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In situ and dynamic observation of coating failure behavior
Progress in Organic Coatings 138 (2020) 105387
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
In situ and dynamic observation of coating failure behavior Jin Gao⁎, Chao Li, Hai-Xiang Feng, Xiao-Gang Li
T
Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Electronic speckle pattern interferometry (ESPI) EIS Organic coatings degradation Under-film corrosion
In this study, the failure behavior of epoxy coating in 3.5% (mass fraction) NaCl solution was investigated by electronic speckle pattern interferometry (ESPI) and electrochemical impedance spectroscopy (EIS). We found that ESPI images changed with the diffusion of corrosion products and the appearance of defects such as blistering. Direct information can be obtained from the images taken at different times. The relation between impedance characteristics and ESPI images was investigated. Experimental results showed that ESPI is an effective technique to detect coating degradation, and it provides a comprehensive approach to studying the process of coating degradation and under-film corrosion.
1. Introduction Organic coating is an effective technique to isolate metal from environmental corrosion [1–4]. The safe service of painted equipment can be severely threatened by coating failure, but the behavior and mechanism of organic coating failure are not fully understood yet. Various research methods, such as electrochemical testing, surface analysis methods, physical performance testing and mathematical methods have been widely used to study the coating failure behavior and predict coating service life [5–8]. Electrochemical impedance spectroscopy (EIS) can determine the coating resistance, coating capacitance, electric double layer capacitance, and other parameters that reflect the protective properties of the coating [9–11]. This technique is advantageous as the disturbance to the test system is small and the detection time is short. However, the EIS measurement has an obvious shortcoming. The impedance results are attributed to the electrochemical response of the whole electrode, which reflects an “averaged” behavior of the macroscopic electrode. An in situ method that continuously monitors the local degradation of coating is also lacking. If the surface changes of coating in the failure process can be observed by an in situ, dynamic, and high-precision method, combined with electrochemical methods, such as EIS, it can provide us a comprehensive and detailed information about the coating failure process. Electronic speckle pattern interferometry (ESPI) is an emerging nondestructive testing technique. Given its full-field, non-contact, highprecision, and real-time measurements, ESPI is increasingly used in the fields of material deformation and defect detection [12,13]. The applications of ESPI in material corrosion have also been
⁎
reported [14,15]. This study attempts to observe the formation and growth of coating defects by ESPI, aiming at providing important information for studies on the mechanism of coating failure. 2. Experimental Fig. 1. shows a diagram of our ESPI apparatus. First, the laser beam is expanded by a 25× microscope objective. Afterward, the laser beam is divided into two beams, namely, a reference beam and an object beam, by a beam splitter. The object beam is scattered in all directions when it is incident to the coating surface, which has a topographical roughness equivalent to the wavelength of the laser beam. The reference beam is reflected by a mirror. The interference between these coherent scattered beams results in an interference pattern. The speckle pattern is recorded on CCD and then transferred to a computer where it is saved. Changes in the coating surface and the concentration of the solution near the coating will lead to an optical path difference. This optical path difference is responsible for the changes in speckle pattern. An original speckle pattern interferometry image is collected in the beginning of the experiment, and an ESPI image is then formed after subtracting the second speckle pattern from the first one. The ESPI images collected at each time can reflect the changes in the coating surface. Princeton PAR2273 was used to measure the impedance spectra via the traditional three-electrode system. A coated steel specimen, as a working electrode, was immersed in 3.5 wt-% NaCl solution. The counter electrode was platinum wire, and the reference electrode was saturated calomel electrode connected with the electrolyte through the salt bridge. EIS measurements were performed by applying a 20 mV
Corresponding author. E-mail address: [email protected] (J. Gao).
https://doi.org/10.1016/j.porgcoat.2019.105387 Received 3 July 2019; Received in revised form 8 August 2019; Accepted 2 October 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 138 (2020) 105387
J. Gao, et al.
immersion with the generation of rust under coating and the deformation of coating. Some stripes became blurred and distorted, and the brightness of some partial regions of the images changed. The diffusion of corrosion products during metal corrosion can vary the wavelength of incident light, resulting in the change of speckle pattern [16]. In Fig. 3, the emergence of the light-colored spots is due to the error caused by the external environment vibration, and deep-colored spots indicate that the under-film corrosion is relatively pronounced and significant coating deformation has occurred. Object light reflected back will change if the coating begins to fail; thus, the interference fringes will correspondingly change. The deep-colored area in Fig. 3(b) is significantly larger than that in Fig. 3(a), which shows that there is a great difference between the ESPI image observed on the eighth day and the initial ESPI image. The reason is that with continuous immersion, corrosion under coatings became more severe and the deformation of coatings became larger (Fig. 4). The object beam reflected back also changed and eventually led to more differences appearing on the ESPI images.
Fig. 1. Schematic of ESPI.
3.1.2. ESPI results and discussion of paint Fig. 5 shows another series of ESPI images of paint samples; compared with those in Fig. 2, stripes observed in varnish experiment are clearer and denser than those in paint experiment. This is because the reflective surface of object light was metal surface in varnish experiment. The metal surface had been polished. Its reflective performance was good, and light reflected back more concentrated; thus, stripes are clearer. However, the reflective rate of the paint surface is poor, and light will be scattered; thus, stripes observed are coarser and fuzzier. Fig. 5 shows that with the extension of immersion time, the fringes become increasingly blurred. The reason is that in the immersion process, the coating surface became rougher, light reflected back became increasingly divergent, and the diffusion of the corrosion product in the solution and a local blistering deformation will cause the change in optical path. Thus, the stripes deform, and the brightness changes. Figs. 6 and 7 indicate that the failure behavior of coating during immersion can be clearly observed by ESPI.
perturbation signal to the electrochemical cell at the open-circuit potential. The frequency range of impedance ranged from 10 mHz to 100 kHz. The impedance data were analyzed by ZSimpWin analytic software. The working electrode was carbon steel coated with a commercial epoxy resin. The surface of the carbon steel was polished using 240–600 grit wet silicon carbide papers and rinsed with acetone and alcohol. The thickness of the coated film was approximately 25 μm. The surface area of the working electrode in contact with the solution was 5 cm2. 3. Results and discussion 3.1. ESPI results and discussion of varnish and paint 3.1.1. ESPI results and discussion of varnish Fig. 2 shows that ESPI images significantly changed during
Fig. 2. EPSI images of varnish collected after (a) 0 day, (b) 4 days and (c) 8 days of immersion. 2
Progress in Organic Coatings 138 (2020) 105387
J. Gao, et al.
Fig. 3. Images of varnish obtained after subtraction with the immersion time of (a) 4 days and (b) 8 days.
Fig. 4. The surface morphology of varnish after (a) 0 day and (b) 8 days of immersion.
Fig. 5. EPSI images of paint collected after (a) 0 day, (b) 4 days and (c) 8 days of immersion.
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Fig. 6. Images of paint obtained after subtraction with the immersion time of (a) 4 days and (b) 8 days.
3.2. Comparative study of EIS and ESPI in the immersion process
Nyquist plots (Fig. 9(c), after 6 days and 8 days of immersion), the highfrequency part of the arc demonstrating the impedance performance of the coating while the low-frequency part demonstrating the process of under-film corrosion. It indicates that the penetration of corrosive media into the coatings and the diffusion of corrosion products into the solution was increasing. And the corrosive media had reached the coating/metal interface and under-film corrosion had already occurred. At this time, the changes in the coating surface were still unable to be observed through the naked eyes. EIS data cannot be well fitted by the former equivalent circuit; thus a typical equivalent circuit of coating, Rs (Qc (R c (Qdl R ct ))) (Fig. 10(b)), in the middle stage of immersion is used. R ct and Qdl represent the charge transfer resistance and double electrolytic layer capacitance, respectively. The coating resistance R c after 6 days of immersion is 5.56 × 106Ω·cm2, which shows that the protective performance of the coating was declining. Under-film corrosion does not become more serious as shown in Fig. 11–14 that the values of R c , phase angles at 10 Hz, R ct and Qdl change slightly. It might be the much more corrosion product blocking the microdefects to reduce the penetration of corrosive media, thus weakening the Redox reaction. A very clear black spot is shown in Fig. 8(e) and (f), indicating that macrodefects, such as blistering, had begun to appear on the coating surface. With the peeling of the coating and the constant diffusion of corrosive media, the corrosion spreads to intact coating area. The blocking effect of the corrosion product is no longer obvious, and the corrosion is more serious. Therefore, as the concentration of the environmental solution near the localized corrosion region changes (shown in Fig. 10(b)), the ESPI pattern also changes significantly. As shown in Figs. 11–14 that the values of R c , phase angles at 10 Hz, R ct and Qdl change substantially, the protective effect of coatings is greatly decreased. Numerous macroscopic defects can be observed on the coating
Fig. 8(a) show a large number of dense points due to the high sensitivity of ESPI. The external vibration and the changes in the coating surface will cause the ESPI image to change. However, no large spot exists in the ESPI image, indicating that the coating is still intact. No macroscopic defect can be observed on the coating surface. The Nyquist plot contains only one high-impedance capacitive arc (Fig. 9(c), after 2 days of immersion), which shows that water penetrated the coating but did not reach the metal interface. Coating is in the early stage of immersion. A typical equivalent circuit Rs (Qc R c ) (Fig. 10(a)) of coating in the early stage of immersion is used to fit the EIS data, and the fitting result proves to be fine. In this work, a constant phase element (CPE) instead of a pure capacitor is encouraged to realize a better fitness, taking into account the roughness and adsorption behavior of the coating surface [17]. Rs , R c and Qc represent the solution resistance, coating resistance, and coating capacitance, respectively. The coating resistance R c reflects the capability to resist electrolyte penetration and may be used to evaluate the protective performance of the coatings. The value of coating resistance R c is 1.049 × 108Ω·cm2, which is much higher than the generally accepted value (106Ω·cm2) for protective coatings. In Fig. 8(b), some large spots are looming in ESPI image. However, the changes in the coating surface could not been seen by the naked eyes. Probably, because corrosive media penetrated into the coatings through the microdefects, causing the coating resistance R c declining and the radius of high-frequency capacitance arc becoming smaller, leading to the concentration of the solution near the coating to change, the ESPI image consequently changed. We can still use Rs (Qc R c ) to fit its EIS data. Fig. 8(c) and (d) show that the fuzzy speckled area gradually becomes larger, and two time constants are obviously displayed in the
Fig. 7. The surface morphology of paint after (a) 0 day and (b) 8 days of immersion.
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Fig. 8. ESPI images of paint collected after (a) 2 days, (b) 4 days, (c) 6 days, (d) 8 days, (e) 10 days, (f) 12 days, and (g) 15 days of immersion.
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Fig. 9. EIS plots of the coatings after different immersion time, (a) Bode plots; (b) Phase angles plots; (c) Nyquist plots. The scatter plots represent the measurement results, while the solid lines represent the fitting results.
surface shown in Fig. 8(g). They could even be seen by the naked eyes. A Warburg arc, which represents diffusion phenomena, and only one time constant are present in its Nyquist plot. The EIS plot mainly reflects the kinetic characteristics of the substrate metal corrosion. Only the equivalent circuit, Rs (Qc (R c W )) (Fig. 10(c)), could be used to fit the EIS data [18]. Meanwhile, Fig. 10(c) shows that macroscopic defects of the coating surface become the primary factors that affect the ESPI results. The entrance of corrosion products to the solution near the coating, the continuing diffusion of corrosive media into the coating through the defects and the changes in the local morphology of the coating surface all have an effect on the ESPI images of the coating surface. Therefore, the localized corrosion microdomains have significant features in the ESPI images so as to better understand the process of under-film corrosion. Figs. 11–14 show the changes in coating resistance R c , phase angles at 10 Hz, double electrolytic layer capacitance Qdl , and charge transfer resistance R ct with the increasing immersion time. The parameter of phase angles at 10 Hz could be used as quick measurements to initially evaluate coating performance [19]. The changing tendencies of phase angle at 10 Hz with immersion time are very close to that of R c . At the early period of immersion. both the coating resistance and phase angles
at 10 Hz decreased slowly. The coating resistance remained high (1.049 × 108Ω•cm2) after immersion for two days, which was much higher than the value that had usually been used to determine whether the coating has protective effects. Ion transfer channel through the coating was not formed. Coating still had an excellent blocking property. However, sharp decreases of phase angles at 10 Hz happened from 4 days to 6 days, which decreased to the range of 20°to 40° [13], and the coating resistance decreased below 107Ω·cm2. Both results indicate that the permeation of coatings by electrolytes and the beginning of corrosion under coatings happened. However, the corrosion areas were very small, and the coating still had a blocking effect. With continuous immersion, coating resistance continued to decline. After immersion for 8 days, coating resistance and charge transfer resistance declined sharply, whereas the double electrolytic layer capacitance increased substantially. The coating resistance was lower than 106Ω•cm2. Meanwhile, macrodefects appeared in the coating, and corrosive media poured into the coating/metal interface. This phenomenon accelerated the corrosion reaction, and the coating lost its protective effects.
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Fig. 12. The variations of phase angles at 10 Hz with immersion time.
Fig. 10. Schematic drawing of observation of the coating failure process by ESPI and corresponding equivalent circuit. (a) early stage of immersion (from 0 day to 4 days); (b) middle stage of immersion (from 4 days to 12 days); (c) late stage of immersion (from 12 days to 15 days).
Fig. 13. The variations of double electrolytic layer capacitance with immersion time.
Fig. 11. The variations of coating resistance with immersion time.
4. Conclusions According to changes in the speckle interference images recorded in the process of coating immersion, the coating failure process can be divided into the early stage, middle stage, and late stage of immersion. The results obtained from ESPI are consistent well with the phenomenon based on the EIS. Coating failure process can be recorded by ESPI, especially in the middle stage of immersion. ESPI can demonstrate the corrosion performance under the coatings by detecting the changes in the solution concentration near the coating. This process cannot be performed by normal photographic observation, while ESPI technology can detect localized corrosion failure under coatings before macroscopic coating failure can be discovered. Our researches indicate that ESPI is a powerful technique that can help us study the progress of coating degradation and under-film corrosion in situ.
Fig. 14. The variations of charge transfer resistance with immersion time.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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Acknowledgments [10]
This work was financially supported by the National Natural Science Foundation of China (Nos. 51771030 and 51071027), and the National Environmental Corrosion Platform (No. 2005DKA10400).
[11]
References
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[1] S.B. Lyon, R. Bingham, D.J. Mills, Advances in corrosion protection by organic coatings: what we know and what we would like to know, J. Prog. Org. Coat. 102 (2017) 2–7, https://doi.org/10.1016/j.porgcoat.2016.04.030. [2] A.O. Abass, Recent advances on organic coating system technologies for corrosion protection of offshore metallic structures, J. Mol. Liq. 269 (2018) 572–606, https:// doi.org/10.1016/j.molliq.2018.08.053. [3] A.A. Nazeer, M. Madkour, Potential use of smart coatings for corrosion protection of metals and alloys: a review, J. Mol. Liq. 253 (2018) 11–22, https://doi.org/10. 1016/j.molliq.2018.01.027. [4] M. Kendig, D.J. Mills, An historical perspective on the corrosion protection by paints, J. Prog. Org. Coat. 102 (2017) 53–59, https://doi.org/10.1016/j.porgcoat. 2016.04.044. [5] V. Upadhyay, D. Battocchi, Localized electrochemical characterization of organic coatings: a brief review, J. Prog. Org. Coat. 99 (2016) 365–377, https://doi.org/10. 1016/j.porgcoat.2016.06.012. [6] S.S. Jamali, D.J. Mills, Steel surface preparation prior to painting and its impact on protective performance of organic coating, J. Prog. Org. Coat. 77 (2014) 2091–2099, https://doi.org/10.1016/j.porgcoat.2014.08.001. [7] S. Mitra, A. Ahire, B.P. Mallik, et al., Investigation of accelerated aging behaviour of high performance industrial coatings by dynamic mechanical analysis, J. Prog. Org. Coat. 77 (2014) 1816–1825, https://doi.org/10.1016/j.porgcoat.2014.06.002. [8] J. Gao, C. Li, Z. Lv, et al., Correlation between the surface aging of acrylic polyurethane coatings and environmental factors, J. Prog. Org. Coat. 132 (2019) 362–369, https://doi.org/10.1016/j.porgcoat.2019.03.041. [9] E.D. Kiosidou, A. Karantonis, G.N. Sakalis, et al., Electrochemical impedance
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spectroscopy of scribed coated steel after salt spray testing, J. Corros. Sci. 137 (2018) 127–150, https://doi.org/10.1016/j.corsci.2018.03.037. C. Vosgien Lacombre, G. Bouvet, D. Trinh, S. Mallarino, S. Touzain, Water uptake in free films and coatings using the Brasher and Kingsbury equation: a possible explanation of the different values obtained by electrochemical Impedance spectroscopy and gravimetry, J. Electrochim. Acta 231 (2017) 162–170, https://doi.org/ 10.1016/j.electacta.2017.02.051. R.L. Sacci, F. Seland, D.A. Harrington, Dynamic electrochemical impedance spectroscopy, for electrocatalytic reactions, J. Electrochim. Acta 131 (2014) 13–19, https://doi.org/10.1016/j.electacta.2014.02.120. T.K. Wen, C.C. Yin, Crack detection in photovoltaic cells by interferometric analysis of electronic speckle patterns, J. Sol. Energy Mater Sol. C 98 (2012) 216–223, https://doi.org/10.1016/j.solmat.2011.10.034. J. Konnerth, W. Gindl, U. Müller, Elastic properties of adhesive polymers. I. Polymer films by means of electronic speckle pattern interferometry, J. Appl. Polym. Sci. 103 (2010) 3936–3939, https://doi.org/10.1002/app.24434. R.K. Singh Raman, R. Bayles, Detection of decohesion/failure of paint/coating using electronic speckle pattern interferometry, J. Eng. Fail. Anal 13 (2006) 1051–1056, https://doi.org/10.1016/j.engfailanal.2005.07.013. N. Andrés, S. Recuero, M.P. Arroyo, M.T. Bona, J.M. Andrés, L.A. Angurel, Fast visualization of corrosion processes using digital speckle photography, J. Corros. Sci. 50 (2008) 2965–2971, https://doi.org/10.1016/j.corsci.2008.08.001. M.F. Wang, X.G. Li, N. Du, Y.Z. Huang, A. Korsunsky, Direct evidence of initial pitting corrosion, J. Electrochem. Commun. 10 (2008) 1000–1004, https://doi.org/ 10.1016/j.elecom.2008.04.032. G.J. Brug, A.L.G. van den Eeden, M. Sluyters-Rehbach, et al., The analysis of electrode impedances complicated by the presence of a constant phase element, J. Electroanal. Chem. 176 (1984) 275–295, https://doi.org/10.1016/0368-1874(84) 83477-2. 张鉴清, 曹楚南. 电化学阻抗谱方法研究评价有机涂层, J. 腐蚀与防护, 19(1998) 99–104. Y. Zuo, R. Pang, W. Li, et al., The evaluation of coating performance by the variations of phase angles in middle and high frequency domains of EIS, J. Corros. Sci. 50 (2008) 3322–3328, https://doi.org/10.1016/j.corsci.2008.08.049.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
In situ and dynamic observation of coating failure behavior Jin Gao⁎, Chao Li, Hai-Xiang Feng, Xiao-Gang Li
T
Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China
ARTICLE INFO
ABSTRACT
Keywords: Electronic speckle pattern interferometry (ESPI) EIS Organic coatings degradation Under-film corrosion
In this study, the failure behavior of epoxy coating in 3.5% (mass fraction) NaCl solution was investigated by electronic speckle pattern interferometry (ESPI) and electrochemical impedance spectroscopy (EIS). We found that ESPI images changed with the diffusion of corrosion products and the appearance of defects such as blistering. Direct information can be obtained from the images taken at different times. The relation between impedance characteristics and ESPI images was investigated. Experimental results showed that ESPI is an effective technique to detect coating degradation, and it provides a comprehensive approach to studying the process of coating degradation and under-film corrosion.
1. Introduction Organic coating is an effective technique to isolate metal from environmental corrosion [1–4]. The safe service of painted equipment can be severely threatened by coating failure, but the behavior and mechanism of organic coating failure are not fully understood yet. Various research methods, such as electrochemical testing, surface analysis methods, physical performance testing and mathematical methods have been widely used to study the coating failure behavior and predict coating service life [5–8]. Electrochemical impedance spectroscopy (EIS) can determine the coating resistance, coating capacitance, electric double layer capacitance, and other parameters that reflect the protective properties of the coating [9–11]. This technique is advantageous as the disturbance to the test system is small and the detection time is short. However, the EIS measurement has an obvious shortcoming. The impedance results are attributed to the electrochemical response of the whole electrode, which reflects an “averaged” behavior of the macroscopic electrode. An in situ method that continuously monitors the local degradation of coating is also lacking. If the surface changes of coating in the failure process can be observed by an in situ, dynamic, and high-precision method, combined with electrochemical methods, such as EIS, it can provide us a comprehensive and detailed information about the coating failure process. Electronic speckle pattern interferometry (ESPI) is an emerging nondestructive testing technique. Given its full-field, non-contact, highprecision, and real-time measurements, ESPI is increasingly used in the fields of material deformation and defect detection [12,13]. The applications of ESPI in material corrosion have also been
⁎
reported [14,15]. This study attempts to observe the formation and growth of coating defects by ESPI, aiming at providing important information for studies on the mechanism of coating failure. 2. Experimental Fig. 1. shows a diagram of our ESPI apparatus. First, the laser beam is expanded by a 25× microscope objective. Afterward, the laser beam is divided into two beams, namely, a reference beam and an object beam, by a beam splitter. The object beam is scattered in all directions when it is incident to the coating surface, which has a topographical roughness equivalent to the wavelength of the laser beam. The reference beam is reflected by a mirror. The interference between these coherent scattered beams results in an interference pattern. The speckle pattern is recorded on CCD and then transferred to a computer where it is saved. Changes in the coating surface and the concentration of the solution near the coating will lead to an optical path difference. This optical path difference is responsible for the changes in speckle pattern. An original speckle pattern interferometry image is collected in the beginning of the experiment, and an ESPI image is then formed after subtracting the second speckle pattern from the first one. The ESPI images collected at each time can reflect the changes in the coating surface. Princeton PAR2273 was used to measure the impedance spectra via the traditional three-electrode system. A coated steel specimen, as a working electrode, was immersed in 3.5 wt-% NaCl solution. The counter electrode was platinum wire, and the reference electrode was saturated calomel electrode connected with the electrolyte through the salt bridge. EIS measurements were performed by applying a 20 mV
Corresponding author. E-mail address: [email protected] (J. Gao).
https://doi.org/10.1016/j.porgcoat.2019.105387 Received 3 July 2019; Received in revised form 8 August 2019; Accepted 2 October 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 138 (2020) 105387
J. Gao, et al.
immersion with the generation of rust under coating and the deformation of coating. Some stripes became blurred and distorted, and the brightness of some partial regions of the images changed. The diffusion of corrosion products during metal corrosion can vary the wavelength of incident light, resulting in the change of speckle pattern [16]. In Fig. 3, the emergence of the light-colored spots is due to the error caused by the external environment vibration, and deep-colored spots indicate that the under-film corrosion is relatively pronounced and significant coating deformation has occurred. Object light reflected back will change if the coating begins to fail; thus, the interference fringes will correspondingly change. The deep-colored area in Fig. 3(b) is significantly larger than that in Fig. 3(a), which shows that there is a great difference between the ESPI image observed on the eighth day and the initial ESPI image. The reason is that with continuous immersion, corrosion under coatings became more severe and the deformation of coatings became larger (Fig. 4). The object beam reflected back also changed and eventually led to more differences appearing on the ESPI images.
Fig. 1. Schematic of ESPI.
3.1.2. ESPI results and discussion of paint Fig. 5 shows another series of ESPI images of paint samples; compared with those in Fig. 2, stripes observed in varnish experiment are clearer and denser than those in paint experiment. This is because the reflective surface of object light was metal surface in varnish experiment. The metal surface had been polished. Its reflective performance was good, and light reflected back more concentrated; thus, stripes are clearer. However, the reflective rate of the paint surface is poor, and light will be scattered; thus, stripes observed are coarser and fuzzier. Fig. 5 shows that with the extension of immersion time, the fringes become increasingly blurred. The reason is that in the immersion process, the coating surface became rougher, light reflected back became increasingly divergent, and the diffusion of the corrosion product in the solution and a local blistering deformation will cause the change in optical path. Thus, the stripes deform, and the brightness changes. Figs. 6 and 7 indicate that the failure behavior of coating during immersion can be clearly observed by ESPI.
perturbation signal to the electrochemical cell at the open-circuit potential. The frequency range of impedance ranged from 10 mHz to 100 kHz. The impedance data were analyzed by ZSimpWin analytic software. The working electrode was carbon steel coated with a commercial epoxy resin. The surface of the carbon steel was polished using 240–600 grit wet silicon carbide papers and rinsed with acetone and alcohol. The thickness of the coated film was approximately 25 μm. The surface area of the working electrode in contact with the solution was 5 cm2. 3. Results and discussion 3.1. ESPI results and discussion of varnish and paint 3.1.1. ESPI results and discussion of varnish Fig. 2 shows that ESPI images significantly changed during
Fig. 2. EPSI images of varnish collected after (a) 0 day, (b) 4 days and (c) 8 days of immersion. 2
Progress in Organic Coatings 138 (2020) 105387
J. Gao, et al.
Fig. 3. Images of varnish obtained after subtraction with the immersion time of (a) 4 days and (b) 8 days.
