Electrochemical and mechanistic insights into the inhibitory properties of a plant extract enriched in oxygenated compounds against the corrosion of C38 steel in 1 M HCl

The electrochemical behavior of C38 steel in 1 M HCl was investigated in the presence of a hydro-alcoholic extract of Ficus pumila Linn. leaves enriched in oxygenated compounds (OCs), hereafter referred to as FPHE, to evaluate its corrosion inhibition performance under acidic conditions. Potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and physicochemical surface characterization techniques (SEM, EDX, and AFM) were used to assess the inhibition efficiency of FPHE and provide insights into the inhibition mechanism. Initial characterization of FPHE by phytochemical screening, FTIR, and GC–MS analyses revealed a significant proportion of polyphenols, glycosylated flavonoids, and other glucosylated derivatives, with total polyphenol and flavonoid contents of 166.381 mg GAE g⁻¹ ES (GAE: gallic acid equivalent; ES: dry extract) and 133.450 mg QE g⁻¹ ES (QE: quercetin equivalent; ES: dry extract), respectively. Electrochemical measurements and surface characterization indicated that the adsorption of OCs onto both cathodic and anodic sites governs the formation of a thin and heterogeneous protective layer on the steel surface, with a predominantly anodic inhibitory character. Adsorption of FPHE molecules occurred predominantly via a physisorption mechanism and followed the Langmuir adsorption isotherm, suggesting a monolayer adsorption pattern. A maximum inhibition efficiency of approximately 79% was achieved at 800–1000 ppm of FPHE, beyond which a decrease in efficiency at 1200 ppm indicated that the desorption of OCs becomes increasingly competitive with adsorption, limiting the overall inhibition performance of this OC-enriched extract.