| The variability and uncertainty associated with chloride thresholds can be partly explained by the surface conditions of carbon steel rebar, in particular, by the presence of mill scale on the steel surface. It has been suggested in the literature that pore solution in the crevices between mill scale and steel surface may be different from that of the bulk pore solution, and this difference may create the necessary conditions for the breakdown of the passive film. To test this hypothesis, a numerical investigation was carried out using a non-linear transient finite element algorithm, which involved the solution of coupled extended Nernst-Planck and Poisson's equations under constraints imposed by the chemical reactions and equilibrium requirements in a domain that represented typical mill scale cracks and crevices. The numerical simulations showed that the chemistry of the pore solution, in particular pH and the chloride-to-hydroxide concentration ratio, Cl-/OH-, within mill scale crevices provided more favourable conditions for depassivation than that of the bulk pore solution. |
