A study of the inhibition of iron corrosion by imidazole and its derivatives self-assembled films

Corrosion Science 51 (2009) 291–300

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A study of the inhibition of iron corrosion by imidazole and its derivatives self-assembled films Zhe Zhang a, Shenhao Chen a,b,*, Yanhui Li a, Shuhuan Li a, Liang Wang a a b

Department of Chemistry, Shandong University, Jinan 250100, PR China State Key Laboratory for Corrosion and Protection, Shenyang 110016, PR China

a r t i c l e

i n f o

Article history: Received 21 June 2008 Accepted 30 October 2008 Available online 20 November 2008 Keywords: A. Iron B. EIS B. XPS B. Polarization

a b s t r a c t The self-assembled (SA) films of imidazole and its derivatives were prepared on the iron surface. The protection abilities of these films against iron corrosion in 0.5 M H2SO4 solution were investigated using electrochemical impedance spectroscopy (EIS) and polarization techniques. The results of EIS and polarization curves demonstrated that films of the imidazole and its derivatives were able to protect iron from corrosion effectively. XPS was also used for the surface analysis, the results from XPS confirmed the adsorption of imidazole derivatives on the iron surface by monitoring the functional group peaks of the compounds. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Interest in the property of thin film organic materials has grown considerably in recent years due to the ease of fabrication, characterization, and manipulation [1,2]. Formation of SA films is based on spontaneous adsorption of the molecules with polar head groups designed to interact and bind specifically and chemically to the solid surface, especially metals [3]. The adsorption behavior of organic molecules on metal surface is often associated with the heteroatoms of the active group such as S, O, N, or P [4–7]. Aramaki et al. studied preparation of two-dimensional polymer films and ultrathin polymer coatings on iron by multistep modification method. They made a p-hydroxymethylbenzene SAM on iron surface via a covalent bond, and then the SAM was multistep modified with tetraethoxysilane and 1,8-octanediol and subsequently with 1,2-bis (triethoxysilyl)ethane and akyltriethoxysilane. The protective ability of the film enhanced with the film thickness [8–10]. Aramaki et al also investigated the self-assembled monolayers on the zinc surface which can protect the zinc from corrosion [11]. Quan studied the adsorption behavior of Schiff base and corrosion protection of resulting films on copper [12,13]. Unfortunately, a large number of the effective inhibitors are toxic. There is a growing trend to study environment friendly inhibitors recently [14]. It has been reported that non-toxic imidazole and its derivatives are well-known corrosion inhibitors for metals and alloys [15–17]. Imidazoles are organic compounds with two nitrogen atoms in * Corresponding author. Address: Department of Chemistry, Shandong University, Jinan 250100, PR China. Tel.: +86 53188564959; fax: +86 53188565167. E-mail address: [email protected] (S. Chen). 0010-938X/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2008.10.040

the heterocyclic ring. The imidazole molecule shows three different possible anchoring sites suitable for bonding: pyridine like nitrogen atom 3 N, pyrrole like nitrogen atom 1 N and aromatic ring itself and probably the substituent may act as an active center [18]. Babic´-Samardzˇija investigated the inhibitive properties and surface morphology of a group of heterocyclic diazoles as inhibitors for acidic iron corrosion [19]. They concluded that the relative position of the two nitrogen atoms appeared to make the most significant difference in inhibition performance. The imidazole derivatives were better inhibition agents compared to the pyrazole derivatives. Both imidazoles and pyrazoles contain two nitrogen atoms, when two nitrogen atoms are adjacent in the ring, the inhibition efficiency is lower than when there is a carbon atom between them. The substituent on carbon atom will affect the electronic properties therefore it will affect the interaction between the compound and iron. The heterocyclic diazoles were added to the electrolyte as inhibitors in their experiments, while in our work, the imidazole and its derivatives were made selfassembled films on iron electrode and the electrolyte was only 0.5 M H2SO4 solution without any additives. The imidazole’s derivatives are completely different and the substituent was all on the nitrogen atom. The purpose of this work is to investigate the protection effects of iron in acidic media by imidazole and a series of its derivatives, either with different length of alkane chains attached or with different functional groups. Electrochemical techniques were used to study the inhibition abilities of SA films of imidazole and its derivatives on iron surfaces in 0.5 M H2SO4 solution. XPS technique was used to investigate the SA films formed on iron surface. The

