Organic synthesis and inhibition action of novel hydrazide derivative for mild steel corrosion in acid solutions

Materials Chemistry and Physics xxx (2016) 1e9

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Organic synthesis and inhibition action of novel hydrazide derivative for mild steel corrosion in acid solutions Zeinab A. Abdallah a, Mohamed S. Mohamed Ahmed a, *, M.M. Saleh a, b a b

Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt Chemistry Department, College of Science, King Faisal University, Al-Hassa, Saudi Arabia

h i g h l i g h t s
One-pot synthesis of novel hydrazinyl hydrazide (HDOP) derivative is described.
Both spectral and elemental analyses are used for characterization of the HDOP.
EIS, Tafel plots, SEM and EDX are used in the corrosion study in HCl and H2SO4.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 7 September 2015 Received in revised form 6 February 2016 Accepted 19 February 2016 Available online xxx

A new derivative of hydrazide family, 2-(2-hydrazinyl-1,6-dihydro-6-oxopyrimidin-4-yl) acetohydrazide (HDOP) is synthesized and used as a corrosion inhibitor for mild steel both in 1 M HCl and 0.5 M H2SO4 solutions. A facial synthesis of a novel hydrazinyl hydrazide derivative is accomplished via a one-pot synthesis. Reaction of ethyl 2-(1,2,3,6-tetrahydro-6-oxo-2-thiopyrimidin-4-yl)acetate (1) with hydrazine hydrate in an ethanol refluxing produces the target hydrazinyl hydrazide derivative in a good yield. Different techniques such as IR, NMR and mass spectroscopy are used for characterization of the obtained products. The inhibition of mild steel corrosion using HDOP in different concentrations is studied by electrochemical impedance spectroscopy and polarization measurements. The inhibition efficiency (IE) of HDOP is found to be higher in HCl than that in H2SO4 solution. This is attributed to the stronger adsorption of Cl on the iron surface which enables better synergism between Cl and the protonated inhibitor. The HDOP acts mainly as a cathodic inhibitor (in both acid solutions) and an inhibition efficiency of ~89% was obtained in the HCl solution. The free energy of adsorption obtained by applying Langmuir adsorption isotherm predicts physisorption of HDOP on the iron surface in the HCl solution. © 2016 Elsevier B.V. All rights reserved.

Keywords: Organic compound Chemical synthesis Adsorption Corrosion

1. Introduction Hydrazides are a class of organic compounds that have been demonstrated as promising materials for applications in biological activities such as anti-inflamtory [1e3], anticonvulsant [4e6], antibacterial [7e9] and antitubercular agents [10e12]. Moreover, they have been used as effective corrosion inhibitors for different metals [13e15]. In this context, a great fraction of the used corrosion inhibitors is organic compounds containing nitrogen, sulfur, oxygen and phosphorus in their molecular structures and are simultaneously bearing aromatic and heterocyclic rings [16e24].

* Corresponding author. E-mail addresses: [email protected] (M.S. Mohamed Ahmed), [email protected] (M.M. Saleh).

The inhibition action of such organic compounds is imparted by adsorption on the metal surface providing a barrier between the metal and the corroding solution. The extent of the inhibition action depends on the nature and surface charge of the metal, composition of the solution and the molecular structure of the inhibitor. The latter factor comprises functional groups, aromaticity, p-electrons characteristics of p-bonding orbitals, steric factor, and electron density at the donor atoms [25e30]. Organic compounds of different structures have been known of their inhibition actions for acid corrosion of different metals. The choice of an organic inhibitor relies on the nature of the metal surface and on the molecular structure of the inhibitor [31e34]. Pickling of metals, chemical cleaning and descaling in acid solution can be made efficient and economically accepted in presence of corrosion inhibitors. This can offer a protection for the underlying metal and less consumption of the used acid. Many categories of

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O O

O

O

O NH2CSNH2

+

O

NH

O O

N H

S

1

NH2NH2

CH3I

O O H2N

O NH

N H

N

NH

O

NH2NH2

NHNH2

O

N

3

SCH3

2

Fig. 1. Synthesis of hydrazinyl hydrazide 3 (HDOP).

