EIS study on degradation of polymer-coated steel under ultraviolet radiation

Corrosion Science 52 (2010) 2080–2087

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EIS study on degradation of polymer-coated steel under ultraviolet radiation Masanori Hattori a,*, Atsushi Nishikata b, Tooru Tsuru b a b

Energy Applications R&D Center, Chubu Electric Power Co., Inc., 20-1 Kitasekiyama, Ohdaka-cho, Midori-ku, Nagoya 459-8522, Japan Graduate School of Science & Engineering, Tokyo Institute of Technology, 2-12-1 Ookayama, Meguro-ku, Tokyo, Japan

a r t i c l e

i n f o

Article history: Received 6 November 2009 Accepted 28 January 2010 Available online 2 February 2010 Keywords: A. Steel A. Organic coating B. EIS B. IR spectroscopy B. SEM C. Polymer coating

a b s t r a c t Degradation of heavy-duty steel coatings when exposed to ultraviolet (UV) radiation was investigated by electrochemical impedance spectroscopy (EIS) and additional techniques in order to clarify the feasibility of evaluation of the UV degradation by EIS. Two coatings were considered: polyester-urethane topcoat plus epoxy primer (PU/E) and epoxy topcoat plus epoxy primer (E/E). Each was applied to a steel substrate and exposed to cyclic wetting–drying under UV radiation. The PU/E coating developed topcoat cracks but did not delaminate from the substrate; capacitive behaviour was evident, and corrosion of the underlying steel was not observed. The E/E coating showed topcoat chalking and partial disappearance, exposing the primer, but corrosion of the underlying steel was not observed. The morphology and chemical changes were compared with the results of EIS. Ó 2010 Elsevier Ltd. All rights reserved.

1. Introduction Polymer-coated steels are often used in the construction of buildings, bridges and similar structures. The polymer (paint) coating acts as a barrier to protect the substrate steel from corrosion by environmental factors such as sunshine (UV), rainfall, daily cycles of temperature and humidity, airborne salts and aggressive gases (SOx, NOx). Dissolution of the steel can occur at defect sites on the paint (anode); the compensating oxygen reduction occurs at the other steel surface under the paint (cathode), where delamination of the paint is induced due to formation of OH. Degradation of the paint itself by environmental factors such as sunshine and wet–dry cycles may enhance the formation of defect sites and thus the onset of corrosion of the underlying steel. Thus it is important to investigate the degradation of paints due to environmental factors as the first stage of corrosion of the underlying steel. In actual steel structures, degradation of the polymer coating is often evaluated by visual examination with the naked eye. However, it is not easy to evaluate the extent of degradation, or to detect corrosion of the underlying steel, simply from appearance, especially for thick, heavy-duty polymer coatings. In laboratory tests, many methods are employed to evaluate polymer degradation. Surface coatings have been examined in detail by optical microscope, scanning electron microscopy (SEM) and atomic force microscopy (AFM) [1–5]. Changes in thickness, mass and colour have been evaluated quantitatively [2,6–10]. Surface roughness

* Corresponding author. Tel.: +81 50 7772 2928; fax: +81 52 624 9207. E-mail address: [email protected] (M. Hattori). 0010-938X/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2010.01.038

of polymer coatings has been measured by laser displacement sensor and AFM [1,2,4,8,11–14]. Chemical changes to the polymer surface due to chemical-bond scission have been analysed by Fourier transform infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) [1,15,16]. Polymer cross-sections have been investigated by combined FT-IR and dielectric sorption analysis (DSA) [4,16–19]. In addition, coatings have been evaluated in terms of their glass transition point, which is related to chemical bonding [1,10,14–16,18,19]. Electrochemical impedance spectroscopy (EIS) is a particularly useful tool for detecting the onset of corrosion of underlying steel [3,5,7,13,14]. However, it is not yet known whether this technique can detect deterioration of the polymer itself, which precedes corrosion of the underlying steel in actual atmospheric environments. In this study, we have investigated the EIS characteristics of polymer coating exposed to UV radiation, which enhances degradation of polymer itself.

