The Inhibitive and Adsorptive Characteristics of Orange Peel Extract on Metal in Acidic Media

Abstract

The potential of orange peel extract (OPE) to inhibit corrosion is investigated to replace the current toxic inhibitors. OPE contains inhibitor molecules that are able to adsorb onto the metal surface and form a protective film, restricting the corrosion attack. In this research, a screening between ethanol and distilled water was conducted for the selection of the best extracting solvent for orange peel extraction. It was found that ethanol extracted more functional groups and enhanced the inhibitive efficiency of OPE as compared to distilled water. The inhibitive performance of OPE was further studied with four parameters, which were orange peel to solvent ratio (0.5:100 – 10:100), contact time (0.5 hr – 3 hr), temperature (30°C – 60°C), and acid concentration (0.1 M – 10 M). It was found that the optimum conditions for the inhibition effects were at orange peel to solvent ratio of 10:100, contact time of 2 hr, temperature of 30°C at acid concentration not more than 1 M. Through the characterization studies, the properties and characteristics of inhibitor molecules in OPE responsible for adsorption were identified through FTIR and phytochemical analysis. FTIR scanning revealed that the OPE contained abundant oxygen containing groups and the functional groups contained heteroatoms that have tendency to donate their unpaired electrons to the metal surface. This could explain the formation of protective film via the adsorption of the inhibitor molecules onto the metal surface, thus reducing corrosion attack. The presence of flavonoids, saponins and phenols was also detected in OPE by conducting phytochemical analysis. These phytochemicals contained heterosides structures that have antioxidant properties which could enhance the adsorption tendency. The experimental data were analyzed by Langmuir, Freundlich, and Temkin isotherm models. The results had shown that the adsorption of OPE obeyed Langmuir adsorption isotherm model with the highest regression coefficient value (R2 = 0.9973). The findings in this research showed that OPE demonstrated the adsorptive and inhibitive characteristics on mild steel in acidic media, suggesting it as a potential green and sustainable option in corrosion inhibition.