Oxidation-Driven Carbon Expulsion and Interfacial Carbon Accumulation in an Iron Carbide Thin Film as Relevant in Passivated Catalysts and Carbon Steels

during oxidation, as the total C 1s intensity is conserved while the speciation changes entirely. Oxygen arriving at the surface displaces surface carbide carbon into the subsurface iron layers; continued oxidation ultimately expels it to the buried Fe/Cu interface, where graphitic C-C bonding develops. DFT shows that coadsorbed oxygen lowers the surface-to-subsurface carbon migration barrier from 1.65 to 0.9 eV, a roughly 7-order-of-magnitude rate enhancement at 550 K. These results connect oxide formation, carbon expulsion, and interfacial carbon accumulation in a single mechanistic framework for iron carbide oxidation.