Chromium vaporisation from Fe,Cr base alloys used as interconnect in fuel cells

Abstract

Fe,Cr base alloys protected by Cr2O3 base oxidation scales are candidate materials for the metallic interconnect of solid oxide fuel cells (SOFC). The operating temperatures of such cells range between 800 and 950 °C. Cr2O3 base oxide scales are necessary since they show sufficient electrical conductivity unlike Al2O3 or SiO2 scales. It is, however, disadvantageous that Cr2O3 base oxide scales form the volatile chromium(VI) species CrO3(g) and CrO2(OH)2(g) under operating conditions at the cathode side of SOFC. The electrochemical reduction of these species forming solid Cr(III) oxides at the three‐phase boundary electrolyte/cathode/oxidant leads to a rapid degradation of the electrical properties of SOFC. The study of the chromium vaporisation of different Fe,Cr base alloys under SOFC operating conditions and its reduction by the coating of the alloy surface are, therefore, of topical interest in SOFC development. The commercial alloys Fe18Cr1Al (DIN 1.4742), Fe25Cr0.7Mn0.5Si (AISI 446), Fe20.4Cr5.7A10.3Si (DIN 1.4767) and the oxide dispersion strengthened (ODS) alloy Cr5Fe1Y2O3 as well as the model alloy FeCrMn (HNA) were investigated. These alloys form the following oxide scales under operating conditions on the cathode side: iron rich chromium oxides (DIN 1.4742), Cr,Mn spinel (AISI 446 and HNA), Al2O3 (DIN 1.4767), and Cr2O3 (Cr5Fe1Y2O3). The alloys DIN 1.4742 and Cr5Fe1Y2O3 were coated with a perovskite layer (25 to 30 μm thickness) made of La0.90Sr0.10CrO3 (LSC) and La0.8Sr0.2MnO3 (LSM) by the use of vacuum plasma spraying (VPS). The vaporisation studies were carried out under non‐equilibrium conditions using the vapour transportation method. The carrier gas consisted of synthetic air with a relative humidity of rH = 60%. Alloy plates of the dimensions 80 · 30 · 5 mm3 with rounded edges and a surface area of 48.5 cm2 were used as samples in the vaporisation experiments carried out at 850 and 950 °C. Typical time periods of the vaporisation measurements were between 20 and 350 h. Alloy Cr5Fe1Y2O3 with Cr2O3 scale showed the highest chromium vaporisation rate among the uncoated samples. The latter showed the following factors for the reduction of the chromium vaporisation rate as compared to Cr5Fe1Y2O3 at 850 °C: 23 for AISI 446, 10 for HNA, 5 for DIN 1.4767, and 2 for DIN 1.4742. The different factors are explained by the different oxide scales mentioned above. The alloys with VPS coatings showed a reduction of chromium release by up to a factor of > 100. Investigations of the microstrucure of the perovskite coating revealed its densification during the transpiration experiments which in turn reduces the chromium vaporisation. Moreover, the Cr2O3 vaporisation was re‐determined under equilibrium conditions by the vapour transportation method leading to new results.