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The inhibition of mild steel corrosion in hydrochloric and sulfuric acid media using thiosemicarbazone derivative Punita Mourya, Sitashree Banerjee, Rashmi Bala Rastogi, and Madan M Singh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/ie4012497 • Publication Date (Web): 12 Aug 2013 Downloaded from http://pubs.acs.org on August 16, 2013

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Industrial & Engineering Chemistry Research

The inhibition of mild steel corrosion in hydrochloric and sulfuric acid media using thiosemicarbazone derivative Punita Mourya, Sitashree Banerjee, Rashmi Bala Rastogi, Madan Mohan Singh*

Department of Applied Chemistry, Indian Institute of Technology, Banaras Hindu University, Varanasi 221005, India.

*Corresponding Author : E-mail address: [email protected], Tel./fax: +91 542 6702859.

1

ACS Paragon Plus Environment


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ABSTRACT The inhibitive effect

of 4-(N,N-dimethylamino)benzaldehyde thiosemicarbazone

(DMABT) on the corrosion of mild steel (MS) in 1N HCl and 1N H2SO4 solution was investigated by weight loss, potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) measurements. It is inferred on the basis of obtained results that DMABT is a mixed type inhibitor predominantly retarding cathodic reaction in both acidic media through adsorption on MS surface. The adsorption obeys a Langmuir’s adsorption isotherm in both acidic media. The observations regarding energy dispersive X-ray (EDX), Scanning electron microscopy (SEM) and Atomic force microscopy (AFM) confirm the existence of a protective film of inhibitor on MS surface. The molecular adsorption of DMABT was ascertained by Density Functional Theory (DFT) data.

KEYWORDS: Mild steel, Corrosion inhibitor, Weight loss, Polarization, EIS, DFT.

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1. INTRODUCTION Acid solutions, especially hydrochloric and sulfuric acids, are widely used in various industrial processes, such as pickling of iron, chemical cleaning, descaling of boilers and oil well acidification in petroleum exploration. The use of inhibitors is one of the most practical methods for corrosion protection of metallic objects in acidic media1, as well as, for reducing acid consumption occurring during the course of corrosion2. Corrosion inhibitors are the substances which minimize or completely prevent corrosion when added at low concentrations in an aggressive environment. The known inhibitors in acidic media are mostly organic compounds containing N, O and S atoms and/or delocalized π-electrons. Effectiveness of the organic inhibitors depends on their adsorption rates and covering capabilities on metal surfaces. It has been realized from many sources3,4 that adsorption depends on the molecular structure, surface charge of a metal and type of electrolytes. In an aqueous solution, inhibitors are adsorbed by replacing water molecules already adsorbed on the surface. For this, electrostatic interaction between an inhibitor molecule and a metal should be more dominant over that between metal and water molecules. In an inhibitor electron densities of different functional groups, their polarizability and electronegativity are main factors for such interactions. In case of organic inhibitors containing N, O and S, inhibition efficiency5,6 generally increases in the order: O85mV with respect to Ecorr in uninhibited solution, the inhibitor can be defined as a cathodic or anodic type52. From the results shown in Table 4, no systematic variation in Ecorr is seen with change in concentration of DMABT and the maximum displacement is observed as 45mV which indicates that the DMABT is a mixed type of inhibitor. From Table 4 it is apparent that icorr decreases considerably in the presence of DMABT at each experimental concentration. Correspondingly, η increases with the inhibitor concentration. The value of η of the inhibitor in 1N HCl which was observed to be 84.7% for 45µM, increases steadily with concentration and approaches to its maximum value 96.2% at 450µM. Similarly the η values increase from 72.6% to 96.6% on increasing the inhibitor concentration in the same range as listed in Table 6. It was observed from linear polarization studies that the polarization resistance (Rp) increases from 31.0 Ω cm2 to 986.0 Ω cm2 and 15.0 Ω cm2 to 372.0 Ω cm2 from blank to the electrolyte containing 450µM of DMABT in 1N HCl and 1N H2SO4 solutions, respectively (Table 4). The increase in polarization resistance in the presence of the inhibitor suggests that a non-conducting physical barrier of DMABT is formed on MS surface giving the highest inhibiting efficiencies of 96.8% and 96.0% at 450µM in 1N HCl and 1N H2SO4, respectively for which the polarization resistance is also the highest. 3.3 Electrochemical impedance spectroscopy (EIS) EIS measurements were performed to determine the impedance parameter of the MShydrochloric acid and MS-sulfuric acid interface in the absence and presence of 270, 360 and 450µM concentrations of DMABT. Figures 7(a,b) show the Nyquist plots at 298K for MS in 1N 19



