Effect of ethylenediamine tetraacetic acid disodium on the corrosion of cold rolled steel in the presence of benzotriazole in hydrochloric acid

Electrochimica Acta 52 (2007) 6811–6820

Effect of ethylenediamine tetraacetic acid disodium on the corrosion of cold rolled steel in the presence of benzotriazole in hydrochloric acid Qing Qu a,∗ , Shuan Jiang a , Wei Bai b , Lei Li c a

Department of Chemistry, Yunnan University, Kunming 650091, China Department of Chemistry, Yunnan Nationalities University, Kunming 650092, China c Laboratory for Conservation and Utilization of Bio-Resources, Yunnan University, Kunming 650091, China b

Received 5 February 2007; received in revised form 26 April 2007; accepted 26 April 2007 Available online 10 May 2007

Abstract The inhibition behavior of cold rolled steel in 0.1 M hydrochloric acid (HCl) by ethylenediamine tetraacetic acid disodium (EDTA) in the absence and presence of benzotriazole (BTA) was investigated with Tafel polarization curve and electrochemical impedance spectroscopy (EIS). The polarization curve results show that the single EDTA acts as an anodic type inhibitor while the combination of EDTA and BTA acts as mixed type inhibitor and mainly inhibits anodic reaction. All impedance spectra in EIS tests exhibit one capacitive loop which indicates that the corrosion reaction is controlled by charge transfer process. Inhibition efficiencies obtained from Tafel polarization, charge transfer resistance (Rt ) are consistent. The corrosion of cold rolled steel in 0.1 M HCl is obviously reduced by EDTA in combination with lower concentrations of BTA. Fourier transform infrared spectroscopy (FTIR) and atomic force microscopy (AFM) were used to characterize the corrosion surface of cold rolled steel. Probable mechanisms are present to explain the experimental results. © 2007 Elsevier Ltd. All rights reserved. Keywords: EIS; Tafel polarization curve; Cold rolled steel; EDTA; BTA

1. Introduction Hydrochloric acid solutions are widely used for acid clearing, industry cleaning, acid descaling and oil-well acidizing, etc. Therefore, corrosion inhibitors for hydrochloric acid have attracted more attention because of wide application [1–6]. In most inhibitor studies, the formation of donor–acceptor surface complexes between free or pi-electrons of an inhibitor and vacant d-orbital of metal was proposed [1,7–9]. Thus, compounds with nitrogen and oxygen function group are considered to be one of the effective chemicals for inhibiting the corrosion of metals [1,8–11]. EDTA is the most commonly used as chelating agent containing nitrogen and oxygen function group, which has been found widespread industrial use in electrodeposition [12], metal recovery [13], separation of V4+ and V5+ [14] and chemical decontamination for metal deposits [15,16] because of its strong

∗

Corresponding author. Tel.: +86 871 5035798; fax: +86 871 5036538. E-mail addresses: [email protected], [email protected] (Q. Qu).

0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.114

metal complexing properties. Due to the widespread application of EDTA, some researchers studied the corrosion of metals in EDTA solutions [17,18]. Yao et al. [17] studied the corrosion behavior of 20A carbon steel in 4% EDTA solution using steadystate polarization curve method at 30 ◦ C and corrosion rates at various temperatures, their results showed that the corrosion progress was controlled by both hydrogen evolution and oxygen reduction reactions in the temperature range of 30–150 ◦ C. Padma et al. [18] investigated the effects of various additives such as PH additive, reducing agent, oxidizing agent and corrosion inhibitor on the corrosion of carbon steel and Monel-400 alloy in EDTA based steam generator cleaning formulations. Furthermore, Studies on the effect of EDTA on the corrosion and film formation of metals in various media have also been reported [19–22]. For example, Miloˇsev et al. [19,20] studied the corrosion of the stainless steel in the physiological solutions, their results indicated that the addition of EDTA induced increase in metal dissolution and disturbed the formation of the passive layer. However, researches by Gadiyar et al. [21] showed that EDTA could inhibit the corrosion of carbon steel, but the inhibitive effect was not very excellent. Capobianco et al. [22]

