**3. Results and discussion**

process taking place, compared to conventional electrical methods that depend on measuring current changes, electrical charges, or the potential of the electrodes as a function of time, which depends on a certain value of its spectrum, such as a cyclic voltage, which includes current changes in hundreds of points for the potential to obtain a specific value for the peak oxidation and return at the corresponding

Among the applications of electromagnetic impedance spectroscopy, in addition to studying corrosion processes, electroplating processes, and semiconductors, it studies surface processes that include oxidation and reduction processes on the electrode surface, adsorption processes, electrical adsorption, and diffusion, as well as the kinetics of reactions in solutions, mass transfer, and resistance to solution, cells, and their electrical properties and batteries, determining the effect of each circuit element on impedance [1, 6]. It is also distinguished by not destroying the

Turmeric root extract was prepared by washing and drying the turmeric root

The iron specimens have a composition (wt%) of 0.200% C, 0.500% Si, 1.600% Mn, 0.035% S, 0.035% P, 0.040% Nb, 0.012% N, 0.020% Al, and remaining Fe. This sample was analyzed in the Atomic Energy Commission and abraded with a series of emery papers 400, 1200, 1500, and 1800 grades. The samples were then

A solution of 0.5 M concentrated acid was prepared using distilled water and

Electrochemical measurements were carried out using a potentiostat IVIUM-STAT.XR (Holland). A three-electrode cell system containing working electrode (iron coupon) of a 1 cm<sup>2</sup> exposed area, saturated (Ag/AgCl) electrode as a reference electrode, and then a platinum wire as auxiliary electrode was used. The electrochemical impedance spectroscopy measurements were carried out using the mentioned electrochemical system. Polarization curves were

spectroscopy measurements were carried out at open-circuit potential over a

frequency range of 1 MHz to 1 Hz. The sinusoidal perturbation has an

. Electrochemical impedance

and grinding it then dissolving 1 g of powder in 100 mL methanol 50% and removing the solvent by placing the solution in a vacuum evaporator, at 60°C.

Distilled water is used in the preparation process [14].

washed thoroughly with distilled water and dried in air.

current and potential value [3].

*Electrochemical Impedance Spectroscopy*

studied sample after testing [13].

**2. Materials and methods**

**2.1 Preparation of plant extract**

**2.2 Preparation of metal specimen**

**2.4 Electrochemical measurements**

recorded at a sweep rate of 50 mV s<sup>1</sup>

**2.3 Test solution**

37% hydrochloric acid.

amplitude of 0.01 mV.

**56**

### **3.1 Mechanism of inhibition process**

Turmeric root extract (TRE) used here as a corrosion inhibitor can serve as a scale inhibitor as well. This plant is characterized by the existence of a percentage of phenolic compounds (categories of curcumin) of a percentage up to 90%. It is a natural, nontoxic, environmentally friendly material. Active compounds in turmeric root extract are attributed to curcumin, demethoxycurcumin, and bisdemethoxycurcumin and to the multiple lone pair of electrons, multiple bonds, and/or conjugated л-type bond system [15]. Adsorption of these active molecules forms thin inhibitor films on the metal surface, which isolate the metal surface from the corrosive environment [16]. The oxygen atoms, the aromatic rings, and the bilateral bond of the aromatic rings boost the electronic pair freedom on the surface of the electrode. These compounds adsorb their free electrons on the surface of the electrode, and the iron is oxidized to form positively charged iron, thus forming a double electrical layer, and difference in voltage arises, as schematically presented in **Figure 3**. The inhibitor enhances the free electrons, which reduces iron corrosion and enhances inhibition.

