ARTICLE pubs.acs.org/IECR
2-Acetylpyridine-N(4)-Morpholine Thiosemicarbazone (HAcpMTSc) as a Corrosion Inhibitor on Mild Steel in HCl Saravana Loganathan Ashok Kumar, Mayakrishnan Gopiraman, Moorthy Saravana Kumar, and Anandaram Sreekanth* Department of Chemistry, National Institute of Technology-Tiruchirappalli, Tiruchirappalli 620015, Tamilnadu, India ABSTRACT: 2-Acetylpyridine-N(4)-morpholine thiosemicarbazone (HAcpMTSc) was synthesized and investigated as an inhibitor for mild steel corrosion in acid media. Various concentrations of the inhibitor in 1 M HCl solution were tested, and the corrosion inhibition efficiency to mild steel was estimated by means of weight loss measurements, potentiodynamic polarization, and electrochemical impedance spectroscopy. The adsorption equilibrium constant (Kads) and standard free energy of adsorption (ΔG°ads) were calculated. The nature of inhibition was studied by various spectroscopic techniques such as UVvisible, FTIR, and EPR spectroscopies and by using surface analysis methods (i.e., SEM-EDS).
1. INTRODUCTION Mild steel is an important raw material used in a variety of industries such as petroleum and power generation. Hydrochloric acid solution is widely used for pickling, decaling, acid cleaning, oil-well acidizing, and other applications; however, it is highly corrosive and very easily causes damage to system components. Hence, the use of an inhibitor is one of the best known methods for corrosion protection from corrosive acid media. Heterocyclic compounds containing multiple heteroatoms such as O, N, and S act as effective inhibitors for the corrosion of steel in pickling acid media and have been the subject of many publications.15 Inhibitors protect steel from corrosion in acid solutions by preliminarily adsorbing onto the steel surface. For the past several decades, thiosemicarbazones and their derivatives have been investigated for their biological properties, and hundreds of transition-metal complexes have been prepared and structurally characterized.6 Thiosemicarbazones have been subject to many reviews wherein different aspects of their coordination behavior leading to excellent properties were discussed.7 Thiourea and thiosemicarbazide have lesser inhibition efficiencies in comparison to the derivative thiosemicarbazone.8 Recently, some thiosemicarbazone derivatives were reported to act as inhibitors of corrosion in mild steel.9,10 It is evident that condensation to an appropriate aldehyde or ketone can markedly improve the inhibition efficiency of thiosemicarbazides. We have previously reported the synthesis and structure of 2-acetylpyridine-N(4)-morpholine thiosemicarbazone (HAcpMTSc).11 The choice of this compound was based on the consideration that it contains good π-electron conjugation, enhancing its coordination, and an abundance of heteroatoms, enhancing its adsorption onto the surface of mild steel. The initial aim of the present investigation was to understand the inhibition properties of HAcpMTSc against mild steel corrosion in pickling acids and a proper understanding of the mechanism of inhibition. The inhibitor action of HAcpMTSc at four different temperatures was followed by means of weight loss and electrochemical techniques such as Tafel polarization and electrochemical impedance spectroscopy r 2011 American Chemical Society
(EIS). Mild steel surface scrapes were subjected to spectroscopic investigation to understand the inhibition mechanism.
2. EXPERIMENTAL SECTION 2.1. Materials and Methods. Mild steel samples having an elemental composition of 0.10 wt % C, 0.34 wt % Mn, and 0.08 wt % P were used. The samples were cut to dimensions of 3 cm 2 cm 0.28 cm, and the surface was abraded with different emery papers up to 4/0 grades and washed with acetone. The cleaned samples were then washed with doubly distilled water and finally dried. Electrochemical experiments were performed using a three-electrode cell assembly with mild steel samples as the working electrode, platinum as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode. AR-grade hydrochloric acid (Merck) and doubly distilled water were used to prepare 1 M acid solutions for all experiments. 2-Acetylpyridine (Fluka), thiourea (Merck), and morpholine (Merck) were used as received. N-Phenyl-N-methyl thiosemicarbazide was prepared according to the reported procedure of Scovil et al.12 and then transaminated using morpholine and condensed with 2-acetylpyridine to yield 2-acetylpyridine-N(4)-morpholine thiosemicarbazone (HAcpMTSc)11 (Figure 1) in good yield (80%); the product was characterized by spectral and elemental analysis methods. Transamination is a process by which the amine group in one molecule is transferred to another molecule, thereby incorporating further diversity into the molecule. Microanalyses (CHNS) were carried out using a Heraeus elemental analyzer at CDRI, Lucknow, India. 2.2. Weight Loss Experiments. Different mild steel samples were immersed in hanging positions in 1 M HCl solutions containing different inhibitor concentrations for 2 h.13 Samples Received: March 12, 2011 Accepted: May 23, 2011 Revised: May 19, 2011 Published: May 23, 2011 7824
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was calculated using the equation IE ð%Þ ¼
Figure 1. Scheme for the synthesis of HAcpMTSc: 1a is subjected to transamination by reaction with morpholine, and the resultant 1b is condensed with 2-acetylpyridine.
