Reactivity of ferritic stainless steels (AISI441 and CROFER22APU) used as electric interconnectors in biogas-fuelled solid oxide fuel cells

Abstract

The objective of this study is to understand the high temperature corrosion behaviour of the ferritic stainless steel type AISI 441 (18CrTiNb), a candidate for SOFC interconnectors, under dry synthetic fermentation biogas (CH4 + CO2 mixtures) possibly used at the anode side of the cell. Thermodynamic analysis showed that, in such mixtures, the partial pressure of oxygen lies in the range of 10–23 to 10–20 bar for temperature between 700 and 900°C and that the formation of solid carbon may take place in several conditions. Kinetic experiments showed linear mass change at temperatures between 700 and 900°C. The modeled kinetics was discussed in this study. In this temperature range, kinetic experiments showed linear mass change. Comparing with the linear rate constants of 441 oxidised in pure CO2, corrosion in biogas is larger and increases with increasing the methane content in the dry biogas. XRD results confirmed the formation of Cr2O3 and Mn-Cr spinel, with a mixture of internal carbides. The surface morphology of the corroded specimens showed a dense oxide scale at temperatures less than 800°C, serving as an efficient barrier to carbon penetration. However, when the temperature reaches 900°C, cracks and pores appear in the oxide scale, carbon can precipitate and diffuse easier than at 800°C and may lead to internal carbide formation. In such biogas atmospheres, 800°C seems the maximum operating temperature of devices containing this ferritic stainless steel.