Adsorption Properties and Inhibition of C38 Steel Corrosion in Hydrochloric Solution by Some Indole Derivates: Temperature Effect, Activation Energies, and Thermodynamics of Adsorption

The corrosion rates in the presence of some indole derivates, namely, 9H-pyrido[3,4-b]indole (norharmane) and 1-methyl-9H-pyrido[3,4-b]indole (harmane), as inhibitors of C38 steel corrosion inhibitor in 1？M HCl solution, were measured by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques, in the range of temperatures from 25 to 55°C. Results obtained revealed that the organic compounds investigated have inhibiting properties for all temperatures. The inhibition was assumed to occur via adsorption of the indole molecules on the metal surface. Adsorption of indole derivates was found to follow the Langmuir isotherm. The apparent activation energies, enthalpies, and entropies of the dissolution process and the free energies and enthalpies for the adsorption process were determined by potentiodynamic polarization and electrochemical impedance. The fundamental thermodynamic functions were used to collect important information about indole inhibitory behaviour. 1. Introduction The effect of temperature on the inhibited acid-metal reaction is highly complex, because many changes occur on the metal surface such as rapid etching and desorption of inhibitor and the inhibitor itself may undergo decomposition and/or rearrangement. Temperature effects on acidic corrosion and corrosion inhibition of iron and steel most often in HCl and H2SO4 solutions had been the object of a large number of investigations [1–12]. However, it was found that few inhibitors with acid-metal systems have specific reactions which are effective (or more) at high temperature as they are at low temperature [13–15]. Thermodynamic parameters such as adsorption heat, adsorption entropy, and adsorption free energy can be obtained from experimental data of the studies of the inhibition process at different temperatures. The kinetic data such as apparent activation energy and preexponential factor at different inhibitor concentrations are calculated, and the effects of the activation energy and preexponential factor on the corrosion rate of steel were discussed [16–20]. Our earlier results obtained for 9H-pyrido 3,4-b indole (norharmane) and 1-methyl-9H-pyrido 3,4-b indole (harmane) as corrosion inhibitors of C38 steel demonstrated that correlation exists between the inhibition efficiency and the chemical structure [21]. It was found that these compounds are good inhibitors in acidic solutions and the inhibition efficiency of these indole-type organic compounds may be explained in terms of electronic properties (the energy of the highest occupied molecular