Influence of benzotriazole derivatives on the dezincification of 65–35 brass in sodium chloride

Applied Surface Science 239 (2005) 182–192

Influence of benzotriazole derivatives on the dezincification of 65–35 brass in sodium chloride R. Ravichandran, N. Rajendran* Department of Applied Sciences and Humanities, MIT Campus, Anna University, Chennai-600044, India Received in revised form 24 May 2004; accepted 24 May 2004 Available online 2 July 2004

Abstract The effect of new corrosion inhibitors namely N-[1-(benzotriazol-1-yl)ethyl]aniline (BTEA), and N,N-dibenzotriazol-1ylmethylaminoethane (DBME) on the dezincification of 65–35 brass in sodium chloride solution was investigated using weightloss measurements and electrochemical techniques such as potentiodynamic polarization and electrochemical impedance spectroscopy. Results obtained revealed that these compounds were very good inhibitors and behaved better in NaCl solution. Polarization studies showed that the BTEA and DBME behave as a mixed-type of inhibitors for 65–35 brass in sodium chloride solution. They decrease the anodic reaction rate more strongly than the cathodic reaction rate and renders the open circuit potential of brass more positive in NaCl solutions. Solution analysis revealed the decrease in dissolution of both copper and zinc in the presence of these inhibitors. # 2004 Elsevier B.V. All rights reserved. Keywords: Brass; Benzotriazole derivatives; Dezincification; Impedance; Polarization

1. Introduction Copper and its alloys are widely used in industry because of their good resistance to corrosion and are often used in cooling water system [1–5]. Brass has been widely used for shipboard condensers, power plant condensers and petrochemical heat exchangers [6–10]. Brass materials are relatively noble. However, 65–35 brass exhibited a-phase and are prone to corrosion attack. If the zinc content increases in the alloy, then the a-phase changes to b-phase which are more prone to corrosion attack. Nevertheless, it reacts

*

Corresponding author. Tel.: þ91-44-22203158; fax: þ91-44-22200660. E-mail address: [email protected] (N. Rajendran).

easily in ordinary environments containing oxygen. Dezincification of brass is one of the well-known and common processes by means of which brass looses its valuable physical and mechanical properties leading to failure of structure [11]. Thus, the study of its corrosion inhibition has attracted much attention. One of the most important methods in the corrosion protection of brass was the use of organic inhibitors. Nitrogen-containing organic heterocyclic compounds may act as inhibitors for corrosion of brass due to chelating action of heterocyclic molecules and the formation a physical blocking barrier on the brass surface [12,13]. Benzotriazole (BTA) has been proved to be one of the most important inhibitors for copper and copper alloy corrosion in neutral, acidic and alkaline solutions [14–16]. The action of BTA as a corrosion

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.05.145

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inhibitor for copper and its alloys in aqueous chloride media has long been of great interest to corrosion scientists and numerous studies have been performed [17–21]. The effectiveness of BTA has been related to the formation of a [Cuþ BTA]n film at low pH and positive electrode potential, the film is considered as being insoluble and polymeric [22,23]. Schweinsberg et al. [24] had done an extensive research on the action of benzotriazole and its derivatives for copper corrosion in acidic solution. Bag et al. [25] investigated the protective action of azole derivatives on the corrosion and dezincification of brass in ammonia solution. Walker [26] has shown that the addition of small amounts of 1,2,3-benzotriazole and 1,2,4-triazole inhibit the corrosion of brass in acidic, neutral and alkali solutions at ambient temperature. Huynh et al. [27] have studied the corrosion protection of octyl esters of carboxy benzotriazole on copper in an aerated sulfate solution. Mansfeld et al. [28] reported that the corrosion of copper in sodium chloride solution would be halted by the presence of BTA. Frignani et al. [29] investigated the influence of alkyl chain on the protective effects of benzotriazole towards copper in acidic chloride solution. Qafsaoui et al. [30] also reported that the growth of protective film on copper in the presence triazole derivatives. Despite these investigations, there remains little information on the mode of action of various functional groups in the benzotriazole derivatives on the corrosion and dezincification of brass. Likewise, there was a little data available on the impedance response of brass in BTA-containing solution. The aim of the present investigation was to study the inhibition efficiencies of N-[1-(benzotriazol-1yl) ethyl] aniline and N,N-dibenzotriazol-1-ylmethylaminoethane on the corrosion and dezincification of 65–35 brass in 3% NaCl solution. Interest in chloride media relates to the applicability of brass as the tubing material for condensers and heat exchangers in cooling water systems. Weight-loss method and electrochemical studies such as potentiodynamic polarization and impedance spectroscopy were used. Dezincification of brass was analyzed using atomic absorption spectroscopy. The composition of brass surface was analyzed using energy dispersive X-ray analysis.

