Alanine as corrosion inhibitor for iron in acid medium: A molecular level study

Abstract

Density functional theory (DFT) calculations have been used to investigate the minimum energy structures of alanine molecules on iron (111). Adsorption of alanine molecule on iron (111) surface has been studied computationally to generate adsorption configurations and to use the force field method to obtain a ranking of the energies for each generated configuration, thereby indicating the preferred adsorption sites. In this article Monte Carlo simulation searches of the configurational space of iron (111) (substarte) - alanine (adsorbate) system as the temperature of the system is slowly decreased is used in order to find low energy adsorption sites on both adsorbate and substrate system. The results indicated that alanine could adsorb on Fe surface through the nitrogen/oxygen atoms with the lone pair electrons in its molecule. The inhibition performance of alanine on iron in normal hydrochloric acid medium (1.0 M HCl) at 25±1°C was tested by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. Alanine inhibits the acidic corrosion even at very low concentrations, reaching a value of inhibition efficiency up to 80% at a concentration of 50 mM. The results obtained from the different corrosion evaluation techniques are in good agreement. Polarization curves indicate that the studied alanine is mixed-type inhibitors, affecting both cathodic and anodic corrosion currents. Data, obtained from EIS measurements, were analyzed to model the corrosion inhibition process through appropriate equivalent circuit model, a constant phase element (CPE) has been used. © 2012 by ESG.