Long-term anticorrosion behaviour of polyaniline on mild Steel

Corrosion Science 49 (2007) 3052–3063 www.elsevier.com/locate/corsci

Long-term anticorrosion behaviour of polyaniline on mild steel Y. Chen, X.H. Wang ¤, J. Li, J.L. Lu, F.S. Wang State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, 5625 Renmin Street, Changchun 130022, PR China Received 31 October 2006; accepted 11 November 2006 Available online 28 December 2006

Abstract Anticorrosion performances of polyaniline emeraldine base/epoxy resin (EB/ER) coating on mild steel in 3.5% NaCl solutions of various pH values were investigated by electrochemical impedance spectroscopy (EIS) for 150 days. In neutral solution (pH 6.1), EB/ER coating oVered very eYcient corrosion protection with respect to pure ER coating, especially when EB content was 5–10%. The impedance at 0.1 Hz of the coating increased in the Wrst 1–40 immersion days and then remained constant above 109  cm2 until 150 days, which in combination with the observation of a Fe2O3/Fe3O4 passive Wlm formed on steel conWrmed that the protection of EB was mainly anodic. In acidic or basic solution (pH 1 or 13), EB/ER coating also performed much better than pure ER coating. However, these media weakened the corrosion resistance due to breakdown of the passive Wlm or deterioration of the ER binder. © 2007 Published by Elsevier Ltd. Keywords: A. Electronic materials; A. Mild steel; B. EIS; C. Alkaline corrosion; C. Polymer coatings

1. Introduction The use of polyanilines (PANIs) for corrosion prevention of iron/steel has attracted considerable attention in the past two decades [1–3]. PANI is a typical conducting polymer existing in various states interchangeable through the redox and acid–base reactions *

Corresponding author. Tel.: +86 431 5262250; fax: +86 431 5689095. E-mail address: [email protected] (X.H. Wang).

0010-938X/$ - see front matter © 2007 Published by Elsevier Ltd. doi:10.1016/j.corsci.2006.11.007

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

3053

Scheme 1. Redox and acid–base reactions of PANI.

shown in Scheme 1 [4]. In addition to the conducting emeraldine salt form of PANI, the non conducting emeraldine base (EB) form of PANI was also extensively studied for corrosion control, nevertheless there is still controversy regarding the eYciency and mechanism of the protection. McAndrew [5] reported that EB had a very high coating resistance (108  cm2) on steel in 3% NaCl solution, and EB blend coating also showed improved corrosion resistance. The high barrier property of EB to corrosive species was attributed to the formation of a dense and strong adherent polymer Wlm [5,6]. Fahlman et al. [7] found that EB protected iron and steel in humidity air even extending to 15 mm distance of the uncoated areas. Talo et al. [8] studied EB blend coating on steel in 0.1 M HCl, 0.6 M NaCl and 0.1 M NaOH solutions, and found eYcient protection in the latter two media even when a hole was drilled in the coating. These Wndings indicate that EB oVered more than simple barrier protection. X-ray photoelectron spectroscopy (XPS) revealed a passive Wlm of Fe2O3/Fe3O4 formed on the metal surfaces, suggesting the protection was anodic [7], which was further conWrmed by the redox catalytic property of EB (Scheme 1) [9–11]. However, Araujo et al. [12] showed that EB, even with an epoxy topcoat, did not protect steel in 0.01 M Na2SO4 due to poor barrier property and adhesion. Kinlen et al. [4] used scanning reference electrode technique to demonstrate that EB did not passivate steel in tap water at deliberately introduced pinholes in the coating, and the entire specimen corroded rapidly. Cook et al. [13,14] studied artiWcial defects in both EB containing coating and EB/topcoat system on steel in 0.1 M HCl and 0.1 M NaCl solutions. They observed no or limited protection and no active–passive transition upon anodic polarization, which led them to conclude that anodic protection of EB could not operate in such media. Wessling [15] and Williams and McMurray [16] also threw doubts on the anodic protection mechanism of EB. It is possible that the above divergences arise partially from the wide variations in experimental conditions used in addition to the test methods, such as EB sample preparation (EB alone, EB blend coating and EB/topcoat combination), the corrosive medium and its pH value. Since EB blend coating has demonstrated long life salt fog performance with superior mechanical properties [9], the pH of a corrosive medium may greatly aVect the protective property of a coating, and earlier accelerated tests by introducing artiWcial defects in coating to initiate corrosion at short term may not reXect the long term protective behaviour

