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UV aging characterization of epoxy varnish coated steel upon exposure to artificial weathering environment
Materials and Design 30 (2009) 1542–1547
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
UV aging characterization of epoxy varnish coated steel upon exposure to artificial weathering environment Jianwen Hu a,b, Xiaogang Li a,*, Jin Gao a, Quanlin Zhao a a b
Materials Science and Engineering School, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Materials Science and Engineering School, Hebei University of Science and Technology, Shijiazhuang 050054, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 6 July 2008 Accepted 30 July 2008 Available online 8 August 2008 Keywords: Aging Epoxy varnish Artificial weathering test EIS SEM Blister .
a b s t r a c t Epoxy varnish coating was exposed to artificial weathering environment produced by fluorescent UV/ condensation weathering equipment for different time periods. The degradation process of epoxy varnish coating was studied by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and adhesion test. The results showed that the electrochemical behavior of aged coating was closely related to the formation and development of blisters on the surface due to coating degradation. The coating resistance, Rp, decreased to a lower value after 28 days of exposure, which indicated significant deterioration of barrier properties. Small blisters were observed on coating surface after 21 days of exposure. With increasing aging time, blisters grow up and subsequently broke. The mechanism of blisters formation and subsequent breakage were suggested. The soluble degradation products penetrate into the coating along with water to form osmotic cells leading to the form of blisters on the coating surface under the alternating wet and UV irradiation condition. The spread of degradation areas caused the growth and development of blisters. With the loss of coating material and embrittlement, cracks appear on the surface and the blisters break, which may result in the significant deterioration of bulk properties of the coatings. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
1. Introduction Organic coatings are widely used in corrosion protection of metallic constructions. It is generally accepted that the coating efficiency is dependent on the barrier properties, the adhesion properties of coating/substrate interface and the degree of environment aggressiveness and so on. For external applications, solar radiation (particularly UV radiation), temperature, water, oxygen and pollutants are main factors inducing degradation that can reduce its anti-corrosive effectiveness. So in many laboratory artificial weathering tests, UV is often associated with condensation or artificial rain in order to simulate water influence to produce a synergic effect to photochemical degradation [1–5]. The degradation of organic coating is often evaluated by visual examination, physical, chemical, mechanical, electrochemical methods and so on [6–8]. Electrochemical impedance spectroscopy (EIS) is commonly used to examine and rank the protective performance of anticorrosion paint qualitatively or semi-quantitatively [9–12]. However, few articles were reported on the relationship between the electrochemical behavior and the formation and development of blisters on UV degraded surface. In order to get more information about the decay of protection properties of or* Corresponding author. Tel.: +86 10 62333931; fax: +86 10 62334005. E-mail address: [email protected] (X. Li).
ganic coating/steel system under UV artificial test in a short term, epoxy varnish was used to investigate the effect of artificial accelerating test on the electrochemical behavior and micro-morphology of degradation coatings so as to reveal the essential reason on deterioration of coating protection property. This paper presented some results concerning the effect of artificial UV degradation on the electrochemical properties, surface micro-morphology and adhesion between coating and steel substrate in order to set up correlation between EIS parameters and photo-degradation degree of coatings. 2. Experimental 2.1. Materials Cold rolled steel plates (100 70 1 mm) were used as substrate. They were polished with an abrasive paper from 80 to 500 grades, degreased in acetone and rinsed with methyl alcohol. Two-pack components epoxy varnishes were purchased from Shijiazhuang Goldfish Painting Factory, China. A component consists of epoxy resin and solvent, while B component is a special solidified agent. The weight ratio of A:B used was 100:30. The coatings were spray applied with the thickness of 45–55 lm. The coated panels were allowed to cure at room temperature for 2 weeks. Three parallel panels were used in each cycle.
0261-3069/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.051
1543
J. Hu et al. / Materials and Design 30 (2009) 1542–1547
Artificial weathering test was carried out by exposing the coated panels in a fluorescent UV/condensation weathering equipment (UV 2000TM, USA). The cycle consisted of 8 h of UV (k = 340 nm) radiation at 60 °C and 4 h of condensation at 50 °C. The irradiance intensity was 0.55 W m 2. The coated panels were exposed for different time intervals up to 35 days.
a
1d immersion 3d immersion
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8.0x10
9
2.3. EIS studies EIS measurements were conducted by using a PARSTAT 2273 system with a conventional three electrode configuration. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a platinum mesh electrode. The area of the working electrode was 3.14 cm2. The electrolyte used was 3.5 wt.% NaCl solution. The frequency range of the applied AC voltage was from 100 kHz to 10 mHz with the sinusoidal perturbation amplitude of 10 mV. All measurements were carried out at the open circuit potential. The software used for equivalent circuit modeling was ZsimpWin software provided by Princeton Applied Research.
