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

Abstract

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 20,000Pa at 1253K. A single, continuous Al2O3 scale with the maximum thickness of about 2000nm 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 180min. With the increase in oxygen partial pressure, the phase transformation time of α-Al2O3 is decreased. The metastable γ-Al2O3 and transition α-(Al0.948Cr0.052)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.