Ferritic stainless steel interconnectors are widely used in solid oxide fuel cells (SOFC) due to a combination of low cost, compatible thermal expansion properties and ease of manufacturing. However, their viability is hindered by some key technical hurdles. Most stainless steels suggested for SOFC applications rely on the formation of a fairly protective Cr₂O₃ scale. However, in air side environments Cr species evaporation leads to material failure and insufficient lifetimes. Coatings offer a way to mitigate Cr evaporation and in some cases decrease oxide scale growth. Numerous research papers have investigated different coating compositions and deposition techniques, however, the most commonly suggested material is (Mn,Co)₃O₄ (MCO). Instead of depositing MCO metallic Co can be applied. During operation Co is converted into Co₃O₄ and subsequently enriched in Mn from the underlying steel substrate. It has been shown that this type of coating effectively impedes Cr volatilization, even when only very thin layers (600 nm) are applied. Moreover the major advantage of metallic Co coatings are cost savings, since Co can be applied in a large scale roll-to-roll process before stamping the interconnect into its final shape. Furthermore, our previous work has shown that an additional layer of 10 nm Ce in combination with the Co coating results in the aforementioned mitigation of volatilization of Cr species, and also effectively reduces oxide scale growth. Oxide scale growth is detrimental for two reasons. First, a depletion of oxide scale forming element (Cr) can result in the formation of poorly protective iron oxide. Second, even a protective Cr₂O₃ scale grows with time. Since Cr₂O₃ is only a moderate electronic conductor this results in an increase of Area Specific Resistance (ASR) as the oxide scale grows in thickness. This eventually creates prohibitively high resistances. Both effects can be mitigated by addition of a thin Ce layer.

In the present work Ce/Co (10/600 nm) coated AISI 441 has been exposed for 4 years (35 000 h) at 800 °C. The mass gain of the samples over the course of exposure has been recorded at selected intervals. After 35 000 h a mass gain of 3.76 mg/cm² has been measured. Over the entire exposure parabolic mass gain kinetics are observed indicating the absence of a poorly protective iron oxide. After 35 000 h the samples were moved to a different setup to measure Cr(VI) evaporation. It was found that despite the fact that the coating was only 600 nm in thickness, it effectively inhibits volatilization of Cr even after 4 years of exposure. Furthermore, samples were removed at selected intervals to measure ASR.