3.5.1. Polarizations

Current–potential plots resulting from cathodic and anodic polarization curves of steel in 1 M HCl in the presence of the studied bio-copolymer at various concentrations were recorded. The Tafel plots, recorded with a cathodic-to-anodic polarization of the system are shown in Figure 9.

**Figure 9.** Cathodic and anodic polarization curves of C-Mn steel in 1 M HCl at different concentrations of St63Gly37. Scan rate 1 V min−1.

The electrochemical parameters and the inhibition e fficiencies (*EI*), determined by the following Equation (5), are presented in Table 3:

$$E\_I \%= \left(1 - \frac{I\_{corr}}{I\_{corr}^{\diamond}}\right) \cdot 100,\tag{5}$$

where *icorr* and *i* ◦ *corr* are the corrosion current density values with and without St63Gly37 bio-copolymer inhibitor, respectively, determined by extrapolation of the cathodicbranch of the Tafel plot.

**Table 3.** Polarization parameters (*Ecorr* and *Icorr*) values and inhibition e fficiencies of C-Mn steel corrosion in 1 M HCl at di fferent concentrations of St63Gly37 bio-copolymer at 298 K.


*Ecorr*—corrosion potential, *icorr*—corrosion current density, *EI*—inhibition e fficiencies, β*c*—Tafel slope constant.

#### 3.5.2. Electrochemical Impedance Spectroscopy

A study of C-Mn steel corrosion behavior in 1 M HCl solution with and without St63Gly37 at 298 K was studied by EIS after an immersion time of 30 min. The purpose was to compare and complete the results obtained by the previous weight loss and polarization methods [22]. Nyquist diagrams obtained in the presence of various concentrations of bio-copolymer are shown in Figure 10. The deduced impedance parameters, as charge transfer resistance *Rt* (Ω cm2), double-layer capacitance *Cdl* (μFcm−2), and inhibition e fficiency (*ERt*%), are shown in Table 4.

**Figure 10.** Nyquist plots for C-Mn steel in 1 M HCl at di fferent concentrations of St63Gly37 bio-copolymer (298 K).

**Table 4.** Impedance parameters for corrosion of C-Mn steel in 1 M HCl at di fferent concentrations of St63Gly37 at 298 K.


*Rt*—charge transfer resistance, *fmax*—maximum frequency, *Cdl*—double-layer capacitance, *ERt*—inhibition e fficiency from the charge transfer resistance.

The charge transfer resistance values were qualitatively estimated from the di fference in impedance at lower and higher frequencies [23]. The double-layer capacitance was obtained at the frequency *fm*, at which the imaginary component of the impedance is maximal (−*Zi,max*) according the following Equation (6):

$$\mathbb{C}\_{dl} = \frac{1}{2\pi f\_m \cdot Rt}.\tag{6}$$

The inhibition e fficiency from the charge transfer resistance was calculated by the following Equation (7):

$$E\_{Rt}(\%) = \frac{R'\_t - Rt}{R'\_t} \times 100,\tag{7}$$

where *R't* and *Rt* are the charge transfer resistances with and without the St63Gly37 bio-copolymer as a corrosion inhibitor, respectively.
