Development of Natural Plant Extracts as Sustainable Inhibitors for Efficient Protection of Mild Steel: Experimental and First-Principles Multi-Level Computational Methods

In the present work, we present the superior corrosion inhibition properties of three plant-based products, Fraxinus excelsior (FEAE), Zingiber zerumbet (ZZAE), and Isatis tinctoria (ITAE), that efficiently inhibit the corrosion of mild steel in phosphoric acid. The anti-corrosion and adsorption characteristics were assessed using a combination of experimental and computational approaches. Weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy methods were used to evaluate the inhibitive performance of the inhibitors on the metal surface. Then, both DFT/DFTB calculations and molecular dynamic simulations were further adopted to investigate the interaction between organic inhibitor molecules and the metal surface. The protective layers assembled using the active constituents, such as carbonyl and hydroxyl groups, of the three plant-based products offer high electrochemical stability at high temperatures and robust protection against aggressive acidic solutions. All electrochemical measurements showed that the inhibition performance of extracts increased by increasing their concentration and improved in the following order: FEAE > ZZAE > ITAE. Further, these extracts worked as mixed-type inhibitors to block the anodic and cathodic active sites on the mild steel surface. Multi-level computational approaches revealed that FEAE is the most adsorbed inhibitor owing to its ability to provide electron lone pairs for electrophilic reactions. The experimental and theoretical results showed good agreement. These results indicate the possibility of replacing conventional compounds with natural substituted organic products in the fabrication of hybrid materials with effective anti-corrosion performance.