Nano-scale study of passive films and chloride-induced depassivation of carbon steel rebar in simulated concrete pore solutions using FIB/TEM

Analytical TEM techniques were used to characterize passive film on carbon steel and how it breaks down with chloride addition. Before chloride addition, the thicknesses of the oxide films on rebars were found to be uniform between 5 and 13 nm. The films consisted mainly of iron oxides that included nano-crystalites of FeOOH with an epitaxial relationship to the metal surface. Elements from the passivating solutions were located in regions of the oxide closest to the free surface. With the addition of chloride at concentrations below depassivation thresholds, there was evidence for change in the oxide closest to the free surface, but generally the oxides were similar to those grown before chloride addition. After exposure to chloride concentrations above depassivation thresholds, the oxide films were no longer uniform, with parts of the surface bare, and the average oxide thickness reduced. Pitting and enhanced corrosion were seen to accompany cementite lamella.