Stability of External alpha-Al2O3 Scales on Alloy 602 CA at 1100-1200 A degrees C

Journal article, 2018

An external ultrathin alpha-Al2O3 scale grown on the Ni-base alloy 602 CA during air oxidation at 800 A degrees C was characterized by means of high-resolution TEM/EDX and electron diffraction. Alloy samples pre-oxidized at 800 A degrees C were subsequently exposed at 1100, 1150 and 1200 A degrees C for up to 100 h. Whereas the external alumina remained stable at 1100 A degrees C, with the increasing exposure temperature, the pre-grown alumina scale tended to break down resulting in an external chromia scale accompanied by internal alumina precipitation. The transition from external to internal Al oxidation was investigated using SEM/EDX/EBSD. The critical Al depletion at the scale-alloy interface during the post-exposure at 1100-1200 A degrees C was modeled using the CALPHAD-based thermodynamic-kinetic approach.