organic compounds have been used as corrosion inhibitors for mild steel [35e41]. However, toxicity of such organic compounds has been a concern and yet there is a great demand for nontoxic corrosion inhibitors. Of these categories, hydrazides represent an important category [42,43]. This can be explained in the light of the fact that hydrazides are considered to be less toxic or relatively ecofriendly than other categories [44,45]. Also, this category of organic compounds gives reasonable inhibition action towards mild steel corrosion in acid solution [46,47]. In this article, a new organic inhibitor belong to hydrazide family is synthesized using one-pot organic synthetic route. The compound is evaluated as a corrosion inhibitor for mild steel in 1 M HCl and 0.5 M H2SO4 solutions. The study of corrosion inhibition was achieved using electrochemical impedance spectroscopy (EIS) and polarizations measurements. 2. Experimental

Mn, 0.19% Si, 0.03% Ni, 0.02% B, 0.06% P and the remaining iron. The molecular structure and synthesis scheme of the used organic compounds is shown in Fig. 1. Stock solution of the organic compound was prepared either in 1 M HCl or 0.5 M H2SO4 and the desired concentrations were obtained by appropriate dilutions. Bidistilled water was used in preparation of the solutions. The temperature was adjusted at 30 ± 0.2  C using a water thermostat. Electrochemical measurements were performed using Gamry potentiostat/galvanostat supported with Gamry electrochemical analysis technique. Electrochemical measurements were carried out in a conventional three-electrode cell. The iron electrode was fitted into a glass tube of proper internal diameter by using epoxy resins. The exposed surface area of the working electrode is 0.50 cm2. The iron electrode was polished gradually with emery paper down to mirror-like surface. It was then washed with bidistilled water and finally degreased by rinsing with acetone and then dried.

2.1. Organic synthesis

2.1.2. Measurements The melting points were measured using Gallenkamp apparatus and were uncorrected. IR spectra were recorded in potassium bromide using PerkineElmer FT-IR 1650 and Pye-Unicam SP300 infrared spectrophotometers. 1H NMR spectra were recorded in deuterated DMSO using Varian Gemini 300 NMR spectrometer. Mass spectra were recorded on a GCMS-QP 1000 EX Shimadzu and GCMS 5988-A HP spectrometer. Elemental analyses were carried out at the Microanalytical Laboratory of Cairo University, Giza, Egypt. 2.2. Electrochemical measurements The iron sample had a chemical composition of 0.19% C, 0.59%

0.0

-0.1

E / V (A g/A gC l/K C l(sat.))

2.1.1. Materials Diethyl 1,3-acetonedicaboxylate (CAS number 165123), iodomethane (CAS number 67692), thiourea (CAS number T8656), potassium hydroxide (CAS number 221473), hydrazine (CAS number 215155) and ethanol were all purchased from Aldrich (USA) and were used without further purification. The progress of the reaction was monitored by thin layer chromatography (TLC) which was purchased from Aldrich (USA).

-0.2

-0.3

-0.4

-0.5 B1

-0.6 A2 -0.7

-7

-6

-5

-4

-3

-2

B2 A1 -1

0

1

log (i / A cm-2) Fig. 2. Tafel Plots of mild steel in 1 M HCl (A1) and 1 M HCl containing 1  102 M HDOP (A2) and 0.5 M H2SO4 (B1) and H2SO4 containing 1  102 M HDOP (B2).

Please cite this article in press as: Z.A. Abdallah, et al., Organic synthesis and inhibition action of novel hydrazide derivative for mild steel corrosion in acid solutions, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.055

