4.6.1. Polarizations

The values reported in Table 3 showed results in agreemen<sup>t</sup> with what was obtained by the weight loss method: Corrosion current densities decrease when the concentration of the inhibitor increases. Maximum value of inhibition efficiency of 91% was obtained for inhibitor concentration of 300 mg L−1.

The obtained polarization curves indicate that the addition of the bio-copolymer influences the kinetics (decrease in reaction rate) but does not modify the mechanism of the cathodic process [22].

From a thermodynamic point of view, at the acidic pH (1 MHCl) and Ecorr (ca. −450 mV vs. SCE) of the investigated system, both the oxygen reduction reaction and hydrogen evolution reaction could potentially take place as the iron Pourbaix diagram states [23].

As for the kinetic aspects related to the cathodic Tafel slope calculation, it is apparent that slope values are markedly different from the theoretical ones expected in the case of pure hydrogen evolution reaction. Tafel slope is in fact known to be 118, 39, or 29.5 mV/decade when the discharge reaction, electrochemical desorption reaction, or recombination reaction is rate-determining, respectively [44]. This might mean that more than one reaction is taking place in the considered potential range.

However, it is shown in Table 3 that Tafel slopes do not remarkably change with addition of different concentrations of inhibitor, thus confirming the hypothesis that the inhibitor does not significantly change the cathodic reaction mechanisms. Therefore, the use of the polarization curves as a tool for deriving quantitative parameters, such as inhibition efficiency, can be justified since it is based on relative calculations (using current densities values in the presence and absence of inhibitor) and does not imply an absolute evaluation.

Considering the anodic reaction, C-Mn steel corrodes in acid medium according to an electrochemical process by anodic dissolution of iron. This phenomenon might be accentuated if other parameters intervene, such as microbiological activity or mechanical stresses. The almost constant value of *Ecorr* at a first sight might give an indication that the inhibitor is of mixed type acting by adsorption mechanism. However, the cathodic branches trend, due to inhibitor concentration increase, and the overlap of all the anodic branches points to the inhibitor being a cathodic one. This is also in agreemen<sup>t</sup> with results obtained at different temperatures. In fact, data showed that the corrosion inhibition mechanism might be more complex than a simple physisorption process on the steel surface.

#### 4.6.2. Electrochemical Impedance Spectroscopy

According to results reported in Figure 10 and Table 4, the impedance diagrams show a semi-circle, indicating that the charge transfer process mainly controls the steel corrosion. By increasing the concentration of St63Gly37, the charge transfer resistance increases and double-layer capacitance (*Cdl*) decreases. A minimum value of *Cdl b* of 31.64 μFcm−<sup>2</sup> was obtained for inhibitor concentration of 300 mg L−1. The decrease in *Cdl* can be explained by the sorption of inhibitor compound on steel surface leading to the protection from the aggressiveness of HCl. The kinetics of the cathodic hydrogen evolution reaction is decreased and the chlorides are prevented from crossing the protective barrier [26,27]. The resulting inhibition e fficiency (*ERt*) increases progressively when the concentration of the inhibitor increases. A maximum value of 90% was obtained for inhibitor concentration of 300 mg L−1. These obtained values are in agreemen<sup>t</sup> with results obtained by weight loss and polarization method.