#### **3.2 Electrochemical impedance spectroscopy measurements**

In the corrosion behavior of iron in 0.5 M HCl solution, in the absence and presence of TRE, it is investigated by the EIS, at 298 K after 1 hour of immersion in the acid solution. The double-layer capacitance (Cdl) and the frequency at which the imaginary component of the impedance is maximum (�Zmax) are found via Eq. (6):

$$\mathbf{C\_{dl}} = \mathbf{1}/\mathbf{w\_{max}}\\\mathbf{R\_{ct}} \text{ where } \mathbf{w\_{max}} = 2\pi \mathbf{f\_{max}}\tag{6}$$

The inhibition efficiency %IERct that resulted from the charge transfer resistance (Rct) is calculated by

$$\text{\textbullet IE}\_{\text{Rct}} = \left[ \left( \text{R}\_{\text{ct}} - \text{R}^{0}\_{\text{ct}} \right) / \text{R}\_{\text{ct}} \right] \* \mathbf{100} \tag{7}$$

#### **Figure 3.** *Schematic presentation of the electric double-layer formation [Chem draw].*

where R<sup>0</sup> ct and Rct are the charge transfer resistance (Rct) in the absence and presence of different concentrations of inhibitor, respectively [17].

Nyquist's and Bode's graphs of the results of the EIS of iron in 0.5 M HCl, in the absence and presence of different concentrations of TRE, were presented in **Figures 4** and **5**, respectively. The big capacitive loop refers to the adsorption of the inhibitor molecules (active compounds) presented in **Figure 2** on the iron samples [18]. Rs represents the resistance of the corrosive solution, and Rct represents the effective resistance of the transport groups of the damper and the adsorption of their electrons onto the metal surface. Cdl is the amplitude of the formed double layer between the surface of the metal and solution.

**Table 1** indicates that the Rct values of the inhibited substrates increased with the concentration of inhibitors and Cdl values decreased because of the increased prevalence of active compounds from the inhibitor and the adsorption of their electrons on the iron surface, which confirms that TRE extract is an effective inhibitor of corrosion of iron in the medium of water chlorine acid [19]. The values of n between 0.7 and 0.98 indicate that the constant phase element Q operates as a capacitance in the equivalent electrical circuit, which indicates a complex. The adsorbent inhibitor is a capacitive capacitor in the electrical circuit equivalent to its positive bus, the surface of the solution, and the negative capacitor, the electrode surface [20]. Resistance values of the solution are small Rs due to the corrosion

**Figure 4.** *Nyquist plots of iron in 0.5 M HCl containing varying concentrations of TRE after 1 hour of immersion in acid solution.*

concentration and temperature of the corrosive medium at different immersion times. Equivalent circuit model used to fit impedance spectra data is presented in

*Electrochemical parameters of iron in 0.5 M HCl solution at different concentrations without and with TRE.*

**Ecorr- (mV/SCE)**

**Icorr (A\*10<sup>4</sup> / cm<sup>2</sup> )**

Blank 0.0 0.677 26.13 0.352 0.183 8.56 —

**βa (mV/ dec)**

2.0 0.3968 10.8 0.155 0.323 3.54 58.67 4.0 0.474 4.7 0.146 0.233 1.55 82.01 6.0 0.453 3.8 0.139 0.179 1.25 85.46 8.0 0.417 3.4 0.131 0.225 1.11 86.99

**βc (mV/ dec)**

**CR (mm/ year)**

**IE%**

Polarization measurements were done in order to know about the kinetics of the cathodic and anodic reactions. The anodic and cathodic current potential curves are extrapolated up to their intersection at a point where corrosion current density (Icorr) and corrosion potential (Ecorr) are acquired [21]. **Table 2** shows the electrochemical parameters (Icorr, Ecorr, βa, βc, and CR) obtained from Tafel plots for the iron electrode in 0.5 M HCl solution without and with various concentrations of

**3.3 Potentiodynamic polarization measurements**

*Equivalent circuit model used to fit impedance spectra data.*

**(g/100 mL)**

**Inhibitor Concentration**

**Inhibitor Concentration**

*DOI: http://dx.doi.org/10.5772/intechopen.92648*

Turmeric root extract

*concentrations of TRE.*

**Table 1.**

**Figure 6.**

Turmeric root extract

**(g/100 mL)**

**Rct (ohm)**

Blank 0.0 20.3 160 4.90\*10<sup>5</sup> 0.73 —

*Electrochemical,Thermodynamic, Surface, and Spectroscopic Study in Inhibition of Iron…*

