Evolution of the Eciency of Divalent Cobalt and Copper Chelates Based on Isatin Derivatives and Thiosemicarbazide Ligands as Inhibitors for the Corrosion of Sabic Iron in Acidic Medium

Divalent cobalt and copper chelates of the two ligands 1-(1- Ethoxycarbonylmethyl-2-oxoindolin-3-ylidene) thiosemicarbazide (EOIT) and 1-(1-Benzyl-2-oxoindolin-3-ylidene) thiosemicarbazide (BOIT) are the target compounds of the current study. Identication of the constitution and geometry of these compounds have been performed using the possible physicochemical and analytical instruments. Elemental analysis, molar conductance and thermal analysis assured the composition of the four chelates to be [Co(POIT)Cl]•1.5H 2 O, [Cu(POIT)Cl], [Co(BOIT)Cl 2 (H 2 O)]•5H 2 O and [Cu(BOIT)Cl]•Cl which was further conrmed by the measurement of mass spectra. The architecture arrangement of the ligand atoms around Co and Cu centers has been determined depending on the UV-Vis spectral measurements and calculation of µ eff values assuring the copper compounds to be square plane whereas the cobalt complexes have tetrahedral or octahedral arrangements. These compounds were examined as corrosion inhibitors for the Sabic steel in 1.0 M HCl utilizing potentiodynamic polarization, electrochemical impedance spectroscopy, mass-loss method and scanning electron microscopy at xed temperature of 298 K. The acquired outcomes disclosed that the inhibition eciencies (% IEs) of the examined compounds were set to be dependent on both the concentrations and composition of the compounds. The examined compounds were set to have high % IEs, which were interpreted by strong adsorption of the compounds’ molecules on the iron surface and such adsorption was discovered to follow Langmuir adsorption isotherm. The investigational results obtained from all utilized techniques were set to be in a good accord with each other.


Introduction
Iron and its alloys have numerous applications in industry and machinery due to their excellent mechanical properties. They are subjected to corrosion phenomenon in various media [1][2][3]. Corrosion phenomenon is a naturally occurring process [4,5], which causes a deterioration of signi cant properties in metals and alloys, weakening of buildings and machinery, corrosion of petroleum pipeline, etc. resulting in dangerous problems for the economy and safety. As an example, iron rust and its alloys is regarded as a serious industrial problem particularly in acidic media [6-8] because acidic media are widely utilized in numerous applications such as industrial cleaning, acid discaling, pickling of steels, petroleum processes, etc. Nevertheless, due to the attack of the acidic media, iron and steel vessels used in these applications are subjected to corrosion. Therefore, it is indispensable to preserve metals and alloys from the harmful effect of corrosion phenomenon in different environments [9].
There is incessant increase in the development of e cient, convenient and environmentally friendly inhibitors to minimize metals electro-dissolution and corrosion [10]. Over the years, plentiful reports have been published to evaluate organic compounds and the product of their combination with inorganic compounds as inhibitors for metallic materials corrosion. Heteroatoms containing systems, such as compounds containing nitrogen, sulphur, oxygen and p-electron systems have been extensively applied as inhibitors for metals corrosion. Former studies have disclosed that these organic substances are adsorbed on the metallic surfaces and hence inhibit corrosion [10][11][12][13]. So, these compounds protect the metals from the corrosive species immediately through formation of resistive layer on the metal surface, which guarantee high resistance to electron transfer reactions [14,15]. Corrosion inhibiting tendency of the inhibitor is extremely affected by molecular orbitals and electron densities surrounding the donor atoms of inhibitor molecules [16][17][18][19].
Over decades, thiosemicarbazides are evidenced to be effective intermediates for the development of so many pharmaceutical and bioactive materials and hence, they are applied expandly in medicinal chemistry. The vast number of heterocycles produced from thiosemicarbazides is related primarily to the fact that these compounds can display the characteristics of N(1)-; N(2)-; N(1); N(4)-; N (2); N(4)-; N(1); S-, N(2); S-and N(4),S-nucleophiles [20]. Among of these heterocycles are the thiosemicarbazones derivatives with isatin counterparts that have attracted the concern of the pharmacy world owing to their extensive range of biological features [21][22][23] and industrial application as corrosion inhibitors [24][25][26][27][28]. Moreover, thiosemicarbazones (TSCs) are termed signi cant ligands with interesting ligating characteristics owing to thione-thiol tautomerism, and therefore bind to metallic centers in monodentate, bidentate or bridging modes [29,30]. The capability of TSCs to form stable complexes with transition metal ions makes them as versatile pharmacophores [31,32] and anti-corrosive agents [33,34].
As a consequence of all the previous reports, the structure of two isatin-thiosemicarbazone based ligands, abbreviated by EOIT and BOIT, motivated us to be used as chelating agent for the synthesis of Co(II) and Ni(II) complexes with extensive insight into their corrosion inhibition for Sabic iron in 1.0 M HCl solutions utilizing alternative methods including potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS), mass-loss (ML) and scanning electron microscopy (SEM). Full structure identi cation of the synthesized compounds has been performed in a trial to study the structure-activity relationship.

