Evaluation of the efficiency of divalent cobalt and copper chelates based on isatin derivatives and thiosemicarbazide ligands as inhibitors for the corrosion of Sabic iron in acidic medium

2.1 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. ¹H NMR spectrum has been measured using Varian Mercury Oxford NMR 300 MHz spectrophotomete and using d⁶-DMSO as the solvent in existence of internal standard, tetramethylsilane. 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 temperature-controlled system. Prior to each experiment, the working electrode (Sabic iron) was prepared as previous reports (Fawzy et al., 2018a, 2018b; 2019a, 2019b) and was rinsed into the corrosive medium (50 mL of 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 cylindrical rods with areas of around 12.6 cm². The samples were initially polished mechanically with different grads emery papers (from 200 to 1000), next washed with double distilled water and lastly with acetone. After precise weighing, Sabic iron samples were dipped in the corrosive media (in the absence of the examined complexes) and at various temperatures. After 6 h, the Sabic samples were taken out, washed with double distilled, dried and weighed precisely. Afterwards, the average mass loss of three parallel Sabic iron samples, in each experiment, were obtained. Surface morphology of Sabic iron surfaces was examined before and after addition of a 200 mg/l of the examined compounds utilizing JEOL Scanning Electron Microscope (SEM) model T-200 with a repeat voltage of 10.0 kV. The surfaces of Sabic iron samples were initially abraded with different emery papers (200 to 1000), rinsed with doubly distilled water. Before examination, the samples were dipped in the investigated medium for 24 h, then washed with double distilled and dried.

2.2 Synthesis of the ligands

In an HP-500 process vial the mixture of 0.003 mol of N-substituted isatin and 0.003 mol 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 800 W power for 30 min. The formed yellow N-substituted-istin-thiosemicarbazones 4 (EOIT) and 5 (BOIT) were collected by usual way and re-crystallized from EtOH.

2.3 Synthesis of metal complexes

3.1 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 reflux for only 10 min under microwaves irradiations as reported previously (Amin et al., 2018). The ¹H NMR spectrum for the EOIT in dimethylsulfoxide‑d₆ is described in Fig. 1. This ¹H NMR data glistened with the four singlet signals at δ = 4.67 (CH₂COOEt), 8.78, 9.13, 12.23 (3NH) ppm, the appearance of the two protons of the amino group (NH₂) with two different chemical shift values at δ = 8.78 and 9.13 ppm this means that they are magnetically different due to intramolecular-hydrogen bond which restricted or slowed rotation about the N–C bond (Alsharekh et al., 2019; Althagafi et al., 2019). In addition, the presence of the two triplet (CH₃) and the quartet (CH₂) for the ethyl ester protons at δ = 1.2 and 4.15 ppm.

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Scheme 1 The synthesis of two ligands thiosemicarbazone derivatives EOIT (4) and BOIT (5).

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Fig. 1 The ¹H NMR of the thiosemicarbazone derivative 4 (EOIT).

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3.2 Comprehensive properties and constitution

All the physical, spectral and analytical data describing the two used ligands, POIT & BOIT and their Co(II) & Cu(II) chelates, POIT-Co, POIT-Cu, BOIT-Co and BOIT-Cu are illustrated in experimental section. Investigation of these results indicated good correlation between the found and calculated percent of carbon, hydrogen, nitrogen and metal content assuring the proposed composition of the metal chelates and also assuring the formation of the complexes in 1:1 (M:L) ration. The molar conductivity of complexes EOIT-Co, EOIT-Cu, BOIT-Co measured from 10⁻⁴ M solution of dimethyl formamide were found to lie within 19.5–26.6 (Ω⁻¹ cm² mol⁻¹) range which strongly supported the formation of non-conductance neutral complexes (Geary, 1971). On the other hand, the quite high value of molar conductance of BOIT-Cu; 85 Ω⁻¹ cm² mol⁻¹, strongly recommended formation of 1:1 (cationic:Cl^–) electrolytic complex. The electrolytic or non-electrolytic behaviors of the complexes were qualitatively proved by the precipitation test with aqueous AgNO₃ solution where only BOIT-Cu formed the white AgCl precipitate. Thus the formed complexes has been corroborated to have the formulae [Co(EOIT)Cl]•1·.5H₂O, [Cu(EOIT)Cl], [Co(BOIT)Cl₂(H₂O)]•5H₂O and [Cu(BOIT)Cl]•Cl for EOIT-Co, EOIT-Cu, BOIT-Co and BOIT-Cu, successively. Solubility test of the metal chelates indicated that all the compounds are readily soluble in most polar solvent while hardly soluble or insoluble in alternative non-polar solvents.

