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EIS study of epoxy resin applied on carbon steel using double-cylinder electrolyte cell
Progress in Organic Coatings xxx (xxxx) xxx–xxx
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EIS study of epoxy resin applied on carbon steel using double-cylinder electrolyte cell ⁎
B. Bíaz, X.R. Nóvoa, C. Pérez , A. Pintos ENCOMAT Group, University of Vigo, E.E.I. Campus Universitario, 36310 Vigo, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Double-cylinder electrolyte cell Parallel and normal conduction Porous film Interfacial properties
The assessment of the protection properties of an organic film applied on metallic substrate is still a challenge. Barrier features are currently studied by electrochemical impedance spectroscopy (EIS) technique, using a classical three electrode arrangement, where current flow normal to the metallic substrate is measured, nevertheless, any information about parallel ionic conductivity is not acquired. The present paper proposes the use of a double cylinder electrochemical cell, in which the combination of three and four electrode arrangements allows the measurement not only the current flow normal to the metallic substrate but also parallel to the metalcoating interface. The EIS experiments are performed in a thick epoxy resin film applied on carbon steel, during 60 days of immersion in a 0.1 M Na2SO4 + NaOH solution. The impedance values measured using the three electrode configuration are much higher than those obtained by the four electrode arrangement. This result can be explained considering that a current fraction flows parallel to the metal-coating interface. The impedance evolution is explained considering the presence of diverse pore families, which evolve in different ways depending on the ionic motion direction, normal or parallel. Changings are more remarkable in the parallel direction, reflecting the anisotropic character of the film.
1. Introduction Organic coatings represent the most common way to protect metallic structures against corrosion. It is generally accepted that the protective action is based on two main properties: barrier (for oxygen, water and aggressive species), and blocking of the ionic paths between anodic and cathodic areas along the metal/polymer interface [1]. The barrier properties deal with the transport of ions and aggressives through the film, in the direction normal to the metallic substrate. The Electrochemical Impedance Spectroscopy (EIS) technique has been used widespread for this study, mainly using the three electrode arrangement, where the reference electrode is place in the test solution. In such setup, the measurements are related to the sorption characteristics of the whole film in this direction [2–6]. This approach does not discriminate between the characteristics of the film at bulk film and at the metal-coating interface region, where chemical and physical interactions are expected. Kittel et al. employed an additional electrode embedded in the coating, which allows the discrimination between the impedance of the inner part in contact with the metal surface and that of the outer part in contact with the electrolyte [7,8]. However, only information about ionic conductivity in normal direction is acquired.
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On the other hand, parallel conductivity has strong influence on the initiation and propagation of corrosion. It is well-known that, for a given coating, the protection effectiveness against corrosion depends on the surface treatment that could lead to distinct adhesive forces between film and metal [9,10]. Luo et al. stated that in the cathodic disbonding process the ionic transport along the coating/metal interface is more important than through the film [11]. Thus, the interest of this aspect is undoubted, nevertheless, the studies found in the literature are scarce, maybe due to the more complex experimental configuration needed. Some researchers use the Fourier Transform InfraredMultiple Internal Reflection (FTIR-MIR) technique for in situ quantifying the water layer at the coating/substrate interface [12]. The main restriction is that it is limited to substrates that produce total internal reflection. Iron is a poorly-reflective metal, which constrains the use of this technology in the corrosion study. An interesting approach is the measurement of impedance between electrodes located at coating/ substrate interface [13–15] or embedded into the polymer [16–18]. However, placing the electrodes is not a simple task, mainly at the interface level, even though the main constrain is that current flow that crosses the coating in a parallel way is minimum in most of the situations, thus much of the information in this direction is lost [17].
