Corrosion inhibition of carbon steel in tetra-n-butylammonium bromide aqueous solution by benzotriazole and Na3PO4

Corrosion Science 51 (2009) 1356–1363

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Corrosion inhibition of carbon steel in tetra-n-butylammonium bromide aqueous solution by benzotriazole and Na3PO4 Song Liu a,*, Nannan Xu a, Jinmei Duan a, Zhenou Zeng a, Ziping Feng b, Rui Xiao b a b

Department of Applied Chemistry, South China University of Technology, Wushan 1, Guangzhou 510640, PR China Guangzhou Institute of Energy Conversion, CAS, Guangzhou 510640, PR China

a r t i c l e

i n f o

Article history: Received 20 November 2008 Accepted 13 March 2009 Available online 27 March 2009 Keywords: A. Steel B. EIS B. SEM B. Weight loss C. Neutral inhibition

a b s t r a c t The corrosion inhibition behavior of benzotriazole, Na3PO4 and their mixture on carbon steel in 20 wt.% (0.628 mol l1) tetra-n-butylammonium bromide aerated aqueous solution was investigated by weightloss test, potentiodynamic polarization measurement, electrochemical impedance spectroscopy and scanning electron microscope/energy dispersive X-ray techniques. The inhibition action of BTA or SP or inhibitors mixture on the corrosion of carbon steel is mainly due to the inhibition of anodic process of corrosion. The results revealed that inhibitors mixtures have shown synergistic effects at lower concentration of inhibitors. At 2 g l1 BTA and 2 g l1 SP showed optimum enhanced inhibition compared with their individual effects. Ó 2009 Elsevier Ltd. All rights reserved.

1. Introduction Tetra-n-butylammonium bromide (TBAB) has been widely used as intermediate, catalyzer in chemistry and pharmacy, and also applied in polarographic analysis. Recently, several investigations were carried out on tetra-n-butylammonium bromide since its clathrate hydrate crystals, formed under a temperature range between 0 and 12 °C depending upon the concentration of the aqueous solution, has advantages as a cold-storage material over ice of water in the air-conditioning system because it has a higher phase-change temperature and because it takes a form of slurry that can be transported directly through pipe lines [1–2]. The phase diagram, latent heat, specific heat, structure, free-growth forms, growth kinetics and growth mechanism of TBAB semi-clathrate hydrate crystals have been studied by many scientists [1–5], but very few investigations have been concerned with the corrosion behavior of carbon steel used to make the pipeline and valve of air-conditioning system in aqueous solution containing aggressive bromide anion. In order to reduce the corrosion of carbon steel an alternative is to use inhibitor. Corrosion inhibition of carbon steel is a result of a barrier film forming between the carbon steel and the environment. The remarkable efficiency of benzotriazole (BTA) [6–18] and Na3PO4 (SP) [18–33] as the corrosion inhibitor of the iron and steel in various solutions has been well established for over 40 years. In neutral solution, the protective action of the former has been attributed to the * Corresponding author. Tel.: +86 020 87114875; fax: +86 020 87112906. E-mail address: [email protected] (S. Liu). 0010-938X/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2009.03.021

formation of compact barrier layer consisting of surface complexes by adsorption on the metal surface [14–16], and of the later, attributed to the formation of the film containing phosphate and cFe2O3 on the carbon steel surface [19–23]. The aim of this paper is to study the action of BTA and SP as the alternative carbon steel corrosion inhibitor in 20 wt.% (0.628 mol l1) TBAB aerated aqueous solution by weight-loss test, potentiodynamic polarization measurement at a scan rate of 0.5 mV s1, electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM)/energy dispersive X-ray (EDX) techniques. The TBAB is soluble in the water, and the highest TBAB concentration in a solution was 8.3 mol kg1 H2O [34]. The pH value and conductivity of 20 wt.% (0.628 mol l1) TBAB in 25 °C are 6 and 1.51 S m1, respectively. 2. Experimental 2.1. Sample preparation The composition of carbon steel used in experiments is given in Table 1. Prior to the measurements, the exposed surface was pretreated by using emery papers grade (200-1200) and then rinsed by double distilled water and finally dried. All other reagents were of analytical grade. 2.2. Weight-loss measurements Weight-loss experiments were performed using specimens of dimension 50 mm25 mm2 mm in triplicate. The tests were