2. Experiments 2.1. Polymer coatings Carbon steel plate (SS400, Nippon Test Panel, JIS G3101, 67 mmW  150 mmH  1.5 mmT) was used as the substrate. Epoxy and polyester-urethane, which have different UV performance, were employed as the polymer coating. Epoxy contains benzene rings, and is well known to have short service life because the benzene rings are easily decomposed by UV radiation. In contrast, polyester-urethane does not contain benzene rings and has long

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M. Hattori et al. / Corrosion Science 52 (2010) 2080–2087 Table 1 Coating specifications. Kind

Thickness (lm)

Colour

Target value

Final value

No. 1

Topcoat Primer

Polyester-urethane Epoxy

45 125

187.1

White Red

No. 2

Topcoat Primer

Epoxy Epoxy

45 125

174

White Red

service lives. To each substrate, primer and topcoat polymer coating were applied in sequence. The primer in each case was 125lm-thick epoxy with red pigment (Fe2O3). Two types of topcoat polymer coating (Dai Nippon Paint Co., Ltd.) were used: polyester-urethane with white pigment (TiO2, Al2O3) and epoxy with white pigment (TiO2, Al2O3). Primer and topcoat specifications are shown in Table 1. Each substrate was ground with #400 abrasive paper, dried, spray-painted on one side with primer and topcoat, and sealed on the other side. Total thicknesses were as follows: for the polyester-urethane/epoxy (PU/E) coating, 190 lm; for the epoxy/epoxy (E/E) coating, 174 lm. Both values are slightly larger than the expected value of 170 lm.

3. Results and discussion 3.1. Characterization of the coating surface

2.2. Accelerated corrosion test Accelerated corrosion tests were performed using a sunshine weather meter (Suga Test Instruments Co., Ltd. S80). Test specimens were exposed to repeated wet–dry cycles under continuous UV radiation provided by a sunshine carbon arc (255 W/ m2 ± 10%, 300–700 nm). Each cycle consisted of wetting by tapwater spray (12 min) followed by drying at 63 °C (48 min). The total test time was 4553 h. Specimens were removed from the test chamber at 1006, 1468, 2459, 3540 and 4553 h, and coating degradation was evaluated by various methods as described below. 2.3. Evaluation of coating degradation Coating surfaces were observed by optical microscope and analysed by SEM/EDX. Changes in chemical bonding were analysed by FT-IR. Coating thickness was measured with an electromagnetic film thickness tester (Kett Electric Laboratory LE300J) at nine random points on the surface and the values averaged. Colour change was quantitatively evaluated with a colour difference meter at three points on the surface. For cross-sectional observation, polymer-coated steel samples were cut into small pieces, immersed in liquid nitrogen for several minutes, covered with a filler cloth to prevent damage to the coating surface, and then crushed with a hammer. Cross-sections were analysed by SEM/EDX. Fig. 1 shows the three-electrode cell in which electrochemical impedance was measured by EIS during the corrosion test. A KCl-

Reference Electrode (SSE)

saturated Ag/AgCl electrode was used as a reference electrode; a platinum electrode was used as a counter electrode. EIS measurements were performed in 3 mass% NaCl solution with a potentiostat (Solatron SI1260) and impedance/gain-phase analyser (Solatron SI1287). The exposed area was approximately 35 cm2. Since the high-impedance limit of the employed instrument was approximately 1  109 X (3  1010 X cm2) in the low-frequency limit, the impedance up to 1  1010 X cm2 was used for the coating evaluation.

Pt counter Electrode

3%NaCl solution

Polymer-coated steel

Fig. 1. Cell for EIS measurements.

