Sustainable Corrosion Inhibition of Carbon Steel in Sulfuric Acid by Using Neolentinus lepideus Extract

In this work, the potential use of Neolentinus lepideus extract as a green corrosion inhibitor for carbon steel in acidic media was investigated, together with its prospects as a renewable and biologically derived corrosion-control material for potential industrial applications. To achieve this, an experimental investigation was conducted using gravimetric measurements, potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS). The results demonstrated that the N. lepideus extract effectively suppressed the corrosion of 1018 carbon steel, with the inhibition efficiency increasing progressively with extract concentration and reaching a maximum value of 89% at 400 ppm. The potentiodynamic polarization curves revealed no evidence of passive film formation and indicated that the extract acted as a mixed-type corrosion inhibitor, with a more pronounced influence on the cathodic reaction. In the absence of the inhibitor, the corrosion process was predominantly governed by charge-transfer reactions and remained essentially under the same control mechanism upon the addition of 400 ppm of Neolentinus lepideus extract. However, at lower concentrations of 100 and 200 ppm, the corrosion process appeared to be affected by the adsorption and desorption of intermediate species at the steel/solution interface. The incorporation of Neolentinus lepideus extract into the corrosive medium resulted in an increase in the charge-transfer resistance and a concomitant decrease in the double-layer capacitance, supporting the formation of a protective interfacial film on the steel surface. The inhibitory action of the extract may be associated with its content of phenolic compounds, amides, and polysaccharides, whose heteroatoms and functional groups can interact with the steel surface and contribute to the formation of a protective adsorbed layer. The calculated Gibbs free energy values of adsorption (ΔG_ads), which were less negative than −20 kJ mol⁻¹, indicated that the adsorption of Neolentinus lepideus constituents onto the steel surface was predominantly physical in nature. Furthermore, the adsorption process was satisfactorily described by the Freundlich isotherm, suggesting heterogeneous adsorption involving surface sites with different adsorption energies.