Thermodynamic Study and Characterization by Electrochemical Technique of Pyrazole Derivatives as Corrosion Inhibitors for C38 Steel in Molar Hydrochloric Acid

The effect of synthesized 1,1'-propane-1,3-diylbis [3-(chloromethyl)-5-methyl-1Hpyrazole], dimethyl 1,1'-butane-1,4-diylbis(5-methyl-1H-pyrazole-3-carboxylate) and 1,1'-butane-1,4-diylbis [3-(chloromethyl)-5-methyl-1H-pyrazole] on corrosion of C38 steel in hydrochloric acid solution has been investigated in the temperature range from 298 K to 328 K by weight loss, potentiodynamic polarization method and modelled with an equivalent electric circuit. All the compounds exhibited more than 90% anticorrosion activity, the highest is 96 % by 1,1'-butane-1,4-diylbis [3-(chloromethyl)5- methyl-1H-pyrazole], and it has been shown that the inhibition efficiencies increased with the concentration of the inhibitors, remaining almost constant in a wide temperature range. Potentiodynamic polarization studies suggested that it is a predominance cathodic type. Nyquist plots showed depressed semicircles with their centre below the real axis. The adsorption on the C38 steel surface followed the Langmuir adsorption isotherm. The thermodynamic parameters for dissolution were investigated at different concentrations and temperature. The theoretical study by modelling the molecules of these inhibitors has been performed by considering the Density Functional Theory (DFT) using the Gaussian 03W suite of programs that can calculate the different quantum parameters such as E HOMO, E LUMO, Δ E HOMO, E LUMO, and μ dipolar moment, allowing us to confirm the results found by the gravimetric and electrochemical methods.