Corresponding author. E-mail address: [email protected] (C. Pérez).
https://doi.org/10.1016/j.porgcoat.2018.02.002 Received 28 August 2017; Received in revised form 13 December 2017; Accepted 6 February 2018 0300-9440/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Bíaz, B., Progress in Organic Coatings (2018), https://doi.org/10.1016/j.porgcoat.2018.02.002
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arrangement, where two platinum wires acted as WE and CE, one in each compartment, respectively. Two Ag/AgCl wires, placed each one in each compartment, were the RE electrodes. It is worthy to highlight that in this arrangement the coated steel is electrically floating. The impedance measured in such configuration, ZTotal, involves the internal and external contributions, as well as the contribution of a fraction of current flowing parallel to the metal-coating interface, as stated elsewhere [19]. The Electrochemical Impedance Spectroscopy, EIS, experiments were performed using an Autolab 30 potentiostat from Ecochemie® equipped with FRA and multiplexer modules. Most of the measurement were done at a frequency scan from 1 MHz down to 10 mHz, even though at longer immersion periods the lowest scanned frequency was 1 mHz, with five points per decade, and amplitude of 100 mVrms at null DC current. The multiplexer module (MUX® control), which is a multichannel device that allows the sequential measurements of the electrode arrangements described above: Zext, Zint and ZTotal, to be completed automatically without manipulation of the set-up. 100 mV amplitude was employed to improve the signal-to-noise ratio of the measurement because of the high coating impedance. The impedance measurements were performed on three replicas. The sample characterization was performed by Optical Microscopy, OM, with an Olympus® GX71/GX51 Inverted Metallurgical Microscope, and by Scanning Electron Microscopy, SEM, and Energy Dispersive Xray, EDX, techniques, employing an Electroscan JSM-54® model JEOL 5410 equipped with an energy dispersive X-ray detector Link ISIS® 300.
In an attempt to overcome these limitations, our research group developed a new experimental design to study the parallel conductivity in a dielectric medium [19]. The experimental setup is based on a double cell used occasionally to study the cathodic protection [11,20]. The design takes advantage of the behaviour of a non-connected electronic conductor (floating electrode) embedded in an ion-conducting medium [21] or contacting it, in the studied case, a polymer film. The general idea is that the current flowing through an ionic-conducting medium that contains a floating electric conductor tends to split in two paths: ionic and electronic, depending on the associated resistances, dielectric and charge transfer resistances, respectively [19,21]. Though the obtained results shown that it was possible the discrimination between parallel and normal current fluxes, a detailed analysis of the parallel impedance evolution at long immersion periods is necessary. Thus, the present study aims on the behaviour of a thick organic film in immersion conditions. For that, a double cylinder electrolyte cell was employed, which allows the tracking of both normal and parallel contributions with immersion time. Two different electrical equivalent circuits were proposed to model the experimental results. 2. Experimental design An adhesive layer based on epoxy resin (Resoltech 3350 HP®), cured at 70 °C during 5 h, was applied on mild steel. This is a structural adhesive used to provide durable bonding of materials such as composites or polymeric layers and aluminium or steel. Thus, the durability of such union depends, among other factors, on the barrier properties of this adhesive. The average thickness was 980 μm. In order to improve the resinmetal adhesion, the metallic substrate was previously grinded with silicon carbide paper 180 grit. Besides, to obtain a uniform coating thickness, the resin surface was also grinded with similar abrasive paper. Fig. 1A depicts a cross-section of the studied samples. The electrochemical measurements were performed using a double cylinder cell, made of Poly-methyl-methacrylate, PMMA. The double cylinders defined inner and outer compartments with the same contact surfaces (S = 2.8 cm2) between the coating and the electrolyte. Details of the geometry are given in Fig. 1B. The electrolyte was a 0.1 M Na2SO4 + NaOH solution in both compartments. NaOH was employed to adjust the pH to 8. The different arrangements of the electrodes employed are depicted in Fig. 2. The three electrode arrangement is illustrated in Fig. 2A, the working electrode (WE) was the metallic substrate coated with the resin, the counter electrode (CE) was a platinum wire, and an Ag/AgCl wire was the reference electrode (RE).This configuration was used in both compartments. The current flows normal to the metallic surface, from CE to WE. The external impedance (Zext) was obtained by placing the RE and CE electrodes in the outer compartment, whereas the internal impedance (Zint) was acquired by placing the RE and CE electrodes in the inner compartment. Fig. 2B shows the four electrode
3. Results and discussion Fig. 3 displays an example of the impedance measurements obtained. It seems striking the value of the total impedance, more than five orders of magnitude lower than the external and internal ones. This behaviour can only be explained assuming that the total impedance is the combination of two contributions: normal impedance, which is the sum of Zext + Zint, and parallel impedance (Zpar) due to a fraction of current that flows along the film, parallel to the metal-coating interface [19]. Thus, the total impedance must be described by a parallel combination of both contributions, as Eq. (1) shows:
ZTotal =
Zpar (Zint + Zext ) Zpar + Zint + Zext
(1)
Taking into account that ZTotal, Zint and Zext are the experimental EIS measurements, the parallel contribution can be obtained from Eq. (2):
Zpar =
1
( ) ( 1
ZTot
−
1 Zext + Zint
)
(2)
This expression indicates that if the normal impedance (Zext + Zint) is high, the total impedance is close to the parallel one, ZTot ≈ Zpar. Fig. 4 corroborates this feature: the total and parallel impedances
Fig. 1. A) Optical picture showing a cross-section of the studied system (50x). B) Sketch showing the dimensions of the double cylinder cell.