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2. Experimental details 2.1. Materials The chemical composition (wt.%) of the brass used in the present study was 65.3% Cu, 34.44% Zn, 0.1385% Fe, 0.0635% Sn and trace amounts of Pb, Mn, Ni, Cr, As, Co, Al and Sr as analyzed by optical emission spectrophotometer. The brass specimens were polished mechanically with different grades of silicon carbide papers (120–1200) and were thoroughly washed with double distilled water then degreased in acetone and dried [24]. The inhibitors N-[1-(benzotriazol-1-yl)ethyl] aniline and N,N-dibenzotriazol-1-ylmethylaminoethane were synthesized according to the reported procedures [31]. The solutions were prepared from analar grade chemicals using double distilled water. The structures of benzotriazole derivatives are shown in Scheme 1. 2.1.1. Synthesis of BTEA and DBME Preparation of N-[1-(benzotriazol-1-yl) ethyl] aniline (BTEA). Benzotriazole (1.19 g, 10 mmol), acetaldehyde (12 mmol) and aniline (10 mmol) were refluxed in ethanol (minimum needed for complete solution) for 10 min. The mixture was kept at 25 8C for 5 h and 5 8C for 16 h. The resulting precipitate was filtered off, washed with diethyl ether and dried in

Scheme 1. Structures of benzotriazole derivatives.

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vacuo to give the crude N-[1-(benzotriazole-1-yl)ethyl]aniline. The product obtained was recrystallized with ethanol (yield 87%). Preparation of dibenzotriazole-1-ylmethylaminoethane (DBME). 1-Hydroxymethyl benzotriazole (2.98 g, 20 mmol), acetic acid (0.57 ml, 10 mmol) and ethanol (30 ml) and ethylamine (12 mmol) were refluxed for 2 min and poured into ice-water. The mixture was extracted with chloroform (50 ml) and the extracts washed with water and dried (MgSO4). Evaporation afforded the crude oily tertiary amine (DBMM). It was dried in vacuo (60 8C/30 mm Hg). The solid product was recrystallized from ether–pentane mixture (yield 76%). 2.2. Weight-loss studies Measurements of weight changes were performed with rectangular brass coupons (5 cm  3 cm 0:3 cm). The coupons were immersed in 300 ml of 3% NaCl solution with and without inhibitors and allowed to stand for 5 days at room temperature. Afterwards, the coupons were rinsed with distilled water and adherent corrosion products were removed by dissolving in 6% H2SO4 for 20 s. Then the coupons were rinsed with water, cleaned with acetone and dried [4]. Duplicate tests were conducted for each experiment. The percentage of inhibition efficiency (IE%) over the exposure period was calculated using the following equation:

1 cm2. The cell assembly consisted of brass as working electrode, a platinum foil as counter electrode and a saturated calomel electrode (SCE) as a reference electrode with a Luggin capillary bridge. Polarization studies were carried out using a potentiostat/galvanostat (Model PGSTAT 12) and the data obtained were analyzed using the GPES software version 4.9. The working electrode was immersed in a 3% NaCl solution and allowed to stabilize for 30 min [4]. Each electrode was immersed in a 3% NaCl solution in the presence and absence of different concentrations of the inhibitors to which a current of 1.5 mA/cm2 was applied for 15 min to reduce oxides. The cathodic and anodic polarization curves for brass specimen in the test solution with and without various concentrations of the inhibitors were recorded at a sweep rate of 1 mV/s. The inhibition efficiencies of the compounds were determined from corrosion currents using the Tafel extrapolation method. 2.5. Electrochemical AC impedance studies

where CRinh and CR are the rate of corrosion of brass with and without inhibitors, respectively.

AC impedance measurements were conducted at room temperature using an AUTOLAB with Frequency response analyzer (FRA), which included a potentiostat model PGSTAT 12. An AC sinusoid of 10 mV was applied at the corrosion potential (Ecorr). The frequency range of 100 kHz–1 mHz was employed. The brass specimen with an exposing surface area of 1 cm2 was used as the working electrode. A conventional three electrode electrochemical cell of volume 100 ml was used [5]. A saturated calomel electrode was used as the reference and platinum plate electrode was used as the counter. All the potentials reported here are with respect to SCE.

