Abstract:
The inhibitory potential of crude ethanolic extract and four fractions of the Alchornea
laxifloraleaves on the corrosion of mild steel in sulphuric acid solution were investigated
using gravimetric method at room temperature. Inhibition efficiency of the ethyl acetate
fraction of Alchornea laxiflora extract was found to be the highest (93%) compared tocrude
extract (69%), aqueous fraction (65%), Butanol fraction (71%) and n-hexane fraction (42%).
Thus, further corrosion studies were carried out on ethyl-acetate fraction using gravimetric,
Fourier Transform Infra- Red (FT-IR), Atomic Absorption Spectrophotometry (AAS),
electrochemical and Scanning Electron Microscopy (SEM) methods. Qualitative investigation
of the phytochemical constituents showed that ethyl acetate fraction of Alchornea laxiflora
extract contains alkaloids, tannin, flavonoid, Saponin, glycosides, steroidsand
phenols.Inhibition efficiency was found to increase with increase in concentration but
decreases with increase in temperature. Thermodynamic studies reveal a mechanism of
physical adsorption of the plant components on the surface of the metal. Activation energies
increase in the presence of the extract.The values for enthalpy of activation were positive;
entropy and Gibbs free energy were negatives. First order kinetic is applicable to the
corrosion of mild steel in the presence of ethyl acetate fraction of Alchornea laxiflora extract.
Calculated half-lives increase as the concentration of the extract increases. The data obtained
were fitted into Langmuir, Temkin and Freundlich adsorption isotherms though the Langmuir
and Temkin models were found to be better fitted.FT-IR resultreveal ethyl acetate fractionof
Alchronea laxiflora contained O-H, C-H, C=C, C=C=C, CH3which meets the general
structural consideration of compounds proven to be good corrosion inhibitors.Polarization
curves reveal that ethyl acetate fraction acts as a mixed-type inhibitor. AAS result reveal
decrease in the concentration of Fe2+ in the electrolyte as the concentration of the inhibitor
increases.The SEM images of the corrosion product confirmed the protection offered by theextract on the surface of the metal immersed in it.
