High temperature corrosion and corrosion protection of metallic interconnects for SOFC
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Reducing solid oxide fuel cells (SOFCs) operation temperature from 900-100 °C to 700-800 °C, has made the substitution of traditional lanthanum chromate (LaCrO3) ceramic interconnect with metallic interconnect possible. At elevated temperature and in oxidant environment, a metal is not stable and will transform into its oxide. Typical metallic materials used as SOFCs interconnects are stainless steels with a Cr-content around 20 wt.%, and with some minor alloying elements like Mn. During oxidation at elevated temperatures, chromium will be preferentially oxidized forming a continuous chromia layer (Cr2O3) to protect the steel against fast oxidation. However, chromia is not stable at these temperatures and in humid cathode environment and it will vaporize by formation of volatile Cr-oxides or hydroxides. These gaseous species are causing chromium poisoning of the SOFC cathode. The chromium poisoning process has an impact on SOFCs performance as the chromium will block the surface active area for electrochemical reaction of oxygen reduction. If there is enough Mn present within the steel alloy, thermally grown (Mn,Cr)3O4 spinel will form on the top of chromia layer to reduce the poisoning effect. Nevertheless, the steel substrate used as SOFCs interconnect should be pre-coated to minimize the poisoning effect. Spinels and perovskites have been reported as potential materials for such coatings, with the spinels as the best candidates. They have high enough electrical conductivity, show matching thermal expansion coefficients with the base materials, and have good chemical stability and compatibility with the other fuel cells part. In this thesis, high temperature corrosion and corrosion protection of metallic interconnects has been investigated. In the first part, Mn2O3 is deposited on steel substrates and used as a model system to study the early stage of protective spinel coating formation on the steel substrate. Phase formation and phase growth after long time exposure to oxidizing environments at high temperature are investigated. The second part of the thesis looks at the phase stability and the formation of secondary phases when interconnect coating materials were in contact with potential cathode materials. From this we look at which materials which are coexistent and can be used together without any destructive phases formed, and which ones cannot be used in the same SOFC fuel cell. In the third part, deposition of MnCo2O4-spinel by the electrophoretic deposition method (EPD) on steel substrates is reported. The method and its parameters is closely investigated to give coatings that are homogenous and without cracks, and therefore can act as a corrosion barrier. The durability and the coating’s adhesion is also brought into discussion.