3. 1. Pdp Test
Polarization plots of HCS in HCl solution (0.5 mol/L) with/ without of different concentrations of CS at 303 K are seen in Fig. 2. The related parameters were calculated from fitting PdP curves are listed in Table 1. The degree of surface coverage (θ) and protection efficiency (%EPdP) were estimated using the following equation [22].
$${\%E}_{PdP}=\left(\frac{{I}_{c}-{I}_{i}}{{I}_{c}}\right)x 100= \theta x 100$$
1
I c & Ii are the corrosion-current densities of without/ with (CS), respectively. As seen in Fig. 2, by adding the extract to the corrosive test solution leads both the anodic and cathodic branches to move to lower corrosion current densities, significantly decreasing the corrosion rate and indicating the inhibitory action of the extract for metal dissolution.
As shown from Table 1, the lower values of (Icorr) in the presence of the extract without appreciable changes in corrosion potential (Ecorr) indicate that the (CS) is a mixed- type inhibitor (that is, inhibit both anodic & cathodic reactions) and is adsorbed on the HCS surface thereby preventing the corrosion reaction sites. Moreover, the fact that the Ɓc is higher than the Ɓa means that the (CS) extract is under mixed control, but that its influence on the catholic reaction is larger than that on the anodic reaction. This behavior may be attributed to the formation of an outer layer on the HCS surface as a result of (CS) extract adsorption, which prevents HCl from attacking the active centers of the HCS surface [23].
Table 1
kinetic parameters obtained from Pdp method.
Conc., mg/L | Icorr,µA cm− 2 | Ecorr,mV | Βa,mV dec− 1 | -Βc,mV dec− 1 | θ | %IEPdP |
HCl (0.5 mol/L) | 627.73 | -528 | 103 | 125 | ----- | ----- |
50 | 89.13 | -524 | 100 | 119 | 0.858 | 85.8 |
100 | 55.24 | -515 | 97 | 122 | 0.912 | 91.2 |
200 | 30.75 | -519 | 91 | 116 | 0.951 | 95.1 |
400 | 15.69 | -525 | 105 | 113 | 0.975 | 97.5 |
3.2. Eis tests
Eis spectra were recorded in (0.5 HCl mol/L) with/ without of different concentrations of CS at 303 K, and Nyquist plots are seen in Fig. 3a. By utilizing an appropriate equivalent circuit (Fig. 3b) to fit the observed Eis data, the inhibition efficiency performance of (CS) extract was evaluated.
As can be seen, the shape of the impedance spectra is similar with and without the addition of (CS) extract, indicating that the nature of the corrosion reaction has not changed. However, the impedance spectra is not a perfect semicircle, which may be due to the frequency dispersion caused by the roughness and unevenness of the solid electrode surface [24]. Additionally, it has been found that as (CS) concentrations increase, the impedance spectrum diameter increases. This indicates that the charge transfer mechanism has been inhibited and confirms that the extract successfully inhibits corrosion of HCS in acidic solutions .The (%EEis) and (θ) are obtained from the (Rct) as the following equation [24]:
$$\%{E}_{Eis}=⌊\frac{{R}_{ct}^{I}-{R}_{ct}^{B}}{{R}_{ct}^{I}}⌋x 100=\theta x 100$$
2
Where, RIct & RBct are the value of charge transfer resistance with/ without of (CS), respectively.
The kinetic parameters derived from analysis of Eis diagrams are listed in Table 2.
It is revealed from Table 2, that when (CS) concentrations increased, (Rct) values increased and (Cdl) values reduced, leading to an increase in (%EEis). This is due to H2O molecules have been replaced by (CS) molecules, increasing the thickness of (Cdl) on the HCS surface. Therefore, in the corrosive media, both HCS corrosion and the charge of electron transfer were reduced [24].
Table 2. kinetic parameters obtained from Eis method
Conc., mg/L | Rct, (Ω cm2) | Cdl, (µF cm-2) | θ | %IEEIS |
HCl (0.5 mol/ L) | 31.58 | 107.54 | ----- | ----- |
50 | 198.8 | 27.8 | 0.841 | 84.1 |
100 | 295.1 | 28.3 | 0.892 | 89.2 |
200 | 401.5 | 28.0 | 0.921 | 92.1 |
400 | 844.5 | 19.8 | 0.962 | 96.2 |
3.3. Adsorption isotherm
The adsorption behavior for the fitting values of θ recorded in Table 1 was examined in the current study using a variety of isotherm models, such Langmuir, Temkin, Frumkin, El-Awady, Flory-Huggins, and Dubinin-Radushkevich. According to the equation below, it is found that the Langmuir model provided the best fit [25].
$$\frac{C}{\theta }=\frac{1}{{K}_{ads}}+C$$
3
Where, Kads is the adsorption process's equilibrium constant, and C, is concentration of extract.
