Corrosion Inhibition of Mild Steel in Hydrochloric Acid Using Extracts of Vernonia amygdalina as Green Inhibitors

Corrosion of mild steel in acidic environments causes substantial economic losses in the Nigerian oil and gas, petrochemical, and industrial sectors. Plant extracts represent eco-friendly, renewable, and inexpensive alternatives to toxic synthetic corrosion inhibitors, and the development of green inhibitors from locally available plants addresses an important industrial chemistry research gap. This study investigated the corrosion inhibition potential of methanol and aqueous extracts of Vernonia amygdalina leaves as green corrosion inhibitors for mild steel in 0.5 and 1.0 M hydrochloric acid solutions. The inhibition mechanism was studied using weight loss measurements, electrochemical impedance spectroscopy, potentiodynamic polarisation, open circuit potential measurements, and quantum chemical calculations. The influence of extract concentration, temperature, immersion time, and acid concentration on inhibition efficiency was evaluated. Inhibition efficiencies up to 94.6 percent were achieved with the methanol extract at 1000 ppm in 0.5 M HCl at 25 degrees Celsius. Polarisation curves showed that the extract functioned as a mixed-type inhibitor, moderately suppressing both anodic and cathodic processes. Impedance spectra revealed an increase in charge transfer resistance and capacitance depression with increasing inhibitor concentration, consistent with adsorption film formation. Quantum chemical parameters calculated using density functional theory at the B3LYP/6-311G level identified sesquiterpene lactones and flavonoids as the most reactive molecular species responsible for metal-surface interaction. Adsorption followed the Langmuir isotherm model. The results validate Vernonia amygdalina as a promising green corrosion inhibitor with direct application relevance for Nigerian industrial acid cleaning operations.

Keywords: corrosion inhibition, mild steel, Vernonia amygdalina, green inhibitor, electrochemical methods