Corrosion protection of mild steel in 1.0 M HCl using quinoxaline derivatives: experimental validation and quantum chemical insight

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This study provides a comprehensive evaluation of the corrosion-inhibiting properties of three quinoxaline derivatives (BNQ, NHQ, and NPQ) on mild steel in 1.0 M HCl, combining experimental and theoretical approaches in a single framework. A multi-technique methodology, integrating electrochemical impedance spectroscopy (EIS), potentiodynamic polarisation (PDP), UV-visible spectroscopy, optical microscopy analysis (OMA), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), and quantum chemical analyses (DFT, MEP, Fukui indices), was employed to correlate molecular structure with inhibition efficiency. Results demonstrate a significant protective effect, with efficiencies of 73%, 82%, and 90% for BNQ, NHQ, and NPQ, respectively, at 10−³ M. All three inhibitors act as mixed-type and follow the Langmuir adsorption isotherm, while UV-visible and surface analyses confirm the formation of a stable protective film. Computational insights further highlight NPQ as the most effective molecule due to its favourable electronic properties and active sites operating in its neutral, non-protonated form despite the strongly acidic environment. By integrating complementary experimental and theoretical tools, this work not only identifies NPQ as a promising corrosion inhibitor but also demonstrates the strength of a synergistic approach for understanding the inhibitor performance.