2.3. Open circuit potential measurement

2.6. Solution analysis

The open circuit potential (OCP) of brass specimen as a function of immersion period in 3% NaCl in the presence and absence of BTA derivatives were measured (versus SCE) till the steady state values were reached.

During the anodic polarization, the metal dissolution takes place releasing considerable amount of metal ions from the material. Hence, the solutions were analyzed to determine the leaching characteristics of the brass alloys. The solution left after polarization measurements were analyzed for copper and zinc by atomic absorption spectrometer to measure the amount of Cu and Zn leached out from the alloys. The solutions containing the optimum concentration of the inhibitor were chosen. A blank solution containing no

IE% ¼

CR  CRinh  100 CR

2.4. Potentiodynamic polarization studies The potentiodynamic polarization studies were carried out with brass strips having an exposed area of

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inhibitors was chosen for comparison. The dezincification factor (z) was calculated using the relation: z¼

ðZn=CuÞsol ðZn=CuÞalloy

where the ratio (Zn/Cu)sol is determined from solution analysis and (Zn/Cu)alloy the ratio of weight-percent in the alloy [32]. 2.7. Surface analysis The composition of the brass surface after polarization measurements was analyzed using Energy dispersive X-ray analysis (EDAX).

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maximum IE% of each compound was achieved at 150 ppm and a further increase in concentration showed only a marginal change in the performance of the inhibitor. Hence, the optimum levels of concentration of the inhibitors were found 150 ppm. The percentage IE of the DBME was higher than that of BTEA. The kinetics and mechanism of anodic dissolution and film formation of brass in neutral chloride solution can be thought of as taking place via the reactions outlined below [33]. In the initial corrosion stage, zinc forms ZnO as a result of Zn2þ þ H2 O ! ZnO þ 2Hþ or Zn þ H2 O ! ZnO þ 2Hþ þ 2e

(1)

and copper forms Cu2O as a result of 3. Results and discussion

2Cuþ þ H2 O ! Cu2 O þ 2Hþ

or

þ

2Cu þ H2 O ! Cu2 O þ 2H þ 2e

3.1. Weight-loss studies The variation of inhibition efficiency (IE%) with inhibitor concentrations are shown in Fig. 1. It has been observed that the inhibition efficiency increases with increase in concentration of the inhibitors. The

After the surface has become covered by both ZnO and Cu2O, CuCl is formed on the surface by the reaction Cuþ þ Cl ! CuCl

(3)

100

BTEA DBME

IE(%)

80

60

40 0

50

100

(2)

150

200

Concentration(ppm)

Fig. 1. Variation of IE% with inhibitor concentration (ppm) in 3% NaCl.

250

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inhibitor on the metal surface. Based on the results, DBME has the highest inhibition efficiency when compared to BTEA. Due to high molecular weight possessed by DBME molecule, the inhibition efficiency of DBME is higher than that of BTEA. Though BTEA have an extra phenyl ring, this ring strongly reduces their solubility and they are sterically hindered from adhering to the metal surface through the nitrogen atom.

which can then undergo the disproportionation reaction [34]: 2CuCl ! Cu þ CuCl2

(4)

or dissolve with the formation of CuCl2 complexes [35,36] via CuCl þ Cl ! CuCl2  3.1.1. Mechanism of corrosion inhibition It is well known that the inhibitive action of organic compound containing S, N and/or O is due to the formation of a co-ordinate type of bond between the metal and the lone pair of electrons present in the additive. The tendency to form co-ordinate bond and hence the extent of inhibition can be enhanced by increasing the effective electron density at the functional group of the additive. In aromatic or heterocyclic ring compounds, the effective electron density at the functional group can be varied by introducing different substituents in the ring leading to variations of the molecular structure. The higher inhibition efficiency of the compounds studied are due to the basis of donor–acceptor interactions between the p electrons of the inhibitor and the vacant d-orbital of copper surface or an interaction of inhibitor with already adsorbed chloride ions [34,35]. The corrosion inhibition property of BTEA and DBME are also due to the presence of heteroatom such as N and p electrons on aromatic nuclei. These factors played a vital role in the adsorption of the

3.2. Open circuit potential measurement The open circuit potential of brass specimens shifted towards more positive direction and attained a steady value after a lapse of 5 min. The steady state OCP values are given in Table 1. The OCP values were slightly affected in the presence of BTA derivatives in NaCl solution, indicating that the inhibitors control both anodic and cathodic reaction. 3.3. Potentiodynamic polarization studies The cathodic and anodic polarization curves of brass in 3% NaCl solution with varying concentrations of BTEA and DBME are shown in Figs. 2–4. The three distinct regions that appeared in the anodic polarization curve were the active dissolution region (apparent Tafel region), the active-to-passive transition region and the limiting current region. It is evident that in the presence of inhibitor, the cathodic and anodic curves were shifted towards

