Influence of Temperature and Water Vapour on the High-Temperature Oxidation and Cr Evaporation Behaviour of High-Cr Alloys for SOFC Cathode Air Pre-Heaters

Abstract

Chromium evaporation and oxide scale growth are among the key degradation mechanisms associated with Solid Oxide Fuel Cells (SOFCs). It is often underestimated how the cathode air pre-heater (CAPH) made of high Cr-containing alloys in SOFC systems can contribute to cathode degradation. This study examines the effect of exposure temperature and water content on both degradation mechanisms in Inconel 625, SS309 and AluChrom 318. For the influence of water content, samples of Inconel 625, SS309 and AluChrom 318 were isothermally exposed at 850°C in dry air and air containing 1%, 3% and 9% of H2O at a high flow rate for 168 hours. For the influence of temperature, samples of Inconel 625, SS309 and AluChrom 318 were isothermally exposed at 650°C, 750°C and 850°C in a 6.0 L/min air stream containing 3% H2O for 168 hours. The evaporated Cr (VI) species were collected using the denuder technique. The formed oxide scales were subsequently analysed using XRD and SEM/EDX. The results of the surface characterisation indicated that the oxide scales formed on Inconel 625, SS309 and AluChrom 318 at high temperatures were Cr2O3, (Cr,Mn)3O4 spinel and Al2O3, respectively.The results of this study show that Cr evaporation and oxidation rates were dramatically reduced at the lower temperature used for Inconel 625 and SS309. AluChrom 318 exhibited an increased oxidation rate with increasing temperature, but it demonstrated a reverse trend to the temperature dependent Cr evaporation compared to Inconel 625 and SS309. It was also found that the amount of Cr evaporated from an alloy solely depended on the partial pressure of Cr presented on the oxide surface. At 850°C, the total mass of evaporated Cr(VI) species was found as follows: Cr2O3 (Inconel 625) > (Cr,Mn)3O4 spinel (SS309) >> Al2O3 (AluChrom 318). However, at 650 and 750°C, the total mass of evaporated Cr(VI) species was found to be: (Cr,Mn)3O4 spinel (SS309) > Cr2O3 (Inconel 625) > > Al2O3 (AluChrom 318). The significant effect of water vapour on the three tested materials appeared to be the further enhancement of Cr2O3 evaporation. At 850°C, the amount of evaporated Cr2O3 from the alloys exposed at the same humidity level corresponded to the Cr partial pressure on the surface, as follows: Cr2O3 (Inconel 625) > (Cr,Mn)3O4 spinel (SS309) > Al2O3 (AluChrom 318). The water vapour slightly increased the growth rate of Cr2O3 and (Cr,Mn)3O4 spinel formed on the Inconel 625 and SS309 surface, respectively. However, no evidence of change in the growth rate of the alumina on AluChrom 318 under the influence of water vapour in oxidising atmosphere was observed for the tested period in this study. Figure 1