**4. Discussions**

## *4.1. St63Gly37 Synthetisis*

The 1H NMR spectrum interpretation of the product St63Gly37 is carried out by comparison with the peaks of the native starch found in the literature [20]. The spectrum of native starch is shown in Table 5. The glycerin has two chemical shifts at 4.3 ppm corresponding to the proton of the CH, and at 4.4 ppm corresponding to the protons of CH2.


**Table 5.** Chemical shifts of native starch [20].

Chemical shifts of native starch and St63Gly37 bio-copolymer are comparable. The starch and glycerin signals are also found together, thus proving the success of the grafting reaction. The glycerin OH signals can be confused with those of the starch OHs (H8, H9).

After concluding that glycerin is well grafted on starch, the question that still remains is how this grafting takes place. Glycerin grafting may occur preferentially on the 7-position of the starch. It could also occur on positions 8 or 9 of the starch (see Figure 2). On the other hand, glycerin can be grafted via its OH-13 or OH-14 functions. Indeed, the possible reacting OH hydrogens are: H7, H8 and H9. For the positions H8 and H9, they are still visible on the 1H NMR spectrum displayed in Figure 3 and Table 3. We can; therefore, conclude that H7 reacted to give modified patterns according to the structure shown in Figure 2. The presence of the peaks at chemical shifts of 4.4 ppm (triplet) and 4.5 ppm (doublet) correspond, respectively, to the CH and CH2 of the structure.

The 1H NMR spectrum also enables the calculation of relative amounts of starch and glycerin in St63Gly37. The copolymer contains 63% starch and 37% glycerin. The chemical shifts are represented in the following Table 6.


**Table 6.** Chemical shifts δ (ppm) of St63Gly37.

The FT-IR technique is largely used for the characterization of starch as a natural polymer [20]. Examination of the FT-IR spectrum confirms the grafting of glycerin on the starch, by the CH2 bands and the characteristic OH bands of glycerin, as reported in Table 7. Figure 4 shows the FT-IR spectra of the native starch in comparison with the St63Gly37 copolymer. It is visible that the C–O–C bond bands change frequency because of the effect of the proportion of glycerin in the copolymer. The two spectra are almost of the same nature. The assignment of the different vibration bands of the native starch have been previously reported [20]. The comparison of the two FT-IR spectra shows a slight displacement of the elongation bands.


**Table 7.** Attribution of the different vibration bands of St63Gly37.

## *4.2. E*ff*ectOfinhibitor Concentration*

The results of C-Mn steel in HCl solution with different concentrations of St63Gly37 at 25 ◦C, using weight loss measurements, are reported in Table 8.

**Table 8.** Data from the literature correlated to the type, concentration, and maximum inhibition efficiency (*Ew*,max) obtained at 25 ◦C in HCl for Carbon-steel.


Results showed that the inhibition efficiency calculated according to Equation (1) increases with increasing inhibitor concentrations. The highest concentration of inhibitor, equal to 300 mg <sup>L</sup>−1, corres ponded to a maximum efficiency of 94%. For comparison, in Table 8, some data from the literature are reported. The cited papers deal with corrosion inhibition in a HCl environment by some investigated bio-based or synthetic compounds. A comparison of reported data demonstrates how the inhibition efficiency varies depending on the species involved in the inhibition mechanism. Moreover, it is also noticeable that the quantity needed to achieve a satisfying corrosion rate control strongly depends on the selected compounds. In a few cases, reported in Table 8, the used quantities are even excessive, losing the reasonable meaning of "corrosion inhibitors" as "compounds added in low amount to the aggressive environment". The concentration of 300 ppm used in this work was not increased more for the purpose of keeping the inhibitor amount in a logical range.

#### *4.3. E*ff*ect of Temperature*

The variation of corrosion rate (*Wcorr*) and inhibition efficiency *Ew* (%) in the temperature range 25–50 ◦C for different concentrations of St63Gly37 bio-copolymer, obtained by weight loss studies, are displayed in Figures 5 and 6.

Results showed that the inhibition efficiency increases with increasing temperature. As expected, corrosion process and inhibition efficiency are significantly dependent on the temperature [31–33]. Optimum temperature was found equal to 323 K, with a maximum efficiency of 98.07% with inhibitor concentration of 300 mg L−1.

In the absence of corrosion inhibitor (blank solution), corrosion rate increases with increasing temperature, but when St63Gly37 bio-copolymeris added, the dissolution of C-Mn steel is widely retarded. These results indicate that the corrosion inhibition mechanism might be more complex than a simple physisorption process on the steel surface. The values of inhibition e fficiency, obtained using the weight loss method in the experimented temperature range, show that higher temperatures might favor the inhibitor sorption onto the steel surface. This might be explained in terms of chemisorption of polymer on the steel surface. In fact, in case of chemisorption, the extent of adsorption increases with rise in temperature, as reported in a previous work [34].

#### *4.4. Thermodynamic and Kinetic Parameters*

Results showed that the corrosion process for C-Mn steel increases more rapidly according to the temperature in the absence of inhibitor, rather than in its presence. This result confirms that the inhibitor acts as an e fficient corrosion inhibitor in the range of temperatures studied.

