Acceleration of Metastable to Alpha Transformation of Al2O3 Scale on Fe–Al Alloy by Pure-Metal Coatings at 900 °C

Abstract

The effect of pure-metal coatings on the transformation of a pre-formed h-Al 2 O 3 scale on an Fe-50Al alloy was investigated. The elements Fe, Cr, Ti, and Ni were deposited on the pre-formed Al 2 O 3 having a thickness of about 50 nm. Ti, Cr or Fe coatings on pre-formed h-Al 2 O 3 were found to accelerate the transformation to a-Al 2 O 3 , while no apparent effect was observed from the Ni coating. Since the corundum crystal structure of Ti 2 O 3 , Cr 2 O 3 and Fe 2 O 3 was indentified on Ti, Cr or Fe coated samples and the transformation to a-Al 2 O 3 started at the outer part of the h-Al 2 O 3 scale, these corundum oxides were considered to act as heterogeneous nucleation sites for a-Al 2 O 3 , therefore promoting the transformation from h-Al 2 O 3 to a-Al 2 O 3 . The growth rate of a-Al 2 O 3 grains and the rate of advance of the h-Al 2 O 3 /a-Al 2 O 3 interface were found to be faster with the Fe coating.

Preparation technique and alloying effect of aluminide coatings as tritium permeation barriers: A review

“…On the other hand, the formation, topography, compactness and crystal type of the Al 2 O 3 film formed on the FeAl layer depend on the alloying elements. Just as argued by Kitajima et al [78], Ti, Fe and Cr could promote the transformation from q-Al 2 O 3 to a-Al 2 O 3 on Fee50Al alloys because their oxides could act as heterogeneous nucleation sites for a-Al 2 O 3 ; while Ni almost had no apparent effect. Since the typical aluminide coatings prepared on steels are composed of a FeAl transition layer and an outer Al 2 O 3 film, it can be inferred from above statements that the alloying elements in the steel substrates must have some influences on the coatings.…”

“…Nevertheless, Fe atoms and other doped alloying elements are also likely to be oxidized to form the corresponding oxides. For example, apart from Al 2 O 3 , NiO and TiO 2 formed on the Ni and Ti predeposited FeAl surface after oxidizing at 900 C, respectively [78]. Zhan et al [82] also found that a little CeO 2 existed in the Al 2 O 3 film formed on CLAM steels with the addition of Ce during the PC aluminization.…”

Metastable-Stable Phase Transformation Behavior of Al2O3 Scale Formed on Fe–Ni–Al Alloys

“…Regarding the effect of Fe and Ni on the phase transformation of Al 2 O 3 scale, we found that a thin-Fe coating, *100 nm, completely suppressed the metastable Al 2 O 3 scale formation on Fe-and Ni-based alloys, but a Ni-coating delayed the transformation to a-Al 2 O 3 [14][15][16]. Initial formation of Fe 2 O 3 , which is isomorphous structure with a-Al 2 O 3 , promoted the transformation, but NiO formation retarded the transformation, which is likely due to the stabilization of metastable Al 2 O 3 [14].…”

Fabrication and Cyclic Oxidation of Y 2 O 3 /CeO 2 -Modified Low Temperature Aluminide Coatings

“…After being incorporated into the growing scale, Y or Ce ions from Y 2 O 3 /CeO 2 nanoparticles in the alumina scale segregate to the scale grain-boundaries and then transport outwards, driven by the oxygen potential gradient between the interfaces of the metal/scale and the scale/gas [16] . In this case, a different effect [3][4][5][6][7][8][9][14][15][16][17][18] on the - transformation occurred. In this experiment, Y significantly hinders the outward short-circuit diffusion of the Al for the growth of the -alumina, thus a lower oxidation rate with shorter -alumina whiskers being observed.…”