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The gas-phase and coal ash-related fireside corrosion behavior of a new type of Ni-Fe-Cr based superalloy GH984G and two other contrast alloys were investigated for its application under advanced ultra-supercritical (A-USC) conditions. Results showed that the oxidation activation energy of the three alloys at 600–780 °C in the flue gas environment can be ranked from large to small: C-HRA-1>GH984G>Alloy 625. In the coal ash environment, GH984G and C-HRA-1 exhibited satisfactory corrosion resistance, while Alloy 625 had weaker corrosion resistance due to the presence of molybdenum. The corrosion mechanism of GH984G and the influence of constituent elements were elucidated. • The oxidation kinetics of the tested alloys followed the Wagner law in the flue gas. • The oxidation activation energy could be ranked: C-HRA-1 >GH984G>Alloy 625. • Alloy 625 showed less corrosion resistance than GH984G and C-HRA-1 due to Mo. • Introduction of Fe might decrease diffusion activation energy of Cr in Ni lattice. • The corrosion mechanism of GH984G was elucidated. [ABSTRACT FROM AUTHOR]