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HCl and 1N H2SO4, respectively. The existence of a single semicircle with its centre below xaxis in both the plots indicates the presence of a single charge transfer process during metal dissolution. The impedance spectra consist of large capacitive loop at high frequencies followed by a small inductive loop at low frequency values. The high frequency capacitive loop is usually related to the charge transfer of the corrosion process and double layer behavior. On the other hand, the low frequency inductive loop may be attributed to the relaxation process of the adsorbed intermediates controlling the anodic process53. The shape of the curve remains unchanged in the two electrolytes both in absence and the presence of inhibitor. This indicates that the mechanism of corrosion is not affected by addition of inhibitor54. These capacitive loops in all the cases are not perfect semicircles which can be attributed to the frequency dispersion effect as a result of the roughness and inhomogenity of electrode surface55. Furthermore, the diameter of the capacitive loop in the presence of inhibitor is bigger than that in its absence and its magnitude is a function of inhibitor concentration. This indicates that the impedance of inhibited substrate becomes larger with the increase in DMABT concentration. The corresponding Bode plots are illustrated in Figure S2 (supporting information) for MS in different media at 298K in the absence and presence of inhibitor. It is apparent from these curves that the addition of inhibitor causes an increase in the interfacial impedance which further increase on increasing the concentration of the inhibitor. The single narrow peak in the phase angle plots [Figure S3 (supporting information)] again indicate a single time constant for the corrosion process at the metal/solution interface in both the cases. The increase in the peak heights indicates a more capacitive response of the interface due to the presence of inhibitor molecules at the interface.

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All the impedance diagrams were analyzed in terms of the equivalent circuit (Figure 8) which is a parallel combination of Rct and the constant phase element of double layer (CPE), both in series with Rs 30,56. CPE is generally introduced to describe the frequency dependence of non ideal capacitive behavior. The impedance of the CPE is mathematically expressed57 as : −1

−n

Z CPE = Y0 (iω )

(29)

where, Y0 is a proportionality factor and ‘n’ has the meaning of phase shift. The value of ‘n’ represents the deviation from the ideal behavior58 and it lies between 0 and 1. The value of double layer capacitance (Cdl) can be calculated from CPE parameter values Y0 and n using the expression59.

C dl =

Y0ω

n −1

(30)

sin( n(π / 2))

The values of Rs, Rct, Cdl, CPE and goodness of fit (chi square) were obtained from the above mentioned equivalent circuit and are presented in Table 5. The quality of fitting to the equivalent circuit was judge by chi square value60. The obtained chi square values (0.000430.0013) in Table 5 indicate a good fitting to the proposed circuit. The value of Rct increases while the double layer capacitance decreases with the concentration of DMABT in both acid solutions. The largest effect was observed at 450µM of DMABT which gives Rct value 812.0 Ω cm2 in 1N HCl and 730.0 Ω cm2 in 1N H2SO4 ; Cdl value 20.1 µF cm-2 in 1N HCl and 26.9 µF cm-2 in 1N H2SO4. The increase in Rct values is attributed to the formation of an insulating protective film at the metal/solution interface. The decrease in Cdl values can be attributed to a decrease in local dielectric constant and/or to an increase in the thickness of the electrical double layer, suggesting

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that the inhibitor molecules are adsorbed at the metal/solution interface56. The values of η increases with the concentration of DMABT, and lies at 96.5% in 1N HCl and 98.2% in 1N H2SO4 solution. These results again confirm that DMABT exhibits good inhibitive performance for MS in both solutions. It is heartening to note that the inhibition efficiencies obtained from the employed techniques, weight loss measurement, electrochemical polarization and linear polarization are reasonably in good agreement as shown in Table 6 for HCl and H2SO4 respectively. Thus, DMABT is a very good inhibitor in both the corrodants, and inhibition efficiency in 1N HCl is higher than that in 1N H2SO4 at lower concentration while at higher concentrations, inhibition efficiencies are almost the same in both solutions. 3.3. Surface analysis 3.3.1. SEM-EDX SEM micrographs (Figure 9 a,b,c,d,e) and atomic percentage [At.(%)] of elements obtained from the EDX analysis of MS surface (Table 7) in 1N HCl and in 1N H2SO4 solutions exhibit the changes which occurred during corrosion process in absence and presence of inhibitor. MS surface in 1N H2SO4 (Figure 9 d) was more damaged in comparison to the 1N HCl (Figure 9 b) solution in absence of inhibitor. However, in presence of 450µM DMABT, the surface was remarkably improved and less damage occurred in comparison to their surfaces in absence of inhibitor. This improvement in surface morphology is due to the formation of a good protective film of DMABT on MS surface which is responsible for inhibition. The obtained At.(%) values of elements from EDX spectra of inhibited MS surface shows more intensity of nitrogen and sulfur because of the nitrogen and sulfur of DMABT. The values of At.(%) due to iron in inhibited MS are comparatively smaller than the abraded and corroded 22