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The surface morphology of epoxy coating was observed by a Cambridge S360 scanning electron microscope (Leica, Germany) with the accelerating voltage of 20 kV.
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2.5. Adhesion tests
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The adhesion between epoxy coating and metal substrate was measured by using a PosiTest Adhesion Tester (DeFelsko, USA). An epoxy adhesive, Aradite AV100 was used to join the stud and the coating and it was cured at ambient temperature for 24 h before testing. At least three locations for one panel were tested in order to get a reliable result. 3. Results and discussion
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Frenquency/Hz Fig. 1. Impedance spectrum of 7 days of exposure on UVA/condensation condition immersion one and three days in 3.5% chloride solution at ambient temperature. (a) Nyquist plot, and (b) Bode plot.
3.1. EIS Figs. 1 and 3 present impedance diagrams (the Nyquist and Bode plots) of the initial stage of immersion (1 d and 3 d) to 3.5% chloride solution at ambient temperature for the samples at different exposure times. Fig. 1 revealed a capacitive behavior over the test frequency subjected to 7 days of exposure. The impedance response is dominated by the dielectric property of coating. There is a slight decrease of capacitive arc at 3 days immersion, while the Bode plots maintained approximately the same order magnitude of low frequency (LF) impedance value |Z| (more than 1010 X cm2), indicating an excellent barrier properties. It also indicated no significant change on coating before this period. The initial drop of capacitive arc at 3 days immersion maybe due to water ingress into the coating through the micro-pores or defects of coating. For 28 days of exposure, there are dramatic differences for EIS spectra (Fig. 2), which shows two semicircles over the frequency range indicating two time constant character in one day immersion. The high frequency (HF) semicircle is attributed to coating pore impedance Rp, while the low frequency semicircle is the impedance response associated with the corrosion reaction occurring at the interface through defects and pores in the coating. A significant decrease of LF impedance value, from 1010 to 106 X cm2 order magnitudes during 28 days of exposure indicated significant drop of barrier properties. LF impedance value of a coating system between 1010 and 1013 X cm2 possess excellent barrier properties, while impedance value between 106 and 108 X cm2,
fair barrier properties, whereas a degraded or failed coated metal shows an impedance value less than 106 X cm2 [13]. At three days immersion, the diameter of HF semi-circle continued to decrease, the whole of the LF semi-circle masked out by a linear part, Warburg impedance. This shows that the corrosion reaction happened at the bases of small defects or pores was hindered by mass transport to the corrosion site. The linear part has an angle of about 80° with respect to the real axis. For 35 days exposure, except for progressively decreasing in HF impedance or semicircle, depressed LF semicircles were obtained. Furthermore there was a rapid decrease in LF semicircle for 3 days immersion which implies faster interface corrosion reaction happened. It may be the poor adhesion underlying the coating caused by periodic accelerating weathering test resulting in a continual decrease of the charge transfer resistance. In general, a physical explanation of EIS spectrum can be given by fitting the experimental impedance data with equivalent circuit model [14]. Fig. 4 shows several common models which were adopted to fit Figs. 1–3 to get the least relative error between experimental and calculated data. EIS plots of 7 days exposure possessing only one time constant were acceptable for using a Randles circuit model (Fig. 4a). The model consists of solution resistance (RS), and a constant phase element (CPE) capacitor (Qc) related to the nonideal capacitive behavior of coating in parallel with the coating resistance (Rp). Data analysis shows high coating resistance values above the order of GX cm2 (6.19 1010 Xcm2) and low coating capaci-
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J. Hu et al. / Materials and Design 30 (2009) 1542–1547
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Frequency/Hz Fig. 2. Impedance spectrum of 28 days of exposure on UVA/condensation condition immersion one and three days in 3.5% chloride solution at ambient temperature. (a) Nyquist plot, and (b) Bode plot.
tance on the order of 10 10 F cm2 (2.82 10 10 F cm2). The higher the coating resistance and the lower the coating capacitance, the better barrier property of the coating possesses. For EIS plots of 28 days exposure, classic model used for coating system with two time constants was adopted (Fig. 4b), where Qdl is the double-layer capacitance and Rct is the charge transfer resistance occurring between organic coating and metal interface. The other element in Fig. 4b has the same meanings as those in (Fig. 4a). In the instance where diffusion controlled the system, the charge transfer resistance Rct was replaced by Rct in series with a Warburg impedance (W). Data analysis showed that the coating resistance dropped quickly to the order of MX cm2 (2.76 106 Xcm2) and the coating capacitance maintained at the same order 10 10 F cm2 (3.01 10 10 F cm2), which suggests coating resistance may be more sensitive than capacitance to evaluate the change of barrier properties of the coating. So the coating resistance is more reliable to evaluate the protective property of UV aged coating. Another equivalent circuit of two time constant (Fig. 4c) was selected to fit EIS plots of 35 days exposure to obtain the best fitting. The physical meaning of this model is associated to the penetration of water and electrolyte and corrosion cell of interface homogeneously. Data analysis showed that more lower coating resistance was on the order of MX cm2 (1.21 106 Xcm2), whereas the capacitance suddenly increased to the order of pF cm2 (1.70 10 6 F cm2). The greater increase of coating capacitance indicated much water ingress or accumulating in the coating or at the inter-
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Frequency/Hz Fig. 3. Impedance spectrum of 35 days of exposure on UVA/condensation condition immersion one and three days in 3.5% chloride solution at ambient temperature. (a) Nyquist plot, and (b) Bode plot.
