Corrosion inhibiting action of tetramethyl-dithia-octaaza-cyclotetradeca-hexaene (MTAH) on corrosion of mild steel in hot 20% sulfuric acid

Materials Chemistry and Physics 77 (2002) 43–47

Corrosion inhibiting action of tetramethyl-dithia-octaaza-cyclotetradeca-hexaene (MTAH) on corrosion of mild steel in hot 20% sulfuric acid M.A. Quraishi∗ , Jaya Rawat Department of Applied Chemistry, Corrosion Research Laboratory, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202 002, UP, India Received 17 April 2001; received in revised form 13 August 2001; accepted 31 August 2001

Abstract Macrocyclic compounds constitute a potential class of inhibitors. In an attempt to develop effective corrosion inhibitor we have synthesized a macrocyclic compound namely 2,3,9,10-tetramethyl-6,13-dithia-1,4,5,7,8,11,12,14-octaaza-cyclotetradeca-1,3,6,8,10,13-hexaene (MTAH) was synthesized to study the corrosion inhibitive effect on pickling of mild steel (MS) in 20% H2 SO4 at 95 ◦ C. The synergistic effect of this compound was studied by weight loss, potentiodynamic polarization, electrochemical impedance and hydrogen permeation studies. The results of investigations in the presence of potassium iodide (KI) as synergent have shown enhancement in inhibition efficiency (IE). Potentiodynamic polarization studies showed that MTAH is mixed in nature. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Sulfuric acid pickling corrosion inhibitor; Electrochemical impedance spectroscopy; Potentiodynamic polarization; Hydrogen permeation; Weight loss; Macrocyclic

1. Introduction Chemical cleaning and pickling are important industrial processes used to remove mill scales (oxide scales) from the metal surface. It is usually conducted at temperatures up to 60 ◦ C in HCl and up to a temperature of 95 ◦ C in H2 SO4 . These conditions require inhibitors that remain effective even under severe conditions of high concentration of acid (20%) and temperatures ranging from 60 to 95 ◦ C. Effective corrosion inhibitors used in sulfuric acid include sulfur and nitrogen/sulfur containing compounds such as sulfoxide, sulfides, mercaptobenzothiazole, thiazole, thiourea derivatives, indole, thiophene and benzothiazoles [1,2]. Survey of literature [3,4] reveals that 2-mercaptobenzothiazole is an effective corrosion inhibitor up to 70 ◦ C. It gives improved performance in combination with alkynols. Sulfonium salts are effective only at ambient temperature. Thiourea derivatives also constitute an important class of corrosion inhibitors but they induce H2 up-take by metals and isomerizes into toxic compounds, hence their use is not safe. Thus, the choice for using inhibitor in strong sulfuric acid at 95 ◦ C is limited. In view of these observations there is a necessity for development of new inhibitor for sulfuric acid. ∗

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In continuation of our work on development of macrocyclic compounds as acid inhibitors [5–7], we have synthesized an organic inhibitor namely tetramethyl-dithiaoctaaza-cyclotetradeca-hexaene (MTAH) with a view to study its inhibiting properties on corrosion of MS in 20% H2 SO4 at 95 ◦ C and to study the synergistic effects of iodide ions on inhibitive performance of MTAH. The corrosion inhibitive performance of MTAH is compared with an Indian patented commercial corrosion inhibitor (Metasave). The selection of this inhibitor is based on the following facts that: 1. It contains 10 heteroatoms (eight nitrogen and two sulfur) as reactive centers through which it can adsorb readily on the metal surface. 2. It is readily soluble in medium. 3. It does not cause any health hazards like other thiourea derivatives, hence its use as corrosion inhibitor is safe. 2. Experimental 2.1. Material preparation Mild steel (MS) strips composed of: 0.14 wt.% C, 0.35 wt.% Mn, 0.17 wt.% Si, 0.025 wt.% S, 0.03 wt.% P and

0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 1 ) 0 0 5 7 6 - 4

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M.A. Quraishi, J. Rawat / Materials Chemistry and Physics 77 (2002) 43–47

two-compartment cell as reported in our previous work [11] hydrogen permeation currents were recorded using a strip chart recorder in the absence and presence of different concentrations of MTAH in 20% H2 SO4 and Metasave.

