Study on influence of nitrite and phosphate based inhibiting admixtures on chloride interaction, rebar corrosion, and microstructure of concrete subjected to different chloride exposures

The study aims to assess the effect of nitrite and phosphate based inhibiting admixtures, i.e., sodium nitrite (NaNO2) and di-Sodium hydrogen phosphate (Na2HPO4) against corrosion of reinforcing steel in concrete exposed to chloride environment with different chloride salts through electrochemical measurements. Further, the influence of inhibiting admixtures on penetration of chloride ions, chloride binding, and changes in microstructure of concrete were investigated. Electrochemical measurements indicated that NaCl is more aggressive in depassivating the rebar early thereby reducing corrosion initiation period, and also increasing corrosion rate compared to CaCl2 followed by MgCl2. The addition of inhibiting admixtures enhanced resistance of concrete against corrosion. NaNO2 exhibited improved resistance against corrosion of rebar compared to Na2HPO4 even in presence of higher amount of chloride ions near rebar level. Corrosion current density in concrete containing NaNO2 was 1.50 times, 1.74 times and 1.89 times lower than Na2HPO4, in case of exposure to NaCl, MgCl2 and CaCl2 solutions respectively. The formation of denser microstructure in concrete made from Portland pozzolana cement was more dominant in minimizing the extent of corrosion over the effect of inhibiting admixtures compared to concrete made from ordinary Portland cement. Microstructure analysis indicated formation of more amount of Friedel's salt in concrete admixed with Na2HPO4 than NaNO2. However, Friedel's salt formation mostly decreased with increase in Na2HPO4 dosage whereas the reverse variation was observed in case of NaNO2.