Mycophenolic acid as a corrosion inhibitor of carbon steel in 3% wt. NaCl solution. An experimental and theoretical study

Abstract

Mycophenolic acid (IUPAC name: (E)-6-(4-hydroxy-6-methoxy-7-methyl-3-oxo-1H-2-benzofuran-5-yl)-4-methylhex-4-enoic acid) is extracted from the fungus Penicillium brevicompactum. In this work, corrosion inhibition under static conditions and turbulent flow was tested in a 3% wt. NaCl solution by means of two techniques: electrochemical impedance spectroscopy (EIS) and polarization curves. The efficiency results showed that low concentrations of mycophenolic acid are effective in static conditions, staying practically constant when the concentration of the compound is increased, with ղ = 90%. The adsorption process is chemisorption at 0 rpm, while for 100 and 1000 rpm, it is a combined process. SEM-EDS confirmed the decrease of the corrosion products in the presence of mycophenolic acid. These results revealed that mycophenolic acid is a good corrosion inhibitor when applied to carbon steel. To obtain insight into this behavior, the interaction between mycophenolic acid (MA) and a steel surface, modeled by a cluster of iron atoms (Fe6), was studied using density functional theory calculations. The calculated MA-Fe6-cluster binding free energy agrees with the experimental results and falls in the chemisorption regime. Using the reactivity Fukui indices, we demonstrated that the metal surface is completely passivated by the inhibitor, which is less reactive to different kind of attacks. After adsorption of the inhibitor molecule, the properties of the iron cluster remained almost unaltered, except for a small amount of charge transferred from the inhibitor to the metal cluster. Global and local parameters were successfully compared to calculated ones for other organic inhibitors obtained from natural sources.