**1. Introduction**

Carbon dioxide corrosion is the most commonly faced problem in the petroleum industry, and thus it has been a hot research topic for many years. Corrosion prevention consists of different methods, but one of these methods, the introduction of organic compounds as corrosion inhibitors, is both effective and cheap [1–3].

The inhibition action of organic compounds depends on the nature of the molecular structure, inhibitor planarity, electron donating functional groups, non-bonding electrons on heteroatoms, i.e., oxygen, nitrogen, and sulfur, and presence of π bonds in the aromatic ring [4]. In recent years, corrosion scientists have been interested in finding green and environment-friendly inhibitors [5]. Benzimidazole derivatives that show antitumor and antimicrobial activities have been categorized in the group of green compounds [6,7].

A survey of the literature reveals that in the past decades, various imidazole and benzimidazole derivatives have been used as anti-sweet corrosion inhibitors [8–15]. However, no literature exits about using benzimidazole as a corrosion inhibitor in a brine solution saturated with carbon dioxide. Benzimidazole has planar structure with two nitrogen atoms that provides a closer approach for interaction with the metal surface, aromatic properties, and an option for introducing different substituents.

The main purpose of the present paper was to elucidate the corrosion inhibition effect of the number of electron donating methoxy groups on the phenyl ring of the three synthesized benzimidazole derivatives, namely, 2-(3,4,5-Trimethoxyphenyl)-1*H*-benzo[*d*] imidazole (TMI), 2-(3,4-Dimethoxyphenyl)- 1*H*-benzo[*d*] imidazole (DMI), and 2-(4-Methoxyphenyl)-1*H*-benzo[*d*] imidazole (MMI), for J55 steel saturated with CO2 in a 3.5% NaCl solution. Corrosion inhibition properties of the benzimidazole derivatives were analyzed using the static weight-loss method and electrochemical methods, i.e., impedance spectroscopy (EIS) and potentiodynamic polarization. Meanwhile, the J55 steel surface was examined by scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS). The potential site for protonation was estimated using density functional theory (DFT). The interaction of the benzimidazole derivatives with the J55 steel surface was studied by molecular dynamic simulation (MD).
