**1. Introduction**

Secondary metabolites, derived from natural plants, of which polyphenols constitute the grand part, have a wide range of activities [1]: biological (antibacterial, antioxidant [2, 3], anti-inflammatory [4], antidiabetic [5], anti-carcinogenic [6], etc.) and chemical (in connection with their chelating power with metals and their reducing properties generated by the hydroxyl functions of their aromatic rings). These molecules are used in several fields: as preservative food additives (constituting an alternative to the use of synthetic ones, such as buthylhydroxyanisol (BHA) and buthylhydroxytoluene (BHT), which have carcinogenic effects [7]; as flavoring in cosmetology [8]; as additives in electrolytic baths during the metals electrodeposition [9–11] and as corrosion inhibitors [12–14].

Even today, these molecules have not revealed all their secrets. Our research aims to understand the power of polyphenols in the field of metal protection against corrosion.

The *Echium italicum L.* is an annual herbaceous plant, available in most of Europe, Asia [15–18], and Africa [19]. This plant widespread in Tebessa area (Algeria) [20], is used traditionally for several infections [15, 17, 18]. Furthermore, the *Echium italicum L.* contains different polyphenolic compounds, known as effective additives in electroplating baths [16, 17, 21].

We continue in this investigation to focus on the *effect* of plant extract obtained from the aerial parts of *Echium italicum L.,* on the corrosion inhibition of carbon steel (CS, API 5 L-X60) against 1 M hydrochloric acid solution, using weight loss, potentiodynamic polarization and electrochemical impedance measurements.

On the other hand, and in order to better understand the underlying reaction mechanisms of the corrosion phenomena to choose which inhibitors to use to combat it, the prediction of the implementation behavior of effective corrosion control measures is paramount. For this purpose, mathematical simulation was used as a powerful method [12, 22] to evaluate the kinetic parameters of both corrosion rate and inhibition efficiency.

### **2. Corrosion**

Corrosion, from the Latin "corrodere" (which means "to attack") is one of the harmful global problems affecting several industrial fields such as maritime installations, petroleum, chemical, civil engineering, electrical, nuclear, sanitary, and other industries, without forgetting the environmental impact [23–25].

The corrosion of metals and their transformation into various compounds cause an alteration in their appearance, either on the surface or in-depth, thus reducing their effectiveness (parts breakage, the toxicity of the resulting metal oxides, etc.).

Several factors come into play in this phenomenon. They can be chemical (water, oxygen, salinity, acidity, etc.), physical (temperature, pressure, etc.), or biological (marine biological deposit of plants or animals, bacteria, etc.).

This phenomenon has Several definitions:


### **3. Corrosion protection**

Corrosion control is the set of measures that can protect metallic materials from the harmful effects of the aggressive environment. Many of these methods were reported in the literature [28, 29]. The first protection is the choice of pure metal or alloys resistant to these attacks [30].

*Mathematical Relationship Based on Experimental Data, for Corrosion Inhibition Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.99616*

After designing the equipment using the appropriate material, it must be protected against corrosion should be considered to avoid many problems and ensure a certain service life. For this, the preferred outlet must comply with environmental protection requirements and allow the recycling or disposal of the various components at the end of their use by applying the following choices [31]:


All these solutions have drawbacks of efficiency over time, cost and environmental pollution, which is why other alternatives are exploited in research for the benefit of sustainable development that respects the environment. To this end, there have recently been some new alternatives products both environmentally friendly and less expensive as the use of inhibitors from naturals origin [32].

### **4. Corrosion inhibitors**

Inhibitors have been successfully applied to prevent corrosion and damage in many and varied technical fields for a long time. These products have been frequently studied because they provide simple solutions to protect metals from corrosion in the aquatic environment. With the originality of being the only means of interference of the corrosive environment with steel, these compounds reduce the rate of corrosion of metals when added in appropriate amounts, without apparent change [9, 14].

