On the high-temperature oxidation behavior of a niobium-bearing high nickel-chromium alloy: microstructural evolution and implications on oxidation mechanisms

The oxidation behavior of a niobium-bearing high nickel-chromium alloy, widely used in the petrochemical industry, is systematically studied at 800 °C using a combination of transmission electron microscopy and atom-probe tomography experimental techniques and thermodynamic calculations. Oxidation leads to complex surface microstructures consisting of a protective chromia layer, an amorphous SiO2 (silica) layer, Fe- and Ni-rich metallic mushroom-like austenitic protrusions/precipitates, and sub-oxide voids with highly faceted surfaces. The thermodynamic equilibrium oxides of this alloy are studied as a function of oxygen activity using Thermo-Calc and compared to the observed microstructures. The growth kinetics of oxide layers are studied in detail.