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.
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This is a manuscript of an article published as Mishra, Pratyush, Denizhan Yavas, Ashraf F. Bastawros, and Kurt R. Hebert. "Electrochemical impedance spectroscopy analysis of corrosion product layer formation on pipeline steel." Electrochimica Acta (2020): 136232. DOI: 10.1016/j.electacta.2020.136232. Posted with permission.