Anomalous oxide scale formation under exposure of sodium containing gases for solid oxide fuel cell alloy interconnects

Reactivity of oxide scale on Fe-Cr alloy with Na-containing gases was examined to estimate the stability against sodium (Na): vapors of NaCl and Na₂SO₄ exposures with air flow at 1073 K. The identified reaction phases were Cr-Mn spinel, Cr₂O₃, and alloy from the X-ray diffraction of surface with no Na-reaction products. However, the protective oxide scales (Mn-Cr spinel and Cr₂O₃ layers) on the Fe-Cr alloy were partially decomposed by reacting with Na to form Na-compounds inside the oxide scale/alloy interfaces. In some parts, anomalous oxide scales were found around the oxide scale/Fe-Cr alloy interfaces, with forming Na-rich compounds: the compounds were distributed inner parts of oxide scales around oxide scale/alloy interfaces. The stability of oxide scales and degradation were discussed based on the observed distribution of elements.