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S. Liu et al. / Corrosion Science 51 (2009) 1356–1363 Table 1 Chemical composition of the carbon steel studied (wt.%). Fe

C

Si

Mn

S

P

Ni

Cr

Cu

Al

Balance

0.60

0.01

0.35

0.015

0.005

0.02

0.03

0.10

0.05

conducted for 140 h by placing them into the test solution. After the stipulated time period, samples were taken out rinsed with distilled water, dried and weighted for change in weight from its initial weight.

that BTA or Na3PO4 or inhibitors mixture can protect carbon steel in TBAB aqueous solution. For instance, during the Na3PO4 addition to the corrosive medium, the corrosion rate decreased from 97.2 (0 g l1) to 17.1 mg m2 h1 (6 g l1), indicating that carbon steel corrosion speed decreased by increasing the inhibition process on the metallic surface (78%). The corrosion inhibition efficiency was calculated as follows [6]:

I:E: ð%Þ ¼ ð1  W=W 0 Þ  100

ð1Þ

where W0 and W are the values of weight-loss of carbon steel after 140 h of immersion in solutions without and with inhibitor, respectively.

2.3. Electrochemical measurements 3.2. Potentiodynamic(Tafel) polarization measurement Electrochemical measurements were performed in three-electrode glass cell at room temperature, a platinum electrode was used as counter electrode, and saturated calomel electrode (SCE) was used as reference electrode. The working electrode in this work was embedded in epoxy resin, leaving a geometrical surface area of 0.1 cm2 exposed to the electrolyte. Potentiodynamic (Tafel) polarization curves and electrochemical impedance measurements were carried out using a computer controlled CHI 660C Electrochemical Workstation, accordingly all experiments were carried out after 1 h immersion of metal specimens into the electrolyte. Tafel polarization cures were obtained using a sweep rate of 0.5 mV s1 in the potential range of ±200 mV with respect to the initial open circuit potential. The linear Tafel segments of the anodic and cathodic curves were extrapolated to corrosion potential to obtain the corrosion current densities. Electrochemical impedance spectra were obtained in the frequency range of 100 kHz to 0.01 Hz with perturbation amplitude of 5 mV at the corrosion potential. 2.4. Surface analysis Examination of carbon steel surface after 24 h exposure to the 20 wt.% (0.628 mol l1) TBAB solution without and with inhibitor(s) was carried out by Hitachi S-550 Scanning Electron Microscope. Rough elemental analyses for the exposed surface were conducted by EDX technique.

The obtained Tafel polarization curves for carbon steel at various concentrations of BTA, Na3PO4 and inhibitors mixture are shown in Fig. 1. Table 3 indicates the Tafel polarization parameters for electrolyte without and with inhibitor(s), such as the corrosion potential (Ecorr), the corrosion current density (Icorr), cathodic and anodic Tafel slopes (bc and ba) and inhibition efficiency (I.E.). The inhibition efficiencies of the inhibitors have been calculated using [17]:

I:E: ð%Þ ¼

I0corr  Icorr I0corr

 100

ð2Þ

where I0corr and Icorr are the corrosion current density values of the specimens in solutions without and with addition of inhibitor. A reduction in the corrosion rate of a different degree was observed in the presence of inhibitors. For example, in the absence of inhibitor, the corrosion current density was 31.6 lA cm2, and drastically reduced to 5.1 lA cm2 in the solution containing 2 g l1 BTA and 2 g l1 Na3PO4. It can be seen from the polarization curves (Fig. 1) that the corrosion potential (Ecorr) of the electrode shifts positively due to addition of BTA or Na3PO4 or inhibitors mixture. Based on the marked decrease of the anodic current density and the positive shift in the corrosion potential upon introduction of BTA or Na3PO4 or inhibitors mixture in the aggressive solution, BTA or Na3PO4 or inhibitors mixture is considered as the inhibitor of predominant anodic effect [35]. 3.3. Impedance spectroscopy

