3.1. Potentiodynamic polarization results
Valuable potentiometric polarization parameters like corrosion current density (iCorr.), corrosion potential (Ecorr.), anodic and cathodic Tafel slopes (βa, −βc) were obtained from the Tafel plots. Which is a plot of Potential vs logi.
The point where the anodic polarization curve and cathodic polarization meet gives the corrosion current (icorr.). The linear part of anodic and cathodic polarization curves gives the anodic(ba) and cathodic Tafel slopes(-bc). The corrosion current density, anodic and cathodic Tafel slopes, and corrosion rate are obtained from the potentiodynamic plots. The results obtained for steel on exposing to water for 3days are tabulated in Tables 1. Fig. 2 represent the Potentiodynamic plots for the steel on exposing to water for 3days.
Table 1: Potentiodynamic polarization measurements for steel in water for 3days.
From the results obtained it was observed that the corrosion rate and corrosion current density increases with adding compounds. This may be because without the compounds, the conductivity of the medium increases, and the number of active centers on the steel surface increases. Hence, increase in metal dissolution occurs. By applying compunds to solution most of the active centers are blocked and hence there is a decrease in corrosion rate and corrosion current density.
As seen, after adding compounds a significant decline in both cathodic and anodic currents was noticed, suggesting that the inhibitors locks the active centers at anode and cathode. The results also show that the inhibition efficiency increased, while the corrosion current density of steel decreased and increased the corrosion inhibition efficiency. This behavior is because of the excellent coverage of the metal surface by schiff-bases which blocked the reaction sites on the metal surface. The corrosion current density is drastically decreased by introducing I3 in the solution. Oxegen and lineer structure increases the force of attraction between schiff-base, thereby not allowing the corroding ions, molecules to enter inside through the coating. Hence increased coating efficiency is observed. There is no change in anodic and cathodic Tafel slopes by changing the compound [30]. This suggests that an change in compound does not change the steps involved in the anodic and cathodic reactions. There is a +2 to + 4mV change in corrosion potential with adding inhibitors for steel in water. With the introduction of coating, the maximum displacement of lower than -85 mV in corrosion potential was observed. These insights that the blocking effect of compounds and with its filler is more at cathode compared to the anode. The corrosion inhibition efficiencies of the ligands were reduced in the following order: I3>I1≈I2.
3.2. Electrochemical impedance spectroscopy
Figure.3 represents the plot obtained by taking an imaginary part of impedance on the Y-axis and the real part of impedance on the X-axis for the corrosion of steel with and without coating.
As observed from Fig. 3, the shape of the curves is half-circle, which implies that transfer of charges takes place between steel and water.
The semi-circles were diminished, this may be due to, non-homogeneity on the metal surface causes the variation in frequency. The non-homogeneity may be due to the deposition of ferric oxide, or other ions on the steel surface. The semi-circle diameter represents the resistance to the exchange of ions at the mild steel-fresh/demineralized water interface (Rct). With adding the inhibitors, the diameter of semicircles increase, this observation may be due to the decrease in the mobility of ions of water and reactivity of the steel surface.
With adding inhibitors, C.E., increased, that shows a increase in adsorbing tendency of the compounds does not efficiently increase the charge transfer at the metal solution interface, or decreased charges at the double layer due to increase in viscosity of the medium, or the number of reactive sites is decreased due to decrease in conductivity of steel. Due to the combined effect of all or any one-two effects may be dominating with adding inhibitors.
The Nyquist plots are evaluated by applying the impedance excel data to the Zimpfin software to get the appropriate equivalent circuits. The satisfactory equivalent circuit is shown in Fig. 4.
Table 2: impedance data for steel in water for 3days.
Rs is the resistance offered by solution present between anode and cathode, constant phase element 1 (CPE 1), the electrical double layer dielectric constant, CPE 2 capacitance of the passive ferric oxide film, Rf film resistance. Due to the irregular surface, the charged layer present on the steel surface and water boundary is not a perfect capacitor. Hence it is replaced by CPE [28-29].