The plot of (C/θ) vs. (C), which give a linear relationship, the correlations (R2) and slopes are were nearly to 1.0, (Fig. 4). The (Kads) of adsorption/ desorption process obtained from the Langmuir plot intercept is related to free energy of adsorption (ΔGads) according to the formula [25]:
$${K}_{ads}={\left(\frac{1}{55.5}\right)e}^{\left(\frac{{-\varDelta G}_{ads}}{RT}\right)}$$
4
Where. (55.5 molar) is H2O concentration, R is universal gas and T is absolute temperature.
The negative sign of (ΔGads) reflects a spontaneous mechanism for the adsorption of (CS) on the HCS surface. The value of (ΔGads) is, however, (-41.6 kJ mol− 1). This reveals that the mixed (physical and chemical) adsorption of (CS) on the HCS surface, with chemical adsorption predominating [25].
3.4. HCS surface morphology investigation
The influence of (CS) extract on the surface morphology of HCS was investigated utilizing AFM spectroscopy. Figures 5a–c show polished (pure) HCS before and after immersion in a solution of (0.5 mol/L) HCl without and with the addition of (CS), respectively. The average deviation of the roughness surface (Ra) for a pure sample was found to be (42.7 nm), as shown in Fig. 5a. The (Ra) of the HCS sample increased (Fig. 5b) as it was dipped in the (0.5 mol/L) HCl and became (77.5 nm). This is a result of the HCS surface was attacked by HCl solution, which caused corrosion and surface damage. The (Ra) value for HCS in HCl solution containing CS, on the other hand, was reduced to (55.2 nm), which corresponds to Fig. 5c. This indicates the formation of a protective thin film of (CS) on the HCS surface, which reduces reactivity of the surface to corrosive media [26].
UV-Visible spectra of (CS) solution and the corrosive solution containing (CS) after immersion of HCS sample for one day, are shown in Fig. 6. The spectra of (CS) solution showed a band at (688, 505 and 435 nm). While, the spectra of corrosive solution containing (CS) after dipping of HCS for one day, showed a shift for peaks to (656, 455 and 445 nm). This due to charge transfer from (CS) to the HCS, and probability of the complex formation between (CS) molecules and the dissolved HCS ions [27].
3.5. Quantum chemical study & Adsorption mechanism
Quantum chemical calculation is a tool that can correlate the relation between the main constituent molecular structures of (CS) and the inhibition performance via the quantitative data obtained from analysis of DFT theory. Figure 7 shows the optimized configuration of (HDB), (HPE) and (HMPE) molecules, as well as the molecular orbital distribution (LUMO & HOMO). The electron of the HOMO and LUMO orbitals of the three molecules are almost distributed on the benzene rings and oxygen atoms as shown in Fig. 7. Therefore, it can be reasonably indicated that these groups of (HDB), (HPE) and (HMPE) can be adsorbed onto the HCS surface by these centers, as well as, are easy to form coordination bonds with Fe atoms via electron donor – accepter. From analysis DFT, it is found that, EHOMO and ELUMO of (HDB), (HPE) and (HMPE) molecules is (-4.87, -4.84, and − 4.86 eV ) and (-2.37, -2.35 and − 2.36 eV), respectively. The energy gap value (∆E = ELUMO -EHOMO) of the (HDB), (HPE) and (HMPE) molecules are (2.5, 2.49 and 2.5 eV), respectively. A high value of EHOMO indicates that the molecule has a greater ability to donate electrons, while a low value of ELUMO indicates that the inhibitor is more likely to receive electrons [28]. Therefore, the performance of the inhibitor molecule is enhanced by a smaller value of ∆E [29]. While the energy gap (∆E) values for (HDB), (HPE), and (HMPE) are quite similar, this means that all three of these molecules can demonstrate high-quality corrosion inhibition.
The corrosion inhibition of HCS in HCl (0.5 mol/L) solution by the (CS) extract can be easily explained by molecular adsorption, according to the experimental data obtained in this study.
The donor-acceptor interaction between the electrons of the main components of (CS) extract, such as (HDB), (HPE), and (HMPE), and the unoccupied d-orbitals of surface HCS atoms would have caused the (CS) extract to be adsorbed on the surface of HCS. Therefore, corrosion inhibition of HCS in HCl solution takes place through the attraction of electrostatic force between the protonated molecules of (CS) and positively charged HCS surface via synergistic effect of the Cl− ions which are attached to HCS surface as negative molecules. Thus is leads to formation of a protective thin film which impede the corrosion of metal in aggressive solution [30]. While, the chemical adsorption can occur via the electrons donation from the lone pair of electrons of O heteroatoms atoms, and π -electrons in aromatic rings to the unoccupied d- orbitals of metal surface [30].