Monitoring corrosion and corrosion control of iron in HCl by non-ionic surfactants of the TRITON-X series - Part III. Immersion time effects and theoretical studies

The inhibition performance of three selected non-ionic surfactants of the TRITON-X series, namely TRITONX-100 (TX-100), TRITON-X-165 (TX-165) and TRITON-X-305 (TX-305), on the corrosion of iron was studied in 1.0 M HCl solutions as a function of inhibitor concentration (0.01-0.20 g L-1) and immersion time (0.0-8 h) at 298 K. Measurements were conducted based on Tafel polarization, LPR and impedance studies. At high frequencies, the impedance spectrum showed a depressed capacitive loop in the complex impedance plane, whose diameter is a function of the immersion time and the type and concentration of the introduced surfactant. In all cases, an inductive loop was observed in the low frequency and this could be attributed to the adsorption behavior. The inhibition efficiency increased with immersion time, reached a maximum and then decreased. This was attributed to the orientation change of adsorbed surfactant molecules. TX-305 inhibited iron corrosion more effectively than TX-100 and TX-165. The frontier orbital energies, the energy gap between frontier orbitals, dipole moments (mu), charges on the C and O atoms, the polarizabilities, and the quantum chemical descriptors were calculated. The quantum chemical calculation results inferred that for the HOMO representing the condensed Fukui function for an electrophilic attack (f(k)(+)), the contributions belong to the phenyl group and the oxygen atom attached to the phenyl group for each tested surfactant. Quantitative structure-activity relationship (QSAR) approach has also been used and a correlation of the composite index having some of the quantum chemical parameters with average inhibition efficiencies (I-av.(%)) was conducted to determine the inhibition performance of the tested surfactant molecules. The results showed that the values of I-av.(%) of the tested inhibitor molecules were closely related to some of the quantum chemical parameters. The calculated I-av.(%) values were found to be close to the experimental ones. Based on the values of coefficients of correlations at 0.02 level concentration. B3LYP/3-21G* method outpaced the other two methods namely B3LYP/6-31G and RHF/6-31G*. The relationship between I-av.(%) and composite index is accounted for by B3LYP/3-21G*. (C) 2011 Elsevier Ltd. All rights reserved.