**5. Conclusions**

C-Mn steel in 1 M HCl solution was investigated to study the corrosion behavior in the temperature range 25–50 ◦C with and without a glycerin-grafted starch as a bio-copolymer working as a corrosion inhibitor.

Results obtained by the weight loss method showed that the inhibition e fficiency increases by increasing the inhibitor concentration. Optimum concentrations of inhibitor equal to 300 mg L−<sup>1</sup> enabled a maximum e fficiency of 94.25%.

The corrosion process and the inhibition e fficiency were found to be significantly dependent on the temperature and concentration of inhibitor. 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. Results indicated that the corrosion inhibition mechanism might be more complex than a simple physisorption process on the steel surface. This might be explained in terms of chemisorption of polymer on steel surface.

The obtained values of corrosion potential and corrosion current density, *Ecorr* and *icorr*, obtained by potentiodynamic polarization, are in agreemen<sup>t</sup> with the weight loss method: The corrosion current densities decrease when the concentration of the inhibitor increases. An analysis of data indicates that the inhibitor might be of the cathodic type.

The decrease in double-layer capacitance values, obtained by EIS method, follows a decrease similar to that obtained for *icorr* by the polarization method. 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.

Results obtained in this study might be developed to supplement corrosion protection in real field applications, injecting the optimum quantities of corrosion inhibitor without any harmful e ffect on the natural environment and human health.

**Author Contributions:** Investigation, B.B.; Methodology, A.M.; Supervision, A.B.; Writing–original draft, S.L.; Writing–review & editing, L.T. and S.M.

**Funding:** This research was funded by Direction Générale de la recherché scientifiqueet du développement technologique, Algerian Ministry of Scientific Research.

**Acknowledgments:** The authors are very grateful to the Algerian Gas Transport TRC for their grea<sup>t</sup> help to make a part of this work by providing us with some steel samples. They also thank all researchers at the LAEPO Research Laboratory from Tlemcen University for their experimental contribution during the corrosion test, and the equipment support from the Algerian University and Society. They also appreciate the contribution of the Department of Environmental Science and Policy from Università degli Studi di Milano.

**Conflicts of Interest:** The authors declare no conflicts of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