Fig. 2 shows photographs of the PU/E coating before exposure and after exposure. At 2459 and 4553 h, the surface colour has changed slightly with exposure, as confirmed with the colour difference meter. SEM images of the PU/E coating before and after exposure are shown in Fig. 3. At 2459 h, microcracks are evident on the surface. At 4553 h, however, the cracks are difficult to observe because the surface is covered with deposits, mainly SiO2, probably originating from the tap-water used for surface wetting in the corrosion test, as described in a later section. Fig. 4 shows photographs of the E/E coating before and after exposure. At 2459 h, the surface colour has changed to yellowish and to reddish at 4553 h, as confirmed with the colour difference meter. In Fig. 5, magnified photographs of the E/E coating before and after exposure are shown. Before exposure, the surface is relatively smooth. At 2459 h, it has become rougher, with the roughening surface and white topcoat distributed in island-like forms on the primer surface. Between 3500 and 4553 h, the primer surface was then exposed directly to the corrosion environment. Fig. 6 shows SEM images of the E/E coating after exposure. At 1006 h, many microcracks are evident on the surface, much earlier than for the PU/E coating. At 4553 h, the surfaces of both the topcoat (Fig. 6b, showing the white part of Fig. 5c) and the primer (Fig. 6c, showing the red part of Fig. 5c1) show many microcracks. Fig. 7 shows FT-IR spectra of the PU/E coating before and after exposure. Before exposure, a typical pattern for polyester-urethane is observed, with the following absorption peaks: 3000– 2900 cm1, ACH2 bonding; 1700 cm1, AC@O ester bonding; 1600–1400 cm1, ACOO bonding; 1300 cm1, ANH group. At 2459 and 4553 h, these peaks have almost disappeared, indicating that the coating surface has deteriorated due to UV radiation. The results of elemental analysis of the coating surfaces by EDX are shown in Table 2. Silicon content increases with exposure time, and therefore peaks in the 1300–1000 cm1 IR region may be attributable to silicon as follows: 1100–1050 cm1, SiAO bonding; 1300 cm1, SiACH3 bonding. The silicon is thought to originate from the spray water of the sunshine weather meter. In contrast, both Al and Ti, which are additives of the coatings, decrease with exposure time. Fig. 8 shows FT-IR spectra of the E/E coating before 1 For interpretation of color mentioned in this figure the reader is referred to the web version of the article.

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Fig. 2. Photographs of the PU/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

Fig. 3. SEM images of the PU/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

Fig. 4. Photographs of the E/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

Fig. 5. Magnified photographs of the E/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

Fig. 6. SEM images of the E/E coating: (a) after exposure for 1006 h; (b) primer after exposure for 4553 h; (c) topcoat after exposure for 4553 h.

and after exposure. The chemical bonding on the surface changes with exposure. Before exposure, absorption peaks are as follows: 3000–2900 cm1, ACH2 bonding; 1500 cm1, benzene ring; 1300–1200 and 950–800 cm1, peroxide. At 2459 and 4553 h, these peaks have almost disappeared. 3.2. Characterization of the coating cross-section Figs. 9 and 10 show SEM images of cross-sections of the PU/E and E/E coatings, respectively. For both coatings, the microstructure of the primer is coarser than that of the topcoat, perhaps due to the pigment. The PU/E coating changes little in thickness with exposure, as confirmed with the electromagnetic film thick-

ness tester. The E/E coating topcoat is roughened by chalking, and seems thicker at the island parts (white areas in Fig. 5c) on the primer surface. The primer itself changes little in thickness even after 4554 h. Figs. 11 and 12 show magnified SEM images of cross-sections of the PU/E and E/E coatings, respectively. For the PU/E coating, the topcoat microstructure is slightly coarse after 4553 h, while the primer microstructure seems less changed, perhaps because the topcoat is slightly damaged by UV radiation but the primer is not. For the E/E coating, the topcoat is severely damaged and the primer is only little damaged. These changes suggest that UV radiation causes only little deterioration for the polyesterurethane topcoat but significant damage for the epoxy topcoat as expected.

M. Hattori et al. / Corrosion Science 52 (2010) 2080–2087

O C

NH

Intensity

O C

CH 2

O

(a)

(b) (c) 3700

3100

2500

1900

1300

700

Wave number / cm-1 Fig. 7. FT-IR spectra of the PU/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

Table 2 Elemental analysis, determined by EDX analysis (except for carbon and oxygen). Exposure time (h)

Al Si Ti

0 (%)

1006 (%)

1468 (%)

2459 (%)

3540 (%)

4553 (%)

11.9 – 88.1

9.56 8.70 81.7

10.2 7.77 82.1

7.94 9.89 82.2

6.69 18.2 75.1

5.53 23.1 71.4

O

Intensity

C

C

CH 2 (a) (b) (c) 3700

3100

2500

1900

1300

700

Wave number / cm -1 Fig. 8. FT-IR spectra of the E/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

3.3. EIS characteristics 3.3.1. PU/E coating EIS of non-deteriorated coatings on steel substrate should show capacitive behaviour (where Cf is the capacitance of polymer film) over the entire frequency region, because the high-impedance lim-

