Understanding the Inhibition Mechanism of a Supramolecular Complex as the Corrosion Inhibitor for Mild Steel in the Condensate Water

This work attempted to elucidate the corrosion inhibition mechanism of a supramolecular complex formed between β-cyclodextrin (β-CD) and octadecylamine (ODA) for mild steel in the condensate water. Molecular mechanics simulation was applied to determine the energy favored configuration of the complex and the possible orientation of ODA inside the β-CD cavity. Based on the results of gravimetric measurements, thermodynamics parameters for the adsorption process were calculated through Arrhenius and transition state equations. Scanning electron microscopy and X-ray photo electron spectroscopy were used for surface characterization. Density functional theory calculations (Mulliken charges and molecular electrostatic potential plot) were performed to clarify the adsorption mechanism of β-CD/ODA complex on the steel surface. It was found that four possible configurations of β-CD/ODA complex might be concurrent in the supramolecular system with the stable state in the aqueous solution. ODA molecules could break away from the β-CD cavity and chemically adsorbed on the metal surface, which obeyed the Langmuir adsorption isotherm. The inherent molecular electrostatic properties resulted in the tilted adsorption of ODA.