Cr Vaporization and Oxide Scale Growth on Interconnects in Solid Oxide Fuel Cells
The vaporization of Cr(VI) species and the greater electrical resistance caused by a growing oxide scale are probably the two most detrimental degradation mechanisms associated with the use of Cr2O3-forming alloys as the interconnect material in a Solid Oxide Fuel Cell (SOFC). High electrical efficiency, clean emissions and the possibility to utilize several types of fuels, such as hydrogen, alcohols and hydrocarbons are some of the great advantages of SOFC technology. However, the limited lifetime and high production costs of a SOFC have limited the commercialization of this technology. Therefore, improving the component materials and reducing production costs is of great importance. This thesis has examined both mechanisms; Cr vaporization and oxide scale growth. The possibility to pre-coat large amounts of steel and deform the material to allow for gas distribution as an alternative method for reducing production costs was also investigated. To achieve these objectives, nano coatings of Co and Ce were applied to Cr2O3-forming ferritic stainless steels to decrease Cr vaporization and improve the oxidation resistance of the interconnect material. All exposures were carried out in a simulated cathode-side environment consisting of air-3% H2O. Cr vaporization was measured using a denuder technique and oxide-scale growth was studied mainly gravimetrically. For chemical, structural and microstructural analysis, SEM/EDX, FIB and XRD techniques were used.
The results presented in this thesis show that high quality coatings that mitigate Cr vaporization are necessary, even if the SOFC operating temperature is decreased to temperatures as low as 650 ºC. By coating a ferritic stainless steel with 600 nm Co, the Cr vaporization rate can be decreased by almost 90 %, and by adding an extra 10 nm layer of Ce, the high temperature corrosion resistance of the interconnect material can be significantly improved, and electrical oxide-scale resistance can be reduced. When the pre-coated steel was mechanically deformed to allow for gas distribution, large cracks formed in the coating. However, upon exposure, the cracks healed and formed a continuous surface oxide rich in Co and Mn. As an effect of this rapid healing, no increase in Cr vaporization was detected for the pre-coated material.