′-((2-methoxy-naphthalene-1-yl)methylene) pyrimidine-5-carbohydrazide (H5) and

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Abstract

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• Structures CS–H4 CS–H5 CS–H6

Structures	CS–H4	CS–H5
CS–H6
Total energy	−244.21576	−267.4052	−246.99164
Adsorption energy	−321.14876	−291.1181	−323.92464
Rigid adsorption energy	−229.01	−196.66	−232.17
Deformation energy	−92.14	−94.46	−91.75
Inh: dEad/dNi	−321.15	−291.12	−323.92

Table 14 Simulation results (total energy, adsorption energy, rigid absorption energy, and deformation energies) in acid solutions

Structures	CS–H4	CS–H5	CS–H6
Total energy	−4019.83	−4002.17	−4028.35
Adsorption energy	−4096.76	−4025.88	−4105.29
Rigid adsorption energy	−4183.44	−4107.05	−4184.89
Deformation energy	86.68	81.17	79.60
INH: dEad/dNi	−323.65	−212.26	−340.60
H2O: dEad/dNi	−8.73	−7.60	−11.69
H3O+: dEad/dNi	−151.74	−154.70	−147.92
Cl−: dEad/dNi	−144.41	−155.94	−152.37

3.9 Inhibitive mechanism

Corrosion happens by two essential reactions, oxidation reaction and reduction of hydrogen. Most possibly, the organic molecules present in the compounds prevent corrosion by reducing both reactions. The mechanism of inhibition of the examined inhibitors cannot be considered as only chemical or physical adsorption because Tafel polarization results demonstrate that (H4 & H5 & H6) appears as mixed type inhibitor. The inhibitive mechanism can be explained with two interpretations; the first is chemisorption which happened when some electron donating groups (EDG) that found in (H4 & H5 & H6) inhibitors give their electrons to the unoccupied orbitals found in carbon steel surface. there are many donating atoms and groups found in (H4 & H5 & H6) with lone pairs to give, such as, O–, OH⁻, NH⁻, NR, OCH₃, alkyl group (CH₃), and a conjugated π-electron. These atoms and functional groups can cover large metallic surface areas of carbon steel and electrons can transfer from them to the empty orbitals of Fe. The second interpretation is the physical attraction between inhibitors and carbon steel surface, Because the C-steel surface has a positive charge, Cl-ions are first adsorbed on the metal surface, then electrostatic interactions between the negatively charged metal surface and the positively charged inhibitor molecules create a protective coating.²⁸ Hence the adsorption of (H4 & H5 & H6) was developed which is approved by AFM, FT-IR and XPS results. Quantum calculations and Monte Carlo simulations have assured the above deduced mechanism with some variations. The investigated inhibitors from previous experiments can be arranged in the order of its inhibition efficiencies as H6 > H4 > H5.

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