3054

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

of the coating, and further the protection mechanisms may be diVerent for defective coatings and intact coatings [17,18], a study on the long term anticorrosion behaviour of EB blend coating in various pH media is of particular importance for further understanding the protective behaviour of EB. In this study, EB/epoxy resin (ER) blend coating was prepared. Its anticorrosion behaviour on mild steel in 3.5% NaCl solution was investigated by electrochemical impedance spectroscopy (EIS) for 150 days. The inXuences of EB content and pH value of the solution on the protective property of the coating were examined. 2. Experimental 2.1. Sample preparation Mild steel plates of 5 £ 5 cm2 were polished with emery paper to 600 grits and degreased in acetone ultrasonically. Mild steel plates of 1 £ 1 cm2 were polished with alumina paste to 1 m and degreased. EB/ER blend was prepared by ball milling calculated amount of EB powder (a product of Ben’an Co. licensed under this lab, its number average molecular weight was 40,000 with polydispersity index of 3.7) with ER (E-51, from Jiangsu Sanmu Group, China) in xylene for 4 h. The blend was mixed with polyamine hardener and cast onto the metals to form an air-dried Wlm of about 20 m thickness. By adjusting the amount of EB used, a series of EB/ER coatings were made with EB contents in the dry Wlm of 1%, 3%, 5%, 7%, 10%, 13% and 15% (w/w). As a reference, pure ER coating was made under similar conditions. 2.2. Measurements Optical micrographs of EB/ER coatings were taken by a LEICA optical microscope. Scanning electron microscopy (SEM) images were recorded on a XL30 ESEM scanning electron microscope. The liquid nitrogen fractured cross-section surfaces were sputtercoated with gold before SEM examination. EIS measurements were performed with a Solartron 1287 electrochemical interface and a Solartron 1255B Frequency Response Analyzer. A three-electrode cell was used employing the coated steel (5 £ 5 cm2) as working electrode with an exposed area of 7.07 cm2, a graphite rod and a saturated Ag/AgCl electrode as counter and reference electrodes, respectively. The tests were performed at room temperature in 3.5% NaCl solution (pH 6.1). To obtain various pH media, the solution was adjusted to pH of 1 and 13 using sulfuric acid and sodium hydroxide, respectively. The impedance data were collected at the open circuit potential with a 20 mV sinusoidal AC perturbation over a frequency range of 100 kHz–0.1 Hz. For SEM/X-ray energy dispersive (EDX) analysis and XPS analysis, coated steel plates (1 £ 1 cm2) were wrapped with paraYn–colophony mixture on the edges and uncoated sides and immersed in the solution (pH 6.1) for certain period; the coating was then removed mechanically to examine the underlying steel surfaces. XPS measurements were conducted on a VG ScientiWc ESCALAB 250 spectrometer with an Al K X-ray source (1486.5 eV). The spectra were collected using pass energy of 20 eV at take-oV angles of 90°. Sputter depth proWles were obtained with a 2 £ 2 mm2 Ar+ beam at 2 keV. Surface charge eVect was compensated by referencing the adventitious C 1s peak at 284.6 eV.

