A theoretical and experimental study of castor oil-based inhibitor for corrosion inhibition of mild steel in acidic medium at elevated temperatures

The castor oil as a cheap and environmentally friendly source was used to prepare a novel green corrosion inhibitor for mild steel in acidic medium. The poor performance at high temperatures and low biodegradability are two important challenges of corrosion inhibitors that castor oil-based corrosion inhibitor (COCI) is designed to overcome them. The inhibition performance of COCI was evaluated by weight loss, open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and polarization techniques. The morphology of MS was examined by field emission scanning electron microscope (FESEM), energy dispersive X-ray (EDX) and atomic force microscopy (AFM). Furthermore, a comprehensive computational study was done to clarify the anticorrosive mechanism of COCI by molecular dynamics (MD) simulation, density functional based tight-binding (DFTB) approach and density functional theory (DFT). Maximum inhibition efficiency of 85 % and 91 % were achieved at 80 °C using 140 μM of COCI in EIS and polarization tests, respectively. COCI acted as a mixed type inhibitor and the adsorption of the inhibitor on the mild steel surface was chemisorption. The obtained results demonstrate that the COCI has a considerable ability to suppress acidic corrosion at all concentrations were studied here, particularly at high temperatures. Given the fact that several articles were reported a significant reduction in performance of corrosion inhibitors by increasing temperature, in this study was found out not only was there no decrease in inhibition efficiency of COCI at higher temperatures, but it depicted good performance at 80 °C. In addition, the high adsorption energy of COCI confirmed a strong interaction between Fe surface and inhibitor molecules and showed a significant effect of the urethane bonds and the triglyceride groups on molecular activity. The Fermi energy state indicated that the chemical adsorption occurred in the interaction of COCI-Fe surface by electron transition from inhibitor to Fe 3d band state. We expect the results of this work provide new opportunities for the design and synthesis of efficient corrosion inhibitors based on vegetable oils.