Role of Nb in promoting protective scale formation on Al2O3-forming austenitic alloys

The high-temperature oxidation behavior of Fe–25Ni–17Cr–7.7Al–0.03Zr alloys with and without 0.5 or 1 at% Nb addition was investigated at 800 °C in air. Nb addition was found to promote the formation of an external Al ₂O ₃ scale. The Nb-free alloy formed a scale consisting of a thick Fe/Ni-rich oxide layer and a Cr ₂O ₃ layer, beneath which Al was internally oxidized. However, a continuous protective Al ₂O ₃ scale was developed below a thin Cr ₂O ₃ layer on the 1 Nb alloy, which significantly decreased the oxidation rate. Examination of the initial oxidation process during heating to 800 °C revealed that all of the alloys formed a Cr ₂O ₃ layer and an amorphous Al ₂O ₃ layer upon heating to 650 °C, but these layers became discontinuous upon further heating to 800 °C for the Nb-free alloy. The beneficial role of Nb was ascribed to its ability to maintain the initially formed thin Cr ₂O ₃ layer by the formation of a Nb-containing Cr-rich oxide layer at the interface between the Cr ₂O ₃ and amorphous Al ₂O ₃ layers. This Nb-containing Cr-rich layer prevented the breakdown of the transient amorphous Al ₂O ₃ layer until its transition to a protective crystalline Al ₂O ₃ scale.