Oxidation and Cr-evaporation behavior of MnCo based spinel and composite coated AISI 430 steel

Abstract

This work investigates the importance of the chemical composition and porosity of the coating on reducing the oxidation and Cr-evaporation rate of AISI 430 steel in O2-5%H2O at 800 °C for solid oxide fuel cell (SOFC) interconnect applications. Electrophoretic deposition (EPD) is used to deposit eight different coatings. Mn1.25−0.5xCo1.75−0.5xMxO4 (M = Fe, Cu, and Fe-Cu; and X = 0 and ~0.3) powders are synthesized by the glycine nitrate process (GNP) and applied as spinel (without La2O3) or composite (with La2O3) coatings. The coatings initially show porous structure, and the coating density improvement depends on coating composition. The coating porosity for composite co-doped Fe-Cu MnCo spinel (MCFeCuL) is 28% less than MnCo spinel (MC) after 504 h. All coated samples show lower oxidation and Cr-evaporation rates than uncoated samples. The parabolic oxidation rates are lower for composite coated than the spinel ones, possibly due to the formation of LaCrO3 reducing Cr outward diffusion. Composite MnCo-La2O3 (MCL) shows the lowest oxidation rate. In contrast, coating effectiveness in reducing Cr-evaporation depends on coating density. Cu-doped coatings show less coating porosity with better oxidation and Cr-evaporation behavior than Fe-doped coatings. All coated samples form a densified layer at the thermally grown oxide (TGO) scale/coating interface, playing a crucial role in oxidation resistance and migration of Cr from the TGO scale to the coating. The changes in Cr-evaporation fluxes depending on coating porosity and pore size are calculated and compared with Cr-loss rate measurements. Among the coatings, composite Cu-doped MnCo spinel (MCCuL) and MCFeCuL show better performance than the other ones. This study shows that it is possible to decrease coating porosity and improve the performance of MnCo spinel coatings against corrosion and Cr-evaporation without two-step sintering (at T ≤ 900 °C) procedures.