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it of a standard EIS instrument is approximately 1  109 X (3  1010 X cm2) in the low-frequency limit. Since the film resistance Rf of the heavy-duty coating used in this study is much higher than the limit, it should be difficult to determine the film resistance of non-deteriorated heavy-duty coatings. Fig. 13 shows equivalent circuits for the various EIS results achieved in this study and described below. The equivalent circuit for non-deteriorated heavy-duty coated steel is shown in Fig. 13a. Fig. 14 shows Bode impedance and change plots for the PU/E coating. As expected from the previous results, the plot shows typical capacitive behaviour, characterized by a straight line with a slope of (o log |Z|/o log f) of 1 and a phase shift of about 90°. There are no EIS changes in the employed frequency range during the corrosion test, even at 5568 h. The EIS can be explained by the equivalent circuit in Fig. 13a, and indicates that the PU topcoat has high resistance to UV radiation. The change in coating capacitance Cf with exposure time for the PU/E coating, measured from the EIS data of Fig. 14, is shown in Fig. 15, together with coating resistance Rf. Rf was beyond the limit (>1010 X cm2) of the employed EIS, even at 5568 h. If the coating deteriorates and absorbs large amounts of water, Cf should increase [13,14]. In fact, Cf initially decreases slightly and, after 1488 h, remains constant at 35 pFcm2. The constant nature of Cf indicates that the PU topcoat suffers no fatal damage by UV radiation. 3.3.2. E/E coating Fig. 16 shows Bode impedance and change plots for the E/E coating. Until 3540 h, impedance for E/E behaves in a manner similar to that for PU/E, indicating simple capacitive behaviour. However, at 4553 h, impedance in the frequency range <1 Hz drops to approximately 4  109 X cm2 and the phase shift changes from 90° to nearly 0°, indicating that Rf decreases to a measureable value (<1010 X cm2 by deterioration due to UV radiation). The equivalent circuit is shown in Fig. 13b. Fig. 17 shows the changes in Cf and Rf with exposure time for the E/E coating, as determined by curve-fitting the EIS data in Fig. 16. At 4553 h, not only the topcoat but also the primer is expected to have deteriorated slightly, although whether or not it has done so is not clear from the cross-section microstructure (Fig. 12). A delamination test with adhesive tape after the corrosion test revealed no delamination and underlying steel corrosion, despite severe degradation of the epoxy topcoat. It is very surprising that delamination does not occur, despite the fact that Rf drops to a measurable level. 3.3.3. Difference between EIS in sunshine weather meter test and immersion test For comparison, an immersion test was performed on a E(100 lm)/E(100 lm) coated sample in 0.5 M NaCl at 45 °C for 4800 h without UV radiation. Fig. 18 shows Bode impedance and phase plots for the coating. Fig. 19 shows a photograph of the coating after the test. Several blisters (cathodic delamination) have formed on the coating surface, and red rust, indicative of underly-

Fig. 9. SEM images of cross-sections of the PU/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

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M. Hattori et al. / Corrosion Science 52 (2010) 2080–2087

Fig. 10. SEM images of cross-sections of the E/E coating: (a) before exposure; (b) after exposure for 2459 h; (c) after exposure for 4553 h.

Fig. 11. Magnified SEM images of cross-sections of the PU/E coating: (a) topcoat before exposure; (b) primer before exposure; (c) topcoat after exposure for 4553 h; (d) primer after exposure for 4553 h.

Fig. 12. Magnified SEM images of cross-sections of the E/E coating: (a) topcoat before exposure; (b) primer before exposure; (b) topcoat after exposure for 3450 h; (d) primer after exposure for 3450 h.

( ) (a Rs

(b) Cf

Rf

Rs Cf

Fig. 13. Equivalent circuits for various coating/substrate systems: (a) non-deteriorated polymer-coated steel; (b) deteriorated-polymer-coated steel. Abbreviations are listed alphabetically as follows: Cdl = electric double-layer capacitance; Cf = film (coating) capacitance; Rp = polarization resistance; Rf = film (coating) resistance; Rs = solution resistance.