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MS samples. The reduction in intensity of At.(%) might be due to overlying inhibitor film. This indicated that MS surface was covered with the protective film of inhibitor molecules. 3.3.2. Atomic Force Microscopy The surface morphology of the MS specimens was further studied by AFM before and after corrosion in the absence and presence of 450µM DMABT. In Figure 10 (a,b,c,d,e) the surface morphology (3D) of the abraded MS sample and in absence and presence of DMABT in 1N HCl and1N H2SO4 are shown. The area and line roughness are listed in Table 8. It is clearly seen from the figure that the steel sample shows a rough surface due to acid corrosion. However, the presence of 450µM of DMABT retarded the corrosion and the surface of the inhibited MS specimen gets smoothened as shown in Figure 10c in HCl with DMABT and Figure 10e in H2SO4 with DMABT. The decrease in roughness was probably due to the formation of adsorbed protective film of DMABT on the MS surface. 3.4. DFT study: The interaction pattern between inhibitor (DMABT) and metal surface The optimized structure of DMABT in its ground state is shown in Figure11. The reactivity of a chemical species is very well defined in terms of frontier orbitals61; the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). According to the frontier molecular orbital theory (FMO) of chemical reactivity, the formation of a transition state is due to an interaction between HOMO and LUMO of reacting species. The smaller the orbital energy gap (∆E) between the participating HOMO and LUMO, the stronger are the interactions between two reacting species62. The results listed in Table 9 show that the interaction between the HOMO of the inhibitor and the LUMO of the metal atom as represented by ∆E2 is stronger than that between the HOMO of the metal atom and the LUMO of the inhibitor (∆E1). In principle the interaction between HOMO of the inhibitor and the LUMO of 23



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the iron atom, ∆E2, should be dominated by the interaction between the HOMO of the Fe atom and the LUMO of the inhibitor, ∆E1, for good inhibitive property63. The adsorption of the inhibitor on the metal surface can occur on the basis of donoracceptor interactions between the lone pair-electron of the hetero atoms present in thiosemicarbazone compound and the vacant d-orbitals of the metal surface atoms64. Substances with high values of EHOMO have a tendency to donate electrons to appropriate acceptors with low energy, empty molecular orbitals64,65. The calculated value of EHOMO (-0.1973 eV) for DMABT being higher than that for Fe (-5.075 eV)63 indicates that DMABT has tendency to donate electrons to vacant d-orbitals of Fe. On the other hand, lower value of ELUMO (-1.747 eV)63 for Fe than that for DMABT (-0.0606 eV) favours Fe to accept electrons. The HOMO and LUMO population of the studied DMABT is shown in Figure 12 (a,b). It could be easily seen that DMABT is fully planar which may result in its significant interaction with metal surface. This observation is in good agreement with the findings of the present work.

4. CONCLUSIONS (1) DMABT acts as a very good inhibitor for the corrosion of MS in both, 1N HCl and 1N H2SO4 solutions. Inhibition efficiency increases with the inhibitor concentration, and the maximum η values are 95.7% (1N HCl) and 97.8% (1N H2SO4) at 450µM. (2) The adsorption of DMABT is a spontaneous process and obeys Langmuir adsorption isotherm irrespective of nature of electrolyte. The free energy of adsorption indicates that the adsorption of DMABT involves both physical and chemical adsorption. (3) DMABT acts as a mixed-type inhibitor, though it retards cathodic reaction to a greater extent. 24


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(4) DFT study and surface characterization support the experimental data and favor the adsorption mechanism of the present inhibitor.