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Z. Zhang et al. / Corrosion Science 51 (2009) 291–300

N N

N

N N N H imidazole

COOH (E)-3-(4-((1H-imidazol-1-yl)methyl)phenyl)acrylic acid

1-benzyl-1H-imidazole

IMZ

BZIM

IMMP N

N N

N N

O S O

N COOCH3 1-buty1-1H-imidazole BTIM

1-tosyl-1H-imidazole (E)-methyl 3-(4-((1H-imidazol-1-yl)methyl)phenyl)acrylate MIMMP TSIM

Fig. 1. The structures, names, and abbreviations of imidazole and its derivatives.

N

Cl

N

+

N

NaH

N H N

Cl

N

+

N

NaH

N H N

Cl O S O N

N O S O

TEN

+ N H

CH3OH HC3 COOH

H2SO4

NBSCCl4 HC3 COOCH3 (

N NH NaH BrCH2

N

N

N

DMF

COOCH3

NaOHH2O COOCH3

O

O) 2

N COOCH3

N

N

HCl

Fig. 2. The synthetic route of imidazole derivatives.

COOH

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Z. Zhang et al. / Corrosion Science 51 (2009) 291–300

150 bare 1h 2h 4h 8h 16h 24h

-Z'' / Ω cm2

100

bare 1h 2h 4h 8h 16h 24h

0.01 M

150

-Z'' / Ω cm2

0.001 M IMZ

100

50

0

50

0 0

50

100

150

200

0

50

100

Z' / Ω cm2

200

250

bare 1h 2h 4h 8h 16h 24h

0.1 M

150

-Z'' / Ω cm2

150

Z' / Ω cm2

100

50

0

0

50

100

150

200

250

Z' / Ω cm2 Fig. 3. Nyquist impedance spectra of bare iron electrode and iron electrodes covered with self-assembled films in 0.5 M H2SO4 solutions with prolonged immersion time in 0.001 M IMZ (a), 0.01 M IMZ (b), and 0.1 M IMZ (c) solutions.

Table 1 Element values of equivalent circuit of Fig. 4 to fit the impedance spectra in Fig. 3 as well as the values of protection efficiency (PE) calculated by formula (2). Inhibitor

Assembling time (h)

2

cm

0.86  106

n

s )

PE (%)

n (0–1) 0.93

48.1

1 2 4 8 16 24

0.75  106 0.80  106 0.79  106 0.80  106 0.74  106 0.80  106

0.92 0.91 0.91 0.90 0.92 0.90

129.7 156.1 179.4 197.1 201.0 206.9

62.9 69.2 73.2 75.6 76.1 76.8

0.01 M IMZ

1 2 4 8 16 24

0.81  106 0.85  106 0.85  106 0.76  106 0.75  106 0.79  106

0.85 0.89 0.89 0.91 0.91 0.86

179.7 190.2 198.8 204.9 214.9 218.0

73.2 74.7 75.8 76.5 77.6 77.9

0.1 M IMZ

1 2 4 8 16 24

0.79  106 0.84  106 0.67  106 0.78  106 0.81  106 0.72  106

0.90 0.89 0.87 0.90 0.89 0.91

196.9 207.5 221.3 230.5 236.7 239.8

75.6 76.8 78.3 79.1 79.7 80.0

Fig. 4. Equivalent circuit used to fit the impedance spectra.