*Impedance parameters of corrosion of iron in 0.5 M HCl at 298 K in the absence and presence of different*

**fmax (Hz)**

2.0 27.7 160 3.59\*10<sup>5</sup> 0.98 26.71 4.0 50.8 100 3.13\*10<sup>5</sup> 0.70 60.04 6.0 73.1 160 1.36\*10<sup>5</sup> 0.70 72.23 8.0 117 160 8.51\*10<sup>6</sup> 0.70 82.65

**Cdl (F/cm<sup>2</sup> )** **ɑ IERct%**

*3.3.1 Influence of concentration*

**Figure 6**.

**59**

**Table 2.**

#### **Figure 5.**

*Bode plots of iron in 0.5 M HCl containing varying concentrations of TRE after 1 hour of immersion in acid solution.*

*Electrochemical,Thermodynamic, Surface, and Spectroscopic Study in Inhibition of Iron… DOI: http://dx.doi.org/10.5772/intechopen.92648*


#### **Table 1.**

where R<sup>0</sup>

*Electrochemical Impedance Spectroscopy*

**Figure 4.**

*solution.*

**Figure 5.**

*solution.*

**58**

ct and Rct are the charge transfer resistance (Rct) in the absence and

Nyquist's and Bode's graphs of the results of the EIS of iron in 0.5 M HCl, in the

**Figures 4** and **5**, respectively. The big capacitive loop refers to the adsorption of the inhibitor molecules (active compounds) presented in **Figure 2** on the iron samples [18]. Rs represents the resistance of the corrosive solution, and Rct represents the effective resistance of the transport groups of the damper and the adsorption of their electrons onto the metal surface. Cdl is the amplitude of the formed double

**Table 1** indicates that the Rct values of the inhibited substrates increased with the concentration of inhibitors and Cdl values decreased because of the increased prevalence of active compounds from the inhibitor and the adsorption of their electrons on the iron surface, which confirms that TRE extract is an effective inhibitor of corrosion of iron in the medium of water chlorine acid [19]. The values of n between 0.7 and 0.98 indicate that the constant phase element Q operates as a capacitance in the equivalent electrical circuit, which indicates a complex. The adsorbent inhibitor is a capacitive capacitor in the electrical circuit equivalent to its positive bus, the surface of the solution, and the negative capacitor, the electrode surface [20]. Resistance values of the solution are small Rs due to the corrosion

*Nyquist plots of iron in 0.5 M HCl containing varying concentrations of TRE after 1 hour of immersion in acid*

*Bode plots of iron in 0.5 M HCl containing varying concentrations of TRE after 1 hour of immersion in acid*

presence of different concentrations of inhibitor, respectively [17].

layer between the surface of the metal and solution.

absence and presence of different concentrations of TRE, were presented in

*Impedance parameters of corrosion of iron in 0.5 M HCl at 298 K in the absence and presence of different concentrations of TRE.*

#### **Figure 6.**

*Equivalent circuit model used to fit impedance spectra data.*


#### **Table 2.**

*Electrochemical parameters of iron in 0.5 M HCl solution at different concentrations without and with TRE.*

concentration and temperature of the corrosive medium at different immersion times. Equivalent circuit model used to fit impedance spectra data is presented in **Figure 6**.

#### **3.3 Potentiodynamic polarization measurements**

#### *3.3.1 Influence of concentration*

Polarization measurements were done in order to know about the kinetics of the cathodic and anodic reactions. The anodic and cathodic current potential curves are extrapolated up to their intersection at a point where corrosion current density (Icorr) and corrosion potential (Ecorr) are acquired [21]. **Table 2** shows the electrochemical parameters (Icorr, Ecorr, βa, βc, and CR) obtained from Tafel plots for the iron electrode in 0.5 M HCl solution without and with various concentrations of

TRE. The Icorr values were used to calculate the inhibition efficiency, IE (%), in **Table 2**, using Eq. (8):

$$\text{IE} = [\text{I} - \text{I}\_{\text{corr}}/\text{I}] \ast \mathbf{100} \tag{8}$$

*3.3.2 Influence of temperature*

onto the metal surface [24].

