*3.4.1 Polarization curves*

**Figures 8** and **9** display the Tafel plots of different rebars in acid (1% HCl) and seawater (3.5% NaCl) solutions. It can be seen from **Figures 8(a)** and **(b)** that in 1% HCl solution, stainless steel rebar undergoes passivation, while other rebars show a constant enhancement of anodic current density with increasing applied potential. Generally, the state of corrosion in rebars can be classified as passive, active or

**Figure 8.** *(a) Tafel plots of different rebars in 1% HCl solution and (b) Tafel plot of stainless steel rebar in 1% HCl solution.*

**Figure 9.** *Tafel plots of the rebars in 3.5% NaCl solution.*

indeterminate (trans passive region), depending on the potential difference between the steel and reference electrode. **Figure 8(b)** shows that the polarization process is divided into the different zones for stainless steel at 1% HCl: (i) Cathodic region: From the corrosion potential of 0.969 V to 0.519 V, current density gradually decreases in the process. (ii) Activation region: Between 0.519 V and 0.275 V, the current density gradually increases in this region as rate of metal dissolution is higher than the rate of metal oxide formation and thereby accelerates the corrosion. (iii) Active passive region: From 0.275 V to 0.176 V, the current density starts decreasing here as the rate of metal oxide formation becomes greater than the rate of metal dissolution and metal slowly starts moving towards the passive region. (iv) Passive region: From 0.176 V to 0.058 V, metal undergoes complete passivation due to formation of chromium oxide layers and the corrosion current density becomes constant and corrosion resistance behaviour increases. (v) Trans-passive region: Above 0.058 V to 1.021 V, the passivation film is punctured due to various anodic processes, such as evolution of oxygen from water and/or the chromium oxide dissolution through Cr3+ to the Cr6+ when metal suffers high anodic polarizations; thereby, the current density increases in this region and corrosion resistance again starts decreasing. It is evident from **Figure 8(b)** that the value of current density is decreasing from point a to b, indicating greater ease of passivation by forming a stable passive film, which is similar to earlier observations [49–51]. In this connection, it is pertinent to mention here that Cr plays a major role during the transition to passive states. It has been reported earlier that increasing the Cr percentage can significantly decrease the corrosion current density values [49]. However, the analysis of the corrosion performance of different stainless steel rebars in strongly acidic environments for long-time exposure is not only limited to the effect of the elements that increase their nobility like Ni but also the effect of elements that promotes their passivation, like Cr [49]. Earlier studies have shown that the addition of both chromium and nickel to iron remarkably increases the ease of passivation [49]. It is noteworthy to mention here that when the corrosion medium changes from 1% HCl to 3.5% NaCl (**Figure 8** *vis à vis* **Figure 9**), the anodic
