Apricot kernel oil as a sustainable corrosion inhibitor for API X65 steel in hydrochloric acid: Experimental, thermodynamic, and quantum chemical insights

Apricot kernel oil (AKO), an agro-industrial byproduct rich in unsaturated and saturated fatty acids, was investigated as a novel, biodegradable corrosion inhibitor for API X65 steel in hydrochloric acid media. Electrochemical techniques, including open circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP), were employed to assess inhibition mechanism and performance under varying concentrations, acid strengths, and temperatures. The results revealed that AKO acts as an efficient mixed-type inhibitor, achieving a maximum inhibition efficiency of 98.9% at 20,000 ppm in 1 M HCl. EIS data demonstrated a substantial increase in charge transfer resistance and a pronounced reduction in double-layer capacitance, indicating the formation of a compact, hydrophobic protective film. The adsorption process followed the Langmuir isotherm, with a calculated free energy of adsorption of –31.78 kJ·mol⁻¹ , suggesting a synergistic physisorption–chemisorption mechanism. FTIR and XRD analyses confirmed the formation of Fe–carboxylate complexes and the preservation of the ferritic microstructure, while SEM images revealed a smooth, pit-free surface. DFT calculations identified 9,17-octadecadienal and oleic acid as the most active constituents, showing strong orbital overlap and dipole-driven adsorption. This study establishes a complete mechanistic model for the corrosion inhibition provided by a complex plant extract, highlighting the critical role of in-situ formed metal-organic complexes. The combined experimental and theoretical results validate AKO as an environmentally benign, thermally stable, and cost-effective inhibitor suitable for acidizing and pickling operations involving pipeline steel. • First report of apricot kernel oil as a high-efficiency (>98%), green corrosion inhibitor for API X65 steel in HCl. • AKO achieves exceptional inhibition via synergistic physisorption–chemisorption, validated by Langmuir adsorption. • In-situ FTIR confirms Fe–carboxylate bond formation, providing direct evidence of chemisorption. • DFT identifies 9,17-octadecadienal and oleic acid as key adsorbates responsible for strong Fe–O coordination. • AKO maintains > 91% efficiency across varied acid strengths (0.01–1 M) and temperatures (25–60 °C).