3. Results 3.1. Weight-loss measurements Table 2 shows the I.E. (inhibition efficiency) and corrosion rate of carbon steel obtained by weight-loss measurements for different concentrations of BTA, Na3PO4 and inhibitors mixture, indicating

Table 2 Corrosion rate of carbon steel and inhibition efficiency for BTA, Na3PO4 and their mixtures for the corrosion of carbon steel in 20 wt.% (0.628 mol l1) TBAB solution obtained from weight-loss measurements. CBTA (g l1)

CSP (g l1)

I.E. (%)

Corrosion rate (mg m2 h1)

0 2 4 6 8 0 0 0 0 1 2 4 6

0 0 0 0 0 2 4 6 8 1 2 4 6

– 44.1 66.2 78.0 78.1 33.0 65.3 77.8 65.8 34.2 84.8 55.0 33.0

97.2 54.3 32.8 21.4 21.3 65.1 33.7 21.6 33.2 63.9 14.8 43.7 65.1

Impedance data in the form of Nyquist plots of carbon steel at the open circuit potential in 20 wt.% (0.628 mol l1) TBAB without and with inhibitor(s) are presented in Fig. 2. The curves have a diagonal line at lower frequencies, which is attributed to the diffusion-related Warburg impedance. The Warburg impedance observed can be attributed to the diffusion of soluble iron species from the electrode surface to the bulk solution or the diffusion of dissolved oxygen or aggressive anions (Br) from the bulk solution to the electrode surface. An equivalent circuit similar to that proposed by Etteyeb et al. [22] shown in Fig. 3 were used to fit impedance data obtain from the 20 wt.% (0.628 mol l1) TBAB aqueous solution in the absence and presence of inhibitor(s). In these figures, Re represents the solution resistance, Rct the charge transfer resistance, Rf the resistance of the film formed; W is the Warburg impedance element, for which a lower value indicates a greater resistance of the diffusion of corrosive agents [36]. Q1 represents the capacitive behavior at the electrolyte/metal interface (i.e. the double layer), Q2 the capacitive behavior of the passive film formed. The symbols Q signify the possibility of a non-ideal capacitance (CPE, constant phase element) with varying n. The appearance of the CPE is often related to the electrode roughness or to the imhomogeneity in the conductance or dielectric constant [12–13,36–39]. The impedance of the CPE is given by [37]:

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S. Liu et al. / Corrosion Science 51 (2009) 1356–1363 -2.0

-2.5

-2.5

-3.0

-3.0

-3.5

-3.5

-2 log (i / A cm )

-2 log (i / A cm )

-4.0 -4.5 -5.0 -5.5 -6.0

-4.0 -4.5 -5.0 -5.5

1

-6.0

1

-6.5

2

-7.0 -0.80

-0.75

-0.70

-0.65

3

-0.60

3

-6.5

4

4

-7.0

5 -0.55

2 5

-0.50

-0.45

-0.85

-0.40

-0.80

-0.75

-0.70

-0.65

E / V (SCE)

E / V (SCE)

(a)

(b)

-0.60

-0.55

-0.50

-0.45

-2.0 -2.5 -3.0

-2 log (i / A cm )

-3.5 -4.0 -4.5 -5.0 -5.5 -6.0

1

-6.5

5

2

-7.0

4

3

-0.85 -0.80 -0.75 -0.70 -0.65 -0.60 -0.55 -0.50 -0.45 -0.40 -0.35 -0.30

E / V (SCE)