### **5. Materials and methods**

#### **5.1 Materials**

The vegetable material was collected during May 2017 from the East-North of Tebessa (Algeria). The extraction and purification of the *n-*butanolic extract obtained from *Echium italicum* L. (BEEI) were performed as reported in litérature [10, 33, 34].

The tested aggressive medium was a chlorhydric acid solution (1 M), and the investigated inhibitors were freshly prepared solutions of BEEI with different concentrations (from 100 to 500 ppm).

Before each mesurements, the substrates were abraded using emery paper with different grades (from 200 to 2000), cleaned with distilled water and then acetone. The used substrates were carbon steels with the following composition: (by weight%): C (0.26%), Mn (1.35%), P (0.03%), S (0.03%) and Fe (98.33%). Weight loss measurements were performed with specimens with dimensions of 1 cm x 1 cm x 1 cm. For the electrochemical experiments, only an exposed surface of 1 cm2 was used. All measurements were conducted in an aerated area.

#### **5.2 Methods**

#### *5.2.1 Electrochemical measurements*

All electrochemical measurements were accomplished through a Voltalab (PGZ 301) potentiostat and controlled by software model (Voltamaster 4) under given conditions. The electrochemical characteristics of CS sample in uninhibited and inhibited solutions were realized in conventional three-electrode cell: CS as working electrode, platinum electrode as counter electrode and saturated calomel electrode (SCE) as a reference.

#### *5.2.1.1 Potentiodynamic polarization*

Potentiodynamic polarization curves were recorded after total immersion of the working electrode (CS) in 1 M HCl solution containing different concentrations of the tested inhibitor by changing the electrode potential from +250 to �250 mV vs. open circuit potential (OCP) with a scan rate of 1 mV/s. The linear Tafel segments of anodic and cathodic curves were extrapolated to corrosion potential ð Þ *Ecorr* to get corrosion current densities *i*ð Þ *corr* . The ð Þ *ηw*% at different concentrations of BEEI were calculated using the formula (1) [35]:

$$\eta\_p(\%) = \frac{i\_{corr}^0 - i\_{corr}}{i\_{corr}^0} \times 100 \tag{1}$$

where *i* 0 *corr* and *icorr* are the corrosion current density in the absence and presence of BEEI, respectively.

#### *5.2.1.2 Electrochemical impedance spectroscopy*

Electrochemical impedance spectroscopy (EIS) experiments were conducted in the frequency range of 100 kHz–10 mHz with a signal amplitude perturbation of 10 mV at open circuit potential (OCP) measured during 60 min of immersion in the tested solutions. The percentage of the inhibition efficiency ð Þ *ηw*% was calculated using the polarization resistance by the following relationship [36]:

$$
\eta\_R = \frac{R\_p - R\_p^0}{R\_p} \times 100\tag{2}
$$

where *R*<sup>0</sup> *<sup>p</sup>* and *Rp* are polarization resistances without and with inhibitor addition, respectively.

#### *5.2.2 Gravimetric method*

For these measurements, the prepared and pre-weighed CS substrates were totally immersed in beakers containing 1 M HCl without and with the addition of diverse concentrations of BEEI. The substrates were taken out after two hours, washed with 20% NaOH solution containing 200 g/l of zinc dust with a brush, rinsed severally with bidistilled water, dried with acetone, washed again with bidistilled water, dried and

*Mathematical Relationship Based on Experimental Data, for Corrosion Inhibition Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.99616*

reweighted [34]. From the weight loss data, the corrosion rate ð Þ *CR* was calculated according to Eq. (3) [37]:

$$CR = \frac{w}{A.t} \tag{3}$$

where *w* is the average weight loss, *A* is the total area of one CS sample, and *t* is the immersion time. The inhibition efficiency *η<sup>w</sup>* can be calculated by Eq. (4) [36]:

$$
\eta\_w \%= \frac{CR^0 - CR}{CR^0} \times 100 \tag{4}
$$

where *CR*<sup>0</sup> and *CR* are the values of corrosion rate in absence and presence of BEEI respectively.