3.3 Mass spectroscopy

Information excluded from the mass spectroscopy of the four isolated chelates have been investigated as it provides sufficient evidence to conclude the formula weights of the compounds in addition to their fragments and hence ensures the formation the complexes in the proposed formulae. The mass spectra of complexes EOIT-Co, EOIT-Cu, BOIT-Co and BOIT-Cu showed the molecular ion peak appearing in the spectra of complexes EOIT-Co, EOIT-Cu, BOIT-Co and BOIT-Cu at m/z = 399.5, 401, 496.02 and 444 amu have been assigned for [M], [M−3], [M+2] and [M], successively, excluding all the hydration water for EOIT-Co and 3 of the hydration water for BOIT-Co supporting the proposed formulae of the metal complexes. The multi‐peaks and other fragments are formed by cleavage at alternative positions in the complexes’ molecules.

3.4 FTIR spectra assignments

Comparison of the IR spectrum of each of EOIT and BOIT with that of their metal chelates is beneficial method to assign the binding function groups of the ligand to the cobalt or copper centers in the metal complexes and hence can help in structure identification of the metal complexes. From the spectrum of EOIT and BOIT, the bands obvious at 3377, 3238, 1743, 1697, 1557 & 1288 cm⁻¹ for EOIT and at 3456, 3257, 1681, 1588 & 1275 cm⁻¹ for BOIT. These bands, respectively, has been attributed to the stretching vibrations of NH(asy), NH(sym), C⚌O, C⚌N, and C⚌S (El-Gammal et al., 2020). All these bands underwent a position movement in complexes spectra owing to different reasons which helped in structure identification through assignment of binding modes. Firstly, the symmetric and asymmetric stretching vibrations of NH of amino group appearing in the ranges 3441–3380 cm⁻¹ and 3257–3220 cm⁻¹, undergoing a shift in their position whenever compared with the same bands in the ligand’s spectra. Such shift is mostly due to the participation of the group in hydrogen bond with the nearest electronegative atoms which coincided with the generation of alternative medium to weak intensity bands inside the range 2831–2710 cm⁻¹ (Yarkandi et al., 2017). On the other hand, the significant shift recorded in the position of C⚌O, C⚌N and CS (C⚌S or C-S) peaks in the spectra of complexes upon comparison with their place in the ligand spectra has been explained by the coordination of the ligand to the Co or Cu centers as OSN tridentate monobasic thiol (i.e. EOIT-Co) or neutral thione (i.e. EOIT-Cu, BOIT-Co and EOIT-Cu) modes of chelation (Haribabu et al., 2021; Takroni et al., 2020). Further support of the coordination of the carbonyl oxygen and azomethine nitrogen to the metal center is the appearance of two bands in the spectra of metal chelates within the ranges 574–510 cm⁻¹ and 470–424 cm⁻¹ with no corresponding bands in the spectra of free ligands. Such bands correspond to the stretching vibrations of M−O and M−N bonds, respectively (Yarkandi et al., 2017).

3.5 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 Fig. 2. 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 five stages (BOIT-Co). The two steps thermogram of EOIT-Cu showed thermal stability up to 205 °C while the first step of degradation occurred within the temperature range 205–238 °C with weight loss of 33.67% (calcd 33.78%) which assigned to the loss of 1/2Cl₂ and the organic fragment C₄H₇NO₂. 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 162 °C and the first step extended to 198 °C corresponding to loss of 8.36% (calcd 7.96%) of the total weight which corresponded to the forfeiture 1/2Cl₂ 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 NH₂ 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.

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Fig. 2 TG thermograms of the metal complexes.

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Fig. 2 TG thermograms of the metal complexes.