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investigated the effect of EDTA-hydroxylamine sulfate-Fe2+ on the corrosion of type 316L stainless steel in the industrial crystallization plant, their results showed that the system was an effective inhibitor of the corrosion of the steel. Additionally, Bennett et al. [23] investigated the antimicrobial properties of triazole compounds (containing BTA); they found that the inhibitory actives of triazole compounds were markedly potentiated when employed in conjunction with EDTA in metalworking fluids. So it is interesting in studying the corrosion of steel in the presence of triazole compounds and EDTA because of their potential use in metalworking fluids. BTA is a heterocyclic compound which can effectively inhibit the corrosion of copper [24–26]. Furthermore, BTA used as an inhibitor for steel in acid solutions has been reported [27,28]. For example, Gomma [27] investigated the corrosion inhibition of steel by BTA in sulphuric acid, their results revealed that the corrosion rate depended on the concentration of BTA and chloride ions along with the sweep rate of polarization, and the maximum inhibition efficiency was obtained at 9 × 10−3 M BTA. A study by Bellaouchou et al. [28] indicated that BTA affected both anodic and cathodic process of corrosion inhibition under heat transfer of 904L stainless steel in phosphoric acid, and the highest inhibition efficiency was obtained at 0.1 M BTA. Generally speaking, higher inhibition efficiency of BTA is only obtained at higher concentration. In order to increase the inhibition efficiency in lower concentration of BTA, some researchers studied the synergistic effects of BTA and other compounds [29,30]. However the combined effect of EDTA and BTA on the corrosion of carbon steel has not yet been reported. The objective of this investigation is to determine the combined effect of EDTA and BTA on the corrosion of cold rolled steel. Meanwhile, probable inhibitive mechanisms are presented to explain the experimental observations.

inum foil and the reference electrode is a saturated calomel electrode (SCE) with a Luggin capillary positioned close to the working electrode surface in order to minimize ohmic potential drop. The working electrodes were immersed in the test solution at open circuit potential for 2 h before measurement until a steady state appeared. All electrochemical measurements were carried out PAR 2263 Potentiostat/Galvanostat (Princeton Applied Research). EIS was carried out in a frequency range of 0.1 Hz to 105 Hz using a 10 mV peak-to-peak voltage excitation. The Tafel polarization scan was carried out by polarizing to ±250 mV with respect to the free corrosion potential (Ec ) at a scan rate of 0.5 mV s−1 . Each experiment was repeated at least three times to check the reproducibility. 2.3. AFM studies Prior to monitor the topographic changes of the electrode surface, the cold rolled steel specimens were abraded with emery paper from 100 to 800 grades, and then washed with distilled water and acetone. After immersion in 0.1 M HCl without and with addition of inhibitors at 20 ◦ C for 6 h, the specimens were cleaned with distilled water and acetone, then dried with a cold air blaster, and then used for a Japan instrument model SPA-400 SPM Unit atomic force microscope examinations. 2.4. FTIR studies

2. Experimental method

In order to investigate the adsorption behavior of inhibitors on the surface of cold rolled steel, Model Magna-IR 560 FTIR combined with InspectIRTM FTIR microanalysis and video imaging accessory was used to measure the spectra of the corroded surface of cold rolled steel in HCl with BTA and EDTA, all the spectra in these experiments were obtained by adding 64 interferograms at a resolution of 8 cm−1 in the region from 650 to 4000 cm−1 .

2.1. Materials

3. Experimental results and discussion

The experiments were performed with cold rolled steel specimens with the following chemical composition (wt.%): C ≤ 0.05, Si ≤ 0.02, Mn ≤ 0.28, S ≤ 0.023, P ≤ 0.019, Fe remainder. Ethylenediamine tetraacetic acid disodium (EDTA), benzotriazole (BTA) and hydrochloric acid (HCl) used were of analytical grade. All solutions were prepared from distilled water.

3.1. Electrochemical studies

2.2. Electrochemical measurements A three-electrode system including a working electrode, an auxiliary electrode and a reference electrode was used for electrochemical measurements in 250 mL solution. The working electrodes were made of the steel specimen in PVC holder using epoxy resin with an exposed area of 1.0 cm2 , polished with emery paper from 100 to 800 grades on the test face, rinsed with distilled water, degreased with acetone (CH3 COCH3 ), and dried with a warm air stream. The auxiliary electrode is a plat-

3.1.1. Tafel polarization The corrosion rates (CR) of cold rolled steel in 0.1 M HCl in the absence and presence of inhibitors were calculated from corrosion current density (Icorr ) values using the following equations (1) and (2): CR (mpy) = C ×