**Figure 7.**

**Figure 8.**

**61**

The surface overage (Θ) was calculated using

*DOI: http://dx.doi.org/10.5772/intechopen.92648*

The inhibition efficiency IE (%) is given by Eq. (8).

*Polarization curves of iron in 0.5 M HCl at different temperatures.*

Polarization curves for the iron in 0.5 M HCl solution are shown in **Figures 7** and **8**

The results of the **Table 2** refer that temperature increase leads to Icorr increase,

while the addition of TRE resulted in the decrease of the Icorr values across the temperature range. The results also indicate that the inhibition efficiencies increased with the concentration of inhibitor but decreased proportionally with temperature. Such behavior can be rationalized that the inhibitor acts by adsorption

*Polarization curves of iron in 0.5 M HCl at different temperatures in the presence of 8 g/100 mL of TRE.*

Θ ¼ IE %ð Þ*=*100 (10)

in two different conditions, with constant concentrations of TRE and in the presence of different concentrations of TRE in the temperature range 283–313 K.

*Electrochemical,Thermodynamic, Surface, and Spectroscopic Study in Inhibition of Iron…*

where Icorr and I are the corrosion current densities with the presence and absence of inhibitor, respectively. The CR values in **Table 2** used the following equation [22]:

$$\text{CR} = \text{3.27} \ast \text{10}^{-3} \text{ i}\_{\text{corr}} \,\text{E}\_{\text{w}} / \text{d} \tag{9}$$

where icorr is the corrosion current density in micro A/cm<sup>2</sup> , Ew is the equivalent weight of the corroding metal in grams, and d is the density of the corroding metal in g/cm<sup>3</sup> .

Under the experimental conditions performed, the cathodic section of the plot represents the hydrogen evolution reaction, while the anodic section represents the iron dissolution reaction. They are determined by the extrapolation of Tafel lines to the respective corrosion potentials.

The results in **Table 3** indicates that the inhibitor reduces the corrosion current value and inhibition; IE (%) increases with the concentration of the inhibitor reaching 88. 90%, at 8 g/100 mL. This result suggests that good inhibition act for TRE [23].


**Table 3.**

*Polarization parameters of iron in 0.5 M HCl at different temperatures with various concentrations of TRE.*

*Electrochemical,Thermodynamic, Surface, and Spectroscopic Study in Inhibition of Iron… DOI: http://dx.doi.org/10.5772/intechopen.92648*

#### *3.3.2 Influence of temperature*

TRE. The Icorr values were used to calculate the inhibition efficiency, IE (%), in

where Icorr and I are the corrosion current densities with the presence and absence of inhibitor, respectively. The CR values in **Table 2** used the following

weight of the corroding metal in grams, and d is the density of the corroding metal

Under the experimental conditions performed, the cathodic section of the plot represents the hydrogen evolution reaction, while the anodic section represents the iron dissolution reaction. They are determined by the extrapolation of Tafel lines to

The results in **Table 3** indicates that the inhibitor reduces the corrosion current

**βa (mV/dec)**

 0.3968 10.8 0.155 0.323 3.54 58.67 0.468 5.9 0.148 0.177 1.933 77.42 0.453 3.8 0.139 0.179 1.25 85.46 0.442 2.9 0.143 0.188 0.979 88.90

 0.379 8.35 0.142 0.328 2.73 73.15 0.458 6.6 0.138 0.205 2.167 78.78 0.458 4.8 0.164 0.206 1.581 84.57 0.441 4 0.131 0.193 1.318 87.14

