## 3 Results and Discussion
### Potentiodynamic Polarization  
#### 3.3.1 Influence of Cl\({}^{-}\) ions on Passivity in Alkaline Solution  
Figure 4 shows potentiodynamic polarization curves for SCPS with and without NaCl. As explained by Pourbaix diagram [34], DP rebar shows spontaneous passivity in the presence of an alkaline medium compared to conventional rebar. High Cl\({}^{-}\) ion concentrations may affect the passive state [35].  
\begin{table}
\begin{tabular}{c c c c c c c c|c c c c} \hline \hline  & & & & & & & & & & & & \\ \hline C & Mn & Si & S & P & V & Mo & Nb & C & Mn & Si & S & P \\
0.20 & 0.70 & 0.172 & 0.027 & 0.020 & \textless{}0.02 & \textless{}0.02 & \textless{}0.01 & 0.10 & 0.90 & 0.30 & 0.010 & 0.02 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Nominal composition of (a) carbon steel and (b) DP steel in (wt %).  
Figure 2: Microstructure of the as-received rebar.  
We observed oxide or passive layer breakdown at Cl\({}^{-}\) ion concentrations of 0.02 and 0.05 M and potential of \(>\)200 mV (SCE). The higher Cl\({}^{-}\) ion concentration shifts the critical pitting potential toward a less noble range, as seen at 0.05 M. It is well known that among different factors contributing to the initiation of pitting corrosion is the damage to passive layer. Moreover, pitting corrosion limits the possibility of re-passivation leading to complete destruction of the oxide layer. In other words, the lower the destruction in the passive layer the better the corrosion resistant property of the material. Thus, it can be confirmed that DP rebar has a better corrosion resistance than the conventional rebar.  
Figure 4: Potentiodynamic polarization curves in simulated concrete pore solution (SCPS) with and without NaCl. SCPS, simulated concrete pore solution.  
Figure 3: Microstructure of DP rebar produced by heat treatment.  
This is clearly seen from the behaviour of these materials as depicted in Figure 4 where the dissolution rate of the passive layer in the DP rebar is far slower than that of conventional rebar.  
Sodium chloride was added to the base electrolyte prior to potentiodynamic polarization, which helped in investigating the susceptibility of the oxide or passive layer formed in the local breakthrough. Low Cl\({}^{-}\) ion concentrations are instrumental in preventing the formation of a protective layer on substrates [36]. The oxide or passive layer dissolves actively with high current densities beginning at the free corrosion potential, which is shown for a Cl\({}^{-}\) ion concentration of 0.05 M for conventional rebar and DP rebar in Figure 4. At all solution concentrations, the corrosion rate was higher for conventional rebar compared to DP rebar, probably because the oxide or passive layer of DP rebar was more stable compared to conventional rebar. By increasing the Cl\({}^{-}\) ion concentration, the free corrosion and passive current densities increased, while the free corrosion potential decreased to a less noble potential. Because of an increase in surface reactivity for conventional rebar in SCPS with increasing Cl\({}^{-}\) ion concentration (0.02 and 0.05 M), we observed current scattering increasing in the current anodic range with high passive current density. The reason was the presence of an oxide or passive layer during anodic polarization, which dissolved and reformed dynamically. At 200 mV (SCE), the current density rapidly increased by 2 orders of magnitude. Oxygen evolution reaction was not the only reason for this increase, because on decreasing the applied potential, the current density did not decrease [37]. Oxide or passive layer breakthrough was evident from the formation of high quantities of flakes on the electrode surface.