face may be due to a fatal breakage of the bulk coating. To the UV aging organic, it maybe means serious degradation appears in almost the whole coating resulting in significant deterioration of bulk properties. Parameters Rp and Qc which obtained by fitting the impedance diagrams with the electrical equivalent circuit model shown in Fig. 4 are illustrated as a function of exposure time in Figs. 5 and 6. In agreement with the observed decrease of the low frequency impedance modulus, the coating resistance Rp progressively decreased with exposure time, especially for samples of exposure time exceed 21 days showing an apparent drop. Rp value at 21 days of exposure is 4.96 107 Xcm2, and drops to more lower impedance of 106 X cm2 order for 28 days and 35 days exposure, which means severe degradation occurred after 28 days of exposure. The coating capacitance value Qc, which reflects water and electrolyte ingress in the coating, is almost constant, maintaining at the same order 10 10 F cm2 before 28 days of exposure. At 35 days of exposure, the value of Qc increased from 10 10 F cm2 to 10 6 F cm2 order magnitudes. The relatively high coating capacitance value may suggest a serious defective coating which expose to electrolyte resulting in much penetration of water or accumulated at the metal/coating interface. Fedrizzi have shown that Qc is not only related to the dielectric properties of the polymer but it becomes a complex parameter affected by water present into the coating (such as clusters or as a film around the pigments)
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Fig. 4. Electrical equivalent circuit models adopted for epoxy varnish coating after different UV aging times (a) for 7 days of exposure, (b) for 28 days of exposure and (c) for 35 days of exposure.
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Exposure time/d Fig. 5. Coating resistance Rp vs. exposure time after one day immersion in 3.5% chloride solution.
and by the water locally accumulated at the metal-coating interface [15]. So it requires further study on the high capacitance value. 3.2. Surface morphology Fig. 7 shows micrographs of epoxy varnish coatings before and after aging for different times in the artificial weathering environment. Compared with the other samples, the surface of the unexposed coating (Fig. 7a) was relatively smooth. After 21 days of exposure, small blisters were observed on the coating surface (Fig. 7b). The formation of blisters had ever discussed by some authors [16–18]. A proposed mechanism of blister formation may be as follows: When the epoxy varnish coating is exposed in the UV/condensation chamber, the coating degraded under the synergic effects of UV, water and oxygen, soluble degradation products penetrate into the coating along with water during the wet period cannot escape during the dry period. As a result, additional water
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Exposure time/d Fig. 6. Coating capacitance Qc vs. exposure time after one day immersion in 3.5% chloride solution.
was adsorbed into the coating which in turn formed osmotic cells under the coating surface layer. Thus, the periodical weathering test causes the osmotic cells continue to develop and the coating surface became blistered over the locally oxidized areas. With increasing aging time, the blisters enlarged and microcracks can be observed on some blisters at 28 days of exposure (Fig. 7c). The growth of blister may be the result of spread of degradation areas to cause more soluble degradation products which result in much water absorbed into the coating. The formation of cracks on some blisters may be attributed to the loss of coating material which decreases in thickness and shrinks to cause embrittlement and cracking [19]. At 35 days of exposure (Fig. 7d), many blisters break to form bigger and deeper pits. The development and breakage of blisters are early stage of coating surface damage under UV/condition accelerating weathering which can result in the significant deterioration of barrier properties. It can be seen that the micro-morphologies of UV aging epoxy varnish coating
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Fig. 7. SEM images showing the blisters formation and progress: (a) before exposure, (b) after 21 days exposure, (c) after 28 days exposure, and (d) after 35days exposure.
condensation but also cause embrittlement due to photo degradation. It also can be seen that the decrease of adhesion with increasing aging time is similar with the variation of coating resistance.
