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Trans. Indian Inst. Met. Vol.57, No. 3, June 2004, pp. 297-306

THE INHIBITION OF CORROSION OF MILD STEEL BY SOME FLUOROQUINOLONES IN SODIUM CHLORIDE SOLUTION S. Acharya and S.N.Upadhyay* Department of Metallurgy, Indian Institute of Science, Bangalore-560012 *Department of Chemical Engineering & Technology, Institute of Technology, Banaras Hindu University, Varanasi-221005 E-mail : [email protected] (Received 10 December 2003 ; in revised form 11 February 2004)

ABSTRACT The corrosion inhibition of mild steel in 3.4% sodium chloride solution containing various concentration of inhibitors such as, ofloxacin, amifloxacin, enofloxacin, pfloxacin, ciprofloxacin and norfloxacin has been investigated. The inhibition efficiency has been evaluated by weight loss and electrochemical polarisation techniques. Morphology of the mild steel specimens were examined using scanning electron microscopy in presence and absence of inhibitors. The inhibition efficiency increased with increasing concentration of inhibitors. At the cocentration of 40 ppm, the inhibition efficiency for mild steel is in the following order : Ofloxacin > amifloxacin > enofloxacin > pfloxacin > ciprofloxacin > norfloxacin. It shows that water soluble thin film of inhibitor is formed on the surface by a surface reaction of metal ion of specimen with adsorbed inhibitor molecules and that film act as a protective barrier against corrosive environment.

1. INTRODUCTION Many researchers have studied the effects of various types of organic inhibitors on the corrosion resistance of mild steel in different environment 1-5. Machu 6 showed that sulphur containing compounds are preferred for H2SO4. Hackermann and Makrides 7 pointed out that sulphur containing organic compounds have better inhibitive efficiency due to better electron donor capacity and easy polarisability. Due to the above reasons, sulphur containing organic compounds have been recognized as better inhibitors for different systems8-13 than the corresponding nitrogen containing compounds14-17. Ayre et al.18 showed that substances containing atoms of both nitrogen and sulphur containing heterocyclic compounds are found to be excellent inhibitors in both 1N H2SO4 and 1N HCl acid solutions. The mild steel when immersed in aqueous environment get corroded due to its thermodynamic instability. Therefore, the corrosion behaviour and inhibition of mild steel in sodium chloride environment have been

given attention by several workers during last few years. The present work includes the study of corrosion behaviour of mild steel in 3.4% sodium chloride solution and its control by using inhibitors like ofloxacin, amifloxacin, enofloxacin, pfloxacin, ciprofloxacin and norfloxacin. The inhibitive efficiencies of various inhibitors were determined by weight loss, electro chemical polarisation and scanning electro microscopy techniques.

2. EXPERIMENTAL PROCEDURE Square (2 cm x 2 cm) specimens were sheared from commercial grade mild steel sheet. The specimens were polished with emery paper ranging from 1/0 to 4/0 followed by washing with distilled water and degreased with acetone. Specimens were dried and stored in vacuum desiccator containing silica gel. A small hole was drilled near one edge of the specimen. The well polished and dried specimens were weighed on an electronic balance(accuracy 0.001gm). After that specimens were suspended with the help of

TRANS. INDIAN INST. MET., VOL. 57, NO. 3, JUNE 2004

polythene threads in conical flask containing 300 ml of corrosive medium. All specimens were kept in an air thermostat. The thermostat equipped with an electronic relay and a contact thermo meter was used to control the temperature. The temperature of thermostat was maintained with an accuracy of 0.2oC. The specimens were removed from the corrosive environment after a definite exposure period, washed with water and rubbed with cork to remove any corrosion product and finally washed with acetone and distilled water. After that the specimens were dried in an oven and reweighed. The change in weight with time can give the value of corrosion rate. The experiments were repeated thrice until reproducible data were obtained. The corrosion rate was calculated from the relation as follows, Corrosion rate = Where, W

534W DAT

= weight loss in milligram,

D

= density in gm/cm2,

A

= Specimen area in square inch,

T

= Exposure period in hours

Instrument Inc., U.S.A). Corrosion rate was observed by the Tafel analysis and potentiodynamic data. The Stren- Geary equation fit into the data with the help of a software provided with the instrument. Surface morphology of the specimen before and after the corrosion experiments was studied using a scanning electron microscope(SEM). Prepared mild steel specimen was dipped in to 3.4 % NaCl solution in presence and absence of fluoroquinolones for period of 30 days. After removing the specimen from the solution, its morphology was examined with SEM.

