Synergistic Corrosion Inhibition of API 5L X65 Steel by Phytochemical Extracts: A Multitechnique Approach With Computational Validation

The synergistic corrosion inhibition of nine phytochemicals from Azadirachta indica (neem) and Dypsis lutescens (golden palm) on API 5L X65 steel in acidic media was studied using DFT, MC, weight loss, EIS, and Tafel (PDP) methods, supported by FE‐SEM and EDX analyses. Quantum chemical parameter analysis indicated synergistic interactions among the phytochemicals via nucleophilic and electrophilic attacks. The orbital matching principle revealed electron donation from the HOMO of 6,11‐dimethyl‐2,6,10‐dodecatrien‐1‐ol, methyl ester (Z), D‐limonene, and 9‐eicosene to the iron Fermi level (−5.177 eV), due to small energy gaps (0.625, 0.738, and −0.249 eV). Electron transfer from the Fermi level of Fe to the LUMO of 9‐eicosene is favored. The remaining six phytochemicals exhibited high hardness and enhanced stability. MC simulations showed an adsorption energy of −4472.220 kcal/mol. The weight loss indicated that GT 85 achieved inhibition efficiencies of 72% at 303 K, 88% at 314 K, and 94% at 328 K after 24 h. The EIS and PDP results showed efficiencies of 94.79 ± 0.0281 and 99.9%, respectively. The Nyquist plots indicate an activation‐controlled mechanism with an increased charge transfer resistance from 2.401 ± 0.032 Ω·cm 2 (blank) to 46.103 ± 0.357 Ω·cm 2 at 60 ppm. FE‐SEM and EDX confirmed the GT 85 protective film. The adsorption process of GT 85 followed the Langmuir and Freundlich isotherms, and thermodynamic analysis revealed a spontaneous physisorption mechanism. The results revealed that GT 85 is thermally stable up to 328 K, anodic, persistent, and effective at low dosages and offers an eco‐friendly alternative green inhibitor with minimal environmental impact.