Table 1 Electrochemical parameters and inhibition efficiency for corrosion of brass in 3% NaCl containing different concentrations of benzotriazole derivatives Inhibitor concentration (ppm)

OCP (mV) vs. SCE

Ecorr (mV) vs. SCE

ba (mV dec1)

bc (mV dec1)

Icorr (mA cm2)

Blank

325

312

175

73

8.61

BTEA 50 100 150 200

276 266 235 237

268 259 230 231

84 58 43 45

39 34 32 34

3.16 2.49 0.83 0.84

63.30 71.08 90.36 90.24

DBME 50 100 150 200

267 254 230 232

259 248 226 227

74 60 41 42

35 32 27 29

2.52 1.87 0.52 0.54

70.73 78.28 93.96 93.73

Inhibition efficiency/(%) –

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Fig. 2. Polarization curves for brass in 3% NaCl containing different concentrations of BTEA.

positive potential region and the shift was found dependent on concentration of the inhibitor. Table 1 illustrates the corresponding electrochemical parameters. The Ecorr values were marginally shifted in

the presence BTEA and DBME. This observation clearly indicated that the inhibitors control both cathodic and anodic reactions and thus acting as mixed-type inhibitors. The current density also decreased with

Fig. 3. Polarization curves for brass in 3% NaCl containing different concentrations of DBME.

Fig. 4. Polarization curves for brass in 3% NaCl containing optimum concentrations of BTEA and DBME.

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Fig. 5. Nyquist diagrams for brass in 3% NaCl containing optimum concentrations BTEA and DBME after 1 h of immersion.

increasing concentrations of the inhibitors. The inhibition efficiency was calculated from Icorr values [37]: IE% ¼

Icorr  IcorrðinhÞ  100 Icorr

where Icorr(inh) and Icorr are corrosion current densities in the presence and absence of inhibitors, respectively. The values of cathodic Tafel slope (bc) and anodic Tafel slope (ba) of benzotriazole derivatives are found to change with inhibitor concentration, which clearly indicates that the inhibitors controlled both the reactions. The inhibition efficiency of BTEA and DBME attained a maximum value of 90.36 and 93.96% at 150 ppm concentration, respectively. The values of

inhibition efficiency increase with increasing concentration of inhibitor, indicating that a higher surface coverage was obtained in a solution with the optimum concentration of inhibitor. 3.4. AC impedance studies The corrosion behavior of brass in NaCl solution in the presence of benzotriazole derivatives was investigated by impedance method at room temperature. The impedance diagrams were not perfect semicircles, which may be attributed to the frequency dispersion [28]. Nyquist plots of brass in NaCl solution with and without optimum concentration of BTEA and DBME after immersion of 1, 24 and 48 h are shown in Figs. 5–7, respectively. IE% of brass was calculated

Fig. 6. Nyquist diagrams for brass in 3% NaCl containing optimum concentrations BTEA and DBME after 24 h of immersion.

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Fig. 7. Nyquist diagrams for brass in 3% NaCl containing optimum concentrations BTEA and DBME after 48 h of immersion.

by Rt as follows [38]: IE% ¼

ðRt Þ1  ðRtðinhÞ Þ1 ðRt Þ1

 100

where Rt(inh) and Rt are the charge-transfer resistance values with and without inhibitors respectively. IE% attained 94.12, 95.92 and 96.60 after immersion of 1, 24 and 48 h immersion, respectively, with optimum concentration of DBME, which was comparatively higher than that of BTEA in 3% NaCl solution. This can be attributed to more surface coverage of DBME on the brass from 3% NaCl solution. After 48 h immersion, the inhibition efficiency increases with increase in inhibitor concentration in 3% NaCl solution. The inhibition efficiency values determined using the polarization curves were lower than those determined by impedance experiments, this difference was probably due the shorter immersion time in the case of the polarization measurements [39]. Impedance parameters derived from these investigations are given in Table 2. In the presence of optimum concentration of

inhibitors, Rt values increased, whereas Cdl values tended to decrease. The decrease in Cdl values was caused by adsorption of benzotriazole derivatives on the metal surface. The relationship between Rt and Cdl values with immersion time of 1, 24 and 48 h are shown in Table 2. Rt values for brass in 3% NaCl increased with increase in immersion time while Cdl values are decreased with increase in immersion time. The tendency to decrease in Cdl, which can result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that the benzotriazole derivatives function by adsorption at the metal–solution interface [40]. The change in Rt and Cdl values was caused by the gradual replacement of water molecules by the anions of the NaCl and adsorption of the organic molecules on the metal surface, reducing the extent of dissolution [41]. 3.5. Solution analysis The results of solution analysis and the corresponding dezincification factor (z) in the presence and

