*3.1. St63Gly37 Characterization*

The grafting of the glycerin may take place preferentially on the 7-position of the starch. It could also occur on positions 8 or 9 of the starch (Figure 2). On the other hand, glycerin can be grafted via its OH-13 or OH-14 functions. For the characterization of our products, we used 1H NMR and Fourier transform infrared (FT-IR) spectroscopy techniques.

#### 3.1.1. 1H NMR Characterization

The 1H NMR spectrum of the starch grafted with glycerin St63Gly37 was recorded on a Brucker spectrometer of 400 MHz and dissolved in DMSO as solvent. Results are displayed in Figure 3.

**Figure 3.** 1H NMR Spectrum of St63Gly37.

3.1.2. FT-IR Characterization

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. Figure 4 shows the FT-IR spectra of the native starch in comparison with the St63Gly37 copolymer.

**Figure 4.** FT-IR spectra of the St63Gly37 copolymer (top) and native starch (bottom).

#### *3.2. Weight Loss Measurements*

#### 3.2.1. Effect of Inhibitor 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 2.


**Table 2.** Inhibition efficiency (*Ew*%) of C-Mn steel in 1 M HCl solution at different concentrations of St63Gly37, measured by weight loss at 25 ◦C.

*Ew*%—Inhibition efficiency, *Wcorr*—corrosion rates.

#### 3.2.2. Effect of Temperature

In order to study the effect of temperature on corrosion inhibition of C-Mn steel in HCl solution after two hours at different bio-copolymer concentrations, weight loss studies were carried out in a temperature range from 25 to 50 ◦C.

The variation of corrosion rate (*Wcorr*) and inhibition efficiency *Ew* (%) with the temperature for different concentrations of St63Gly37 bio-copolymer are displayed in Figures 5 and 6, respectively.

**Figure 5.** Variation of corrosion rate (*Wcorr*) as a function of temperature for different concentrations of St63Gly37 bio-copolymer.

**Figure 6.** Variation of inhibition efficiency *Ew* (%) as a function of temperature for different concentrations of St63Gly37 bio-copolymer.

#### *3.3. Thermodynamic and Kinetic Parameters*

Thermodynamic and kinetic parameters, such as activation energy *Ea*, enthalpy, and entropy of adsorption of St63Gly37 on steel, were calculated building the Arrhenius plot. Figure 7 presents the Arrhenius plots of corrosion rate logarithm vs. 1000/T related to blank solution and bio-copolymer solution.

**Figure 7.** Arrhenius plots of the corrosion rate for both the blank solution and the solution of bio-copolymer.

The activation parameters for the corrosion process can be regarded as an Arrhenius-type process according to the following Equations (2) and (3):

$$
\ln(W\_{corr}) = \frac{-E\_a}{RT} + A.\tag{2}
$$

$$
\ln(\mathcal{W}'\_{corr}) = \frac{-E'\_a}{RT} + A.\tag{3}
$$

*Ea* and *E'a* are the apparent activation energies with and without the bio-copolymer, respectively. *T* is the absolute temperature, *A* is a constant, and *R* is the universal gas constant. *Wcorr* and *W'corr* are the steel corrosion rates in the absence and presence of the bio copolymer inhibitor, respectively.

## *3.4. Adsorption Isotherms*

Figure 8 shows the results of adsorption isotherms. Linear plots were obtained in the studied temperature range.

**Figure 8.** (**A**) Freundlich adsorption plot at different temperatures in the studied range; and (**B**) linearization of Freundlich isotherm.

The equilibrium constant of adsorption K is related to the standard free energy Δ *Gads* [21]. Δ *Gads* values at di fferent temperatures can be calculated by Equation (4):

$$K = \frac{1}{55.5} \exp(\frac{-\Delta G\_{ads}}{RT}),\tag{4}$$

where 55.5 represents the concentration of water in solution expressed in mol L−1.

#### *3.5. Electrochemical Tests*
