Inhibition Of Corrosion Of Carbon Steel In A Well ...

Inhibition by SM – Zn2+ system

E-ISSN: Epshiba et al.

Inhibition Of Corrosion Of Carbon Steel In A Well Water By Sodium Molybdate – Zn2+ System R.Epshiba [a]*, A.Peter Pascal Regis [b] and S.Rajendran [c]

The inhibition efficiency of sodium molybdate (SM) in controlling corrosion of carbon steel in well water, in the presence of sodium molybdate (SM) and Zn 2+ has been evaluated by the weight loss method. The formulation consisting of 250 ppm of SM and 50 ppm of Zn2+ offers 92% inhibition efficiency to carbon steel. A synergistic effect exists between SM and Zn2+. Polarization study reveals that SM-Zn2+ system functions as an anodic inhibitor, and the formulation controls the anodic reaction predominantly. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra reveal that the protective film consists of Fe2+-MoO42- complex and Zn(OH)2. Keywords: carbon steel, corrosion inhibition, sodium molybdate, well water, synergistic effect. * Corresponding Authors Fax: E-Mail: [email protected] [a] Department of Chemistry, Sri Muthukumaran Institute of Technology, Chennai, Tamil Nadu, India 600026. [b] Department of Chemistry, St.Joesph’s College, Trichy, Tamil Nadu, India 624005; Email: [email protected]. [c] Department of Chemistry, RVS School of Engineering and Technology Dindigul, Tamil Nadu, India 624005; Email: [email protected].

1. INTRODUCTION Mild steel has many industrial applications because of its easy availability, low cost, uncomplicated fabrication of it into water pipelines [1,2], cooling water systems [3], boilers etc., However, they are susceptible to different forms of corrosion inducted by chloride and so on. One of the most important methods in corrosion protection is to use inhibitors [4-5]. Inhibitors should be of low toxicity and easily biodegradable in order to meet environmental protection requirements. Several oxyanions such as molybdate, tungstate, phosphate, and chromate have been used as corrosion inhibitors in cooling water. Molybdate, chromate, technitate, and nitrite oxyanions are known to act as inhibitors of metal corrosion in neutral and alkaline media. Their inhibiting action is often associated with the formation of insoluble compounds, “healing” the passive film on the metal. The passivation of metal is facilitated by using oxyanions of the oxidative type. Development of passivation coatings by cathodic reduction in sodium molybdate has been investigated [6]. Sodium molybdate shows corrosion inhibition in tap water and in pure water [7,8]. The influence of sodium molybdate on the semiconducting properties of the passive films formed on 304 stainless steel in 0.1 NaOH has been reported [9]. Li Yuchun et al. have described inhibitive mechanisms of molybdate used in combination with phosphate in neutral environments [10]. Protection of corrosion of carbon steel by inhibitors in chloride containing solutions has been reported [11]. Sodium molybdate has been used to control pitting corrosion of aluminum in the presence of chloride ion at pH 3.7 and 10.0. Polarization study and AC impedance have been used. A passiviting mechanism has been proposed [12]. Corrosion of carbon steel is controlled by sodium molybdate along with calcium gluconate. It was observed that sodium molybdate and calcium gluconate are nontoxic inhibitors [13]. Moreover, the potentiodynamic polarization and electrochemical impedance spectroscopy tests have been done to study the effect of molybdate concentration and hydrodynamic effect on mild steel corrosion inhibition in simulated cooling water [14]. Molybdate and tungstate were found to be effective inhibitors for cold rolling steel in hydrochloric acid solution [15]. Sodium molybdate can also be used to control corrosion of some iron-based ternary alloys. It was found that sodium molybdate and cobalt were used to control Int. J. Nano. Corr. Sci. Engg. 1(1) (2014)

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corrosion of iron-based FeCoC ternary alloys in a non-deaerated solution of NaHCO3 (10-3 M) and Na2 SO4 (10-3 M) at 25°C [16]. The present work is undertaken 1) to evaluate the inhibition efficiency (IE) of the sodium molybdate - Zn2+ system in controlling corrosion of carbon steel immersed in well water in the absence and presence of Zn2+ by the weight loss method; 2) to investigate the influence of the immersion period on the IE of the system; 3) to study the mechanism of corrosion inhibition by polarization study and AC impedance spectra; 4) to analyze the protective film by FTIR spectra , and, 5) to propose the mechanism of corrosion inhibition based on the above results.

2. EXPERIMENTAL 2.1. Preparation of the Specimen Carbon steel specimens (0.026 % S, 0.06 % P, 0.4 % Mn, 0.1 % C, and the rest iron) of the dimensions 1.0 cm × 4.0 cm × 0.2 cm were polished to a mirror finish and degreased with trichloroethylene and used for the weight loss method and surface examination studies.

