Quaternium-22 as a High-Performance Corrosion Inhibitor for Carbon Steel in Acidic Media: Experimental and Theoretical Insights

This work provides an integrated experimental and computational evaluation of the cationic surfactant Quaternium-22 (Q-22) as a potentially eco-compatible corrosion inhibitor for carbon steel (CS) in 1 M hydrochloric acid. Gravimetric analysis and electrochemical techniques, including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP), were employed over a temperature range of 20–50 °C. Q-22 exhibited mixed-type inhibition behavior, with efficiency rising to 97% at an optimal concentration of 277 μmol L−1. Performance was concentration-dependent but diminished with increasing temperature, indicating partial inhibitor desorption at elevated temperatures. Thermodynamic evaluation confirmed a spontaneous adsorption process consistent with the Langmuir isotherm, involving a combined physisorption and chemisorption mechanism. Surface characterization via scanning electron microscopy (SEM), atomic force microscopy (AFM), contact angle (CA) measurement, and X-ray photoelectron spectroscopy (XPS) confirmed the formation of a coherent, hydrophobic inhibitor layer that substantially reduced surface roughness and corrosion damage. Theoretical investigations using density functional theory (DFT), natural bond orbital (NBO) analysis, and molecular dynamics (MD) simulations revealed strong adsorption energies and favorable electronic properties consistent with the inhibitor’s high experimental efficacy. Overall, the results demonstrate that Q-22 is a highly effective, eco-compatible corrosion inhibitor for CS in acidic environments, operating through a stable adsorptive film-forming mechanism.