Synthesis and Properties of a New Environmentally Friendly Bicyclic Imidazoline Quaternary Ammonium Salt as a Corrosion Inhibitor of Carbon Steel

A new corrosion inhibitor, environment-friendly bicyclic imidazoline quaternary ammonium salt (DL-IM), was synthesized using DL-aspartic acid, hydroxyethyl ethylenediamine, and benzyl chloride as the main raw materials. The amidation, cyclization, and quaternization reactions were performed at 140°C for 4 h, 220°C for 3 h, and 70°C for 3 h, respectively. The structure of DL-IM was characterized by infrared spectroscopy and nuclear magnetic hydrogen spectroscopy. The corrosion experiments of carbon steel sheets (American Petroleum Institute, API 5CT N80, primarily used in well drilling operations to protect the wellbore), protected with or without DL-IM, demonstrated that DL-IM had a good anticorrosion performance (with the inhibition efficiency and corrosion rate of 97.13% and 0.6528 mm/a, respectively) in the 1 M HCl solution at 60°C. DL-IM was a kind of cathodic protection-type corrosion inhibitor confirmed by Tafel curves. The analysis of adsorption kinetics by quantum chemical simulations demonstrated that the imidazoline ring in the DL-IM molecule can form covalent bonds with the empty d-orbitals of Fe through charge sharing. Moreover, the dipole moment of DL-IM is 1.3931 D (1 D = 3.34 × 10⁻³⁰ C/m), suggesting a good hydrophobicity of DL-IM. Therefore, the adsorption film formed by DL-IM on the surface of N80 can effectively isolate the corrosive medium from N80. Thus, the successful synthesis of DL-IM provides new insight into the design and synthesis of an environment-friendly corrosion inhibitor with a stronger corrosion inhibition effect.