Abstract:

Dithio-N,N'-dimethoxypropyldipropionamide (DPDA) was investigated as a corrosion inhibitor for mild steel in 1 mol·L-1 hydrochloric acid using weight loss test, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS), quantumchemical study, and Raman spectroscopy. The weight loss test results showed that DPDA was an excellent inhibitor for mild steel in acid media with an inhibition efficiency of 90.2% at the DPDA concentration of 1×10-3 mol·L-1. The polarization curves indicated that DPDA behaved as a mixed-type inhibitor while the single capacitive loop in the Nyquist plots revealed that the corrosion was a charge transfer controlled process. Inhibition efficiencies increased with the increase of DPDA concentrations, and the values of the inhibition efficiencies, obtained from the weight loss test, potentiodynamic polarization, and EIS, were in reasonable agreement. The absorption of DPDA was found to obey the Langmuir adsorption isotherm. The free energy of adsorption (△G0ads) was found to be -38.65 kJ·mol-1 and this value indicated that the adsorption was mainly chemisorption. This results from the sharing or/and transfer of electrons from the DPDA molecules to the metal surface to form coordinate bonds. The negative △G0ads value revealed that the adsorption of DPDA was a spontaneous process. Raman spectra suggest that DPDA adsorbed well onto the mild steel surface, and the quantumchemical study showed that the sulphur atoms in the DPDA molecule were the main active sites that resulted in absorption on the surface of the mild steel.

Key words: Inhibitor, Dithioamide derivative, Hydrochloric acid, Absorption, Potentiodynamic polarization, Electrochemical impedance spectroscopy