3. Results and discussion 3.1. Weight loss studies

Fig. 1. Name and molecular structure of inhibitor used.

balance Fe were used. For weight loss measurements, MS strips of 36 cm2 (5.76 in.2 ) were used. For potentiodynamic polarization, electrochemical impedance and hydrogen permeation studies, MS strips with an exposed area of 1 cm2 (0.16 in.2 ) were used. MS specimens were polished mechanically with emery papers of 1/0–4/0 grades. They were degreased subsequently with trichloroethylene (CCl2 =CHCl). Double distilled water and analytical reagent-grade H2 SO4 were used for preparing solutions. Inhibitor was synthesized following procedures reported previously [8]. The molecular structure of inhibitor is shown in Fig. 1. 2.2. Weight loss measurement Weight loss experiments were done according to ASTM methods described previously. Tests were conducted in 20% H2 SO4 for 600 and 45 s at 95 ± 2 ◦ C [9,10]. 2.3. Potentiodynamic polarization measurement Polarization curves were recorded potentiodynamically (1 mV S−1 ) using an EG&G Princeton Applied Research (PAR) model 173 Potentiostat/Galvanostat, a model 175 universal programmer and a model RE 0089 X–Y Recorder at 35 ± 2 ◦ C. The cell assembly consisted of an MS as working electrode, platinum as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. 2.4. Electrochemical impedance measurement Nyquist plots for MS in 20% H2 SO4 containing different concentrations of MTAH were recorded using an EG&G PAR model M 6310 system with M398 software at 35 ± 2 ◦ C. Impedance plots were obtained over a wide range of frequencies (i.e., from 10 kHz to 100 mHz). The alternating current (AC) amplitude was 10 mV root mean square (rms). 2.5. Hydrogen permeation measurement Hydrogen permeation studies were carried out at 35±2 ◦ C using an adaptation of the modified Devanathan–Stachurski

The values of metal loss (%) and inhibition efficiency (IE) obtained from weight loss measurement for different concentrations of MTAH in absence and presence of KI and commercial corrosion inhibitor in 20% H2 SO4 for 600 and 45 s are given in Table 1. It is seen that percentage metal loss for synthesized inhibitor, viz. MTAH + 0.25% KI is two times less than that of commercial inhibitor in 20% H2 SO4 at 95 ± 2 ◦ C. The high IE of MTAH may be attributed to presence of lone pair of electrons on 10 heteroatoms (eight nitrogen and two sulfur atoms) through which it can adsorb strongly on the metal surface. In addition to presence of lone pair of electrons it can also adsorb through protonated specimen. It is also seen from the results that IE of MTAH increases on addition of small amount of KI owing to synergism, according to which iodide (I− ) ions are initially adsorbed on the metal surface thereby increases negative charge on the metal surface which facilitates adsorption of protonated macrocyclic compound leading to reduction in metal loss [12]. The synergistic parameter (Si ) was calculated by using the relation (Eq. (1)) given by Aramaki and Hackerman [13]: Si =

1 − I1+2  1 − I1+2

(1)

where I1+2 = (I1 + I2 ) − (I1 I2 )

Table 1 Corrosion parameters obtained from weight loss measurements in 20% H2 SO4 containing Metasave and different concentrations of MTAH with and without 0.25% KI at 95 ± 2 ◦ C Inhibitor concentration (ppm)

Immersion time (s)

Metal loss (%)

IE (%)

