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Introduction
The use of synthetic corrosion inhibitors are significant products widely used in various industrials sectors especially in the pharmaceutical, petrochemical, generation power industries and water treatment plants. [1] In most case, heterocyclic organic compounds have been reported as effective inhibitors against corrosion phenomena for mild steel in acidic media [2] mainly depends on their abilities to be adsorbed on the metal surface with the aromatic rings and polar functional groups acting as reactive centers. [3] At a recent time, 5-Chloroisatin and its derivatives are ones of the conventional organic compounds [4-9] which have temped much attention for their anticorrosion properties in various media [10] and other applications. [11-14]
The aim of this present study was to compare the anticorrosion potential of these three synthesized 5-Chloroisatin derivatives TZ ACI, TZ CDI and TZ COI in Hydrochloric acid (1.0 M) media using corrosion rate method, electrochemical research-impedance and polarization measurements. Morphological study has been done using scanning electron microscope (SEM).
Experimental
Inhibitor synthesis
The inhibitors used were 1-allyl-5-chloro-indoline-2,3-dione (TZACI), [15] 5-chloro-1- (2- (dimethylamino) ethyl) indoline-2,3-dione (TZCDI), and 5-chloro-1-octylindoline-2,3-dione (TZCOI). The inhibitors were synthesized in the laboratory according to the published method [16-19] and they are represented in the table 1 below.
Materials
Mild steel specimens used as test materials contain the following composition (in wt%): 0.370 % C, 0.230 % Mn 0.680 % Si, 0.016 % S, 0.077 % Cr, 0.011 % Ti, 0.059 % Ni, 0.009 % Co, 0.160 % of Cu and the remainder iron (Fe). The steel samples were pre-treated prior to the experiments by grinding with emery paper (SiC) (400, 600 and 1200), rinsed with distilled water, degreased in acetone, washed again with bidistilled water and then dried at room temperature before use.
Results and discussion
Weight loss studies
The values of corrosion inhibition efficiency ( η (%)) and corrosion rate ( C R ) with and without different concentrations of 5-Chloroisatin derivatives for mild steel in 1.0 M HCl are presented in Tables 2.
Table 2 Weight loss data in the absence and presence of inhibitors in 1.0 M HCl at different concentrations.
| Concentration (M) | C R (mg.cm2.h1) | η (%) | |
| TZACI | 1.0 M HCl | 0.82 | -- |
| 10-3 10-4 10-5 10-6 |
0.07 0.15 0.21 0.32 |
91 82 74 61 |
|
| TZCDI | 1.0 M HCl | 0.45 | -- |
| 10-3 10-4 10-5 10-6 |
0.04 0.07 0.22 0.24 |
91 83 51 46 |
|
| TZCOI | 1.0 M HCl | 0.82 | -- |
| 10-3 10-4 10-5 10-6 |
0.08 0.13 0.23 0.34 |
90 84 72 59 |
It is clear from the Table 2 that, as the concentration reduced as, the corrosion rate decreased, and inhibition efficiency increased for all the three inhibitors. The increased inhibition efficiency at higher inhibitor concentration may be due to the adsorption of inhibitor molecules on mild steel surface. [23]
The inhibition efficiency of the three studied molecules follows the order: TZ CDI>TZ ACI >TZ COI. This order can be explained by the presence of the end-group attached to the phenyl ring of the Indole moiety. [24]
Electrochemical measurements
Polarization studies
In this study, the potentiodynamic polarization experiments were performed to achieve information about the kinetics of anodic and cathodic reactions[25].
Potentiodynamic polarization curves with and without different concentrations of TZ ACI, TZ CDI and TZ COI at 308 K are shown in Fig. 1, and their electrochemical corrosion kinetic parameters such as corrosion potential ( E corr), corrosion current ( I corr), cathodic Tafel slopes (β c) and percentage of E p are given in Table 3.
Fig. 1 Potentiodynamic polarization curves for mild steel in 1.0 M HCl in the absence and the presence of different concentrations of the three compounds.
