ABSTRACT
The influence of two plant extracts, Parkia biglobosa (African locust bean) and Vitex doniana (Black plum) on electrochemical corrosion behaviour of as-cast AA2024 aluminium alloy exposed to artificial sea water (3.5%NaCl solution) was investigated. The results of the 1H NMR-based metabolomics of leave extracts of Parkia biglobosa (PBE) and Vitex doniana (VDE) revealed aromatic and other functional groups such as -CH3, and R-OH which are attributes of a good inhibitor. The performances of these extracts were evaluated individually, synergistically and under the influence of Boehmite Nanoparticles in different amounts of 0.1, 0.2, 0.3, 0.4 and 0.5g. Parkia biglobosa extracts gave better Inhibition Efficiencies (IE%) than Vitex doniana extracts whether alone or in synergistic combinations; this could probably be due to the high density/concentration of aromatics and other functional groups as revealed by 1H NMR-based spectrum. Comparison of the performances of the different extract-extract and extract-Nanoparticle systems were also made on the basis of Inhibition Efficiencies (IE%), determined from both chemical (Gravimetric-based Mass Loss) and electrochemical techniques (Linear Polarisation Resistance and Electrochemical Impedance Spectroscopy). Only PBE+VDE+Boehmite Nanoparticle synergistic system was found to compare favourably with Sodium chromate, the reference inhibitor against which the effectiveness of the different inhibitor systems (single, binary and ternary) was compared. An optimum IE% of 92.15% for PBE+VDE+Boehmite Nanoparticles system compared favourably with IE% of 92.57% for Sodium Chromate. The adsorbed films of Sodium Chromate and PBE+VDE+Boehmite Nanoparticles systems were found to be stable around -0.6V when evaluated by Chronopotentiometry. The synergism parameters were found to be greater vii than unity, affirming synergy among the inhibitor molecules/ions. Kinetic and thermodynamic data generated suggested co-adsorption through a combination of both Physisorption and Chemisorption mode of interaction between the inhibitors and the substrates. The linearity of Langmuir adsorption Isotherms further supported adsorption proposition, while the El-Awady's adsorption Isotherms consolidated the Langmuir's model by calculating the number of active sites; obtained values greater than unity inferred that the inhibitor molecules occupied more than one site. SEM/EDX panorama of the inhibited samples revealed evidences of interference of corrosion products with the additive substances to form new phases, as revealed by X-Ray Diffractograms, which was a plausible mechanism for the action of the inhibitors, and not just adsorption or coadsorption. Raman spectroscopic analyses were found to be particularly helpful when examining the surfaces of the substrates inhibited by the plant extracts. Variations in Raman shifts were indicative of surface interactions; due either to adsorption of organic molecules onto the alloy surfaces or release of certain metal ions into solution, leading to appearance of certain peaks. All the characterisation techniques, using SEM/EDX, XRD and Raman Spectroscopy, gave insights into the influence of the additive substances on the electrodes/substrates. This was achieved through morphological revelations, phase identifications and other thin corrosion products elucidation. IE% results obtained from Linear Polarisation Resistance, Electrochemical Impedance Spectroscopy and Gravimetric-based mass loss were compared using ANOVA statistical tool and found to be in good agreement. In conclusion, PBE+VDE+Boehmite Nanoparticles system could serve as an ecofriendly alternative to Sodium Chromate.