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

3055

3. Results and discussion 3.1. Characterization of the coating Optical micrographs of EB/ER coatings with diVerent EB contents are shown in Fig. 1. When EB content was 1%, 5% and 10%, EB was well dispersed in ER binder as particles of typically 10–15 m in diameter with little aggregation (Fig. 1a–c). When EB content increased to 15%, large EB aggregates were formed, indicating less eVective dispersion (Fig. 1d). SEM images of cross-sections of pure ER coating and EB/ER coatings with 5%, 10% and 15% EB are shown in Fig. 2. The ER coating had a smooth and compact microstructure (Fig. 2a). Though EB/ER coating revealed more coarse structure (Fig. 2b–d) than pure ER coating, it was still quite compact when EB content was 5% and 10%. When EB content increased to 15%, large cracks appeared on the cross-section (Fig. 2d), indicating the coating was no longer compact, consistent with the above optical micrographic result. 3.2. Corrosion resistance of coating in neutral solution 3.2.1. EIS measurements Fig. 3 shows Bode impedance plots obtained for pure ER coating and EB/ER coatings with EB contents of 1%, 3%, 5%, 7%, 10%, 13% and 15% during immersion in 3.5% NaCl solution (pH 6.1). According to the literature [19–24], the impedance modulus at 0.1 Hz (|Z|0.1 Hz) is an appropriate parameter for characterization of the protective properties of the coatings. Good correlation between the low frequency (LF) impedance value and coating

Fig. 1. Optical micrographs of EB/ER coatings with EB contents of: (a) 1%, (b) 5%, (c) 10% and (d) 15%.

3056

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

Fig. 2. SEM images of cross-sections of ER coating (a) and EB/ER coatings with EB contents of 5% (b), 10% (c) and 15% (d).

protective performance has been reported, and it is considered that a coating with good anticorrosion performance should show a LF impedance above 108  cm2, whereas a poor or failed coating shows a LF impedance less than 106  cm2 [21,22]. The plots of |Z|0.1 Hz against immersion time for ER and EB/ER coatings are shown in Fig. 4. The |Z|0.1 Hz for ER coating was 7.8 £ 107  cm2 after 2 h of immersion, whereas for EB/ER coatings with 1%, 3%, 5%, 7% and 10% EB was 8.9 £ 108, 7.0 £ 108, 1.1 £ 109 2.3 £ 109 and 1.6 £ 109  cm2, respectively. At the initial immersion, the |Z|0.1 Hz value reXected the pore resistance of the coating resulting from water and electrolyte penetration [19,20]. Therefore, EB/ER coating had much better barrier property to corrosive ions than pure ER coating, and with increasing EB content in the coating the barrier property enhanced, which was consistent with earlier work by McAndrew [5]. However, when EB content increased further to 13% and 15%, the |Z|0.1 Hz decreased signiWcantly to 2.0 £ 108 and 4.4 £ 106  cm2, respectively, indicating the coating became porous, in agreement with the above SEM observations. When immersion time increased, the |Z|0.1 Hz of ER coating decreased rapidly to 1.9 £ 107  cm2 at 1 day, then reached a plateau up to 10 days and then decreased rapidly again. These decreases in impedance resulted from water and electrolyte penetrating the coating and subsequently corrosion progressing on steel as well as delamination of the coating [19]. At 40 days, the |Z|0.1 Hz fell below 106  cm2, indicating failure of protection of the coating. In comparison, the |Z|0.1 Hz of EB/ER coating with 1% EB decreased to 2.7 £ 107  cm2 at 1 day, but then increased gradually to 1.3 £ 108  cm2 at 30 days. After that, it began to decrease but was 6.0 £ 106  cm2 at the end of 150 test days, indicating

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063 10

10

10

2

7

10

6

10

8

10 2

8

10

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

9

10

|Z| (Ω•cm )

9

|Z| (Ω•cm )

10

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

10

5

7

10

6

10

5

10

10

4

4

10

10

3

3

10

-1

10

0

10

1

10

2

10

3

10

4

10

10

5

-1

10

10

0

10

Frequency (Hz)

2

7

10

6

10

8

10 2

8

10

|Z| (Ω•cm )

4

10

5

10

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

9

10

|Z| (Ω•cm )

9

7

10

6

10

5

5

10

4

10

10

4

10

3

3

10

-1

10

0

10

1

10

2

10

3

10

4

10

10

5

-1

10

10

0

10

Frequency (Hz)