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Thermal decomposition of the EOIT-Co and BOIT-Co occur according to the following Schemes

3.6 Electronic spectra and magnetic moments

UV–Vis spectra is regarded as one of the most helpful tools to assign the geometrical architectures around the metal centers and also to assure the binding modes of the ligands to the central metal ion. The UV–Vis of the two ligands EOIT & BOIT and their Co(II) & Cu(II) chelates, EOIT-Co, EOIT-Cu, BOIT-Co and BOIT-Cu were measured from DMSO solutions. Investigation of the spectra of the ligands indicated that spectral bands apparent at 265 & 370 nm and at 264 & 375 nm in the spectrum of EOIT & BOIT, respectively, are assignable to π → π* and n → π* transitions, respectively (Yarkandi et al., 2017; Chioma et al., 2018). 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 ⁴A₂→⁴T₁ (υ₂) and ⁴A₂→⁴T₁(P) (υ₃) transitions, respectively, assuring four coordinate tetrahedral stereochemistry around Co(II) ions (Chioma et al., 2018). For the six coordinated Co(II) chelate and BOIT-Co, The spectrum exhibited low intensity bands at 588 nm assigned to ⁴T_1g(F)→⁴A_2g(F). The shoulder band appearing at 483 nm that can be assigned to ⁴T_1g(F)→⁴T_1g(p). Such transitions are characteristic for octahedral Co(II) chelates (Saad et al., 2019). The values of the µ_eff was measured to be 4.25 and 4.63 B.M. for EOIT-Co and BOIT-Co, successively, which are close to reported values for high spin tetrahedral (Sadeek et al 2020) and octahedral (Fawzy et al., 2019a, 2019b) Co(II) complexes.

The spectra of Cu(II) complexes; EOIT-Cu and BOIT-Cu complexes also allowed broad with low intensity bands at the low energy region which centered at 737 and 676 cm⁻¹, respectively corresponding to ²B_1g→²A_1g transitions special for square planar divalent copper chelates (Chioma et al., 2018). The two complexes afforded µ_eff values of 1.82 and 1.88 B.M. for EOIT-Cu and BOIT-Cu, respectively, which supports the results of electronic spectra.

3.7 X-ray diffraction analysis

We recorded the XRD patterns for the four N-substituted-isatin-thiosemicarbazone complexes EOIT-CoCl₂, BOIT-CoCl₂, EOIT-CuCl₂ and BOIT-CuCl₂ to investigate their crystal structures and their size. The first look to the charts of the XRD of the tested complexes indicated that the two complexes of CoCl₂ are amorphous and on the opposite side the two Cu-complexes reflected the excellent nano-size of the solid samples. From the calculation the size of the investigated tow crystalline Cu-complexes from Fig. 3 using FWHM method and Deby–Scherrer and Bragg equations (Al-Ne’aimi and Al-Khuder, 2013). The size of the particles was found in the nanometer range: for complex EOIT-CuCl₂ its size equal 22.15 nm and BOIT-CuCl₂ its size equal 26.60 nm.

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Fig. 3 The XRD for the two complexes EOIT-CuCl₂ (a) and BOIT-CuCl₂ (b).

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According to all the previous measurements, the structures and geometry of the metal complexes can be formulated as depicted in Scheme 2

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Scheme 2 Structures and geometries of the synthesized metal complexes.

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3.8 Assessment of the corrosion rate of Sabic iron and the inhibition efficacy of the interested compounds

3.8.1

3.8.1 PDP measurements

PDP measurements of Sabic steel in 1.0 M HCl solution without and with various concentrations (100–400 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 Fig. 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 2. From Fig. 4 and the determined corrosion parameters listed in Table 1, it can be realized that adding the examined compounds to the corrosive solution (blank, 1.0 M HCl) shifted both anodic and cathodic branches of the polarization curves of the corrosive medium to less current densities indicating delay of both anodic and cathodic reactions and thus inhibition of Sabic iron corrosion. The movement of the E_corr value of Sabic iron in the blank solution to positive directions by the addition of the investigated compounds indicates that these compounds perform as mixed-type inhibitors with a major anodic one (Mahmoud et al. 2020a, 2020b). The values of β_a and β_c did not show obvious change in blank solution and when adding the complexes indicating that the adsorbed compounds’ molecules did not alter the anodic metal dissolution or cathodic hydrogen evolution. In addition, the value of i_corr of Sabic iron in the blank solution was found to decrease with raising the compounds concentration, which indicates inhibition effects. The acquired results supported that, under similar experimental conditions, the inhibition efficiencies of the investigated inhibitors were set to slightly increase according to the order: EOIT-Cu > BOIT-Cu > EOIT-Co > BOIT-Co which can be explained on the basis of thermal stability of the two Cu(II) complexes over the Co(II) complexes as concluded from the results of TGA.

Table 2 Corrosion parameters obtained from the polarization curves in the corrosion of Sabic iron in 1.0 M HCl solution without and with various concentrations of the investigated compounds at 298 K.

