Heteroatom-containing phosphoramides as carbon steel corrosion inhibitors: Density functional theory and molecular dynamics simulations

In an attempt to introduce a new class of carbon steel corrosion inhibitors, the anti-corrosion properties of two heteroatom-containing phosphoramides; N-(5-methylisoxazol-3-yl)-P,P-diphenylphosphinic amide (MAPP) and diphenyl (5-methylisoxazol-3-yl)phosphoramidate (PMAP) were theoretically investigated. Density functional theory (DFT) calculations were performed using B3LYP functional and 6-31+G(d,p) basis set to evaluate the inhibition efficacy of MAPP and PMAP in the presence of HCl molecule. Based on the frontier molecular orbitals energy gap (E-g), dipole moment, global hardness, electrophilicity, and nucleophilicity values, we concluded that PMAP acts as a strong anti-corrosion. With the aim of identifying the most stable configuration of inhibitor-HCl, the HCl molecule was approached to all probable reactive sites of inhibitors determined by molecular electrostatic potential plots. It was revealed that molecule PMAP forms a stronger hydrogen bond with the HCl molecule and has a higher Delta H (-4.33 kcal.mol(-1)) than MAPP (Delta H = -3.50 kcal.mol(-1)). Adsorption behavior of MAPP and PMAP on a Fe (1 1 0) superlattice was studied using molecular dynamics (MD) simulations at T = 298, 308, and 318 K and in the presence of n = 1-4 molecules of MAPP and PMAP. Radial pair distribution function analysis revealed that molecules MAPP and PMAP have the ability to form a chemical bond with metal, and thus protect it from corrosion. The best anti-corrosion performance occurs at T = 298 K and higher concentrations of inhibitors. The adsorption energy of the PMAP molecule on the steel surface (-258.8 kcal.mol(-1) at 298 K and for n = 4) is more negative than that of MAPP (-239.5 kcal.mol(-)(1)) at the same conditions, indicating that PMAP can be used as an effective corrosion inhibitor for carbon steel in acidic media.