Effect of alloy composition on the oxide scale formation and electrical conductivity behavior of Co-plated ferritic alloys

Several ferritic alloys, including Crofer 22 APU, ZMG232L, SS430, and three model alloys were electrodeposited with a 5-μm Co layer, and then oxidized at 800 °C in air for 100 h. The phase structure, surface morphology, and cross-sectional features of the surface oxide layers formed on different substrates were characterized. An outer Co3O4 spinel layer and an inner Cr2O3 layer were achieved for all the studied alloys. Outward diffusion of Mn from the substrate was restricted to the Cr2O3 layer and no noticeable incorporation of Mn in the Co3O4 layer was detected, while inward diffusion of Co into the substrates was observed except for SS430. For the three model alloys, the alloying addition of La and Mn altered the scale growth behavior of the Co-plated substrates, La leading to a more adherent scale/metal interface and a combination of La and Mn leading to a thinner Cr2O3 layer. For the commercial alloys, the Cr2O3 layer was much thinner and the area-specific resistance was lower than that of the three model alloys. The scale area-specific resistance of the different alloys was found to be a linear function of the thickness of the Cr2O3 layer.