1

10

2

10

3

10

4

10

5

10

Frequency (Hz)

10

10

10

10

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

2

7

10

6

10

8

10 2

8

10

2 hours 1 days 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

9

10

|Z| (Ω•cm )

9

10

|Z| (Ω•cm )

3

10

10 2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

10

5

7

10

6

10

5

10

10

4

4

10

10

3

3

10

-1

10

0

10

1

10

2

10

3

10

4

10

10

5

-1

10

10

0

10

Frequency (Hz)

1

10

2

10

3

10

4

10

5

10

Frequency (Hz)

10

10

10

10

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

7

10

6

10

8

10 2

8

10

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

9

10

|Z| (Ω•cm )

9

10

2

2

10

10

10

|Z| (Ω•cm )

1

10

Frequency (Hz)

10

5

7

10

6

10

5

10

10

4

4

10

10

3

10

3057

3

-1

10

0

10

1

10

2

10

3

10

Frequency (Hz)

4

10

5

10

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

Frequency (Hz)

Fig. 3. Bode plots at diVerent immersion time in 3.5% NaCl solution for steel coated with ER coating (a), EB/ER coatings with EB contents of 1% (b), 3% (c), 5% (d), 7% (e), 10% (f), 13% (g) and 15% (h).

3058

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

10

10

9

10

8

10

7

10

6

10

5

10

4

|Z|0.1Hz ,Ω • cm

2

10

0

20

40

60

80

100

120

140

160

Immersion time, days Fig. 4. Evolution of |Z|0.1 Hz with immersion time in 3.5% NaCl solution for steel coated with ER coating (䊊), EB/ER coatings with EB contents of 1% (䉱), 3% (4), 5% (䊉), 7% (䉫), 10% (䊏), 13% (䊐) and 15% (5).

eYcient protection of the coating. Again, the initial decrease in |Z|0.1 Hz was due to water and electrolyte penetrating the coating, which may initiate corrosion on steel, whereas the following increase in |Z|0.1 Hz reXected formation of protective passive Wlms on the metal surface [24]. As EB content increased to 3%, the |Z|0.1 Hz decreased to 8.9 £ 107  cm2 at 1 day, then increased to 2.5 £ 108  cm2 at 10 days and remained constant until 50 days. It was 9.8 £ 106  cm2 at 150 days, indicating more eYcient passivation of steel and therefore, more eYcient corrosion protection of the coating. When EB content was 5%, 7% and 10%, the |Z|0.1 Hz decreased at 1 day, then increased up to 5–40 days and remained constant around 2 £ 108 or 3 £ 109  cm2 until 150 days, indicating even more eYcient steel passivation and coating protection. However, when EB content increased further to 13%, the |Z|0.1 Hz increased from a decreased value of 1.6 £ 108  cm2 at 1 day to 4.8 £ 108  cm2 at 10 days, then decreased till 3.1 £ 107  cm2 at 150 days. When EB content reached 15%, the |Z|0.1 Hz increased sharply to 3.1 £ 107  cm2 at 1 day and further to 3.5 £ 107  cm2 at 5 days, but fell below 106  cm2 at 100 days. Thus, though signiWcant passivation of steel also occurred when EB content was above 10%, the protective performance of the coating decreased remarkably corresponding to its porous structure. 3.2.2. Surface analysis In order to further understand how EB/ER coating aVected the underlying steel surface, SEM/EDX and XPS analysis were performed on the surface after the coating was peeled oV. Fig. 5 shows SEM images of a freshly prepared steel surface and EB/ER coating with 10% EB coated steel surfaces after immersion in 3.5% NaCl solution for various days. The freshly prepared steel surface was rough with Wne polishing grooves (Fig. 5a). After coated with the EB/ER coating and immersed for 5 days, the surface became smooth with only a few grooves (Fig. 5b). After immersion for 10 and 20 days, the surface became more smooth and the polishing grooves were nearly invisible (Fig. 5c and d, respectively). Corre-