The CPE impedance is calculated using equation (1):
Z= Q-1 (iw)-n (1)
The Q is the proportionality coefficient; w=2πfmax is the angular frequency; fmax is the frequency at which the imaginary component of the impedance is maximum, i is the imaginary number, and the power n is associated with the phase shift. If n = 1, then the charged double layer at the boundary behaves like an ideal capacitor. The correction in the capacitance to its real value is calculated using equation (2).
Cdl = Q(w)n-1 (2)
The Cdl at the steel-water with and without coatings was evaluated using (3).
Cdl =1/2πfmax. Rp (3)
The polarization resistance Rp at the steel-water with and without coatings was calculated using (4).
Rp = Rct + R f + Rs (4)
From table.2 and Fig. 3, it is observed that Rs with and without inhibitors in water have the nearly same value. By introducing inhibitors on steel Rp value rises, and Cdl values suppress. It confirms the blocking of active centers presents on steel surface by the molecules of Schiff-bases. The inhibition effect of compounds was enhanced by adding I3. At the steel surface and water interface net exchange of charges decreases (resistant to charge transfer Rct increases) and thickness of the electrical double layer increases due to the presence of inhibitors big Schiff-base molecules. Therefore, by introducing inhibitors capacitance rises, and resistance to shifting equilibrium potential from the net corrosion current (Rp) rises at the steel and water interface increases.
3.4. DFT
In the preliminary step of the approach, a conformer search [42,43] was performed utilizing Boltzmann jump search method, using 2000 as a number of searched conformer structures and the COMPASS III forcefield [44]. This was done in order to acquire the lowest feasible start energy for the molecule while also quickening the DFT computations. DFT calculations began by picking the conformer with the lowest energy level, as illustrated in Figure 5.
The sigma-profile charge density curve is created using calculations based on the COSMO model. The electrostatic potential in COSMO is represented by the use of partly charged atomic nuclei [39,45,46]. Figure 6 is an illustration that depicts how an inhibitor can perform either the role of an acceptor or a donor of H-bonds.
H-bond acceptor/donor interactions between water molecules occur when an inhibitor is dissolved in water. This property controls how soluble the inhibitor is [47,48].
As seen in Figure 7, HOMO in the inhibitor molecules is positioned on one side of the ring containing O atoms, whereas LUMO is located on the opposite side of the ring, implying that these part of the molecule can are prone to electron tranfer to/from the metal's surface [49–51].
This exchange of electrons consequently results in the generation of a protective organic layer that coats the surface of the metal and preserves it from corrosion [43,48,52–54]. Electron acceptors, also called LUMOs, are parts of an inhibitor's structure that are responsible to take electrons from a surface with a lot of electron density, like the surface of a metal [47,49,50]. As a result of the exchange of lone pair electrons between heteroatoms (N and O) and the vacant iron d-orbital, which consequences in a moderate rise in surface absorption potential, it is presumed that adsorption on the surface of the metal will be significantly greater. It is because the exchange of lone pair electrons results in a moderate rise in surface absorption potential [53–55].
Table 3. Calculated theoretical chemical parameters for the inhibitors.
Descriptor
|
I1
|
I2
|
I3
|
HOMO
|
-7.5650
|
-7.2570
|
-7.2310
|
LUMO
|
0.6590
|
0.8420
|
0.8450
|
∆E(HOMO-LUMO)
|
6.906
|
6.415
|
6.386
|
Ionization energy (I)
|
7.5650
|
7.2570
|
7.2310
|
Electron affinity A)
|
-0.6590
|
-0.8420
|
-0.8450
|
Electronegativity (Χ)
|
3.4530
|
3.2075
|
3.1930
|
Global hardness (η)
|
4.1120
|
4.0495
|
4.0380
|
Chemical potential (π)
|
-3.4530
|
-3.2075
|
-3.1930
|
Global softness (σ)
|
0.2432
|
0.2469
|
0.2476
|
Global electrophilicity (ω)
|
1.4498
|
1.2703
|
1.2624
|
Electrodonating (ω-) power
|
3.6903
|
3.3802
|
3.3637
|
Electroappcepting (ω+) power
|
0.2373
|
0.1727
|
0.1707
|
Net electrophilicity (∆ω+-)
|
0.0337
|
0.1231
|
0.1266
|
Fraction of transferred electrons (∆N)
|
-0.0271
|
0.0028
|
0.0046
|
Energy from Inhb to Metals (∆N)
|
0.0030
|
0.0000
|
0.0001
|
∆E back-donation
|
-1.0280
|
-1.0124
|
-1.0095
|
Table 3 contains the most regularly used descriptors, which are organised by frequency of usage (the equations used to calculate them can be found in the following references) [51,56,57]. DFT simulations of inhibitor adsorption can help understand inhibitor adsorption mechanisms. Numerous studies suggest that the inhibitors' ability to exchange electrons with Fe(110) supports its adsorption. The inhibitors' low electron affinity and large ionization potential sustenance this interpretation (Table 3) Chemical softness and hardness predict the inhibitor's metal-surface adsorption affinity [40,58,59].