Table 3 Potentiodynamic polarization parameters for the corrosion of mild steel in 1.0 M HCl in absence and presence of different concentrations of TZ ACI, TZ CDI and TZ COI at 308 K.
| Inhibitors | Concentration (M) | -E corr (mV/SCE) | I corr (μA/cm2) | -βc (μA/cm²) | Ep (%) |
|---|---|---|---|---|---|
| Blank | 1 | 464 | 1386 | 184 | -- |
| 10-6 | 452 | 239 | 159 | 81 | |
| TZACI | 10-5 | 454 | 206 | 173 | 83 |
| 10-4 | 459 | 198 | 174 | 85 | |
| 10-3 | 453 | 157 | 184 | 88 | |
| Blank | 1 | 506 | 172.4 | 3818 | -- |
| 10-6 | 503 | 2114 | 174 | 44 | |
| TZCDI | 10-5 | 499 | 1885 | 175 | 51 |
| 10-4 | 498 | 1138 | 186 | 70 | |
| 10-3 | 483 | 761 | 197 | 80 | |
| Blank | 1 | 465 | 1387 | 184 | -- |
| TZCOI | 10-6 | 461 | 612 | 177 | 56 |
| 10-5 | 459 | 401 | 193 | 71 | |
| 10-4 | 455 | 273 | 178 | 80 | |
| 10-3 | 457 | 143 | 172 | 90 |
It is clear from these polarization plots that introduction of inhibitors into corrosive medium exerted significant effect on both anodic and cathodic reactions indicating that these 5-Chloroisatin derivatives reduced the anodic mild steel dissolution and also retarded the cathodic hydrogen evolution reaction. [26] Further, from Table 3 it can also be seen that the presence of inhibitors decreases the value of corrosion current density ( I corr); this decrease highlights the inhibition property E p (%) of these molecules. Also in the presence of 5-Chloroisatin derivatives the shift in E corr values is towards more negative side, but it is less than 85 mV, which proposed mixed type behaviour of inhibitors. [27]
Electrochemical impedance spectroscopy
Impedance spectra of mild steel in 1.0 M HCl containing various concentrations of the three inhibitors at 308 K are shown in Fig. 2.
Fig. 2 Impedance diagram for mild steel in 1.0 M HCl in the presence and absence of different concentrations of the inhibitors.
The Nyquist plots for inhibited and uninhibited specimens give semicircles which indicate that studied inhibitors molecules behave as interface inhibitors. They inhibit corrosion by adsorbing at metal/electrolyte interface. [28] The diameter of capacitive loop increased with increase in inhibitor concentration, suggesting the improved inhibition efficiency of these molecules TZ ACI, TZ CDI and TZ COI at higher inhibitor concentrations.
Table 4 showed the electrochemical impedance spectroscopy (EIS) parameters in the absence and the presence of the three corrosion inhibitors of mild steel in 1.0 M HCl. It can be observed that the charge transfer resistance ( R t ) value increased with an increase in the concentration of the TZ ACI, TZ CDI and TZ COI inhibitors, while the values of the double-layer capacitance ( C dl ) of the interface started decrease, with an increase in the inhibitors concentrations. The rise in R ct data, and as a result of the inhibition efficiency E Rt (%), may be due to the gradual change molecules of water by the inhibitor molecules adsorption on the mild steel surface to form a film adherent on surface. [29] For this reason, the decrease in C dl for all molecules indicates a reduction in the local dielectric constant or an increase in the thickness of the electrical double layer. [30]
Table 4 Impedance parameters for the corrosion of mild steel in 1.0 M HCl in absence and presence of different concentrations of inhibitors.
| Inhibitors | Concentration (M) | R t (Ω.cm2) | C dl (μF.cm-2) | ERt (%) |
|---|---|---|---|---|
| 1.0 M HCl | 1 | 10 | 200 | -- |
| TZACI | 10-6 | 60 | 74 | 83 |
| 10-5 | 110 | 46 | 91 | |
| 10-4 | 175 | 38 | 94 | |
| 10-3 | 250 | 31 | 96 | |
| 1.0 M HCl | 1 | 4.88 | 66 | -- |
| TZCDI | 10-6 | 2.39 | 68 | 59 |
| 10-5 | 2.67 | 62 | 68 | |
| 10-4 | 1.08 | 59 | 82 | |
| 10-3 | 2.92 | 46 | 85 | |
| 1.0 M HCl | 1 | 14.78 | 71 | - |
| TZCOI | 10-6 | 52.35 | 69 | 72 |
| 10-5 | 57.51 | 61 | 74 | |
| 10-4 | 65.51 | 52 | 77 | |
| 10-3 | 97.09 | 50 | 85 |
Adsorption studies
The anticorrosion activities of organic inhibitors mainly depend on the adsorption of inhibitor molecules on the surface metallic to construct a compact barrier film. [31] Hence, it is essential to know the adsorption isotherm that can give valuable information on the interaction of inhibitor and metal surface.
