ABSTRACT

Inhibiting effects of Barteria fistulosa and Spondias mombin leaves extract on the corrosion of low carbon steel and aluminium alloy (AA2024) in 1M HCl and 0.5 M NaCl solution was investigated using gravimetric and electrochemical methods. The results obtained from the weight loss experiment performed at various temperatures (30, 45, and 60 ⁰C) show that inhibition efficiency values decreased with temperature rise but increased as the inhibitor concentrations were increased. However, in all cases, the inhibition efficiency values of the Barteria fistulosa (BF) extracts were found to be greater than those of Spondias mombin (SM). For low carbon steel in 1 M HCl, optimum inhibition efficiency value of 95.27% and 80.32% were respectively obtained for BF and SM at 30 ⁰C while for AA2024 in 1M HCl, an optimum inhibition efficiency value of 69.28 % and 78.06 % were respectively obtained for BF and SM at same temperature. Similarly, for low carbon steel in 0.5 M NaCl, optimum inhibition efficiency value of 78.06 % and 67.25 % were respectively obtained for BF and SM at 30 ⁰C. The values obtained for the standard enthalpy of adsorption  were all positive which signifies endothermic adsorption of the inhibitors on the metals’ surface. The values obtained for the standard entropy of adsorption   were all negative. Negative value of entropy is associated with decrease in disorderliness and implies that the activation complex encourages association instead of dissociation. For all cases, the difference between the corresponding values of  and E_a is approximately 2.64 kJ/mol. More so, results obtained from the isotherm study show that Langmuir isotherm best fitted the adsorption of both leaves extract on low carbon steel in the aggressive media. However, the adsorption of the inhibitors’ extract on AA2024 in the acidic medium best fitted the Freundlich isotherm. The tafel extrapolation plots obtained from the PDP experiments show a decrease in the values of corrosion current density (I_corr) and corrosion potential (E_corr) as the concentration of the inhibitors (SM and BF) was increased. This implies that the anodic dissolution of mild steel and cathodic reduction of hydrogen ions was inhibited leading to reduction of the corrosive active surface area. The trend of the Nyquist plots obtained from the EIS experiment reveals that the higher the concentration of the inhibitor, the more the increase in radius the semi-circular loops. This confirms that BF and SM are adsorption inhibitors for mild steel and AA2024 in both acidic and saline solutions.