Adhesion strenth/MPa
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4. Conclusion From the results of EIS, SEM and adhesion tests, the following conclusions can be obtained:
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Exposure time/d Fig. 8. Effect of exposure time on the adhesion between the epoxy varnish coating and substrate.
are in agreement with the previous statements which gave an effective interpretation of the variation of parameters Rp, and also the EIS spectrum. 3.3. Adhesion measurement Fig. 8 presents the effect of exposure time on the adhesion between the epoxy varnish coating and substrate. In the first 21 days of exposure, the adhesion strength decreased quickly from 1.35 MPa to 0.46 MPa and then leveled off at the relative lower value in the following days. The decrease of adhesion is related to the process of photo degradation under UV radiation/condensation weathering environment resulted in the break of polymeric chains with the formation of small molecules such as ketones, alcohols and acids [20]. These oxidized products evaporated or were washed away by moisture contact to decrease the coating thickness. So, it can not only increase the internal stress of coating because of swelling and shrink during periodically UV radiation and
(1) EIS is a very useful technology which allows us to obtain valuable information on the evolution of both the organic coating barrier properties and the degree of photo degradation under UV/condensation artificial weathering environment. Compared with the coating capacitance, the coating resistance is more sensitive to be used to assess the barrier properties of the aged coating in artificial weathering environment. (2) SEM indicated that EIS characteristics and variations of Rp for epoxy varnish coating are related to the formation and development of blisters under UV/condensation artificial weathering environment. (3) The mechanism of blisters formation and development can be described as following: the epoxy varnish coating degraded under the synergic effects of UV, water and oxygen, soluble degradation products penetrate into the coating along with water to form osmotic cells under the coating surface layer. The cycling of wet and UV irradiation cause osmotic pressure effects leading to the form of blisters over the locally oxidized areas of the coating surface. The spread of degradation areas causes more degradation products which result in water absorption into osmotic cells that cause the growth and development of blisters. With the loss of coating material, the thickness of the coating decreased. The serious breakage of blisters results in the significant deterioration of bulk properties of coatings. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China, whose registered
J. Hu et al. / Materials and Design 30 (2009) 1542–1547
number is 50571013. The authors are also thankful for the financial support of the National R&D Infrastructure and Facility Development Program of China, whose registered number is 2005DKA10400. References [1] Jacques LFE. Accelerated and outdoor/natural exposure testing of coatings. Prog Polym Sci 2000;25(9):1337–62. [2] Johnson BW, McIntyre R. Analysis of test methods for UV durability predictions of polymer coatings. Prog Org Coat 1996;27(1):95–106. [3] Yang XF, Tallman DE, Bierwagen GP, et al. Blistering and degradation of polyurethane coatings under different accelerated weathering tests. Polym Degrad Stab 2002;77(1):103–9. [4] Valentinelli L, Vogelsang J, Ochs H, et al. Evaluation of barrier coatings by cycling testing. Prog Org Coat 2002;45(4):405–13. [5] Skerry BS, Simpson CH. Accelerated test method for assessing corrosion and weathering of paints for atmospheric corrosion control. Corrosion 1993;49(8):663–74. [6] Perrin FX, Irigoyen M, Aragon E, et al. Evaluation of accelerated weathering tests for three paint systems: a comparative study of their aging behavior. Polym Degrad Stab 2001;72(1):115–24. [7] Yang XF, Li J, Croll SG, et al. Degradation of low gloss polyurethane aircraft coatings under UV and prohesion alternating exposures. Polym Degrad Stab 2003;80(1):51–8. [8] Irigoyen M, Bartolomeo P, Perrin FX, et al. UV ageing characterization of organic anticorrosion coatings by dynamic mechanical analysis, Vickers microhardness, and infra-red analysis. Polym Degrad Stab 2001;74(1): 59–67.
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[9] Poelman M, Olivier M-G, Gayarre N, et al. Electrochemical study of different ageing tests for the evaluation of a cataphoretic epoxy primer on aluminum. Prog Org Coat 2005;54(1):55–62. [10] Amirudin A, Thierry D. Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals. Prog Org Coat 1995;26(1):1–28. [11] Deflorian F, Fedrizzi S, Rossi S, et al. Organic coating capacitance measurement by EIS: ideal and actual trends. Electrochim Acta 1999;44(24):4243–9. [12] McIntyre JM, Pham HQ. Electrochemical impedance spectroscopy; a tool for organic coatings optimizations. Prog Org Coat 1996;27(1):201–7. [13] Bierwagen GP, He L, Li J, et al. Studies of a new accelerated evaluation method for coating corrosion resistance – thermal cycling testing. Prog Org Coat 2000;39(1):67–78. [14] Vogelsang J, Strunz W. New interpretation of electrochemical data obtained from organic barrier coatings. Electrochim Acta 2001;46(24– 25):3817–26. [15] Fedrizzi L, Bergo A, Fanicchia M. Evaluation of accelerated aging procedures of painted galvanized steels by EIS. Electrochim Acta 2006;51(8–9):1864–72. [16] Wicks ZW, Jones FN, Peppas SP. Organic coatings: science and technology. 2nd ed. New York: John Wiley & Sons; 1994. [17] Lemaire J, Arnaud R, Gardette JL. The role of hydroperoxides in photooxidation of polylefines, polyamides and polyurethane elastomers. Pure Appl Chem 1983;55(10):1603–14. [18] Yang XF, Vang C, Tallman DE, et al. Weathering degradation of a polyurethane coating. Polym Degrad Stab 2001;74(2):341–51. [19] Armsteron RD, Jenkins ATA, Johnson BW. An investigation into the UV breakdown of thermoset polyester coatings using impedance spectroscopy. Corros Sci 1995;37(10):1615–25. [20] Deflorian F, Fenrizzi L, Bonora PL. Influence of the photo-oxidative degradation on the water barrier and corrosion protection properties of polyester paints. Corros Sci 1996;38(10):1697–708.
