The corrosion of carbon steel in contact with synthetic calcareous natural waters containing sulphate ions was investigated. Since the diffusion process is a determining factor in the corrosion kinetics through the reduction of dissolved oxygen, use was made of a rotating disc electrode. The development of the open circuit corrosion potential showed two distinct time domains indicating two corrosion processes: one at the bare electrode and another at the accumulation of corrosion products. Electrochemical impedance spectroscopy measurements at the open circuit potential allowed study of three main phenomena relating to the corrosion of mild steel and the influence of sulphate ions: (i) the formation of a poorly protective surface layer, the double layer capacitance indicating the accumulation of corrosion products; (ii) the charge transfer resistance, allowing evaluation of the corrosion rate; and (iii) mass transport processes through the hydrodynamic layer in the vicinity of the electrode surface, and through the corrosion products.