High-resolution characterization reveals the role of Al content in the evolution of oxide scales formed on alumina-forming alloy exposed to supercritical water

Abstract

The oxidation behavior of alumina-forming austenitic (AFA) stainless steels exposed to 600 °C supercritical water (SCW) was studied via several high-resolution characterization techniques. The addition of 1.5–3.5 wt% aluminum (Al) significantly affected the microstructure of AFA alloys compared with Al-free alloys, with the formation and re-distribution of precipitates, including Laves, B2-NiAl, and δ-ferrite. After exposure to SCW, a typical duplex oxide scale was formed on the surface, and the oxide thickness decreased with increasing Al content, which was ascribed to the formation of a protective inner oxide layer. The threshold Al content was determined to be 3.5 wt%, as only the AFA alloy with a 3.5 wt% Al addition formed a distinct alumina scale and exhibited the lowest weight gain. The relationship between the Al content, precipitates, and mechanism of oxide film formation was revealed. In the AFA alloys, the volume fraction of B2-NiAl increased with increasing Al content, resulting in a more continuous alumina film formation. The addition of Al promoted the spheroidization of the needle-like Laves phase, leading to the formation of a protective Si-containing oxide film. Additionally, the higher content of Al addition also favored the formation of the δ-ferrite phase, which formed a slightly thicker but uneven alumina film after exposure to SCW.