Enthalpy and entropy of the corrosion process may be evaluated from the e ffect of temperature by an alternative formulation of transition state, as displayed in the following Equation (8) [35].

$$\mathcal{W} = \frac{RT}{Nh} \exp\left(\frac{\Delta S^{\circ}\_{\,\,a}}{R}\right) \exp\left(-\frac{\Delta H^{\circ}\_{\,\,a}}{RT}\right) \tag{8}$$

where *h* is Plank's constant, *N* is Avogadro number, and Δ*S*◦ *a* and Δ *H*◦ *a* are the entropy and enthalpy of activation, respectively. Table 9 presents the calculated values of *Ea*, Δ*S*◦ *a*, and Δ *H*◦ *a* in inhibited and uninhibited corrosive solutions. It is observed that the activation energy value is higher in the presence of the bio-copolymer inhibitor than in the uninhibited solution. The obtained activation energy value of the corrosion process in the inhibitor's presence, compared to its absence, can be attributed to its sorption. It is the result of electrostatic attraction between charged metal surface and charged species in solution and/or chemical interaction between polymer and metal.

**Inhibitor Concentration (mg L**−**1)** *Ea* **(kJmol**−**1) Δ***Ha* **(kJmol**−**1) Δ***Sa* **(Jmol**−**1)** 0 40.9 38.5 −36.7 5 36.6 34.1 −37.1 10 35.7 33.3 −37.2 50 35.3 32.9 −37.4 100 26.5 24.0 −37.5 200 27.5 25.0 −38.0 300 28.8 26.3 −38.1

**Table 9.** Calculated parameters at di fferent concentrations of the bio-copolymer.

(*Ea*—activation energy, Δ*S*◦ *a* and Δ*H*◦ *a*—the entropy and enthalpy of activation).

The values of Δ *H*◦ *a* are reported in Table 9. The positive sign of the reflects the endothermic nature of the steel dissolution process and values vary in the same way with inhibitor concentration and acid solutions [36].

On the other hand, values of are more positive in the uninhibited solutions and decrease by increasing the inhibitor concentration. Large and negative values of entropies imply that the activated complex in the rate-determining step represents an association rather than a dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex. A similar observation has been reported in the literature [37].

One can notice that *Ea* and Δ *H*◦ *a* values vary in the same way (Figure 11). This result allows for the verification of the known thermodynamic relationship between the *Ea* and Δ *H*◦ *a*, as shown in Equation (9).

$$
\Delta H^{\circ}\_{\text{a}} = E\_{\text{a}} - RT \tag{9}
$$

**Figure 11.** *Ea* and Δ*H*◦ variation depending on the corrosion inhibitor concentration.

## *4.5. Adsorption Isotherm*

As reported above, data indicate that a complex interaction mechanism takes place between the steel and the inhibitor, and a chemisorption might occur. Given this awareness, the analysis, reported in this paragraph, follows anyway the literature approach of applying isotherm adsorption models to verify the most experimental and realistic data. Adsorption isotherms are in fact used to understand the mechanism of metal–inhibitor interaction. The most frequently used isotherms are Langmuir, Frumkin, Temkin, and Parson [38,39]. The type of adsorption isotherm provides information about the interaction among both the adsorbed molecules themselves and their interactions with the metal surface. The most widely used isotherm employs the Langmuir model, whose primary assumptions are: (i) Adsorbate molecules attach to the active sites of the adsorbent surface, (ii) the Langmuir equation assumes that adsorption is monolayer, and (iii) all the sites on the solid surface are equal in size and shape and have equal affinity for adsorbate molecules [40]. However, the last two conditions are hard to fulfill in the corrosion studies, and this is the main weak point in terms of using the Langmuir model, as already evidenced by the literature [8]. Many practical cases, in fact, cannot be described the Langmuir model. The more complex adsorption models take into consideration factors such as the surface heterogeneity and the presence of areas having different adsorption energy or interactions between the adsorbed molecules.

The Freundlich isotherm is empirical and very widely used to describe the adsorption characteristics when the energy of adsorption on a homogeneous surface is independent of surface coverage. Its linearized form is described by the following Equation (10):

$$\ln \theta = \ln K + \frac{1}{n} \ln \mathcal{C}\_{\epsilon} \tag{10}$$

where θ is the surface coverage (calculated as *Ew*/100), *Ce* is the adsorbate concentration in solution at equilibrium (in mg <sup>L</sup>−1), Kis the equilibrium constant, and 1/*n* is a measure of intensity of adsorption. If the plotln θ vs. ln *Ce* displays a linear trend, it means that the adsorption process can be described by the Freundlich model [41].

If Δ*Gads* is lessthan −10.40 kJmol−1, it can be inferred that the inhibitor interacts on the C-Mn steel surface by electrostatic effect.

The negative values of Δ*G*◦*ads*, displayed in Table 8, confirmed the spontaneity of the process and stability of the adsorbed layer on the steel surface [42,43]. The obtained values of Δ*G*◦*ads* show the dependence of Δ*G*◦*ads* on temperature (Table 10), indicating a strong interaction between the bio-polymer molecules and the metal surface.

Thermodynamically, Δ*G*◦*ads* is related to the standard enthalpy and entropy of the adsorption process, Δ*H*◦*ads* and Δ*S*◦*ads*, respectively, via the Equation (11):

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

The positive value of Δ*H*◦ suggests an endothermic nature of the metal dissolution process in the presence of the inhibitor. The positive value of Δ*S*◦ indicates that the adsorption process is accompanied by an increase in entropy, which is the driving force for the adsorption of the inhibitor onto the metal surface [31].

**Table 10.** Variation of the thermodynamic parameters according to the Langmuir isotherm at the studied temperatures.


*Kads*—adsorption parmeter, <sup>Δ</sup>*G*◦*ads*—adsorption enthalpy, <sup>Δ</sup>*H*◦*ads*—enthalpy, Δ*S*◦*ads*—entropy of the adsorption process.