#### *5.2.3 Mathematical regression*

A mathematical model is used to correlate variables by fitting an equation to experimental data. When using two variables, one of them is considered as explanatory, whereas the other is considered as a dependent. The linear regression of *y* is given as a function of *x* in the equation *y* ¼ *ax* þ *b* which minimize the value Δð Þ *a*, *b* [38]:

$$\Delta(a,b) = \sum\_{i=1}^{i=N} \left[ y\_i - (a\mathbf{x}\_i + b) \right]^2 \tag{5}$$

where *a* and *b* can be calculated as follow:

$$a = \frac{\sum\_{i=1}^{i=N} (\mathbf{x}\_i - \overline{\mathbf{X}}) \left( y\_i - \overline{\mathbf{Y}} \right)}{\sum\_{i=1}^{i=N} \left( \mathbf{x}\_i - \overline{\mathbf{X}} \right)^2} \tag{6}$$

and

$$b = \overline{Y} - a\,\overline{X}.\tag{7}$$

### **6. Results and discussion**

#### **6.1 Electrochemical impedance spectroscopy**

EIS was performed to estimate CS corrosion behavior in the presence of 500 ppm of BEEI at 298 K.

The inspection of Nyquist plots presented in **Figure 1** shows only one depressed capacitive imperfect semicircle at the higher frequency range, indicating that the corrosion reaction is controlled by the charges transfer process on a heterogeneous and irregular steel surface electrode [39–41].

The electrical equivalent circuit model (EEC) adjusted by fitting from the resulting impedance diagrams for Nyquist plots is reported in literature [12].

From **Table 1**, a noticeable increase in Rct values and a decrease in Cdl at 500 ppm of BEEI due to the formation of protective diapers on the CS surface are observed. This phenomenon can be explained by the higher adsorption of phytochemical

**Figure 1.**

*Nyquist spectra for carbon steel in HCl solution in the absence and presence of 500 ppm of BEEI.*


#### **Table 1.**

*EIS parameters of CS in HCl solution in the absence and presence of 500 ppm of BEEI.*

components above CS upon BEEI addition which ultimately reduces the charge transfer between the CS surface and the corrosion medium [42]. Consequently, we noticed that the *η<sup>R</sup>* reached 74% at 500 mg.L<sup>1</sup> .

#### **6.2 Potentiodynamic polarization measurement**

To further evaluate the efficiency of the expected green inhibitor, the polarization technique was studied due to its excellent reliability. **Figure 2** shows the parameters given in **Table 2** and extracted from the CS electrode polarization curves for 500 ppm BEEI in the presence of 1 M HCl solution, at 293 K.

As can be seen in this figure, at 500 ppm of BEEI, the cathodic and anodic corrosion current densities will decrease. This comportment can be attributed to the adsorption of the inhibitor at the carbon steel interface [43] by reducing the dissolution of steel and delaying the hydrogen evolution reaction [44].

In our study, the maximum displacement in Ecorr value for the optimum concentration of BEEI was 13.3 mV (< 85 mV), which exhibits that the inhibitor acts as a mixed type [43, 45]. The highest inhibition efficiency was 75% at a concentration of 500 ppm.

#### **6.3 Weight loss measurements**

To study the behavior of the steel protection in the presence of an aggressive solution, the weight loss method was used. Based on this approach, the *CR*, the *η<sup>w</sup>* and *Mathematical Relationship Based on Experimental Data, for Corrosion Inhibition Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.99616*

#### **Figure 2.**

*Potentiodynamic polarization curves for carbon steel in HCl solution with and without inhibitor addition.*


#### **Table 2.**

*Potentiodynamic polarizations parameters of CS in HCl solution in the absence and presence of 500 ppm of BEEI.*


#### **Table 3.**

*Corrosion parameters obtained from weight loss measurements of CS in 1 M HCl with different concentrations of BEEI at different temperatures.*

the degree of surface coverage *θ* were calculated and are very useful for discussing adsorption properties.

