Experimental, quantum chemical studies of oxazole derivatives as corrosion inhibitors on mild steel in molar hydrochloric acid medium

The corrosion inhibition performances of (4-ethyl-2-phenyl-4,5-dihydro-1,3-oxazol-4-yl)-methanol (C1); 4-{[(4-ethyl-2-phenyl-4,5-dihydro-1,3-oxazol-4-yl) methoxy]methyl}benzene-1-sulfonate (C2) and 4-[(azidoxy) methyl]-4-ethyl-2-phenyl-4,5-dihydro-1,3-oxazole (C3) mild steel in molar hydrochloric solution have been evaluated by using gravimetric, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques as well as quantum chemical calculations. Results obtained reveal that these compounds reduce significantly the corrosion rate of mild steel, their inhibition efficiencies increased with inhibitor concentration. This behavior means that the inhibitive effect of the studied oxazole derivatives occur through the adsorption of inhibitor molecules on the metal surface. Polarization curves reveal that both compounds C1 and C3 act essentially as mixed type inhibitors with cathodic predominance effect, while the compound C2 can be classified as cathodic type inhibitor. EIS spectra obtained show a typical Nyquist plot with single semicircles shifted along the real impedance of x-axis. Impedance data are analyzed in term of the simple modified Randles equivalent circuit with one relaxation time constant. Indeed, a Constant phase element, CPE, is introduced in the circuit instead of a pure double layer capacitor in order to take into account the electrode surface heterogeneity. Adsorption of these inhibitors on the mild steel surface was found to obey the Langmuir adsorption isotherm. Some thermodynamic parameters were calculated and discussed. The correlation between inhibition efficiency and molecular structure of oxazole derivatives was theoretically studied via quantum chemical calculations using density functional theory (DFT) at B3LYP/ 6-31G (d,p). Results showed a general correlation between the computed descriptors and the experimental data.