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A supramolecular complex (HPDA) based on (2-hydroxypropyl)-β-cyclodextrin (HP-β-CD) and octadecylamine (ODA) exhibited a favourable inhibiting effect on the corrosion of mild steel (MS) in the condensate water. The structural properties of HPDA, including the apparent association constant (κ) and the optimal spatial configuration, were clarified using phase solubility simulation and molecular mechanics calculations. The results indicated that HPDA was fairly stable in water with a κ value of 9199 mol−1, and its four possible configurations might coexist. The corrosion inhibition effect of HPDA was collectively evaluated by both experimental and theoretical methods. Weight loss measurements showed that the inhibition efficiency of HPDA depended on the concentration and temperature, and the maximum value could reach 92.6% with 50 mg L−1 concentration at 313 K. The potentiodynamic polarization tests showed that HPDA was a mixed type inhibitor with a predominantly anodic type. Also, the polarization resistance was effectively enhanced in the presence of HPDA according to the results of electrochemical impedance spectroscopy. Adsorption experiments revealed that HPDA inhibited the corrosion of MS by a chemisorption mechanism, which was well described by the Langmuir model. Surface analyses based on X-ray photoelectron spectroscopy and Auger electron spectroscopy disclosed that the chemisorption of HPDA on the MS surface resulted from the self-assembly of guest molecules (ODA) with a tilted orientation. In addition, an apparent connection was ascertained between the adsorption measurements and the theoretical parameters (Mulliken charges and molecular electrostatic potential plot) using quantum chemical calculations.

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.