Mentha pulegium Extract as a Natural Product for Inhibition of Corrosion. Part I: Electrochemical studies Abdelkader Khadraouia*, Abdelah Khelifaa, Hocine Boutoumia and Belkheir Hammoutib

The inhibition effect of Mentha pulegium extract (MPE) on the acid corrosion of steel in 1 M HCl solution was investigated using potentiodynamic polarisation and electrochemical impedance spectroscopy. The results shown that the Mentha pulegium extract is a good inhibitor, the inhibition efficiency depended on the concentration of the plant extract. Efficiency is highest, 88 % for 33% (v/v) of Mentha pulegium extract. Polarization measurements also show that Mentha pulegium extract is a good mixed inhibitor. The remarkable inhibition efficiency of MPE was discussed in terms of blocking of electrode surface by adsorption of inhibitor molecules through active centres. MPE obeys the Timkin adsorption isotherm.

Natural plant; Mentha pulegium; Extract; Green Inhibitor; adsorption; Steel.

The electrochemical study was carried out using a EG&G potensiostat/Galvanostat (PAR 273A) piloted by CORR III software. This potentiostat is connected to a cell with three electrode thermostats with double wall. A saturated calomel electrode (SCE) and platinum electrode were used as reference and auxiliary electrodes, respectively. The material used for constructing the working electrode was the same used for gravimetric measurements. The surface area exposed to the electrolyte is 0.64 cm². Potentiodynamic polarization curves were plotted at a polarization scan rate of 1 mV/s. Before all experiments, the potential was stabilized at free potential during 30 min. The polarization curves are obtained from −750 mV to -250mV at 298 K. The solution test is there after de-aerated by bubbling nitrogen. Gas bubbling is maintained prior and through the experiments. The data in Tafel region have been processed for evaluation corrosion kinetic parameters by plotting the polarization curves. The linear Tafel segments, in a large domain of potential, of the cathodic curves were extrapolated to the corresponding corrosion potentials to obtain the corrosion current values.

The electrochemical impedance spectroscopy (EIS) measurements are carried out with the Analyzer of function of transfer model 5210, piloted by Powersuite software. After the determination of steady-state current at a corrosion potential, sine wave voltage (10 mV) peak to peak, at frequencies between 100 kHz and 10 mHz are superimposed on the rest potential. Computer programs automatically controlled the measurements performed at rest potentials after 30 min of exposure at 298 K. The impedance diagrams are given in the Nyquist representation. Experiments are repeated three times to ensure the reproducibility.

Figure S1: Potentiodynamic polarisation curves for steel in 1 M HCl in the presence of different percentage concentration of MPE.