In this study, carbon nanotubes (CNTs) were formed on a high-carbon 25Cr35Ni–Nb cast alloy using a laboratory-scale experimental set-up that simulated favorable conditions for CNT growth in the presence of ethane and water vapor. After 45 min of exposure to the reactive atmosphere, the entire sample surface was covered with multiwalled nanotubes with an average diameter of approximately 50 nm, indicating the strong catalytic activity of the alloy surface. Transmission electron microscopy combined with energy-dispersive X-ray spectroscopy analyses revealed the presence of iron-containing Ni3C-based catalytic particles at the nanotubes tip. The origin of the nickel in the system was then investigated via cross-sectional observations and discussed. A mechanism similar to the so-called “ex-solution” process was proposed to explain the presence of catalytic particles, while their stability was attested by thermodynamic considerations. A scenario describing the different steps involved in the CNTs formation on the oxide scale is finally proposed.