Chemicals and methods
All chemicals applied in the current study and solvents were purchased from Sigma-Aldrich, Merck and/ or across companies in Japan and used as received. The applicable corrosive medium (1.0 M HCl) which prepared by attenuation of HCl (37%) via bi distilled water. Solutions of the investigated compounds (inhibitors) were prepared using the organic solvent DMF and their concentrations range used was: 100 to 400 mg/l. Corrosion experiments were conveyed out on Sabic iron specimens (SABIC Company, Saudi Arabia) with the composition (wt. %): 0.076 C, 0.125 Mn, 0.0126 Cr, 0.034 Cu, 0.012 Si, 0.008 S, 0.009 P, 0.003 Mo, 0.027 Ni and the remainder is iron.
About the measurements and analysis, percent of C, H and N elements have been analyzed using Perkin-Elmer 2400 CHN Elemental analyzer. IR spectra have been recorded by Perkin-Elmer 1430 IR spectrophotometer within 4000-400 cm -1 range in the form of KBr disks. EI-MS of the ligand was recorded at 70 eV. 1 H-NMR spectrum has been measured using Varian Mercury Oxford NMR 300 MHz spectrophotomete and using d 6 -DMSO as the solvent in existence of internal standard, tetramethylsilane.

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523 conductivity bridge has been used for the measurement of molar conductance at 25 °C using 10 -3 M solution of each complex dissolved in DMF. TG-50 thermogravimetric instrument has been used for the thermogravimetric analysis of the metal chelates under 10 °C /min heating rate and nitrogen atmosphere from room temperature up to 800 °C . The UV-Vis spectra were recorded on a Shimadzu UV-3600 spectrophotometer. The magnetic susceptibilities of the solid metal chelates were measured at 25 °C using Gouy's method.
PDP and EIS experiments were performed on the PGSTAT30 potentiostat/galvanostat with a temperaturecontrolled system. Prior to each experiment, the working electrode (Sabic iron) was prepared as previous reports [35][36][37] and was rinsed into the corrosive medium (1.0 M HCl solution) without and with the required quantity at open circuit potential (OCP) which reached after about 30 min. of rinsing in the solution. In PDP, the potential was automatically altered within (-200 to + 200 mV vs. OCP) with a scan rate of 2.0 mV/s. In EIS, the experiments were functioned within frequency range: 100 kHz to 0.1 Hz, and with the amplitude was 4.0 mV (peak to peak) utilizing AC signals at OCP. ML experiments were carried out in a temperature-controlled system. Sabic iron samples were prepared for these experiments also as reported [35][36][37]. Surface morphology of Sabic iron surfaces was examined before and after addition of a

Synthesis of the ligands
In an HP-500 process vial the mixture of 0.003 mole of N-substituted isatin and 0.003 mole of thiosemicarbazone and 15 mL of EtOH with 0.5 ml of HCl were mixed and subjected to microwaves irradiation using pressurized conditions at 110 °C with 800W power for 30 min. The formed yellow Nsubstituted-istin-thiosemicarbazones 4 (EOIT) and 5 (BOIT) were collected by usual way and recrystallized from EtOH.