Si

Blank

600 45

5.94 1.45

– –

– –

500 MTAH

600 45

1.42 0.072

76.1 95.0

– –

1000 MTAH

600 45

0.658 0.055

88.9 96.2

– –

500 MTAH + KI

600 45

0.040 0.022

99.3 98.5

8.56 1.80

Metasave

600 45

0.190 0.088

96.6 93.2

– –

M.A. Quraishi, J. Rawat / Materials Chemistry and Physics 77 (2002) 43–47

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Fig. 2. Typical potentiodynamic polarization curves for MS in 20% H2 SO4 containing different concentrations of inhibitors: (1) blank, (2) 500 ppm MTAH, (3) 1000 ppm MTAH, (4) 500 ppm MTAH + 0.25% KI.

I1 is the IE of the anion, I2 the IE of the cation, and  I1+2 the measured IE for the cation in combination with anion. Table 1 shows the Si values of 500 ppm MTAH with 0.25% KI for 600 and 45 s in 20% H2 SO4 . All Si values are more than unity, thereby suggesting that the phenomenon of synergism is existing between macrocyclic compound and iodide ions [14]. The results obtained from weight loss studies show that 500 ppm MTAH + 0.25% KI has shown about four times less metal loss than Metasave.

3.3. Electrochemical impedance studies

3.2. Potentiodynamic polarization studies

Electrochemical impedance measurements were carried over the frequency range from 10 kHz to 100 mHz. The simple equivalent Randles circuit [15] for impedance studies is shown in Fig. 3, where R represents the solution and corrosion product film; the parallel combination of resistor, Rt and capacitor Cdl represents the corroding interface. Cdl is the electrochemical double layer capacity resulting from adsorbed ions and water molecules and Rt the charge transfer resistance. Nyquist plots obtained from the impedance studies in 20% H2 SO4 containing different concentrations of MTAH are shown in Fig. 4(a) and (b). Various parameters

The polarization behavior of MS in 20% H2 SO4 in the absence and presence of different concentrations of MTAH are shown in Fig. 2. Electrochemical parameters such as corrosion current density (Icorr ), corrosion potential (Ecorr ), Tafel slope constants (bc and ba ) calculated from Tafel plots are given in Table 2. It is apparent from the results that Icorr value synthesized inhibitor is about 3.5 times less than Metasave. The shifting of Ecorr indicated that MTAH and Metasave both are mixed inhibitor in 20% H2 SO4 .

Fig. 3. A simple Randles-type equivalent circuit for electrochemical impedance measurements.

Table 2 Electrochemical polarization parameters for MS in 20% H2 SO4 containing Metasave and different concentrations of MTAH with and without 0.25% KI at 35 ± 2 ◦ C Inhibitor concentration (ppm)

Blank 500 MTAH 1000 MTAH 500 MTAH + KI Metasave

Ecorr (mV)

−524 −518 −528 −516 −522

Icorr (mA cm−2 )

16.00 0.60 0.34 0.20 0.76

Tafel slope

IE (%)

bc (mV per decade)

ba (mV per decade)

132 120 110 115 112

70 56 50 60 52

– 96.3 97.9 98.8 95.25

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Fig. 4. Nyquist plots for MS in 20% H2 SO4 containing different concentrations of inhibitors: (a) blank; (b) 1: 500 ppm MTAH, 2: 1000 ppm MTAH, 3: 500 ppm MTAH + 0.25% KI.

such as charge transfer resistance (Rt ), double layer capacitance (Cdl ) and IE were obtained from impedance measurements are given in Table 3. Rt values were calculated from the difference in impedance at lower and higher frequencies as reported elsewhere [16]. Cdl values were calculated from the frequency at which the imaginary component of impedance was maximum (Zim max ) using the following relation: 1 1 Cdl = (2) 2πfmax Rt Table 3 Electrochemical impedance parameters for MS in 20% H2 SO4 containing Metasave and different concentrations of MTAH with and without 0.25% KI at 35 ± 2 ◦ C Inhibitor concentration (ppm)

