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

Corrosion behavior of grade 550 (X-80) pipeline steel with coating dis bondments was studied in a range of bicarbonate (HCO3-) solutions usin g a potentiodynamic polarization technique. Corrosion morphology of th e steel was examined using optical and scanning electron microscopy. T he shape of the polarization curves and the sensitivity of the steel t o pitting corrosion varied with HCO3- concentration. Increases in HCO3 - concentration resulted in a wider passive region and a more noble br eakdown potential (E(b)), resulting in greater resistance to pitting c orrosion. Surface morphology showed no pit formation in solutions with HCO3- concentration > 0.05 M, but pits developed at a concentration o f 0.05 M. Size and density of corrosion pits increased with further de creases in HCO3- concentration. The graph of corrosion potential (E(co rr)) vs HCO3- concentration showed a peak at 0.05 M, whereas the corro sion current density increased monotonically with increased HCO3- conc entration. Low concentrations of chloride ion (Cl-) greatly affected p olarization characteristics and caused elimination of passivity in spe cific solutions.