Electrochemical impedance spectroscopy (EIS) and polarization curve techniques were used to study the early stages of hydrogen sulfide (H2S) corrosion. Results revealed the formation of different iron sulfide compounds during the early stages of corrosion. Meanwhile, Warburg impedance characteristics were determined through EIS. Numerous flake-like mackinawites were observed after 15 h. The diffusion-controlled mechanism gradually disappeared during the process. After immersion for 15 h to 7 d in aqueous H2S, the flake-like mackinawites grew larger and adhered, which increased the thickness of corrosion product films. The EIS and polarization curve tests proved that the increase in thickness of corrosion product films enhances corrosion resistance. (C) 2014 Elsevier B.V. All rights reserved.