Identification and localization of organic coating ...

Polymer Degradation and Stability 95 (2010) 960e964

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Identification and localization of organic coating degradation onset by impedance imaging ski*, K. Darowicki, K. Schaefer M. Szocin sk 80-233, Poland Gdansk University of Technology, Chemical Faculty, Department of Electrochemistry, Corrosion and Materials Engineering, 11/12 G. Narutowicza Str., Gdan

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 February 2010 Accepted 15 March 2010 Available online 21 March 2010

The aim of this work was to demonstrate the potential of a localized impedance measurement technique to identify and spatially localize the onset spots of polymeric coating degradation. The technique, which has not yet been applied in the field of organic coatings, utilizes atomic force microscopy (AFM) in contact mode. During the scan a single-frequency voltage perturbation signal is applied between the AFM tip and the coated metal substrate. A current response signal is registered. As a result an impedance map of the scanned region is created. The method was applied to investigation of acrylic coating degradation during exposure to UV radiation. Localized topography and impedance images revealed formation of micro-cracks in the coating layer, which gradually converted into through-the-coating defects with an increase in the irradiation time. Thus the method allowed early identification and localization of the sites of degradation onset, which was not possible using classical impedance measurement. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Localized impedance measurements Atomic force microscopy Organic coating Local defects UV degradation

1. Introduction Durability of any protective coating is limited. The coating’s ultimate lifetime depends on chemical and physical properties of the polymer material, kinds and intensity of degradation factors acting in service environment as well as their mutual interaction. From the point of view of effectiveness of anticorrosion protection a detection of the moment of degradation onset is crucial. It allows estimation of the coating durability in a given environment, monitoring the progress of degradation once it has been initiated and, what seems to be of the highest importance, prediction of the time horizon of any necessary correction and preventive measures such as strip painting or total repainting. For many years electrochemical impedance spectroscopy (EIS) has been used to detect the degradation onset of polymer coatings. It lends the advantage of its non-destructiveness and ability to reveal the first signs of coating degradation far earlier than the macroscopic defects are evident. Mansfeld et al. evaluated degradation of various polymer coating systems on steel exposed to natural and seawater using electrochemical impedance spectroscopy and electrochemical noise analysis [1]. Biodegradation of polyamide coatings due to a fungal culture was investigated with EIS by Gu and co-workers [2]. Water uptake by organic coating, the

* Corresponding author. Tel.: þ48 58 347 14 40; fax: þ48 58 347 10 92. ski). E-mail address: [email protected] (M. Szocin 0141-3910/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2010.03.016

acceleration of which is believed to be the first sign of coating degradation, was also monitored using impedance spectroscopy [3]. An influence of temperature extremes and cyclic temperature changes was the focus of many EIS investigations in the field of polymer coatings [4e6]. Electrochemical impedance spectroscopy was used for identification of organic coating degradation caused by factors of mechanical origin, such as tensile stress or abrasion [7e9]. EIS was also found to be a useful tool in monitoring of polymer coating degradation on accelerated aging involving UV radiation, mechanical deformation and thermal cycling [10e12]. Oliveira and Ferreira used electrochemical impedance spectroscopy to rank paint systems with respect to their susceptibility to degradation when tested in 3% NaCl solution at ambient and elevated temperatures [13,14]. However, a detection of the degradation onset is not only evaluation of the time-to-failure but also identification of the sites of coating degradation. This allows finding the interaction between particular regions of the polymer coating with the environment and identification of the weakest points in the coating structure, which are the ones deciding about resistance of the entire protective layer. Obtaining such information can contribute to improvement of the coating material by strengthening the weakest points being the preferred sites of degradation. Classical impedance measurements reveal a shortage in this field as the information obtained via such approach is averaged over a surface area subject to investigation, which is typically from a cm2 order. Accordingly, it is not possible to spatially localize the spots of coating degradation.