High values of chemical softness are also expected to show the inhibitor's affinity for the metal surface. Inhibitor have an ∆E value in the range of -1, which reflects their aptitude to receive electrons from the Fe(110) surface.
The Mulliken Atomic Charges (MAC) are a key factor in determining which atoms, typically known as inhibitory sites, are involved in the process of metal adsorption [47,53,54,60].
Numerous studies have shown that it is more probable for Fe(111) O surface atoms and inhibitor molecules to interact when the inhibitor atoms have a negative atomic charge (MAC). This has been shown both experimentally and theoretically. The MAC value of the inhibitors are displayed in Figure 8, and these are the atomsthat we are interested in. The atoms of oxygen and nitrogen in the inhibitors have significant negative charges, which indicates that these centers contain the greatest electron density and are thus able to adhere to metal surfaces with the greatest degree of success. Figure 7 presents the molecular electrostatic potential, also known as MEP, of the inhibitors at varying concentrations (area in red) [41,45,61,62].
Monte Carlo and Molecular dynamic simulations
Throughout this case, the adsorption energy may be easily calculated by beginning with the Fe(110) surface. Adsorption energy may be calculated using the following equation: (Eads) [63–68] :
where is the total energy of the simulated system, EFe, and is the total energy of the Fe(110) surface and the corresponding free inhibitor molecules.
As a last step in verifying the accuracy of the MC calculations, the inhibitor's adsorption geometry was examined in detail. Checking whether equilibrium can be reached in the MC simulation using the steady-state energy levels. As the simulation progressed, the system eventually settled into its lowest-energy condition. Inhibitor arrangement on a possible Fe (110) plane is shown in Figure 5. The backbone of an inhibitor molecule adhering to the surface atoms of the Fe (110) plane may be responsible for the observed adsorption pattern (directed by heteroatoms, mainly O) [69,70].
Adsorption is caused when molecules exhibit their heteroatoms and electron rings at the surface, which is what gives them the capacity to absorb. Adsorption is what gives molecules their ability to absorb [43,48]. The adsorption of inhibitors results in the formation of massive Eads (Figure 10) on the surface of the metal. These inhibitor compounds have a significant adsorption interaction with the metal due to the exceptionally high adsorption energies that they possess. By making use of this contact, a protective layer may be formed, which serves to shield the metal surface from the effects of corrosion. Based on the Eads. values that were found through MC, the order of corrosion inhibition performance of the inhibitors is: I3>I1≈I2. The MD model of adsorption dynamics is generally acknowledged to be the more accurate of the two [40,42,43,45,50]. After several hundreds of ps of NVT simulation, it is clear that the inhibitors Figure 9 adopt a horizontal structure on one side of the molecule rings onto the metal surface and is substantially adsorbed onto the Fe surface.
Adsorption processes are generally portrayed on the RDF graph of metal surfaces if peaks form at a specific distance from the metal surface [71]. When the heights are in the zone of 1–3.5 Å in range, it is thought to indicate a chemisorb able process. On the other hand, for physical adsorption, RDF peaks are expected to be present at distances larger than 3.5 Å [47–51].
Fe surface and inhibitor O an N atoms had RDF peak values at distances of less than 3.5 Å – except the I1 molecule, where it seem that the N atoms have insignificant adsorption contribution to this molecule (Figure 11) [43,53,54]. As seen by its comparatively high negative energy value and RDF peaks, in this instance the inhibitors seem to be significantly interacting with the surface of the metal through the O/N atoms.