Rt ( cm2 )

Cdl (␮F cm2 )

IE (%)

Blank 500 MTAH 1000 MTAH 500 MTAH + KI Metasave

6.0 1570 1860 1906 1520

259652 1046 924 884 1086.36

– 99.6 99.8 99.9 99.60

MTAH has exhibited more than 99% IE as calculated from Rt values. The decrease in Cdl values in the presence of inhibitors indicated that these inhibitors inhibit corrosion by adsorption mechanism [17]. 3.4. Hydrogen permeation studies It is found from the results that reduction of hydrogen permeation current is four times more for MTAH + 0.25% KI than Metasave in 20% H2 SO4 solution. The reduction of hydrogen uptake could be attributed to adsorption of Table 4 Hydrogen permeation parameters for MS in 20% H2 SO4 containing Metasave and different concentrations of MTAH with and without 0.25% KI and at 35 ± 2 ◦ C Inhibitor concentration (ppm)

Permeation current (␮A)

Percent reduction

Blank 500 MTAH 1000 MTAH Metasave

62.6 38.0 32.0 52.6

– 39.3 48.9 15.97

M.A. Quraishi, J. Rawat / Materials Chemistry and Physics 77 (2002) 43–47

Macrocyclic compound on MS surface, which prevented permeation of hydrogen into metal (Table 4). 4. Conclusions 1. MTAH exhibited 95% IE in 20% H2 SO4 at 95 ◦ C and its IE further increased in the presence of KI due to synergistic effect. 2. MTAH significantly reduced permeation of hydrogen through MS in 20% H2 SO4 . 3. MTAH significantly reduced the double layer capacitance thereby suggesting that inhibition of corrosion taking place by adsorption mechanism. References [1] G. Moretti, G. Quartarone, A. Tassan, A. Zingales, Br. Corros. J. 31 (1) (1996) 49. [2] A. Singh, R.S. Chaudhary, Br. Corros. J. 31 (4) (1996) 300.

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[3] W.W. Frenier, F.B. Growcock, in: A. Raman, P. Labine (Eds.), Rev. on Corros. Inhib. Sci. Technol., Vol. I, NACE, Houston, TX, 1993, pp. 1–28. [4] G. Schmidt, Br. Corros. J. 19 (1984) 165. [5] M. Ajmal, J. Rawat, M.A. Quraishi, Anti-Corros. Meth. Mater. 45 (6) (1998) 419. [6] M. Ajmal, J. Rawat, M.A. Quraishi, Electrochem. Soc. India 48 (1999) 70. [7] M. Ajmal, J. Rawat, M.A. Quraishi, Bull. Electrochem. 14 (6–7) (1998) 199–203. [8] O.P. Pandeya, S.K. Sengupta, S.C. Tripathi, Synth. React. Met. Org. Chem. 17 (1987) 567. [9] ASTM, Standard Practice for Laboratory Immersion Corrosion Testing of Metals, G31-72, Philadelphia, PA, 1990, p. 401. [10] A.D. Mercer, Br. Corros. J. 20 (1985) 61. [11] S. Muralidharan, M.A. Quraishi, S.V.K. Iyer, Corros. Sci. 37 (11) (1995) 1739. [12] M.A. Quraishi, J. Rawat, M. Ajmal, Corrosion 54 (12) (1998) 996. [13] K. Aramaki, N. Hackerman, J. Electrochem. Soc. 116 (5) (1969) 568. [14] M.A. Quraishi, J. Rawat, M. Ajmal, Corrosion 55 (10) (1999) 919. [15] K. Hladky, L.M. Callow, J.L. Dawson, Br. Corros. J. 15 (1980) 20. [16] M. Ajmal, J. Rawat, M.A. Quraishi, Br. Corros. J. 34 (1999) 220. [17] M.A. Quraishi, J. Rawat, Mater. Chem. Phys. 70 (2001) 95–99.