The study of the effects of flavanoid derivatives as corrosion inhibitors for zinc and aluminium metals in sulphuric and hydrochlorine acids

Abstract

In this study, the corrosion inhibition potentials of three flavonoid derivatives namely Naringenin (NRNG), Morin hydrate (MNHD) and 6-Hydroxyflavone (6-HFN) for zinc and aluminium metals was investigated using gravimetric analysis, electrochemistry, and surface characterization techniques in three aggressive acidic environments of 0.5 M and 1.5 M hydrochloric acid and 1.5 M sulphuric acid at a temperature range from 30 – 60°C. All the three inhibitors managed to inhibit the corrosion of zinc and aluminium metals from the acid attack by adsorption of their molecules onto the metals surface. The adsorption of the inhibitors onto the zinc metals was found to be both physisorption and chemisorption. However, in the presence of aluminium metals, the inhibitors were found to adsorb through physisorption only. The experimental data obtained from gravimetric analysis obeys Langmuir adsorption isotherms. Atomic Absorption Spectroscopy (AAS) was used to find the concentration of the metal ions leached into the solution after gravimetric analysis in the absence and presence of the inhibitors. The lowest concentration was obtained when the three inhibitors were used in the presence of sulphuric acid in zinc, which suggest that the inhibitors managed to inhibit the metal dissolution to a much greater extent as compared to when the acid was used alone. Fourier Transform Infrared Spectroscopy (FTIR) was utilised to understand the functional groups that disappeared or formed during the adsorption process of the inhibitor’s molecules on the metals surface. The Ar - OH functional group was the worst affected since only 6-hydroxyflavone was the only inhibitor to retain it after gravimetric analysis in the presence of hydrochloric acid. The C=C functional group of the aromatic ring was retained by all the inhibitors in all the metals studied. A two-dimensional (2D) and three-dimensional (3D) microscopy was also used to study the extent to which the metals were damaged by the acid and the type of corrosion that resulted thereafter, in the absence and presence of the three inhibitors. The results obtained from Potentiodynamic Polarization (PDP) indicated that the compounds are mixed-type inhibitors. The results from PDP also indicated that the use of three flavonoid derivatives compounds as corrosion inhibitors managed to significantly reduce the corrosion current densities for the cathodic and anodic half reactions, which indicate that, the dissolution and cathodic reduction of the hydrogen ions were inhibited. The order of the inhibition efficiency at 1.8x10-3 M for the maximum temperature (60°C) was 6-HFN (86.54 and 77.24 %) ˃ MNHD (81.37 and 73.30 %) ˃ NRNG (78.07 and 72.47 %) for zinc metals in sulphuric acid and hydrochloric acid, and NRNG (91.48) ˃ 6-HFN (90.12) ˃ MNHD (81.59) for aluminium metals in hydrochloric acid. In Electrochemical Impedance Spectroscopy (EIS), the charge transfer resistance increased with an increase in inhibitors concentration, while the constant phase element was found to decrease with an increase in inhibitors concentration. This observation is an indication that the inhibitors adsorbed onto the metal surface and formed a film layer protecting the metal from corrosion. The percentage inhibition efficiency of Electrochemical Impedance Spectroscopy (%IEEIS) was found to correlate with those obtained from PDP, AAS and gravimetric measurements.