3. RESULTS AND DISCUSSION Corrosion of mild steel was studied over a period of 10 to 50 days. The inhibitors used were fluoroquinolone derivatives in the concentration range of 20 to 50 ppm. Their structural formula are given in Fig. 1. 3.1 Weight Loss studies

For electrochemical polarisation studies, flag shaped metal specimens with sufficiently long tail were cut from the metal sheet. The specimens were polished as described in the preceding section. Leaving on working area of 1 cm2 on both sides of the flag and small portion at the tip of the tail for providing electrical contact, rest of the surface was coated with enamel lacquer. The test specimen was connected to the working electrode holder through the tip of the tail. About 250 ml of 3.4 % NaCl solution was taken in the test cell. This volume was sufficient enough to permit immersion of electrodes. The electrochemical cell which is used for present investigation is essentially three electrode system. The saturated calomel electrode (SCE) is placed in the luggin capillary. Second electrode is the counter electrode and is made of a high density graphite rod. Third electrode is the working electrode. After assembling the system, the potentiodynamic scan system was energized and the complete polarization curve were obtained with a microprocessor based corrosion measurement system(CMS- 100, Gamry

The measured weight loss and corrosion rate of mild steel in 3.4% sodium chloride solution at different exposure periods are shown in Fig. 2. These values are indicated that the weight loss increases linearly with increase in exposure period and corrosion rate decreases slowly with increase of exposure period. The effect of fluoroquinolone derivatives on weight loss and their corrosion inhibition efficiencies at different temperatures are given in Table 1. The addition of trace amount (ppm) of these fluoroquinolone derivatives brings a significant change in the corrosion rate of mild steel. The percentage of inhibition efficiency and surface coverage 19 were calculated by using equations (1) and (2). % Inhibition efficiency = ( %IE)

Uninhibited weight loss – Inhibhited weight ———————————100 (1) Uninhibited weight loss

Uninhibited weight loss Surface – Inhibhited weight coverage (S) = ——————————— Uninhibited weight loss

298

(2)

ACHARYA, et al., : THE INHIBITION OF CORROSION OF MILD STEEL BY SOME FLUOROQUINOLONES IN SODIUM CHLORIDE SOLUTION

efficiency is obtained at 40 ppm. The efficiency of different fluoroquinolone derivatives decrease in the following order:

Norfloxacin

Ofloxacin > amifloxacin > enofloxacin > pfloxacin > ciprofloxacin > norfloxacin

Ciprofloxacin

Ofloxacin has the largest molecular size than other used fluoroquinolone derivatives. Therefore, it easily shields the surface of mild steel and shows the highest inhibition efficiency. Trabanelli 21 has reported that the efficiency can be increased by increasing the size of inhibitor molecule. Pefloxacin

Amifloxacin

3.2 Langmuir adsorption isotherm

Enoxacin

Ofloxacin

Fig. 1 : Molecular structure of some fluoroquinolones used as corrosion inhibitors in 3.4% NaCl solution.

It may be observed that the inhibition efficiency increases with temperature and inhibitors concentration. The increase in efficiency with increase in temperature is due to sluggish diffusion of large inhibitor molecule as compared to water molecules 20. The increase in concentration of inhibitors in 3.4% sodium chloride solution causes increase in number of ions. This may be produce steric hindrance to the dissolution of mild steel. The maximum 250

Weight loss (mg/cm2) Corrosion rate (mpy)

200

150

100

 θ log 1−θ S A C

 ∆H ads     = log A + log C −    2.303RT 

= = =

∆H ads

(3)

surface coverage by inhibitor , temperature dependant constant, bulk concentration of inhibitor, = heat of adsorption .

From the plots of Langmuir adsorption isotherm, it is seen that (Figs. 3-5) the slopes are never unity, contrary to what is expected from the ideal isotherm equation. The departure in the values of slope from unity is due to the mutual interaction of adsorbed molecules in close vicinity 20. It is also observed from the plots that the deviation of some points from linearity. This may be due to attraction or repulsion between the adsorbed molecules. This gets reflected in the heat of adsorption 20. The heat of adsorption may be calculated from slope of the plot of log

50 0

The characteristics of inhibitors have been studied by Langmuir adsorption isotherm 22,23-25 . The compound should obey Langmuir adsorption isotherm26 which assumes that the % area covered by the inhibitor is directly related to decrease in corrosion rate.

10

20 30 40 50 Exposure Period (days) Fig. 2 : Average weight loss and corrosion rate of mild steel in 3.4% of NaCl solution at various exposure periods.

θ 1−θ

vs

1 for fluoroquinolones T

and are listed in Table 2. It can be observed that norfloxacin has most negative value and ofloxacin has the least negative value of heat of adsorption. Ofloxacin adsorbed more strongly on the surface of

299


(a)

(a)

(b)

(b)

Fig. 3 : Langmuir adsorption isotherms of mild steel for (a) norfloxacin and (b) ciprofloxacin.

