Pipeline steels exhibit intergranular corrosion (IGC) and stress corrosion cracking at active dissolution potentials in carbonate-bicarbonate solutions. The evolution of electrochemical behavior of API X70 pipeline steel during active dissolution in 1 M NaHCO3 was investigated by electrochemical impedance spectroscopy (EIS). Electrochemical modeling of EIS revealed that the metal rate is limited by slow diffusion of CO3−2 ions through a porous precipitated corrosion product layer to the steel surface. Further, the porosity of the carbonate layer decreases over time as its thickness increases, both factors contributing to a strongly suppressed corrosion rate due to impeded CO3−2 diffusion. Decreasing steel corrosion rates with time in carbonate-bicarbonate solutions can be understood on this basis. Growth of the carbonate layer at the steel-carbonate interface intensifies tensile wedging stress in corroded grain boundaries, thereby facilitating intergranular crack initiation.