层的发展具有重要的意义。