Phytochemical Characterization, Antioxidant Proprieties and Electrochemical Investigations of Methanolic Extract of Rubia t.L. Roots for LC-Steel Corrosion Protection in 1 M HCl Medium

The current study aims to valorize the roots of a medicinal plant of the family Rabeacea called Rubia tinctorum L. ( Rubia t.L. ), by phytochemical analyses, and by pointing out the antioxidant and corrosion inhibitory proprieties of the soxhlet extract using different solvent. The content of polyphenols and flavonoids was investigated using colorimetric assays Folin-Ciocalteu, which showed a higher TPC (total phenolic contents) in methanolic extract (129.40 ± 1.78 mg GAE/g), while the flavonoid fraction represents the minor proportion with 63.32 ± 1.41 mg GAE/g. The optimized extract was characterized by UV–vis spectrophotometric analysis and FTIR spectrometric analysis. The antioxidant activities were evaluated using TAC (total antioxidant capacity), DPPH (diphenyl picryl-hydrazyl), free radical scavenging and FRAP method; the obtained results are more significant than those of BHT, ascorbic acid and vitamin E, used as standard. The soxhlet methanolic extract of Rubia tinctorum L. roots was also used as green corrosion inhibitor for low carbon steel (LC-steel) in acid environment (1 M HCl), using the mass loss method and both techniques PDP (potentiodynamic polarization) and EIS (electrochemical impedance spectroscopy) for the electrochemical tests. PDP results reveal that the Rubia t.L. extract functions as a mixed inhibitor. Nyquist plots show that the resistance of charge transfer increases by increasing the inhibitor concentration, whereas the capacity of the double layer decreases. The adsorption of the extract follows the Langmuir isothermal model. The inhibition efficiencies found in the presence of the tested extract through the three electrochemical methods are consistent and are around the value of 95% at the optimum concentration (1 g/l) and a temperature of 298 K.