were weighed before and after immersion, and the weight differences were determined. The experiments were done in triplicate at the same time, and the weight losses were averaged. Variable concentrations of inhibitor starting from 50 ppm by weight in the range of 100, 200, 300, 400, 500, and 600 ppm were used, and it was inferred that the inhibition efficiency reached an optimum value at 300 ppm by weight of the inhibitor in 1 M HCl. The degree of surface coverage (θ) and percentage inhibition efficiency (IE, %) were calculated from the equations14,15 inhibition efficiency ðIE; %Þ ¼ surface coverage ðθÞ ¼
W0 W 100 W0
ð1Þ
W0 W W0
ð2Þ
where W0 and W are the weight losses of mild steel without and with the inhibitor, respectively, and it was assumed that the surface was saturated with adsorbed inhibitor molecules, that is, θ f 1. Within the limits of this assumption, a measurement of the corrosion rate yields a measurement of the coverage of the adsorbed species. Frequently, an adsorption/desorption equilibrium is obtained and described by a simple Langmuir adsorption isotherm such that θ ¼ K°Cinh =ð1 þ K°Cinh Þ
ð3Þ
where K° is the equilibrium constant for adsorption/desorption and Cinh is the inhibitor concentration.16 This experiment was repeated at the four different temperatures of 300, 310, 320, and 330 K to determine the temperature dependence of the inhibition efficiency. 2.3. Tafel Polarization Studies. Electrochemical measurements were carried out in a conventional three-electrode cylindrical glass cell, using a CH electrochemical analyzer model 604B electrochemical workstation maintained at a temperature of 300 K. A platinum electrode was used as a counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. Before recording of the polarization curves, the solution was deaerated for 20 min, and the working electrode was maintained at its corrosion potential for 10 min until a steady state was obtained. The mild steel surface was exposed to various concentrations (50300 ppm by weight) of HAcpMTSc in 100 mL of 1 M HCl at 300 K. The inhibition efficiency (IE, %)
I0 I 100 I0
ð4Þ
where I0 and I are the corrosion current density without and with the inhibitor, respectively The potentiodynamic currentpotential curves were recorded by changing the electrode potential automatically from 750 to þ150 mV vsrsus the open-circuit potential at a scan rate of 20 mV/min. The same experiment was repeated at a scan rate of 600 mV/min. The corresponding corrosion current (Icorr) was recorded. Tafel plots were constructed by plotting E versus log I; corrosion potential (Ecorr), corrosion current density (Icorr), and cathodic and anodic slopes (βc and βa) were calculated according to known procedures.1719 2.4. Electrochemical Impedance Spectroscopy (EIS). Impedance measurements were carried out in the frequency range from 0.1 to 10000 Hz using an amplitude of 20 mV and 10 mV peak to peak with an ac signal at the open-circuit potential. The impedance diagrams were plotted in the Nyquist representation. Charge-transfer resistance (Rct) values were obtained by subtracting the high-frequency impedance from the low-frequency impedance. The percentage inhibition efficiency (IE, %) was calculated from the equation IE ð%Þ ¼
Rct R0ct 100 Rct
ð5Þ
where R0 ct and Rct are the corrosion current of mild steel without and with the inhibitor, respectively. The double-layer capacitance (Cdl) calculated18 from the equation Cdl ¼
1 2πfmax Rct
ð6Þ
where fmax is the frequency at the maximum on the Nyquist plot. 2.5. Surface Analysis and Spectroscopy. Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) was measured in a Hitachi SU 6600 instrument at NIT Calicut, Kerala, India. Necessary samples were prepared by dipping the mild steel sample in HCl and HCl containing 300 ppm inhibitor. Electronic spectra and IR spectra in the mid- and far-IR regions were recorded using samples that were scraped off the surface of the mild steel it had been dipped in HCl containing 300 ppm inhibitor for 2 h. The X-band continuous-wave (CW) electron paramagnetic resonance (EPR) spectrum of the scraped sample (hereafter FeTSc) was also recorded in dimethylformamide (DMF) solution at 77 K, using a modulation amplitude of 0.1 mT and a modulation frequency of 100 kHz. Infrared spectra were recorded on a Shimadzu DR8001 series FTIR instrument as KBr pellets in the range of 4004000 cm1 and far-IR spectra were recorded in the range of 50500 cm1 on a Nicolet MAGNA 550 FTIR spectrometer using polyethylene pellets at SAIF, IIT, Bombay, India. Electronic spectra were recorded in the 900250-nm range on a PG Instruments T90þ UVvisible spectrophotometer in DMF solution. 1H NMR spectra were obtained in a Bruker DRX 300 MHz instrument using CDCl3 as the solvent and tetramethylsilane (TMS) as the internal reference at CARISM, SASTRA University, Tanjore, Tamilnadu. 7825