From **Table 3**, it can be observed that the *CR* values reduced progressively with increasing BEEI concentration, reaching 0.0334 g cm�<sup>2</sup> h�<sup>1</sup> for 500 ppm at 293 K. This result indicates that the addition of this inhibitor slows down the dissolution process of the CS. There was also a *CR* increase with increasing temperature.

This behavior can be attributed to the removal of some adsorbed molecules contained in the BEEI, through thermal energy-induced mechanical vibration [46]. As the inhibition efficiency is derived from the weight-loss method, the highest inhibition efficiency was 72.88% for 500 ppm of BEEI at 293 K.

#### *6.3.1 Thermodynamic parameters*

The good fitting for experimental data of gravimetric measurements at all measured temperatures supports the applicability of Freundlich adsorption model expressed by the subsequent Equation [47, 48]:

$$
\log \theta = \log K\_{ads} + a \log \text{C} \tag{8}
$$

where,

*Kads* is the adsorption constant which could be taken as a measure of the strength of the adsorption forces between the inhibitor molecules and the metal surface [49].

*α* is the parameter taking into account the heterogeneity of the intermolecular interactions in the adsorbed layer and the steel surface.

*Kads* represent the values calculated from the intercept lines of *logθ* versus Log C.

The standard adsorption free enthalpy Δ*G*° *ads* can be calculated from the equation given below [50, 51]:

$$
\Delta \mathbf{G}\_{ads}^{\prime} = -RT \ln(\mathbf{C}\_{H\_2O} \times \mathbf{K}\_{ads}) \tag{9}
$$

where

*R* is the gas constant,

*T* is the absolute temperature,

*CH*2*<sup>O</sup>* is the water concentration expressed in mg L�<sup>1</sup> with an approximate value of 10<sup>6</sup> . [52].

The standard adsorption enthalpy Δ*H*° *ads* can be calculated by the relationship (10) [53]:

$$
\ln K\_{ads} = \frac{-\Delta H\_{ads}^{\circ}}{RT} + I \tag{10}
$$

where

*I* is the integration constant.

The Δ*H*° *ads* values were calculated from the intercept lines of *ln Kads* versus 1*=T*. The standard adsorption entropy Δ*S*° *ads* was calculated using the equation below:

$$
\Delta \mathbf{S}\_{ads}^{\circ} = \frac{\Delta \mathbf{H}\_{ads}^{\circ} - \Delta \mathbf{G}\_{ads}^{\circ}}{T} \tag{11}
$$

The obtained results for *Kads*, Δ*G*° *ads*, Δ*H*° *ads* and Δ*S*° *ads* are presented in **Table 4**. From **Table 4**, we noticed the following:


**Table 4.**

*Thermodynamic parameters of the adsorption of BEEI on the CS in 1 M HCl at different temperatures.*

*Mathematical Relationship Based on Experimental Data, for Corrosion Inhibition Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.99616*


#### *6.3.2 Activation parameters of the corrosion process*

The Arrhenius-type process was used to calculate the activation energies ð Þ *Ea* between the corrosion rate of CS in acidic solution, from the equation given below [61]:

$$
\ln \text{CR} = -\frac{E\_a}{RT} + \ln D \tag{12}
$$

Where

*Ea* is the apparent activation energy of the CS dissolution.

*D* is the Arrhenius pre-exponential factor.

The logarithm of the *CR* versus 1*=T* can be branded by straight lines. The activation energy values were calculated from Arrhenius plots *lnCR* versus 1/T and registered in **Table 5**.

For the evaluation of the enthalpy Δ*Ha* as well as the standard adsorption entropy Δ*Sa* of the corrosion activation process, the alternative formulation of Arrhenius equation was employed [62, 63]:

$$\ln\frac{CR}{T} = \left[\ln\frac{R}{hN\_a} + \frac{\Delta S\_a}{R}\right] - \frac{\Delta H\_a}{RT} \tag{13}$$

where

h is the Planck's constant,

Na is the Avogadro's number.

