**4. Aliphatic compounds as an organic corrosion inhibitor**

Jia-jun Fu et al. carried out experimental, MD simulations and DFT calculations to investigate the corrosion inhibition properties of four amino acid compounds namely L-cysteine, L-histidine, L-tryptophan, and L-serine amino acid for mild steel in hydrochloric acid solution [154]. The optimization of the amino acid compounds has been performed using B3LYP Density Functional Theory (DFT) with 6-311G (d,p) basis set with the Gaussian 03 W program. This basis set will provide EHOMO, ELUMO, ΔE, and μ. The Fukui function calculations were performed with DMol3 version 4.3 available in Material Studio software (Accelrys, San Diego, CA), using a PBE (Perdew, Burke, and Enzerhof) functional and a double-numeric quality basis set with polarization functions (DND). Dmol3 uses a Mulliken population analysis.

The optimized structures of the four amino acid compounds were shown in **Figure 1**. Additionally, the orbital density distributions of EHOMO and ELUMO for amino acid compounds were shown in **Figure 2**. From this DFT calculation of these four amino acids, the EHOMO, ΔE and ΔN provide that the order of the inhibition efficiency of these inhibitors follows the sequence: L-Try>L-His>L-Cys > L-Ser.

Mohammed et al. [155] carried out a study of the inhibition of iron corrosion in HCl solutions by three amino acids, namely alanine (Ala), cysteine (Cys), and S-methyl cysteine (S-MCys). The effectiveness of amino acids, alanine (Ala), cysteine (Cys), and S-methyl cysteine (S-MCys) as safe corrosion inhibitors for iron in aerated stagnant 1.0 M HCl solutions have been evaluated by Tafel polarization and

**Figure 1.** *Optimized structures of four amino acid compounds.*

**Figure 2.** *The frontier molecular orbital distribution of four amino acids compounds.*

impedance measurements. They have also performed molecular dynamics (MD) and density functional theory (DFT) for their inhibition efficiency. The electronic density distribution of HOMO (the highest occupied molecular orbital) and LUMO (the lowest unoccupied molecular orbital) for the studied amino acids were shown in **Figure 3.** From these studies, Cys has the lowest EHOMO and the highest ELUMO then S-MCys and Ala. The experimental data and computational study has been in agreement that Cys has a higher inhibition efficiency than alanine (Ala) and S-methyl cysteine (S-MCys).

Obot et al. studied the mechanism of 2-mercaptobenzimidazole adsorption on Fe (110), Cu (111), and Al (111) surfaces by applying DFT and molecular dynamics simulations [156]. The DFT study was performed using B3LYP, 6-31G (d,p) basis set in the gas phase. The inhibition was performed by MD simulations, optimization using the COMPASS force field under the NVT ensemble and PBC at a simulation time of 50 ps. The mechanism of corrosion inhibition of 2-mercaptobenzimidazole (2-MBI) on Fe, Cu, and Al surfaces Density functional theory (DFT) and molecular dynamics (MD) simulations.

Canul and Rosado studied the Inhibition Effect of Sodium Glutarate toward Carbon Steel Corrosion in Neutral Aqueous Solutions [157]. They investigated the Inhibition Effect of Sodium Glutarate in a near-neutral 0.02 M NaCl solution at ambient temperature for a range of concentrations (1–100 mM) by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) measurements. Sodium glutarate showed a poor inhibitive action for corrosion of carbon steel in a 0.02 M NaCl solution when used in concentrations of 1 mM and 5 mM. However, open circuit potential and polarization curve measurements give evidence that full chemical passivation is accomplished for concentrations of 32 mM or higher; a significant improvement in protective is achieved. Investigation of the effect of temperature showed that increasing the temperature from 22 to 55°C decreases the inhibition efficiency from 96 to 89%, indicating good stability of the protective film in this temperature range.

Ambat and co-workers reported electrochemical and molecular modeling studies of CO2 corrosion inhibition characteristics of alkanolamine molecules (ethanolamine, diethanolamine, and triethanolamine) for the protection of 1Cr steel [158]. The DFT

**Figure 3.** *Structure of Ala, S-Myc, and Cys.*

calculations were performed with the Quantum Espresso package on a Fe (110) surface and a FeCO3 (104) surface. The experimental results and molecular modeling calculations using DFT found that the inhibitor efficiency for the ethanolamine on Fe(110), FeCO3(104), and Fe3C(001) was the highest.