ing steel corrosion, is also evident. During immersion, Rf drops to 4  107 X cm2 at the onset of delamination and corrosion of the underlying steel. The EIS can be expressed by the equivalent circuit of Fig. 13b. In further lower frequency region (<102 Hz), a parallel combination of double-layer capacitance Cdl and polarization resis-

tance Rp at the steel/electrolyte interface under the delaminated coating film may appear. The difference between the EIS measurements of Fig. 16 (sunshine weather meter test) and Fig. 18 (immersion test) can be explained as follows. In both cases, Rf finally decreases to a measurable value, indicating deterioration of the coating. In the sunshine weather meter test, the topcoat is partially removed and the primer is directly exposed; in the immersion test, the coating does not degrade significantly, but does finally blister. Fig. 20 shows schematic cross-sections of coatings after both types of test. For the sunshine weather meter test (Fig. 20a), multiple water and oxygen micropaths form in the primer due to UV radiation, causing Rf to drop. However, the primer does not delaminate from the substrate steel, for reasons described in a later section. In contrast, for the immersion test (Fig. 20b), micropaths are expected to form on the surface, but to a considerably smaller extent than for the sunshine weather meter test because of the absence of UV radiation. The primer delaminates from the substrate steel because local cells, which induce delamination, form between

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M. Hattori et al. / Corrosion Science 52 (2010) 2080–2087

10

8

10

7

10

6

10

5

10

4

Exposure time ((h)) 1006 2459 3540 4553 5568

10 10 10 10 10

0 -20 -40 40 -60

2459h 3540h -80 80 4553h h 5568h

-100 -2 10

10

1006h

-1

10

0

10

1

10

2

10

3

10

4

5

10

10

9 8

Exposure time (h)

7

1006 1468 2459 3540 4553

6 5 4

0 -20 20 -40 -60 60 -80 -100 100 -2 2 10

-1 1

0

10

10

F Frequency, f /Hz /H

8

80 60

4

40

2

20

0

0

1500

3000

4500

3

10

4

10

5

10

6000

Fig. 16. Bode impedance and phase plots for the E/E coating, measured by EIS.

Coatting g re esis stance e, Rf / GΩ Ω c cm2

100

Co oating g ca apa acitancce, Cf / pF p cm m-2

m2 C Coa atin ng res r ista ancce, Rf / GΩ cm

10

0

2

10

Frequency, f /Hz

Fig. 14. Bode impedance and phase plots for the PU/E coating, measured by EIS.

6

1

10

Exposure time, t / h

10

100

8

80

6

60

4

40

2

20

0 0

Fig. 15. Plot of change in coating capacitance Cf with exposure time for the PU/E coating, measured by EIS.

the anode and cathode on the coating surface, as described in the next section. It can be concluded that degradation of the polymer coating itself decreases Rf to a measurable value in the sunshine weather meter test, while delamination of the coating reduces it in the immersion test. 3.4. Mechanism of degradation Polymer-coated steels are used mostly in atmospheric environments. The degradation process in a general atmosphere can be divided into two stages: Stage I, degradation of the polymer coating; and Stage II, corrosion of the underlying steel. During Stage I, degradation of the coating occurs mainly by sunlight (UV and heat) in atmosphere. Wet/dry cycles may enhance degradation by causing cyclic swelling/shrinking due to water absorption/desorption. Degradation of the polymer creates diffusion paths for molecules of water and oxygen, and diffusion rates increase gradually with exposure time. During wet conditions, local cells form as water pass through coating defects between the

1000

2000

3000

4000

0 5000

Co oating capaccita anc ce, Cf / pF c cm--2

9

10

10

Im mpe eda ancce, IZI / Ω c cm 2

10

2459h 3540h 4553h 5568h

1006h

P ase Pha e sh hift, θ / d degree e

P ase Pha e sh hift, θ / degree e

Imped dan nce e, IZI / Ω cm m2

10

10

Exposure time, t /h Fig. 17. Plot of changes in film (coating) capacitance Cf (circles) and film (coating) resistance Rf (squares) with exposure time for the E/E coating, determined by curvefitting the EIS data in Fig. 16.

anode and cathode on the surface of the substrate steel. Local cell current, however, should be negligible during this stage and may increase very slowly as coating degradation progresses. During Stage II, further degradation of the polymer coating, having increased the local cell current, now finally induces delamination of the coating. During wet conditions, iron at the anode may dissolve at a considerable rate into water under the deteriorated coating, and iron hydroxide (FeOOH) and magnetite (Fe3O4) may form:

Fe ! Fe2þ þ 2e 2þ

2Fe

ð1Þ þ

þ O2 þ 2H2 O ! 2FeOOH þ 2H

3Fe2þ þ 1=2O2 þ 3H2 O ! Fe3 O4 þ 6Hþ

ð2Þ ð3Þ

At the cathode, oxygen is reduced on the surface of the substrate steel:

O2 þ 2H2 O þ 4e ! 4OH

ð4Þ

P ase sh Pha hift, θ / de egrree

10

10

10

9

10

8

10

7

10

6

10

5

10

4

C ating cap Coa c paciitan nce e, Cf / pF cm m-2

M. Hattori et al. / Corrosion Science 52 (2010) 2080–2087

Imp I ped dan nce, IZ I / Ω cm m2

2086

0

50 absorption desorption absorption desorption

40 30 20 10

0

4

8 Time, t/h

12

16

Fig. 21. Plot of change in film (coating) capacitance Cf with exposure time, starting just after immersion in 0.5% NaCl solution, measured by EIS.