2. Experiment 2.1. Materials The working electrode was made of an iron rod (Aldrich, 99.9%, 2.0 mm diameter), and the iron rod specimen was embedded in epoxy resin in a glass tube, leaving its cross-section only to contact the solution. Before each experiment, the exposed surface was polished with 1600# and 2000# emery paper until its surface became smooth and mirror-like bright, then it was cleaned by ultra pure water and anhydrous ethanol as quick as possible. The structures of the imidazole and its derivatives are shown in Fig. 1. The imidazole is analytical grade and used as purchased without further purification. The imidazole derivatives were synthesized especially for this research [20,21]. A synthetic scheme

Y0 (X

1

0.001 M IMZ

Bare

surface morphologies of iron were characterized by SEM after the iron electrodes were corroded in H2SO4 solutions.

Rct (X cm2)

CPE

of the representative synthesis is shown in Fig. 2. The imidazole and its derivatives were dissolved in anhydrous ethanol solution to form the assembling solutions. Testing electrolyte of 0.5 M H2SO4 aqueous solution was prepared by diluting sulfuric acid with ultra pure water.

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Z. Zhang et al. / Corrosion Science 51 (2009) 291–300 Table 2 Element values of equivalent circuit of Fig. 4 to fit the impedance spectra in Fig. 5 as well as the values of protection efficiency (PE) calculated by formula (2).

150 bare 0 ºC

Inhibitor

20 ºC

-Z'' / Ω cm2

100

Assembling temperature (°C)

CPE

0 20 40 60

0.81  106 0.85  106 0.85  106 0.86  106

40 ºC 0.01 M IMZ

60 ºC

50

0 0

50

100

150

200

Z' / Ω cm2 Fig. 5. Nyquist impedance spectra of bare iron electrode and iron electrodes covered with films in 0.5 M H2SO4 solutions with the immersion time of 1 h in 0.01 M IMZ at different assembling temperatures.

1

Y0 (X

2

cm

n

s )

Rct (X cm2)

PE (%)

132.0 174.2 186.8 191.1

63.6 72.4 74.2 74.8

n (0–1) 0.88 0.89 0.90 0.90

(1.0 cm  1.8 cm). Electrochemical measurements were performed by IM6 electrochemical workstation (ZAHNER, Germany). Impedance measurements were performed under the corrosion potential with a sinusoidal potential perturbation of 5 mV in amplitude and the frequency was from 60 kHz to 0.02 Hz. The polarization curves were measured by scanning the potential at 1 mV s1 from 0.8 V to 0.2 V versus SCE. The data of impedance spectra and polarization curves were fitted using the software set in IM6 system. Experimental temperature was 25 ± 1 °C.

2.2. Electrochemical experiment 2.3. XPS and SEM experiment The electrochemical experiments were carried out using a traditional three-electrode cell. The working electrode was iron electrode. The reference electrode was a saturated calomel electrode (SCE) and the counter electrodes were two platinum foils

0.001 M

-2

log [ i (A/cm2) ]

log [ i (A/cm2) ]

-2

Samples for XPS and SEM experiments are iron sheets (12 mm  12 mm  2 mm). The polishing method used was the same as above. After 16 h of immersion, the iron sheet was rinsed

-4 bare 1h 2h 4h 8h 16h 24h

-6 -0.8

-0.6

-0.4

0.01 M

-4 bare 1h 2h 4h 8h 16h 24h

-6

-0.2

-0.8

-0.6

E (V/SCE)

-0.4

-0.2

E (V/SCE) 0.1 M

log [ i (A/cm2) ]

-2

-4

bare 1h 2h 4h 8h 16h 24h

-6

-0.8

-0.6

-0.4

-0.2

E (V/SCE) Fig. 6. Polarization curves of bare iron electrode and iron electrodes covered with self-assembled films in 0.5 M H2SO4 solutions with prolonged immersion time in 0.001 M IMZ (a), 0.01 M IMZ (b), and 0.1 M IMZ (c) solutions.

