Chloride Corrosion Threshold of Reinforcing Steel in Alkaline Solutions—Open-Circuit Immersion Tests

Reinforcing steel bars with three types of surface conditions (as-received mill scale, prerusted in 3.5% sodium chloride [NaCl] solution, or sandblasted) were exposed in open-circuit immersion (OCI) tests in pH 12.6 saturated calcium hydroxide (Ca[OH]2) solution (SCS) and pH 13.3 (SPS1) and pH 13.6 (SPS2) simulated concrete pore solutions. These solutions were free of chlorides at the beginning of the test to allow the steel specimens to passivate. Then the chloride concentration ([Cl–]) was increased in several steps. The corrosion potential was monitored, and electrochemical impedance spectroscopy tests were performed periodically on selected specimens. It was found that active corrosion for all three surface conditions took place in the pH 12.6 SCS and pH 13.3 SPS1, when the [Cl–] reached threshold levels. However, sustained active corrosion of steel was not found in pH 13.6 SPS2 in a testing period of ~ 2 years, although the final [Cl–] in that solution reached 3.0 M. Removing the mill scale or rust from the surface by sandblasting was beneficial in elevating the chloride corrosion threshold in alkaline solutions, although the corrosion rate of sandblasted steel after pitting initiation was higher than those of the other two groups. The threshold chloride-to-hydroxide ratio ([Cl–]/[OH–]) increased significantly with pH, suggesting a stronger inhibiting effect of hydroxide ions (OH–) at high levels. Results from the present investigation were compared with data in the literature. Significant differences between previously reported threshold [Cl–]/[OH–] values were explained based on methodology variations. Estimated ranges of the total chloride corrosion threshold in concrete based on the liquid solution measurements and on available chloride binding data were in general agreement with values observed in practice.