## 3 Results and Discussion
### Capacitance  
The oxide or passive layer formed on metals significantly affects corrosion resistance [41]. The layer usually exhibits semiconducting properties [42,43]. The most common technique for investigating the electronic properties of an oxide or passive layer is Mott-Schottky plot measurement, which characterizes the oxide or passive layer generated on metal surfaces. In this study, we used electrochemical capacitance measurements to characterize the electron properties of the oxide or passive layer formed on conventional rebar and DP rebar. Capacitance \(C\) is equal to the space charge capacitance \(C_{\text{sc}}\). Referring to the Mott-Scottky principle, the relation between 1/\(C^{2}\) and potential \(E\) can be written as follows [43]:  
\[\frac{1}{C^{2}}=\frac{2}{\varepsilon\varepsilon_{0}qNA^{2}}(-E+E_{fb}+\frac{kT} {e}) \tag{6}\]  
where \(q\) is the electron charge, \(E_{\text{fb}}\) the flat band potential, \(\varepsilon_{0}\) the vacuum permittivity, \(\varepsilon\) the dielectric constant, \(T\) the absolute temperature, \(k\) the Boltzmann constant, \(A\) the electrode area and \(N\) the carrier concentration. We can calculate the semiconductor type from the \(V\) versus 1/\(C^{2}\) plot. A p-type semiconductor can be detected from the negative slope, while an n-type semiconductor is associated with the positive slope of the oxide or passive layer. Figure 10 shows the Mott-Schottky plot of the oxide or passive layers for conventional rebar and DP rebar. The positive slopes obtained were associated with n-type semiconductors of the oxide or passive layer of both conventional rebar and DP rebar. Specifically, the rebar phase did not affect or change the semiconductor type. The slope of the \(V\) versus 1/\(C^{2}\) plot was contrariwise relational to the carrier concentration. The slope of DP rebar was larger compared to conventional rebar, indicating that the donor concentration in the oxide or passive layer of DP rebar is lower compared to conventional rebar. \(N_{\text{d}}\) was obtained from the slopes: \(1.67\times 10^{22}\) cm\({}^{-3}\) for conventional rebar and \(9.07\times 10^{20}\) cm\({}^{-3}\) for DP rebar. EIS results showed that passive layer formation of conventional rebar and DP rebar was diffusion controlled. During passive layer formation, the electrochemical stage is faster than the diffusion stage. The electrochemical stage is activated at the layer-metal interface, where metal ionization occurs. Oxygen vacancies or metal ions predominate in the oxide or passive layer because of a rapid electrochemical process. The oxide or passive layer is an n-type semiconductor, so the oxide or passive layers of conventional rebar and DP rebar are n-type semiconductors. The EF of the electrode decreases  
Figure 10: Mott–Schottky plot of DP rebar and conventional rebar. if anode polarization is applied to the passive electrode, which bends the energy band upward to the side of the electrolyte. In conventional rebar, the electron (e\({}^{-}\)) originated from the conductive range in the oxide or passive layer, which resulted in increasing the magnitude of \(N_{\mathrm{d}}\). The more the donors the oxide or passive layer has, the easier the anodic reaction process is. In addition, the passive current density for conventional rebar is larger compared to DP rebar. The main conclusion is that in conventional rebar, an inner layer from pearlite oxide is formed in the oxide or passive layer, increasing the passive current density and forming an unstable oxide or passive layer. As it can be seen from Table 2, the current density is becoming higher with increasing chlorine percentage but increasing rate higher in case of conventional rebar compare to DP steel, which is in accordance with the values of E\({}_{\mathrm{corr}}\). For the present study, the polarization curves are important because the anodic and cathodic slopes \(\beta\)a and \(\beta\)c were determined to be used in the calculation of corrosion rates. The Tafel regions in the polarization curves are well defined.