Contents lists available at ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
UV aging characterization of epoxy varnish coated steel upon exposure to artificial weathering environment Jianwen Hu a,b, Xiaogang Li a,*, Jin Gao a, Quanlin Zhao a a b
Materials Science and Engineering School, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China Materials Science and Engineering School, Hebei University of Science and Technology, Shijiazhuang 050054, People’s Republic of China
a r t i c l e
i n f o
Article history: Received 6 July 2008 Accepted 30 July 2008 Available online 8 August 2008 Keywords: Aging Epoxy varnish Artificial weathering test EIS SEM Blister .
a b s t r a c t Epoxy varnish coating was exposed to artificial weathering environment produced by fluorescent UV/ condensation weathering equipment for different time periods. The degradation process of epoxy varnish coating was studied by electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and adhesion test. The results showed that the electrochemical behavior of aged coating was closely related to the formation and development of blisters on the surface due to coating degradation. The coating resistance, Rp, decreased to a lower value after 28 days of exposure, which indicated significant deterioration of barrier properties. Small blisters were observed on coating surface after 21 days of exposure. With increasing aging time, blisters grow up and subsequently broke. The mechanism of blisters formation and subsequent breakage were suggested. The soluble degradation products penetrate into the coating along with water to form osmotic cells leading to the form of blisters on the coating surface under the alternating wet and UV irradiation condition. The spread of degradation areas caused the growth and development of blisters. With the loss of coating material and embrittlement, cracks appear on the surface and the blisters break, which may result in the significant deterioration of bulk properties of the coatings. Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved.
1. Introduction Organic coatings are widely used in corrosion protection of metallic constructions. It is generally accepted that the coating efficiency is dependent on the barrier properties, the adhesion properties of coating/substrate interface and the degree of environment aggressiveness and so on. For external applications, solar radiation (particularly UV radiation), temperature, water, oxygen and pollutants are main factors inducing degradation that can reduce its anti-corrosive effectiveness. So in many laboratory artificial weathering tests, UV is often associated with condensation or artificial rain in order to simulate water influence to produce a synergic effect to photochemical degradation [1–5]. The degradation of organic coating is often evaluated by visual examination, physical, chemical, mechanical, electrochemical methods and so on [6–8]. Electrochemical impedance spectroscopy (EIS) is commonly used to examine and rank the protective performance of anticorrosion paint qualitatively or semi-quantitatively [9–12]. However, few articles were reported on the relationship between the electrochemical behavior and the formation and development of blisters on UV degraded surface. In order to get more information about the decay of protection properties of or* Corresponding author. Tel.: +86 10 62333931; fax: +86 10 62334005. E-mail address: [email protected] (X. Li).
ganic coating/steel system under UV artificial test in a short term, epoxy varnish was used to investigate the effect of artificial accelerating test on the electrochemical behavior and micro-morphology of degradation coatings so as to reveal the essential reason on deterioration of coating protection property. This paper presented some results concerning the effect of artificial UV degradation on the electrochemical properties, surface micro-morphology and adhesion between coating and steel substrate in order to set up correlation between EIS parameters and photo-degradation degree of coatings. 2. Experimental 2.1. Materials Cold rolled steel plates (100 70 1 mm) were used as substrate. They were polished with an abrasive paper from 80 to 500 grades, degreased in acetone and rinsed with methyl alcohol. Two-pack components epoxy varnishes were purchased from Shijiazhuang Goldfish Painting Factory, China. A component consists of epoxy resin and solvent, while B component is a special solidified agent. The weight ratio of A:B used was 100:30. The coatings were spray applied with the thickness of 45–55 lm. The coated panels were allowed to cure at room temperature for 2 weeks. Three parallel panels were used in each cycle.
0261-3069/$ - see front matter Crown Copyright Ó 2008 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.07.051
1543
J. Hu et al. / Materials and Design 30 (2009) 1542–1547
Artificial weathering test was carried out by exposing the coated panels in a fluorescent UV/condensation weathering equipment (UV 2000TM, USA). The cycle consisted of 8 h of UV (k = 340 nm) radiation at 60 °C and 4 h of condensation at 50 °C. The irradiance intensity was 0.55 W m 2. The coated panels were exposed for different time intervals up to 35 days.
a
1d immersion 3d immersion
9
8.0x10
9
2.3. EIS studies EIS measurements were conducted by using a PARSTAT 2273 system with a conventional three electrode configuration. The reference electrode was a saturated calomel electrode (SCE) and the counter electrode was a platinum mesh electrode. The area of the working electrode was 3.14 cm2. The electrolyte used was 3.5 wt.% NaCl solution. The frequency range of the applied AC voltage was from 100 kHz to 10 mHz with the sinusoidal perturbation amplitude of 10 mV. All measurements were carried out at the open circuit potential. The software used for equivalent circuit modeling was ZsimpWin software provided by Princeton Applied Research.
10
1.0x10
Zi
2.2. Artificial weathering test
6.0x10
9
4.0x10
9
2.0x10
0.0 9 -1x10
9
0
9
9
9
9
9
9
9
1x10 2x10 3x10 4x10 5x10 6x10 7x10 8x10
Zr
b
10
10
9
10
2.4. Surface morphology
Z /Ohms
8
The surface morphology of epoxy coating was observed by a Cambridge S360 scanning electron microscope (Leica, Germany) with the accelerating voltage of 20 kV.
10
7
10
6
2.5. Adhesion tests
10 5
The adhesion between epoxy coating and metal substrate was measured by using a PosiTest Adhesion Tester (DeFelsko, USA). An epoxy adhesive, Aradite AV100 was used to join the stud and the coating and it was cured at ambient temperature for 24 h before testing. At least three locations for one panel were tested in order to get a reliable result. 3. Results and discussion
10 4
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0
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3
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4
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Frenquency/Hz Fig. 1. Impedance spectrum of 7 days of exposure on UVA/condensation condition immersion one and three days in 3.5% chloride solution at ambient temperature. (a) Nyquist plot, and (b) Bode plot.
3.1. EIS Figs. 1 and 3 present impedance diagrams (the Nyquist and Bode plots) of the initial stage of immersion (1 d and 3 d) to 3.5% chloride solution at ambient temperature for the samples at different exposure times. Fig. 1 revealed a capacitive behavior over the test frequency subjected to 7 days of exposure. The impedance response is dominated by the dielectric property of coating. There is a slight decrease of capacitive arc at 3 days immersion, while the Bode plots maintained approximately the same order magnitude of low frequency (LF) impedance value |Z| (more than 1010 X cm2), indicating an excellent barrier properties. It also indicated no significant change on coating before this period. The initial drop of capacitive arc at 3 days immersion maybe due to water ingress into the coating through the micro-pores or defects of coating. For 28 days of exposure, there are dramatic differences for EIS spectra (Fig. 2), which shows two semicircles over the frequency range indicating two time constant character in one day immersion. The high frequency (HF) semicircle is attributed to coating pore impedance Rp, while the low frequency semicircle is the impedance response associated with the corrosion reaction occurring at the interface through defects and pores in the coating. A significant decrease of LF impedance value, from 1010 to 106 X cm2 order magnitudes during 28 days of exposure indicated significant drop of barrier properties. LF impedance value of a coating system between 1010 and 1013 X cm2 possess excellent barrier properties, while impedance value between 106 and 108 X cm2,
fair barrier properties, whereas a degraded or failed coated metal shows an impedance value less than 106 X cm2 [13]. At three days immersion, the diameter of HF semi-circle continued to decrease, the whole of the LF semi-circle masked out by a linear part, Warburg impedance. This shows that the corrosion reaction happened at the bases of small defects or pores was hindered by mass transport to the corrosion site. The linear part has an angle of about 80° with respect to the real axis. For 35 days exposure, except for progressively decreasing in HF impedance or semicircle, depressed LF semicircles were obtained. Furthermore there was a rapid decrease in LF semicircle for 3 days immersion which implies faster interface corrosion reaction happened. It may be the poor adhesion underlying the coating caused by periodic accelerating weathering test resulting in a continual decrease of the charge transfer resistance. In general, a physical explanation of EIS spectrum can be given by fitting the experimental impedance data with equivalent circuit model [14]. Fig. 4 shows several common models which were adopted to fit Figs. 1–3 to get the least relative error between experimental and calculated data. EIS plots of 7 days exposure possessing only one time constant were acceptable for using a Randles circuit model (Fig. 4a). The model consists of solution resistance (RS), and a constant phase element (CPE) capacitor (Qc) related to the nonideal capacitive behavior of coating in parallel with the coating resistance (Rp). Data analysis shows high coating resistance values above the order of GX cm2 (6.19 1010 Xcm2) and low coating capaci-
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Frequency/Hz Fig. 2. Impedance spectrum of 28 days of exposure on UVA/condensation condition immersion one and three days in 3.5% chloride solution at ambient temperature. (a) Nyquist plot, and (b) Bode plot.