Fig. 4 : Langmuir adsorption isotherms of mild steel for (a) pefloxacin and (b) ofloxacin.

mild steel than any other fluoroquinolones. The adsorbe ability decreases in the sequence as follows:

3.3 Open Circuit Potential(OCP)

Ofloxacin >amifloxacin > enofloxacin > pfloxacin >ciprofloxacin > norfloxacin Table 2 VALUE OF HEAT OF ADSORPTION FOR DIFFERENT FLUOROQUINOLONE DERIVATIVES

Inhibitor

Concentration (ppm)

-&Hads (cal/mole)

The open circuit potential of mild steel in 3.4% sodium chloride solution in presence and absence of fluoroquinolone derivatives are shown in Fig. 8. All the fluoroquinolones were maintained at 40 ppm. The steady state corrosion potential is obtained after 30 minutes of exposure. From the Fig. 6, it is seen that OCP of mild steel has more negative value than in the presence of fluoroquinolone derivatives. This may indicate that the corrosion process is getting hindered in the presence of inhibitors due to the formation of protective barrier layer.

Norfloxacin

20

3.266

Ciprofloxacin

20

2.852

Pefloxacin

20

2.53

3.4 Potentiodynamic Measurement

Ofloxacin

20

1.61

Amifloxacin

20

2.30

Enofloxacinm

20

2.438

Potentiodynamic polarisation of mild steel was carried out in 3.4% sodium chloride solution with various fluoroquinolone derivatives. The polarisation curves 300


(a)

T (b)

T (c) Fig. 5 : Langmuir adsorption isotherms of mild steel for (a) amifloxacin and (b) enofloxacin.

were obtained at the scan rate of 5 mv/sec for different fluoroquinolone derivatives. The value of electrochemical parameters and the percentage of efficiency are shown in Table 3. The polarisation curves of mild steel in the presence and absence of various inhibitors at different concentrations are represented in Figs. 7 to 9. It may be observed from the figure that both anodic and cathodic polarisation curves shift towards the low current density region. The current density decreases with increase of inhibitor concentration. The corrosion potential value shifted in the positive direction with the increase of inhibitor concentration. The efficiency of different fluoroquinolone derivatives decrease in the following order: Ofloxacin > pfloxacin > ciprofloxacin > norfloxacin

Fig. 6 : Heat of adsorption plots for (a) norfloxacin, (b) ciprofloxacin and (c) pefloxacin

The percentage of efficiencies calculated through electrochemical technique are slightly higher than those obtained from the weight loss measurements. This may be attributed to the improper cleaning of the surface of specimen. 3.5 Corrosion Morphology of Mild steel by SEM Surface morphology of mild steel specimen exposed to 3.4% sodium chloride solution with and without

301


(e)

( Potential (V) vs. SCE )

(a)

(b)

(d)

(c) (b)

(a)

( Time (sec) ) Fig. 8 : Variation of corrosion potential of mild steel exposed to 3.4% NaCl solution with fluoroquinolone derivatives at 40 ppm (a) control (b) norfloxacin (c) ciprofloxacin (d) pefloxacin (e) ofloxacin.

(c)

Fig. 9 : Potentiodynamic polarisation curves of mild steel exposed to 3.4% NaCl solution with different concentration of norfloxacin (1) control, (2) 20 ppm, (3) 30 ppm, (4) 40 ppm. Fig. 7 : Heat of adsorption plots for (a) ofloxacin, (b) amifloxacin and (c) enofloxacin

ofloxacin are shown in Figs. 13(a)-(d). Figure13(a) shows the formation of protective filim on the surface of mild steel specimen in the presence of ofloxacin. Figure 13(b) represents the corrosion products deposit on the surface of mild steel in absence of inhibitor, ofloxacin. Figure 13(c) shows the effective protection of mild steel specimen due to the use of inhibitor, ofloxacin. In the absence of ofloxacin, large number of pits appear as shown in Fig. 13(d). Organic compounds containing hetero atoms (N,S and O) act as inhibitors because of the electron donating ability of theses atoms. Hackerman 27

Fig. 10: Potentiodynamic polarisation curves of mild steel exposed to 3.4% NaCl solution with different concentration of ciprofloxacin (1) control, (2) 20 ppm, (3) 30 ppm, (4) 40 ppm.