The plot of lnCR*=*T vs. 1*=*T gave a straight line with a slope of ð Þ �ΔHa*=*R and an intercept of ln Rð *=*hNa þ ΔSa/R), from which the values of ΔSa and ΔHa were calculated and registered in **Table 5**.


**Table 5.**

*Activation parameters for carbon steel in HCl with 500 ppm of BEEI.*

The activation energy values were higher in the presence of 500 ppm of BEEI than in its absence, which proved the adsorption of BEEI molecules on the substrate by electrostatic bonds (physisorption) [62, 64]. The positive signs of Δ*Ha* indicate the endothermic nature of the dissolution process [65].

The BEEI adsorption process is accompanied by a decrease in its entropy. This can be explained that before the adsorption of the extract on the steel, the disorder degree of the inhibitor molecules is high, but when the molecules are adsorbed on the surface of the substrate, there is a decrease the in the disorder (i.e. a decrease in the entropy) [66].

#### **6.4 Mathematical regression**

This investigation focused on how could the used inhibitor (BEEI) decrease the corrosion rate ð Þ *CR* and offer a good inhibition efficiency ð Þ *η<sup>w</sup>* .

For that, a mathematical method was employed to predict the influence of a variable ð Þ *x* which is the BEEI concentration as a function of different variables *yi* which are *CR* and *η<sup>w</sup>* for 293 K, 303 K, and 313 K [51, 67]. The building of a mathematical model that can correlate between three variables (*CR*, *ηw*, and *C*) was also highlighted.

Based on experimental data of the CS behavior in the presence of BEEI at various temperatures, a linear regression relation between *CR* and *ηw*, was performed.

As reported by Khadom et al. [67], when the correlation coefficient of the correlation is < 0.30, the correlation is weak and when this coefficient is between 0.50 and 0.70, the correlation is important, while if it is >0.90, the correlation will be strong.

According to the plots shown in **Figure 3**, a decrease in the CR with a raise in the ηw, when adding BEEI with various concentrations at different temperatures was distinguished, and based on correlation coefficients *r*<sup>2</sup> *CR* and *r*<sup>2</sup> *<sup>η</sup><sup>w</sup>* given in **Table 6**, the concordance between the experimental and the simulated results is very strong.

**Figure 3.** *Mathematical relationship between corrosion rate and inhibitory efficiency at different temperatures.*

*Mathematical Relationship Based on Experimental Data, for Corrosion Inhibition Mechanism… DOI: http://dx.doi.org/10.5772/intechopen.99616*


**Table 6.**

*Correlation coefficients evaluated from linear regression of CR and η<sup>w</sup> for CS with and without BEEI solution.*

As we can see from the obtained results, the correlation coefficients were almost >0.90 for considered temperatures. However, as the temperature rises the correlation coefficients go up. The Eqs. (12)–(14) construe the mathematical expression obtained for the relationship between the *CR* and the *η<sup>w</sup>* at different temperatures.

This mathematical model exhibits a decrease in the corrosion rate when inhibition efficiency rises, with reducing in temperature. These results indicate the powerful concordance between experimental and predicted results.

T = 293 K

$$
\eta\_w = 99.9849 - 811.470.CR \tag{14}
$$

T = 303 K

$$
\eta\_w = 99.9977 - 594.869.CR \tag{15}
$$

T = 213 K

$$
\eta\_w = \text{99.9910} - \text{582.713.CR} \tag{16}
$$

### **7. Conclusion**

In order to estimate the multiple benefits of phenolic compounds, a phenolic *Echium italicum* L. extract was evaluated as an efficient corrosion inhibitor. The subsequent conclusions can be pointed out:


• The *Echium italicum* L. extract has prooved to be an effective corrosion inhibitor, and hence the phenolics can be useful on various existing industries to prevent losses caused by corrosion of steel without harming the environment.

For a more extensive valuation of phenolic products, many perspectives are envisaged, namely:

