Combined experimental and computational analysis of green Vachellia Nilotica as a mild steel corrosion inhibitor in acidic medium

For mild steel (MS), Vachellia nilotica leaf extract (VNL) was used as a corrosion inhibitor in 1 M HCl. Mass loss (ML) and electrochemical methods were used in the inquiry. Increasing the VNL concentration and decreasing the temperature will increase the inhibition efficiency. Electrochemical techniques, including potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS), revealed that VNL effectively inhibited both cathodic and anodic corrosion reactions. With a good fit, VNL follows the Langmuir isotherm. At 150 ppm, the inhibition efficiency of the VNL inhibitor reaches 91.8% at 298 K. However, it decreased with elevating temperatures and prolonged exposure. A high activation energy (55.1 kJ/mol) for the inhibited solution than that of the blank solution (29.5 kJ/mol) and the free energy of adsorption (-19.3 to -18.4 kJ mol-1) provides evidence for physical adsorption mechanisms in the interaction of the VNL inhibitor with the steel surface. The efficiency of the extract improved with concentration, achieving optimal values of 93.9% (POD) and 92.2% (EIS) at 150 ppm. Furthermore, the negative free energy values confirm the spontaneous nature of this adsorption process. To find out how the VNL inhibitor affected the MS surface, researchers employed Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), and Atomic Force Microscopy (AFM), which confirmed the formation of a protective film on the metal surface. Optimized molecular structures of phytochemicals confirmed their inhibitory properties via Quantum chemical calculations (DFT), which showed the molecular inhibitory action of VNL.