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Abstract: The electrochemical behavior of metallic passive film on rebar in concrete is characterized by its semiconductive nature. The charge distribution at the interface between a semiconductor and an electrolyte is often determined by measuring the capacitance of the space-charge layer (C_SC) as a function of the electrode potential (E). When the space charge-layer serves as the depletion layer, the relation of C_sc^-2 vs E resembles a Mott-Schottky plot (M-S plot). The semiconductive properties of the passive film on rebar in concrete were analyzed with M-S plots to study the effect of chloride ions and mineral admixtures on rebar passive films. Some rebar electrodes were immersed in simulated concrete pore solutions, while others were embedded in concrete with/without mineral admixtures. In saturated Ca(OH)₂ solutions, the relation of C_sc^-2-E of rebar electrodes shows linear Mott-Schottky relationship indicating that the passive film on rebar is a highly disordered n-type semiconductor, with donor density (N_D) in the order of 10²⁶ m⁻³. After adding chloride ions (Cl⁻ wt%<0.2%) in system solutions, the M-S plot slopes significantly decreased and N_D increased, suggesting that chloride ion will cause passive film corrosion and breakdown. The M-S plots of the passive film on rebar electrodes embedded in concrete were similar to those immersed in simulated system solution. However, N_D of those in concrete with mineral admixtures tended to be a little smaller, indicating that introducing proper quantity admixtures into concrete could make the rebar passive film have a thicker space-charge layer and therefore a thicker passive film layer.

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