Effect of Fe and partial pressure of oxygen on the formation and phase transformation behavior of Al₂O₃ scale

Abstract

The effect of oxygen partial pressure on the phase transformation of Al2O3 scale on various Fe–Al alloys with and without very thin (∼100 nm) Fe coating was investigated. Fe‐coating on Fe–Al alloys can effectively suppress metastable Al2O3 formation, but little effect was observed when the samples were oxidized in a low partial pressure of oxygen. Under the low ${\rm P}_{{\rm O}_{{\rm 2}} } $ atmosphere, metastable to stable α‐Al2O3 scale phase transformation on both Fe‐coated and non‐coated Fe–Al alloys was significantly delayed. The lattice spacing of α‐Al2O3 scale formed in air decreased with increasing alloy Al content. Further decrease in the lattice spacing of α‐Al2O3 scale was observed when the alloy was oxidized in low ${\rm P}_{{\rm O}_{{\rm 2}} } $. The results obtained clearly indicated that the formation of Fe2O3 or Fe3+ in metastable Al2O3 accelerated the metastable to stable α‐Al2O3 scale transformation.

Transition metal attenuated mechanism for protective alumina formation from first principles

“…Intermediate is the transformation from the mentioned initial transient oxide to the one that satises said local equilibrium conditions. 7,[14][15][16][17][18][19][20][21][22] The properties of the transient oxide are indeed crucial for the alumina forming process, partial control of which may be gained by the so-called 3rd element effect. As rst observed in the Cu-Zn-Al system, 23 the 3rd element effect is understood to initiate and maintain formation of a protective a-Al 2 O 3 scale.…”

The Transition from Transient Oxide to Protective Al2O3 Scale on Fe–Cr–Al Alloys During Heating to 1000 °C

“…The in situ high-temperature X-ray diffraction measurement clearly showed that the a-Al 2 O 3 , formed initially on alloys with higher Al content, had larger d-spacing and FWHM, because the a-Al 2 O 3 contained higher Fe and/or Cr contents. This Al 2 O 3 , which has larger d-spacing, was reported on Fe-high Al binary alloys [10,11]. Cr 2 O 3 and Al 2 O 3 form a solid-solution [12], but solubility of Fe 2 O 3 in Al 2 O 3 is relatively small, 5.3 mol% at 1000°C [13], thus the d-spacing of initially formed a-Al 2 O 3 on higher Cr alloys could be larger than that formed on a lower Cr alloy.…”

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

“…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]. We also found that the transformation to a-Al 2 O 3 was promoted on binary Fe-Al alloys with higher Fe content [17]. P. Brito et al also found same beneficial effect of Fe on promoting a-Al 2 O 3 scale formation on the binary Fe-Al alloys at lower temperature, 700°C [18].…”

“…In our previous study, we used binary Fe-Al alloys with different Al contents [17] (i.e., different Fe contents), and we found initial Fe 2 O 3 formation prior to development of an a-Al 2 O 3 scale on the alloys with lower Al (higher Fe). The peak positions of the initially formed Fe 2 O 3 moved toward higher-2h angle during heating.…”