**1. Introduction**

Hypoeutectoid C-Mn steels are typically employed in many structural applications and, being commonly exposed to near-neutral aerated environment, undergo general corrosion in the form of rust. Cathodic protection and/or coating, together with the use of adequate corrosion allowance, enables the phenomenon to be controlled, thus reducing the risk of failure [1]. The use of C-Mn steel

in a broader context, where the steel comes in contact with more aggressive solutions (for example, during pickling or oil well acidizing treatments as well as acid-releasing processes [2,3], might need diverse technologies, such as addition of inhibitor products.

In this work, we propose to investigate a method to control and minimize the steel corrosion in the presence of HCl by adding a corrosion inhibitor of bio-polymeric nature. The test solutions are meant to mimic the aggressiveness of the environment due to in-situ generation of harsh byproducts (i.e., during plastic or biomass combustion) or due to an environment that is aggressive itself in the exercise conditions (i.e., pickling or acidizing treatments).

The corrosion inhibition strategy is well-known to help in reducing the economic input of corrosion damage [4,5]. Moreover, through a proper selection of the nature of the used inhibitor compounds, the related toxicological aspects can be controlled. Efficient inhibitors are compounds whose activity is related to the presence in their structure of hetero atoms like nitrogen, oxygen, or sulfur [6]. It was observed that adsorption depends mainly on the presence of lone pair electrons, electron-donating groups and π-orbital character of the molecule [7]. However, even if many synthetic organic compounds are effective inhibitors, regarding their cost and toxic nature, researches were compelled [8,9]. The concern is not only related to the environmental poisoning, needing effective procedures of inhibitor removal before flowing the used solution out of the industrial plant, but also to the contact of the protected metal, for example, with food, beverages, and medicals, during its lifecycle. In the last decade, many research works were focused on the use of "green" corrosion inhibitors, lacking in the adverse health issues connected to the organic compounds used in the past [8,9]. The use of biomaterials as corrosion inhibitors takes considerable attention due to their inherent stability, availability, cost effectiveness, and environmentally-friendly nature [10,11].

The protection of a steel structure against corrosion has already been studied by means of biopolymers as corrosion inhibitors [12,13]. Bio-copolymers derived from starch, already used in the literature as a corrosion inhibitor, motivated our research [14,15].

The present work reports on the establishment of different possibilities of grafting glycerin on starch. We looked at the different grafting sites on amylopectin that can receive glycerin molecules. By the conditions, this was possible by eliminating amylose chains. The obtained product was used as corrosion inhibitor of C-Mn steel in 1 M HCl solution in the temperature range 25–50 ◦C. The corrosion inhibition was investigated by weight loss measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS). Appropriate corrosion parameters such as corrosion current density (*icorr*), polarization resistance (*Rp*), and parameters derived from EIS were derived to characterize the corrosion process. The kinetic and thermodynamic parameters were determined and discussed.

#### **2. Materials and Methods**