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

3059

Fig. 5. SEM images of a freshly prepared steel surface (a) and EB/ER coating with 10% EB coated steel surfaces after immersion in 3.5% NaCl solution for 5 days (b), 10 days (c) and 20 days (d).

sponding EDX tests on these underlying steel surfaces found no chloride or sodium element. Fig. 6 shows XPS Fe 2p spectra of the corresponding steel surfaces. The freshly prepared steel showed two peaks at 706.8 and 719.9 eV corresponding to Fe 2p3/2 and 2p1/2 lines of metallic iron (Fe0), respectively. For steel coated with EB/ER coating and immersed for 5 days, another two peaks appeared at 710.7 and 724.1 eV assignable to Fe 2p3/2 and 2p1/2 lines of trivalent iron (Fe3+) in Fe2O3, respectively. After immersion for 10 days, these two peaks intensiWed remarkably, indicating the oxide layer was thickened. After immersion for 20 days, these two peaks dominated the spectrum with the Fe0 contribution disappearing and the broad satellite around 719 eV being distinguishable, indicating a dense oxide layer of Fe2O3 was formed on the steel surface hereto. Fig. 7 shows the Fe 2p depth proWles of this surface. Upon etching, the Fe0 peaks at 706.8 and 719.9 eV emerged gradually and Wnally dominated the spectrum after 6 min. The peaks related to Fe2O3 at 710.7 and 724.1 eV shifted to 709.4 and 722.8 eV, respectively, after 2 min with another satellite feature emerging around 716 eV, suggesting an inner oxide layer of Fe3O4 [7]. The above SEM/EDX and XPS data in combination with the EIS results indicate that a dense, stable passive oxide Wlm of Fe2O3/Fe3O4 was formed gradually on the steel surface beneath EB/ER coating during the early immersion period, which contributed to the initial increase and following prolonged maintenance of corrosion resistance of the coated metal upon immersion. The results support earlier work by Fahlman et al. [7] and ourselves [9,10], further conWrming that the protection of EB was predominately anodic.

3060

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

Intensity (a.u.)

(d) (c) (b) (a) 740

735

730

725

720

715

710

705

700

Binding Energy (eV) Fig. 6. XPS Fe 2p spectra of a freshly prepared steel surface (a) and EB/ER coating with 10% EB coated steel surfaces after immersion in 3.5% NaCl solution for 5 days (b), 10 days (c) and 20 days (d).

6 min

Intensity (a. u.)

5 min 4 min 3 min 2 min 1 min 0 min

740

735

730

725

720

715

710

705

700

Binding Energy (eV) Fig. 7. XPS Fe 2p spectra against sputtering time for steel coated with EB/ER coating with 10% EB after 20 days of immersion in 3.5% NaCl solution.

3.3. Corrosion resistances of coating in acidic and alkaline solutions Figs. 8 and 9 show Bode plots for ER coating and EB/ER coating with 5% EB on steel in 3.5% NaCl solution with pH of 1 and 13, respectively. The |Z|0.1 Hz of the coatings against immersion time in the various pH media are summarized in Fig. 10. Similar to that observed in NaCl solution with pH of 6.1, after 2 h of immersion, the |Z|0.1 Hz for EB/ER coating in NaCl solutions with pH of 1 and 13 were 5.2 £ 107 and 3.9 £ 108  cm2, respectively, whereas for ER coating were only 1.9 £ 107 and 5.8 £ 107  cm2,

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063 10

10

10

9

10

8

10

7

10

6

5

10

5

4

10

4

10

3

10

8

10 2

7

10

6

10 10 10

3

10

-1

10

10

0

10

1

10

2

10

3

10

4

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

2

9

|Z| (Ω•cm )

10

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

|Z| (Ω•cm )

10

3061

10

5

-1

10

0

10

Frequency (Hz)

1

10

2

10

3

10

4

10

5

10

Frequency (Hz)

Fig. 8. Bode plots for steel coated with (a) ER coating and (b) EB/ER coating with 5% EB at diVerent immersion time in 3.5% NaCl solution with pH value of 1. 10