Obot and co-workers performed density functional theory and molecular dynamics simulation for study the corrosion inhibition for three amine derivatives, namely, N1-(2-aminoethyl)ethane-1,2-diamine (DETA), N1-(2-(2-aminoethylamino)ethyl) ethane-1,2-diamine (TETA) and N1-(2-(2-(2-(2-aminoethylamino)ethylamino) ethylamino)ethyl)ethane-1,2-diamine (PEHA) on the steel surface [159]. They have calculated EHOMO, ELUMO, energy gap (ΔE), electron affinity (A), electronegativity (χ), global hardness (η), softness (σ), and the fraction of electron transferred (ΔN) from the inhibitor molecule to the metal surface. The quantum chemical parameters EHOMO, ELUMO, ΔE, hardness (η), softness (σ), and fraction of electron transferred (ΔN) are in good agreement with the experimental findings. The nucleophilic and electrophilic characteristic of inhibitor molecules were analyzed from Fukui indices. MD simulation performed by COMPASS force field. The interaction energies at 298 K temperature are �355.30, �455.93, and � 784.74 kJ/mol for DETA, TETA, and PEHA, respectively, on the Fe (1 1 0) surface. PEHA has maximum interaction energy value and possesses better inhibition effectiveness than DETA and TETA. The corrosion inhibition property increases from DETA to PEHA. The binding energy values of inhibitor molecules are in the order PEHA > TETA > DETA which also supports the experimentally obtained results.

Bingul and co-workers studied the inhibition effects of methionine and tyrosine on the corrosion of Iron in HCl Solution by electrochemical and quantum-chemical method [160]. The quantum chemistry calculation for inhibition of corrosion has been performed with the 6-311G(d,p) basis set for methionine and tyrosine. The quantum chemical calculations were performed for methionine and tyrosine, as well as, for their protonated structures. The inhibition effect of methionine ion on Fe in HCl solution is better than that of tyrosine and is in agreement with experimental and theoretical data.

Kumari and Kumar have reported experimental and theoretical investigations of 3,3<sup>0</sup> -diamino dipropyl amine: highly efficient corrosion inhibitor for carbon steel in 2 N HCl at normal and elevated temperatures [161]. The theoretical study were performed by B3LYP using 6-31G(d) basis set and with the help of Gaussian 09, 6 Wallingford CT, the USA software. The adsorption of DADPA on carbon steel has been found from the data on global hardness, electronegativity, softness, chemical potential, ionization potential, maximum charge transfer, electron transfer, electrophilicity, nucleophilicity, electron releasing and accepting tendency, work function, back donation energy, proton affinity, metal inhibitor interaction, and binding energies. The strong adsorption of the inhibitor molecule was further supported by the suitable linear molecular configuration of DADPA, the high value of electronegativity, global softness parameter, and high negative value of interaction parameter.

Wang et al. studied experimental and theoretical investigation on corrosion inhibition of AA5052 aluminum alloy by L-cysteine in alkaline solution [162]. AA5052 aluminum alloy contains Mg, Mn, and Zn. The inhibition efficiencies of L-cysteine in 4 M NaOH solution were studied by polarization curves, electrochemical impedance spectroscopy, and quantum chemical calculation. The DFT calculations were performed using the Dmol3 package with numerical atomic orbital basis sets on Al (111) surface, double-numerical plus polarization (DNP) basis sets were used in the expansion of the Kohn-Sham orbitals and the orbital confining cut-off was set as 4.8 Å.

The adsorption of L-cysteine on an aluminum surface followed the Langmuir adsorption isotherm and the polarization curves showed that the L-cysteine acts as a cathodic inhibitor to inhibit the cathodic reaction.

The adsorption energy was calculated on the surface of Al(1 1 1) and the L-cysteine molecules found negative values. The adsorption energy decreases in the order, (e) < (a) < (d) < (c) < (b) indicates that inhibitor molecules adsorbs on the surface of aluminum instead of water molecules and the adsorption is mainly based on the reactive groups (**Figure 4**). The interaction between inhibitor molecules and Al surface was further understood by calculations of charge density (**Figure 5**).

Marcus et al. reported corrosion inhibition of locally de-passivated surfaces by DFT study of 2-mercaptobenzothiazole on copper [163]. They have studied the adsorption of the organic inhibitor 2-mercaptobenzothiazole (MBT) on the incompletely passivated or locally depassivated copper surface based on quantum chemical DFT calculations. They investigated quantum chemical DFT calculations on the adsorption of MBT in thione or thiolate forms on a model of the copper surface incompletely covered by a surface oxide film, Cu(111) surface covered by an ultrathin Cu2O oxide film (Cu(111)∣∣Cu2O(111)). The calculation of DFT has been performed by periodic plane-wave basis set implemented in Vienna Ab initio Simulation Package (VASP) [70–73] with projector-augmented wave potentials using a 450 eV plane wave cutoff [74, 75]. Electron exchange and correlation terms were treated within the generalized gradient approximation (GGA) of Perdew–Burke–Ernzehof (PBE) functional [76, 77]. They used a Methfessel–Paxton smearing [78] with smearing value of 0.1 eV. They started from Cu (111)∣∣Cu2O (111) model followed by the construction of the hole in the oxide consists in removing certain copper and oxygen atoms from the surface and interface oxide layers. The hole in the oxide has been generated by removing eight atoms of copper and two atoms of oxygen, which correspond to four copper and one oxygen atom in each oxide layer. This study suggests that the partially oxidized copper surface blocks the initiation of pitting corrosion by strong interaction with the MBT molecule.

