Experimental and theoretical studies of the inhibition potential of Lippia javanica plant extracts for the corrosion of aluminium, mild steel, and zinc metals in acidic medium

PhD (Chemistry)Department of ChemistryCorrosion is a natural phenomenon considered a chemical and an electrochemical process of metals interacting with the surrounding corrosive environment. Inhibition is a preventive measure used by corrosion engineers to reduce the effects of corrosion on metals, the environment, society, and the economy. The current study investigates the use of the Lippia javanica plant as an ecofriendly green inhibitor for mild steel (MS), aluminium (Al), and zinc (Zn) corrosion in a 1 M HCl environment. The leaf extracts were prepared by the Soxhlet extraction method using methanol, ethanol, and acetone as solvents. The leaf extracts of L. javanica were characterized using Fouriertransform infrared spectroscopy (FT-IR) and Liquid chromatography-mass spectrometry (LC/MS) analysis. The inhibitory potential of L. javanica extracts was established by performing weight loss measurements, electrochemical methods such as potentiodynamic polarization (PDP), and electrochemical impedance spectroscopy (EIS) techniques. The weight loss assessment was carried out at different temperatures (303 to 333 K) and varying concentrations of the extracts from 200 to 800 ppm. The technique was also used to determine the stability of the extract with varying immersion times. Weight loss measurements showed that the inhibition efficiency increased with increasing concentration of the extracts up to 800 ppm for all three metals. Increasing the temperature of the corrosive environment resulted in a decrease in the inhibition efficiency of Al and MS corrosion, with that of Zn increasing with temperature. The variation of inhibition efficiency with time showed a similar trend, with the protective efficiency of Al and MS decreasing with time and Zn increasing with immersion time. Increasing Zn’s inhibition efficiency with temperature and immersion time implies a chemical protection mode. The reduction in the inhibition efficiency with temperature and immersion time for Al and MS suggests a physical protection mode. According to EIS measurements, the extracts adsorb onto Al, Zn, and MS surfaces to create a protective coating with pseudo-capacitive properties. The only component of the Zn and MS Nyquist plots in the high frequency was a capacitive loop, but the Al plots also showed an inductive loop at a lower frequency. The higher frequency loop represents the resistance for the charge transfer during the corrosion process. In contrast, the lower frequency loop represents the relaxing of hydrogen ions and the adsorption of corrosive chloride ions onto the oxide film. The PDP results revealed that for Al and MS, the three extracts had a similar impact on both the anodic and cathodic half-reactions. In contrast, both half-reactions were affected for Zn, but the cathodic area was more significantly impacted. Undulation Tafel curves for MS and Zn were observed with and without the extracts; however, a longer passive region was detected for Al, particularly in the presence of the plant extracts. The investigational extracts function as mixed-type corrosion inhibitors for Al, Zn, and MS, as indicated by the control of both the anodic and cathodic areas with the introduction of the extracts. Among the many plotted isotherms for the three extracts on the metal surfaces, the Langmuir adsorption isotherm was determined to be the best-fit isotherm. The isotherm confirmed the mechanism of adsorption, which was a mixed-type adsorption for Al, Zn, and MS. Spectroscopy studies revealed that the interaction of the three extracts with Al, Zn, and MS resulted in the formation of metal-inhibitor complexes, which slowed the corrosion process. Scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDS) studies demonstrated that L. javanica leaf extracts form a protective film on Al, Zn, and MS surfaces, protecting them from the corrosive environment. Theoretical simulations showed that the primary extract constituent (verbascoside) had binding energy greater than 13 kcal/mol on the surfaces of Zn(110), Al(111), and Fe(110). The high binding energy indicates a mixed-type binding process that includes chemisorption and physisorption. All corrosion experiments revealed that the three extracts exhibited superior inhibition performance for all the three metals studied with comparable results in 1 M HCl corrosive solution.National Research Foundation (NRF