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. (C) The Author(s) 2016. Published by ECS. All rights reserved.