ski et al. / Polymer Degradation and Stability 95 (2010) 960e964 M. Szocin

A few authors carried out research aimed at obtaining local impedance characteristics of organic coatings. Wittmann et al. used a five-electrode system with a split micro-reference electrode to provide local electrochemical impedance mapping of coated steel substrates [15]. In this way they detected various types of intentional local heterogeneities such as oil absorbed in coating, blisters, underfilm salt deposits, pinholes and directional under-film corrosion. Local under-coating corrosion of aluminium in chloride rich solution was also the object for impedance mapping [16,17]. A correlation was obtained between the under-film chemical characteristics, the electrochemical impedance, and the visual appearance of the blisters. Jorcin and co-workers used local electrochemical impedance to observe changes of delaminated area around a scribe in coating exposed in a salt spray chamber [18]. Corrosion mechanism within pinhole defect in coating was also investigated by localized EIS [19]. The phenomenon of corrosion inhibition in coating defects due to application of cathodic protection was the object of localized impedance research performed by Dong et al. [20]. Many investigations, including the ones mentioned above, employs a method of measuring local impedance described by Lillard et al. in which the local impedance is acquired by measuring local ac solution current density above the investigated surface using two micro-electrodes [21]. This method was also used by Zou and Thierry for assessment of coating blistering [22]. In this paper the authors want to contribute to the attempts of identification and spatial localization of coating degradation onset sites with a novel local impedance imaging technique, based on the approach proposed and developed by O’Hayre [23,24], which has not so far been applied in the field of organic coatings.

Atomic force microscopy (AFM) in contact mode is a basis for the proposed method of localized impedance measurement. Impedance is determined between the two electrodes e the conductive AFM tip and examined sample, in that particular case metal substrate covered with polymer coating. A single-frequency voltage perturbation signal is applied between the tip and the sample and a current response signal is registered. When the tip is moved over the investigated surface a localized impedance characteristics of the material is acquired. An output of such measurement is an impedance map of the scanned area along with other surface features available via classical AFM measurements such as height profile. The resolution of this method depends on the size of the AFM tip, frequency of the perturbation signal and the scan rate. Decreasing a radius of the AFM tip one obtains higher resolution because then an impedance response of the material is more localized corresponding to the volume in direct neighbourhood of the tip/sample contact point. On the other hand small area of contact can yield high impedances, difficult to be measured. The localized response was estimated by O’Hayre and others providing the spreading resistance formula for a circular point contact of radius r between the tip and sample [23]:

r 4r

The materials exhibiting dielectric properties, for instance polymer coatings with good barrier properties, should be investigated with the perturbation of high frequency and amplitude. For dielectric materials the current response signal to a sinusoidal voltage perturbation is as follows:

DIzjuC DE

(2)

where u is the signal frequency, DE is the signal amplitude and C is the material capacitance. Alternating response current is imaginary in character for relatively high frequency. From the above it can be seen that for dielectric materials the response is capacitive. Low impedance systems, such as defective polymer coatings, call for a perturbation of lower frequency and amplitude. Then the response signal can be given by the formula:

1 R

DIz DE

(3)

where DE is the signal amplitude and R is the material resistance. Thus the response describes resistive properties of the material. That is why depending on a character of investigated material or state of material at a given stage of service a suitable value of perturbation signal frequency and amplitude can be selected. The perturbation signal frequency and thus the measurement resolution is also influenced by the scan rate. The relation between signal frequency and scan rate is crucial as it determines the spatial localization. Following the uncertainty principle [25] the frequency resolution and time selection are combined in the relationship:

Dt Df

1 4p

(4)

where Df is the frequency resolution and Dt is the time selection. For a scan rate v this equation takes the form:

2. Measurement method

RSR ¼

961

(1)

where r designates the resistivity of the sample. It is not possible to evaluate the exact volume of material contributing to impedance response. Nevertheless, this volume remains similar for all following positions of the tip. Accordingly, such localized impedance measurement provides a map of impedance changes rather than of absolute impedance values. Selection of frequency and amplitude of the perturbation signal depends on the expected impedance of the measured material.

