The thickness of the passive layer on carbon steel can vary from 3 to 15 nm and constitutes different metal hydroxides and metal oxides as mentioned in eqs 6-9. Typical bilayer nature with distinct characteristics has been observed for the passive layer. The metal oxides are the main constituents of the inner layer while the outer layer is comprised of different precipitates such as -OH and -OOH species. For instance, distinct regions have been identified in a perfect passive layer for iron surface based on its constituents: the inner layer mainly consisting of FeO whilst the outer layer consisting of Fe3O4 (FeO•Fe2O3) and FeOOH (Figure (a)). This passive film act as physical barrier, preventing further oxidation and corrosion of embedded steel bars in concrete. However, the passive layer is sensitive to environmental factors such as pH and aggressive ions (Cl -, SO4 2-), which can alter its chemical, physical, and protective properties.