 0.452 13.5 0.167 0.249 4.4 60.06 0.426 9 0.132 0.263 2.97 73.37 0.474 7 0.152 0.193 2.29 79.29 0.498 5.7 0.14 0.16 1.87 83.14

 0.477 14.6 0.188 0.237 4.788 59.67 0.419 11 0.135 0.28 3.631 69.61 0.379 7.9 0.133 0.339 2.6 78.18 0.468 5.9 0.148 0.177 1.933 83.70

**βc (mV/dec)**

value and inhibition; IE (%) increases with the concentration of the inhibitor reaching 88. 90%, at 8 g/100 mL. This result suggests that good inhibition act for

> **Icorr (μA/cm2 )**

283 Blank 0.677 26.13 0.352 0.183 8.56 —

293 Blank 0.661 31.1 0.316 0.174 10.19 —

303 Blank 0.677 33.8 0.331 0.168 11.07 —

313 Blank 0.714 36.2 0.378 0.119 11.84 —

*Polarization parameters of iron in 0.5 M HCl at different temperatures with various concentrations of TRE.*

where icorr is the corrosion current density in micro A/cm<sup>2</sup>

IE ¼ ½ � I � Icorr*=*I ∗ 100 (8)

CR <sup>¼</sup> <sup>3</sup>*:*<sup>27</sup> <sup>∗</sup> <sup>10</sup>�<sup>3</sup> icorr Ew*=*<sup>d</sup> (9)

, Ew is the equivalent

**CR mm/year** **IE%**

**Table 2**, using Eq. (8):

*Electrochemical Impedance Spectroscopy*

the respective corrosion potentials.

**Ecorr- (mv/SCE)**

**C (g/100 mL)**

equation [22]:

in g/cm<sup>3</sup> .

TRE [23].

**T (K)**

**Table 3.**

**60**

Polarization curves for the iron in 0.5 M HCl solution are shown in **Figures 7** and **8** in two different conditions, with constant concentrations of TRE and in the presence of different concentrations of TRE in the temperature range 283–313 K. The surface overage (Θ) was calculated using

$$\Theta = \text{IE} \left( \% \right) / \mathbf{100} \tag{10}$$

The inhibition efficiency IE (%) is given by Eq. (8).

The results of the **Table 2** refer that temperature increase leads to Icorr increase, while the addition of TRE resulted in the decrease of the Icorr values across the temperature range. The results also indicate that the inhibition efficiencies increased with the concentration of inhibitor but decreased proportionally with temperature. Such behavior can be rationalized that the inhibitor acts by adsorption onto the metal surface [24].

**Figure 7.** *Polarization curves of iron in 0.5 M HCl at different temperatures.*

**Figure 8.** *Polarization curves of iron in 0.5 M HCl at different temperatures in the presence of 8 g/100 mL of TRE.*

Ea, ΔS\*, and ΔH\*, for both corrosion inhibition and corrosion of iron in 0.5 M HCl in the presence and absence of TRE at different concentrations between 283 and 313 K, were calculated from an Arrhenius-type plot (Eqs. (11) and (12)) [25]:

$$\text{Log}\,(\text{I}\_{\text{corr}}) = -\text{E}\_{\text{a}}/2.\text{303RT} \tag{11}$$

where Icorr is the corrosion current density (taken from averaged polarization), Ea is the activation of ion energy, and R is the universal gas constant.

$$\mathbf{I} = \frac{\mathbf{R}\mathbf{T}}{\mathbf{N}\mathbf{h}} \exp\left(\frac{\Delta\mathbf{S} \ast \mathbf{s}}{\mathbf{R}}\right) \exp\left(-\frac{\Delta\mathbf{H} \ast \mathbf{s}}{\mathbf{R}\mathbf{T}}\right) \tag{12}$$

pyrolysis process is endothermic. The decreasing entropy of the reaction at the formation of the complex indicates that the adsorbent inhibitor complex has a

*Plots of Langmuir adsorption isotherm of TRE on iron surface at different temperatures.*