Dx

v 4pKf

(5)

where K ¼ Df =f is the relative frequency resolution, Dx ¼ vDt is the spatial localization and v is the scan rate. The above equation shows that spatial localization is a function of both scan rate and perturbation signal frequency. For a constant scan rate high spatial resolution could be ensured by application of a relatively high measurement frequency. This approach can be successfully implemented to investigation of dielectric materials, where the response current can be relatively easily measured reflecting capacitive characteristics of the specimen (Eq. (2)). The materials revealing resistive character require the perturbation of relatively low frequency. Thus, to obtain high spatial resolution one has to decrease the scan rate. The presented method of localized impedance measurements seems to be the right tool for investigation of very thin polymer films, which can be encountered in nanotechnology, electronics or dielectric devices. In these fields investigation of very small areas with high accuracy and spatial resolution is a must. Another advantage of this technique is the possibility of the localized impedance investigations to be carried out without a presence of electrolyte. In practice it can be the case that the coatings are not expected to be in direct contact with electrolyte throughout their service, for instance in electronic industry. Thus impedance testing in the electrolyteless conditions provides more reliable results than the conventional impedance measurements, which require immersion of the sample. The potentialities of the presented technique in detection of coating degradation onset have been verified and are presented on the example of acrylic coating degradation upon exposure to UV radiation.

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3. Experimental The investigated specimen was a transparent acrylic coating on carbon steel substrate. The coating was applied using air spray technique and its thickness was 25 mm 2 mm. The steel substrate was of circular shape of diameter 1 mm. The tested specimen is schematically presented in Fig. 1. The specimen was exposed to UV radiation generated by a 320 nm wavelength lamp providing a radiation intensity of 3.6 W/m2 at a distance of 1 m. Two processes have been proposed in a photodegradation of acrylic resins [26]. A side-chain scission, that plays a role in the photolysis at shorter wavelengths (260 and 280 nm), is followed by the main-chain scission at the wavelength close to 320 nm. The total period of exposure was 800 h. During that time the sample was subjected to classical impedance measurements on a global scale as well as to localized impedance measurements in AFM mode at regular time intervals (every 7 days). Classical impedance measurements on a global scale were carried out in a two-electrode system, where the substrate was a working electrode and platinum mesh was a counter electrode. The investigation was performed in immersion in 3% sodium chloride solution. The examined area was equal to 0.8 mm2. The experimental set-up was composed of a Schlumberger 1255 frequency response analyser coupled to the high input impedance buffer Atlas 9181. Impedance spectra were registered within the 1 MHze1 mHz frequency range. Ten points were recorded for each frequency decade. Amplitude of the perturbation signal was equal to 120 mV. Localized electrical and topographic measurements were performed with the SPM Integra Aura system by NT-MDT Co. Immediately after each classical impedance measurement the specimen was investigated with atomic force microscopy in contact mode. The scans were performed at 10 locations distributed all over the surface of the specimen.

cm2 Fig. 1. Scheme of the test sample.

8

Z"/G

10

4

0.002 Hz

6

2 0 0

2

4

6

Z'/G

8

10

12

14

2

cm

Fig. 3. Global impedance spectrum of acrylic coating prior to UV exposure.

The height profile, local impedance and spreading resistance images were collected for each location. During the impedance scan a single-frequency sinusoidal voltage signal of 3 kHz was applied between the tip and the steel substrate and the current response of the sample was registered. An amplitude of the perturbation signal was 2 V. Scanning frequency was equal to 1 Hz. Spreading resistance measurements were performed for a bias voltage within the range 2e10 V. The tip was a silicon one coated with platinum in order to ensure its conductivity. Image registration, processing and analysis were performed using Nova software by NT-MDT Co.

4. Results and discussion Fig. 2 presents an exemplary topographic AFM image (a) and localized impedance image (b) of the acrylic coating surface prior to the exposure to UV radiation. The surface exhibits some hills and valleys with the maximum height difference not exceeding 2 mm. There is a strong correlation between the topographic image and the impedance one. Elevated areas correspond to high impedance regions whereas depressions are characterized by lower impedance. Such correlation can be explained by the fact that at this stage the polymer coating character is capacitive and its response to the

Fig. 2. AFM images of acrylic coating surface prior to UV exposure: (a) topographic image, (b) localized impedance image.


