(Invited) Role of Transient Oxide Scale for Development and Growth of Al2O3
Authors: Hayashi Shigenari , Yoneda Suzue 
Published: February 2020

It is well-known that Cr addition to Al2O3 forming alloys and coatings are beneficial to decrease the critical Al content for an external Al2O3 scale formation. This beneficial Cr effect is known as a Third Element Effect, TEE. The TEE explains that initial Cr2O3 scale formation in the transient oxidation period reduces the oxygen potential at the surface of alloy, promoting the exclusive Al2O3 scale formation. However, our previous study of oxidation of Fe-low Cr-Al alloys found that an Al2O3 scale was formed on alloys with lower Al content regardless initial Cr2O3 scale formation, which suggests that decreased oxygen potential due to initial Cr2O3 scale formation is not the essential reason for decreasing the critical Al content for an external Al2O3 scale formation. In this study, we investigated the initial oxidation behavior of Fe-(6~10)Al, Fe-(4~24)Cr, and Fe-(4~24)Cr-(6~10)Al (in at.%) model alloys and tried to understand the role of not only Cr but also Fe and Al on development of the Al2O3 scale. Particular attention was paid for the effect of the initial transient oxide scale on development of an external Al2O3 scale.
The formation of initially formed transient oxide(s) and its transition to α-Al2O3 during heating, followed by isothermal oxidation up to 10~60min at 1000°C were characterized by in-situ high-temperature X-ray diffraction by means of Synchrotron radiation. The oxide scale formed at different temperatures during heating was observed by TEM. The distribution of each element in the oxide scale was measured by STEM-EDS. The oxide scale initially observed on all alloys by XRD was α-Fe2O3. The transition from initial transient oxide, Fe2O3, to Cr-rich Cr2O3-Fe2O3 solid-solution during heating on the alloys with higher Cr content was confirmed by an in-situ high-temperature XRD experiments. Beneath this transient oxide scale, Al-rich amorphous oxide scale was found to develop on the alloys with Al addition. No Cr effect was observed in terms of the amorphous layer formation. This Al-rich amorphous layer prevented the growth of outer Fe-rich transient oxide. Therefore, the oxidation mass gain of alloys with Al at relatively lower temperatures, ~700°C was much smaller than that of binary Fe-Cr alloys. Break down of the amorphous layer resulted in rapid oxidation of alloys. The time for break down of the amorphous layer became longer with increasing alloy Cr content, which indicates that Cr helped to maintain the Al-rich amorphous layer for a longer oxidation time at lower temperatures, ~700°C. The amorphous layer crystalized to α-Al2O3 on the alloys with higher Al content, when the sample temperature became about 900 to 1000°C. The crystallization to α-Al2O3 occurred continuously from the interface between an outer transient/amorphous layers to the amorphous/alloy interface. However, on the alloy with lower Al content, amorphous internal Al2O3 precipitates were developed and the amorphous layer disappeared accompanied by formation of internal precipitates. Although the volume fraction of internal precipitates increased with increasing alloy Cr content, higher Cr addition did not prevent internal oxidation of Al. During heating, amorphous internal precipitates transformed to α-Al2O3. After the transformation to α-Al2O3, the internal precipitate grew larger and connected each other, forming an α-Al2O3 scale below the transient oxide scale. No apparent effect of the transient oxide scale on formation of the amorphous internal oxide and its transition to an α-Al2O3 layer was observed on alloys with lower Al content.
The Al-rich amorphous layer was found to have important role for preventing Fe-rich oxide scale formation for all alloys with Al. The Fe- or Cr-rich transient oxide, which formed in the initial oxidation stage acted as the template for the transformation from amorphous to α-Al2O3 for the alloys with higher Al content, but it did not have apparent role for formation of the α-Al2O3 scale on the alloys with lower Al content.