EW × Icorr d

(1)

where EW is the equivalent weight of the sample in g, Icorr is the corrosion current density in A cm−2 , d is the density of sample in g cm−3 and C is a conversion constant equal to 1.287 × 105 , respectively. The inhibition efficiencies (IEp ) from Tafel polarization were obtained by the following relationship [1]: IEp =

Icorr(0) − Icorr(inh) × 100 Icorr(0)

(2)

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Table 1 Electrochemical parameters from Tafel polarization curves and inhibition efficiencies in 0.1 M HCl at 30 ◦ C

Fig. 1. Polarization curves for the corrosion of cold rolled steel in 0.1 M HCl with different concentrations of EDTA (a) in the absence of BTA and (b) in the presence of 10 mg L−1 BTA at 30 ◦ C.

where Icorr(0) and Icorr(inh) are corrosion current density values in the absence and presence of inhibitors, respectively. The polarization curves for the corrosion of cold rolled steel in 0.1 M HCl with different concentrations of EDTA without and with 10 mg L−1 BTA at 30 ◦ C were shown in Fig. 1a and b, respectively. The polarization curves in the presence of 8 mg L−1 BTA are the same as those in the presence of 10 mg L−1 BTA and not presented here. As can be seen from Fig. 1a, the corrosion potentials are shifted to more positive direction with the increase of concentrations of EDTA in the absence of BTA, and the anodic branches are shifted to more positive potential direction while the cathodic branches are not affected obviously. These results reveal that EDTA acts as an anodic type inhibitor. However, the addition of BTA induces significant changes in the polarization characteristics, which can be easily seen from Fig. 1b, the anodic branches are shifted to more positive potential direction, and the cathodic branches are shifted to more negative potential direction, which indicates that the combination of EDTA and BTA acts as a mixed type inhibitor. The potentiodynamic polarization parameters including corrosion potential (Ecorr ) and corrosion current density (Icorr ) for

CEDTA (mg L−1 )

CBTA (mg L−1 )

Ecorr (mV)

Icorr (␮A cm−2 )

CR (mpy)

IEp (%)

0 10 25 40 50 0 10 25 40 50 0 10 25 40 50

0 0 0 0 0 8 8 8 8 8 10 10 10 10 10

−511.31 −485.75 −481.58 −478.82 −475.25 −491.10 −504.82 −507.68 −496.40 −468.54 −499.78 −480.64 −485.29 −481.20 −461.73

323.30 173.20 170.80 157.80 154.80 85.64 50.40 28.91 24.45 41.88 82.42 40.98 25.66 21.83 40.40

149.00 79.80 78.70 72.71 71.32 39.46 23.22 13.32 11.27 19.30 37.98 18.88 11.83 10.06 18.61

– 46.43 47.17 51.19 52.12 73.51 84.41 91.06 92.44 82.59 74.51 87.32 92.06 93.25 83.07

the corrosion of cold rolled steel in 0.1 M HCl at 30 ◦ C with different concentrations of EDTA in the presence and absence of BTA were calculated by Tafel plots and listed in Table 1. The corrosion rates (CR) and inhibition efficiencies (IEp ) which were respectively recalculated by means of Eqs. (1) and (2) were also listed in Table 1. It is clear that the corrosion potentials are shifted from −511.31 to −475.25 mV with increasing of the concentration of EDTA in the range of 0–50 mg L−1 in the absence of BTA, accordingly, the Icorr values decrease from 323.30 to 154.80 ␮A/cm2 , and the IEp values increase from 46.63% to 52.12%. These results show that the single EDTA can inhibit the corrosion of cold rolled steel in 0.1 M HCl somewhat but the inhibition efficiencies do not change obviously with increasing of the concentration of EDTA. Comparing the IEp of EDTA in the absence of BTA with those in the presence of BTA, substantial improvements in inhibition efficiencies are observed in the presence of BTA. That is to say, the combination of EDTA and BTA is effective inhibitor for the corrosion of cold rolled steel in 0.1 M HCl. 3.1.2. EIS In order to compare the corrosion behavior of different solutions, Fig. 2a and b show the Nyquist diagrams of cold rolled steel obtained for EDTA as an inhibitor in 0.1 M HCl at 30 ◦ C without and with 10 mg L−1 BTA, respectively. The EIS in the presence of 8 mg L−1 BTA are the same as those in the presence of 10 mg L−1 BTA and also not presented here. All impedance spectra obtained in 0.1 M HCl with and without inhibitors exhibit one capacitive loop. However, these diagrams are not perfect semicircles and this is attributed to frequency dispersion [31]. The fact that impendence diagrams have a semicircular appearance shows that the corrosion of steel is controlled by a charge transfer process and the presence of inhibitor does not change the mechanism of dissolution of cold rolled steel [31]. Comparing the semicircles in the presence of BTA with those in the absence of BTA, it is easy to see that the diameters of the capacitance loops in the presence of BTA are bigger than those in the

