Indole Derivatives as Organic Corrosion Inhibitors of Low Carbon Steel in HCl Medium-Experimental and Theoretical Approach

Corrosion remains a major problem in the manufacturing industries and the whole world at large, hence the need to control it. Organic molecules have been used in the past and recent times because they have certain advantages over inorganic molecules. Information on the use of indole derivatives as anti-corrosive agents is scanty. In this work therefore, two indole derivatives, 6-benzyl-2H-indol-2-one and 3-methylindole were investigated for their ability to provide protective coverage on the surface of low carbon steel (LCS) and serve as anti-corrosive agents. Their corrosion rates and inhibition efficiencies were determined by weight loss, gasometric, electrochemical and theoretical methods. Images of LCS in the absence of inhibitor and acid, in the presence of acid only and in the presence of acid and inhibitors were recorded with the aid of Scanning Electron Microscopy (SEM). Quantum mechanical calculations on the energies of the frontier molecular orbitals (FMOs) were performed using the density functional theory. The FMOs energies were used to calculated the reactivity descriptors which inform about the inhibition capacity of the molecules. Monte Carlo simulation was used to perform the adsorption of the molecules on the surface of Fe(110). These molecules inhibit the corrosion of LCS with inhibition efficiencies of the inhibitors increasing at lower temperature, with all the inhibitors having their highest inhibition efficiencies at 303 K and at the concentration of 10 x 10(-5) M. Potentiodynamic polarisation curves revealed that both inhibitors are mixed-type inhibitors. The SEM images revealed that the surface of the low carbon steel was protected by these organic inhibitors. The activation energies (12.21-55.40 kJ/mol) and the Gibb's free energy values (- 4.51 to - 8.58 kJ/mol) obtained from the weight loss method supported a physical adsorption mechanism. The adsorption was found to obey Langmuir adsorption isotherm. All theoretical results are consistent with experimental results. These molecules could be adopted for use as anti-corrosive agents in the oil, gas and manufacturing industries.