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Fig. 2. Electrode arrangements designed to perform the impedance measurements. A) Three electrode configuration to measure the impedance in the inner and the outer compartments with the coated steel as working electrode. B) Four electrode arrangement for measuring the total impedance, where the metallic substrate is electrically floating. For more details see the text.
follow the same pattern. 3.1. Evolution of normal impedances (Zext, Zint): three electrode arrangement The impedance measurements acquired in the inner and outer compartments are quite similar, following the same tendency not only at the beginning (see Fig. 3) but also at the end of the immersion time, as Fig. 5 illustrates. As it can be seen, impedance values overlap in almost all the frequency scanned, which suggests that the coating is a homogeneous film. The small differences observed at frequencies higher than 1 kHz could be attributed to stray current at high frequencies [22], as discussed later. Therefore, in this section the impedance study will be focused on one compartment (the external one, Zext), assuming that the same interpretation can be extrapolated to the internal one. The impedance evolution with immersion time for the external compartment is depicted in Fig. 6. Huge impedance values are measured even at the end of the immersion period, which is not a surprise taking into account the high film thickness. Nonetheless, a clear impedance decrease is observed as the immersion time elapses. On the other hand, it is noticeable, mainly in phase angle, the two contributions located at frequencies higher than 100 Hz.
Fig. 4. Nyquist plots of total (ZTotal) and parallel (Zparallel) impedances of the studied sample after 9 days immersion in the electrolyte.
The observation of the applied film morphology has been helpful to interpret the impedance spectra in terms of adequate electrical equivalent circuit. Fig. 7A–C correspond to SEM images of the epoxy resin at different magnifications. The main characteristic is the presence of numerous pores or bubbles of varying sizes, from microns down to nanometres. Thus, the model should consider not only their presence but also the
Fig. 3. Impedance modulus of the external (●), internal (★) and total (■) after 9 days of immersion in the electrolyte. The corresponding phase angle is depicted in the insert.
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Fig. 5. Bode plots of the external (●) and internal (★) compartments after 60 days of immersion in the electrolyte.
Fig. 6. Impedance spectra measured in the external compartment at different immersion times.
Fig. 7. SEM images showing a cross-section of the epoxy resin at A) 150x, B) 7.500 x and C) 16.000x. D) Physical interpretation of the electrical equivalent circuit proposed.
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Fig. 8. Experimental and fitted data using the equivalent circuit inserted (see the text for more details) corresponding to Zext after 22 days of immersion.
Fig. 9. Evolution of CfRf parameters with immersion time.
Fig. 10. Nyquist plot measured after 53 days of immersion in the electrolyte. The new EEC is included, as well as the best fitting results obtained.
