Investigation of the efficacy of Derris elliptica extract as a green corrosion inhibitor for mild steel in hydrochloric acid solutions

The oil and gas industry commonly uses a mild steel as transportation pipeline material, however it needs to be protected from corrosion. To protect mild steel from corrosion, a green organic inhibitor, Derris elliptica extract (DEE), is used and its effectiveness has been investigated in this work. Through potentiodynamic polarization and Electrochemical Impedance Spectroscopy (EIS) analysis, the efficacy of the DEE corrosion inhibitor was assessed. The results demonstrated that DEE has a 94.1% inhibitory efficiency at 500 ppm, 30 °C, and 30 min of immersion. DEE has a mixed-type adsorption with the standard Gibbs free energy of adsorption values obtained at temperatures of 30, 40, and 50 °C are −24.76 kJ mol⁻¹, −27.08 kJ mol⁻¹, and −28.52 kJ mol⁻¹, respectively. Through the physisorption-chemisorption process, the DEE molecules were adsorbed into the steel surface, forming stable, long-lasting inhibitive and protective films at the metal/solution interface. Surface morphology analysis (SEM, AFM, and XRD) further supports this by showing that the mild steel surface becomes smoother upon the addition of the DEE inhibitor.