Two Co-alloy-coated steels with 1.0 and 3.0 wt.% Mn content were subjected to 1000 h of thermal exposure (750°C) in air. The coated alloys displayed an increased oxidation rate as the Mn content varied between 1.0 and 3.0 wt.% and Co coating was thermally converted into a three-layer structure composed of Cr-rich oxide (inner layer), mixed oxide with Co, Fe, Mn and Cr (middle layer), and doped Co3O4 spinel oxide (outer layer). Regardless of the Mn content, only a small amount of Mn was incorporated in the Co3O4 spinel. The area specific resistance for the Co-coated Fe-Cr containing 1.0 wt.% Mn was as low as 5.37 mΩ cm2 subsequent to oxidation (lasting for 1000 h). Compared to the cathode with the uncoated alloy present, the polarization resistance of the cathode in the presence of the Co-coated alloy increased very slowly under the conditions of open circuit potential set as 750°C and 200 h, and Cr deposition on the cathode surface declined noticeably. This showed that the Co coating effectively inhibited outward diffusion of Cr to poison the cathode during long-term exposure.