Experimental and Theoretical Studies of Mono- and Bis-Pyridinium-Based Ionic Liquids as Corrosion Inhibitors for Mild Steel in Aggressive Acidic Media

The present work reports the synthesis and spectroscopic characterization (IR, ¹H NMR and ¹³C NMR) of mono- and bis-pyridinium-based ionic liquids as corrosion inhibitor compounds through quaternization of 4-(N,N-dimethylamino)pyridine with 1,4-dibromobutane. These compounds are referred as 1,1′-(butane-1,4-diyl)bis(4-(dimethylamino)pyridin-1-ium)bromide (D1) and 1-(4-bromobutyl)-4-(dimethylamino) pyridin-1-ium bromide (M1). The inhibition efficiency of the synthesized materials was assessed using weight loss, thermodynamics and electrochemical methods such as electrochemical impedance spectroscopy (EIS) and linear polarization resistance (LPR). Furthermore, AFM and SEM were also used to examine the surface morphology of the CS. The results demonstrated that the investigated liquids are effective corrosion inhibitors, with higher concentrations exhibiting an increasing inhibitory efficiency (%IE). At concentrations as low as 3.0 mM, the inhibitory efficiency (IE%) reached about 93.2% for D1 and 91.5% for M1, which indicate that both IL molecules possess a powerful inhibitory activity. Additionally, the percentage of inhibitory activity (IE) decreased with increasing temperature, indicating that physisorption is involved in the adsorption of inhibitor molecules on the surface of the CS. Furthermore, the inhibitor molecules adsorb onto the CS surface in accordance with the Langmuir adsorption isotherm. LPR revealed the mixed-type inhibitory effect of D1 and M1 through blocking both cathodic and anodic corrosion sites on carbon steel, reducing the current densities of both Tafel branches without changing the corrosion mechanism. The EIS findings validated the two ionic liquid adsorption mechanism, which involved replacing the originally adsorbed water molecules and creating a protective film. With increasing concentrations, this film increased the charge transfer resistance, due to a decrease in double-layer capacitance. Density functional theory was performed to optimize the geometry and explore the anti-corrosion capability of the title compounds through the assessment of FMOs and related reactivity parameters. Besides, the adsorption energy, stability and compactness of interaction were computed by adsorption locator and molecular dynamics simulations.