Understanding early-stage oxidation mechanism of Crofer 22 APU solid oxide fuel cell steel interconnect using glow discharge optical emission spectroscopy and grazing incidence X-ray diffraction

Abstract

Crofer 22 APU is used as metallic interconnects in stacking solid oxide fuel cells (SOFCs) operated at elevated temperatures (above 700 °C) owing to their excellent oxidation resistance. Understanding the protective layer formation in the initial oxidation stage would be useful for optimizing and designing protective coatings for extended life. Initial stage oxidation of Crofer 22 APU steel using surface analytical tools such as glow discharge optical emission spectroscopy (GD-OES), grazing incidence X-ray diffraction (GIXRD), Raman spectroscopy, and atomic force microscopy (AFM) are studied in the paper. An oxidation test on as-received Crofer 22 APU steel was carried out in a controlled atmosphere (0.01 Pa) in an in-situ high-temperature X-ray diffraction (XRD) stage at 950 °C. Normal XRD showed no indication of oxidation, while GIXRD revealed the formation of two-layer oxides: Top layer spinel MnCr<inf>2</inf>O<inf>4</inf> and fine-grained inner layer Cr<inf>2</inf>O<inf>3</inf>, which was confirmed and quantified by GD-OES depth profiling. The Cr<inf>2</inf>O<inf>3</inf> formed initially led to the formation of MnCr<inf>2</inf>O<inf>4</inf> spinel during the initial stage. The rapid diffusion of Mn through the fine-grained Cr<inf>2</inf>O<inf>3</inf> layer results in an increased growth rate of MnCr<inf>2</inf>O<inf>4</inf> spinel on the top of the fine-grained Cr<inf>2</inf>O<inf>3</inf> layer. © 2023 Elsevier B.V.