Inhibition of API 5L X70 steel corrosion in 1 mol L ⁻¹ HCl using <i>Cerinthe major L.</i> hydromethanolic extract

This study investigates the corrosion inhibition potential of the hydromethanolic extract of Cerinthe major L. on API 5L X70 carbon steel (CS) in 1 mol L⁻ 1 HCl solution. Electrochemical techniques, including Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS), along with weight loss measurements, were employed to evaluate its performance at extract concentrations ranging from 100 to 400 ppm. The extract exhibited a maximum inhibition efficiency of 88.11%, 89.07%, and 88.63% according to weight loss, EIS, and PDP, respectively, at 400 ppm. EIS results showed that the charge transfer resistance (R ct ) increased from 86.17 Ω cm 2 (blank) to 788.3 Ω cm 2 , while the double-layer capacitance (C dl ) decreased from 133.10 to 45.94 μF cm⁻ 2 , confirming the formation of a protective film on the steel surface. The thermodynamic analysis revealed that the adsorption of the extract followed the Langmuir isotherm, the negative ΔG°ads values (−24.92 to −26.19 kJ mol⁻ 1 ), indicating that the adsorption process on the CS surface occurs spontaneously and involves a mixed adsorption mechanism predominantly governed by physisorption. FT-IR analysis displayed the presence of characteristic bands capable of adsorbing onto the steel surface, effectively blocking active sites and enhancing corrosion resistance. SEM micrographs supported these findings, showing a severely corroded, pitted surface in the uninhibited sample, while the inhibited steel exhibited a smoother, uniform morphology with minimal defects. Overall, Cerinthe major L. extract demonstrated high efficiency and strong adsorption capacity, highlighting its potential as a sustainable, green, and eco-friendly corrosion inhibitor for carbon steel in acidic environments.