Table 2 Impedance measurements and inhibition efficiency in 3% NaCl containing optimum concentrations of benzotriazole derivatives after 1, 24 and 48 h immersion Inhibitors

Blank BTEA DBME

Cdl (mF cm2)

Rt (104 O cm2)

Inhibition efficiency (%)

1h

24 h

48 h

1h

24 h

48 h

1h

24 h

48 h

0.196 2.13 3.38

1.02 12.7 24.9

1.89 24.8 54.8

3.325 0.371 0.263

0.613 0.079 0.033

0.415 0.033 0.012

– 90.78 94.12

– 91.94 95.92

– 92.40 96.60

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Table 3 Effect of optimum concentrations of benzotriazole derivatives on the dezincification of brass in 3% NaCl solution Inhibitors

Blank BTEA DBME

Solution analysis Cu (ppm)

Zn (ppm)

0.680 0.081 0.059

16.32 1.43 0.89

absence of benzotriazole derivatives at their optimum concentration level in 3% NaCl solution for brass are given in Table 3. The concentration of copper and zinc leached out from brass in the presence and absence of BTA derivatives are shown in Figs. 8 and 9, respectively. The results reveal that both copper and zinc were present in the solution. The copper/zinc ratio in solution was found smaller than that of the bulk alloy. This indicates that the growth of surface film and the dissolution of the alloy were controlled by diffusion [42], which is related to the difference between the ionic radii of Zn2þ and Cuþ ions, 0.07 and 0.096 nm, respectively. The results indicated that the inhibitors are able to minimize the dissolution of both copper and zinc. The percent inhibition efficiency against the

Dezincification factor (z) 44.57 32.78 28.01

Percent inhibition Cu

Zn

– 88.09 91.32

– 91.24 94.55

dissolution of zinc was correspondingly higher as compared to the dissolution of copper. This observation suggests that the benzotriazole derivatives excellently prevent the dezincification of brass in 3% NaCl solution, which is also reflected in the values of dezincification factor. 3.6. Surface composition analysis The surface composition (wt.%) of the alloy in the presence and absence of inhibitors are given in Table 4. In the absence of inhibitors, the wt.% of Cu and Zn are present in the surface were reduced due to the leaching of metal ions in 3% NaCl solution. Moreover, the higher concentration of chloride ions on the surface shows the penetration of Cl ions into the alloy.

Fig. 8. Concentration of copper leached out from brass at optimum concentrations of BTEA and DBME.

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Fig. 9. Concentration of zinc leached out from brass at optimum concentrations of BTEA and DBME.

Table 4 Surface composition (wt.%) of brass in 3% NaCl after polarization with optimum concentrations of inhibitors Inhibitors

Cu (wt.%)

Zn (wt.%)

Cl (wt.%)

Alloy Blank BTEA DBME

65.3 59.62 64.78 65.10

34.44 23.23 33.35 33.79

– 17.15 1.87 1.11

However, in the presence of BTEA and DBME, the wt.% of Cu and Zn is closer to that of the bulk composition of the alloy. Based on the surface analysis, these inhibitors exhibited excellent inhibition efficiency in sodium chloride solution.

4. Conclusions 1. Both BTEA and DBME show good inhibition efficiency in NaCl solution. The percent IE of DBME was higher than that of BTEA. 2. The polarization data indicate suppression of both the partial corrosion processes in the presence of BTA derivatives. These inhibitors behave as

mixed-type. They decrease the anodic reaction rate more strongly than the cathodic reaction rate and renders the open circuit potential of brass more positive in NaCl solutions. 3. The inhibitors easily adsorb on the brass surface at the corrosion potential and form a protective complex with the Cu(I) ion, preventing brass from corrosion. 4. Electrochemical impedance spectroscopy shows that Rt values increase, while Cdl values decrease in the presence of benzotriazole derivatives. 5. Solution analysis reveals that the benzotriazole derivatives excellently prevent the dezincification of brass.

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