Onion Peel Extract/Copper Oxide Nanoparticles as Corrosion Inhibitors for Carbon Steel in Hydrochloric Acid: Extraction, Characterization, Electrochemical Study, and Theoretical Explorations

The effectiveness of onion peel extract (OPE) as a corrosion inhibitor for carbon steel in a 1 M hydrochloric acid solution was investigated using weight loss (WL) techniques, potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), and surface morphological examination. Additionally, this process was characterized by Fourier transform infrared (FT-IR) spectroscopy, energy dispersive X-ray spectroscopy (EDX), thermogravimetric analysis (TGA), atomic force microscopy (AFM), and transmission electron microscopy (TEM). According to weight loss measurements, the protective efficiency of OPE increases with rising OPE concentration and decreases with increasing temperature of the corrosive solution. Polarization curves indicate that OPE acts as a mixed-type inhibitor in hydrochloric acid. The adsorption mechanism, supported by EIS results, shows that charge-transfer resistance increases and double-layer capacitance decreases with higher inhibitor concentration. The adherence of OPE to carbon steel follows the Langmuir isotherm, indicating a physical adsorption process, and the spontaneous adsorption of the inhibitor molecules is evidenced by the negative values of Gibb's free energy of adsorption. The synergistic effect of copper oxide nanoparticles (CuO–NPs) in combination with OPE on the corrosion inhibition of carbon steel was also evaluated. The results demonstrated that the inhibition efficiency of OPE is enhanced in the presence of CuO–NPs due to synergistic interactions between OPE extract molecules and CuO–NPs. The electron-donating capacity of the chemical components in OPE was confirmed through theoretical studies employing quantum chemistry methods.