10 2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

9

10

8

10

7

10

6

5

10

5

4

10

4

10

3

8

10 2

|Z| (Ω•cm )

10

10

7

10

6

10

2

9

10

|Z| (Ω•cm )

10

10 10

3

10

-1

10

10

0

10

1

10

2

10

3

Frequency (Hz)

10

4

10

5

2 hours 1 day 5 days 10 days 20 days 40 days 60 days 80 days 100 days 120 days 150 days

-1

10

0

10

1

10

2

10

3

10

4

10

5

10

Frequency (Hz)

Fig. 9. Bode plots for steel coated with: (a) ER coating and (b) EB/ER coating with 5% EB at diVerent immersion time in 3.5% NaCl solution with pH value of 13.

respectively, which again conWrmed the improved barrier property of EB/ER coating than pure ER coating. As time increased, the |Z|0.1 Hz of ER coating in acidic solution decreased to 7.8 £ 106  cm2 at 1 day, and further to 7.9 £ 105  cm2 at 20 days indicating failed protection of the coating. In contrast, the |Z|0.1 Hz of EB/ER coating in the same solution decreased slightly to 4.1 £ 107  cm2 at 1 day and to 1.1 £ 107  cm2 at 40 days, then remained constant around 5 £ 106  cm2 until 150 days, indicating high protective property of the coating. Nevertheless, the absence of an initial increase in the |Z|0.1 Hz value reXected no signiWcant passivation of steel occurred in the medium. In alkaline solution, the |Z|0.1 Hz of ER coating decreased to 9.2 £ 106  cm2 at 1 day, then remained steady until 60 days and then decreased quickly below 106  cm2 at 80 days.

3062

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

9

10

8

|Z|0.1Hz ,Ω • cm

2

10

10

7

10

6

10

5

10

4

10

3

0

20

40

60

80

100

120

140

160

Immersion time, days Fig. 10. Evolution of |Z|0.1 Hz with immersion time for steel coated with ER coating and EB/ER coating with 5% EB in 3.5% NaCl solutions of various pH values: (䊐) ER in pH 1 solution, (䊏) EB/ER in pH 1 solution; (䊊) ER in pH 6.1 solution, (䊉) EB/ER in pH 6.1 solution; () ER in pH 13 solution, (䉱) EB/ER in pH 13 solution.

The |Z|0.1 Hz of EB/ER coating decreased to 7.8 £ 107  cm2 at 1 day, then increased gradually to 5.8 £ 108  cm2 at 20 days, indicating the coating was also able to passivate steel in high eYciency in this medium. Though upon longer immersion the |Z|0.1 Hz decreased rapidly to 9.5 £ 106  cm2 at 80 days, it regained later and was 2.0 £ 107  cm2 at 150 days, indicating high protective property of the coating. As shown in Fig. 10, at early immersion stage (630 days) the protective property of EB/ ER coating decreased with decreasing pH value of the solution, whereas at late immersion stage (>30 days) it decreased in order of pH 6.1 > pH 13 > pH 1. The relatively poor property of the coating in acidic solution may be caused by the diYculty in inducing the formation of a stable passive oxide Wlm on steel surface due to fast dissolution of the metal in this medium, as suggested by Pourbaix diagram [25]. The earlier best property of the coating in alkaline solution may be due to the formation of a more stable passive oxide Wlm on steel, while the later declination in the performance was most likely related to loss of adhesion of the coating to the metal resulting from “saponiWcation” of the ER binder in the medium [26], though the deterioration of EB itself may be another factor, since it is well known that PANI also experienced degradation in strong basic environment. 4. Conclusions EB/ER coating oVered eYcient corrosion protection on mild steel in 3.5% NaCl solution (pH 6.1), especially when EB content was 5–10%. The |Z|0.1 Hz of the coating increased in the Wrst 1–40 immersion days and then remained constant above 109  cm2 until 150 days, which in combination with the observation of a dense Fe2O3/Fe3O4 passive Wlm formed gradually on steel during the early immersion period conWrmed that the protection of EB/ER coating was predominately anodic during the long term immersion. In acidic or basic 3.5% NaCl solution (pH 1 or 13), EB/ER coating also exhibited excellent anticorro-

