Inter-diffusion between Co₃O₄ coatings and the oxide scale on Fe-22Cr

The oxidation behaviour of a commercially available Fe-22Cr alloy coated with a Co3O4 layer by spray-painting or plasma-spraying was investigated at 1173K in air with 1% H2O and compared to the oxidation behaviour of the non-coated alloy. The oxide morphology was examined with X-ray diffraction, electron microscopy, and energy dispersive X-ray spectroscopy. Cr2O3 developed in-between the Co3O4 coating and the alloy, whilst alloying elements of the substrate were incorporated in the coating, regardless of the deposition method. The coatings reduced the growth rate of the Cr2O3 layer as well as the evolution in contact resistance with time between two Fe-22Cr plates sandwiched around a Pt-mesh. SiO2 developed as particles within the alloy during oxidation of the Co3O4 spray-painted samples, whereas SiO2 was identified as an interfacial layer between Cr2O3 and the alloy after oxidation of the Co3O4 plasma-sprayed and the non-coated samples. The difference in morphology is suggested to be an effect of SiO2 nucleation assisted by Kirkendall void formation.