Abstract In the present work the influence of the two organic inhibitors, namely : phenyl (2- (3,4,5 trimethoxyphenyl) -1H-benzo [d] imidazol-1-yl) methanone (P3) and ( 2- (4 -methoxyphenyl) -1H-benzo [d] imidazol-1-yl) (phenyl) methanone (P4) on the ordinary steel corrosion in 5.0 M HCl solution was investigated by using electrochemical measurements, scanning electron spectroscopy (SEM) coupled with Energy Dispersive Spectroscopy (EDX), UV-visible spectroscopy and theoretical study. The obtained results showed that these compounds act as mixed inhibitors. It is obtained that the inhibition efficiency increases with their concentrations and reaches 97.0% and 91.3% at 10−3 M of P3 and P4, respectively. These findings were confirmed by the electrochemical impedance spectroscopy measurements, where the transfer resistance values increase with the concentrations. In addition, the effect of temperature range from 298 K to 328 K was investigated and indicated that P3 and P4 keep their performance at height temperature. Moreover, density functional theory explained the relationship between the molecular structure of imidazole derivatives and inhibition performance obtained experimentally. Additionally, the morphological surface and EDX analyses indicated that these compounds act by the formation of a protective layer on the ordinary steel surface. In order to confirm the possibility of the formation of P3-Fe and P4-Fe complexes, the UV–Visible absorption spectra was used. The obtained results indicated the formation of a complex between Fe2+ and molecules inhibitors.

The corrosion inhibition of mild steel in a 1.0 M hydrochloric acid solution by 2-(4,5diphenyl- 4,5-dihydro-1h-imidazol-2-yl)-5-methoxyphenol (P1) has been studied in relation to the concentration of the inhibitor, as well as to the temperature, using chemical (weight loss) and electrochemical techniques. All the employed methods were in reasonable agreement. The protection efficiency increased with an increased inhibitor's concentration. The obtained thermodynamic adsorption parameters (ΔG*ads, ΔH*ads, ΔS*ads) indicated that this polymer retarded both cathodic and anodic processes through physical adsorption, and blocked the active corrosion sites. It was also found that this compound obeyed the Langmuir's adsorption isotherm.

New quinoline, namely 5-(ethoxymethyl)-8-quinolinol (M-QN), has been synthesized and characterized by different spectral methods, such as 1H NMR, 13C NMR and IR spectra. Its inhibitive action against the corrosion of carbon steel in 1.0 M hydrochloric acid solution was investigated at different temperatures in the range from 25±2 to 55±2 °C by a series of known techniques, such as weight loss, open circuit potential (OCP), electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements. The inhibition efficiencies obtained from all employed methods are in good agreement with each other. The obtained results show that M-QN compound is a very good inhibitor with an efficiency of 97.7 % at 10-3 M additive concentration in a 1.0 M HCl solution. The inhibition efficiency increased with an increase of the inhibitor's concentration. Changes in impedance parameters (Rct and Cdl) were indicative of adsorption of the compound (M-QN) on the metal surface, leading to the formation of a protective film. Tafel polarization measurements showed that M-QN inhibitor is of a mixed type. The adsorption of the inhibitor on the surface of carbon steel in a 1.0 M HCl solution was found to obey Langmuir's adsorption isotherm. The kinetic and thermodynamic parameters for carbon steel corrosion and inhibitor adsorption, respectively, were determined and discussed. On the bases of thermodynamic adsorption parameters, comprehensive adsorption (physisorption and chemisorption) for the studied inhibitors on carbon steel surface was suggested.