Y. Chen et al. / Corrosion Science 49 (2007) 3052–3063

3063

sion property. However, these media weakened the corrosion resistance due to breakdown of the passive Wlm or deterioration of the ER binder. Acknowledgement The Wnancial support from Natural Science Foundation of China (Grant No. 20225414) is gratefully acknowledged. References [1] D.E. Tallman, G. Spinks, A. Dominis, G.G. Wallace, J. Solid State Electrochem. 6 (2002) 73–84. [2] G.M. Spinks, A.J. Dominis, G.G. Wallace, D.E. Tallman, J. Solid State Electrochem. 6 (2002) 85–100. [3] P. Zarras, N. Anderson, C. Webber, D.J. Irvin, J.A. Irvin, A. Guenthner, J.D. Stenger-Smith, Radiat. Phys. Chem. 68 (2003) 387–394. [4] P.J. Kinlen, V. Menon, Y.W. Ding, J. Electrochem. Soc. 146 (1999) 3690–3695. [5] T.P. McAndrew, Trends Polym. Sci. 5 (1997) 7–12. [6] L.C. Lei, M. Wu, H.M. Shu, H.Y. Li, Q.D. Qiao, Y.L. Du, Polym. Adv. Technol. 12 (2001) 720–723. [7] M. Fahlman, S. Jasty, A.J. Epstein, Synth. Met. 85 (1997) 1323–1326. [8] A. Talo, O. Forsen, S. Ylasaari, Synth. Met. 102 (1999) 1394–1395. [9] J.L. Lu, N.J. Liu, X.H. Wang, J. Li, X.B. Jing, F.S. Wang, Synth. Met. 135 (2003) 237–238. [10] X.H. Wang, J.L. Lu, J. Li, X.B. Jing, F.S. Wang, Acs Symp. Series 843 (2003) 254–267. [11] D.E. Tallman, Y. Pae, G.P. Bierwagen, Corrosion 55 (1999) 779–786. [12] W.S. Araujo, I.C.P. Margarit, M. Ferreira, O.R. Mattos, P.L. Neto, Electrochim. Acta 46 (2001) 1307–1312. [13] A. Cook, D. Gabriel, D. Siew, N. Laycock, Curr. Appl. Phys. 4 (2004) 133–136. [14] A. Cook, D. Gabriel, N. Laycock, J. Electrochem. Soc. 151 (2004) B529–B535. [15] B. Wessling, ACS Symp. Series 843 (2003) 34–73. [16] G. Williams, H.N. McMurray, Electrochem. Solid State Lett. 8 (2005) B42–B45. [17] C.G. Oliveira, M.G.S. Ferreira, Corros. Sci. 45 (2003) 123–138. [18] C.G. Oliveira, M.G.S. Ferreira, Corros. Sci. 45 (2003) 139–147. [19] A. Amirudin, D. Thierry, Prog. Org. Coat. 26 (1995) 1–28. [20] J.N. Murray, Prog. Org. Coat. 31 (1997) 375–391. [21] L.G.S. Gray, B.R. Appleman, J. Prot. Coat. Linings 20 (2003) 66–74. [22] D. Loveday, P. Peterson, B. Rodgers, JCT Coat. Tech. 2 (2005) 22–27. [23] A. Shi, S. Koka, J. Ullett, Prog. Org. Coat. 52 (2005) 196–209. [24] J.H. Park, G.D. Lee, A. Nishikata, T. Tsuru, Corros. Sci. 44 (2002) 1087–1095. [25] A.J. Dominis, G.M. Spinks, G.G. Wallace, Prog. Org. Coat. 48 (2003) 4349. [26] R. Lambourne, T.A. Strivens, Paint and Surface Coatings – Theory and Practice, second ed., Woodhead Publishing, 1999.