As can be seen from Fig. 3, straight lines were obtained by plotting C/θ vs. C at all concentrations studied.
Fig. 3 Langmuir adsorption isotherm plots for the adsorption of TZACI, TZCDI and TZCOI on mild steel surface in 1.0 M HCl at 308 K temperature.
This indicates that the three inhibitors TZ ACI, TZ CDI and TZ COI are adsorbed on the mild steel surface, according to the Langmuir adsorption isotherm as the best fit. The data of DG° ads and K ads were calculated and are reported in Table 5.
Table 5 Adsorption thermodynamic parameters in the absence and presence of various concentrations of inhibitors.
| Inhibitor | Slope | R2 | K ads (M-1) | DG° ads (kJ mol -1 ) |
| TZACI | 1.08 | 0.999 | 1.9 105 | -17.97 |
| TZCDI | 1.06 | 0.999 | 1.6105 | -40.45 |
| TZCOI | 1.10 | 0.999 | 2.6105 | -42.26 |
Meanwhile, the negative values of DG° ads suggest a spontaneous adsorption process[32]. The examination of data listed in Table 5 revealed that the adsorption process of the TZ CDI and TZ COI molecules (<_40 kJ mol_1) on mild steel surface involves chemisorption[33]. On the other hand the measured data of free energy of the inhibitor TZ ACI is 20 kJ mol-1 or less, which lead to that adsorption of this organic assembled inhibitor on mild steel surface takes place via physisorption[34].
Morphology examination of mild steel by SEM
Optical microscopic surface analysis [35] with and without inhibitors TZ ACI, TZ CDI and TZ COI at 308 K are presented in Fig. 4((a)-(e)). The polished and smooth surface of mild steel before immersion in 1.0 M HCl is shown in Fig. 4(a). It can be clearly observed from Fig. 4(b) that the mild steel surface processed without the inhibitors is severely corroded and a number of pits are distributed over the metal surface. However, the surface images in the presence of inhibitors (Figures 4(c)-(e)) show close resemblance to bare the outside of the mild steel, which indicate that in the presence of inhibitors, the corrosion process has been decreased to a great extent. This observation demonstrates the good protective potential of TZ ACI, TZ CDI and TZ COI to behave as good inhibitors for the surface tested in acidulous media. [36]
Conclusion
From the above studies on 5-Chloroisatin derivatives as corrosion inhibitors using gravimetric, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, scanning electron microscopy (SEM), it is presented the following conclusions:
The expired three compounds 1-allyl-5-chloro-indoline-2,3-dione (TZ ACI), 5-chloro-1-(2-(dimethylamino) ethyl) indoline-2,3-dione (TZ CDI) and 5-chloro-1-octylindoline-2,3-dione (TZ COI) acts as good and efficient corrosion inhibitors for the corrosion of mild steel in 1.0 M HCl acid medium.
The corrosion inhibition efficiency of the three inhibitors increases sharply with increasing concentration.
Adsorption of these three inhibitions follows Langmuir isotherms indicate that the adsorption of inhibitor on surface is a spontaneous process, involving Physisorption for TZ ACI and chemisorption for the both TZ CDI and TZ COI .
Polarization studies revealed their mixed type nature inhibition for the three tested inhibitors.
EIS method shows that the charge transfer resistance ( R ct ) increases and double-layer capacitance ( C dl ) decreases in presence of the three inhibitors, suggesting the increasing surface coverage by the inhibitors molecules on the surface of mild steel.
SEM confirmed the absorption of 5-Chloroisatin derivatives molecules on the surface.