2.2. Weight-Loss Method Carbon steel specimens were immersed in 100 ml of the well water, containing various concentrations of the inhibitor (sodium molybdate) in the absence and presence of Zn2+ for 3 days. The weights of the specimens before and after immersion were determined using a Digital Balance Model AUY 220 SHIMADZU. The corrosion products were cleaned with Clarke’s solution. It can be prepared by dissolving 20 gms of Sb 2O3 and 50 gms of SnCl2 in one litre of Conc.HCl of specific gravity (1.9)[17]. The corrosion IE was then calculated using the equation. IE = 100 [1-(W2/W1)] % where W1 is the corrosion rate in the absence of inhibitor and W 2 is the corrosion rate in the presence of inhibitor. Corrosion rate was calculated using the formula Millimetre per year = 87.6 W / DAT W = weight loss in milligrams D = density of specimen g/cm3 = 7.87 gm/cm3 A = area of specimen = 10cm2 T = exposure in hours = 72 hrs.

2.3. Surface Examination Study The carbon steel specimens were immersed in various test solutions for a period of 3 days. After 3 days, the specimens were taken out and dried. The nature of the film formed on the surface of the metal specimen was analyzed by various surface analysis techniques.

2.4. Fourier Transform Infrared Spectra These spectra were recorded in a Perkin-Elmer-1600 spectrophotometer using KBr pellet. The FTIR spectrum of the protective film was recorded by carefully removing the film, mixing it with KBr, and making the pellet.

Int. J. Nano. Corr. Sci. Engg. 1(1) (2014)

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2.5. Potentiodynamic Polarization Polarization studies were carried out in a CHI-electrochemical work station with impedance model 660A. It was provided with IR compensation facility. A three- electrode cell assembly was used. The working electrode was carbon steel. A SCE was the reference electrode. Platinum was the counter electrode. From polarization study, corrosion parameters such as corrosion potential (E corr), corrosion current (Icorr), Tafel slopes anodic = ba, and cathodic = bc were calculated and a linear polarization study was done. The scan rate (V/S) was 0.01. Hold time at (Efcs) was zero and quiet time (s) was two.

2.6. AC Impedance Spectra The instrument used for polarization study was used to record AC impedance spectra also. The cell set up was also the same. The real part (Z’) and imaginary part (Z’’) of the cell impedance were measured in ohms at various frequencies. Values of charge transfer resistance (Rf) and the double layer capacitance (Cdl) were calculated. AC impedance spectra were recorded with initial E (v) = 0, high frequency (Hz) = 1x105, low frequency (Hz) = 1, amplitude (V) = 0.005, and quiet time (s) = 2.

2.7. Synergism Parameter The synergism parameter can be calculated by using the equation indicates the synergistic effect existing between the inhibitors [18-20]. SI value is found to be greater than one suggesting that the synergistic effect between the inhibitors is SI=1-I1+2 /1-I’1+2 where I1 = inhibition efficiency of substance 1 I2 = inhibition efficiency of substance 2 I’1+2 = combined inhibition efficiency of substance 1 and 2. If synergistic effect exists between the inhibitors, S1 value will be greater than one.

2.8. Analysis of Variance (F-Test) An F-test was carried out to investigate whether the synergistic effect existing between inhibitor systems is statistically significant [21]. If F-value is greater than 5.32 for 1,8 degrees of freedom, the synergistic effect proves to be statistically significant. If it is less than 5.32 for 1,8 degrees of freedom, it was statistically insignificant at a 0.05 level of significance.

3. RESULTS AND DISCUSSION 3.1 Weight-Loss Study The physicochemical parameters of well water are given in Table 1. The corrosion inhibition efficiencies and the corresponding corrosion rates (millimetre per year) of sodium molybdate-Zn2+ systems are given in Table 2. Table 1. Physico-Chemical Parameters of Well Water

Parameter

Value

pH

8.38

Conductivity

3110µmhos/cm

Chloride

665 ppm

Sulphate

14ppm

TDS

2013ppm

Total hardness

1100ppm

The inhibition efficiency (IE) of sodium molybdate (SM) in controlling corrosion of carbon steel immersed in well water for a period of 3 days in the absence and the presence of Zn 2+ is given in Table 2. It can be seen Int. J. Nano. Corr. Sci. Engg. 1(1) (2014)

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from the data that SM alone shows some IE, whereas Zn2+ alone is found to be corrosive. When SM is combined with Zn2+ ions, it is found that the IE increases with the concentration of Zn 2+ ions. For example, 250 ppm SM has only 64% IE and 50 ppm of Zn2+ has only 10% IE. Interestingly, their combination shows 92% IE. This suggests a synergistic effect between the binary inhibitor formulation SM and Zn 2+ ions; SM is able to transport Zn2+ towards the metal surface. Table 2. Corrosion Rates (CR) of Carbon Steel in Well Water, in the Absence and the Presence of Inhibitors and Inhibition Efficiencies (IE) Obtained by Weight-Loss Method