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tion resistance (Rs ) and charge transfer resistance (Rt ). It is worth mentioning that the double layer capacitance (Cdl ) value is affected by imperfections of the surface, and that this effect is simulated via a constant phase element (CPE) [33,34]. A constant phase element composed of a component Qdl and a coefficient α is required, and α quantifies different physical phenomena like surface inhomogeneousness resulting from surface roughness, inhibitor adsorption, porous layer formation, etc. So the capacitance is deduced from the following relation [34]: Cdl = Qdl × (2πfmax )α−1

(3)

where fmax represents the frequency at which imaginary value reaches a maximum on the Nyquist plot. If the electrode surface is homogeneous and plane, α is equal to 1 and the electrode surface can be treated as an ideal capacitance. Parametrical adjustment of this circuit with experimental impedance spectra gives access to the double layer capacitance and charge transfer resistance. Rt , Cdl fitted from EIS in 0.1 M HCl at 30 ◦ C were listed in Table 2. The inhibition efficiencies from Rt were calculated and also listed in Table 2 using the following equation: IERt (%) =

Fig. 2. EIS of the corrosion of cold rolled steel in 0.1 M HCl with different concentrations of EDTA (a) in the absence of BTA and (b) in the presence of 10 mg L−1 BTA at 30 ◦ C. The points represent experimental data and continuous line in figure corresponds to the EIS diagram generated using the electric circuit of Fig. 3.

absence of BTA, which suggests that BTA can greatly promote anti-corrosion performance of EDTA. The EIS results were simulated using the equivalent circuit shown in Fig. 3 to pure electronic models that could verify or role out mechanistic models and enable the calculation of numerical values corresponding to the physical and/or chemical properties of the electrochemical system under investigation [32]. The circuit employed allows the identification of both solu-

Fig. 3. Electric equivalent circuit of EIS.

Rt(0) − Rt(inh) × 100 Rt(inh)

(4)

where Rt(0) and Rt(inh) are charge transfer resistance for cold rolled steel in 0.1 M HCl in the absence and presence of inhibitors, respectively. The electric equivalent circuit suggests that, the single capacitance loop can be attributed to the charge transfer that has taken place at electrode/solution interface, the transfer process controls the corrosion reaction of cold rolled steel, and the protective film on the surface of steel can quickly form [31]. As can be seen from Table 2, with increasing concentration of EDTA from 10 to 50 mg L−1 without BTA, Cdl values decrease and Rt values increase to some extent, as a result, the inhibition efficiencies also increase to some extent, which reveals that single EDTA exhibits inhibition for the corrosion of cold rolled Table 2 Electrochemical parameters from EIS and inhibition efficiencies in 0.1 M HCl at 30 ◦ C CEDTA (mg L−1 )

CBTA (mg L−1 )

Rt ( cm2 )

IERt (%)

Cdl (␮f cm2 )

0 10 25 40 50 0 10 25 40 50 0 10 25 40 50

0 0 0 0 0 8 8 8 8 8 10 10 10 10 10

44.2 85.3 90.3 98.3 99.2 188.7 429.6 543.6 609.1 492.5 200.6 453.6 636.5 711.7 503.5

– 48.18 51.05 55.04 55.44 76.58 89.71 91.87 92.74 91.02 77.97 90.26 93.06 93.79 91.22

73.61 23.69 22.38 20.56 20.37 10.71 7.40 3.72 3.32 4.70 9.64 4.45 3.17 2.84 4.01

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Fig. 5. Relationship between the inhibition efficiencies and the test temperatures.