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3. RESULTS AND DISCUSSION
corresponding to the azomethine group was found around 333 nm as a shoulder. The NMR spectrum of HAcpMTSc in CDCl3 showed no resonance peak at δ 4.11 ppm assignable to S—H proton, but it did show a peak at δ 7.57 ppm assignable to the secondary N—H protons. This peak disappeared upon addition of D2O, also confirming it as an exchangeable proton. This also supports the fact that, in solution, the predominant tautomer is also the thione form under the measurement conditions. A singlet of three protons at δ 2.6 ppm was attributed to the methyl protons, which are chemically and magnetically equivalent. Aromatic protons appear as a multiplet in the δ 7.27.4 ppm range. The X-ray crystal structure of the compound reported elsewhere by one of us11 confirms the spectral data assignments. 3.2. Corrosion Inhibition Studies 3.2.1. Weight Loss Experiments. The weight losses of the mild steel samples due to corrosion obtained from different experiments upon the addition of different concentrations of HAcpMTSc and thiourea (50600 ppm by weight) with a 2-h immersion time in 1 M HCl solutions at 300 K are reported in Table 1. The experiments were done simultaneously in triplicate and were averaged. The degree of surface coverage (θ) was calculated from eq 2. The inhibition efficiency was calculated using eq 1. Even though experiments were conducted at higher concentrations of HAcpMTSc (i.e., 400, 500, and 600 ppm), there were no improvement in the IE, and 300 ppm was found
3.1. Characterization of HAcpMTSc. Synthesis of 2-acetylpyridine-N(4)-morpholine thiosemicarbazone (HAcpMTSc) was achieved using reported procedures. Elemental analysis data were in good agreement to the calculated data showing sufficient purity in the material. CHNS (%) found (calcd): C 54.4 (54.52), H 6.12 (6.10), N 21.01 (21.19), S 11.9 (12.13). IR (KBr) cm1: 1303 and 892 (CdS stretch), 1627 (CdN stretch), 1104 (N—N stretch), 2847 (C—H stretch), 1593 (CdC stretch). The Schiff bases of thiosemicarbazone type contain the thioamide function —NH—C(S)—NR2; consequently, they exhibit thionethiol tautomerism. In the solid state, the compound HAcpMTSc remains in the thione form, as indicated by the absence of the ν(S—H) band expected to be at 2600 cm1 in the IR spectrum of the compound.20 A sharp ν(CdS) band and a low-intensity δ(CdS) band are seen at 1380 and 892 cm1, respectively, also confirming the presence of the thione form in the solid state. The azomethine ν(CdN) stretching vibration for HAcpMTSc was found at 1627 cm1. The ν(N—N) stretching band was observed with medium intensity at 1104 cm1. The band at 2847 cm1 was due to the C—H bond stretching. The electronic spectrum of the HAcpMTSc ligand was recorded in 102 M DMF solution (Figure 7a). Bands around 274 and 341 nm are attributed to n f π* and π f π* transitions arising from the thiocarbonyl group. The second n f π* band
Table 1. Inhibition Efficiency Obtained by Weight Loss of Mild Steel in 1 M HCl Containing Various Concentrations of HAcpMTSc and Thiourea at 300 K HAcpMTSc
thiourea weight
concentration weight lossa
(ppm by weight)
lossa
IE
2
θ
(mg cm ) (%)
σ
b
IE
(mg cm2) (%)
θ
σb
0.05
blank
31.40
50
12.10
61.5 0.62
0.09
31.42 18.22
42.0
0.42
100
10.04
68.0 0.68
0.09
16.13
48.6
0.47
0.03
200
7.52
76.0 0.76
0.06
15.12
51.9
0.52
0.06
300
5.73
81.7 0.82
0.03
13.11
58.3
0.58
0.07
400
5.70
81.8 0.82
0.07
12.32
60.8
0.61
0.09
500 600
5.67 5.45
81.9 0.82 82.6 0.83
0.05 0.03
11.23 11.02
64.3 64.9
0.64 0.65
0.01 0.03
a Weight losses reported are means of triplicate measurements. b Standard deviation (σ) calculated for IE for different temperatures.
Figure 2. Tafel polarization curves of mild steel in 1 M HCl at 300 K in the presence of HAcpMTSc at different concentrations.
