Prickly Pear Peel Extract as a Green Corrosion Inhibitor for API 5L X70 Steel in Hydrochloric Acid Environment

This study investigates the corrosion inhibition efficiency of prickly pear peel extract (PPPE), an agro-industrial residue rich in polyphenolic compounds, on API 5L X70 pipeline steel exposed to 1 M hydrochloric acid. Electrochemical techniques, including electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP), were employed alongside surface characterization (SEM–EDX) and FTIR analysis. Results demonstrate that PPPE acts as a mixed-type inhibitor, significantly reducing both anodic dissolution and cathodic hydrogen evolution. Inhibition efficiency increased with PPPE concentration, reaching 77% at 1 g/L, beyond which saturation effects were observed. Adsorption followed the Langmuir isotherm model with a high adsorption constant (Kads = 28.25 L/g) and a standard free energy of adsorption ( $$\Delta G_{{{\text{ads}}}}^{^\circ }$$ = –18.2 kJ mol–1), indicative of spontaneous physisorption. SEM–EDX analysis confirmed the formation of an organic protective film that suppressed chloride adsorption and oxide formation on the steel surface. The findings highlight PPPE as a promising, low-cost, and environmentally benign corrosion inhibitor for pipeline steel in acidic environments, offering both technical effectiveness and sustainability advantages for industrial applications.