Live observation of the oxidation of coated interconnects with environmental electron microscopy

Coatings on SOFC interconnect steels are complex and challenging to investigate due to the high number of chemical elements which are present and interact with each other. In this work, advanced electron microscopy techniques are used to study the steel oxidation in its initial stage (first days), where scale is built up and first elemental diffusions and interactions may be clearly seen. First the surface of a coated steel piece is observed in an environmental scanning electron microscope (E-SEM) at high temperature (880°C) under a pressure of pure oxygen (40Pa) allowing a direct observation of its morphological evolution during oxidation for 48h (Fig.1 (a)-(c)). For post data treatment, images were segmented using a machine-learning plug-in available in ImageJ (Fig.1 (d)), and the size of the grains on the surface quantified as a function of time. After the oxidation experiment in the E-SEM, a focused ion beam (FIB) was used to extract a TEM lamella from the exact same zone as that observed during the in situ exposure (rectangle in Fig.1 (a)-(d)), targeting specific grains where a two-stage growth could be observed. This post-mortem TEM cross section observation (Fig. 1 (e)-(f)) allows correlations of the sub-surface microstructure and grains with the surface structures seen during the in situ oxidation process. The system studied so far concerns Ce-Co coated SSHT steel (Sandvik).