Z.A. Abdallah et al. / Materials Chemistry and Physics xxx (2016) 1e9

3

3. Results and discussion

300

A

3.1. Organic synthesis and characterization

Zim / ohm.cm

2

200

A2

100

B2 0

A1

B1 0

100

200

300

400

Zreal / ohm.cm2 3

B A2

80

A2 B2 A1

2

A1

60

B1

B1

40

θ / degree

log Z / ohm cm

2

B2

1

3.1.1. Synthesis The synthesis strategy adopted for the preparation of the intermediate and the target molecule (hydrazinyl hydrazide) is depicted in Fig. 1. Heating of diethyl 3-oxopenteanedioate with thiourea in presence of potassium hydroxide in ethanol afforded ethyl 2-(1,2,3,6-tetrahydro-6-oxo-2-thiopyrimidin-4-yl)acetate (1). The latter compound was easily methylated via treatment with methyl iodide in alcoholic sodium acetate solution yielding ethyl 2(1,6-dihydro-2-(methylthio)-6-oxopyrimidin-4-yl) acetate (2), which in turn was reacted with hydrazine hydrate in ethanol to give 2-(2-hydrazinyl-1,6-dihydro-6-oxopyrimidin-4-yl) acetohydrazide (3). The FT-IR spectrum of this compound showed the presence of absorption bands at 1636, 1670, 3256 and 3333 cm1 due to (C¼Ohydrazide), (C¼Opyrimidine) and (NH2, NH) functions, respectively. Also IR spectrum clearly revealed the absence of an absorption band at 1724 cm1 which is characteristic of C¼Oester group. Its 1H NMR spectrum reveals three singlet signals corresponding to CH2, pyrimidine-C5 and amino protons at 3.08, 5.44 and 9.07 ppm, respectively. Also, 1H NMR spectrum clearly reveals that there are no signals for the CH3CH2- ester group. Furthermore, the structure of compound 3 was established through its independent synthesis via reflux of compound 1 with hydrazine in ethanol which afforded a product identical in all aspects (mp and IR) with that obtained previously from the reaction of compound 2 with hydrazine hydrate.

20

0

1

2

3

4

0

log f / Hz Fig. 3. (A) Nyquist plots of mild steel in 1 M HCl (A1) and 1 M HCl containing 1  102 M HDOP (A2) and 0.5 M H2SO4 (B1) and H2SO4 containing 1  102 M HDOP (B2). (B) Bode plots at the same notations of 3A.

The counter electrode was made of a platinum sheet. The reference electrode was Ag/AgCl/KCl(Sat.) with a Luggin probe positioned near the electrode surface. The potential in the polarization curves and throughout the text is referred to the above reference electrode. The iron electrode was immersed for 30 min at the free corrosion potential, Ecor in the solution before the electrochemical measurements were recorded. Measurements involving electrochemical impedance spectroscopy, EIS were performed under free corrosion potential. Potentiodynamic polarization curves were recorded using a potentiodynamic technique with a constant scan rate equals 2 mV s1. The EIS measurements were carried out in the frequency range 10 mHze100 kHz and using a signal of amplitude 5 mV peak-to-peak. The measurements were repeated to test the reproducibility of the results. The reproducibility of the measurements for the inhibition efficiency (cf. Table 2 and Fig. 6) was determined by estimating the relative standard deviation (RSD) with (n ¼ 3, i.e., three replicate measurements) and the values of RSD are given (Table 2). The surface of iron sample in absence and presence of the inhibiter was investigated using scanning electronic microscopy (SEM, Model JEOL JSM 5410, Japan) supplied with an energy dispersive X-ray analysis unit (EDX).

3.1.2. Synthesis and structural characterization 3 .1. 2 .1. S y nt h e s i s o f e t hyl 2 - ( 1, 2 , 3, 6 - t e t ra hyd ro - 6 - o xo - 2 thiopyrimidin-4-yl) acetate (1). To a stirred solution of KOH (5.6 g, 0.1 mol) in ethanol, diethyl 1,3-acetonedicaboxylate (20.2 g, 0.1 mol) and thiourea (7.6 g, 0.1 mol) were added at room temperature. The resulting solution was heated under reflux condenser for 3 h. This period was found to be enough for reaction completion as monitored by the thin layer chromatography (TLC). The whole mixture was then left for few minutes to cool. It was then poured onto water and acidified by using concentrated hydrochloric acid solution. The resulting mixture was then left overnight in a refrigerator. The precipitated solid was filtered, washed with water and finally recrystallized from ethanol. White crystals, 70%, mp 150  C (EtOH), IR n (KBr): nNH 3416 cm1, nCO-ester 1713 cm1, nCO1 1 pyrimdinone 1663 cm . H NMR (DMSO-d6, ppm): d 12.48 (s, 1H, NH, D2O exchangeable), 12.37 (s, 1H, NH, D2O exchangeable), 5.84 (s, 1H, pyrimidin-H5), 4.25 (q, 2H, CH2), 3.57 (s, 2H, CH2), 1.21 (t, 3H, CH3). Mass m/z (%): 214 (Mþ, 3%), 142 (100%). Anal. Calcd for C8H10N2O3S: C, 44.85; H, 4.70; N, 13.08; found, C, 45.07; H, 4.66; N, 12.93. 3.1.2.2. Synthesis of ethyl 2-(1,6-dihydro-2-(methylthio)-6oxopyrimidin-4-yl) acetate (2). To a stirred solution of compound 1 (2.12 g, 10 mmol) in ethanolic sodium acetate solution (984 mg, 12 mmol in 30 mL ethanol), iodomethane (excess) was added. The resulting solution was refluxed for 2 h. The excess iodomethane was removed under vacuum. After cooling, the above solution was poured onto ice-water mixture and the solid product was collected by filtration. The so formed solid product was washed with water, dried and then recrystallized from ethanol-water mixture to give compound 2. The latter was in the form of white crystals (60%, mp 128  C). IR n (KBr): nNH 3427 cm1, nCO-ester 1724 cm1, nCO-pyr1 1 imdinone 1687 cm . H NMR (DMSO-d6, ppm): d 12.60 (s, 1H, NH, D2O exchangeable), 6.09 (s, 1H, pyrimidin-H5), 4.12 (q, 2H, CH2), 3.55 (s, 2H, CH2), 2.12 (s, 3H, SCH3), 1.21 (t, 3H, CH3). Mass m/z (%):