Fig. 4. The surface morphology of varnish after (a) 0 day and (b) 8 days of immersion.
Fig. 5. EPSI images of paint collected after (a) 0 day, (b) 4 days and (c) 8 days of immersion.
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Progress in Organic Coatings 138 (2020) 105387
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Fig. 6. Images of paint obtained after subtraction with the immersion time of (a) 4 days and (b) 8 days.
3.2. Comparative study of EIS and ESPI in the immersion process
Nyquist plots (Fig. 9(c), after 6 days and 8 days of immersion), the highfrequency part of the arc demonstrating the impedance performance of the coating while the low-frequency part demonstrating the process of under-film corrosion. It indicates that the penetration of corrosive media into the coatings and the diffusion of corrosion products into the solution was increasing. And the corrosive media had reached the coating/metal interface and under-film corrosion had already occurred. At this time, the changes in the coating surface were still unable to be observed through the naked eyes. EIS data cannot be well fitted by the former equivalent circuit; thus a typical equivalent circuit of coating, Rs (Qc (R c (Qdl R ct ))) (Fig. 10(b)), in the middle stage of immersion is used. R ct and Qdl represent the charge transfer resistance and double electrolytic layer capacitance, respectively. The coating resistance R c after 6 days of immersion is 5.56 × 106Ω·cm2, which shows that the protective performance of the coating was declining. Under-film corrosion does not become more serious as shown in Fig. 11–14 that the values of R c , phase angles at 10 Hz, R ct and Qdl change slightly. It might be the much more corrosion product blocking the microdefects to reduce the penetration of corrosive media, thus weakening the Redox reaction. A very clear black spot is shown in Fig. 8(e) and (f), indicating that macrodefects, such as blistering, had begun to appear on the coating surface. With the peeling of the coating and the constant diffusion of corrosive media, the corrosion spreads to intact coating area. The blocking effect of the corrosion product is no longer obvious, and the corrosion is more serious. Therefore, as the concentration of the environmental solution near the localized corrosion region changes (shown in Fig. 10(b)), the ESPI pattern also changes significantly. As shown in Figs. 11–14 that the values of R c , phase angles at 10 Hz, R ct and Qdl change substantially, the protective effect of coatings is greatly decreased. Numerous macroscopic defects can be observed on the coating
Fig. 8(a) show a large number of dense points due to the high sensitivity of ESPI. The external vibration and the changes in the coating surface will cause the ESPI image to change. However, no large spot exists in the ESPI image, indicating that the coating is still intact. No macroscopic defect can be observed on the coating surface. The Nyquist plot contains only one high-impedance capacitive arc (Fig. 9(c), after 2 days of immersion), which shows that water penetrated the coating but did not reach the metal interface. Coating is in the early stage of immersion. A typical equivalent circuit Rs (Qc R c ) (Fig. 10(a)) of coating in the early stage of immersion is used to fit the EIS data, and the fitting result proves to be fine. In this work, a constant phase element (CPE) instead of a pure capacitor is encouraged to realize a better fitness, taking into account the roughness and adsorption behavior of the coating surface [17]. Rs , R c and Qc represent the solution resistance, coating resistance, and coating capacitance, respectively. The coating resistance R c reflects the capability to resist electrolyte penetration and may be used to evaluate the protective performance of the coatings. The value of coating resistance R c is 1.049 × 108Ω·cm2, which is much higher than the generally accepted value (106Ω·cm2) for protective coatings. In Fig. 8(b), some large spots are looming in ESPI image. However, the changes in the coating surface could not been seen by the naked eyes. Probably, because corrosive media penetrated into the coatings through the microdefects, causing the coating resistance R c declining and the radius of high-frequency capacitance arc becoming smaller, leading to the concentration of the solution near the coating to change, the ESPI image consequently changed. We can still use Rs (Qc R c ) to fit its EIS data. Fig. 8(c) and (d) show that the fuzzy speckled area gradually becomes larger, and two time constants are obviously displayed in the
Fig. 7. The surface morphology of paint after (a) 0 day and (b) 8 days of immersion.
