Design of a Pyrrolidine–Quaternary Ammonium Cyclocopolymer as a Nonbiocidal Corrosion Inhibitor for Mild Steel in 15 wt % HCl

Abstract

15 wt % HCl used in oil and gas acidizing operations causes severe corrosion to operational pipelines and tubing. In this work, a pyrrolidine- and quaternary ammonium salt-based cyclocopolymer (PCP) has been synthesized as an efficient corrosion inhibitor for mild steel under oil- and gas-acidizing conditions. PCP was characterized by 1H and 13C NMR, FT-IR, and TGA. PCP, with thermal stability up to ≈300 °C, retarded corrosion of mild steel in 15 wt % HCl by ≈90% at 100 ppm at 298 K, as confirmed via weight loss (WL), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), and linear polarization resistance (LPR) techniques. The adsorption of PCP onto mild steel followed the Langmuir adsorption isotherm. ΔGads° values (≈42 kJ mol–1) calculated using surface coverage (θ) data indicated a mixed physico-chemisorption mode of adsorption with chemisorption predominance, further supported via detailed discussion on kinetic and activation parameters (Ea, ΔH‡, and ΔS‡). PDP study demonstrated that PCP acted as a mixed-type inhibitor with anodic predominance. SEM-EDX and XPS studies provided strong evidence for PCP adsorption onto the mild steel surface. Antibacterial studies showed that PCP, at concentrations up to 100 ppm, did not interfere with E. coli growth up to 12 h. Computational toxicity prediction, including several organ toxicity and toxicity end points, proved PCP to be much less toxic (LD50 775 mg kg–1) than efficient small-molecule inhibitors. A reasonable explanation for the mechanism of inhibition has been proposed based on the experimental and DFT findings.