Green Synthesis of Rhodanine-Derived Inhibitors for Carbon Steel Corrosion Protection: DFT, Thermodynamic, and Surface Analysis

Abstract

This study presents a green synthesis of three of (Z)-5-arylidene-rhodanine derivatives in an aqueous medium and evaluates their efficacy as corrosion inhibitors for carbon steel in 0.5 M H₂SO₄. The synthesis, optimized using ammonium acetate as a catalyst, yielded ten derivatives (3a–3c) with excellent efficiency (86–90% yield). Spectroscopic techniques (¹H NMR, MS) and theoretical calculations confirmed the (Z)‑configuration of the products. Corrosion inhibition performance was assessed via weight loss studies, and scanning electron microscopy (SEM). Among the tested compounds 3c, 5-(4-hydroxy-3-methoxybenzylidene)-3-phenyl-2-thioxothiazolidin-4-one, exhibited superior inhibition efficiency (99.65% after 72 h), attributed to its strong chemisorption (ΔG_ads = −43.70 kJ/mol) and high equilibrium constant (K_ads = 617.26 m³/mol). Density functional theory (DFT) calculations, revealing high E_HOMO (–5.83 eV), low E_LUMO (–2.37 eV), low energy gap (ΔE_gap = 3.46 eV), and significant electron transfer (ΔN = 3.21 eV) for 3c, elucidated the molecular interactions promoting adsorption. Thermodynamic and kinetic analyses revealed an endothermic, non-spontaneous adsorption process, with 3c significantly increasing the activation energy (E_act = 85.40 kJ/mol). SEM confirmed the formation of a protective layer, minimizing surface degradation. These findings highlight the potential of rhodanine derivatives as eco-friendly, high-performance corrosion inhibitors for industrial applications.