Low‐Concentration Corrosion Inhibition of Carbon Steel in 1.0 M HCl by a Tricationic Pyridinium Surfactant: Experimental and DFT Insights

A tricationic surfactant containing pyridinium and quaternary ammonium centers, denoted mono16‐diN + ‐PyN + , was synthesized and evaluated as a corrosion inhibitor for carbon steel in 1.0 M HCl. Its structure was confirmed by FT‐IR and 1 H NMR spectroscopy, while its inhibition performance was investigated by gravimetric, electrochemical, contact‐angle, adsorption, surface, and density functional theory analyses. Gravimetric measurements showed inhibition efficiencies of 98.80%–99.48% over the investigated immersion periods, even at very low inhibitor concentrations. Electrochemical impedance spectroscopy revealed an increase in charge–transfer resistance from 165.5 Ω for the blank solution to 1238 Ω at 10 ppm, accompanied by a marked decrease in double‐layer capacitance, indicating formation of a protective adsorbed layer. Potentiodynamic polarization confirmed mixed‐type inhibition: at 10 ppm, the corrosion current density decreased from 492 to 37.4 μA cm −2 and the corrosion rate from 5.71 to 0.43 mm year −1 , corresponding to an inhibition efficiency of 92.4%. The water contact angle increased from 17.15° to 71.50°, demonstrating enhanced surface hydrophobicity after adsorption. The adsorption behavior was best described by the Langmuir isotherm, and negative Δ G ° ads values indicated spontaneous adsorption involving electrostatic and charge–transfer interactions. SEM, EDS, and Gwyddion‐derived 3D surface profiles showed reduced surface damage and fewer corrosion products in the presence of the inhibitor. DFT results further indicated that the heteroatom‐rich head group, pyridinium unit, cationic centers, and hydrophobic alkyl chain contribute synergistically to adsorption and barrier formation. These findings demonstrate that mono16‐diN + ‐PyN + provides highly effective corrosion protection at low concentrations.