A numerical investigation was conducted to test the hypothesis that the composition of the pore solution in mill scale crevices on carbon steel rebar surfaces in concrete might be different from that of the bulk concrete pore solution, and this difference may create the necessary conditions for the premature breakdown of the passive film. The modeling was performed using a non-linear transient finite element algorithm, which involved the solution of coupled extended Nernst-Planck and Poisson's equations in a domain that represented typical mill scale crevices on carbon steel rebar. The numerical simulations showed that the chemistry of the pore solution, in particular pH and Cl − /OH − , within mill scale crevices provided more favorable conditions for depassivation than the bulk concrete pore solution. Local acidification and increase in Cl − /OH − within crevices were observed in all simulations, albeit to different degrees. Crevice geometry has been found to be the most important parameter affecting local acidification and the increase in Cl − /OH − . Simulations supported the hypothesis that the chemical composition of the pore solution within the crevices differs from that of the bulk solution through a process similar to the suggested mechanism of typical crevice corrosion. Within the alkaline environment of concrete, carbon steel reinforcement is protected against corrosion by a passive film, but chlorides from deicing chemicals or marine salts might cause the loss of this film when their concentrations exceed threshold values. [1][2][3][4][5][6][7] Whether they are represented in terms of total or free chloride concentrations, or chloride-to-hydroxide concentration ratio, Cl − /OH − , reported values of chloride thresholds for carbon steel reinforcement in concrete cover a wide range and have a large degree of variability. 9,10The surface conditions of rebar, in particular, the presence of mill scale on the steel surface, is one of the potential causes of the reported uncertainty and variability in chloride thresholds. 8,[11][12][13][14][15] It has been widely reported that higher chloride thresholds were observed for rebars with modified surfaces (i.e., without mill scale) through sandblasting, polishing or pickling than those in as-received conditions with mill scale. 12,13,15,16 In some cases, it was observed that corrosion did not initiate in highly-polished rebar even after the specimens were exposed to chloride concentrations in excess of that is typically found in sea water.11 In addition, modifying the rebar surface leads to reduced variability and fluctuations in electrochemical measurements, which can be explained by the fact that modified surfaces are nearly uniform, whereas as-received surfaces are locally much more diverse and complex due to the presence of mill scale. 17 These results suggest that the variability associated with the reported chloride thresholds may be partially attributed to the variability in mill scale properties resulting from the variability in manufacturing.In a recent...

No abstract