Table 2. Inhibition Efficiency Obtained by Weight Loss of Mild Steel in 1 M HCl Containing Various Concentrations of HAcpMTSc at Different Temperatures 310 K
a
320 K
a
HAcpMTSc
weight loss
(ppm by weight)
(mg cm2)
blank
73.10
50
330 K
a
a
weight loss IE (%)
θ
σb
42.39
42.0
0.42
0.09
100 200
39.89 34.12
45.4 53.3
0.45 0.53
300
32.19
56.0
0.56
(mg cm2)
weight loss IE (%)
θ
σb
61.54
34.3
0.34
0.04
0.03 0.08
59.54 48.35
36.4 48.3
0.36 0.48
0.07 0.03
0.08
47.47
49.3
0.49
0.02
93.67
(mg cm2)
IE (%)
θ
σb
76.94
28.1
0.28
0.04
71.80 64.54
32.9 39.7
0.33 0.40
0.05 0.06
59.45
44.4
0.44
0.01
107.0
Weight losses reported are means of triplicate measurements. b Standard deviation (σ) calculated for IE for different temperatures. 7826
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to be optimal. The thiourea derivative of thiosemicarbazone HAcpMTSc (82%, 300 ppm) had a higher inhibition efficiency than thiourea (58%, 300 ppm). The addition of inhibitor led to a decrease of the corrosion rate even at elevated temperatures of 310, 320, and 330 K (50300 ppm), as summarized in Table 2. The increase in corrosion rate was more pronounced with the rise in temperature for uninhibited acid solution. 3.2.2. Tafel Polarization. The potentiodynamic polarization curves of mild steel in 1 M HCl solution in the presence and absence of different inhibitor (HAcpMTSc) concentrations is shown in Figure 2. As can be seen from the figure, after addition of inhibitor, a decrease in both cathodic and anodic currents was observed. The electrochemical corrosion kinetic parameters, namely, the corrosion potential (Ecorr), cathodic and anodic Tafel slopes (βc, βa), and corrosion current density (Icorr), obtained from the Tafel polarization curves are listed in Table 3. Percentage inhibition efficiency (IE, %) was calculated from eq 4. It can be clearly seen that the inhibition efficiency Table 3. Inhibition Efficiency Obtained by Tafel Polarization of Mild Steel in 1 M HCl Containing Various Concentrations of HAcpMTSc at 300 K inhibitor Ecorr
βc
βa
(ppm by
Icorr
(mV vs
(mV/
(mV/
IE
weight)
(μA cm2)
SCE)
dec)
dec)
(%)
concentration
θ
HAcpMTSc (scan rate = 20 mV/min) blank 50
630.95 120.22
495 491
125 100
120 121
80.9 0.809
100
79.43
498
134
130
87.4 0.874
200
70.79
503
78
64
88.8 0.888
300
50.11
510
66
69
92.1 0.921
HAcpMTSc (scan rate = 600 mV/min) blank
645
520
200
276
50
125
515
155
260
80.6 0.806
100 200
85 78
540 532
140 158
106 232
86.8 0.868 87.9 0.879
300
51
539
141
229
92.1 0.921
increased with increasing concentration of the inhibitor. The maximum inhibition efficiency (92%) was reached at an optimum concentration of 300 ppm of inhibitor and was found to be almost unchanged at a high scan rate of 600 mV/min, as well as at a slow scan rate of 20 mV/min, as suggested by the ASTM standards. This indicates a superior coordination of the inhibitor to the surface of the mild steel. The fact that the polarization curves show a clear shift to more negative potential with increasing inhibitor concentration indicates that HAcpMTSc acts as cathodic-type inhibitor in 1 M HCl.21 3.2.3. Electrochemical Impedance Spectroscopy (EIS). The effect of the inhibitor (HAcpMTSc) concentration on the impedance behavior of mild steel in 1 M HCl solution at 300 K is presented as Nyquist plots in Figure 3a. The Nyquist plots contain a depressed semicircle, whose size increased with increasing inhibitor (HAcpMTSc) concentration. The electrochemical corrosion kinetic parameters, namely, the charge-transfer resistance (Rct) and double-layer capacitance (Cdl), were calculated from the Nyquist plot and are listed in Table 4. The percentage inhibition efficiency (IE, %) was calculated from eq 5. The maximum inhibition efficiency (89.7%) was achieved at an inhibitor concentration of 300 ppm. The fact that the chargetransfer resistance (Rct) value increased with increasing inhibitor concentration indicates considerable surface coverage by the inhibitor and strong bonding to the surface. The results are comparable to those obtained for previously reported heterocyclic systems.9,22,23 On the other hand, the values of Cdl decreased with increasing inhibitor concentration, which is probably due to a decrease in the local dielectric constant and/ or an increase in the thickness of the electrical double layer, suggesting that the inhibitor strongly adsorbed to the surface of steel. Thiourea was found to have a lower inhibition efficiency (65%) and larger Cdl value compare to thiosemicarbazone (HAcpMTSc) (Table 4). Therefore, a good inhibition efficiency could be obtained at relatively low concentration of the inhibitor thiosemicarbazone (HAcpMTSc). 3.2.4. Adsorption Isotherm Behavior. The adsorption of an organic substance (surfactant in this case) at the metal/ solution interface can be written according to the displacement reaction organicðsolnÞ þ nH2 OðadsÞ f organicðadsÞ þ nH2 OðsolnÞ
Figure 3. Nyquist plots of EIS measurements of mild steel in 1 M HCl at 300 K: Different concentrations of (a) HAcpMTSc and (b) thiourea. 7827
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where n is the number of water molecules removed from the metal surface for each molecule of inhibitor adsorbed. Clearly, the value of n depends on the cross-sectional area of the inhibitor molecule. Adsorption of the inhibitor molecule occurs because of the interaction energy between the inhibitor and the metal surface. A correlation between surface coverage (θ) and inhibitor concentration (Cinh) in the electrolyte can be represented by the Langmuir adsorption isotherm Cinh 1 ¼ þ Cinh Kads θ
ð7Þ
where Kads is the adsorption constant.
