Bis-isoxazolidines: A new class of corrosion inhibitors of mild steel in acidic media

Two new bis-isoxazolidines were synthesized in excellent yields via cycloaddition reaction of 1-pyrroline 1-oxide with 2,7-di(10-undecenyloxy)naphthalene and 1,4-di(10-undecenyloxy)benzene. One of the bis-isoxazolidines, on reaction with two equivalents of 1-bromododecane, afforded a bis-quaternary ammonium salt. All three inhibitor molecules in the presence of 400 ppm at 60 °C achieved inhibition efficiencies (IE) in the ranges 97–98% and 92–96% as determined by gravimetric method for corrosion of mild steel in 1 M HCl and 0.5 M H2SO4, respectively. The results obtained by the electrochemical methods using Tafel plots and electrochemical impedance spectroscopy corroborated the findings of the weightloss method. Both bis-isoxazolidines demonstrated remarkable efficiency in the lower range of inhibitor concentrations – in the presence of a meager 1.5 ppm of the inhibitor molecules, IEs of ∼70% in 1 M HCl were achieved. Thermodynamic parameters (ΔGads0, ΔHads0, ΔSads0) for the adsorption process in the presence of the bis-isoxazolidines were determined. The ΔGads0 values of around –90 kJ/mol in 1 M HCl and −50 kJ/mol in 0.5 M H2SO4, pointed towards the chemisorption of the inhibitor molecules, especially in HCl media. While the corrosion inhibition by these molecules was predominantly under cathodic control in 1 M HCl, the inhibition in 0.5 M H2SO4 was found to be under anodic control. The bis-isoxazolidines were found to provide a suitable inhibition mechanism for the corrosion inhibition in HCl as well as in H2SO4 media.