## 3 Results and Discussion
### Discussion on Corrosion Inhibitors Comparison
At approximately \(-0.9\) V, oxidation peak A1 is formed, which is related with the transformation of Fe to Fe(OH)\({}_{2}\), according to Equation (2):  
\[\mathrm{Fe}+2\mathrm{OH}^{-}\leftrightarrow\mathrm{Fe(OH)}_{2}+2\mathrm{e}^{-} \tag{2}\]  
As the passive film forms progressively with the process of the oxidizing reaction, the peak associate with the formation of Fe\({}_{3}\)O\({}_{4}\) appears in A2, according to Equations (3) and (4) [40]:  
\[3\mathrm{Fe(OH)}_{2}+2\mathrm{OH}^{-}\leftrightarrow\mathrm{Fe_{3}O_{4}}+2 \mathrm{e}^{-} \tag{3}\]  
\[3\mathrm{FeO}+2\mathrm{OH}^{-}\leftrightarrow\mathrm{Fe_{3}O_{4}}+2\mathrm{e}^ {-} \tag{4}\]  
Oxidation peak A3 is formed at 0.3 V, which is related with the transformation of magnetite (Fe\({}_{3}\)O\({}_{4}\)) to lepidocrocite (FeOOH) (Equation (5)) and \(\gamma\)-Fe\({}_{2}\)O\({}_{3}\) (Equation (6)):  
\[\mathrm{Fe_{3}O_{4}}+\mathrm{OH}^{-}+\mathrm{H_{2}O}+2\mathrm{e}^{-} \leftrightarrow 3\gamma-\mathrm{FeOOH}+\mathrm{e}^{-} \tag{5}\]  
\[2\mathrm{Fe_{3}O_{4}}+2\mathrm{OH}^{-}+2\mathrm{H_{2}O}\leftrightarrow 3 \gamma-\mathrm{Fe_{2}O_{3}}+2\mathrm{e}^{-} \tag{6}\]  
Beyond peak A3, oxygen absorption corrosion occurred. In the reverse scan, the reduction peaks C1 and C2 correspond to the reaction of Fe\({}^{3+}\) to Fe\({}^{2+}\)[51]. A wide passivate interval located between C1 and C2 is captured, following the process of adsorption of Ca\({}^{2+}\) ions as a calcium oxide gel in the presence of calcium hydroxide [20]. This wide passivation interval is advantageous to the formation of the passive film. The passive film was broken over the C2 peak, and hydrogen evolutional corrosion occurred. Similar results can be found in Ref. [20]. Chloride ions can exert ruinous effects on passive films, generating much more porous corrosion product, leading to the formation of non-protective passive films on steel bar surfaces. CV curves of steel bars with chloride also demonstrate three oxidation peaks (defined as A1, A2, and A3) in the oxide scan and two reduction peaks (defined as C1 and C2) in the reverse scan. As shown in Figure 8, the passivate interval between A1, A2, and A3 contracts sharply, and the reaction intensity of this area is significantly increased. The chloride ions can be adsorbed on the oxide layer and produce a complex loose interface to the oxide matrix. The mechanism of pitting initiation is the competitive  
Figure 8: Cyclic voltammograms (1 st and 5 th cycles) of steel bars in SCP solution with chloride. chemisorption of chloride and OH' over passive film. When chlorides substitute OH\({}^{-}\) in a sufficient number of adjacent sites, the rupture of the passive film occurred with the formation of nascent pits. Finally, it leads to an enhancement of anodic dissolution, and even the protective film is still unable to be produced and sustained [40]. \[\mathrm{Fe(OH)}_{2}+\mathrm{Cl}^{-}\rightarrow\mathrm{FeOCl}+\mathrm{OH}^{-}+ \mathrm{e}^{-} \tag{7}\]  
\[\mathrm{FeOCl}+\mathrm{OH}^{-}\rightarrow\mathrm{Fe(OH)}_{2}+\mathrm{Cl}^{-} \tag{8}\]  
In the eroded areas, OH\({}^{-}\) and dissolved oxygen restore the passive film on the inner surface of the primitive pit again. Consequently, the substitution between chloride and OH\({}^{-}\) is a dynamic state process. The corrosion resistance effect of calcium nitrite for blocking chloride attacks is verified by voltammograms and shown in Figure 9. The CV curves in Figure 9 are similar to Figure 10, which consist of three oxidation peaks and two reduction peaks, being close to the passivation status. This result proves that the addition of calcium nitrite is effective for corrosion resistance. In addition, calcium nitrite hydrolysis produced NO\({}^{2}\) and reacted with Fe(OH)\({}_{2}\), forming Fe\({}_{3}\)O\({}_{4}\) and promoting the formation of inner passive film [2], as shown in Equation (9). The presence of calcium nitrite accelerates the development of a homogenous layer and swarm with Fe\({}_{3}\)O\({}_{4}\) (magnetite), which could block the chloride ions and steel to exacerbate the corrosion [52].