PITTING CORROSION OF PIPELINE STEEL IN DILUTE BICARBONATE SOLUTION WITH CHLORIDE-IONS

Corrosion behavior of grade 550 (X-80) pipeline steel with coating disbondments was studied in a range of bicarbonate (HCO3-) solutions using a potentiodynamic polarization technique. Corrosion morphology of the steel was examined using optical and scanning electron microscopy. The shape of the polarization curves and the sensitivity of the steel to pitting corrosion varied with HCO3- concentration. Increases in HCO3- concentration resulted in a wider passive region and a more noble breakdown potential (E(b)), resulting in greater resistance to pitting corrosion. Surface morphology showed no pit formation in solutions with HCO3- concentration > 0.05 M, but pits developed at a concentration of 0.05 M. Size and density of corrosion pits increased with further decreases in HCO3- concentration. The graph of corrosion potential (E(corr)) vs HCO3- concentration showed a peak at 0.05 M, whereas the corrosion current density increased monotonically with increased HCO3- concentration. Low concentrations of chloride ion (Cl-) greatly affected polarization characteristics and caused elimination of passivity in specific solutions.