Inhibition of Tryptophan on AA 2024 in Chloride-Containing Solutions

The inhibitory effects of tryptophan on the corrosion of AA 2024 in 1 M HCl, 20% (wt.%) CaCl(2), and 3.5% (wt.%) NaCl solutions were investigated via polarization techniques, electrochemical impedance spectroscopy, and weight loss methods. The scanning electron microscope technique was employed to observe corrosion morphology. The results suggest that AA 2024 was corroded in these three corrosive media to some extent and that tryptophan can significantly inhibit the corrosion of aluminum alloys. The inhibition efficiency (eta) increased with increasing concentrations of tryptophan, and the best inhibition efficiency exhibited was about 87% in 1 M HCl solution with 0.008 M tryptophan. Tryptophan acted as a cathodic corrosion inhibitor and affected the hydrogen evolution reaction, which was the main electrode reaction in the 1 M HCl solution. In solutions with 20% CaCl(2) and 3.5% NaCl, tryptophan was adsorbed onto anodic areas, thus increasing the activation energy of the interface reaction as an anodic corrosion inhibitor. The Dmol(3) program of Material Studio 4.0 was used to obtain the optimized geometry of the tryptophan inhibitor and some quantum-chemical parameters. Front orbital distributions and Fukui indices indicate that the molecular active reaction zones were located in the indole ring of tryptophan.