Hydrochloric-acid treatment is used to clean up the mineral deposits on steel-based desalination and water purification systems. However, the same medium promotes carbon steel dissolution by the proton reduction mechanism, chloride-enhanced corrosion and stripping of thin oxide layer. This work is aimed at finding out whether \textit{Equisetum arvense} aerial part (EAAP) extract stabilizes the organic-metal interface under the conditions of \SI{2}{\molar} HCl solution and what experimental criteria distinguish a transition from partial to significant inhibition. For that purpose the following parameters were examined: chemical composition of the EAAP, gravimetric weight loss, potentiodynamic polarization measurements, electrochemical impedance spectroscopy, temperature effect, adsorption properties, quantum-chemical descriptors, molecular dynamics adsorption energy and morphological structure of the surface. It is shown that HPLC analysis revealed twelve oxygenated phytochemicals, such as caftaric acid, kaempferol glycosides, quercetin-3,7-di-\textit{O}-glucoside, isoquercetin, astragalin, and cichoric acid. IR bands at \SI{3260}{\per\centi\meter}, \SI{2169}{\per\centi\meter}, \SI{1606}{\per\centi\meter}, \SI{1518}{\per\centi\meter}, \SI{1440}{\per\centi\meter} correspond to hydroxyl, aliphatic, carbonyl and deformation vibrations characteristic of adsorption active groups. With a concentration of \SI{300}{\milli\gram\per\liter} at \SI{298}{\kelvin}, EAAP inhibits the corrosion process by reducing the corrosion rate from $12.23 \pm 0.33~\si{\micro\gram\per\centi\meter\squared\per\hour}$ to $0.58 \pm 0.02~\si{\micro\gram\per\centi\meter\squared\per\hour}$, lowers the corrosion current density from \SI{511.5}{\micro\ampere\per\centi\meter\squared} to \SI{13.2}{\micro\ampere\per\centi\meter\squared}, and increases the charge-transfer resistance from \SI{35.7}{\ohm\centi\meter\squared} to \SI{1054.9}{\ohm\centi\meter\squared}. This major change occurs within the range of \SI{50}{\milli\gram\per\liter} and \SI{100}{\milli\gram\per\liter}, with all three corrosion parameters changing at the same time. According to the activation energy, Langmuir, DFT, MD, and SEM data, the inhibition of the EAAP corrosion is caused by a mixed layer of physical and chemical adsorption where glycosylated flavonoids provide extensive surface contact while phenolic acids help occupy local sites. The combined results indicate that high inhibition cannot be described only by dosage; it is the result of interfacial consolidation where molecular identity, surface coverage, and charge-transfer limitation interact with each other.

carbon steel,equisetum arvense,hydrochloric acid,green corrosion inhibitor,electrochemical impedance spectroscopy,adsorption; molecular dynamics