Inh.	Inhibitor Conc. (mg/l)	-Ecorr (mV(SCE))	βa (mV/dec.)	-βc (mV/dec.)	icorr (µA/cm2)	% IE	θ
	0	469	123	118	365	--	--
EOIT-Co	100	453	129	117	175	52	0.52
	200	462	125	121	113	69	0.69
	300	449	131	123	77	79	0.79
	400	447	126	119	62	83	0.83
BOIT-Co	100	465	119	113	186	49	0.49
	200	460	124	121	124	66	0.66
	300	463	127	115	91	75	0.75
	400	449	124	109	69	81	0.81
EOIT-Cu	100	457	127	116	175	52	0.52
	200	458	121	111	95	74	0.74
	300	448	125	114	63	83	0.83
	400	451	129	118	47	87	0.87
BOIT-Cu	100	466	132	122	168	54	0.54
	200	453	127	117	106	71	0.71
	300	455	118	120	66	82	0.82
	400	446	123	116	61	83	0.83

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Fig. 4 Potentiodynamic polarization curves of in the corrosion of Sabic iron in 1.0 M HCl solution without and with various concentrations of EOIT-Co complex at 298 K.

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3.8.2

3.8.2 EIS measurements

Corrosion of Sabic iron was studied in 1.0 M HCl solution in the lack and existence of alternative concentrations of the examined compounds at 298 K after immersion of the iron samples in the corrosive medium for around 30 min. by EIS technique. The obtained Nyquist and Bode plots of the complex EOIT-Cu (as a representative example) are shown in Fig. 5 (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 (Mu et al., 2006). The acquired communal profile 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 (Bessone et al., 1983). 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 (Reis et al., 2006). Additionally, the Bode phase plots, Fig. 5(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 (Hsu and Mansfeld, 2001).

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Fig. 5 a) Nyquist plot and b) Bode plots (magnitude and phase) for the corrosion of Sabic iron corrosion in 1.0 M HCl solution in absence and existence of various concentrations of EOIT-Cu complex at 298 K.

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Fig. 6 Electrochemical equivalent circuit used to fit the EIS output data for Sabic iron in 1.0 M HCl solution without and with the investigated compounds.

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Analysis of the impedance spectra were done through illustrating the model of the equivalent circuit shown in Fig. 6. 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 significantly 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 investigated compounds’ molecules on the iron/solution interface (Muhsen et al., 2019) 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 efficiencies were set to increase confirming that these compounds are regarded as efficient inhibitors for the corrosion of Sabic iron in 1.0 M HCl solution.

Table 3 Values of the impedance parameters of the corrosion of Sabic iron in 1.0 M HCl solution without and with various concentrations of the investigated synthesized complexes at 298 K.

Inhibitor	Inhibitor Conc. (mg/l)	Rs (ohm cm2)	Rct (ohm cm2)	CPE (µF/cm2)	% IE	θ
	0	1.23	71	224	–	–
EOIT-Co	100	2.24	127	122	44	0.44
	200	3.07	207	77	66	0.66
	300	2.18	321	62	78	0.78
	400	3.74	423	59	83	0.83
BOIT-Co	100	2.31	119	133	40	0.40
	200	1.91	187	95	62	0.62
	300	2.07	298	76	76	0.76
	400	1.82	384	59	82	0.82
EOIT-Cu	100	1.05	131	122	46	0.46
	200	0.96	244	73	71	0.71
	300	3.20	368	54	81	0.81
	400	2.33	497	46	85	0.85
BOIT-Cu	100	0.95	142	112	50	0.50
	200	3.40	211	84	66	0.66
	300	2.51	316	63	78	0.78
	400	1.71	458	48	84	0.84

Experimental error ± 4%.

3.8.3

3.8.3 ML measurements

Mass loss (ML) recording of Sabic iron in 1.0 M HCl solution were carried out at specified time intervals in the absence and presence of certain concentrations of the tested compounds at 298 K. Fig. 7 shows only the mass-loss versus immersion time plots of the complex BOIT-Co. The same plots were acquired for other investigated compounds but not shown here. Values of the corrosion rates (CR), θ and %IE of the examined compounds are also inserted in Table 4. The data listed in Table 4 indicated that the values of CR were decreased while the inhibition efficiencies were enhanced with raising the inhibitors’ concentrations which attributed to augmenting adsorption coverage of the inhibitor molecules on the steel surface with rising concentration leading to decrease of the corrosion rates of Sabic iron. Therefore, the examined compounds are regarded as effective inhibitors for Sabic iron corrosion in 1.0 M HCl solution. In consistence with both PDP and EIS techniques, at similar inhibitors concentration, the % IEs are slightly increased in the order: EOIT-Cu > BOIT-Cu > EOIT-Co > BOIT-Co. A comparison of the change of the % IEs of the examined compounds with their concentrations at 298 K, obtained from all used techniques, PDP, EIS and ML, is shown in Fig. 8 indicating that the results concluded from all employed techniques are in a good consistent with each other’s.

