Oxidation activation energy of manganese-cobalt-cerium oxide coated AISI 430 steel for SOFC interconnect application

The formation of oxide scales is one of the critical issues for applying Fe–Cr alloys in solid oxide fuel cells (SOFCs) operated at 773 K–1073 K (intermediate operation temperature SOFCs) [1-2]. The growth of oxide scale is dominated by the diffusion of elements (cations and oxygen) in the scales and the microstructures of scales [3]. One of the most effective approaches to improve the interconnect properties is to apply surface coatings to provide better conductivity, reduced scale growth and Cr volatility. Manganese-Cobalt coatings are promising candidates for SOFC interconnect applications because of their high conductivity and good oxidation resistance. In this study, the electrodeposition method was employed to coat manganese, cobalt and cerium oxide onto AISI 430 steel. For the direct current (DC) electrodeposition process, a one-compartment cell was used. Platinum foil was used as the anode and it was placed in the same compartment as the working electrode. All experiments were carried out in 100 ml electrolytes prepared with deionized water. Ammonia hydroxide or 20 vol% H2SO4 was used to adjust the electrolyte pH. Bath electroplating with composition of 0.5 M MnSO4 + 0.10M CoSO4 + CeO2 40 g/L + 1 M H3BO3 + 0.7 M NaC6H11O7 + 0.1 M (NH4)SO4 at pH 2 and current density of 600 mA.cm-2 was used to electrodeposition of Mn-Co-CeO2 coating. Oxidation resistance of manganese-cobalt-CeO2 coated and uncoated samples was studied at 800 ºC for 100 h. The corrosion resistance was remarkably reduced by the application of coating layer. The bare substrate had a weight change of 0.407 mg.cm-2 after 100 h of isothermal oxidation, while 14th Annual Electrochemistry Seminar of Iran Materials and Energy Research Center (MERC), 12- 13 Dec, 2018 163 the coated samples had a weight change of 0.181 mg.cm-2. The coating effectively protected the substrate from the corrosion at high temperatures. In order to measure the oxidation activation energy of Mn-Co-CeO2 coating, uncoated and coated samples were subjected to isothermal at 800 °C, 900 °C and 1000 °C for 20 h. [4]. The parabolic rate constants (kp), can be fit into an Arrhenius expression to determine the activation energies for the oxide formation process on each of the samples (Fig. 1). Activation energies were calculated 118 and 49 kJ mol−1h for uncoated and Mn-Co-CeO2 coated specimens, respectively. The lower activation energy for the Mn-Co-CeO2-coated samples would imply that oxide formation is easier during isothermal oxidation. This is an interesting result, considering the fact that the oxides on the Mn-Co-CeO2 coated samples are thinner than the oxides on the uncoated AISI 430 samples.