Several SOFC metallic interconnect coatings are based on cobalt, which is efficient to prevent chromium evaporation and to maintain the contact resistance low after prolonged exposure. The addition of a very thin cerium layer beneath the Co coating seems to reinforce its properties. The oxidation of such coated SOFC FeCr steels, which contain a considerable number of alloy additives, becomes difficult to interpret.

In this study, surface oxidation of Sanergy SSHT steel with Ce-Co coating, frequently used as interconnect material, was first observed at high temperature (900°C) under an oxygen flow, in a modified environmental scanning electron microscope (ESEM), for different durations (0-48h). After these live observations, lamellas of the observed area were precisely cut using a focused ion beam (FIB). This allowed to evaluate the final composition of the oxidation layers of the steel by scanning transmission electron microscopy (STEM) and energy dispersive x-ray analysis (EDX), to try to link the chemistry to the structures observed during in-situ observation in the ESEM.

In-situ observation up to 48h was conducted. Results show the diffusion of manganese towards the cobalt coating to form a cobalt-manganese spinel oxide layer. Under this top layer, chromia scale forms. The very thin cerium layer does not prevent the diffusion of other elements from the steel. Niobium and titanium form oxidized precipitates in the steel just below the chromia scale.

Results from shorter exposure/observation times present a time-lapse picture of the initial oxidation for a more detailed understanding. Ex-situ experiments complement the results.