Alkyl-Triethylenediamine Bromide Ionic Liquids as Corrosion Inhibitors for Mild Steel in 1 M HCl Solution: Experimental and Theoretical Calculations

Alkyl-triethylenediamine bromide ionic liquids ([CnDABCO]Br ILs) were synthesized via a self-quaternization method and innovatively applied as environmentally friendly and nontoxic corrosion inhibitors for mild steel in 1 M HCl solution. The composition and corrosion inhibition performance of [CnDABCO]Br were comprehensively studied by using Fourier transform infrared spectroscopy, weight loss tests, and electrochemical measurements. The results showed that [CnDABCO]Br is an efficient mixed-type corrosion inhibitor, with its inhibition efficiency increasing with alkyl chain length and concentration and decreasing with rising temperature. Under optimal conditions (298 K and 2 mM), the inhibition efficiency of [C12DABCO]Br reached 91.5%, while for [C4DABCO]Br and [C8DABCO]Br, the inhibition efficiencies were 85.1 and 87.2%, respectively. Surface analysis (scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy) combined with quantum chemical calculations provided evidence that [CnDABCO]Br can form a protective layer on mild steel, following the Langmuir adsorption model, with significantly higher Fe content observed after inhibitor adsorption. Molecular dynamics simulations further verified the adsorption mode, illustrating the process of both chemical bonding and nonbonding interactions with the Fe(110) surface. This study provides new insight into the structure–activity relationship of triethylenediamine-based ILs, establishing a clear link between alkyl chain length and inhibition efficiency, and offering guidance for the molecular design of high-performance corrosion inhibitors in acidic environments.