Surface coverage values (θ) for the inhibitor are reported in Tables 1 and 2. The best-fit straight line was obtained for the plot of Cinh/θ versus Cinh with a slope of around unity. The correlation coefficient (r2) was used to choose the isotherm type that best fits the experimental data. These plots suggest that the adsorption of HAcpMTSc onto a metal surface follows the Langmuir adsorption isotherm (Figure 4a). From the intercepts of the straight lines with the Cinh/θ axis, the values of Kads were calculated and are summarized in Table 5. The standard free energy of adsorption (ΔG°ads) and the adsorption constant (Kads) are related by the equation ΔG°ads ¼ RT lnð55:5Kads Þ
Table 4. Inhibition Efficiency Obtained by EIS Measurements of Mild Steel in 1 M HCl Containing Various Concentrations of HAcpMTSc and Thiourea at 300 K
ð8Þ
blank
16.19
54.25
50
26.18
33.56
38.16
0.382
100
32.90
26.71
50.79
0.508
Negative values of ΔG°ads indicate the stability of the adsorbed layer on the steel surface and the spontaneity of the adsorption process.24 The dependence of ΔG°ads on temperature can be explained by two cases as follows: (a) ΔG°ads might increase (become less negative) with increasing temperature, which indicates the occurrence of an exothermic process. (b) ΔG°ads might decrease (become more negative) with increasing temperature, indicating the occurrence of an endothermic process. Generally, the magnitude of ΔG°ads around 20 kJ mol1 or less negative is assumed for cases with electrostatic interactions existing between the inhibitor and the charged metal surface (i.e., physisorption). Those around 40 kJ mol1 indicate chemisorption.25 The plots and the data listed in Table 5 reveal that the adsorption of HAcpMTSc on mild steel is an exothermic process. The value is not dramatically changed with increasing temperature, indicating less desorption even at elevated temperatures. The ΔG°ads value is around 30 kJ mol1, indicating that the adsorption is closer to chemisorption.26 The weight loss test slope was not equal to unity, as expected for the ideal case. This phenomenon is probably due to the interaction between the adsorbed molecules on the metallic surface.27,28 The other thermodynamic functions can also be calculated from the equation
200 300
46.15 46.51
19.04 18.89
64.92 65.19
0.649 0.652
ΔG°ads ¼ ΔH°ads TΔS°ads
inhibitor concentration (ppm by weight)
Rct (Ω cm2)
Cdl (μF)
IE (%)
θ
HAcpMTSc (amplitude = 20 mV) blank
16.84
52.18
50
67.02
13.11
74.9
0.749
100
106.96
8.22
84.3
0.843
200
124.49
7.06
86.5
0.865
300
163.22
5.38
89.7
0.897
HAcpMTSc (amplitude = 10 mV) blank 50
16.19 68.09
54.25 12.89
76.2
0.762
100
103.50
8.46
84.4
0.844
200
130.80
6.71
87.6
0.876
300
145.20
6.04
88.8
0.888
Thiourea (amplitude = 10 mV)
ð9Þ
Figure 4. (a) Langmuir adsorption plots for HAcpMTSc on mild steel in 1 M HCl at different temperatures: EIS (300 K) and Tafel polarization (300 K). (b) Dependence of ΔG°ads on temperature for mild steel in 1 M HCl containing HAcpMTSc. 7828
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where ΔH°ads and ΔS°ads are the enthalpy and entropy of adsorption, respectively. The calculated values of free energy were plotted against temperature; straight lines were obtained, as shown in Figure 4b, and ΔH°ads = 31.94 kJ mol1 was found from the graph. The negative value of ΔH°ads reflects the exothermic nature of the adsorption process on steel.29 3.2.5. Surface Analysis. SEM micrographs and the corresponding EDS spectra of the surfaces of the mild steel samples were recorded in order to see the changes that occurred during the corrosion of mild steel in the presence and absence of the inhibitor. Figure 5a shows that the mild steel sample immersed in 1 M HCl was highly damaged; it can be concluded that the mild steel surface was highly corroded in aggressive acid media. This type of corrosion is typical in aggressive acid solutions. Figure 5b shows a smooth surface with deposited inhibitor on the surface of mild steel after the addition of 300 ppm inhibitor to the 1 M HCl solution. It is clearly seen from the SEM images that the irregularities in the surface due to corrosion are absent in the inhibited surface and the surface is almost free from corrosion. The EDS data indicate a correspondingly lower chloride concentration on the inhibited surface (Table 6, Figure 6), along with a decreased surface-exposed Fe composition and a percentage CHNS matching that of HAcpMTSc. This type of Fe composition is typical in complexes of iron. From these results, it can be clearly concluded that HAcpMTSc forms a uniform adsorbed layer on the surface by means of coordination and retards the corrosion. 3.2.6. Spectroscopy. The scraped sample FeTSc was found to be soluble in common organic solvents, giving reddish brown solutions. FeTSc was insoluble in water, and all the test solutions used for the inhibition analysis were colorless, indicating that soluble complexes were not formed during the inhibition. FeTSc Table 5. Estimation of the Equilibrium Adsorption Constant (Kads) and the Free Energy of Adsorption (ΔG°ads) of the HAcpMTSc on Mild Steel Surface Immersed in 1 M HCl Solution