tance on the order of 10 10 F cm2 (2.82 10 10 F cm2). The higher the coating resistance and the lower the coating capacitance, the better barrier property of the coating possesses. For EIS plots of 28 days exposure, classic model used for coating system with two time constants was adopted (Fig. 4b), where Qdl is the double-layer capacitance and Rct is the charge transfer resistance occurring between organic coating and metal interface. The other element in Fig. 4b has the same meanings as those in (Fig. 4a). In the instance where diffusion controlled the system, the charge transfer resistance Rct was replaced by Rct in series with a Warburg impedance (W). Data analysis showed that the coating resistance dropped quickly to the order of MX cm2 (2.76 106 Xcm2) and the coating capacitance maintained at the same order 10 10 F cm2 (3.01 10 10 F cm2), which suggests coating resistance may be more sensitive than capacitance to evaluate the change of barrier properties of the coating. So the coating resistance is more reliable to evaluate the protective property of UV aged coating. Another equivalent circuit of two time constant (Fig. 4c) was selected to fit EIS plots of 35 days exposure to obtain the best fitting. The physical meaning of this model is associated to the penetration of water and electrolyte and corrosion cell of interface homogeneously. Data analysis showed that more lower coating resistance was on the order of MX cm2 (1.21 106 Xcm2), whereas the capacitance suddenly increased to the order of pF cm2 (1.70 10 6 F cm2). The greater increase of coating capacitance indicated much water ingress or accumulating in the coating or at the inter-
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Frequency/Hz Fig. 3. Impedance spectrum of 35 days of exposure on UVA/condensation condition immersion one and three days in 3.5% chloride solution at ambient temperature. (a) Nyquist plot, and (b) Bode plot.
face may be due to a fatal breakage of the bulk coating. To the UV aging organic, it maybe means serious degradation appears in almost the whole coating resulting in significant deterioration of bulk properties. Parameters Rp and Qc which obtained by fitting the impedance diagrams with the electrical equivalent circuit model shown in Fig. 4 are illustrated as a function of exposure time in Figs. 5 and 6. In agreement with the observed decrease of the low frequency impedance modulus, the coating resistance Rp progressively decreased with exposure time, especially for samples of exposure time exceed 21 days showing an apparent drop. Rp value at 21 days of exposure is 4.96 107 Xcm2, and drops to more lower impedance of 106 X cm2 order for 28 days and 35 days exposure, which means severe degradation occurred after 28 days of exposure. The coating capacitance value Qc, which reflects water and electrolyte ingress in the coating, is almost constant, maintaining at the same order 10 10 F cm2 before 28 days of exposure. At 35 days of exposure, the value of Qc increased from 10 10 F cm2 to 10 6 F cm2 order magnitudes. The relatively high coating capacitance value may suggest a serious defective coating which expose to electrolyte resulting in much penetration of water or accumulated at the metal/coating interface. Fedrizzi have shown that Qc is not only related to the dielectric properties of the polymer but it becomes a complex parameter affected by water present into the coating (such as clusters or as a film around the pigments)
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Fig. 4. Electrical equivalent circuit models adopted for epoxy varnish coating after different UV aging times (a) for 7 days of exposure, (b) for 28 days of exposure and (c) for 35 days of exposure.
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and by the water locally accumulated at the metal-coating interface [15]. So it requires further study on the high capacitance value. 3.2. Surface morphology Fig. 7 shows micrographs of epoxy varnish coatings before and after aging for different times in the artificial weathering environment. Compared with the other samples, the surface of the unexposed coating (Fig. 7a) was relatively smooth. After 21 days of exposure, small blisters were observed on the coating surface (Fig. 7b). The formation of blisters had ever discussed by some authors [16–18]. A proposed mechanism of blister formation may be as follows: When the epoxy varnish coating is exposed in the UV/condensation chamber, the coating degraded under the synergic effects of UV, water and oxygen, soluble degradation products penetrate into the coating along with water during the wet period cannot escape during the dry period. As a result, additional water
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Exposure time/d Fig. 6. Coating capacitance Qc vs. exposure time after one day immersion in 3.5% chloride solution.