Fig. 5 shows the relationship between the inhibition efficiencies of EDTA obtained from Tafel polarization curves in the absence and presence of BTA and the test temperatures. It is worth noting that the inhibition efficiencies decrease with increasing of the test temperature in the presence of single EDTA while there is a tendency that the inhibition efficiencies increase with increasing of the test temperature in the presence of EDTA and BTA. The results may be interpreted via the difference of the adsorption mechanism between single EDTA and the combination of EDTA and BTA. 3.2. Synergistic parameter Fig. 4. (a) Tafel polarization curves and (b) EIS of cold rolled steel in 0.1 M HCl in the presence of 40 mg L−1 EDTA + 10 mg L−1 BTA at test temperatures. The points represent experimental data and continuous line in (b) corresponds to the EIS diagram generated using the electric circuit of Fig. 3.

steel somewhat but the variation of EDTA concentration affects the corrosion inconspicuously. Table 2 also shows that the addition of BTA to 0.1 M HCl containing EDTA further enhances Rt values and reduces Cdl values. In accordance, the inhibition efficiencies increase markedly in the presence of BTA. This can be attributed to the synergistic effect of BTA and EDTA. These results are well in agreement with the results obtained from Tafel polarization curves. 3.1.3. Effect of temperature Fig. 4a and b give the Tafel polarization curves and EIS of the combined formulation of 40 mg L−1 EDTA and 10 mg L−1 BTA at the test temperatures, respectively. Fig. 4a shows that the corrosion potentials shift to more negative direction and the Icorr values increase with the raising of temperature. Fig. 4b shows that all the impedance spectra are only one capacitance loop at all test temperatures, and the diameter of the loop decreases with increasing of the test temperature. Fig. 4a and b further reveal that the mechanism of corrosion reaction of cold rolled steel in 0.1 M HCl containing BTA and EDTA does not change obviously with the variation of temperature.

Tables 1 and 2 reveal that the corrosion rates of cold rolled steel in 0.1 M HCl decrease while the inhibition efficiencies increase more greatly in the presence of EDTA and BTA than those in the presence of single EDTA, which suggests that the synergistic effect may exist between EDTA and BTA. The synergistic parameter (S) can be further used to reveal the effect between two compounds [1,35,36]. More recently, Lalitha et al. [1] also used the synergistic parameter (S) to show the synergism between azoles and surfactants. Generally speaking, S is a good parameter to investigate the synergistic inhibition of two inhibitors. For the interaction of two inhibitors, this value of S can be described as follows [1,36]: S=

Icorr(inh1) × Icorr(inh2) Icorr1,2

(5)

where Icorr1,2 is the corrosion current density of combining formulation of inhibitor 1 and 2, Icorr(inh1) and Icorr(inh2) are the corrosion current density of inhibitor 1 and 2, respectively. Consequently, S approaches the value of 1 when no interaction between inhibitors exists. While S > 1, synergistic interaction of two inhibitors prevails. In opposition to S > 1, the antagonistic interaction exits between two inhibitors in the case of S < 1, which may be attributed to competitive adsorption [36]. The values of S between BTA and EDTA were calculated via Eq. (5) and listed in Table 3. As can be seen from this table, the

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Table 3 Synergism parameters between EDTA and BTA in 0.1 M HCl at test temperatures CEDTA (mg L−1 )

CBTA (mg L−1 )