ACKNOWLEDMENTS Authors are highly thankful to Prof. G.V.S. Shastri, Department of Metallurgical Engineering, Indian Institute of Technology (BHU), Varanasi for providing SEM/EDX facilities. Authors are also thankful to the Head, Chemistry Department, Faculty of Science, Banaras Hindu University, Varanasi, for carrying out theoretical calculations.

SUPPORTING INFORMATION Detail of DMABT adsorption in both the acid, the plots of lnKads versus T‑1 (Figure S1), the representation of Z Bode spectra (Figure S 2) and phase angle Bode spectra (Figure S 3) of the corrosion of MS in acidic media without and with different concentrations of DMABT are given in supporting information. This material is available free of charge via the internet at http://pubs.acs.org.

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Captions of Figures

Figure

1.

Chemical

molecular

structure

of

4-(N,N-dimethylamino)benzaldehyde

thiosemicarbazone [DMABT].

Figure 2. Variation of inhibition efficiency (η) and corrosion rate (CR) with 180 and 360µM of DMABT obtained from weight loss experiments at different temperature in 1N HCl and 1N H2SO4 solutions.

Figure 3. Arrhenius plots for MS corrosion rates (CR) in acidic media in absence and presence of 180 and 360µM of DMABT (a) 1N HCl (b) 1N H2SO4.

Figure 4. Transition-state plots for MS corrosion rates (CR) in acidic media in absence and presence of 180 and 360µM of DMABT (a) 1N HCl (b) 1N H2SO4.

Figure 5. Langmuir adsorption isotherm plot of MS in 1N HCl and 1N H2SO4 containing different concentration of DMABT at 298K.

Figure 6. Anodic and cathodic polarization curves for MS in acidic media in absence and presence of various concentrations of DMABTat 298K (a) 1N HCl (b) 1N H2SO4.

Figure 7. Nyquist plots of the corrosion of MS in acidic media without and with different concentrations of DMABT at 298K (a) 1N HCl (b) 1N H2SO4.

Figure 8. Equivalent circuit for MS in both the electrolyte. Figure 9. SEM micrographs of MS surface (a) abraded MS, (b) Blank in 1N HCl, (c) with DMABT in HCl, (d) Blank in 1N H2SO4, (e) with DMABT in H2SO4.

Figure 10. AFM Micrographs of MS surface (a) abraded MS, (b) Blank in 1N HCl, (c) with DMABT in HCl, (d) Blank in 1N H2SO4, (e) with DMABT in H2SO4.

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Figure 11. Optimized structure of DMABT calculated with the B3LYP/6-31G(d) model chemistry.

Figure 12. Frontier molecular orbital diagrams (a) HOMO and (b) LUMO of DMABT by the B3LYP/6-31G(d) model chemistry.

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Captions of Tables Table 1. Variation of inhibition efficiency with different concentration of DMABT obtained from weight loss experiments at 298K temperature in 1N HCl and 1N H2SO4 solutions.

Table 2. Thermodynamic activation parameters of MS in 1N HCl and 1N H2SO4 solutions obtained from weight loss method.

Table 3. Thermodynamic parameters for adsorption of DMABT on MS surface in 1N HCl and 1N H2SO4 acidic solutions at different temperatures.

Table 4. Electrochemical parameters obtained from the polarization curves of DMABT in 1N HCl and 1N H2SO4 solutions at 298K.

Table 5. EIS parameters for the corrosion of MS in 1N HCl and 1N H2SO4 solutions containing DMABT at 298K.

Table 6. Variation of η for MS in acidic media with different concentrations of DMABT by weight loss and electrochemical methods in 1N HCl and 1N H2SO4.

Table 7. Atomic (%) of elements obtained from EDX spectra of MS surfaces in 1N HCl and 1N H2SO4 solutions without and with DMABT at 298K.

Table 8. The area and line roughness obtained from AFM of MS surfaces in 1N HCl and 1N H2SO4 solutions without and with DMABT at 298K.

Table 9. Energy order of the frontier orbital (eV).