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Fig. 11. Optical images showing the aspect of the resin at the end of the test at A) 60x and B) 1000x magnifications. C) EDX corresponding to the damaged area displayed in the SEM picture (1700x) inserted.
of the resin, about 1 mm and the huge low frequency impedance values. An example of the fitted quality is depicted in Fig. 8. The interpretation given is not new, Diguet et al. [24] proposed the same model to characterize the delamination of a PTFE coating applied on a 2024 aluminium alloy. Le Thu et al. [20] used equivalent circuits, which include a vacuole branch to account the pre-existing holes in thick coatings. More recently, other authors [25,26] used the same concept to study the water uptake process in organic coatings. In addition, our research group employs this equivalent circuit to model the pore structure of cementitious materials [27,28]. The evolution of the resistance and the capacitance of the resin is depicted in Fig. 9. Rf undergoes a fast decay, more than two orders of magnitude, mainly during the first 42 days of immersion. Surprisingly, only small changes are observed in the capacitance values over the same period. This evolution reflects the electrolyte uptake through preexisting channels, however, little physical-chemical interaction occurs with the resin network [24]. Capacitance values vary from 50 to 70 pF, normalizing by the geometric factor of the flat condenser (d/S) the permittivity ranges from 20 to 27. The initial values seems too high for a dry epoxy resin, although it should be kept in mind that these values are in the order of the potentiostat input capacitance, so the recorded values may be affected by the instrumentation employed [29]. Therefore, although the capacitance values may not be directly related to the coating capacitance, the observed evolution, a regular increase, should correspond to the dielectric behaviour of the resin.
several pore families in terms of size and depth associated. Fig. 7D is a sketch of the proposed electrical equivalent circuit, EEC, and its physical interpretation. Rf represents the resistance of the ionic conduction through the film that reach the metallic substrate (percolating pores) and Cf the dielectric capacitance of the resin. Whereas R2C2 and R3C3 are related to pore families with different morphologies both in size and in depth. Thus, R2 and R3 account the ionic resistances inside of the pores (non-percolating pores), and C2 and C3 represent the double layer capacitance at the pore walls. The impedance is given by Eq. (3):
Z (ω) = R e +
Z1 Z4 Z1 + Z4
(3)
Where
Z1 =
Rf 1 + (jωRf Cf )α1
(
Z2 = R2 1 +
and Z4 =
1 (jωR2 C2)α2
)
Z2 Z3 Z2 + Z3
with
(
and Z3 = R3 1 +
1 (jωR3 C3)α3
)
Re accounts the electrolyte resistance, α1, α2, α3 are the Col-Cole dispersion parameters associated to the corresponding time constants. The other elements have been already explained. A detailed explanation of the fitting procedure has already been provided elsewhere [23]. In this model, the low frequency limit is given by Rf (film resistance), and no interfacial phenomena at the metal-resin interface is considered. This assumption seems reasonable due to the high thickness 6
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Fig. 12. A) Resistances and B) capacitances evolution with immersion time for an epoxy resin applied on carbon steel using a three electrode arrangement. The R2, R3, C2 and C3 meaning is explained in the text.
chloride that suggest the appearance of corrosion products. It is noticeable the presence of a peak corresponding to Si, that probably comes from the silicon carbide abrasive paper used to obtain the flat resin surface. Regarding to the carbon peak, it is compatible with the organic nature of the film. The evolutions of R2, R3, Rct and C2, C3, Cdl parameters are shown in Fig. 12. As stated above, R2C2 and R3C3 refer to the same morphological feature: the presence of different pore families. An estimation of the pore size of both families is possible, based on Eq. (5) [30,31]:
After 42 days of immersion a continuous increase in capacitance is observed, that can be related to the corrosion onset when the electrolyte reaches the metal surface. Fig. 10 displays the Nyquist plot obtained after 53 days of immersion in the tested solution. An additional time constant is clearly distinguished in the low frequency domain, in the range of 10–1 mHz. In this situation, the EEC must include a new time constant related to the corrosion process (RctCdl) that develops at the bottom of the percolating pores. The impedance is also given by Eq. (3), although Z1 now is:
Z1 =
Rf (jωRf Cf )αf +
1 1 + ⎛⎜Z0 Rf ⎞⎟ ⎝ ⎠
with
Z0 =
R ct 1 + (jωR ct Cdl)αdl
τ= (4)
δ2 D
(5)
Where τ (s) is the time constant associated with the ionic motion inside of the non-percolating pores, D (cm2 s−1) corresponds to the diffusion coefficient and δ (cm) the pore length in the current flow direction. Most literature reports diffusion coefficients ranging from 10−12 up to 10−8 cm2 s−1 for ionic species diffusion through organic films
The new EEC and the best fitting values obtained are also shown in Fig. 10. Additionally, visual examination at the end of the experiment reveals signs of corrosion, as Fig. 11A and B illustrate. The corresponding EDX analysis (Fig. 11C) reveals the presence of iron, oxygen and 7
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Fig. 13. Nyquist (A) and Bode (B) plots corresponding to the parallel contribution at different immersion times.
