Behaviour of ferritic stainless steels subjected to dry biogas atmospheres at high temperatures

International audienceThe 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 (CH(4) + CO(2) 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 degrees C and that the formation of solid carbon may take place in several conditions. XRD results confirmed the formation of Cr(2)O(3) and Mn-Cr spinel, with a mixture of internal carbides. In this temperature range, kinetic experiments showed linear mass change. Comparing with the linear rate constants of 441 oxidised in pure CO(2), corrosion in biogas was larger and increased with increasing the methane content in the biogas. The surface morphology of the corroded specimens showed a dense oxide scale at temperatures less than 800 degrees C, serving as an efficient barrier to carbon penetration. However, when the temperature reaches 900 degrees C, cracks and pores appear in the oxide scale, carbon can precipitate and diffuse easier than at 800 degrees C and may lead to internal carbide formation. In such biogas atmospheres, 800 degrees C seems the maximum operating temperature of devices containing this ferritic stainless steel