Synthetic routes and structures of the two ligands EOIT and BOIT
The two valuable thiosemicarbazone ligands EOIT and BOIT (compounds 4 & 5, respectively in Scheme 1) were synthesized as sketched in Scheme 1 from the condensation of N-substituted isatin derivatives 2 and 3 with the thiosemicarbazide 1 under the known acidic method in EtOH/HCl under re ux for only 10 min under microwaves irradiations as reported previously [39]. The 1 H NMR spectrum for the EOIT in dimethylsulfoxide-d 6 is described in Figure 1. This 1 H NMR data glistened with the four singlet signals at δ = 4.67 (CH 2 COOEt), 8.78, 9.13, 12.23 (3NH) ppm, the appearance of the two protons of the amino group (NH 2 ) with two different chemical shift values at δ = 8.78 and 9.13ppm this means that they are magnetically different due to intramolecular-hydrogen bond which restricted or slowed rotation about the N-C bond [40,41]. In addition, the presence of the two triplet (CH 3 ) and the quartet (CH 2 ) for the ethyl ester protons at δ = 1.2 and 4.15 ppm.

Thermogravimetric analysis
Thermal responses of the metal chelates have been evaluated through TG analysis (thermogravimetric analysis). The TG thermograms of the four compounds are illustrated in Figure 4. From these thermograms it is obvious that the metal complexes decomposed within either two steps (EOIT-Cu), three steps (BOIT-Cu), four steps (EOIT-Co) or within ve stages (BOIT-Co). The two steps thermogram of EOIT-Cu showed the rst step of degradation within the temperature range 25-238 °C with weight loss of 33.67% (calcd 33.78%) which assigned to the loss of 1/2Cl 2 and the organic fragment C 4 H 7 NO 2 . The rest of organic moiety completely lost giving Cu metal as residual product. For BOIT-Cu, the decomposition of the complex that occurred within three stages started at room temperature where the rst step extended to 198 °C corresponding to loss of 8.36% (calcd 7.96%) of the total weight which corresponded to the forfeiture 1/2Cl 2 of coordinated chloride. The following step which occurred within v198-262 °C range assigned to the loss of 11.93% (calcd 11.67) of the total weight and within this stage, the counter chloride anion get lost in addition to NH 2 group. The last and third step appeared in the range 262-566 °C and appointed to the forfeiture of the rest of organic ligand with mass loss of 62.11% (calcd 62.57%) leaving CuO as residual product.
Thermal decomposition of the EOIT-Co and BOIT-Co occur according to the following Schemes (see respectively, are assignable to π→π * and n→π* transitions, respectively [43,45]. Such transitions underwent a movement in their place in the spectra of metal chelates (Table 1) assuring the attachment of the azomethine nitrogen to the metal centers.
Addition to the to π→π * and n→π* transition bands appearing in the spectra of all complexes, the spectrum of the Co(II) complex EOIT-Co showed the medium to low intensity bands in the visible region at 528 & 736 nm, for EOIT-Co which are credited to 4 A 2 → 4 T 1 (υ 2 ) and 4 A 2 → 4 T 1 (P) (υ 3 ) transitions, respectively, assuring four coordinate tetrahedral stereochemistry around Co(II) ions [46]. For the six coordinated Co(II) chelate and BOIT-Co, The spectrum exhibited low intensity bands at 588 nm assigned to 4 T 1g (F)→ 4 A 2g (F). The shoulder band appearing at 483 nm that can be assigned to 4 T 1g (F)→ 4 T 1g (p).