295

Z. Zhang et al. / Corrosion Science 51 (2009) 291–300 Table 3 Polarization parameters for bare iron electrode and the iron electrodes covered with IMZ films in different inhibitor concentration solutions as well as the protection efficiency calculated by formula (3). 2

Ecorr (mV)

Icorr (lA cm

549

7.80

1 2 4 8 16 24

540 537 536 536 535 531

2.76 2.50 2.08 2.01 1.90 1.87

64.6 67.9 73.3 74.2 75.6 76.0

0.01 M IMZ

1 2 4 8 16 24

540 533 532 530 515 509

2.13 1.98 1.92 1.87 1.78 1.74

72.7 74.6 75.4 76.0 77.2 77.7

0.1 M IMZ

1 2 4 8 16 24

538 537 536 533 526 523

1.95 1.84 1.66 1.67 1.59 1.59

75.0 76.4 78.7 78.6 79.6 79.6

Inhibitor

Assembling time (h)

Bare 0.001 M IMZ

)

PE (%)

with anhydrous ethanol and ultra pure water. Then it was dried with a steam of nitrogen. The XPS spectra were recorded by PHI 5300 ESCA System (Perkin–Elmer, USA). The excitation source was Al Ka radiation (photoelectron energy 1253.6 eV). Take-off angles of 45° from the surface were employed. All the analyses were performed at the pressure below 1  108 Torr. JEOL JSM-6700F Field emission scanning electron microscope was used to observe the surface morphology of the iron sheets including bare iron sheet and TSIM-modified iron sheet after corroded in 0.5 M H2SO4 solutions for 2 h. 3. Results and discussion 3.1. IMZ self-assembled films 3.1.1. Electrochemical impedance spectra The electrochemical impedance spectroscopy was used for the characterization of SA films-modified iron electrode. Fig. 3 shows the Nyquist plots of bare iron electrode and iron electrodes modified by imidazoles at various concentrations (0.001 M, 0.01 M, and 0.1 M). A series of experiments of different immersion time (from 1 to 24 h) were carried out at each concentration: 0.001 M (Fig. 3a), 0.01 M (Fig. 3b), and 0.1 M (Fig. 3c). The loops are not exact semicircles but depressed to some extent. This is attributed to the dispersion effect and the state of the electrode surface [22]. The intercept of the loops with horizontal axis, as well as the diameter of the capacitive loop, represents the charge-transfer resistance (Rct). A large Rct indicates a strong resistance against corrosion. To determine the impedance parameters of the pure iron specimen and the IMZ-modified iron specimen, the measured impedance data were analyzed with Complex Nonlinear Regression Least Squares (CNRLS) Fitting of IM6 software for the electric equivalent circuits that are given in Fig. 4 [23]. In the circuit, Rs and Rct represent the resistance of solution between the iron electrode and the reference electrode and the charge-transfer resistance corresponding with the corrosion reaction at metal substrate/solution interface, respectively. The double-layer usually behaves as a constant phase element (CPE) rather than a pure capacitor. The CPE is substituted for the capacitor to fit the semicircle more exactly. The admittance and impedance of a CPE are defined as below, respectively:

Y CPE ¼ Y 0 ðjxÞn

and Z CPE ¼

1 ðjxÞn Y0

ð1Þ

where Y0 is the modulus, x is the angular frequency, and n is the phase. The protection efficiency (PE) is calculated by the following equation:

PE ð%Þ ¼

R0ct  Rct  100% R0ct

ð2Þ

where Rct and R0ct represent the charge-transfer resistance of bare electrode and SA films-modified electrodes, respectively. Table 1 shows the fitted data and the calculated results from the impedance plots of bare electrode and the electrodes modified with IMZ self-assembled films formed at different concentration and at different immersion time. It is shown that when the concentration of the assembling solution increases, the Rct values increase, which indicates the higher protection efficiency. It is also shown that the iron electrode with longer immersion time has a larger Rct value and has better protection ability. However, when the immersion time was prolonged beyond 16 h, the Rct for the iron electrode covered with IMZ films remains unchanged. The phenomenon can be explained as the following: the IMZ molecules adsorbed on the bare iron surface quickly at the initial 1 h. With the increasing in immersion time, more and more molecules were adsorbed on the surface to form a dense film. When the film reached the maximal coverage, the quality of the film will not be improved significantly by further prolonging the immersion time. The IMZ molecule contains two nitrogen atoms and p-electrons which enable IMZ molecule to be adsorbed on the iron surface via lone-pair electrons. The EIS tests showed that prolonging the immersion time will increase the PE of the SA films, which indicated that some IMZ molecules must be adsorbed on the iron surface. Before each experiment, the iron electrode was washed with ultra pure water and anhydrous ethanol, only the chemisorbed IMZ molecules left on the iron surface. Although the compound coverage on the iron surface may change over the time due to the solubility property of IMZ in water, especially in acid solution, the EIS tests showed that the IMZ molecules can be adsorbed on the iron surface and form SA films which protect iron from corrosion. Fig. 5 shows the Nyquist impedance spectra for SA film-modified iron electrode in 0.01 M IMZ solution at different temperature, the immersion time was 1 h. The higher assembling temperature gave the larger loop, and a better protection efficiency was obtained. The element values of equivalent circuit to fit the impedance spectra in Fig. 5 as well as the PE values calculated by formula (2) are shown in Table 2. The PE of the IMZ SA films is 63.6% at the assembling temperature at 0 °C. When the temperature was raised to 20 °C, the PE reached to 72.4%. While the assembling temperature increased to 40 and 60 °C, the PE values were 74.2% and 74.8%, respectively. The above data also indicated that, the PE has a little change when the assembling temperature exceeds 20 °C. As a result, the working temperature was set at 25 °C. 3.1.2. Polarization curves measurements The polarization curves for the bare iron electrode and the iron electrode covered with IMZ films with different immersion time in 0.5 M H2SO4 solution are given in Fig. 6. Values of the associated electrochemical parameters, such as corrosion potential (Ecorr) and corrosion current density (icorr) obtained by extrapolation of the Tafel curves and the calculated PE are listed in Table 3. PE (%) are calculated as

PE ð%Þ ¼

1

icorr 0

icorr

!  100%

ð3Þ

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Z. Zhang et al. / Corrosion Science 51 (2009) 291–300

100

bare 1h 2h 4h 8h 16h 24h

200

-Z'' / Ω cm2

150

-Z'' / Ω cm2

0.01 M BZIM

bare 1h 2h 4h 8h 16h 24h

0.01 M BTIM

150

100

50 50

0

0 0

50

100

150

200

250

0

50

100

Z' / Ω cm2

150

200

250

300

Z' / Ω cm2 250

200

-Z'' / Ω cm2

150

100

0.01 M IMMP

bare 1h 2h 4h 8h 16h 24h

200

-Z'' / Ω cm2

bare 1h 2h 4h 8h 16h 24h

0.01 M TSIM

50

150

100

50

0

0 0

50

100

150

200

250

300

350

0

50

100

150

Z' / Ω cm2 250

250

300

350

bare 1h 2h 4h 8h 16h 24h

0.01 M MIMMP

200

-Z'' / Ω cm2

200

Z' / Ω cm2

150

100

50

0 0

50

100

150

200

250

300

350

400

Z' / Ω cm2 Fig. 7. Nyquist impedance spectra of bare iron electrode and iron electrodes covered with self-assembled films in 0.5 M H2SO4 solutions with prolonged immersion time in 0.01 M BTIM (a), 0.01 M BZIM (b), 0.01 M TSIM (c), 0.01 M IMMP, (d) and 0.01 M MIMMP (e) solutions.

0

where icorr and icorr represent the corrosion current densities of unmodified and modified electrodes, respectively. It is observed in Fig. 6 that the formation of IMZ SA films affects both anodic dissolution of iron and cathodic reduction reaction of hydronium. Both cathodic and anodic slopes decrease with the increased inhibitor concentration and assembling time, which indicates that the anodic dissolution of iron and hydrogen evolution are both suppressed. Therefore, the IMZ behave as a mixed inhibitor [24]. In each case, the curves of modified electrodes move toward lower current density values compared to the bare electrode. For the bare iron electrode the icorr is 7.80 lA cm2, when the iron electrode is immersed in the 0.01 M IMZ solution for 24 h,

icorr decreases to 1.74 lA cm2, which indicates that IMZ SA films reduce the dissolution rate of iron in 0.5 M H2SO4 solution remarkably. The results implied that IMZ SA films have protection ability to iron corrosion. The PE values are consistent with the results calculated from the electrochemical impedance spectra. 3.2. IMZ derivatives self-assembled films Fig. 7 shows the impedance spectra of different IMZ derivatives SA films-modified iron electrode, the assembling solution concentrations were all 0.01 M. The assembling time ranged from 1 to

