*2.1. Reagents*

The used reagents were maize starch (St), glycerin (Gly), hydrochloric acid 37%, and sodium hydroxide. They were supplied by the Aldrich Company. The only solvent used was bi-distilled water.

#### *2.2. C-Mn Steel*

Tests were conducted on C-Mn steel samples obtained and treated according to the procedure described in our previous work [4].

Atomic composition was compared to the one quoted in the material test certificates [16]. Elemental composition of the cutting samples (Table 1) was determined by emission spectroscopy analysis type "Spectro RP 212". Composition and microstructure can vary significantly, resulting in substantial differences in corrosion performances in a corrosion regime. Chemical composition of steel, displayed in Table 1, was in compliance with the API norm (American Petroleum Institute).


**Table 1.** Chemical composition of C-Mn steel.

We observed a low carbon composition (0.129%), providing high chemical resistance, high concentration of manganese (1.590%), and low sulfur and phosphorus content. This composition can lead to the formation of manganese sulfide (MnS) inclusions, which are not desirable in the microstructure, being able to start corrosion pitting [7,17].

The surfaces of steel samples were observed under an optical microscope and SEM (Figure 1). The metallographic images revealed a fine microstructure of ferrito-perlitictype, with ferritic prevalencein the presence of clusters of pearlite in the grain boundaries withsome inclusion fields.

**Figure 1.** Micrographs of ×60 steel. (**a**) Optical microscope observation; (**b**) SEM imaging (scale bar = 5 μm).

The refinement of ferritic size was obtained by di fferent hardening and precipitation mechanisms based on a dislocation movement that increases elasticity limit and steel resistance. The SEM image (Figure 1b) confirms the fine microstructure of steel.

## *2.3. Test Solution*

The chosen solution for corrosion tests was 1 M HCl solution prepared from analytical grade 37% (Aldrich) by dilution with bi-distilled water.

#### *2.4. Synthesis of Bio-Copolymer (St63Gly37)*

Anew glycerin-starch bio-copolymer (abbreviated from now on by (St63Gly37) was synthesized by modification of maize starch and used as corrosion inhibitor. Glycerin grafting on the starch takes place in three stages. The first and second steps consisted of the preparation of aqueous solutions of 50% glycerin (A) and 50% starch (B), respectively. In the third step, the glycerin (A) and starch (B) solutions were poured into a 200 mL Erlenmeyer flask, to which 3 mL of 1 M HCl were added under stirring and heating to evaporate all the water. Heating and stirring were continued for the sole purpose of evaporating all the water contained in the solution. The obtained mixture was neutralized

by adding 3 mL of 1 M NaOH. The molecular structures of the involved species are displayed in Figure 2.

**Figure 2.** (**A**)Chemical structure of glycerin; (**B**) chemical structure of native starch; (**C**) chemical structure of St63Gly37.

#### *2.5. 1H NMR Characterization of St63Gly37*

The 1H NMR spectrum of the starch grafted with glycerin St63Gly37 was recorded on a Brucker spectrometer of 400 MHz and dissolved in DMSO-*d*6 as solvent. The interpretation of the 1H NMR results of the product St63Gly37 was carried out by comparison with the peaks of the native starch found in the literature [18,19].

#### *2.6. FT-IR Characterization of St63Gly37 Bio-Copolymer*

FT-IR spectra were recorded on a Fourier transform infrared (FT-IR) spectrometer Agilent Technologies Cary 600 Series with a resolution of 4 cm<sup>−</sup><sup>1</sup> in the LAEPO laboratory, University of Tlemcen. They were employed to observe the characteristic transmittance bands in St63Gly37. The FT-IR spectra were recorded over the frequency range between 500 and 4000 cm<sup>−</sup>1.

#### *2.7. Corrosion Inhibitor Tests*

The weight loss (2 cm<sup>2</sup> apparent surface area) and electrochemical tests (1 cm<sup>2</sup> exposed surface to the corrosive solution) were carried out according to procedures described in previous works [4,13]. St63Gly37 testing was carried out in 1 M HCl solution, varying the concentration of the inhibitor between 5 and 300 mg L−1. The temperature range was 25–50 ◦C.

The inhibition efficiency (*Ew %*) was calculated using the following Equation (1):

$$E\_{\overline{w}} = (1 - \frac{\mathcal{W}\_{corr}}{\mathcal{W}\_{corr}^{\circ}}) \times 100,\tag{1}$$

where *Wcorr* and *W*◦*corr* are the corrosion rates of steel samples in the absence and presence of St63Gly37, respectively.

A potentiostat (Amel 549) and linear sweep generator (Amel 567) were used to record the current–voltage curves. The scan rate was 1 V min−1. The reference electrode was a saturated calomel electrode (SCE); the counter electrode was the platinum electrode. The working electrode was polarized at 800 mV for 10 min before recording the cathodic curves. For the anodic curves, the potential of the electrode was swept from its open circuit value after 30 min. A Voltalab PGZ-100 electrochemical system was used for the determination of the electrochemical impedance spectroscopy (EIS) at *Ecorr* after immersion in solution. After determination of the steady-state current at a given potential, sine wave voltage (10 mV) peak to peak, at frequencies between 100 kHz and 10 mHz, was superimposed on the rest potential. The measurements performed at rest potentials after 30 min of exposure were automatically controlled by computer programs. EIS diagrams are detailed in the Nyquist representations.