was subjected to thorough spectroscopic analysis by means of UVvis, FTIR, and EPR spectroscopies. UVvisible spectral bands between 250 and 500 nm in the spectra of HAcpMTSc and FeTSc (Figure 7a) were assigned to π f π* and n f π* transitions arising from the ligand moieties and were discussed in section 3.1. In the spectrum of FeTSc, there was a hypsochromic shift in the absorbance, indicating that the same region was being masked by metal-to-ligand charge-transfer (MLCT) bands, and this phenomenon is typical of coordination complexes. d f d transitions of d5 spin-paired systems normally have several bands in various regions of the UVvisible range. A sharp 650-nm absorption (Figure 7b) assigned to the d f d transition provides a confirmation of our assumptions of coordination of HAcpMTSc to the surface. However, the electronic spectra of low-spin iron(III) compounds have not been adequately studied. Table 7 shows the FTIR data of both FeTSc and the inhibitor. The presence of functional-group peaks whose absorption frequencies correspond to thiocarbonyl CdS (shifted from 1380 to 1273 cm1), imine CdN (shifted from 1627 to 1578 cm1), N—N (shifted from 1104 to 1112 cm1), and aromatic C—H stretch (shifted from 2362 to 3065 cm1) indicates strongly that the surface has been coordinated by the ligand.20 In addition, the far-IR stretching of FeL data also confirms a strong coordination on the surface. One of us previously synthesized, structurally characterized, and reported many Fe(III) complexes.30,31 The present sample FeTSc shows remarkable similarities in the spectral data to those of the reported complexes. We attempted the isolation of single crystals of FeTSc but failed. Magnetic moments were determined for FeTSc at room temperature in the polycrystalline state. Iron(III) is known to exist in three states with S = 5/2 (6A1, μ = 5.92 μB), S = 3/2 (4A2, μ = 4.00 μB), and S = 1/2 (2T2, μ = 2.6 μB). The magnetic moment of FeTSc was found to be 1.59 μB, indicating a typical low-spin Fe(III) system. This was confirmed from the X-band CW EPR signal with rhombic features and three g values: gx =1.99, gy = 2.08, and gz = 2.13
slope
r2
Kads (L mol1)
ΔG°ads (kJ mol1)
300
1.14
0.99
3.38 103
30.29
310
1.66
0.99
2.01 103
30.19
320
1.75
0.99
1.75 103
30.14
330
1.97
0.99
1.05 103
30.12
300 (Tafel)
1.06
0.99
1.02 103
33.06
blank
300 (EIS)
1.08
0.99
7.87 103
32.39
300 ppm HAcpMTSc
temperature (K)
Table 6. EDS Analysis Data for Mild Steel in 1 M HCl in the Absence and Presence of HAcpMTSc composition (wt %) Fe 95.88 2.89
O
C
S
N
Cl
4.12
5.95
60.52
0.07
30.24
0.33
Figure 5. SEM micrographs of the surface of mild steel after 2 h of immersion in 1 M HCl: (a) without HAcpMTSc, showing the damaged surface in acid solution, and (b) with HAcpMTSc, showing the inhibited smooth surface. 7829
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Figure 6. EDS spectra of mild steel in 1 M HCl in the (a) absence and (b) presence of HAcpMTSc.
Figure 7. (a) UVvisible spectra of (1) FeTSc and (2) HAcpMTSc recorded in DMF. A hypsochromic shift is visible in FeTSc due to MLCT. (b) Visible region of the FeTSc spectrum in DMF showing the d f d transition.
Table 7. FTIR Data for HAcpMTSc and FeTSc Indicating the Coordination of the Inhibitor to the Mild Steel Surface HAcpMTSc (cm1)
FeTSc (cm1)
1380 1627
1273 1578
functional group CdS stretch CdN stretch
1104
1112
N—N stretch
2847
3065
C—H stretch
2362
2369
C—N stretch
1593
1511
CdC stretch
3402
H2O
ν(Fe—Nazo)
ν(Fe—Npy)
ν(Fe—S)
515 w
354 w
452 w
(marker gTCNE = 2.00277, where TCNE = tetracyanoethylene). The absence of a half-field signal indicates that the surface is completely low-spin Fe(III) and not Fe(II) or high-spin Fe(III). This suggests that the initial surface exposed to HCl gets oxidized
Figure 8. Tentative structure of the complex FeTSc. HAcpMTSc acts as a monoprotic tridendate ligand, and the Fe3þ shown is surface Fe.
to Fe(II) and the presence of oxygen converts the Fe(II) to Fe(III). This exposed surface otherwise is corroding, as the surface oxides of Fe(III) are not passive. However, the presence of HAcpMTSc as a strongly coordinating ligand induces a strong surface protection layer and inhibits the surface through the coordination of Fe(III) to HAcpMTSc. As coordination 7830
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Industrial & Engineering Chemistry Research complexes are normally unstable in acids at elevated temperatures, the high-temperature weight loss measurements (Table 1) show a significant reduction in the inhibition percentage. Based on all of these data and our previous reports of Fe(III) complexes, we suggest a tentative structure of the mild steel surface after inhibition that contains a complex of Fe(III) with the ligand inhibitor. HAcpMTSc acts as a monoprotic tridendate ligand on the surface and forms a complex with the tentative structure shown in Figure 8.