was adsorbed into the coating which in turn formed osmotic cells under the coating surface layer. Thus, the periodical weathering test causes the osmotic cells continue to develop and the coating surface became blistered over the locally oxidized areas. With increasing aging time, the blisters enlarged and microcracks can be observed on some blisters at 28 days of exposure (Fig. 7c). The growth of blister may be the result of spread of degradation areas to cause more soluble degradation products which result in much water absorbed into the coating. The formation of cracks on some blisters may be attributed to the loss of coating material which decreases in thickness and shrinks to cause embrittlement and cracking [19]. At 35 days of exposure (Fig. 7d), many blisters break to form bigger and deeper pits. The development and breakage of blisters are early stage of coating surface damage under UV/condition accelerating weathering which can result in the significant deterioration of barrier properties. It can be seen that the micro-morphologies of UV aging epoxy varnish coating
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Fig. 7. SEM images showing the blisters formation and progress: (a) before exposure, (b) after 21 days exposure, (c) after 28 days exposure, and (d) after 35days exposure.
condensation but also cause embrittlement due to photo degradation. It also can be seen that the decrease of adhesion with increasing aging time is similar with the variation of coating resistance.
Adhesion strenth/MPa
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4. Conclusion From the results of EIS, SEM and adhesion tests, the following conclusions can be obtained:
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Exposure time/d Fig. 8. Effect of exposure time on the adhesion between the epoxy varnish coating and substrate.
are in agreement with the previous statements which gave an effective interpretation of the variation of parameters Rp, and also the EIS spectrum. 3.3. Adhesion measurement Fig. 8 presents the effect of exposure time on the adhesion between the epoxy varnish coating and substrate. In the first 21 days of exposure, the adhesion strength decreased quickly from 1.35 MPa to 0.46 MPa and then leveled off at the relative lower value in the following days. The decrease of adhesion is related to the process of photo degradation under UV radiation/condensation weathering environment resulted in the break of polymeric chains with the formation of small molecules such as ketones, alcohols and acids [20]. These oxidized products evaporated or were washed away by moisture contact to decrease the coating thickness. So, it can not only increase the internal stress of coating because of swelling and shrink during periodically UV radiation and
(1) EIS is a very useful technology which allows us to obtain valuable information on the evolution of both the organic coating barrier properties and the degree of photo degradation under UV/condensation artificial weathering environment. Compared with the coating capacitance, the coating resistance is more sensitive to be used to assess the barrier properties of the aged coating in artificial weathering environment. (2) SEM indicated that EIS characteristics and variations of Rp for epoxy varnish coating are related to the formation and development of blisters under UV/condensation artificial weathering environment. (3) The mechanism of blisters formation and development can be described as following: the epoxy varnish coating degraded under the synergic effects of UV, water and oxygen, soluble degradation products penetrate into the coating along with water to form osmotic cells under the coating surface layer. The cycling of wet and UV irradiation cause osmotic pressure effects leading to the form of blisters over the locally oxidized areas of the coating surface. The spread of degradation areas causes more degradation products which result in water absorption into osmotic cells that cause the growth and development of blisters. With the loss of coating material, the thickness of the coating decreased. The serious breakage of blisters results in the significant deterioration of bulk properties of coatings. Acknowledgements The authors are grateful for the financial support of the National Natural Science Foundation of China, whose registered
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number is 50571013. The authors are also thankful for the financial support of the National R&D Infrastructure and Facility Development Program of China, whose registered number is 2005DKA10400. References [1] Jacques LFE. Accelerated and outdoor/natural exposure testing of coatings. Prog Polym Sci 2000;25(9):1337–62. [2] Johnson BW, McIntyre R. Analysis of test methods for UV durability predictions of polymer coatings. Prog Org Coat 1996;27(1):95–106. [3] Yang XF, Tallman DE, Bierwagen GP, et al. Blistering and degradation of polyurethane coatings under different accelerated weathering tests. Polym Degrad Stab 2002;77(1):103–9. [4] Valentinelli L, Vogelsang J, Ochs H, et al. Evaluation of barrier coatings by cycling testing. Prog Org Coat 2002;45(4):405–13. [5] Skerry BS, Simpson CH. Accelerated test method for assessing corrosion and weathering of paints for atmospheric corrosion control. Corrosion 1993;49(8):663–74. [6] Perrin FX, Irigoyen M, Aragon E, et al. Evaluation of accelerated weathering tests for three paint systems: a comparative study of their aging behavior. Polym Degrad Stab 2001;72(1):115–24. [7] Yang XF, Li J, Croll SG, et al. Degradation of low gloss polyurethane aircraft coatings under UV and prohesion alternating exposures. Polym Degrad Stab 2003;80(1):51–8. [8] Irigoyen M, Bartolomeo P, Perrin FX, et al. UV ageing characterization of organic anticorrosion coatings by dynamic mechanical analysis, Vickers microhardness, and infra-red analysis. Polym Degrad Stab 2001;74(1): 59–67.
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