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Table 1 WEIGHT LOSS (mg) OF MILD STEEL IN 3.4 % SODIUM CHLORIDE SOLUTION FOR 30 DAYS

Conc.(ppm)

Control

250C

350C

450C

Wt.loss (mg) IE(%)

Wt.loss(mg) IE(%)

Wt.loss (mg) IE(%)

194.5

232.8

271.3

Norfloxacin 20 30 40 50

31.2 23.6 19.1 22.3

83.95 87.86 90.17 88.53

33.5 27.8 21.3 22.7

85.60 88.05 90.85 90.24

35.1 27.4 23.9 25.1

87.06 89.90 91.19 90.74

Ciprofloxacin 20 30 40 50

29.9 22.8 18.3 20.8

84.62 88.27 90.59 89.30

32.1 26.3 20.1 20.3

86.21 88.70 91.32 91.24

33.4 25.8 22.7 23.4

87.68 90.49 91.63 91.37

Pefloxacin 20 30 40 50

28.7 22.9 17.7 19.5

85.24 88.72 90.89 89.97

29.6 22.3 19.1 20.3

87.28 90.42 91.79 91.28

31.9 26.7 21.4 22.3

88.24 90.15 92.11 91.78

Ofloxacin 20 30 40 50

25.2 20.8 14.7 15.9

87.04 89.30 92.44 91.82

27.4 22.9 16.3 17.1

88.23 90.16 92.99 92.65

30.2 25.4 18.7 20.2

88.86 90.63 93.10 92.55

Amifloxacin 20 30 40 50

26.3 21.4 15.4 17.1

86.47 88.99 92.08 91.20

28.5 23.1 17.5 17.9

87.75 90.07 92.48 92.31

30.3 25.9 19.7 21.1

88.83 90.45 92.73 92.22

Enofloxacin 20 30 40 50

27.8 21.6 16.4 18.3

85.70 88.89 91.56 90.59

29.5 23.5 18.9 19.5

87.32 89.90 92.05 91.62

31.8 24.3 20.8 21.1

88.27 91.04 92.33 92.2

proposed that the inhibitive properties of amine type compounds are attributed to the adsorption of inhibitor on the metal surface through an unshared lone pair of electrons belonging to the nitrogen atom. According to Machu 28, the inhibitor molecules are uniformly adsorbed over the entire surface. The possible mechanism of corrosion inhibition of mild

steel in 3.4% NaCl solution can be explained on the basis of above mentioned concept. The adsorption of fluoroquinolones molecules on the metal surface through lone pair of electrons of nitrogen and oxygen atom decreases the dissolution rate of mild steel. The presence of fluoroquinolones leads to the formation of a complex film over the mild steel

303


Fig. 11: Potentiodynamic polarisation curves of mild steel exposed to 3.4% NaCl solution with different concentration of pefloxacin (1) control, (2) 20 ppm, (3) 30 ppm, (4) 40 ppm.

Fig. 12: Potentiodynamic polarisation curves of mild steel exposed to 3.4% NaCl solution with different concentrations of ofloxacin (1) control, (2) 20 ppm, (3) 30 ppm, (4) 40 ppm.

Fig. 13: SEM microphotographs of mild steel exposed to 3% NaCl solution (a) protective film (40 ppm, ofloxacin), (b) control, (c) effective protection, (d) localized attacked.

304


Table 3 POTENTIODYNAMIC POLARIZATION PARAMETERS OF MILD STEEL IN 3.4% NACL SOLUTION CONTAINING VARIOUS CONCENTRATION OF INHIBITORS

Conc (ppm)

Ecorr (mV)

Icorr DC DA (A/cm2) (mV/Decade) (mV/Decade)

RP (ohm cm2)

Corr. Rate (mm/y)

IE (%)

(control) -582.0

8.754E-05

656.7

336.7

1.104E+03

2.769

——

Norfloxacin 20 -570.5 30 -569.6 40 -569.6

1.937E-05 1.395E-05 1.009E-05

1.4068 872.4 584.2

312.8 269.3 214.8

5.738E+03 6.405E+03 6.764E+03

0.234 0.168 0.122

91.5 93.9 95.5

Ciprofloxacin 20 -532.5 30 -532.3 40 -532.1

8.908E-06 5.673E-06 4.145E-05

612.4 224.3 335.5

210.0 170.9 133.2

7.6623E+03 9.326E+03 1.122E+04

0.107 0.068 0.050

96.1 97.5 98.1

Pefloxacin 20 -569.5 30 -569.5 40 -569.5

6.602E-06 5.264E-06 4.229E-06

411.1 358.5 321.3

171.5 158.1 146.2

7.960E+03 9.050E+03 1.032E+03

0.080 0.063 0.051

97.1 97.7 98.1

Ofloxacin 20 -796.0 30 -796.2 40 -796.4

5.811E-06 4.104E-06 2.494E-06

145.6 123.6 104.0

294.5 249.2 212.3

7.281E+03 8.745E+03 1.216E+04

0.070 0.049 0.030

97.4 98.2 98.9

surface. This complex film act as a good barrier between the mild steel surface and aggressive environment. Therefore it prevents the corrosion.

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4. CONCLUSIONS

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