Polyphenol-Driven Interfacial Control: How Achillea millefolium Extract Modulates Mild Carbon Steel Corrosion in Acid and Neutral Media

Plant-derived corrosion inhibitors are increasingly investigated due to their rich content of adsorption-active phytochemicals. Four extracts obtained from Achillea millefolium were biochemically characterized through spectrophotometric and chromatographic analyses, confirming a substantial polyphenolic content and associated antioxidant capacity. In addition, the hydroethanolic extract (1:1) was examined for its ability to inhibit the corrosion of S235 mild steel in 1 M HCl and neutral medium of 3.5% NaCl by gravimetry, potentiodynamic polarization, open-circuit potential, and electrochemical impedance spectroscopy methods, suggesting that its antioxidant molecules may contribute to the passivation of the metal surface, but in different mechanistic ways. The inhibitory efficiency determined by both the gravimetric method and the Taffel polarization curve method reaches values of 78.53% in HCl 1 M and of 79.65% in NaCl 3.5%, thus demonstrating the contribution of polyphenols from the Achillea millefolium extracts to the inhibition of corrosion.