Kinetic Studies Of Selected Corrosion Inhibitors of Aluminium and Mild Steel in Acid Medium

Abstract

The corrosion inhibition of aluminium and mild steel in H2SO4 in the presence of Bromothymol blue, BTB, Dimethyl yellow, DMY, Methyl orange, MO, Titanium (IV) oxide, TO and Cassia Siamea Lam, CSL leaves extract at 303 – 313K was studied using “Absorbance difference” technique. The results showed that inhibition efficiency (%IE) increased with increase in inhibitor concentration (except for BTB, where beyond a “critical” concentration of 0.0007M, it reversed its function and became an accelerator) and decreased with increase in temperature within the range 303 – 313K studied in the absence and presence of inhibitors and halides. The efficiencies increased in the order: DMY [65.03% (49.72%)] < BTB [67.82% (56.37%)] < CSL [68.79% (58.75%)] < MO [71.98% (57.91%)] < TO [73.34% (59.62%)]. The effect of halides, namely NaCl, NaBr and NaI is also reported. The addition of halides enhanced the inhibition efficiency in the order: I- < Br- < Cl-. The synergism parameter (SI) evaluated are found to be less or equal to unity for aluminium corrosion and greater or equal to unity for mild steel corrosion due to the addition of halides. The enhanced inhibition efficiency on addition of halides is ascribed to antagonism (or passivity) and synergism for aluminium and mild steel respectively. BTB, DMY, MO, TO and CSL were found to obey Freundlich and Langmuir adsorption isotherms at all concentrations studied for aluminium and mild steel corrosion. Physical adsorption is proposed from the activation energy, (Ea < 40kJmol-1) obtained for aluminium and mild steel. The results obtained from the study were treated kinetically and values of Qads (a thermodynamic parameter) were evaluated and reported. xx The kinetic data obtained indicates that the adsorption follows a first order type of reaction. The rate laws for the inhibition process involving BTB, DMY, MO and TO as well as the mechanism of inhibition of CSL on aluminium and mild steel surfaces have been proposed. The difference in inhibitor behaviour has been explained on the basis of their molecular structures, number of substituent(s)/functional groups, electronic effects and molecular masses. Phytochemical screening of CSL leaves extract revealed the presence of alkaloids, saponins, tannins, phenols (having –C=O, -C=N, C-OH, >C=C< etc functional groups) which were found to contribute greatly to the inhibitory properties of CSL on aluminium and mild steel.