Engineering a 3D-structured quaternary ammonium hydrogel as a high-efficiency interfacial barrier for mild steel corrosion in marine environments.

The development of sustainable, high-performance polymeric barriers is critical for extending the service life of mild steel in chloride-rich environments. This study introduces a novel cross-linked quaternary ammonium-based acrylamide hydrogel (QAAHG), synthesized via free-radical copolymerization of acrylamide (AAm) and (4-vinylbenzyl)trimethylammonium chloride (VBTAC) and evaluates its performance as a corrosion inhibitor for mild steel in 3.5 wt% NaCl solution. Beyond standard inhibition, the hydrogel's 3D network provides a unique structural framework for the dense orientation of quaternary ammonium groups at the metal-electrolyte interface. Fourier-transform infrared spectroscopy (FTIR) confirmed the presence of amide carbonyl and quaternary ammonium C-N functional groups, while X-ray diffraction (XRD) confirmed the entirely amorphous nature of the hydrogel matrix. At the optimal concentration of 100 ppm, QAAHG achieved inhibition efficiencies of 90.98% (gravimetric) and 93.8% (potentiodynamic polarization), suppressing corrosion current density from 112.2 to 6.95 µA cm⁻² and produced a corrosion potential shift of 69 mV, indicating mixed-type inhibition with a predominant anodic contribution. Electrochemical impedance spectroscopy (EIS) confirmed a peak impedance inhibition efficiency of 95.3% at 100 ppm, with charge transfer resistance (Rct) rising from 438.2 to 9319.1 Ω cm², and a concurrent drop in Constant Phase Element (CPE) from 36.5 to 1.6 µF cm⁻², consistent with the formation of a compact, resistive protective film. Adsorption studies confirm that the process follows the Temkin isotherm model, with a standard free energy of adsorption of -19.64 kJ mol-1. This value indicates spontaneous physisorption mechanism driven by electrostatic interactions between the cationic hydrogel network and the steel surface. Notably, scanning electron microscopy/ energy dispersive X-ray spectroscopy (SEM/EDX) characterization confirmed heterogeneous polymer adsorption and partial barrier film formation, while the observed efficiency threshold at 200 ppm provides new insights into the interfacial aggregation behavior of polyelectrolyte inhibitors. These findings establish QAAHG as a high-efficiency, structurally optimized alternative to conventional small-molecule inhibitors for marine infrastructure.