The inhibitive effect of bicarbonate and carbonate ions on carbon steel in simulated concrete pore solution: Fid-Bau Portal

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

Tan, Yong Teck / Wijesinghe, Sudesh L. / Blackwood, Daniel J.

The effects of chloride ingress and carbonation of concrete pore solution have been characterised through various electrochemical techniques on AISI 1020 carbon steel in simulated concrete pore solutions of pH 9, 10, 11 and 12.5 with varying chloride and bicarbonate/ carbonate concentrations. The presence of bicarbonate/carbonate ions increases the polarisation and charge transfer resistances of carbon steel, with the polarisation and charge transfer resistances at pH 9 being double those at pH 10 and pH 11 at high chloride concentrations. The higher R_p and R_ct with increasing bicarbonate/carbonate concentration imply a lower corrosion rate. Additional peaks in the potentiodynamic polarisation curves suggests that bicarbonate helps stabilise an Fe(II) film. The critical chloride threshold concentration necessary for pitting has been identified as between 8.5 and 56 mM Cl at pH 12.5 and between 0 and 0.8 mM Cl at pH 11. In the presence of NaHCO3, the threshold concentration was between 0.8 and 2.8 mM Cl at pH 11, between 0.3 and 2.8 mM Cl at pH 10 and between 2.8 and 28 mM Cl at pH 9. Bicarbonate/carbonate ions have an inhibitive effect on pitting corrosion. This inhibitive effectiveness was found to be around one order of magnitude lower relative to hydroxide, but can be dominant when the hydroxide concentration is much lower than the bicarbonate/carbonate concentration. Interpreting the pitting potentials in terms of the Point Defect Model suggests a direct positive correlation between the polarizability and the bicarbonate/ carbonate concentration that may indicate a thinner barrier layer with increasing bicarbonate/carbonate concentration or decreasing hydroxide concentration.