Synergistic Corrosion Inhibition of Mild Steel in 0.5 M H₂SO₄ by Sodium Benzoate, Onion Extract, and Chitosan: Mechanistic and Electrochemical Insights

The corrosion inhibition performance of eco-friendly hybrid inhibitors based on benzoate, onion extract, and chitosan for mild steel in 0.5 M H₂SO₄ was investigated using OCP, Tafel polarization, and EIS techniques. The ternary Benzoate–Onion extract–Chitosan (BOC) system exhibited the most positive and stable OCP, indicating effective surface passivation. Polarization studies showed a pronounced reduction in corrosion current density with a shift of E ₍corr₎ toward nobler potentials, confirming strong inhibition. EIS results revealed substantially increased charge-transfer resistance for chitosan-containing systems, with BOC showing the largest Nyquist semicircle. The inhibition efficiency followed the order: BOC > BC > OC > BO > blank. The superior performance of BOC arises from synergistic adsorption of benzoate carboxylates, chitosan amino groups forming a polymeric barrier, and sulfur- and phenolic-rich compounds from onion extract that enhance film stability. The BOC hybrid system offers an efficient, biodegradable, and sustainable solution for mild steel protection in acidic environments.