4. CONCLUSIONS HAcpMTSc can act as an effective corrosion inhibitor for mild steel in 1 M hydrochloric acid. The inhibition efficiency was measured through weight loss, potentiodynamic polarization, and EIS. The Tafel data indicate that the inhibition is cathodic in nature and suggest a coordination mechanism. Chemisoption of HAcpMTSc to the metal surface was confirmed by the Langmuir adsorption isotherm. The surface protection was clearly visible in the SEM and EDS images, which gave clear information about the corrosion inhibition of the surface. HAcpMTSc acts as a cathodic inhibitor, retarding the corrosion of the surface through strong coordination, as evidenced by the spectral data of the scraped surface. ’ AUTHOR INFORMATION Corresponding Author
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[email protected]. Tel.: þ91 431 2503642. Fax: þ91 431 2500133.
’ ACKNOWLEDGMENT A.S. thanks the Department of Science and Technology (DST SR/S1/IC-36/2008) for a research grant. S.L.A. acknowledges MHRD, Government of India, for a research fellowship, and M.S. acknowledges CSIR for an SRF fellowship [CSIR No. 01(2304)/ 09/EMR-II]. ’ REFERENCES (1) Bentiss, F.; Traisnel, M.; Vezin, H.; Hildebrand, H. F.; Lagren, M. 2,5-Bis(4-dimethylaminophenyl)-1,3,4-oxadiazole and 2,5-bis(4-dimethylaminophenyl)-1,3,4 thiadiazole as corrosion inhibitors for mild steel in acidic media. Corros. Sci. 2004, 46, 2781. (2) Prabhu, R. A.; Venkatesha, T. V.; Shanbhag, A. V.; Praveen, B. M.; Kulkarni, G. M.; Kalkhambkar, R. G. Quinol-2-thione compounds as corrosion inhibitors for mild steel in acid solution. Mater. Chem. Phys. 2008, 108, 283. (3) Lowmunkhong, P.; Ungthararak, D.; Sutthivaiyakit, P. Tryptamine as a corrosion inhibitor of mild steel in hydrochloric acid solution. Corros. Sci. 2010, 52, 30. (4) Bentiss, F.; Jama, C.; Mernari, B.; Attari, H. E.; Kadi, L. E.; Lebrini, M.; Traisnel, M.; Lagrenee, M. Corrosion control of mild steel using 3,5-bis(4-methoxyphenyl)-4-amino-1,2,4-triazole in normal hydrochloric acid medium. Corros. Sci. 2009, 51, 1628. (5) Abboud, Y.; Abourriche, A.; Saffaj, T.; Berrada, M.; Charrouf, M.; Bennamara, A.; Hannache, H. A novel azo dye, 8-quinolinol-5-azoantipyrine as corrosion inhibitor for mild steel in acidic media. Desalination 2009, 237, 175. (6) Lobana, T. S.; Sharma, R.; Bawa, G.; Khanna, S. Bonding and structure trends of thiosemicarbazone derivatives of metals—An overview. Coord. Chem. Rev. 2009, 253, 977.
ARTICLE
(7) Padhye, S.; Kauffman, G. B. Transition metal complexes of semicarbazones and thiosemicarbazones. Coord. Chem. Rev. 1985, 63, 127. (8) Ebenso, E. E.; Ekpe, U. J.; Ita, B. I.; Offiong, O. E.; Ibok, U. J. Effect of molecular structure on the efficiency of amides and thiosemicarbazones used for corrosion inhibition of mild steel in hydrochloric acid. Mater. Chem. Phys. 1999, 60, 79. (9) Jacob, K. S.; Parameswaran, G. Corrosion inhibition of mild steel in hydrochloric acid solution by Schiff base furoin thiosemicarbazone. Corros. Sci. 2010, 52, 224. (10) Khaled, K. F.; Mghraby, A. E.; Ibrahim, O. A.; Elhabib, O. A.; Ibrahim, M. A. M. Inhibitive effect of thiosemicarbazone derivative on corrosion of mild steel in hydrochloric acid solution. J. Mater. Environ. Sci. 2010, 1, 139. (11) Fun, H. K.; Chantrapromma, S.; Suni, V.; Sreekanth, A.; Sivakumar, S.; Kurup, M. R. P. 4-Morpholinyl{2-[1-(2-pyridinyl) ethylidene]diazan-2-iumylidene}methanethiolate. Acta Crystallogr. E. 2005, 61, 1337. (12) Scovill, J. P.; Klayman, D. L.; Franchino, C. F. 2-Acetylpyridine thiosemicarbazones. 