RESUMEN
X80 steel has great risk of corrosion in high voltage direct current (HVDC) interference cases. In this study, the anodic polarization behavior of X80 steel under high potential and current density in Na2SO4 solution was investigated. The I × R drop was eliminated using current interrupt technique during the potentiodynamic measurement. Therefore, the real polarization curve was obtained. The corrosion behavior was investigated by galvanostatic polarization, scanning electron microscopy, and X-ray photoelectron spectroscopy. The results show a new form of passivation route. The steel dissolved actively below -0.388 VSCE, then became partly passivated from -0.388 to 1.448 VSCE, and fully passivated above 1.448 VSCE. The passive film was formed containing Fe2O3 and FeOOH, and resistant to SO42- ions. It not only blocked the direct dissolution of steel, but also facilitated oxygen evolution. The corrosion rates of steel samples decreased after the passivation.
RESUMEN
Pipeline steel has considerable risk of corrosion in the high voltage direct current interference cases. Thus, under high potential/current density conditions, the anodic polarization behaviour of X80 steel in Na2SO4 solution and the influence of Cl- ions were investigated using reversed potentiodynamic polarization, the current interrupt method, galvanostatic polarization, scanning electron microscopy, and X-ray photoelectron spectroscopy. In the Na2SO4 solution free of Cl- ions, steel was passivated above 0.120 A cm-2 and the potential increased from -0.32 V to 1.43 V. The passive film was composed of Fe3O4, γ-Fe2O3, and FeOOH. The addition of Cl- ions observably influenced the passivation by attacking the passivate film. Low concentration of Cl- ions (<5 mg L-1 NaCl) could set higher demands of current density to achieve passivation and increase the requirement of potential to maintain passivation. A high concentration of Cl- ions (>5 mg L-1 NaCl) completely prevented passivation, showing strong corrosiveness. Thus, the X80 steel was corroded even under high-current-density conditions. The corrosion products were mainly composed of Fe3O4, α-Fe2O3, and FeOOH.