Manganese-Cobalt Spinel Coatings for SOFC Metallic Interconnects Manufactured by Conventional Plasma Spraying (PS) and Suspension Plasma Spraying (SPS)

Protective coatings are used on ferritic stainless steel interconnects of solid oxide fuel cells (SOFCs) to prevent the oxidation and evaporation of volatile chromium compounds. Oxide scale is formed of chromium oxide (Cr₂O₃) which tends to react with the oxygen and water, forming chromium trioxide (CrO₃) and chromium hydroxides (Cr₂(OH)₂). These compounds will migrate to the triple phase barrier (TPB) of a cathode and reduce back to Cr₂O₃. This reaction pathway is a notable reason for the degradation phenomena of the cell. Plasma spraying (PS) and suspension plasma spraying (SPS) were studied as possible manufacturing processes for thin Mn-Co-(Fe) spinel coatings. Powder for PS was manufactured by using a solid state reaction method from carbonates and oxide to form a MnCo_1.8Fe_0.2O₄ and powder for SPS by co-precipitation process from nitrates to form a MnCo₂O₄ spinel structure. Using PS, coatings with thin and relatively dense structures were obtained. The composition of the coatings was homogeneous although, the decomposition of the spinel structure was noticed. The crystal structures of the PS coatings were partially restored by a separate annealing process. The spray parameters had a strong influence on the coating structure and the composition when SPS was used. The most homogenous coating structure was formed when low energy spraying parameters were used, whereas high energy parameters formed a columnar structure with larger cobalt rich areas. The decomposed spinel structure of the SPS coatings were fully restored by the annealing treatment. In SPS, more process optimization is needed to improve the coating quality and especially denseness.