High Temperature Oxidation Behaviour of MnCo2O4 Coating on Crofer 22 APU Manufactured by a Novel Solution Precursor Plasma Spray Process (SPPS)

Abstract

MnCo2O4 spinel coatings are designed to be used on metallic interconnectors in SOFC devises to decrease oxidation rate of the metallic interconnect and to prevent the evaporation of harmful CrO3 and Cr2(OH)2 compounds. These Cr-compounds degrade the long-term performance of the SOFC by migrating to the triple phase barrier (TPB) of the cathode and reduce back to Cr2O3. MnCo2O4 spinel coatings, used in this study, were manufactured by using a novel solution precursor plasma spray (SPPS) process and heat treated in oxidizing environment. Deionized water based solutions of Mn(NO3)2•4H2O and Co(NO3)2•6H2O were used as a feedstock material. Concentration of the metal cations in the solutions was adjusted to 3 M. Ferritic stainless grade Crofer 22 APU with the thickness of 0.5 mm and surface roughness of Ra < 0.5 μm was used as a substrate material. The coatings were manufactured using a Sulzer Metco A3000S plasma spray system with F4-MB plasma gun with modified solution feeding hardware. Coatings with different microstructures were sprayed using different spraying parameters, e.g. the type of plasma gases used. The as-sprayed coatings were aged at 700 °C for 500 h in oxidizing environment, in order to study the stability of the coating, the growth of the Cr-scale and the Cr-transport through the spinel coatings. The microstructural characterization for the as-sprayed and the oxidized coatings were done using a field-emission scanning electron microscopy (FESEM) with SE-mode. The quantitative analyses were executed with energy dispersive spectroscopy (EDS), and in addition X-ray diffraction (XRD) was used for qualitative studies. The coatings with various microstructures were sprayed. The densest microstructure was sprayed using Ar-He plasma gas. Also the crystallographic equivalence for MnCo2O4 was achieved when Ar-He plasma was used with 40 mm spraying distance. Ageing caused the increase in structural porosity. On the interface between the coating and the interconnect, a dense spinel layer was formed which effectively prevented the Cr-transport forming approximately 500 nm thick Cr-rich sub-scale.