Study Of Electrodeposited Ni-Co Alloy Coating On Ferritic Stainless Steel

Progress in fabrication technology of solid oxide fuel cell (SOFC) has enabled cell operation at lower temperatures (e.g.,650-850℃), which made metals can be practically utilized as interconnects. Ferritic stainless steels have become the standard material for SOFC interconnect applications due to their CTE match with ceramics, excellent formability and low cost. Nevertheless, under SOFC operating conditions (high-temperature, wet and O2), ferritic stainless steels suffer from several shortcomings. These problems include rapidly increasing contact resistance and volatilization of Cr from the oxide scales, resulting in cathode Cr poisoning and cell malfunction. Numerous coatings, surface treatments, and alloy bulk composition modifications and developments have been considered as potential remedies in order to overcome these issues.Spinel coatings have attracted significant attention recently. Because spinels have good electronic conductors and show excellent CTE match with the ferritic stainless substrate and other cell components. Spinels coatings have also shown excellent capability for absorbing Cr. Slurry coating methods including spraying or screen printing and plasma spraying have been the main application techniques for spinel coatings in the past few years. Recently, electrodepositon of metals followed by heattreatment/oxidation has been considered as a novel technique for spinel coating.This dissertation investigated electroplating Ni-Co alloy on ferritic stainless steels and followed by heattreatment in air at800℃in order to form a (Ni,Co)3O4spinels coating on the surface of the interconnect. The effects of pH and temperature of plating solution as well as the cathode current density on the quality of Ni-Co alloy coatings were studied. The area specific resistance (ASR) of the oxide scale was measured. Morphologies of the coating and its oxide scale were observed using SEM、EDS and their phase structures were identified by XRD.Furthermore, numerous studies proved that rare earths can improve the properties of the coatings on the aspect of decrease the thickness of the oxide scale and the rate of oxidation. In this work, electrodeposited Ni-Co-CeO2composite coating on ferritic stainless steels was investigated.The major results are shown below:(1) In this study, pH=3.0, T=50℃with the cathode current density1.43A/dm2were the suitable deposition conditions for obtaining good performance of coatings within the condition ranges.(2) After oxidized for ten weeks in air at800℃, the surface of ferritic stainless steels were seriously oxidized, spallation and cracks were found on the oxidized surface of the substrate. It indicated that uncoated ferritic stainless steels can not work well in SOFC for a long time.(3) After oxidized for ten weeks in air at800℃, the surface of the steel with Ni-Co alloy coating was covered with uniform and dense oxide crystals and no spallation or cracks were observed. The ASR of steel coated by Ni-Co alloy coating was lower than that of uncoated ferritic stainless steels.(4) After oxidized for six weeks in air at800℃, the thickness of the oxide scale of Ni-Co-CeO2composite coating was thinner than that of Ni-Co alloy coating and the ASR of the former was also lower than that of the latter. It demonstrated that rare earths can decrease the rate of oxidation and the scale ASR.