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2010, 40, 2135–2143. (35) Avci, G. Corrosion inhibition of indole-3-acetic acid on mild steel in 0.5 M HCl. Colloids Surf. A, 2008, 317, 730-736. (36) Solmaz, R.; Kardas, G.; Culha, M.; Yazici, B.; Erbil, M. Investigation of adsorption and inhibitive effect of 2-mercaptothiazoline on corrosion of mild steel in hydrochloric acid media. Electrochim. Acta. 2008, 53, 5941-5952. (37) Migahed, M.A.; Nassar, I.F. Corrosion inhibition of Tubing steel during acidization of oil and gas wells. Electrochim. Acta. 2008, 53, 2877-2882. (38) Benali, O.; Larabi, L.; Traisnel, M.; Gengembra, L.; Harek, Y. Electrochemical, theoretical and XPS studies of 2-mercapto-1-methylimidazole adsorption on carbon steel in 1 M HClO4. Appl. Surf. Sci. 2007, 253, 6130-6139.

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(55) Lebrini, M.; Legrenee, M.; Vezin, H.; Traisnel, M.; Bentiss, F. Experimental and theoretical study for corrosion inhibition of mild steel in normal hydrochloric acid solution by some new macrocyclic polyether compounds. Corros. Sci. 2007, 49, 2254-2269. (56) Badr, G.E. The role of some thiosemicarbazide derivatives as corrosion inhibitors for Csteel in acidic media. Corros. Sci. 2009, 51, 2529–2536. (57) Yuan, S.; Pehkonen, S.O.; Liang, B.; Ting, Y.P.; Neoh, K.C.; Kang, E.T. Superhydrophobic fluoropolymer-modified copper surface via surface graft polymerisation for corrosion protection. Corros. Sci. 2011, 53, 2738–2747. (58) Singh, S.K.; Tambe, S.P.; Gunasekaran, G.; Raja, V.S.; Kumar, D. Electrochemical impedance study of thermally sprayable polyethylene coatings. Corros. Sci. 2009, 51, 595– 601. (59) Quartarone, G., Battilana, M.; Bonaldo, L.; Tortato, T. Investigation of the inhibition effect of indole-3-carboxylic acid on the copper corrosion in 0.5 M H2SO4. Corros. Sci. 2008, 50, 3467–3474. (60) Lavos-Valereto, I.C.; Wolynec, S.; Ramires, I.; Guastaldi, A.C.; Costa, I. Electrochemical impedance spectroscopy characterization of passive film formed on implant Ti-6Al-7Nb alloy in Hank’s solution. J. Mater. Sci. 2004, 15, 55-59. (61) Popova, A.; Sokolova, E.; Raicheva, S.; Christov, M. AC and DC study of the temperature effect on mild steel corrosion in acid media in the presence of benzimidazole derivatives. Corros. Sci. 2003, 45, 33–58. (62) Wang, L.S.; Han, S.K. Structure, properties and activity of molecules. Beijing: Chemical Engineering Industry Press, 1997, 60-68.

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(63) Ogretir, C.; Mihci, B.; Bereket, G. Quantum chemical studies of some pyridine derivatives as corrosion inhibitors. J Mol Struct: Theochem. 1999, 488, 223-231. (64) Huang, W.; Tan, Y.; Chen, B.; Dong, J.; Wang, X. The binding of antiwear to iron surfaces: quantum chemical calculations and tribological test. Tribology International,

2003, 36, 163-168. (65) Ebenso, E. E.; Kabanda, M.M.; Murulana, L.C.; Singh, A.K.; Shukla, S.K. Electrochemical and Quantum Chemical Investigation of Some Azine and Thiazine Dyes as Potential Corrosion Inhibitors for Mild Steel in Hydrochloric Acid Solution. Ind. Eng. Chem. Res., 2012, 51 (39), 12940–12958.

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Table 1. Variation of inhibition efficiency with different concentration of DMABT obtained from weight loss experiments at 298K temperature in 1N HCl and 1N H2SO4 solutions.

Cinh

1N HCl

1N H2SO4

Wt. loss

CR

η

Wt. loss

CR

η

mg cm-2

mg cm-2 h-1

(%)

mg cm-2

mg cm-2 h-1

(%)

00

20.35

0.848

--------

76.65

3.194

--------

45

3.530

0.147

82.5

25.85

1.077

66.2

90

1.992

0.083

90.1

9.240

0.385

88.0

180

1.560

0.065

92.2

8.760

0.365

88.6

270

1.464

0.061

92.8

3.216

0.134

95.7

360

1.248

0.052

94.0

2.280

0.095

97.0

450

0.888

0.037

95.7

1.512

0.063

97.8

Mx10-6

38


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Thermodynamic activation parameters of MS in 1N HCl and 1N H2SO4 solutions obtained from weight loss method.

