ABSTRACT
In this study, the effects of methanolic leave extracts of Combretum micrantum (CM), Combretum Glutinosum (CG), Ficus exasperate (FE) and Ficus platyphylla (FP) on the corrosion of A356.0 type Al-Si-Mg alloy in 3.5 % NaCl aqueous solution were determined. It evaluated the inhibition efficiency of each of the inhibitors and determined their mechanism of inhibition. It also characterized the microstructures of the corroded surfaces. All these were done with a view to determining the inhibitive potentials of all the inhibitors with regards to the corrosivity of A356.0 Al-Si-Mg alloy in marine environment. Weight loss and electrochemical techniques were used for corrosion study of the alloy. Thin Layer Chromatography (TLC), Fourier Transform-Infrared Spectroscopy (FT-IR), Gas Chromatography-Mass Spectroscopy (GC-MS) were used to characterize the plant extracts in order to identify the chemical bonding and functional groups. Some physicochemical properties such as viscosity, solubility, density, pH value and Biodegradability tests of the plant extracts were also determined. The weight loss method of corrosion tests were carried out at 1, 2, 3, 4, and 5 hours of exposure time using various concentration of extracts (0.2, 0.4, 0.6, 0.8 and 1.0) v/v at different temperatures 30 OC, 50 OC, and 70 OC in a static solution. The effect of co-inhibition was also studied, and the surface morphology of the coupons before and after corrosion tests was characterized by Optical Microscope (OPM) and Scanning Electron Microscopy (SEM) techniques. A statistical design for the corrosion process was carried out to determine which parameters (Inhibitor concentration, exposure time and temperature) are statistically significant using ANOVA and F-test distribution at 95% confidence level.The phytochemical screening (Qualitative and Quantitative) and thin layer chromatography results revealed the presence of tannins, saponins, flavonoids and alkaloids in the plant extracts. The results of FT-IR and GC-MS analyses detected all organic species. Each peak area in the chromatogram was proportional to the amount of the organic compound(s) forming the peak. The plant extracts displayed high level of solubility in different solvents, low viii densities and less viscous at the studied temperatures. From the result, the corrosion rates decreases with increase in inhibitor concentration for all the inhibitor types. The maximum inhibitor efficiency of the inhibitors are 94.44 % at 0.8 v/v for CM, 81.25 % at 0.4 v/v for CG, 84.62 % at 0.4 v/v for FE, 94.74 % at 1.0 v/v for FP and 94.64 % at 1.0 v/v for the Sodium chromate (control). The co-inhibitor shows that the best combination occurred at 1.0 v/v of CG and FE with 95.13 % IE and that of surfactant was at 1.0 v/v FE and TEA with 84.20 % IE. The potentiodynamic polarization results revealed a change in the anodic and cathodic regions for the corroded samples indicating a mixed type inhibitors. The percentage inhibition efficiency (IE%) of the linear polarization resistance (LPR) for the various plant extracts are 90.11 % CM, 91.01 % CG, 92.69 % FE, 91.34 % FP and 96.19 % Na2CrO4 with inhibitor concentration of 0.8, 1.0, 0.4, 0.8 and 1.0 v/v respectively. The kinetic and thermodynamic studies showed that activation energy (Ea) in the presence of inhibitor is greater than in the absence of inhibitor at all the temperatures studied. The mechanism of physical adsorption was proposed for the extracts as inhibitors with the temperature range investigated since the Ea and �”Gads values are lesser than 80 kJ/mol.and -20 kJ/mol, respectively. The surface morphology of some of the samples indicated that the alloy in the presence of the inhibitors had smooth surfaces compared to the alloy that has no inhibitor. The results of the ANOVA show that inhibitor concentration is the significant corrosion parameter.