Design and synthesis of a novel corrosion inhibitor embedded with quaternary ammonium, amide and amine motifs for protection of carbon steel in 1 M HCl

Methyl 11-bromoundecanoate [Br (CH₂)₁₀CO₂Me] (1) on treatment with tripropylamine gave quaternary salt [Pr₃N⁺(CH₂)₁₀CO₂Me]Br⁻ (2) which on treatment with diethylenetriamine afforded [Pr₃N⁺(CH₂)₁₀CONH(CH₂)₂NH(CH₂)₂NH₂] Br⁻ (3) containing inhibitive motifs of ammonium, amide and amine motifs embedded in a single frame. The precursor salt 2 and its derivative 3 were successfully synthesized in excellent yields and characterized using different spectroscopic techniques. For the first time, a detailed study on the corrosion inhibition behavior of corrosion inhibitors 2 and 3 for mild steel in 1 M HCl was carried out using electrochemical measurements and comprehensive computational analysis. Both the studied inhibitors showed excellent aqueous solubility. The high inhibition efficiency of 91% and 93% at a concentration of 200 mg L⁻¹ was obtained for corrosion inhibitors 2 and 3, respectively. The adsorption of the corrosion inhibitors exhibited the Langmuir isotherm with a mixture of physical and chemical modes of adsorption. The impedance studies showed a rise in the polarization resistance with increasing concentration of the inhibitors. Polarization measurements demonstrated that the inhibitors displayed a mixed-mode of inhibition with primarily cathodic nature. Surface analytical studies supported the inhibitor adsorption and a protective film formation on the carbon steel surface, which improved the surface smoothness of the steel surface. The DFT based quantum chemical calculations supported the experimentally obtained results and showed that the inhibitor 3 displays superior inhibition in comparison to the inhibitor 2. The Monte Carlo simulations revealed higher adsorption energy for the inhibitor 3 compared to 2.