Investigation on the corrosion inhibition of two newly-synthesized thioureas to mild steel in 1 mol/L HCl solution

Investigation of the corrosion inhibition potentials of some 2-(4-(substituted)arylidene)-1H-indene-1,3-dione derivatives: density functional theory and molecular dynamics simulation

Background

Corrosion is a threat to material strength and durability. Electron-rich organic inhibitor may offer good corrosion mitigation potentials. In this work, anti-corrosion potentials of nine derivatives of 1H-indene-1,3-dione have been investigated using density functional theory (DFT) approach and molecular dynamics (MD) simulation. Chemical reactivity descriptors like energies of lowest unoccupied molecular orbital (ELUMO), highest occupied molecular orbital (EHOMO), electron affinity (A), ionization potential (I), energy gap (ΔEgap), global hardness (η), global softness (σ), electronegativity (χ), electrophilicity (ω), number of transferred electrons (ΔN) and back-donation (ΔEback-donation) were computed at DFT/B3LYP/6-31G(d) theoretical level. The local reactive sites and the charge partitioning on the compounds were studied using Fukui indices and molecular electrostatic potential (MEP) surface analysis. The adsorption behavior and the binding energy of the inhibitors on Fe (110) surface in hydrochloric acid solution were investigated using MD simulation.

Results

The high chemical reactivity, kinetic instability and good corrosion inhibition potentials demonstrated by the inhibitors are rationalized based on their high EHOMO, A, σ, ΔN, ΔEback-donation, and low ΔEgap, ELUMO, I and η. A wide difference of approximately 2.4–3.2 eV between the electronegativities of iron and the 1H-inden-1,3-diones suggests good charge transfer tendency from the latter to the low-lying vacant d-orbitals of iron. The heteroatoms (O and N) and the aromatic moieties are the nucleophilic sites on the inhibitors for effective adsorption on the metal surface as shown by condensed Fukui dual functions and MEP analysis. The MD simulation shows good interaction and strong binding energy between the inhibitor and Fe (110) surface.

Conclusions

Effective surface coverage and displacement of H3O+, Cl− and water molecules from Fe (110) surface by the inhibitors indicate good corrosion inhibition properties of the inden-1,3-diones. 2-((4,7-dimethylnaphthalen-1-yl)methylene)-1H-indene-1,3(2H)-dione display low energy gap, strongest binding interaction and most stabilized iron-inhibitor configuration, hence, the best anti-corrosion potential.

Graphical abstract

A Study of 1-Benzyl-3-phenyl-2-thiourea as an Effective Steel Corrosion Inhibitor in 1.0 M HCl Solution

1-Benzyl-3-phenyl-2-thiourea (BPTU) was studied as a steel corrosion inhibitor in 1.0 M HCl solution. Experimental methods were conducted including potentiodynamic polarization measurement (PPM), electrochemical impedance spectroscopy (EIS), and scanning electron microscopy (SEM) analysis. Quantum calculations were performed at B3LYP/6-311G(d,p). Hexamethylenetetramine (URO) was selected for comparison with BPTU. The results showed that BPTU with the concentration of 2 × 10−4 M and at the temperature of 30°C could protect the steel surface with the highest inhibition efficiency of 94.99% and 94.30% according to EIS and PPM, respectively. High temperature decreased BPTU’s ability to inhibit the steel corrosion. The adsorption of BPTU on the steel surface is followed by the modified Langmuir isotherm. Quantum chemical calculations showed that the thiourea functional group is the main adsorption center of BPTU. The experimental results are completely consistent with theoretical calculations.

Structural investigation of N-[2-(4-fluoro-3-phen-oxy-benzo-yl)hydrazinecarbo-thio-yl]benzamide and N-[2-(4-fluoro-3-phen-oxy-benzo-yl)hydrazinecarbo-thio-yl]-4-meth-oxy-benzamide

The compound N-[2-(4-fluoro-3-phen-oxy-benzo-yl)hydrazinecarbo-thio-yl]benzamide, C21H16FN3O3S, crystallizes in the monoclinic centrosymmetric space group P21/c and its mol-ecular conformation is stabilized via an intra-molecular N-H⋯O hydrogen bond. The corresponding para-meth-oxy derivative, namely, N-[2-(4-fluoro-3-phen-oxy-benzo-yl)hydrazinecarbo-thio-yl]-4-meth-oxy-benzamide, C22H18FN3O4S, crystallizes in the monoclinic centrosymmetric space group C2/c. The supra-molecular network mainly comprises N-H⋯O, N-H⋯S and C-H⋯O hydrogen bonds, which contribute towards the formation of the crystal structures for the two mol-ecules. The different inter-molecular inter-actions have been further analysed using Hirshfeld surface analysis and fingerprint plots.