**Inhibitor C (g/100 mL) Ea (kJ/mol) ΔH\* (kJ/mol) ΔS\* (J/mol)** 0.5 M HCl — 7.69 5.22 �275.37 Turmeric root extract (TRE) 2.0 10.02 7.55 �275.83

*Electrochemical,Thermodynamic, Surface, and Spectroscopic Study in Inhibition of Iron…*

*Values of activation parameters ΔS\* and ΔH\* for iron 0.5MHCl in the presence and absence of various*

4.0 15.99 13.52 258.93 6.0 18.99 16.52 �251.65 8.0 18.43 15.96 �255.51

The study of adsorption behavior of active compounds on the iron surface shows

Study the effect of turmeric root extract concentration on the inhibition efficacy by acting C\θ in terms of C. Gibbs standard free energy (ΔGads) is calculated as a single molecule of water, which replaces a molecule. One of the inhibitor molecules has a ratio of 1/55.5, from relationships (13) and (14), according to the isotope of the immersion in adsorption. Langmuir adsorption isotherm is represented by

where θ is the degree of surface cover with the inhibitor, K ads is the adsorption equilibrium constant, and C is the concentration of inhibitor used in the corrosive

adsnRT (13)

Cnθ ¼ 1*=*Kads þ C (14)

Log K <sup>¼</sup> log 1ð Þ <sup>n</sup>55*:*<sup>5</sup> –ΔG0

coherent structure [26–28].

*3.3.3 Adsorption isotherm*

Rearrangement gives Eq. (14):

the following data:

**Figure 11.**

**Table 4.**

*inhibition concentrations.*

*DOI: http://dx.doi.org/10.5772/intechopen.92648*

Eq. (13) [29]:

medium (**Figure 11**).

**63**

where h is Plank's constant, N is Avogadro's number, ΔH\*a is the enthalpy of activation, and ΔS\*a is the entropy of activation.

The plots of Log (Icorr) vs. 1/T and Log (Icorr/T) vs. 1/T gave straight lines with the slope of -Ea/R and -ΔH\* /R, respectively. The intercepts were A and [Ln (R/ Nh) + (ΔS\*/R)] for the Arrhenius and transition state equations, respectively (**Figures 9** and **10**). The calculated values of the activation energy Ea, the entropy of activation ΔS\* , and the enthalpy of activation ΔH\* are presented in **Table 4**.

The activation energy values with TRE film are greater than the absence as shown in **Table 4**. This result is consistent with previous results, since adsorption of active compounds on the metal surface impeded the corrosion reaction as if an additional energy barrier had arisen to dampen the corrosion reaction. The positive signal for a change in the enthalpy of the corrosion reaction indicates that the

**Figure 9.** *Arrhenius plots of log (Icorr) versus 1/T at various concentrations of TRE.*

**Figure 10.** *Variation of log (Icorr/T) versus 1/T at various concentrations of TRE.*

**Inhibitor C (g/100 mL) Ea (kJ/mol) ΔH\* (kJ/mol) ΔS\* (J/mol)** 0.5 M HCl — 7.69 5.22 �275.37 Turmeric root extract (TRE) 2.0 10.02 7.55 �275.83 4.0 15.99 13.52 258.93 6.0 18.99 16.52 �251.65 8.0 18.43 15.96 �255.51

*Electrochemical,Thermodynamic, Surface, and Spectroscopic Study in Inhibition of Iron… DOI: http://dx.doi.org/10.5772/intechopen.92648*

#### **Table 4.**

Ea, ΔS\*, and ΔH\*, for both corrosion inhibition and corrosion of iron in 0.5 M HCl in the presence and absence of TRE at different concentrations between 283 and 313 K, were calculated from an Arrhenius-type plot (Eqs. (11) and (12)) [25]:

where Icorr is the corrosion current density (taken from averaged polarization),

ΔS ∗ R 

where h is Plank's constant, N is Avogadro's number, ΔH\*a is the enthalpy of

The plots of Log (Icorr) vs. 1/T and Log (Icorr/T) vs. 1/T gave straight lines with the slope of -Ea/R and -ΔH\* /R, respectively. The intercepts were A and [Ln (R/ Nh) + (ΔS\*/R)] for the Arrhenius and transition state equations, respectively (**Figures 9** and **10**). The calculated values of the activation energy Ea, the entropy