297

Z. Zhang et al. / Corrosion Science 51 (2009) 291–300 Table 4 Element values of equivalent circuit of Fig. 4 to fit the impedance spectra in Fig. 7 as well as the values of protection efficiency (PE) calculated by formula (2). Rct (X cm2)

PE (%)

0.90 0.91 0.91 0.90 0.90 0.89

204.4 215.3 224.2 235.7 251.4 253.1

76.5 77.7 78.6 79.6 80.9 81.0

0.88 0.85 0.82 0.78 0.76 0.78

0.89 0.90 0.90 0.91 0.90 0.89

234.5 240.4 257.1 271.4 291.7 308.8

79.5 80.0 81.3 82.3 83.5 84.4

1 2 4 8 16 24

0.85 0.76 0.73 0.71 0.43 0.83

0.90 0.89 0.91 0.91 0.90 0.90

240.5 254.8 266.1 276.1 298.2 312.0

80.0 81.1 81.9 82.6 83.9 84.6

1 2 4 8 16 24

0.75 0.82 0.86 0.75 0.83 0.80

0.91 0.91 0.89 0.91 0.90 0.89

244.1 258.6 269.4 280.2 312.0 333.3

80.3 81.4 82.1 82.8 84.6 85.6

1 2 4 8 16 24

0.86 0.74 0.76 0.79 0.81 0.65

0.89 0.89 0.88 0.89 0.90 0.90

246.8 276.7 300.8 315.1 348.4 363.8

80.5 82.6 84.0 84.7 86.2 86.8

Assembling time (h)

CPE

0.01 M BTIM

1 2 4 8 16 24

0.86 0.83 0.81 0.82 0.83 0.69

0.01 M BZIM

1 2 4 8 16 24

0.01 M TSIM

0.01 M IMMP

0.01 M MIMMP

1

Y0 (X

2

cm

n

s )

n (0–1)

80

60

PE / %

Inhibitor

100

40

IMZ BTIM

20

TDIM BZIM TSIM IMMP MIMMP

0 0

8

16

24

immersion time /h Fig. 8. The intuitionistic plot showing the effect of immersion time on the PE of different imidazole derivatives.

24 h. From the spectra it can be seen that when the iron electrode was covered with different IMZ derivatives films, the shape of the impedance spectra did not change significantly, whereas their size increases in a varied degrees. The capacitive loop diameters of the iron electrode covered with IMZ derivatives films were larger than that of the IMZ films. The fitted and calculated results are listed in Table 4. The protection abilities of IMZ and IMZ derivatives molecules in corrosion of iron were found in the order of IMZ < BTIM < BZIM < TSIM < IMMP < MIMMP. Fig. 8 is an intuitionistic plot showing the effect of immersion time on the protection efficiency, in which the general increasing tendency of PE with the immersion time is clearly indicated. When the immersion time was gradually rising to 1 h, the IMZ and its derivative molecules in the solution adsorb on the bare iron surface quickly. Subsequently, the absorbed molecules will rearrange to form a denser and more ordered film gradually. With the prolonging of immersion time, more and more molecules were adsorbed on the surface, accordingly the protection ability of the films became stronger and stronger until the amount of adsorbate is saturated and the PE values are almost invariable. In addition, by comparing the plots of IMZ films and IMZ derivative films, the PE of MIMMP increased remarkably, and after 24 h immersion the MIMMP modified films reached the maximal protection efficiency of 86.8%. Comparing with the IMZ, the BTIM molecules are not soluble in water, so the BTIM SA films on the iron surface will be more stable than that of the IMZ. Increased alkyl chain length can increase the thickness of the SA films, which can enhance the protection ability of the films. All the other four IMZ derivatives have phenyl ring that may act as adsorbing center via their p-electrons. The phenyl ring makes the SA films more dense and stable, it improve the protection efficiency as well. It is also found that the

Fig. 9. Wide-scan XPS spectrum of TSIM-modified iron surface.