(c) Fig. 1. (a) Tafel polarization curves for carbon steel in 20 wt.% (0.628 mol l1) TBAB solution in the presence of the following concentrations of BTA (g l1): (1) blank; (2) 2; (3) 4; (4) 6; (5) 8. (b) Tafel polarization curves for carbon steel in 20 wt.% TBAB solution in the presence of the following concentrations of Na3PO4 (g l1): (1) blank; (2) 2; (3) 4; (4) 6; (5) 8. (c) Tafel polarization curves for carbon steel in 20 wt.% TBAB solution in the presence of inhibitors mixtures: (1) blank; (2) 1 g l1 BTA +1 g l1 Na3PO4; (3) 2 g l1 BTA +2 g l1 Na3PO4; (4) 4 g l1 BTA +4 g l1 Na3PO4; (5) 6 g l1 BTA +6 g l1 Na3PO4.

Table 3 Polarization parameters of BTA, Na3PO4 inhibitors and their mixtures in 20 wt.% (0.628 mol l1) TBAB solution. CBTA (g l1)

CSP (g l1)

Ecorr (mV (SCE))

Icorr (lA cm2)

ba (mV dec1)

Anodic Tafel region (mV (SCE))

Raa

bc (mV dec1)

Cathodic Tafel region (mV (SCE))

Rcb

I.E. (%)

0 2 4 6 8 0 0 0 0 1 2 4 6

0 0 0 0 0 2 4 6 8 1 2 4 6

698.0 610.0 573.0 567.0 533.0 683.0 694.0 650.0 668.0 628.0 585.0 591.0 682.0

31.6 19.0 10.5 8.5 9.5 26.1 13.0 9.7 15.3 16.6 5.1 17.0 20.4

149 ± 6.0 124 ± 1.2 95 ± 1.9 94 ± 9.4 125 ± 5.0 141 ± 3.0 114 ± 10.0 100 ± 15.0 166 ± 11.0 118 ± 25.0 321 ± 64.0 182 ± 5.3 123 ± 8.6

790 to 760 545 to 480 500 to 450 530 to 471 469 to 421 621 to 580 623 to 582 610 to 578 582 to 551 600 to 552 540 to 499 461 to 391 560 to 492

0.9986 0.9999 0.9998 0.9970 0.9996 0.9996 0.9968 0.9940 0.9979 0.9825 0.9130 0.9992 0.9974

88 ± 8.8 130 ± 2.0 122 ± 12.0 147 ± 15.0 152 ± 6.3 114 ± 6.8 66 ± 3.0 143 ± 6.0 102 ± 10.0 116 ± 6.0 133 ± 4.0 166 ± 16.0 105 ± 10.0

640 to 710 to 690 to 64 0to 640 to 780 to 800 to 770 to 770 to 740 to 720 to 700 to 740 to

0.9980 0.9999 0.9961 0.9952 0.9991 0.9993 0.9996 0.9989 0.9931 0.9991 0.9994 0.9985 0.9942

– 40.0 66.8 73.0 70.0 17.4 58.8 69.0 51.6 47.5 83.8 46.2 35.4

a b

Ra, the linear regression coefficient of anodic Tafel region. Rc, the linear regression coefficient of cathodic Tafel region.

561 670 630 600 590 750 760 710 740 690 650 670 690

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S. Liu et al. / Corrosion Science 51 (2009) 1356–1363 250

40 36

200

32

2

-Z'' (ohm cm )

2

-Z'' (ohm cm )

28 24 20 16

150

100

12

1

8

2

50

4 0

0 0

5

10

15

20

25

30

35

40

45

50

55

0

50

100

2

150

(a)

250

300

(b) 300

250

250

200

200

2

-Z'' (ohm cm )

300

2

-Z'' (ohm cm )

200 2

Z' (ohm cm )

Z' (ohm cm )