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Fig. 4. SEM images and EDX charts for mild steel in 1 M HCl (A) and 0.5 M H2SO4 (B) 1 M HCl containing 1  102 M HDOP (C) and H2SO4 containing 1  102 M HDOP (D).

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5

Fig. 6. Equivalent circuit used to fit the EIS data in Fig. 5.

3.1.2.3. Synthesis of 2-(2-hydrazinyl-1,6-dihydro-6-oxopyrimidin-4yl) acetohydrazide (3). Method (A): To a solution of compound (2) (1.14 g, 5 mmol) in 20 mL ethanol, hydrazine hydrate (2 mL) was added. The resulting solution was refluxed for 6 h. The excess hydrazine was removed under vacuum. After cooling, it was poured onto ice-water mixture and the solid product was collected by filtration. The so formed solid product was washed with water, dried and then recrystallized from water to give compound 3 in the form of white crystals (70%, mp 227  C). IR n (KBr): nNH, NH2 3333, 3256 cm1, nCO-hydrazide 1636 cm1, nCO-pyrimdinone 1670 cm1. 1H NMR (DMSO-d6, ppm): d 9.07 (s, 1H, NH, D2O exchangeable), 5.44 (s, 1H, pyrimidin-H5), 3.08 (s, 2H, CH2). Anal. Calcd for C6H10N6O2: C, 36.36; H, 5.09; N, 42.41; found, C, 36.60; H, 5.35; N, 42.17. Method (B): To a solution of compound (1) (1.06 g, 5 mmol) in 20 mL ethanol, hydrazine hydrate (2 mL) was added. The resulting solution was refluxed for 12 h until no H2S is evolved. The excess hydrazine was removed under vacuum. After cooling, it was poured onto ice-water mixture and the solid product was collected by filtration. The so formed solid product was then washed with water and dried then recrystallized from water to give compound 3 in the form of white crystals (73%, the resulted solid compound was identical in all spectral and physical data). Fig. 5. (A) Nyquist plots of mild steel in 1 M HCl containing different concentrations of HDOP. 1) blank, 1 M HCl, 2) 1  104 M, 3) 5  104 M, 4) 1  103 M, 5) 5  103 M, 6) 1  102 M and 7) 5  102 M HDOP. (B) Bode plots at the same notations of 5A.

