Computational, theoretical and experimental studies of four amino acids as corrosion inhibitors for brass in 3% NaCl medium

The corrosion inhibition performance of some amino acids such as L-cysteine (L-cys), L-methionine (L-met), Ltyrosine (L-tyr), and L-histidine (L-his) as environmentally safe corrosion inhibitors for brass in aerated and stirred 3 % NaCl solution was studied by Potentiodynamic Polarization and impedance measurements. Cys showed better corrosion inhibition efficiency (88 %) and prevented the formation of corrosion products responsible for the corrosion of brass. Protonated cysteine demonstrated a high corrosion inhibition performance by providing strong interactions between the lone pairs of its sulfur and nitrogen atoms and the vacant d-orbitals of copper atoms. Corrosion products and the protective layer were analyzed using SEM-EDS techniques. Molecular structures of the four studied amino acids have been geometrically optimized by DFT/ B3LYP method, with the function 6-311++G(d, p). Molecular electrostatic potential (MEP) and Mulliken charge analysis were used to predict the adsorption sites of the studied amino acids. Some confusing electronic parameters have been resolved using non-covalent interactions (NCI) method, which revealed the presence of intramolecular interactions and steric hindrance at the molecular level. Conformational analysis using Monte Carlo (MC) simulations method was performed in order to give better insight into the adsorption mechanism of the protonated cysteine on the metal surface via its thiol and amino groups.