150

3

100

150

2 100

4 50

50

1 0

0 0

50

100

150

200

250

300

350

400

0

450

50

100

150

200

250

300

350

2

Z' (ohm cm )

Z' (ohm cm2)

(c)

(d) 350

384

315

336 280 245

240

2

-Z'' (ohm cm )

2

-Z'' (ohm cm )

288

192 144

4

96

210 175

2 140 105 70

3 48

1

35

0

0

0

48

96

144

192

240

288

336

384

432

480

0

50

100

150

200

250

300

350

Z' (ohm cm )

Z' (ohm cm2)

(e)

(f)

2

400

450

500

550

200 180 160

2

Z'' (ohm cm )

140 120 100 80 60

3

40

4

20 0 0

30

60

90

120

150

180

210

240

270

300

2

Z' ( ohm cm )

(g) 1

Fig. 2. Nyqusit plots for carbon steel in 20 wt.% (0.628 mol l ) TBAB solution. (a) without inhibitor; (b), (c) in the presence of the following concentrations of BTA (g l1): (1) 2, (2) 4, (3) 6, (4) 8; (d), (e) in the presence of the following concentrations of Na3PO4 (g l1): (1) 2, (2) 4, (3) 6, (4) 8; (f), (g) in the presence of inhibitors mixtures: (1) 1 g l1 BTA +1 g l1 Na3PO4, (2) 2 g l1 BTA +2 g l1 Na3PO4, (3) 4 g l1 BTA +4 g l1 Na3PO4, (4) 6 g l1 BTA +6 g l1 Na3PO4. The points represent experimental data and the solid lines are spectra simulations based the proposed electrical equivalent circuit shown in Fig. 3.

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Fig. 3. Equivalent circuit used to fit the EIS data on carbon steel surface in 20 wt.% (0.628 mol l1) TBAB solution without and with inhibitor(s).

Q ¼ Z CPE ðxÞ ¼ ½Cjxn 1

ð3Þ

For n = 1, the Q element reduces to a capacitor; for n = 0, to a simple resistor and for n = 0.5, to a Warburg impedance element. The points in the Fig. 2 represent experimental data and the solid lines are spectra simulations based the proposed electrical equivalent circuit shown in Fig. 3. The fitted parameters are summarized in Table 4. The I.E. (%) of the inhibitors has been calculated using [39,40]:

I:E: ð%Þ ¼

Rct  R0ct  100 Rct

ð4Þ

where R0ct and Rct are charge transfer resistance for carbon steel in 20 wt.% (0.628 mol l1) TBAB, in the absence and presence of inhibitor, respectively. As it is observed in Table 4, an addition of BTA or Na3PO4 or their mixture caused an increase in charge transfer resistance (Rct), an increase of resistance of the film formed (Rf), a sharp decrease in capacitive of the double layer (Q1) and a decrease of Warburg impedance element (W). These results indicate that BTA or Na3PO4 or their mixture is efficient carbon steel corrosion inhibitor in TBAB solution. For instance, during the BTA addition to corrosive medium, Rct increased from 37.7 (0 g l1) to 219.8 ohm cm2 (6 g l1), Rf increased from 0.3 to 15 ohm cm2, Q1 decreased from 248.5  104 to 8.5  104 ohm1 sn cm2, and W decreased from 19.6  102 to 1.8  102 ohm1 s0.5 cm2. Decrease in Q1, which can result from a decrease in local dielectric constant and/or an increase in the thickness of electrical double layer, suggests that the BTA functions by adsorption at the steel–solution interface [6,41]. As shown in Tables 2–4, the corrosion inhibition efficiencies determined by the three methods (weight-loss test, Tafel polarization and impedance measurements) are in reasonably good agreements. 3.4. Surface analysis The surface highly corroded of the carbon steel panel was observed after 24 h of immersion in a solution of 20 wt.%

Fig. 4. SEM image (a) and EDX spectrum (b) of carbon steel immersed for 24 h in 20 wt.% (0.628 mol l1) TBAB aqueous solution without inhibitor (magnification for image = 1000).

