The inhibitive effect of bicarbonate and carbonate ions on carbon steel in simulated concrete pore solution

• Presence of NaHCO3 increases the critical chloride threshold for pitting.
• For pitting, inhibitive effect of HCO3−/CO32− is 10−1 that of OH− at high [Cl−].
• Inhibitive effect of HCO3−/CO32− overwhelms that of OH− when [OH−] is low.
• HCO3−/CO32− increases Rp and Rct at high [Cl−].

The critical chloride threshold concentration and the polarizability of the barrier layer/outer layer (as defined by the Point Defect Model) have been determined for AISI 1020 carbon steel in simulated concrete pore solutions of pH 9, 10, 11 and 12.5 with varying chloride and bicarbonate/carbonate concentrations. It was found that bicarbonate/carbonate ions have a beneficial effect on the charge transfer resistance, the critical chloride threshold concentration and the pitting potential. When the critical chloride threshold concentration for pitting is exceeded, the inhibitive effectiveness of bicarbonate/carbonate against pitting corrosion is around one order of magnitude lower compared to hydroxide.