Evaluation of Henna Extract Performance on Corrosion Inhibition of API 5L Steel in H2S-Containing Medium and DFT Quantum Computing of Its Constituents

In this work, the effect of Henna extract as an inhibitor on the corrosion behavior of API 5L Gr.B steel in H2S-containing solution at 25 ˚C was investigated. The electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization tests were used for determination of corrosion parameters in the blank sample and samples with different concentrations of the extract. The obtained results indicated that with increasing Henna extract concentration, the corrosion current density decreased and the inhibition efficiency increased. The calculated efficiency of EIS and polarization tests showed good agreement. The studied inhibitor in this work follows the Langmuir adsorption isotherm and the type of surface adsorption is physical. Quantum chemical parameters such as energy gap ( $$\Delta{\text{E}}$$ = EHOMO – ELUMO), lowest unoccupied molecular orbital energy (ELUMO), highest occupied molecular orbital energy (EHOMO), hardness ( $$\eta$$ ) and electronegativity ( $${\chi}_{\text{inh}}$$ ) for the main constituents of Henna extract were calculated. The results of quantum chemistry proved that the main constituent of Henna extract and the inhibition agent was Lawsone.