(0.628 mol l1) TBAB without inhibitor (Fig. 4a), and EDX analysis showed the presence of Fe, O and Br (Fig. 4b) of corrosion product. As shown in Fig. 5a, in the presence of BTA (6 g l1), the rate of corrosion is suppressed, and the reduced intensity of oxygen peak of EDX spectrum (Fig. 5b) is due to the decrease of corrosion products (iron oxides). For the solution containing Na3PO4 (6 g l1), we

Table 4 Impedance parameters during corrosion inhibition of carbon steel in 20 wt.% (0.628 mol l1) TBAB solution with and without inhibitor(s). CBTA (g l1)

CSP (g l1)

Q1  104 (ohm1 sn cm2)

n1

Rf (ohm cm2)

Q2  106 (ohm1 sn cm2)

n2

Rct (ohm cm2)

W  102 (ohm1 s0.5 cm2)

I.E. (%)

0 2 4 6 8 0 0 0 0 1 2 4 6

0 0 0 0 0 2 4 6 8 1 2 4 6

248.5 15.3 11.5 8.5 11.2 79.6 33.4 34.0 52.8 63.2 57.3 98.9 24.5

0.50 0.67 0.67 0.65 0.67 0.57 0.66 0.56 0.60 0.75 0.58 0.62 0.75

0.3 9.8 19.3 15.0 16.3 3.3 4.4 5.1 3.7 4.5 11.2 4.7 4.9

27.7 19.9 14.2 17.0 17.2 24.2 20.2 7.1 6.5 26.5 19.3 24.3 17.2

0.83 0.99 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

37.7 72.4 111.0 219.8 229.2 55.5 88.0 144.1 108.2 60.1 408.4 84.9 54.1

19.6 2.7 2.7 1.8 4.3 4.3 1.8 1.6 1.3 2.8 2.6 3.7 2.3

– 47.9 66.0 82.8 83.5 32.1 57.1 73.8 65.1 37.2 90.8 55.6 30.3

S. Liu et al. / Corrosion Science 51 (2009) 1356–1363

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Fig. 5. SEM image (a) and EDX spectrum (b) of carbon steel immersed for 24 h in 20 wt.% (0.628 mol l1) TBAB aqueous solution containing 6 g l1 BTA (magnification for image = 1000).

Fig. 6. SEM image (a) and EDX spectrum (b) of carbon steel immersed for 24 h in 20 wt.% (0.628 mol l1) TBAB aqueous solution containing 6 g l1 Na3PO4 (magnification for image = 1000).

observed a much lower density of pits at the materials surface (Fig. 6a) compared with the inhibitor-free solution, and the EDX spectrum showed extra P which could be attributed to the presence of phosphate on the carbon steel surface (Fig. 6b). The SEM image of the surface of the panel formed in the presence of inhibitors mixture in proportion of 2 g l1 BTA and 2 g l1 Na3PO4 are shown in Fig. 7a. The surface showing the clear polishing lines is almost free from corrosion, and the EDX spectrum showed P and N which could be attributed to the presence of the phosphate and BTA molecules on the carbon steel surface (Fig. 7b).

complex polymer of [Fen(BTA)p]m is formed, which may suppress the dissolution and oxidation of Fe effectively [14–16]. As indicated in Tables 2–4, the inhibition efficiency increased with the increase of BTA concentration and began to stabilize at 6 g l1 level, indicating that a higher surface coverage was obtained in a solution with a higher concentration of inhibitor (6–8 g l1). The corrosion inhibition can be attributed to adsorption of BTA molecules at steel/TBAB solution interface. Trisodium phosphate is widely used industrial inhibitor whose inhibition effect is due to specific passivation of carbon steel by deposition the metal phosphate from the solution, the accumulation on the carbon steel surface of a poorly soluble Fe phosphate creates conditions favorable for ordinary oxide passivation. The protective film formed on the carbon steel surface in the presence of Na3PO4 consists of Fe phosphate and c-Fe2O3 [18–33]. Moreover, competitive adsorption exists between PO4 3 ions and aggressive Br ions on the passive electrode surface and this retards their corresponding destructive action. As it is observed in Tables 2–4, the inhibition efficiency increases with the increase of Na3PO4 concentration from 2 to 6 g l1, and shows a maximum value at 6 g l1 concentration. But, an adverse effect on corrosion inhibition is observed when the Na3PO4 concentration increased to higher values.