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Fig. 8. ESPI images of paint collected after (a) 2 days, (b) 4 days, (c) 6 days, (d) 8 days, (e) 10 days, (f) 12 days, and (g) 15 days of immersion.
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Fig. 9. EIS plots of the coatings after different immersion time, (a) Bode plots; (b) Phase angles plots; (c) Nyquist plots. The scatter plots represent the measurement results, while the solid lines represent the fitting results.
surface shown in Fig. 8(g). They could even be seen by the naked eyes. A Warburg arc, which represents diffusion phenomena, and only one time constant are present in its Nyquist plot. The EIS plot mainly reflects the kinetic characteristics of the substrate metal corrosion. Only the equivalent circuit, Rs (Qc (R c W )) (Fig. 10(c)), could be used to fit the EIS data [18]. Meanwhile, Fig. 10(c) shows that macroscopic defects of the coating surface become the primary factors that affect the ESPI results. The entrance of corrosion products to the solution near the coating, the continuing diffusion of corrosive media into the coating through the defects and the changes in the local morphology of the coating surface all have an effect on the ESPI images of the coating surface. Therefore, the localized corrosion microdomains have significant features in the ESPI images so as to better understand the process of under-film corrosion. Figs. 11–14 show the changes in coating resistance R c , phase angles at 10 Hz, double electrolytic layer capacitance Qdl , and charge transfer resistance R ct with the increasing immersion time. The parameter of phase angles at 10 Hz could be used as quick measurements to initially evaluate coating performance [19]. The changing tendencies of phase angle at 10 Hz with immersion time are very close to that of R c . At the early period of immersion. both the coating resistance and phase angles
at 10 Hz decreased slowly. The coating resistance remained high (1.049 × 108Ω•cm2) after immersion for two days, which was much higher than the value that had usually been used to determine whether the coating has protective effects. Ion transfer channel through the coating was not formed. Coating still had an excellent blocking property. However, sharp decreases of phase angles at 10 Hz happened from 4 days to 6 days, which decreased to the range of 20°to 40° [13], and the coating resistance decreased below 107Ω·cm2. Both results indicate that the permeation of coatings by electrolytes and the beginning of corrosion under coatings happened. However, the corrosion areas were very small, and the coating still had a blocking effect. With continuous immersion, coating resistance continued to decline. After immersion for 8 days, coating resistance and charge transfer resistance declined sharply, whereas the double electrolytic layer capacitance increased substantially. The coating resistance was lower than 106Ω•cm2. Meanwhile, macrodefects appeared in the coating, and corrosive media poured into the coating/metal interface. This phenomenon accelerated the corrosion reaction, and the coating lost its protective effects.
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Fig. 12. The variations of phase angles at 10 Hz with immersion time.
Fig. 10. Schematic drawing of observation of the coating failure process by ESPI and corresponding equivalent circuit. (a) early stage of immersion (from 0 day to 4 days); (b) middle stage of immersion (from 4 days to 12 days); (c) late stage of immersion (from 12 days to 15 days).
Fig. 13. The variations of double electrolytic layer capacitance with immersion time.
Fig. 11. The variations of coating resistance with immersion time.
4. Conclusions According to changes in the speckle interference images recorded in the process of coating immersion, the coating failure process can be divided into the early stage, middle stage, and late stage of immersion. The results obtained from ESPI are consistent well with the phenomenon based on the EIS. Coating failure process can be recorded by ESPI, especially in the middle stage of immersion. ESPI can demonstrate the corrosion performance under the coatings by detecting the changes in the solution concentration near the coating. This process cannot be performed by normal photographic observation, while ESPI technology can detect localized corrosion failure under coatings before macroscopic coating failure can be discovered. Our researches indicate that ESPI is a powerful technique that can help us study the progress of coating degradation and under-film corrosion in situ.
Fig. 14. The variations of charge transfer resistance with immersion time.
Data availability The raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.
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Acknowledgments [10]
This work was financially supported by the National Natural Science Foundation of China (Nos. 51771030 and 51071027), and the National Environmental Corrosion Platform (No. 2005DKA10400).
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