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Ferritic stainless steels have nowadays been used as materials for interconnectors in solid oxide fuel cells (SOFCs) at intermediate temperatures (800°C). Their degradation in contact with dry synthetic biogas used instead of other fuel gas has already been studied. In such biogas atmosphere, humidity may play an important role. The objective of this study is therefore to understand the effect of H2O on the corrosion kinetics of the ferritic stainless steels type AISI441 (18CrTiNb) under synthetic biogas (70%CH4 and 30%CO2) mixed with 3%H2O. The thermodynamic analysis by FactSage was used to determine the partial pressure of oxygen and the activity of carbon in the humid biogas. The results showed that the partial pressure of oxygen is in the range 10–24.8 to 10–21.2 bar for temperatures between 600-800°C and that the formation of solid carbon can occur in these conditions. This was not different compared with the conditions in dry biogas. These conditions lead to the stability of some important oxides such as Cr2O3 and Cr-Mn spinel and to carbon deposition and/or carbide formation. The surface morphology of 441 subjected to humid biogas showed oxide scale composed mainly of Cr2O3 topped with Cr-Mn spinel. Some carbide such as Cr7C3 was found in chromia scale. Kinetic experiments under both dry and humid biogas at temperatures between 600 and 800°C showed linear weight changes. Arrhenius law was followed and the rate-determining steps were identified as parallel oxidation and carburization limited by oxide-gas interface reactions.

The tensile test, accompanied by the corresponding theoretical model, has been developed to quantify the mechanical adhesion energy of the oxide scale on metallic substrate in our previous works. The method to quantify the adhesion energy took into account the effect of residual stress. The effect of the variation of the measured residual stress on the quantified adhesion energy is assessed in this paper. For the scales failed at strains initiating the spallation of 0.018 and 0.011 followed by the transverse crack, it was found that the quantified adhesion energy of the oxide is not sensitive to the variation of the residual stress measured in the range from 0.5 to 2.0 GPa. This is due to the compensation of the decrease in stored energy due to the stress applied in the loading direction (x direction) and the increase in stored energy due to the stress applied in the direction perpendicular to the loading direction (y direction) when the residual stress increases. For the scale failed by the transverse crack followed by the spallation, the quantified adhesion energy tends to be sensitive to the variation of the measured residual stress. The assumption of energy relaxation during the tensile test is alternatively proposed. It is assumed that the energy stored due to the stress in x and y directions is totally released at the first crack. The energy stored due to the stress in y direction from strain initiating the crack to strain initiating the spallation is used in the quantification of the adhesion energy. The scatter of the adhesion energy values quantified by this method and those measured by the inverted-blister test is reduced comparing to the results reported in the previous work.