*Study on the Perspective of Mechanical Properties and Corrosion Behaviour of Stainless… DOI: http://dx.doi.org/10.5772/intechopen.101388*

current density decreases as depicted by a shift of the curve along the X-axis (current density) from a value of 3 A/cm<sup>2</sup> to values of ((4) to (5)) A/cm<sup>2</sup> due to neutral nature of 3.5% NaCl solution and thereby showing enhanced corrosion resistance of the different types of rebars in 3.5% NaCl solution. It is evident from **Figure 9**, that the current density of Fe 600 rebar is more than the plain rebar; that is, the corrosion rate of Fe 600 rebar is greater than plain rebar. Therefore, it can be assumed from **Figures 8** and **9** that although the corrosion resistance of Fe 600 rebar is higher than plain rebar in the acid medium and it is reversed in the case of seawater. It has been also observed that the corrosion resistance of plain rebar is more than galvanized rebar because the zinc coating at the surface of galvanized rebar gets damaged due to penetration of chloride ions. In this context, it cannot be overruled that the galvanized rebar undergone through thermomechanical treatments leads to the formation of a finer rim of tempered martensite, which increases the grain boundary area, thereby leading to a higher solution attack at the grain boundary and reducing the corrosion resistance of galvanized rebars.

The corresponding polarization parameters determined by extrapolation methods from the polarization curves (**Figures 8** and **9**) are summarized in **Tables 5** and **6**. From **Table 5**, it can be seen that the corrosion potential values were more positive for Fe 600, followed by plain and stainless steel rebars in 1% HCl solution, whereas in the case of 3.5% NaCl solution, more positive corrosion potential values have been observed for stainless steel followed by plain and Fe 600 rebars as evident in **Table 6**. It is well established that the higher values of corrosion potentials lead to enhancement in the corrosion resistance properties [50, 52]. From **Tables 5** and **6**, it is noticeable that the values of icorr were minimum for Fe 600 followed by plain, stainless steel and Galvanized rebars in both 1% NaCl and 3.5% HCl solutions. It is well known that higher icorr values represent the higher corrosion resistance behaviour of metal. **Tables 5** and **6** also summarize the estimated values of corrosion rates that were calculated by using Eq. (2) for all the experimental rebars in both 1% HCl and 3.5% NaCl solutions. It has been observed that the


**Table 5.**

*Tafel plot data of five different rebars in 1% HCl solution.*


### **Table 6.**

*Tafel plot data of five different rebars in 3.5% NaCl solution.*

corrosion rate of all the experimental rebars is almost the same and very small in 3.5% NaCl as compared with 1% HCl solution. However, when the solution is 1% HCl, mm/y values obtained from **Table 5** show an increase for all the experimental rebar samples, which is well expected because the acid solutions generally possess a higher corrosive environment due to their acidic nature and pH lowering properties along with the presence of water and air, which is not seen in case of 3.5% NaCl solution due to its neutral nature [34, 53]. It is well known that a lower value of corrosion rate depicts higher corrosion resistance behaviour of the samples. According to Faraday's law, the corrosion rate can be calculated by using the corrosion current density as follows [49, 52].

$$\text{Corrosion Rate} \left(\frac{\text{mm}}{\text{year}}\right) = \left(3.16 \times 10^8 \text{i}\_{\text{corr}} \text{M}\right) / (\text{zF} \rho) \tag{2}$$

where icorr is the corrosion current density in (A/cm<sup>2</sup> ), M is the molar mass of steel in (g/mol), F is the Faraday's constant (96,500 C/mol), z is the number of electrons transferred for each metal atom and ρ is the metal density (g/cm<sup>3</sup> ).where β<sup>a</sup> = Anodic Tafel slope (mV/dec) and β<sup>c</sup> = Cathodic Tafel slope (mV/dec).

