Experimental and theoretical studies on corrosion inhibitory action of Peganum harmala (L.) seeds extract on carbon steel in 1M HCl

The objective of this study is to test the inhibitory efficiency of the methanol extract of P. harmala against the corrosion of carbon steel C38 in 1M hydrochloric acid by gravimetric method and electrochemical measurements. The results indicated that the methanol extract of P. harmala is a good inhibitor of carbon steel. The inhibition efficiency increases with the increase of the concentration of the inhibitor. The nature of adsorption of the extract on carbon steel surface was in conformity with Langmuir isotherm. Both kinetic and thermodynamic parameter of adsorption allows us to suggest chemical adsorption of this inhibitor on the metal. Potentiodynamic polarization measurements show that P. harmala acts as mixed type inhibitor.   The result of EIS was correlating with the result of polarization measurement. A theoretical study was done in solution for major alkaloids isolated from P. harmala seeds, and was investigated using density functional theory (DFT) at the B3LYP/6-31G(d,p) basis set level through the relationship between their molecular and electronic structure. The quantum chemical parameters such as E HOMO (highest occupied molecular orbital energy), E LUMO (lowest unoccupied molecular orbital energy), energy gap (ΔE), dipole moment (μ), absolute hardness (η), absolute softness (S), the absolute electronegativity (χ), the fractions of electrons transferred from the inhibitor molecule to the metallic atom (ΔN) and the electrophilicity index (ω) were calculated. The local selectivity and reactivity has been analyzed through the Fukui function indices in order to compare the possible sites for nucleophilic and electrophilic attacks.