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Fig. 7 Mass loss (ML) versus immersion time in the corrosion of Sabic iron in 1.0 M HCl solution in the absence and presence of various concentrations of BOIT-Co complex at 298 K.

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Table 4 Values of CR (mpy) of Sabic iron, % IE and θ of various concentrations of the investigated compounds in 1.0 M HCl solution at 298 K.

Inhibitor	Inhibitor Conc. (mg/l)	CR	% IE	θ
	0	166	–	–
EOIT-Co	100	80	52	0.52
	200	48	71	0.71
	300	33	80	0.80
	400	28	83	0.83
BOIT-Co	100	81	52	0.52
	200	53	68	0.68
	300	40	76	0.76
	400	35	79	0.79
EOIT-Cu	100	75	55	0.55
	200	46	72	0.72
	300	29	83	0.83
	400	25	85	0.85
BOIT-Cu	100	75	55	0.55
	200	51	69	0.69
	300	30	83	0.83
	400	23	86	0.86

Experimental error ± 3–4%.

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Fig. 8 Variation of the %IEs of the complex BOIT-Cu with its concentration obtained from PDP, EIS and ML measurements, in the corrosion of Sabic iron in 1.0 M HCl solution at 298 K.

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3.8.4

3.8.4 Adsorption isotherms

The results of the inhibition efficiencies of the tested compounds obtained from ML measurements were interpreted based on their adsorption on the metal surface. Some adsorption isotherms such as Langmuir, Frumkin, Freundlich, Temkin, etc. have been widely studied to investigate the kind of adsorption of the tested inhibitor molecules on the metallic surfaces and hence the mechanism of corrosion inhibition. In our investigation, Langmuir adsorption isotherm, the relation between the fractional surface coverage ([Inh.]/θ) of the examined compounds versus their concentrations [Inh.], according to the following equation (Christov and Popova. 2004),

(1)

$$\frac{lnh}{\theta }=\frac{1}{K_{\mathit{ads}}}+\left[lnh\right]$$ where K_ads is the absorptive equilibrium constant, was fitted and is illustrated in Fig. 9. This indicates that the inhibitors adsorption on the surface of Sabic iron was correlated to the Langmuir adsorption isotherm.

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Fig. 9 Langmuir adsorption isotherms for the investigated compounds adsorbed on the surface of Sabis iron in 1.0 M HCl solution at 298 K.

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The values of standard free energy of adsorption (ΔG^o_ads) were computed using the following equation (Shukla and Quraishi, 2009),

(2)

$$\mathrm{\Delta }{G}_{\mathit{ads}}^0=-RTln\left(55.5K_{\mathit{ads}}\right)$$ where, the value 55.5 is the molar concentration of water in solution.

The values of both K_ads and ΔG^o_ads for the four examined complexes were calculated at 298 K and were inserted in Table 5.

Table 5 Values of the adsorptive equilibrium constant (K_ads) for the corrosion of Sabic iron in 1.0 M HCl solution in the presence of a 400 mg/l of the investigated compounds at 298 K.

Value \ Inh.	EOIT-Co	BOIT-Co	EOIT-Cu	BOIT-Cu
10−3 Kads (l mol−1)	4.35	4.23	4.72	4.67
ΔGoads (kJ mol−1)	30.98	30.92	31.19	31.17

3.8.5

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 Fig. 10(a–f). Fig. 10 (a) and (b) show a polished Sabic iron surface before and after 24 h immersion in the corrosive medium, successively. Fig. 10(b) shows a strong destruction of the surface of iron specimen due to its exposure to the corrosive medium. Fig. 10(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 detected 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 efficient corrosion inhibition.

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Fig. 10 SEM images for Sabic iron surfaces: (a) before dipping in the corrosive solution (1.0 M HCl), (b) after 24 h dipping in 1.0 M HCl, (c), (d), (e) and (f) after 24 h dipping in the corrosive media involving 200 mg/l of the synthesized complexes: EOIT-Co, BOIT-Co, BOIT-Cu and EOIT-Cu, successively, at 298 K.

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