Therefore, it is evident from these experimental results that the estimated values of corrosion rate were the lowest for Fe 600 followed by plain, stainless steel and galvanized rebars in acid solutions, but in the case of 3.5% NaCl solution, these values were significantly reduced for all the rebars, which is an indication of better corrosion resistance properties under marine environment.

### *3.4.2 Electrochemical impedance spectroscopy (EIS)*

**Figures 10** and **11** show the open-loop potential stabilities in the Nyquist plots of the experimental rebars in 1% HCl and 3.5% NaCl solutions, respectively. To compare with the results obtained from the polarization test (**Figures 8** and **9**) and also to get a better correlation, EIS studies were further performed. It is evident from

**Figure 10.** *Nyquist plots of the experimental rebars in 1% HCl solution.*

*Study on the Perspective of Mechanical Properties and Corrosion Behaviour of Stainless… DOI: http://dx.doi.org/10.5772/intechopen.101388*

**Figure 11.** *Nyquist plots of the rebars in 3.5% NaCl solution (seawater).*

**Figure 10** that except stainless steel rebar, all other rebars are showing only one semi-circular loop in the electrochemical impedance spectra, thereby indicating a capacitive semicircle in the high-medium frequency range. The occurrence of two semicircles in the electrochemical impedance spectra for stainless steel in 1% HCl is due to passivation of the rebar surface, thereby indicating a capacitive semicircle and an inductive loop in the high-medium and low-frequency ranges, respectively [52]. The capacitive semicircle represents the active state of the interface between the rebar and acid solution when the carbon steel is exposed to the corrosive solutions [52, 54]. It is well known that stainless steel will form a trans-passive zone after the damage of the passive layer, which is also evident in **Figure 8(b)**. This is due to the evolution of oxygen from water and/or the chromium oxide dissolution through Cr3+ to the Cr6+ when metal suffers high anodic polarization [49–51]. For this reason, in the Nyquist plot two loops are visible (one is bigger and the other one is smaller). However, it cannot be overruled that the open-loop diameters in **Figure 11** were maximum for the plain rebar sample, followed by Fe 600 rebar, stainless steel, and galvanized rebar. In this connection, it is noteworthy to mention here that the capacitive arcs overlap in the Nyquist plots (**Figures 10** and **11**) since concrete is a heterogeneous material and many intermixed interfacial regions influence the impedance spectra. It has been already reported that these imperfect interfaces and electrode surface roughness require a constant phase element (CPE) instead of a pure capacitor to accurately model an equivalent circuit [55].

**Tables 7** and **8** summarize the values of the resistance of the electrolyte solution (Rs), the resistance of reinforced concrete (Rf) and CPE for all the rebars. It is well established that polarization resistance (Rp = Rf + Rct) is an indicator to study the corrosion resistance behaviour of carbon steel in corrosive environments [52]. It cannot be overruled that except for stainless steel all the plots in **Figures 10** and **11** exhibit only single-loop curves for all the other rebars, which generally do not involve the charge transfer resistance (Rct). Thus, the higher values of Rf reveal a higher value of corrosion resistance for the sample [52, 56]. Furthermore, the arc diameter in the Nyquist diagrams can be considered as Rf and the reduction of the


**Table 7.**

*EIS plot data of five different rebars in 1% HCl solution.*


**Table 8.**

*EIS plot data of five different rebars in 3.5% NaCl solution.*

arc diameter reveals a decrease in Rf values [52]. It is evident from **Table 7** that the Rf values were maximum for Fe 600, followed by plain, stainless steel, and then galvanized rebars in 1% HCl solution, whereas in 3.5% NaCl solution (**Table 8**), it is showing maximum values for plain rebar followed by Fe 600 and stainless steel rebars with a slight variation (≈0.07) and then galvanized rebar, which is also in agreement with **Figures 10** and **11**. Therefore, all the experimental evidence obtained from the polarization and EIS studies under acidic and seawater solutions are in good agreement and the corrosion resistance of all the experimental rebars is higher in 3.5% NaCl than 1% HCl solutions.
