Reinforcing steel bars (-12mm diameter and 150mm long) were used in cyclic polarization tests in saturated Ca(OH)z solution and simulated concrete pore solution (SPS) with various levels of sodium chloride addition. Below a limiting chloride level (-O.004M [Cl-] in Ca(OH)z solution and -0.4M [Cl-] in SPS solution), steel was not found to undergo pitting corrosion even if it was polarized to the oxygen evolution potential (-O.6V/SCE). At higher NaCl addition, pitting corrosion could often be initiated but the pitting potential was non-deterministic to a great extent. In Ca(OH)z solution~ the average pitting potential was found to be strongly dependent on chloride concentration when [C1-]>0.008M. In SPS solution, the average pitting potential was almost independent of the chloride concentration when [C1-]>0.8M. The repassivation potential was found to be a strong function of the severity of corrosion attack that has occurred on the steel surface before repassivation, rather than a function of the chloride content of the bulk solution. The pitting tendency in chloride-containing SPS and Ca(OH)z solutions was interpreted on a statistical basis. The threshold thus determined good agreement with other values reported in the literature.
Reinforcing steel bars (rebar) in concrete are often corroded due to the chloride-induced breakdown of the protective passive film, when the chloride content of the concrete is high enough. It is generally believed that the free chloride in the concrete pore solution is responsible for the corrosion initiation. Hydroxide ions in the pore solution play an opposing role and tend to stabilize the passive film, However, it is difficult to accurately determine the free hydroxide (OH_) and free chloride (Cl-) concentration in the concrete pore solution, especially at the rebar level. To gain insight on the relative proportions leading to corrosion initiation, several laboratory investigations( 1-4)of the effect of OH and Cl- on the corrosion behavior of reinforcing steel have been carried out in model alkaline solutions such as saturated Ca(OH)2, pure NaOH or KOH solution, and simulated concrete pore solutions (SPS). Although it would seem easier to define the chloride corrosion threshold in a solution with controlled [OH-] and [Cl-] content, reported threshold values from different investigators have been in poor agreement. For example, steel remained passive in a pH 13.2 NaOH solution with 0.25M NaCl according to Hausmann(l), whereas Gouda(2) stated that the maximum tolerable chloride concentration was only 0.085M and O.115M for NaOH solution with pH as high as 13.3 and 13.9, respectively. Discrepancies of this order merit additional study. This investigation used the cyclic polarization technique to determine the pitting and repassivation potentials of reinforcing steel in saturated Ca(OH)z solution and SPS solutions with various levels of NaC1. A statistical interpretation of the pitting potential was used to evaluate the chloride corrosion threshold concept and to establish a comparison with previous findings in the literature.