IMMP and MIMMP have two O atoms and the length of the two molecules are longer compare to the other compounds, we propose that with the addition of the adsorption atoms and the increasing of the SA films’ thickness the PE of the SA films may be improved.

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Z. Zhang et al. / Corrosion Science 51 (2009) 291–300

Fig. 10. High-resolution XPS spectra for (a) S2p, (b) N1s, (c) O1s, (d) C1s, and (e) Fe2p.

Z. Zhang et al. / Corrosion Science 51 (2009) 291–300

299

of the electrochemical measurements. In order to further confirm the corrosion resistance ability of the SA films, scanning electron microscopy was applied here to study the surface morphology of the bare and TSIM-modified iron sheets after corroded in 0.5 M H2SO4 solutions for 2 h. The modified iron sheet was immersed in TSIM solution for 16 h. As can be seen from the Fig. 11, there are distinct differences between the two iron sheets after the corrosion in acid solution. As for bare iron (see Fig. 11a), the surface was seriously damaged as a great deal of deep cavities and drawbacks were found. However, under the same corrosion circumstance, the surface of TSIM-modified iron sheet (Fig. 11b) was smooth with a few small notches. The phenomenon implied that the presence of TSIM films can protect the iron from corrosion efficiently. 4. Conclusions The corrosion efficiency of the imidazole and its derivatives was studied on iron in 0.5 M H2SO4 solution. Analysis of the experimental data suggests that the protection efficiency was strongly determined by the immersion time, concentration of the assembling solution and assembling temperature. The protection efficiency in 0.5 M H2SO4 solution increased in the sequence: IMZ < BTIM < BZIM < TSIM < IMMP < MIMMP. The best iron corrosion inhibition effect, as determined by electrochemical methods, was achieved by the MIMMP. This study showed that the imidazole and its derivatives are good inhibitors for iron corrosion in acid solution. Acknowledgements We thank the Chinese National Science Fund (20573069) and Special Funds for the Major State Basic Research Projects (2006CB605004) for their support of this research. References Fig. 11. SEM images of the surface for iron after being corroded in 0.5 M H2SO4 solution for 2 h: (a) bare iron sheet; (b) TSIM-modified iron sheet.

3.3. X-ray photoelectron spectroscopy X-ray photoelectron spectroscopy is a useful method in detecting the compositions of the surface coating [25,26]. In order to further clarify the formation of the SA films, XPS measurements were carried out with surfaces treated with the key assembling solution: TSIM. The XPS spectra were obtained from the iron surface which was treated by 0.01 M TSIM solution after 16 h of immersion. Wide-scan XPS spectra of the iron surface are shown in Fig. 9. The low-resolution XPS survey spectrum of the TSIM SA films confirms the presence of the desired elements in the SA films. Highresolution XPS spectra of S2p (Fig. 10a), N1s (Fig. 10b), O1s (Fig. 10c), and C1s (Fig 10d) regions of the TSIM SA films on iron surface were recorded to reveal the chemical states of S, N, O, and C atoms in the SA films. The two main signals located at around 724.90 and 709.12 eV can be ascribed to the photoemission peaks from Fe2p1/2 and Fe2p3/2 (Fig 10e), which indicates a certain degree of oxidation of Fe. The presence of an N1s peak at 399.96 eV, an O1s peak at 531.10 eV and a small S2p peak at 167.55 eV provides evidence that the TSIM was adsorbed on the iron surface [27]. 3.4. SEM characterization With the presence of imidazole and its derivatives, SA films are able to protect iron from corrosion according to the results

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