Table 1 Equivalent circuit parameters corresponding to EIS data in Fig. 5. C/M 0 1 5 1 5 1 5

     

104 104 103 103 102 102

Rct/U cm2

Yo (Qdl)/mF cm2

Rs/U cm2

Cdl/mF cm2

a

50.3 63.1 77.8 103.7 208.7 369.8 558.8

147.7 137.4 114.6 109.5 103.6 93.0 83.9

0.90 0.95 0.91 0.82 0.86 0.87 0.87

566.1 468.2 382.7 291.6 153.4 93.9 67.9

0.869 0.887 0.887 0.895 0.906 0.908 0.915

Table 2 Electrochemical parameters derived from polarization curves (Fig. 8). C/M 0 1 5 1 5 1 5

     

104 104 103 103 102 102

Ecor/V (SCE)

icor/A cm2 (x106)

Bc/mV dec1

%IEicor

RSD %

0.331 0.357 0.356 0.355 0.360 0.355 0.350

513 431 363 265 102 71 52

121 118 126 129 116 126 123

e 15.9 29.2 48.4 80.2 86.2 89.8

e 2.2 2.1 2.2 1.9 1.2 1.1

228 (Mþ, 20%), 186, 156, 135 (100%). Anal. Calcd for C9H12N2O3S: C, 47.35; H, 5.30; N, 12.27; found, C, 47.07; H, 5.45; N, 12.85.

3.2. Electrochemical and corrosion study 3.2.1. Comparative corrosion study An attempt was done to test the difference in the inhibition action of HDOP for mild steel corrosion in HCl and H2SO4 solutions. This was done by testing Tafel and Nyquist plots for mild steel in 1 M HCl and 0.5 M H2SO4 containing the same concentration of the inhibitor (HDOP). Fig. 2 shows Tafel plots for mild steel in 1 M HCl and 0.5 M H2SO4 in absence of the inhibitor (curves A1 and B1, respectively) and in presence of 1  102 M HDOP (curves A2 and B2, respectively). In both media, the cathodic Tafel lines were shifted to lower values in presence of the HDOP and hence to lower currents with respect to that in blank solutions. However, the shift in case of HCl is higher than that in case of H2SO4 indicating a more inhibition action of HDOP to mild steel corrosion in HCl compared to that in H2SO4. Fig. 3A shows Nyquist plots for mild steel in 1 M HCl and 0.5 M H2SO4 in absence of the inhibitor (A1 and B1, respectively) and in presence of 1  102 M HDOP (A2 and B2, respectively). The Nyquist plot reveals a depressed semicircle with the size depends on the acid type and on the presence of the HDOP. The diameter of the semicircle depends on the medium. A higher diameter is obtained in presence of the inhibitor. The increase in the diameter is more pronounced in case of the HCl, indicating higher impedance to charge transfer and hence stronger inhibition action of the HDOP for mild steel corrosion in HCl compared to that in case of H2SO4. The inhibition efficiency of HDOP (see Eq. (4)) driven from Nyquist plots (i.e., from Rct values) was found to be 85.4 and 72.3

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respectively in, HCl and H2SO4 containing 1  102 M of HDOP. This confirms the lower extent of adsorption of HDOP on the mild steel in H2SO4 than that in the HCl solution. This may be illustrated as follows. HDOP is considered to have different positively charged moieties due its protonation in the acid medium. Specific adsorption of chloride ions on the iron surface modifies the surface charge to negatively charged surface [48,49]. The latter surface facilitates adsorption of the protonated organic compound. We may conclude that the medium of choice is going to be HCl solution and yet it is of practical importance since pickling of mild steel is usually done in HCl solutions. Fig. 3B shows the corresponding Bode Diagrams at the same notations of Fig. 3A. The value of the phase angle, q provides an indication of the inhibition action. The Bode diagrams in Fig. 3B give one time constant (single maximum) at moderate frequencies. Meanwhile, a broadening of the maximum in presence of the inhibitor is attributed to the adsorption of the HDOP at metal/solution interface [23,24]. The values of log Z and phase angle fall to zero at the side of high frequency which is a characteristic resistive of the solution existed between the reference electrode and the mild steel electrode. A linear relationship between log Z with log f with a slope value close to 1 and phase angle value < 70 has been observed whereas an ideal capacitor is characterized by a value of phase angle and slope equal to 90 and 1, respectively [23,24]. The mechanism of iron dissolution in acid chloride solution can be written as; [50,51].

Fe þ Cl 4 FeCl ads

(1.a)

 FeCl ads 4FeClads þ e

(1.b)

FeClads /FeClþ þ e ads

(1.c)

FeClþ 4Fe2þ þ Cl ads

(1.d)

Also, the mechanism of iron dissolution in acid sulfate solution can be given as; [50,51].

