The corrosion behavior of the 18Ni 300-grade maraging steel in 0.6 mol L-1 NaCl solution, containing 1 mmol L-1 H2S, and saturated with CO2, was investigated by measurement of the open circuit potential, potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The maraging steel was solution-treated at 840 degrees C for 1 h and aged at 650 degrees C for 3 h. All corrosion electrochemical tests proved that the austenitic phase formed during the aging thermal treatment was detrimental to corrosion resistance of the 18Ni 300-grade maraging steel. The analysis of the XPS spectra suggested that FeCO3, Fe2O3, FeS2, FeSO4, NiO, Ni(OH)(2), NiS, NiSO4, MoO2, MoO3 and MoS2 were the corrosion products deposited on the surface of both maraging steel samples after 29 days of immersion in the testing solution. Lastly, for industrial application requiring the corrosion resistance property of the 18Ni 300 grade maraging steel, it is recommended that it be thermal solution-treated before its use. (C) 2018 Elsevier Ltd. All rights reserved.