ABSTRACT

The inhibition efficiencies of tryptophan and histidine Schiff base of para-dimethylamino benzaldehyde as corrosion inhibitors on mild steel in 1.0 M H₂SO₄ medium at 303K, 313K, 323K and 333K at concentration range of 0.1 to 0.4 % using weight loss technique were investigated. The surface characterization was done using Fourier transform infrared (FT-IR) spectroscopic technique and scanning electron microscope (SEM). FT-IR spectra revealed some of the functional groups responsible for the adsorption of the inhibitors on the metal surfaces. Comparison of the inhibition efficiencies of the two Schiff bases shows that the tryptophan Schiff is a better inhibitor than histidine and with maximum inhibition efficiency of tryptophan Schiff base reaching 96.43% at the inhibition concentration of 0.4%. The Schiff bases of tryptophan and histidine of para-dimethyl amino benzaldehyde gave good yields of 55.3 and 59.79 % respectively. The solubility tests showed that the tryptophan Schiff base is soluble in ethanol, chloroform and ethyl acetate but insoluble in hexane while the solubility tests for the histidine Schiff base showed that it is soluble in chloroform and ethyl acetate but partly soluble in ethanol and insoluble in hexane.   The negative and low values of the reaction constant (B) of both Schiff bases on mild steel indicate that the corrosion rate is inversely proportional to the concentration of the inhibitors. The increase in rate constant with increase in temperature indicates the increase in corrosion rate with temperature. The activation energies in the presence of inhibitors were found to be higher than the uninhibited solution. This implies that the energy required for corrosion to occur in the inhibitor molecule is higher. The adsorption parameters showed that the adsorption of trytophan and histidine Schiff bases on mild steel fitted perfectly into Freundlich adsorption isotherm model. The values of Gibbs free energy calculated were found to be negative indicating the spontaneity of the corrosion inhibition process on mild steel in both Schiff bases in 1.0 M H₂SO₄ medium. Values of entropy and enthalpy of activation were found to be positive and negative for both Schiff bases in 1.0 M H₂SO₄ medium on mild steel. Positive values of enthalpy indicate endothermic nature of mild steel dissolution process. Negative values of entropy of activation are indication of formation of activated complex in the rate determining step which represents association rather than disassociation step. It can be seen that uninhibited mild steel surface in 1.0 M H₂SO₄ medium has a very rough surface morphology due to corrosion by H₂SO₄ solution   while the mild steel surface after inhibition by the Schiff bases showed a lesser degree of roughness in comparison to the mild steel surface in the uninhibited acid solution. This could be attributed to the adsorption of the inhibitor on the metal surface thereby preventing aggressive corrosion on the metal. Attempts should be made to synthesize other novel Schiff bases from tryptophan, histidine and para-dimethyl amino benzaldehyde and their corrosion inhibitory properties on the metal surfaces should be investigated.