Quantum mechanical research and molecular modeling of lignin monolignol as low carbon steel inhibitors in sulfuric acid media.

Computational quantum mechanics research and organic molecular modeling can provide information about the characteristics of organic compounds and the adsorption mechanism of corrosion inhibitors. Lignin extracts from various plant parts acted as corrosion inhibitors and have reduced the corrosion rate (2.51-15.26%) in steel. The aims of this study were (1) to predict the molecular structure of protonated lignin using density functional theory; (2) predict the reactivity of lignin species and the interfacial adsorption properties of low carbon steels; and (3) investigate the interaction of neutral and protonated lignin compounds with atoms in H2SO4. This study was conducted using computational modelling. including (1) calculation of the molecular structure optimization of monolignol lignin monomers (p-caumaryl-, coniferyl-, and sinapyl alcohol) as neutral and protonated inhibitor species and (2) modeling of corrosion inhibitor molecules adsorbed on the surface of low carbon steel in H2SO4 medium. The predicted result of the protonated structure shows the tendency of the H+ proton to be bound to the O-phenolic and O-methoxy atoms. The interaction of lignin monolignol molecules with the adsorbed sulfate ion result in physisorption interactions. In contrast, the chemisorption interaction of lignin species occurs through the transfer of heteroatom electrons and the heterocyclic π-ring from the inhibitor to the empty d-orbital of Fe in sulfuric acid. The quantum mechanics calculations results show that the p-coumaryl alcohol molecule has the highest inhibition efficiency value of 23.41%. The results of the Monte Carlo (MC) calculation, enthalpy of inhibitor-Fe 19.673 kcal/mol. At the same time, acid-Fe -126.082 kcal/mol shows that the function of acid as an adsorption medium is stronger than ionized Fe. Based on the calculation of DFT/B3LYP 6-31G(d,p) the adsorption of p-coumaryl alcohol in H2SO4 has the highest inhibition efficiency with a bond energy of 78.543 kJ/mol. [ABSTRACT FROM AUTHOR]