-20 20 -40 40 -60 60 -80 80

-100

10

-2

10

-1

10

0

10

1

10

2

10

3

10

4

10

5

Frequency, f /Hz Fig. 18. Bode impedance and phase plots for a PU/E coating without UV radiation, measured by EIS.

Fig. 19. Photograph of a PU/E coating after the immersion test.

The cathode surface thus alkalizes, inducing delamination of the coating from the substrate steel [20,21]. During Stage II, as described above, formation of local cells enhances delamination of the coating at the cathode and corrosion of the underlying steel at the anode. During this stage, adhesiveness of the polymer coating to the substrate steel may be an important factor.

This study employed wet–dry conditions of 12-min wetting followed by 48-min drying, and the time of wetness (TOW) was thus 20% of the total exposure time. TOW is well known to be an important factor in the atmospheric corrosion of metallic materials, and a value of 20% is not extremely short compared to general atmospheric conditions. As described previously, however, after UV exposure in the sunshine weather meter test for even 4533 h, delamination and corrosion of the underlying steel are not evident, even though the E/E topcoat has deteriorated significantly. Apparently, for polymer-coated steel, the length of the wet period in a wet–dry cycle is more important for the onset of delamination and underlying steel corrosion than the TOW. TOW is often defined as the period of time during which relative humidity (RH) exceeds 80% [22], and corrosion of uncoated metallic materials is assumed to start when RH exceeds 80% RH. For polymer-coated steels, however, it takes a certain amount of time for water to reach the steelcoating boundary, establish local cells between the anode and cathode, and thus enhance delamination and corrosion. If the drying stage of a cycle starts before water can penetrate the boundary, delamination and corrosion do not occur. The wetting period of 12 min per cycle may thus be too short for the onset of delamination and corrosion. To confirm the time period of water absorption, a cycle test of water absorption/desorption using the PU(100 lm)/E(100 lm) coating was performed by alternate exposure to 0.01 M LiCl solution (4 h) and 10 M LiCl solution (4 h) at 60 °C. A LiCl-saturatedsolution was selected for water desorption because it has the lowest activity of H2O among chloride-saturated solutions. Fig. 21 shows the change in coating capacitance. The coating capacitance was monitored by continuous measurements of impedance at

Fig. 20. Schematic cross-sections of a coating after both types of corrosion test: (a) sunshine weather meter test; (b) immersion test.

M. Hattori et al. / Corrosion Science 52 (2010) 2080–2087

1 kHz [23]. The increase in capacitance in the dilute solution (activity of water, aH2O  1) is attributed to water absorption into the polymer coating, while the decrease in capacitance in the concentrated solution (aH2O  0.15) is due to water desorption. From the monitoring result, it is found that it takes about 4 h for water to saturate the coating, confirming that the wetting time of 12 min each cycle is too short for the onset of film delamination and corrosion of the underlying steel. 4. Conclusions Accelerated corrosion tests of PU/E- and E/E-coated steels were performed under exposure to UV radiation for about 4500 h, and degradation was evaluated by SEM, FT-IR, EDX and EIS. The following conclusions were drawn: (1) The PU/E coating deteriorates only a very little. In contrast, the E/E topcoat deteriorates significantly due to chalking, and partially exposes the primer. (2) The PU/E coating shows only capacitive behaviour, which does not change with exposure time. In contrast, the E/E coating shows resistance Rf in the low-frequency range of the Bode plot, indicating topcoat degradation, in good agreement with SEM observations. (3) The E/E coating, despite significant topcoat deterioration and corresponding decrease in Rf, does not delaminate from the substrate steel, and corrosion of the steel does not occur. The decrease in Rf is due to deterioration of the polymer coating. (4) EIS is an effective technique for evaluating polymer coating degradation as well as for detecting the onset of corrosion of underlying steel.

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