Electrochemical Investigations of Galium verum Ethanolic Extract as a Steel Corrosion Eco-Inhibitor in the Acid Media: An Unexpected Versatility of Plant Chemistry

Corrosion inhibitors are substances that reduce or eliminate the corrosion of a metal in a certain environment. Corrosion inhibitors act by several mechanisms, including adsorption, film formation, passivation, and oxygen scavenging. Due to their toxicity, classic corrosion inhibitors affect the environment. Therefore, in recent years, more and more studies have focused on the development of eco-friendly inhibitors for the environment. In this study, ethanolic extract of <i>Galium verum</i> (GV) was tested for the inhibition of steel corrosion in 1 M HCl medium using electrochemical methods: open circuit potential (OCP), potentiodynamic polarization (PP), and electrochemical impedance spectroscopy (EIS). Reverse-phase liquid chromatography (HPLC) and gas chromatography mass spectrometry (MS-GC) previous studies state that GV extract contains polyphenols and other chemical species responsible for the inhibitory effect. Corrosion investigations have highlighted the influence of the concentration of the GV extract, in the range of 50 ÷ 400 ppm G.A.E./mL, as well as the influence of temperature in the range of 20 ÷ 50 °C. The corrosion inhibitory efficiency of the <i>Galium verum</i> ethanolic extract had a maximum value of 91.82% for a concentration of 400 ppm polyphenol content, demonstrating the inhibitory potential of this green product in an acidic environment for mild steel. Statistical calculus on the obtained values of EIS inhibitor efficiency showed that the effect of the extract becomes stronger at higher concentrations.