SM (ppm)

2+

Zn 0

(ppm)

25

50

75

CR(mmy) 0

0.1174

0.1115

0.1056

0.0927

50

0.0950

0.0845

0.0763

0.0481

100

0.0845

0.0704

0.0493

0.0258

150

0.0751

0.0493

0.0340

0.0187

200

0.0587

0.0352

0.0211

0.0117

250

0.0422

0.0293

0.0093

0.0058

IE(%) 0

-

5

10

21

50

19

28

35

59

100

28

40

58

78

150

36

58

71

84

200

50

70

82

90

250

64

75

92

95

3.1.1. Synergism parameter The values of synergism parameters are shown in Table 3. The values of SI are greater than one, suggesting a synergistic effect. SI approaches 1 when no interaction exists between the inhibitor compounds. When S I >1, this points to the synergistic effect. In the case of SI0.05

3.2. Analysis of FTIR Spectra FTIR spectra have been used to analyze the protective film formed on metal surface [22,23]. FTIR spectrum of pure sodium molybdate is given in Figure 1a. The Mo-O stretching frequency appears at 824 cm-1. The FTIR spectrum of the film formed on the metal surface after immersion in the well water for 3 days containing 250 ppm of sodium molybdate and 50 ppm of Zn2+ is shown in Figure 1b. The Mo-O stretching frequency has decreased from 824 cm-1 to 700 cm-1 [24,25]. This indicates that the electrons present on the oxygen of molybdate have coordinated with Fe2+ formed on the metal surface resulting in the formation of Fe 2+-MoO42− complex on the anodic sites of the metal surface. The peak at 3404 cm-1 is due to Zn(OH)2 formed on the cathodic surface [26]. ACIC St.Joseph's College ( Autonomous) Trichy-2 Spectrum Name: SM.sp 100.0 95 90 85 80 1800.02

75 70

1384.54

65

2223.73

60 55 50

1698.45

%T 45

637.71 545.40

1678.04

40 35 902.39

30 25 3303.51

20

858.36 824.64

15 10 5 0.0 4000.0

3600

3200

2800

2400

2000

1800

1600

1400

1200

1000

800

600

400.0

cm-1

(a)


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Epshiba et al. ACIC St.Joseph's College ( Autonomous) Trichy-2

Spectrum Name: 250PPM SM- 50PPM Zn.sp 100.0 95

930.25

3913.30

1723.97

90 85 1816.72 1893.88

3699.10

80

700.24 1030.55

75

2189.89 3779.68 2096.51

70

481.74 855.70

65 60 %T 55 50

2928.03 1590.25

45 40 35 1479.12

30 3404.48

25 20.0 4000.0

3600

3200

2800

2400

2000

1800

1600

1400

1200

1000

800

600

400.0

cm-1

(b) Figure 1. FTIR Spectrum (a) Pure sodium molybdate (b) Film formed on metal surface after the immersion in well water containing 250ppm SM and 50 ppm of Zn2+

3.3. Analysis of Potentiodynamic Polarization Curves A polarization study has been used to detect the formation of protective film on the metal surface [27,28]. The potentiodynamic polarization curves of carbon steel immersed in well water medium are shown in Figure 2. The corrosion parameters of carbon steel immersed in various test solutions obtained by polarization study are given in Table 6. When mild steel is immersed in well water, the corrosion potential is ─763 mV vs SCE. The formulation consisting of 250 ppm SM + 50 ppm Zn2+ shifts the corrosion potential to ─625 mV vs SCE. This suggests that the anodic reaction is controlled predominantly, since more molybdate ions are transported to the anodic sites in the presence of Zn2+. The corrosion current Icorr for well water is 3.92x10-5 A/cm2. When SM (250 ppm) and Zn2+ (50 ppm) are added, it decreases to 2.062x10-5 A/cm2. The significant reduction in corrosion current indicates a decrease in corrosion rate in the presence of the inhibitor. Both the anodic and cathodic Tafel slopes are shifted in the presence of the inhibitor. However, a shift of 29 mV in the anodic Tafel slope (ba) is much higher as compared to a shift of 11 mV in the cathodic Tafel slope for the blank and SM (250 ppm)-Zn2+ (50 ppm) formulations. Thus, it can be concluded that this inhibitor formulation acts as a anodic inhibitor. This indicates that a protective film is formed on the metal surface. This study clearly reveals the fact that the addition of increasing concentrations of SM-Zn2+ moves the corrosion potential towards less value and reduces the corrosion rate. In the presence of these combinations, Icorr value decreases. Table 6. Corrosion Parameters of Carbon Steel in Well Water in the Presence of Inhibitors, Obtained by Polarization Study