Fe þ H2 O4 FeOHads þ Hþ þ e rds

(2.a)

FeOHads ƒ! FeOH þ þ e

(2.b)

FeOHþ þ Hþ 4 Fe2þ þ H2 O

(2.c)

The values of the anodic Tafel slopes for mild steel in HCl and H2SO4 do not change in presence of the HDOP. For instance, in analysis of the Tafel plots in Fig. 2, it is found to be 121 and 132 mV dec1 in HCl and H2SO4 and 123 and 139 mV dec1 in presence of 5  102 M of HDOP in HCl and H2SO4, respectively. The above results indicate that the presence of the inhibitor does not change the mechanism of iron dissolution at the present conditions. Hydrochloric acid is widely used in acid pickling since it is more favorable than other mineral acid from both economic and technical point of view [52]. The main advantage of this acid over other acids is concerned with its ability to form metal chloride, which is easily soluble in aqueous solutions, compared to sulfate, phosphate and nitrate. The higher solubility of chloride salt results in lowest extent of polarizing effect and does not hinder the rate of corrosion [53,54]. The above discussion for the different corrosion inhibition behaviors in HCl and H2SO4 can be further examined by taking SEM images and EDX charts. Fig. 4 shows SEM images (A-D) and EDX charts (in the same panel) for the mild steel samples after immersion for 2 h in (A) blank HCl, (B) blank H2SO4, (C)

HCl þ 5  102 M HDOP and (D) H2SO4 þ 5  102 M HDOP. The images in blank (A,B) show higher extent of surface damage, cracks and fractures than that revealed in the presence of the inhibitor (C,D). The EDX chart in HCl (A) shows stronger peak of Cl and that in H2SO4 (B) shows a weaker peak of S. This is due to higher adsorbability of the Cl ions than that of the SO2 4 ions. In presence of the inhibitor (images C,D), better surface integrity is revealed which confirm the inhibition action of the HDOP. A better morphology in case of HCl (image C) is revealed which points to better inhibition action of HDOP in HCl than in H2SO4. The appearance of N (nitrogen element) peaks in EDX charts (C,D) (i.e., in presence of the inhibitor in solution) is evident for the adsorption of the HDOP on the metal surface. 3.2.2. Results in HCl solution The EIS measurements were achieved to determine important impedance parameters of the metal/solution interface in presence and absence of the inhibitor. Fig. 5A shows Nyquist plots for mild steel in 1 M HCl containing different concentrations of the HDOP. The Nyquist plots are; 1) blank, 1 M HCl, 2) 1  104 M, 3) 5  104 M, 4) 1  103 M, 5) 5  103 M, 6) 1  102 M and 7) 5  102 M HDOP in 1 M HCl. The plots reveal depressed semicircle with a size dependent on the inhibitor concentration. It is obvious that the semicircle size increases with the inhibitor concentration indicating an increase in the charge transfer resistance of the metal/ solution interface. The EIS data in Fig. 5 was analyzed using Gamry electrochemical software. The equivalent circuit used for fitting the EIS data in Fig. 5 is displayed in Fig. 6. It consists of Rs, CPE and Rct which respectively represent, the solution resistance, a constant phase element corresponding to the double layer capacitance and the charge transfer (polarization) resistance associated with the electrochemical process. A simulation of the impedance data in Fig. 5 is performed by replacing the capacitor, C with the CPE in the equivalent circuit (Fig. 6). This CPE is denoted as Qdl in Table 1. This is in agreement with the approaches found in literature [55,56]. The use of CPE in modeling is to account for frequency dispersion behavior corresponding to some physicochemical processes such as surface inhomogeneity resulting from surface roughness, distribution of the active sites, dislocations, impurities and adsorption of inhibitors [55,56]. The impedance (ZCPE) of a constant phase element is defined as ZCPE ¼ [Yo(ju)a]1, where 1  a  1, j ¼ (1)1/2 and u ¼ 2pf is the angular frequency in rad/s, f is the frequency in Hz ¼ s1. Note that a is a fitting parameter which is an empirical exponent equals; 1 for a perfect capacitor, 0 for a perfect resistor, 1 for inductor and 0.5 for Warburg impedance. In this complex formula an empirical exponent (a) (see Table 1) varying between 0 and 1, is introduced to account for the deviation from the ideal capacitive behavior due to surface heterogeneity, roughness factor and adsorption effects [55,56]. In all cases, good agreement between theoretical and experimental Nyquist plots was obtained for the whole frequency range. The estimated parameters are given in Table 1. The range of the a values obtained from the fitting procedures is 0.80e0.90 which points to the fact that the iron/electrolyte interface does not behave as a perfect capacitor. The value of the double layer capacitance of the metal/solution interface, Cdl is listed in Table 1 and it is estimated from the relation;

