Inhibitory effects of Ficus sycomorus leaves extracts on the corrosion of mild steel and aluminium in 1 M hydrochloric acid (HCl) solution was studied at temperature of 30°C using gravimetric technique. Corrosion rates of mild steel and aluminium in the aggressive medium were found to increase as temperature increased, but decreased upon the addition of leaves extract of Ficus sycomorus compared to the blank. At 30°C, the inhibition efficiency increased with increase in inhibitor concentration reaching 87.84% for mild steel and 98.92% for Aluminium in the presence of 5 g/L of Ficus sycomorus leaves extracts. The results obtained show that aluminium had correlation coefficients (R 2 ) of 0.999 and 0.876 for Langmuir and Temkin isotherms respectively while Mild steel had correlation coefficients of 0.989 and 0.751 for Langmuir and Temkin isotherms respectively. However, the inhibitor adsorption was found to fit Langmuir adsorption isotherm better than Temkin isotherm. The free energy of adsorption (∆G°a ds ) has negative values and this indicates that the adsorption of Ficus sycomorus on the metal surface follows a spontaneous process.

The inhibiting effect of Spondias mombin (SM) leaves extract on the corrosion of AA2024 and mild steel in 0.5 M NaCl environment was investigated using electrochemical techniques. Tafel plots obtained from the potentiodynamic polarization (PDP) experiments indicate that for both metals, the curves shifted toward more negative values of the corrosion current density (Icorr) as the inhibitor concentration increased. More so, for all concentrations of the inhibitor, the values of maximum displacement shift in corrosion potential (Ecorr) were found to be less than 85 mV. This shows that SM behaves like a mixed-type corrosion inhibitor. The electrochemical impedance spectroscopy (EIS) study shows that for both metals, the charge transfer resistance (Rct) values increased with an increase in concentration of the inhibitor. The increase in values of Rct can be attributed to the formation of protective film at the metal-solution interface. The Nyquist plots show semi circles whose diameter increased as the inhibitor concentration is increased and this is suggestive of retardation in corrosion process. The inhibition efficiency values obtained in the EIS experiment were found to be in close conformity with those of the PDP experiment.