Experimental and Theoretical Studies of the Corrosion Inhibition Performance of a Quaternary Phosphonium-Based Ionic Liquid for Mild Steel in HCl Medium

The corrosion inhibition performance of a quaternary phosphonium-based ionic liquid, i.e., hexadecyltriphenylphosphonium bromide (HPP), on mild steel in 1 M HCl solution was investigated by electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) methods. The surface characterization of mild steel was examined by scanning electron microscopy with energy-dispersive X-ray spectroscopy (SEM-EDS). The results revealed that the inhibition efficiency increases with its increasing concentration, and it can reach up to 99.1% at the concentration of 0.07 mM HPP. PDP data showed that the absorption of HPP conformed to Langmuir adsorption, which served as a mixed-type inhibitor, involving chemisorption and physisorption. SEM analysis confirmed the formation of barrier film on the metal surface, inhibiting the acid attack. Moreover, density functional theory (DFT) calculations and molecular dynamics (MD) simulations were conducted to elucidate the adsorption mechanism of inhibitor molecules on the mild steel surface. A match between the experimental and theoretical findings was evidenced.