4. Discussion It is well-known that the inhibitive action of organic compounds containing N is due to the formation of co-ordinate type bond between the metal and the lone pair of electrons of the additive. BTA is described as good inhibitor for metals [6–18,37,42–44]. Some scientists thought the protective action of this inhibitor has been attributed to the formation of a polymeric film of metalBTA on the metal surface. Recently, The results from surface-enhanced Raman scattering (SERS) show that BTA interacts with Fe surface through its two N atoms of the triazole ring and surface

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and strong adsorption of BTA on the passive layer or/and incorporation of BTA into the passive layer. However, an adverse effect on inhibitors combinations was observed when the concentration of BTA and Na3PO4 increased to higher values. Antagonism was observed at 4 g l1 BTA, 4 g l1 Na3PO4 and 6 g l1 BTA, 4 g l1 Na3PO4. This finding may be interpreted by hypothesising that, unfavorable scale inhibitive influence of BTA on Na3PO4 corrosion inhibition exists which forms over the surface of corrosion products. The similar unfavorable scale inhibitive influence of 1-hydroethyidene-1,1-diphosphonic acid on sodium silicate corrosion inhibition has already observed in the soft water media [39]. Further investigation is needed. 5. Conclusions The corrosion inhibition of carbon steel in 20 wt.% (0.628 mol l1) tetra-n-butylammonium bromide (TBAB) aqueous solution with and without different concentration of BTA, Na3PO4 and their mixtures have been investigated by weight-loss test, Tafel polarization curves, impedance spectroscopy (EIS) and SEM/ EDX techniques and the main conclusions are:

Fig. 7. SEM image (a) and EDX spectrum (b) of carbon steel immersed for 24 h in 20 wt.% (0.628 mol l1) TBAB aqueous solution containing 2 g l1 BTA and 2 g l1 Na3PO4 (magnification for image = 1000).

This may be due to the formation of looser Fe phosphate layer resulting from the higher nucleation rate of Fe phosphate when the Na3PO4 concentration is higher. As shown in Tables 2–4, the inhibitors combinations resulted in higher efficiencies compared to their individual addition for the solution containing 2 g l1 BTA and 2 g l1 Na3PO4. The results of Banczek et al. [45] also showed better corrosion resistance for the steel phosphated with BTA comparatively to that without BTA. Rammelt et al. [12] have investigated the synergistic effect of benzoate and BTA on passivation of mild steel in near neutral air saturated aqueous solution. They think benzoate acts primarily on the rate of iron dissolution by plugging the pores in the air formed oxide layer with insoluble ferric benzoate complexes, whereas BTA is strongly adsorbed on the oxide layer. The synergistic effect in the corrosion is due to a more favorable adsorption of BTA compared with benzoate. Insofar that the conclusions of the cited investigation [12] can be transposed to the present case of BTA and Na3PO4 on the carbon steel, this finding implies that the main function of Na3PO4 seems to form the layer containing Fe phosphate and c-Fe2O3, whereas BTA can stabilize this layer by incorporation into it or/and adsorption on it conjectured by the N and P signals of EDX spectrum (Fig. 7b) so that passivation occurs after immersion of carbon steel into the solution containing both inhibitors. A synergistic effect in the corrosion can be attributed to a blocking of surface sites for anodic dissolution by Na3PO4

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