The activation energy values with TRE film are greater than the absence as shown in **Table 4**. This result is consistent with previous results, since adsorption of active compounds on the metal surface impeded the corrosion reaction as if an additional energy barrier had arisen to dampen the corrosion reaction. The positive signal for a change in the enthalpy of the corrosion reaction indicates that the

, and the enthalpy of activation ΔH\* are presented in **Table 4**.

Ea is the activation of ion energy, and R is the universal gas constant.

<sup>I</sup> <sup>¼</sup> RT Nh exp

*Arrhenius plots of log (Icorr) versus 1/T at various concentrations of TRE.*

*Variation of log (Icorr/T) versus 1/T at various concentrations of TRE.*

activation, and ΔS\*a is the entropy of activation.

*Electrochemical Impedance Spectroscopy*

of activation ΔS\*

**Figure 9.**

**Figure 10.**

**62**

Log Ið Þ¼� corr Ea*=*2*:*303RT (11)

(12)

exp � <sup>Δ</sup><sup>H</sup> <sup>∗</sup> RT 

*Values of activation parameters ΔS\* and ΔH\* for iron 0.5MHCl in the presence and absence of various inhibition concentrations.*

**Figure 11.** *Plots of Langmuir adsorption isotherm of TRE on iron surface at different temperatures.*

pyrolysis process is endothermic. The decreasing entropy of the reaction at the formation of the complex indicates that the adsorbent inhibitor complex has a coherent structure [26–28].

The study of adsorption behavior of active compounds on the iron surface shows the following data:

#### *3.3.3 Adsorption isotherm*

Study the effect of turmeric root extract concentration on the inhibition efficacy by acting C\θ in terms of C. Gibbs standard free energy (ΔGads) is calculated as a single molecule of water, which replaces a molecule. One of the inhibitor molecules has a ratio of 1/55.5, from relationships (13) and (14), according to the isotope of the immersion in adsorption. Langmuir adsorption isotherm is represented by Eq. (13) [29]:

$$\text{Log K} = \log \left( \mathbf{1} \,\text{(55.5)} \mathbf{-} \Delta \mathbf{G}^{0}\_{\text{ads}} \,\text{/RT} \right) \tag{13}$$

Rearrangement gives Eq. (14):

$$\mathbf{C} \backslash \boldsymbol{\theta} = \mathbf{1} / \mathbf{K}\_{\text{ads}} + \mathbf{C} \tag{14}$$

where θ is the degree of surface cover with the inhibitor, K ads is the adsorption equilibrium constant, and C is the concentration of inhibitor used in the corrosive medium (**Figure 11**).


the decrease in contact between the iron and the aggressive medium. Thus, a good adsorptive protection layer that was formed by the inhibitor can efficiently inhibit

*Electrochemical,Thermodynamic, Surface, and Spectroscopic Study in Inhibition of Iron…*

The following table shows the percentages of the studied elements in the pres-

*EDX of (a) polished iron, (b) after 1 hour of immersion in HCl, and (c) treated iron in the presence of 1 g/*

corrosion of steel.

**Figure 13.**

**65**

*100 mL extract.*

ence and absence of TRE.

*DOI: http://dx.doi.org/10.5772/intechopen.92648*

**Table 5.**

*Calculated parameters of Langmuir adsorption isotherm.*

#### **Figure 12.**

*SEM of polished iron (a) before immersion (b) after 1 hour of immersion in HCl 1 g/100 mL of TRE and (c) treated iron in the presence of 1 g/100 mL extract.*

The value of <sup>Δ</sup>Ga is less than 20 kJ mol<sup>1</sup> which is an indication that physical adsorption is dominant [30] (**Table 5**).
