Static and Dynamic Corrosion Characteristics of Plasma-Sprayed Fe-70Cr Coatings by Molten Sulfur and Sodium Polysulfides in Sodium Sulfur Cells

The sodium sulfur (NaS) cell comprises of sodium and sulfur electrodes separated by a beta alumina solid electrolyte (BASE). To avoid radical oxidation of the sodium and sulfur by oxygen or other oxidants in ambient environments, all the electrochemical components need to be protected by a hermetically sealed metal container. Joints between the adjacent structural parts are bonded by advanced gastight sealing technologies, such as electron beam welding (EBW), thermal compression bonding (TCB), and glass sealing for metal-metal, metal-ceramic, and ceramic-ceramic joints, respectively. For a contemporary sodium central type tubular cell for energy storage systems (ESS) applications, the metal container is made of aluminum alloys, and its inner surface is exposed to molten sulfur and/or sodium polysulfides (Na₂S_x), at its operating temperatures of 300~350^oC. However, since aluminum sulfides (AlS_x), corrosion products of the aluminum alloys formed on the inner surface, may lead to an abrupt increase in cell resistance due to their high electrical resistivities, and cause cell failure by catastrophic corrosion, the inner surface needs to be appropriately coated with a conductive material. The coating material should have a high electrical conductivity, but also form a continuous conductive layer at a sufficiently slow growth rate during service. In this study, an Fe-(65~75)Cr alloy has been coated on the inner surface of the aluminum container via the argon shrouded plasma spray process. The coated containers were used to construct different types of sodium sulfur cells, and static and dynamic corrosion experiments have been conducted for up to 9 months. Results from the experiments will be discussed focusing on the characterization of corrosion products, and corrosion kinetics in order to predict the lifetime of the coating which is critical to ensure the long term durability of sodium sulfur cells.