Cost-effective new Mannich bases 1-(pyridin-4-yl­(pyrrolidin-1-yl)­methyl)­urea (UPyP), 1-(morpholino­(pyridin-4-yl)­methyl)­urea (UMP), and 1-(piperidin-1-yl­(pyridin-4-yl)­methyl)­urea (UPP) were synthesized, characterized, and examined as corrosion inhibitors for the corrosion of a mild steel surface in 1.0 M hydrochloric acid (HCl) solution using weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) measurements. The X-ray structures of UMP and UPP are reported. The inhibition efficiency increases with increasing inhibitor concentration and decreases with increasing solution temperature. Potentiodynamic polarization measurements showed that all inhibitors are mixed type. The values of thermodynamic and activation parameters were calculated and discussed. Adsorption of inhibitors on the mild steel surface in the presence of HCl follows Langmuir’s adsorption isotherm. The relationship between molecular structure and their inhibition efficiencies has been studied using density functional theory (DFT) calculations. Experimental and theoretical results are in good agreement.

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