Cinh

Thermodynamic Parameters 1N HCl

Mx10-6

1N H2SO4

Ea

∆H a

− ∆S a

Ea

∆H a

− ∆S a

kJ mol-1

kJ mol-1

J mol-1 K-1

kJ mol-1

kJ mol-1

J mol-1 K-1

00

47.3

47.2

87.7

25.5

19.7

168.2

180

68.1

68.0

40.0

67.7

54.2

66.01

360

63.1

63.0

58.6

101.2

87.5

-32.5

39



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Table 3. Thermodynamic parameters for adsorption of DMABT on MS surface in 1N HCl and 1N H2SO4 acidic solutions at different temperatures.

Temperature (K)

DMABT in HCl Kads

DMABT in H2SO4

∆Gads

Kads

∆Gads

-1

(kJ mol ) (M) (kJ mol-1) -39.74 54.5x103 -36.97

298

(M) 167x103

308

115.2x103

318

3

-40.12

14.7x103

-34.85

100.5x10

-41.07

3

9.12x10

-34.72

328

71.7x103

-41.44

4.49x103

-33.89

338

4.89x103

-41.63

1.79x103

-32.33

40


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48


Table 4. Electrochemical parameters obtained from the polarization curves of DMABT in 1N HCl and 1N H2SO4 solutions at 298K.

Cinh

1N HCl

1N H2SO4

Tafel

Mx10-6

LPR

Tafel

LPR

-Ecorr

βa

-βc

icorr

Rp

-Ecorr

βa

-βc

icorr

Rp

mV

mV dec-1

mV dec-1

µA cm-2

Ω cm2

mV

mV dec-1

mV dec-1

µA cm-2

Ω cm2

00

446

151

117

917

31

458

130

157

2049

15

45

480

102

158

140

192

499

122

141

560

51

90

475

109

150

93

296

488

121

154

238

119

180

491

105

138

76

341

478

70

130

163

124

270

484

117

131

59

460

465

85

137

84

278

360

469

118

135

53

516

472

109

130

82

306

450

462

154

166

35

986

471

92

164

69

372

1



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Page 42 of 62

Table 5. EIS parameters for the corrosion of MS in 1N HCl and 1N H2SO4 solutions containing DMABT at 298K.

1N HCl

1N H2SO4

EIS

EIS

Cinh Mx10-6

Rs

Rct

Cdl

CPE Y0

n

Ω cm2

Ω cm2

µF cm-2

µ Ω sn cm-2

00

7.47

28

99.2

234

0.85

45

8.04

220

89.5

99

90

8.34

340

46.7

180

8.45

357

270

8.40

360 450

Chi Square

Rs

Rct

Cdl

CPE Y0

n

Chi Square

Ω cm2

Ω cm2

µF cm-2

µ Ω sn cm-2

0.00041

4.27

13

304.5

328

0.95

0.00099

0.89

0.0050

4.44

53

267.8

76

0.88

0.0013

68

0.90

0.0048

4.56

67

63.1

69

0.93

0.0037

45.2

60

0.90

0.0054

4.40

201

39.3

60

0.95

0.0028

581

39.3

56

0.91

0.0029

5.08

373

37.1

48

0.95

0.0048

8.60

632

33.9

46

0.91

0.0033

5.77

412

29.6

36

0.97

0.0014

8.76

812

20.1

21

0.89

0.0016

5.73

730

26.9

32

0.94

0.0026

2


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Table 6. Variation of η for MS in acidic media with different concentrations of DMABT by weight loss and electrochemical methods in 1N HCl and 1N H2SO4.

Cinh

Inhibition Efficiency (η%) 1N HCl

Mx10-6

1N H2SO4

Wt. Loss

Tafel Extrapolation

LPR

EIS

Wt. Loss

Tafel Extrapolation

LPR

EIS

45

82.5

84.7

83.7

87.3

66.2

72.6

70.6

75.5

90

90.1

89.9

89.4

92.0

88.0

88.4

87.4

80.6

180

92.2

91.7

90.8

92.1

88.6

92.0

88.0

93.5

270

92.8

93.6

93.2

95.2

95.7

95.9

94.6

96.5

360

94.0

94.2

94.0

95.5

97.0

96.0

95.1

96.8

450

95.7

96.2

96.8

96.5

97.8

96.6

96.0

98.2

1



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Table 7. Atomic percentage of elements obtained from EDX spectra of MS surfaces in 1N HCl and 1N H2SO4 solutions without and with DMABT at 298K.

