The corrosion mechanism of the N10276 alloy before and after long-term aging treatment in aqueous solutions containing CO₂/Cl^- and CO₂/H₂S/Cl^- was studied through the electrochemical impedance plots and cyclic potentiodynamic polarization. Results reveal that a not-well-defined capacitive loop appears in the low-frequency range and it changes into Warburg's impedance as the H₂S concentration increases (10~100 μL/L) due to the large quantities of H₂S-related adsorbed species in the film. In addition, H₂S concentration plays a key role in increasing corrosion rate. In comparison, the action of long-term aging is more significant on pitting corrosion. The surrounding sites of the precipitates become the preferred corrosion area since the austenite microstructure of long-term aged N10276 alloy includes lots of precipitated second phases (μ phase with accumulation of Mo and Ni elements), leading to the occurrence of pitting defects.