Ecorr

ba

bc

LPR

I corr

mV vs SCE

mV

mV

Ω cm2

A/cm2

Well water (WW)

-763

148

64

496.9

3.92 x 10-5

WW + SM (250ppm) + Zn2+ (50ppm)

-625

119

75

973.5

2.062 x 10-5

System


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Figure 2. Polarization curves of carbon steel immersed in various test solutions a) Well water b) Well water containing 250 ppm of sodium molybdate and 50 ppm of Zn 2+

3.4. Analysis of AC Impendance Spectra AC impedance spectra have been used to detect the formation of film on the metal surface. If a protective film is formed, the charge transfer resistance increases and double layer capacitance value decreases [29,30]. The AC impedance spectra of carbon steel immersed in various solutions are shown in Figure 3. The AC impedance parameters, namely charge transfer resistance (Rt) and double layer capacitance (Cdl), are given in Table 7. When carbon steel is immersed in well water, the Rt value is 79.71 Ω cm2 and the Cdl value is 5.21 × 10-8 μF/0.5 cm2. When MoO42─ and Zn2+ are added to well water, the Rt value increases tremendously from 79.71 Ω cm2 to 183.52 Ω cm2 and the Cdl decreases from 5.21 ×10-8 μF/0.5 cm2 to 2.28 ×10-13. This suggests that a protective film is formed on the surface of the metal. This accounts for the very high IE of MoO 42- -Zn2+ system. The impedance value increases from 2.007 to 2.414. This accounts for the high inhibition efficiency of SM and Zn 2+ system and a protective film is formed on the mild steel surface. This is also supported by the fact that for the inhibitor system the phase angle increases from 29.33° to 33.26°.

Table 7. Impedance parameter of carbon steel in well water in the absence and presence of inhibitors, obtained by AC Impedance method

System

Rt, ohm cm2

Cdl, F cm-2

Impedance, log(Z ohm-1)

Phase angle

Well water (WW)

79.71

5.21x10-8

2.007

29.33°

WW + SM (250ppm) + Zn2+ (50ppm)

183.52

2.26x10-8

2.414

33.26°


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(a)

(b) Figure 3. Nyquist plots of carbon steel immersed in various test solution a) Well water b) Well water + SM 250 ppm + Zn2+ 50 ppm

(a)


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(b) Figure 4. Bode plots of carbon steel immersed in various test solution a) Well water b) Well water + SM 250 ppm + Zn2+ 50 ppm

3.5. Mechanism of Corrosion Inhibition Weight loss study reveals that the formulation consisting of 250 ppm of molybdate and 50 ppm of Zn2+ has 92% inhibition efficiency. Polarization studies reveal that the formulation SM-Zn2+ functions as an anodic inhibitor. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra indicate that the protective film consists of Fe2+-MoO42− complex and Zn(OH)2 . In order to explain all these observations in a holistic way, the following mechanism of inhibition of corrosion has been proposed: 1) When carbon steel specimen is immersed in an aqueous solution, the anodic reaction is: Fe → Fe 2++2e and the cathodic reaction is: 2H2O + O2 + 4e- → 4OH-. 2) When the system containing 250 ppm of SM and 50 ppm of Zn2+ is prepared, there is formation of Zn2+ – molybdate complex: Zn2++-MoO42− → Zn2+ -MoO42. 3) When carbon steel is immersed in the solution, the Zn2+-molybate diffuses from the bulk of the solution to the metal surface. 4) On the metal surface, Zn2+-MoO42− complex is converted into Fe2+- MoO42− complex on the anodic region, as the latter is more stable than the former. Zn2+-MoO42− + Fe2+ → Fe2+-MoO42− + Zn2+ 5) The released Zn2+ is deposited as Zn(OH)2 on the cathodic sites. Zn2+ + 2OH− → Zn(OH)2 ↓ 6) Thus the protective film consists of Fe2+-MoO42− complex and Zn(OH)2 . .

4. CONCLUSIONS The present study leads to the following conclusions: 

The Zn2+-MoO42−system shows a synergistic effect in controlling the corrosion of carbon steel immersed in well water .


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   

Epshiba et al.

The formulation consisting of 250 ppm of sodium molybdate and 50 ppm of Zn2+ has 92% inhibition efficiency. AC impedance spectra reveal that a protective film is formed on the metal surface. FTIR spectra reveal that the protective film consists of Fe2+-MoO42− complex and Zn(OH)2. Polarization studies reveal that the formulation MoO4-Zn2+ functions as an anodic inhibitor.

ACKNOWLEDGMENT Though only my name appears on the cover, a great peoples have contribute to this production. I owe my credit to all those people who have made this subject possible and because of whom my graduate experience has been one that I will cherish forever. The authors are thankful to their respective managements for their help and encouragement and to the University Grants Commission for financial assistance.

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Received: (08-08-2014) Accepted: (11-09-2014)


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