Solvent and catalyst-free synthesis, corrosion protection, thermodynamic, MDS and DFT calculation of two environmentally friendly inhibitors: Bis-phosphonic acids.

Solvent and catalyst-free synthesis of two bis-phosphonic acids were realized adopting one-pot reaction of 2,6-bis (hydroxymethyl)-4-methoxyphenol or 4-chloro- 2,6-bis (hydroxymethyl) phenol with trialkyl phosphite. Their structures were confirmed by 1H, 13C, 31P NMR and IR spectroscopies. The molecular structures have been optimized using DFT, and the related calculated vibrational frequencies and NMR shift confirm the proposed structures. Chemical and electrochemical tests evaluated the inhibitive capabilities on the XC48 steel corrosion against 1 M HCl. The weight loss (chemical one) shows that the adsorption on the XC48 steel surface obeys to Langmuir isotherm, while the polarization measurements suggest that the inhibition process is mixt, the results of Electrochemical Impedance Spectroscopy (EIS) prove an increase of the inhibition efficiency with the increase of inhibitor concentrations. The surface topographies were analyzed by the Atomic Force Microscopy (AFM). The Density Functional Theory (DFT) with B3LYP as correlation function and 6-31G (p, d) as basis sets were used to determine some theoretical physic-chemical parameters affecting the corrosion inhibition and on the other hand the DFT develop the Frontier Molecular Orbitals (FMOs) and the Molecular Electrostatic Potentials (MEP) and through a discussion was made about their relationship with the inhibition of the corrosion. At the end, Molecular Dynamics Simulations (MDS) based on Monte-Carlo calculations were accomplished to simulate the interface configuration between the inhibitor molecules and Fe (110)/H 2 O. Image 1 • Bis-phosphonic acids were synthesized and experimentally/theoretically identified. • Inhibition efficiency increases with concentration but decrease with temperature. • They inhibit the corrosion by the chemical adsorption via the Langmuir isotherm. • Polarization studies showed that inhibitors act as mixt inhibitors. • Quantum chemical parameters were in good correlation with experimental results. [ABSTRACT FROM AUTHOR]