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 alpha-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 alpha-Al2O3 is decreased. The metastable gamma-Al2O3 and transition alpha-(Al0.948Cr0.052)(2)O-3 phases were formed in the earlier oxidation process and finally transformed to the stable alpha-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 alpha-Al2O3. (C) 2015 Elsevier B.V. All rights reserved.