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Fig. 4. AFM images of acrylic coating after 470 h of UV exposure: (a) topographic image, (b) localized impedance image.

perturbation follows Eq. (2). Coating capacitance is described by the formula:

C ¼

330 A d

(6)

where 3 is the dielectric permittivity of coating material, 30 is the dielectric permittivity of vacuum, A is the area exposed to investigation and d denotes the coating thickness. There is an inverse proportionality between coating thickness and capacitance, which explains the topographyeimpedance correlation. Scanning spreading resistance (SSR) measurements, which had also been carried out, did not reveal any DC conductivity, what was evidence of lack of through-the-coating defects on the investigated surfaces. This observation was confirmed by the global impedance measurements using the classical approach. Fig. 3 shows a global impedance spectrum registered at the beginning of the exposure. The coating resistance is of GU order proving its strong barrier and dielectric properties. In Fig. 4 one can see the images of the coating surface after 470 h of UV irradiation. The surface of the coating got covered with a web

of micro-cracks clearly visible on the topographic image as well as the correlated impedance image. Simultaneously the spreading resistance measurements still did not reveal any breakthrough in the protective layer. The coating resistance assessed from the global impedance measurements maintained at GU level indicating no deterioration of protective properties of the coating. However, the web-like micro-cracks were suspected to be the sites of coating degradation onset indicating the sites where degradation could occur in future. The authors’ predictions occurred to be correct as far as localization of the degradation sites is concerned. The cracks were propagating and becoming deeper with the time of exposure to UV radiation (ca. 3.5 mm of maximum depth after 800 h of UV irradiation as compared to ca. 2 mm after 470 h of exposure to UV). Moreover, after 800 h of irradiation they became the places where the coating lost its tightness and allowed penetration of corrosion agents to the metal substrate. Such situation was confirmed by the scanning spreading resistance measurements. They disclosed DC current conductivity within the majority of micro-cracks visible on the AFM topographic image (Fig. 5). It means that at these locations coating

Fig. 5. AFM images of acrylic coating after 800 h of UV exposure: (a) topographic image, (b) scanning spreading resistance image.


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The proposed technique of localized impedance measurements seems to be especially well suited for a very thin polymer films, like the ones used in the field of nanotechnology, electronics or dielectric devices, where degradation over small areas must be investigated with high accuracy yielding spatially resolved results. Moreover, the method described makes it possible to perform the localized impedance investigations in an electrolyteless mode, unlike the global scale measurements, which in certain cases reflects the real conditions of coating service in a better way.

Z" [k

cm2]

1200

800

100 Hz

400

0 0

400

800

1200

1600

2000

Z' [k cm2] Fig. 6. Global impedance spectrum of acrylic coating after 800 h of UV exposure.

did not constitute a barrier for DC current, thus indicating the coating damage. The global impedance spectrum depicted in Fig. 6 is characteristic for the coating with poor protective properties exhibiting the resistance of kU; order. The shape of the spectrum takes the form of a depressed semicircle turning into the second time constant. It suggests degradation of the coating and electrochemical reaction at the coating/substrate interface. However, the impedance diagram does not possess a local character and does not provide any information about the form and extent of the coating degradation. The presumptions stemming from the impedance data were confirmed and supplemented by the proposed method of localized AFM investigations e the presence of numerous micro-cracks and through-the-coating defects in the polymer protective layer. The method allowed spatial localization of the defected areas and assessment of the mode of degradation (uniform on the entire surface, not a single point one). Such information was unattainable via classical approach to impedance measurements on a global scale. Obviously, neither visual nor microscopic observation using a conventional optical microscope allowed detection of the mentioned degradation pattern. 5. Conclusions The performed investigation allows drawing the following conclusions: The proposed technique of localized impedance measurements allowed detection of micro-cracks in acrylic coating formed due to exposure to UV radiation, while at the same time the classical impedance measurements on a global scale did not reveal any deterioration of the coating. As the time of exposure to UV radiation increased, the detected micro-cracks turned into through-the-coating defects, which was confirmed by the scanning spreading resistance measurements. Due to the above it can be stated that the proposed technique allows monitoring of a state of the protective polymer film and identification of the sites of onset of coating degradation. The spatial localization and distribution of defects over the investigated surface can be determined. Such early detection and localization of potential sites of coating degradation cannot be done with the classical impedance measurements performed on a global scale as they give only surface averaged results. The obtained results suggest that the presented technique can be helpful in prediction of polymer coating durability, time-tofailure and degradation mode upon action of a given degradation factor.

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