10 25 40 50 10 25 40 50

8 8 8 8 10 10 10 10

Synergism parameter (S) 10 ◦ C

20 ◦ C

30 ◦ C

40 ◦ C

0.632 0.494 0.486 0.371 0.637 0.436 0.432 0.364

0.625 0.628 0.564 0.389 0.570 0.525 0.509 0.290

0.910 1.565 1.710 0.728 1.077 1.697 1.843 0.721

2.010 2.697 2.766 1.923 2.114 2.710 2.919 1.720

values of S < 1 at 10 and 20 ◦ C, which reveals that the antagonistic interaction exits between BTA and EDTA. However, when the temperature is raised to 30 or 40 ◦ C, most values of S are greater than unity, and the maximum value is 2.919, which suggests that the improvement in inhibition efficiency is due to a synergistic effect generated by the addition of BTA to 0.1 M HCl containing EDTA [37]. 3.3. AFM studies AFM provides a powerful technique of investigating the surface microstructure [34,38]. The AFM surface topography images of cold rolled steel in 0.1 M HCl in the absence and presence of inhibitors were shown in Fig. 6, the corresponding two-dimensional images and three-dimensional images were showed in Figs. 7 and 8, respectively. The AFM images shown in Figs. 6–8(A), are the corroded surface of cold rolled steel in the absence of inhibitors. From these pictures, it is clearly found that the surface looks relatively uneven and appears potholed shape. The AFM images shown in Figs. 6–8(B) are the corroded surface of cold rolled steel in the presence of 40 mg L−1 EDTA. These images clearly show that the surface looks more uniform than the images in the absence of inhibitors except some sunken places. Additionally, there are some claviform particles adhered to the sunken places in Figs. 6–8(B) which do not exist in Figs. 6–8(A). Therefore, it may be concluded that these particles are the adsorption film of EDTA, which inhibits the active corrosion of cold rolled steel to some extent. The AFM images shown in Figs. 6–8(C) are the corroded surface of cold rolled steel in the presence of the combination of 40 mg L−1 EDTA and 10 mg L−1 BTA. As can be seen from these figures, the surface becomes more flat and close. And the spherical and elliptical particles distribute on the surface more evenly, which reveals the formation of complete and protective film. These facts suggest that EDTA in the presence of BTA can form close and protective film on the surface and inhibit the corrosion of cold rolled steel evidently. 3.4. FTIR studies Several researchers [1,39] have proved that FTIR spectrometer is a powerful instrument that can be used to determine the type of bonding for organic surfactants absorbed on the surface

Fig. 6. AFM surface topography images in 0.1 M HCl: (A) in the absence of inhibitors; (B) in the presence of 40 mg L−1 EDTA and (C) in the presence of 40 mg L−1 EDTA + 10 mg L−1 BTA.

Q. Qu et al. / Electrochimica Acta 52 (2007) 6811–6820

Fig. 7. AFM two-dimensional images in 0.1 M HCl: (A) in the absence of inhibitors; (B) in the presence of 40 mg L−1 EDTA and (C) in the presence of 40 mg L−1 EDTA + 10 mg L−1 BTA.

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Fig. 8. AFM three-dimensional images in 0.1 M HCl: (A) in the absence of inhibitors; (B) in the presence of 40 mg L−1 EDTA and (C) in the presence of 40 mg L−1 EDTA + 10 mg L−1 BTA.

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Fig. 9. FTIR spectra of (A) EDTA and (B) the corrosion surface of cold rolled steel in 0.1 M HCl in the presence of EDTA.

Fig. 11. FTIR spectra of the corrosion surface of cold rolled steel in 0.1 M HCl containing EDTA and BTA.

of a solid. In this paper, FTIR spectrometer was used to identify whether there was adsorption and to provide new bonding information on the steel surface after corrosion. Figs. 9–11 show the FTIR spectra of EDTA, BTA and the corrosion surfaces of cold rolled steel in 0.1 M HCl in the presence of different inhibitors, respectively. Fig. 9(A) is the FTIR spectra of EDTA, the broader bands around 3124 cm−1 are attributed to O–H stretching in –COOH which indicates that many EDTA molecules aggregate through H-bonds, the band at 2810 cm−1 is due to aliphatic –CH2 asymmetric and symmetric stretching vibrations, the band 1400 cm−1 can be assigned to COO− stretching vibration, the absorption bands at 1272 cm−1 and 1207 cm−1 can be assigned to C–N stretching vibrations and the bands at 1013 and 1088 cm−1 can be assigned to C–O stretching vibrations. In Fig. 9(B), the adsorption bands around 3124 and 1400 cm−1 become weaker which suggests the disjunction of Hbands, the presence of C–O stretching vibrations at 1091 cm−1 and C–N stretching vibrations at 1314 cm−1 reveal the fact that EDTA also gets absorbed on the surface of cold rolled steel, comparing with the FTIR spectra of pure EDTA the C–O and

