ABSTRACT

The efficacy and behaviour of different concentrations of Treculia africana (TA) and Newbouldia laevis (NBL) leaves' extracts, as corrosion inhibitors for aluminium alloy AA7075-T7351 in saline (0.5 M NaCl) and acidic (1.0 M HCl) environments, have been investigated, using electrochemical impedance spectroscopy (EIS), potentiodyanamic polarization (PDP), thermodynamic and gravimetric techniques at ( ); 298, 313, 323, 333, 338 ^oK. In 1.0 M HCl corrosive environments the respective average maximum and minimum inhibition efficiencies obtained through gravimetric methods, were 82.48% and 67.21%  for NBL extracts, whereas 81.71% and 63.29% were recorded for TA extracts. Elsewhere, in 0.5 M NaCl, TA performed better with respective maximum and minimum inhibition efficiencies of 85.47% and 77.71%, while NBL extract recorded, equally good pair of values of 82.96% and 77.19%. At tested concentration of 600 mg/L, both extracts' inhibition efficiencies were reduced with temperature increments. TA and NBL efficiencies reduced to 55.0% and 36.2% respectively, after 1 h and 51.4% and 29.0% respectively after 3 h, in 0.5 M NaCl. In 1.0 M HCl environment the results were 10.4% and 75.7%, after 1 h and 23.3% and 67.5%, after 3 h respectively for TA and NBL. PDP and EIS results showed that, in 1.0 M HCl environments both TA and NBL acted as mixed-type corrosion inhibitor with dominant cathodic behaviour. Also, in 0.5 M NaCl environments, TA worked like a mixed-type inhibitor with dominant anodic character, while NBL was concentration sensitive - being anodic for concentrations; 100, 200, 500 mg/L NBL and cathodic for the concentration, 1000 mg/L. In all cases, corrosion inhibitions were achieved by the obstruction/restriction/reduction of charge flow through the formation of a thin protective metal complex layer with the adsorbates, on the AA7075-T7351 surfaces. The inhibitory actions were achieved through easily attached, endothermic, strong and spontaneous reactions that fitted nicely to Langmuir, Freundlich and Temkin models. The adsorption coefficients, , and the Gibb's free energy, , values obtained from Temkin isotherms, for both inhibitors were respectively lower and higher than the values for the other two models. These proved that the adsorption of TA and NBL on AA7075-T7351 surfaces were through chemisorption and physisorption processes. Bode plots revealed that increases in concentration, increased the slopes of their linear sections and the corresponding values of the phase angles.