Pitting corrosion of X70 pipeline steel in the simulated wet storage environment

Pitting mechanism and behaviour of X70 pipeline steel in humid storage environments were investigated using electrochemical polarization curves, electrochemical impedance spectrums (EIS), immersing corrosion tests and corrosion morphology observation through SEM. It was demonstrated that pitting of X70 pipeline steel occurred in simulated moist storage environments, for which the corrosive substances came from the residual species in laminar cooling water introduced during steel manufacture processes. HCO₃^- and NO₃^- are passivating agents, Cl^- and SO₄^2- would destroyed the passivation layer, which could lead to pitting. In solution with 0.5 mol/L NaHCO₃, 0.02 mol/L Cl^- was enough to break the passivation layer. Cl^- concentration is a key factor for pitting initiation and propagation. When the Cl^- concentration was relatively low, pitting could initiate but was hard to grow up. When the Cl^- concentration was moderate (about 0.149 mol/L), pitting sensitivity was the highest because pitting was easy to grow up. However, if the concentration of Cl^- was too high, uniform corrosion occurred.