X-ray diffraction analysis
We recorded the XRD patterns for the four N-substituted-isatin-thiosemicarbazone complexes EOIT-CoCl 2 , BOIT-CoCl 2 , EOIT-CuCl 2 and BOIT-CuCl 2 to investigate their crystal structures and their size. The rst look to the for charts of the XRD of the tested complexes indicated that the two complexes CoCl 2 are amorphous and on the opposite side the two Cu-complexes re ected the excellent nano-size of the solid samples. From the calculation the size of the investigated tow crystalline Cu-complexes from Figure 5 using FWHM method and Deby-Scherrer and Bragg equations [49]. The size of the particles was found ppm) of the tested compounds were performed at 298 K and the PDP curves of the complex EOIT-Co (as a representative example) are shown in Figure 6. The corrosion parameters, viz. corrosion potential (E corr ), anodic and cathodic Tafel slopes (β a , β c ), corrosion current density (i corr ), %IE and θ of the organic ligands were determined and located in Table 1. From Figure 6 and the determined corrosion parameters listed in Table 1 Figure 7 (a, b). It was observed from the Nyquist (a) and Bode plots (b) that the resulted impedance spectra comprised of only depressed capacitive loops in addition to one-time constants, correspondingly, suggesting that adsorption of the tested compounds happens through covering the surface and the corrosion is managed by the process of charge transfer [51]. The acquired communal pro le of the plots was similar in both lake and existence of the compounds at the alternative concentrations employed revealing that there was no alter in Sabic iron corrosion mechanism [52]. It was realized from the Nyquist plots that the size of the capacitive semicircle increased by addition of the examined compounds revealing a reduce in the corrosion rate and an increase in the %IEs and the later were increased as the concentrations of the examined compounds increased [53]. Additionally, the Bode phase plots, Figure 7(b), showed that the phase angle was increased with increasing the compounds' concentrations. This indicated that the metal surface was considerably changed to smooth because of formation of a protecting layer of inhibitors' molecules on the Sabic iron surface resulting in a decrease in the corrosion rate [54].
Analysis of the impedance spectra were done through illustrating the model of the equivalent circuit shown in Figure 8. Impedance parameters values such as solution resistance (R s ), charge transfer resistance (R ct ), constant phase element (CPE), % IE and θ were evaluated from the impedance spectra and were tabulated in Table 3. From these results it is obvious that the addition of the examined compounds to the blank solution leads to increasing the value of R ct of the corrosive medium and this behavior was set to signi cantly increased with increasing inhibitors' concentrations. This was associated with a reduce in CPE value as a result of a reduce in the dielectric constant and/or an increase in the double-layer thickness. This indicated adsorption of the investigatd compounds' molecules on the iron/solution interface [55] resulting in the safeguard of the Sabic iron surface from the attack of the corrosive medium. With increasing the concentration of the examined complexes, the inhibition e ciencies were set to increase con rming that these compounds are regarded as e cient inhibitors for the corrosion of Sabic iron in 1.0 M HCl solution. Variation of inhibition e ciencies with the concentrations of the investigated compounds was illustrated in Figure 9.    Table 4. The data listed in Table 4   where K ads is the absorptive equilibrium constant, was tted and is illustrated in Figure 12. This indicates that the inhibitors adsorption on the surface of Sabic iron was correlated to the Langmuir adsorption isotherm.
3.8.5. Surface morphology SEM images of Sabic iron specimens in a free 1.0 M HCl (corrosive medium) and with addition of 200 mg/l of the investigated compounds are shown in Figure 13(a-f). Figure 13 (a) and (b) show a polished Sabic iron surface before and after 24 hours immersion in the corrosive medium, successively. Figure   13(b) shows a strong destruction of the surface of iron specimen due to its exposure to the corrosive medium. Figure 13(c) to (f) shows SEM images after addition of a 200 mg/l of the investigated compounds: EOIT-Co, BOIT-Co, EOIT-Cu and BOIT-Cu, correspondingly, to the corrosive medium. It can be detect that, the surface of Sabic iron specimens were considerably covered with the investigated compounds on the most surface areas which was attributed to strong adsorption of the compounds' molecules on the iron surface, leading to protecting the iron surfaces from the medium, and hence display an e cient corrosion inhibition. The compounds were set to have high inhibition e ciencies, which were interpreted by strong adsorption of the compounds' molecules on the iron surface and such adsorption was set to follow Langmuir adsorption isotherm. The inhibition e ciencies of these compounds were set to depend on their concentrations and structures. At the same concentration, the inhibition e ciencies are slightly increased in the order: EOIT-Cu > BOIT-Cu > EOIT-Co > BOIT-Co. Finally, the investigational results obtained from all employed techniques were found to be in a good consistent with each other.

Declarations Declaration of Interest
The authors declare that they have no known competing nancial interests or personal relationships that could have appeared to in uence the work reported in this paper. Figure 1 The 1H NMR of the thiosemicarbazone derivative 4 (EOIT) Figure 2 EI-mass spectrum of EOIT-Co

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