Effect of chloride and conjoint chloride–sulfate ions on corrosion of reinforcing steel in electrolytic concrete powder solution (ECPS)

Abstract

In the present work an experimental investigation has been carried out to assess the corrosion behaviour of bare steel in electrolytic concrete powder solution (ECPS) prepared from various concrete mixes. The corrosion behaviour of steel has been evaluated through anodic polarization study by conducting potentiodynamic linear sweep test on bare steel specimens immersed in electrolytic concrete powder solutions contaminated with different concentrations of chloride and sulfate ions. The electrolytic concrete powder solution extracts were prepared from concrete mixes made with OPC (ordinary Portland cement) and PPC (Portland pozzolana cement). Sodium chloride and sodium sulfate were used as the source of chloride ions and sulfate ions respectively. From experimentally obtained polarization curves, different zones of corrosion of steel reinforcement were identified. XRD analysis was also conducted on concrete powder samples to analyze the products formed due to presence of chloride and composite chloride–sulfate ions. The compressive strength results indicated that, specimens made with PPC exhibited higher compressive strength as compared to those made with OPC in both chloride and composite chloride–sulfate contaminated concrete. In addition, strength was more in chloride contaminated concrete as compared to that in mixed chloride–sulfate contaminated concrete for both OPC and PPC. The polarization study results indicated that, passivity range of steel reinforcement decreased with increase in free chloride ion concentration in both chloride and composite chloride–sulfate contaminated ECPS. The range of passive zone was less in PPC as compared to that in OPC in both chloride and composite chloride–sulfate contaminated concrete, which is attributed to lower chemical chloride binding in PPC than that in OPC and this was confirmed through appearance of lower intensity peaks of Friedel’s salt (calcium chloroaluminate) in XRD profiles of PPC than that of OPC. Further sulfate ions mitigated the effect of chloride ions in reducing the passivity range of steel, in the conjoint presence of both ions. PPC showed slightly higher chloride tolerance against chance of pitting corrosion as compared to OPC. It was observed that, presence of higher concentration of sulfate ions may mitigate the chance of pitting corrosion as chloride tolerance against the risk of pitting corrosion increased with increase in sulfate ion concentration in the conjoint presence of both ions.