Metastable Al 2 O 3 scale formation on NiCoCrAl coated layers was investigated during initial-stage reaction for up to 100 h exposure at 800 °C in air. NiCoCrAl was diffusion-coated onto low carbon steel by electroplating a NiCo layer followed by pack-cementation deposition of Cr and Al. The microstructural characterization of as-coated and oxidized samples was carried out at the micro- and nano-scales. The XRD and diffraction pattern in TEM analysis revealed the presence of metastable Al 2 O 3 , spinel NiAl 2 O 4 and cubic (Cr,Al) 2 O 3 in the oxide. The phase identification using XRD and electron diffraction analysis associated with the coating composition was plotted on an isothermal oxidation map.