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Quantum chemical
calculations using the density functional theory (DFT) have
been applied to the five kinds of polydentate Schiff base
compounds (PSCs), act as inhibitors for iron in aerated 2.0
M HNO3 and 2.0 M NaOH media. The structural parameters, such
as the frontier molecular orbital energy HOMO (highest
occupied molecular orbital), LUMO (lowest unoccupied
molecular orbital), energy gap ∆E (ELUMO - EHOMO), the
charge distribution, the absolute electronegativity (χ), the
fraction of electrons transfer (∆N) from inhibitors to iron,
the dipole moment (µ), the global hardness (η) and the total
energy (Etotal) were also calculated and correlated with
their inhibition efficiencies (%IE). The inhibition effects
of (PSCs) may be explained in terms of electronic
properties. The results showed that the (%IE) of PSCs
increased with the increase in EHOMO and decrease in ELUMO -
EHOMO. The inhibitor molecules were first adsorbed on the
iron surface and blocking the reaction sites available for
corrosive attack; and the areas containing N and O atoms are
most possible sites for bonding by donating electrons to the
iron surface through interaction with π-electrons of the
aromatic rings, and the azo methine group. Also, the
adsorbed Schiff base molecules interact with iron ions in
the corrosive media leading to neutral and cationic
iron-Schiff base complexes. Adsorption process is
spontaneous, exothermic and obeyed Temkin isotherm and
regarded as physical as chemical mechanism. The polarization
studied indicated that the inhibitors act as a mixed type
inhibitor in HNO3; cause only inhibition of the cathode
process in NaOH, and the magnitude of the displacement of
the Tafel plot is proportional to its concentration. Models
for the inhibition corrosion behaviour were developed based
upon statistical analyses of the experimental data. Some
thermodynamic and kinetic parameters (Kads, ∆Gads) were
estimated. Both experimental and quantum theoretical
calculations are in excellent agreement. The inhibition
efficiency increase in the order of: PSC_1 > PSC_2 > PSC_3 >
PSC_4>PSC_5. Thus, DFT study gave further insight into the
mechanism of inhibition action of PSCs. This research might
provide a theoretical inhibition performance evaluation
approach for homologous inhibitors.
Keywords: Corrosion inhibitor, Quantum chemical, DFT method,
Schiff bases adsorption, Molecular orbital, Iron electrode,
Temkin isotherm
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