Formation of α-alumina scales in the Fe–Al(Cr) diffusion coating on China low activation martensitic steel

To study the formation mechanism of stable α-Al₂O₃ scales, the oxidation behavior of Fe–Al(Cr) diffusion coating on China low activation martensitic steel has been investigated under the oxygen partial pressure ranging from 1 to 20,000 Pa at 1253 K. A single, continuous Al₂O₃ scale with the maximum thickness of about 2000 nm was formed on the Fe–Al(Cr) diffusion layer. The phase transformation of alumina scales on the surface of Fe–Al(Cr) layer was studied at different oxidation times ranging from 3 to 180 min. With the increase in oxygen partial pressure, the phase transformation time of α-Al₂O₃ is decreased. The metastable γ-Al₂O₃ and transition α-(Al_0.948Cr_0.052)₂O₃ phases were formed in the earlier oxidation process and finally transformed to the stable α-Al₂O₃ phase, which were detected by grazing incidence angle X-ray diffraction and confirmed by transmission electron microscopy. This implies that Cr shows the third element effect and serves as a template for the nucleation of the stable α-Al₂O₃