Study on the corrosion behavior of reinforcing steel is one of the most significant research topics in the field of corrosion and protection of metals. The scanning microelectrode technique is a powerful tool to study localized corrosion behavior of reinforcing steel in a corrosive solution. In this work, the corrosion behavior of reinforcing steel in a simulated concrete pore solution with Cl^- and the inhibition effect of a mixture of NaNO₂ and Na₂SiO₃ on the steel were studied by the scanning microelectrode technique assisted by polarization curve measurements and surface analyses.

Q235 reinforcing steel was used as the tested material. The cylindrical steel specimen with dimension of Æ1.14 cm ´ 0.50 cm was sealed in a PVC holder enveloped with epoxy resin, and only a cross section (1.00 cm²) of the steel served as the working surface. Before experiments, the steel specimen was sequentially polished by silicon carbide sandpaper up to 1500 grit and 0.3 µm Al₂O₃ powder, and then ultrasonically cleaned in deionized water and ethanol.

A saturated Ca(OH)₂ solution was used as the simulated concrete pore (SCP) solution. The simulated polluted concrete pore (SPCP) solution was prepared from SCP solution by adjusting its pH to 11.50 with deionized water and adding 0.05 M NaCl. NaNO₂ and Na₂SiO₃ with different concentrations as corrosion inhibitors were added to the SPCP solution.

The potential distribution on the reinforcing steel surface was measured by a home-built scanning microelectrode measurement system. Ag/AgCl electrodes were used as a scanning microprobe and a micro-reference electrode. Polarization curve measurements were carried out by an Autolab Potentiostat Galvanostat. A three–electrode cell was used with the steel specimen as the working electrode, a saturated calomel electrode (SCE) as the reference electrode and a platinum electrode as the counter electrode. All the measurements were performed at ambient temperature.

The results showed that there were no stable large potential peaks on the steel surface in the pure Ca(OH)₂ solution with pH 12.50 or 11.50, indicating that the steel surface was in a passive state and no corrosion took place. For the steel in the SPCP solution with 0.050 M NaCl and pH 11.50, a stable and large local potential peak occurred on its potential map after 20 min immersion. This result indicated that the steel was active and localized corrosion occurred resulting from the Cl^- attack. For the SPCP solution with 0.004 M NaNO₂ and 0.001 M Na₂SiO₃, there were no stable potential peaks on the potential map of the steel immersed in the solution for 8 h, indicating that no localized corrosion took place. Therefore, the mixture of NaNO₂ with Na₂SiO could protect the reinforcing steel from corrosion. The polarization curve measurements and surface analyses also confirmed the above results.

This work was supported by the National Natural Science Foundation of China (Nos. 21073151, and 21173177).