Cdl ¼

ðYo xRct Þ1=a Rct

(3)

As the [HDOP] increases, the values of Rct increases and Cdl decreases pointing to the inhibition action of the HDOP for acid corrosion of mild steel. The change in values of Rct and Cdl with the inhibitor concentration (see Table 1) may be attributed to the

Please cite this article in press as: Z.A. Abdallah, et al., Organic synthesis and inhibition action of novel hydrazide derivative for mild steel corrosion in acid solutions, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.055

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 %IE ¼

1

-0.1

-0.2

E / V (A g/A gC l/K C l(sat))

gradual replacement of water molecules with the inhibitor molecules on the iron surface. Consequently, the number of active sites necessary for corrosion process decreases. Formation of a protective layer and an increase in the thickness of the protective film on the iron surface results in an increase in the Rct and a decrease in Cdl. Fig. 5B shows Bode diagrams of the mild steel in 1 M HCl containing different concentrations of the HDOP. As the [HDOP] increases, the broadening of the phase angle at moderate frequencies increases. The deviation from the ideal capacitive behavior (since q ¼ (60) to (70) ) is attributed to the roughness of the mild steel surface. The Bode plots demonstrate that the increase in the impedance and the shifts in the phase angle in the presence of the HDOP are attributed to the inhibition action of the inhibitor [23,24]. The inhibition efficiency, IE% can be determined from the above EIS parameters using the following equation;

7

-0.3

-0.4

-0.5

-0.6

 Rct ð1Þ  100 Rct ð2Þ

(4)

where Rct(1) and Rct(2) are the charge transfer resistances in absence and presence of the inhibitor, respectively. Fig. 7 depicts the relation between the IE% and the inhibitor concentration. As the [HDOP] increases, the IE% increases before it reaches a nearly constant value at [HDOP]  1  102 M. At low concentration the inhibitor molecules replace the adsorbed water molecules on the iron surface. This replacement extent increases with the [HDOP] and more HDOP molecules replace more of the adsorbed water molecules on the metal surface. At high concentration, [HDOP]] > 1  102 M, the iron surface maybe saturated (covering most of the corrosion active sites) with the inhibitor molecules and yet the IE% does not increase anymore even at higher [HDOP]. Fig. 8 shows Tafel plots for mild steel in 1 M HCl containing different concentrations of the HDOP. The presence of the inhibitor shifts the cathodic lines to lower values. This indicates lower currents and hence lower corrosion rates of mild steel in presence of the HDOP compared to that in blank. The inhibitor does not show any significant effects on the anodic lines. It can be concluded that the inhibitor acts mainly as a cathodic inhibitor in the HCl solution.

7 -8

-7

-6

-5

-4

-3

1 6 5 4 32 -2

-1

-2

log (i / A cm ) Fig. 8. Polarization curves of mild steel in 1 M HCl containing different concentrations of HDOP. 1) blank, 1 M HCl, 2) 1  104 M, 3)5  104 M, 4) 1  103 M, 5) 5  103 M, 6) 1  102 M and 7) 5  102 M HDOP.

The electrochemical parameters of mild steel in presence of different inhibitor concentrations are listed in Table 2. These include the free corrosion potential, Ecor, corrosion current density, icor, cathodic Tafel slope, Bc and inhibition efficiency, %IEicor. The cathodic Tafel lines show similar slope either in presence or absence of HDOP. It indicates that the mechanism of the cathodic reaction does not change in presence of the inhibitor and the inhibition action is achieved by simple blocking of the iron surface. It should be pointed out that the inhibition efficiency, %IEicor of the inhibitor was determined from the corrosion currents, icor in presence and absence of the inhibitor. It can be determined from icor values according to the following equation;

 %IEicor ¼

icor1  icor2 icor1

  100

(5)

100

where icor1 and icor2 are the corrosion current densities in absence and presence of the inhibitor, respectively. In general, the values of IEicor can be compared to the corresponding values obtained from the EIS data (see Fig. 7).

