Steam oxidation of aluminide coatings under high pressure and for long exposures

Abstract

B2-FeAl-based aluminide coatings were elaborated on P92 ferritic-martensitic steels by slurry. Under atmospheric pressure of steal and 650°C, a thin protective α-Al 2 O 3 oxide layer formed and was maintained until 5000 h. With increasing pressure to 300 bar, the morphology of α-Al 2 O 3 changed markedly. Iron oxide clusters started to appear at the surface of the scale after 5000 h of exposure and kept on growing till 10000 h. It appears that pressure increased the outward diffusion of Fe through the coating and through the oxide scale and fostered the propagation of the tensile cracks of the coating.

Oxidation Behavior of Fe–Al, Fe–Si and Fe–Al–Si Intermetallics

“…Despite these problems, we formulated a theory of how the silicon influences the oxidation. Aluminum oxide is the main product of oxidation of Fe–Al-based alloys, as confirmed by numerous research works [5,10,11,12,13]. Therefore, aluminum diffuses to the surface during oxidation and, hence, the material below the oxide layer becomes increasingly depleted by aluminum.…”

Effect of Total Pressure and Furnace Tube Material on the Oxidation of T22 in Humidified Air

“…In some of the mentioned work, an increase in oxide scale growth at elevated pressure was found [17,24,27,28], while other studies found similar or enhanced oxide scale growth at atmospheric pressure [23,25,26]. A possible explanation for enhanced scale growth at elevated pressure is an increase of diffusion rates in the oxide and alloy [24,28,31]. However, when the stresses caused by the applied pressure are high enough to induce lattice defects such as dislocations, simple compression of the crystal lattice reduces the diffusion rates in most metals [32].…”

Scale Formation and Degradation of Diffusion Coatings Deposited on 9% Cr Steel in Molten Solar Salt

“…In addition, despite the coating manufacturing process conducted in a reducing atmosphere (Ar + 5 vol.% H 2 ), one could expect the formation of a native Al 2 O 3 scale on the surface of Al-rich intermetallic phases (i.e., FeAl 2 , Fe 2 Al 5 as well as Cr 5 Al 8 for the Cr/Al-coating) considering the coating temperature around 1000 • C (see Table 2). The beneficial influence of the pre-oxidation of aluminide coatings upon aluminizing on the growth and sustainment of α-Al 2 O 3 scales has been reported by Boulesteix et al in steam exposure [49]. Furthermore, as aforementioned alumina particles can be incorporated into the coating due to the in-pack cementation process, as highlighted in [30].…”

“…A large fraction of the surface, appearing dark in the secondary electron image (representatively marked as spot 1), is covered with a very thin oxide scale (see Figure 9). The corresponding Raman spectrum (Figure 11c) exhibited the characteristic fluorescence doublet of α-Al 2 O 3 at a frequency around 1400 cm −1 [49]. Local protrusions, appearing bright in the secondary electron image (representatively marked as spot 2), were also observed on the surface.…”