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

Abstract

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 (Eg), 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 ΔH (-4.33 kcal.mol−1) than MAPP (ΔH = -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.