80

3.2.3. Adsorption isotherm Adsorption of organic inhibitor on metal surface is always accompanied by replacement of water molecules adsorbed on the metal surface with organic molecules according to the following equation;

IE / %

60

40

OrgðsolÞ þ nH2 OðadsÞ4OrgðadsÞ þ nH2 O 20

0 -4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

-1.0

log ([HDOP] / mol L-1) Fig. 7. Inhibition Efficiency, %IE of mild steel corrosion in presence of different concentrations of HDOP in 1 M HCl.

(6)

where Org(sol) and Org(ads) are the organic molecules present in the aqueous solution and adsorbed on the metallic surface, respectively, H2O(ads) is the absorbed water molecules on the metal surface and n is the size ratio representing the number of water molecules replaced by one molecule of the organic adsorbate. The values of surface coverage, q corresponding to some extent of the inhibitor adsorption can be estimated at a specific cathodic potential (0.55 V in our case) under the condition of equal Tafel slopes [57]. The latter condition is valid as evidence from Fig. 8 and

Please cite this article in press as: Z.A. Abdallah, et al., Organic synthesis and inhibition action of novel hydrazide derivative for mild steel corrosion in acid solutions, Materials Chemistry and Physics (2016), http://dx.doi.org/10.1016/j.matchemphys.2016.02.055

8

Z.A. Abdallah et al. / Materials Chemistry and Physics xxx (2016) 1e9

Table 2 (where Bc was found to be equal in absence and presence of HDOP). The surface coverage, accordingly is given by; [57].

q¼1

i1 i2

(7)

where i1 and i2 are the current densities in presence and absence (blank) of the organic inhibitor, respectively at 0.55 V. The values of q were obtained at different inhibitor concentrations. The data of the surface coverage were found to fit with Langmuir isotherm which is given by:

q ¼ KC 1q

(8)

  DGoads 1 exp  K¼ 55:5 RT

(9)

where C is the concentration of the inhibitor in the bulk of the solution, DGoads is the standard free energy of adsorption. Fig. 9 shows Langmuir adsorption isotherm for HDOP on mild steel. A reasonable straight line is obtained (r2 ¼ 0.98) with a slope equal z 1.0. The standard free energy of adsorption, DGoads was determined from the intercept of the plot in Fig. 9. It is found to be 25.1 kJ mol1. The above value may suggest physisorption of HDOP on the iron surface. In this context, it is believed that in this acid HCl solution, the organic molecules is protonated and HDOP molecules acquire positive charge. In the other hand, the adsorption of Cl ions on the metal surface turns the surface charge to negative surface which facilitates electrostatic attraction (physisorption) between the positive moieties on the HDOP molecules and the negative iron surface. 4. Conclusions A hydrazide derivative (HDOP) was synthesized using an organic synthesis route and was applied as a corrosion inhibitor for mild steel corrosion in acid solutions. The organic synthesis process involved a facial synthesis of novel hydrazinyl hydrazide derivative.

1.2

log[θ/(1-θ)]

0.8

0.4

0.0

-0.4

-0.8 -3.5

Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.matchemphys.2016.02.055.

where

-4.0

The synthesis process has been accomplished via a one-pot synthesis. The reaction of ethyl 2-(1,2,3,6-tetrahydro-6-oxo-2-thiopyrimidin-4-yl)acetate (1) with hydrazine hydrate in ethanol refluxing produced the target hydrazinyl hydrazide derivative in a good yield. HDOP showed higher inhibition efficiency in HCl compared to that in H2SO4 owing to possible pre-adsorption of the Cl on the iron surface which facilitates adsorption of the protonated HDOP in such acid media. The results in HCl showed that HDOP is a cathodic inhibitor and its data was fitted with Langmuir adsorption isotherm. The inhibitor showed physical adsorption characteristics as revealed from the value of the standard free energy of adsorption.

-3.0

-2.5

-2.0

-1.5

-1

log ([HDOP] / mol L ) Fig. 9. Langmuir isotherm for the adsorption of HDOP on mild steel at 30  C.

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