Optimization of electrophoretic deposition of modified ceramic protective coatings for SOC stack metallic interconnects

Solid oxide cells (SOCs) devices are of great interest for efficient and clean power generation. An important component of the SOC stack is the metallic interconnect. It allows the fuel and oxygen gas separation and provides an electrical connection between the anode and the cathode of two neighboring cells. Chromia-forming ferritic stainless steels are widely used as interconnect materials in solid oxide cells. However, chromium evaporation and corrosion may negatively affect the component durability and efficiency. Ceramic coatings play a key role to limit these degradation issues. In particular, Mn-Co spinel coatings and Mn-Co doped spinel coatings are proposed as a viable solution, thanks to their high electrical conductivity and Cr retention capability. Electrophoretic deposition (EPD) is a low cost, simple and fast technique to deposit ceramic particles on metallic interconnects. In this work different Mn-Co spinel-based coatings were produced for different metallic interconnects by the electrophoretic deposition technique. Parts of an interconnector supplied by Ames company were used for the first experiments, that allowed to identify the best deposition parameters and to analyze the effect of heat treatments. The suspension was prepared with a solution of acetone and ethanol, 0.5g/L I2 and 15g/L MC12. The deposition of a uniform and homogeneous coating was performed with a voltage of 50V for 30s. Coated samples were heat treated with a two-step sintering technique. The first step was a reducing treatment, with two variations, followed by an oxidizing treatment. The first reducing treatment was performed at T > 1000°C, whereas the second reducing treatment at 1000°C. The following oxidizing step was conducted at 800°C for 5h. Once the optimal deposition parameters were assessed, the electrophoretic deposition was performed on both faces of real-size interconnects. It was possible to demonstrate how the EPD technique is an easy, economical, and suitable to scaling up from laboratory scale to industrial one. The same MC12 powder was used for the study of a third heat treatment, the rapid thermal treatment (RTP), which allows an even faster treatment through infrared radiation. In this case the EPD coated substrate was the Crofer22APU steel. ASR tests at 800°C for 1000h, and postmortem characterization were performed on these samples. The second part of the work focused on the characterization and preparation of Ce-doped MCO coatings in different percentages. The objective was to begin to understand the behavior of ceria in the MCO coating and the positive effects related to the increase in conductivity and preventing the steel corrosion. Crofer22 APU was deposited with two different suspensions 10CeMCO and 5CeMCO. “As deposited” samples were sintered in a two-step heat treatment, first in a reducing atmosphere and then in an oxidizing one. The suspension for 10CeMCO samples was composed of ethanol and distilled water, to which 90wt.%MCO and 10wt.%CeO2 powders were added, for a total amount of 37,5g/L. The deposition took place at 50V for 30s. Different multilayers were also produced by using suspensions of MCO, 5CeMCO, CeO2 and the electrolytic deposition of Ce(NO3)3 6H2O. The heat treatments in a reducing and oxidizing atmosphere and the following characterization were then performed. The data obtained in this MSc thesis highlight the importance of EPD as a fast, suitable, and viable technique to deposit ceramic coatings on metallic interconnects for solid oxide cells.