4. Complexes with transition metals as antimalarial and antileukemic agents. J. Med. Chem. 1982, 25, 1261. (13) Rehim, S. S. A. E.; Ibrahim, M. A. M.; Khalid, K. F. The inhibition of 4-(20 -amino-50 -methylphenylazo) antipyrine on corrosion of mild steel in HCl solution. Mater. Chem. Phys. 2001, 70, 268. (14) Emregul, K. C.; Atakol, O. Corrosion inhibition of iron in 1 M HCl solution with Schiff base compounds and derivatives. Mater. Chem. Phys. 2004, 83, 373. (15) ASTM G-31-72, Standard recommended practice for the laboratory immersion corrosion testing of metals. In Annual Book of ASTM Standards; ASTM International: West Conshohocken, PA, 1990; p 401. (16) Volovitch, P.; Gazizzullin, I.; Ruel, F.; Ogle, K. An atomic emission spectroelectrochemical study of corrosion inhibition: The effect of hexamethylenetetramine on the reaction of mild steel in HCl. Corros. Sci. 2011, 53, 1362. (17) Sorkhabi, H. A.; Shaabani, B.; Seifzdeh, D. Effect of some pyrimidinic Schiff bases on the corrosion of mild steel in hydrochloric acid solution. Electrochem. Acta. 2005, 50, 3446. (18) Sorkhabi, H. A.; Shaabani, B.; Seifzdeh, D. Corrosion inhibition of mild steel by some Schiff base compounds in hydrochloric acid. Appl. Surf. Sci. 2005, 239, 154. (19) Hosseini, M.; Mertens, S. F. L.; Ghorbani, M.; Arshadi, R. M. Asymmetrical Schiff bases as inhibitors of mild steel corrosion in sulphuric acid media. Mater. Chem. Phys. 2003, 78, 800. (20) Campbell, M. J. M. Transition metal complexes of thiosemicarbazide and thiosemicarbazones. Coord. Chem. Rev. 1975, 15, 279. (21) Khaled, K. F. New Synthesized Guanidine Derivative as a Green Corrosion Inhibitor for Mild Steel in Acidic Solutions. Int. J. Electrochem. Sci. 2008, 3, 462. (22) Mernari, B.; Elattari, H.; Traisnel, M.; Bentiss, F. Inhibiting effects of 3,5-bis(N-pyridyl)-4-amino-1,2,4-triazoles on the corrosion for mild steel in 1 M HCl medium. Corros. Sci. 1998, 40, 391. (23) Ramesh, S. V.; Adhikari, A. V. N0 -[4-(diethylamino)benzylidine]3-{[8-trifluoromethyl) quinolin-4-yl]thio}propano hydrazide) as an effective inhibitor of mild steel corrosion in acid media. Mater. Chem. Phys. 2009, 115, 618. (24) Abdallach, M. Rhodanine azosulpha drugs as corrosion inhibitors for corrosion of 304 stainless steel in hydrochloric acid solution. Corros. Sci. 2002, 44, 717. (25) Noor, E. A. Temperature Effects on the Corrosion Inhibition of Mild Steel in Acidic Solutions by Aqueous Extract of Fenugreek Leaves. Int. J. Electrochem. Sci. 2007, 2, 996. (26) Migahed, M. A. Electrochemical investigation of the corrosion behaviour of mild steel in 2 M HCl solution in presence of 1-dodecyl-4methoxy pyridinium bromide. Mater. Chem. Phys. 2005, 93, 48. (27) Touhamia, F.; Aounitia, A.; Abeda, Y.; Hammoutia, B.; Kertitb, S.; Ramdanic, A.; Elkacemid, K. Corrosion inhibition of armco iron in 1 M HCl media by new bipyrazolic derivatives. Corros. Sci. 2000, 42, 929. 7831
dx.doi.org/10.1021/ie200487g |Ind. Eng. Chem. Res. 2011, 50, 7824–7832
Industrial & Engineering Chemistry Research
ARTICLE
(28) Scendo, M. The influence of adenine on corrosion of copper in chloride solutions. Corros. Sci. 2008, 50, 2070. (29) Ergun, U.; Yuzer, D.; Emregu, K. C. The inhibitory effect of bis2,6-(3,5-dimethylpyrazolyl)pyridine on the corrosion behaviour of mild steel in HCl solution. Mater. Chem. Phys. 2008, 109, 492. (30) Sreekanth, A.; Fun, H. K.; Kurup, M. R. P. Structural and spectral studies of an iron(III) complex [Fe(Pranthas)2][FeCl4] derived from 2-acetylpyridine-N(4),N(4)-(butane-1,4-diyl) thiosemicarbazone (HPranthas). J. Mol. Struct. 2005, 737, 61. (31) Sreekanth, A.; Kurup, M. R. P. Synthesis, EPR and M€ossbauer spectral studies of new iron(III) complexes with 2-benzoylpyridineN(4),N(4)-(butane-1,4-diyl) thiosemicarbazone (HBpypTSc): X-ray structure of [Fe(BpypTSc)2]FeCl4 3 2H2O and the free ligand. Polyhedron 2004, 23, 969.
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