Sustainable corrosion protection of mild steel in 5% HCl using Comfrey leaves extract

An aqueous extract of Comfrey leaves (CLE), obtained from a certified source (Vital Herbs, New Delhi, India), characterized using spectral techniques and evaluated as a sustainable corrosion inhibitor for mild steel (MS) in 5 % HCl solution. Corrosion inhibition performance was investigated using weight loss (WL), electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS), atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and contact angle analyses. CLE exhibited a maximum inhibition efficiency of 93.7 % at 600 ppm and 333 K in WL experiments, confirming its strong protective capability at low dosage. EIS results showed a significant increase in polarization resistance (R p = 926.73 Ω·cm²) and decrease in double-layer capacitance, indicating the formation of a stable, protective film. PDP studies revealed a mixed-type inhibition mechanism, affecting both anodic and cathodic reactions, with 91.08 % inhibition efficiency at 600 ppm. Surface analyses confirmed the adsorption of CLE constituents on MS, forming a smooth, hydrophobic barrier. The adsorption followed the Langmuir isotherm, indicating monolayer coverage. Notably, CLE demonstrated better inhibition performance than many reported plant extracts, even under aggressive and elevated temperature conditions. These findings underscore the academic novelty and technical advantage of CLE as a low-cost, eco-friendly, and thermally stable inhibitor, offering a viable alternative to conventional toxic inhibitors for industrial acid cleaning processes.