Comparative analysis of corrosion inhibition efficiency of Silybum marianum L. parts on mild steel in 1 M HCl solution

Silybum marianum L. known as milk thistle has been evaluated as anti-corrosion agent on mild steel in 1 M HCl. Research mainly targets its medicinal uses, with little attention given to its anticorrosive potential. The objective of this study is to compare the inhibition efficiency of its different parts such as seeds, leaves and stems, knowing that the phytochemicals differ between plant parts. This allows us to understand how variations in phytochemicals across different parts influence their adsorption behavior on metal surfaces which enables to evaluate whether the entire plant can be utilized as a renewable and effective green anti-corrosion agent. For this purpose, the plant parts extracts were evaluated as corrosion inhibitors by employing potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) analysis. The study demonstrated that all the extracts exhibited a good inhibition efficiency with the effectiveness increasing with increasing the concentration. Specifically, the inhibition efficiency reached 95.5 % for the seeds extract, 94.1 % for the leaves extract and 95.1 % for the stems extract obtained at 2g/l. It follows the order: Seeds extract ≈ Stems extract > Leaves extract. Polarization studies showed that the extracts are mixed-type inhibitors, predominantly affecting the cathodic reaction. The extracts were found to follow the Langmuir adsorption isotherm, involving mainly physical adsorption. Additionally, SEM/EDX analysis provided clear evidence of surface-protective layer formed on the mild steel surface, which serves as a physical barrier against corrosion. These findings strongly indicate that milk thistle part extracts could serve as a green alternative to chemical anti-corrosive agents.