C–N stretching vibrations shifting to higher wavenumbers may be due to formation of the chelate compound (Fe-HEDTA)− . In Fig. 10(A), N–H stretching vibration exhibits absorption bands in the region from 3525 to 3130 cm−1 , C–H stretching vibration in benzene ring shows the adsorption at 3026 cm−1 , framework vibration of benzene ring in heteroaromatic compound gives rise to absorption bands at 1603 and 1486 cm−1 , the band at 1410 cm−1 is due to the framework vibration of heteroaromatic ring, and C–N stretching vibration exhibits weak absorption band at 1320 cm−1 . While in Fig. 10(B), the fact that bands from 3525 to 3130 cm−1 become very weak is due to the protonated amine. The elevating of framework vibration bands of benzene ring in heteroaromatic compound from 1603 to 1608 cm−1 , from 1486 to 1496 cm−1 and the disappearance of the band at 1410 cm−1 are also due to the fact that BTA gets protonated in acidic solutions. Comparing Fig. 10(A) and (B), it can be suggested that BTA can be absorbed on the cold rolled steel surface after being protonated. It is easy to see from Fig. 11 that the reflection absorption spectrum of the film formed on cold rolled steel in 0.1 M HCl containing EDTA and BTA gives an evidence for the adsorption of BTA along with EDTA. In Fig. 11, the disappearance of N–H stretching bands, the weakening of the benzene ring bands, the appearance of C O vibration at 1668 cm−1 , the heightening of the band of COO− stretching vibration from 1400 to 1408 cm−1 are due to the electrostatic attraction effects between the protonated BTA and negative EDTA ions. The appearance of aliphatic –CH2 stretching vibration bands and lowering of C–N stretching band from 1320 to 1315 cm−1 due to negative EDTA ions further suggest the chemisorption of BTA along with EDTA. 4. Explanation for synergism In 0.1 M HCl, iron dissolves into solutions and forms Fe2+ , is apt to form chelate compound (Fe-EDTA)2− and the iron-hydride anion (Fe-HEDTA)− in the presence of EDTA, (FeHEDTA)− is more stable than (Fe-EDTA)2− at low pH [22]. Fe2+

Fig. 10. FTIR spectra of (A) BTA and (B) the corrosion surface of cold rolled steel in 0.1 M HCl in the presence of BTA.

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That is Fe2+ + EDTA4− → (Fe-EDTA)2−

then EDTA can be easily adsorbed through electrostatic interaction.

(a)

Acknowledgements

At low pH: (Fe-EDTA)2− + H+ → (Fe-HEDTA)−

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(b)

And (Fe-HEDTA)− ions can be absorbed onto the surface of cold rolled steel through the transference of lone-pair electrons of nitrogen and oxygen atoms to the d-orbital in iron atom. But the chloride ions being specifically adsorbed on steel create an excess negative charge towards the solution [40–42]. Thus, it is assumed that chloride ions are firstly adsorbed on the steel/solution interface, (Fe-HEDTA)− ions are rather difficult to be adsorbed due to the electrostatic repulsion interaction. Therefore, due to the competitive adsorption between chloride ions and EDTA, EDTA can inhibit the corrosion of cold rolled steel to some extent, but the inhibition efficiency is not too high. Previous studies confirm the fact that the triazole molecules get protonated in acidic solution [25]. When low concentrations of BTA are added into 0.1 M HCl containing EDTA, the protonated BTA cations are able to be absorbed electrostatically on the steel surface previously covered with chlorides. With increasing concentration of BTA, large amount of BTA ions will accumulate gradually closely to the steel/solution interface; thus BTA ions are likely to be absorbed not only through electrostatic interaction on the steel surface previously covered with chlorides but also through the transference of lone-pair electrons of nitrogen atoms to the d-orbital in iron atom [40]. The above adsorption models will reduce the excess negative charge on the steel/solution interface and make the steel/solution interface positively charged. So (FeHEDTA)− ions can be easily absorbed onto the steel surface due to electrostatic attraction effects. This may be why the inhibition efficiencies in the presence of BTA and EDTA are significantly higher than those in the presence of single EDTA. 5. Conclusions (1). EDTA in the absence of BTA acts as anodic type inhibitor in 0.1 M HCl. The combination of EDTA and BTA acts as mixed type inhibitor, and mainly inhibits the anodic reaction. EIS shows that the fact the corrosion reaction is controlled by charge transfer. The inhibition efficiencies obtained from Tafel polarization, charge transfer resistance are consistent. (2). Single EDTA can inhibit the corrosion of cold rolled steel in 0.1 M HCl, but the inhibition efficiencies are not too high. However, the inhibition efficiencies markedly increase with the addition of lower concentration of BTA to 0.1 M HCl containing EDTA. (3). The combination of EDTA and BTA shows strong synergism effects at 30 and 40 ◦ C. (4). During the adsorption process, protonated BTA cations can be firstly absorbed onto the surface of cold rolled steel,

This work was financially supported by the Natural Science Foundation of Yunnan University under the Grant No. 2003Q004A and the Natural Science Foundation of Yunnan Province under the Grant No. 2006E0008Q. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35]

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