Inhibitor

Fe

C

Si

S

Mn

P

N

Abraded MS

95.4

18.6

2.32

0.37

0.27

0.21

1.1

1N HCl

90.0

22.5

0.55

0.44

0.27

0.35

1.3

1N H2SO4

90.2

25.3

0.62

0.34

0.38

0.50

2.7

1N HCl

74.1

26.2

0.59

0.44

0.34

0.29

2.6

1N H2SO4

64.2

29.3

0.75

0.47

0.32

0.41

4.6

Blank

DMABT

2


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Table 8. The area and line roughness obtained from AFM of MS surfaces in 1N HCl and 1N H2SO4 solutions without and with DMABT at 298K.

AFM Data

Area Roughness

Abraded MS

1N HCl

1N H2SO4

Blank

DMABT

Blank

DMABT

42.23

626.9

124.6

660.0

131.9

53.15

586.4

157.3

518.9

67.9

(nm) Line Roughness (nm)

3



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Page 46 of 62

Table 9. Energy order of the frontier orbital (eV).

EHOMO

ELUMO

∆E1=ELUMO(DMABT) -EHOMO(Fe)

∆E2=ELUMO(Fe)-EHOMO(DMABT)

Fe5

-5.075

-1.747

------

------

DMABT

-0.1974

-0.6068

5.014

-1.550

4


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S CH N HN C NH 2

H 3C

N

CH 3

Figure1. Chemical molecular structure of 4-(N,N-dimethylamino)benzaldehyde thiosemicarbazone [DMABT].

5



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Page 48 of 62

Figure 2. Variation of inhibition efficiency (η) and corrosion rate (CR) with 180 and 360µM of DMABT obtained from weight loss experiments at different temperature in 1N HCl and 1N H2SO4 solutions.

6


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Figure 3 (a)

(b)

Figure 3. Arrhenius plots for MS corrosion rates (CR) in acidic media in absence and presence of 180 and 360µM of DMABT (a) 1N HCl (b) 1N H2SO4. 7



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Page 50 of 62

Figure 4(a)

(b)

Figure 4. Transition-state plots for MS corrosion rates (CR) in acidic media in absence and presence of 180 and 360µM of DMABT (a) 1N HCl (b) 1N H2SO4.

8


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Figure 5. Langmuir adsorption isotherm plot of MS in 1N HCl and 1N H2SO4 containing different concentration of DMABT at 298K.

9



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Page 52 of 62

Figure 6(a)

(b)

Figure 6. Anodic and cathodic polarization curves for MS in acidic media in absence and presence of various concentrations of DMABT at 298K (a) 1N HCl and (b) 1N H2SO4.

10


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Figure 7(a)

(b)

Figure 7. Nyquist plots of the corrosion of MS in acidic media without and with different concentrations of DMABT at 298K (a) 1N HCl (b) 1N H2SO4.

11



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Page 54 of 62

Figure 8. Equivalent circuit for MS in both the electrolyte.

12


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Figure 9 (a)

(b)

13



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Page 56 of 62

(c)

(d)

14


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(e)

Figure 9. SEM micrographs of MS surface (a) abraded MS, (b) Blank in 1N HCl, (c) with DMABT in HCl, (d) Blank in 1N H2SO4, (e) with DMABT in H2SO4.

15



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Page 58 of 62

Figure10(a). MS abraded

(b) MS HCl Blank

16


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(c) MS in HCl with DMABT

(d) MS in H2SO4 Blank

17



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Page 60 of 62

(e) MS in H2SO4 with DMABT

Figure 10. AFM Micrographs of MS surface (a) abraded MS, (b) Blank in 1N HCl, (c) with DMABT in HCl, (d) Blank in 1N H2SO4, (e) with DMABT in H2SO4.

18


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Figure11. Optimized structure of DMABT calculated with the B3LYP/6-31G(d) model chemistry.

19



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Page 62 of 62

Figure 12 (a)

(b)

Figure 12. Frontier molecular orbital diagrams (a) HOMO and (b) LUMO of DMABT by the B3LYP/631G(d) model chemistry.

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Inhibition of Mild Steel Corrosion in Hydrochloric ... - ACS Publications

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