Effect of Sulfate on Corrosion Behavior of Reinforcing Steel in Simulated Concrete Pore Solutions
Authors: Du Rong-Gui , Dong Shi-Gang , Gao Ying-Bo , Liang Yan , Guan Zi-Chao 
Published: February 2020

Reinforcing steel in concrete is normally passivated against corrosion in the concrete pore solution with high alkalinity (often pH ≥ 12.5) resulting from the cement hydration. However, the carbonation of concrete and the presence of aggressive species may induce the breakdown of the passive film on the steel surface and the occurrence of steel corrosion. In some environments, such as marine and saline-alkali land environments, sulfate ions (SO4
2-) also is a key factor affecting the corrosion behavior of reinforcing steel in concrete. Researchers have studied the chloride and/or carbonation induced steel corrosion, explored some corrosion mechanisms, and developed many anticorrosion techniques. Additionally, it is necessary to study the corrosion behavior of reinforcing steel in the sulfate environment for the corrosion protection of the steel in concrete. In this work, the corrosion behavior of reinforcing steel in the simulated concrete pore solution containing sulfate was studied by electrochemical techniques (electrochemical impedance spectroscopy and potentiodynamic polarization measurements) and surface analyses.

All corrosion test specimens (Ø 11.2 mm × 4 mm) were cut from a Q235 reinforcing steel bar. Before testing, the specimens with a working surface of 1.00 cm2 were polished down to 1500 grade SiC paper and rinsed with deionized water. A saturated calcium hydroxide solution served as the simulated concrete pore solution, and was used as the test solution after the solution pH was adjusted to 11.00 with a 0.8 M NaHCO3 solution and NaSO4 with different concentrations was added. Electrochemical measurements were carried out in the test solutions by an Autolab Potentiostat/Galvanostat. The steel specimen, a saturated calomel electrode, and a platinum sheet served as the working electrode, the reference electrode and the counter electrode, respectively. The surface topography and chemical composition of the steel specimen were characterized by SEM/EDS/Electron Micro-Probe Analyzer.

The results indicated that the Na2SO4 concentration had an effect on the corrosion behavior of reinforcing steel. Based on the Nyquist plots of the steel in the test solution, when the Na2SO4 concentration was lower than 0.01 M, the impedance arc radius of the steel was large, and its charge transfer resistance (R
ct) was about 300 kΩ cm2, which was similar to that in the solution without SO4
2-. This indicated that the steel might be in a passive state. However, when the Na2SO4 concentration reached 0.05 M or higher, the impedance arc radius of the steel became small, and its R
ct value decreased to below 50.00 kΩ cm2, showing that the passive film of the steel might be destroyed and the steel corrosion occur in the solution with high Na2SO4 concentrations. The polarization curve measurements showed that when the Na2SO4 concentration was higher than 0.05 M, the corrosion potential of the steel decreased dramatically and no a stable passive region occurred in the curves, indicating that the corrosion activity of the steel occurred. Based on the electrochemical test results, the critical SO4
2- concentration for corrosion of the reinforcing steel in the simulated concrete pore solution with pH 11.0 was 0.05-0.10 M. The SEM observations showed that the general corrosion of the steel occurred after it was immersed in the test solution with 0.5 M Na2SO4 for 1 h.