Allah and Moustafa studied quantum chemical calculations for amino pyrazole derivatives as inhibitors of corrosion of zinc, copper, and α-brass in an aqueous acid chloride solution [164]. They have performed the quantum mechanical calculations using the Dewar LCAO-SCF MO semi-empirical method, MNDO (modified neglect of differential overlap). The inhibition efficiency of the amino pyrazole derivatives depends upon the position of the substituent group with respect to the pyrazole ring, that is, the para, meta, or ortho positions of the arylazo group.

**Figure 4.** *Optimized structure and adsorption energy modes of L-cysteine molecules on Al (1 1 1).*

**Figure 5.** *Charge densities difference of different adsorption modes of L-cysteine molecules on Al (1 1 1).*

Zhang et al. reported QSAR study on N-containing corrosion inhibitors: Quantum chemical approach assisted by the topological index [165]. They have studied the inhibition efficiency used DFTB3LYP/6-31G\* level using GAUSSIAN 98 program package to carried out Ehomo, Elumo, energy gap, charge distribution, Dipole, topological index which refers to the steric hindrance of molecules.

Khadom has studied quantum chemical calculations of some amines corrosion inhibitors/copper alloy interaction in hydrochloric acid [166]. The quantum chemical calculations have been performed by Argus Lab 4.0.1 package, estimation by PM3 and AM1 method. The structural parameters, such as the frontier molecular orbital (MO), HOMO (highest occupied molecular orbital), LUMO (lowest unoccupied molecular orbital), dipole moment (μ), and ΔN were calculated. The inhibition efficiency calculation performed by AM1 and PM3 are in agreement with the experimental results.

Liang and Gao reported electrochemical and DFT studies of β-amino-alcohols (1-diethylamino-propan-2-ol (EAP) and 1,3-bis-diethylamino-propan-2-ol (DEAP)) as corrosion inhibitors for brass [167]. The DFT calculation was carried out with the Gaussian 98 program, B3LYP, and the 6-311G\*\* orbital basis sets. The interactions of the inhibitor molecules with the metal surface were studied by the frontier orbital energy such as EHOMO, ELUMO, and energy difference (ΔE) and dipole moment (μ). The EHOMO of DEAP (0.205 a.u.) and EAP (0.222 a.u.) and ΔE for EAP and DEAP are 0.258 and 0.242 a.u., respectively. The dipole moment (μ) for DEAP and EAP are 2.32 and EAP 2.95, respectively. The theoretical calculations carried out for EAP

and DEAP found that the inhibition efficiency of DEAP is higher than that of EAP which was in agreement with e experimental results potentiodynamic curves and electrochemical impedance spectroscopy (EIS).

Elhenawy and co-workers studied the understanding the adsorption performance of two glycine derivatives (bicine (N,N-bis(2-hydroxyethyl)glycine) and tricine (N-(tri(hydroxymethyl)methyl) glycine) as novel and environmentally safe anti-corrosion agents for copper in chloride solutions: experimental, DFT, and MC studies [168]. The quantum chemical indices (QCIs) for bicine, tricine, and glycine were performed using the density functional theory (DFT) with DMol3 module in Materials Studio software (Accelrys Inc.), the generalized gradient approximation (GGA) of the BOP function was used through the double-numeric basis set (DNP 3.5).

The Monte Carlo (MC) simulations studies were performed on Cu (111) surface, COMPASS force field was done for minimum energy calculation. The energies were calculated from the geometry optimized structure (with the energy minima), Mulliken charges were computed at the GGA/DNP 4.4 level for the atoms in bicine, tricine, and glycine molecules. Tricine was found higher inhibition efficiency than bicine or glycine as tricine DE values (2.9194, 3.0000, and 0.5920) and lower than those of bicine (3.9193, 3.5117, and 1.0343) and glycine (4.6342, 4.1916, and 1.1731) in the gas, solvated, and protonated molecules, respectively. Also, the ΔN values found from theoretical data are the order tricine > bicine > glycine. The adsorption energy against the Cu (111) surface for tricine was found more than that for bicine from Monte Carlo (MC) simulations. The experimental results were in agreement with computational indicating higher efficiency for tricine over that for bicine.