beginning down to 200 nm at the end of the immersion test. This decreasing can be related to pore blockage (pore splitting) or to full filling with the electrolyte, both processes lead to time constant decreasing because of associated resistance decreasing. On the contrary, size of pore family II (R3C3) has no a clear tendency, ranging between 200 nm to 20 nm. This reflects a dynamic situation probably related to blocking and unblocking processes. It is worth remembering that for the electrolyte accesses to the pores, air must be evacuated via dissolution. As the electrolyte is air saturated, bubbles can easily develop, blocking the pores entry, until further collapse and unblocking. At the low frequency limit, in the order of mHz, the time constant associated with the corrosion process can be distinguished. The high values obtained for charge transfer resistance, about 1 GΩ cm2 indicate the presence of an incipient corrosion process, which is corroborated by the small active surface, estimated from capacitance as 0.02%. Fig. 14. Parallel impedance obtained after 9 days of immersion in the electrolyte together with the fitting result obtained using the EEC depicted in the insert.
3.2. Evolution of parallel impedance (Zpar): four electrode arrangement As above mentioned, the total and parallel impedances follow the same pattern (see Fig. 4). Therefore and in order to study just the parallel contribution, the results presented hereinafter have been obtained from Eq. (2), using the impedance spectra obtained with three (Zext and Zint) and four (Ztot) electrode configurations. Thus, Fig. 13 depicts the parallel impedance evolution with immersion time. As stated before, the obtained values are lower, more than five orders of magnitude, than those measured using a three electrode configuration. Besides, two capacitive loops are clearly distinguished from the
[2,5,32]. These values are obtained by macroscopic experiments, which assume that whole the surface is “active surface”. However, the electrolyte transport only goes through the pores network. In this situation, the diffusion coefficient should be closer to the corresponding in the solution instead of in the polymer matrix bulk [33]. Based on this approach, a diffusion coefficient value of 10−5 cm2 s−1 has been considered to estimate the size of both pore families, named I (R2C2) and II (R3C3). The size of the pore family I (R2C2) evolves from 300 nm at the 8
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Fig. 15. Impedance plots (Zpar) obtained after 60 days of immersion, where resistive behaviour is observed at frequencies lower than 1 Hz.
Fig. 15 corroborates. The impedance spectra show a resistive behaviour at frequencies lower than 1 Hz, only scattered points are recorded, without traces of the presence of double layer like capacitance. The lack of interfacial phenomenon suggests good adherence between the resin and the metallic substrate. The evolution of the resistances and the capacitances obtained from fitting is shown in Fig. 16. As expected, the C1 capacitance, associated with the inner cylinder wall, keeps constant during all the immersion period, indicating no interaction with the electrolyte. It is remarkable the evolution of film resistance, Rf, distinct than that observed in the normal impedance measurements. The values remain quite stable, even though a slight increasing tendency can be observed initially, and decreasing trend at longer immersion periods. The reason for such evolution is not clear, although it could be related to the viscoelastic nature of the polymer. The section considered for the parallel ionic motion is located under the inner cylinder wall, which is submitted to the mechanical pressure necessary to guarantee the electrolyte confinement. Under these conditions, polymer could undergo creep deformation, which tends to compress the film and so, to increase the resistance to ionic motion along the film. The interpretation of R2C2 and R3C3 can be done in the same terms used for normal impedance, even though some differences have to be highlighted. Firstly, the second pore family (R3C3) was not detected initially. Its occurrence can be explained taking into account the plasticizing effect induced by absorbed water that stimulates the local mobility of the polymer [34]. As a result, the initial morphology evolves developing a new pores family. An estimation of pore size for both families can be done using Eq. (5). The pore family I (R2C2) results of bigger size, from 1.7 μm down to 250 nm, whereas family II (R3C3) ranges from 200 nm down to 10 nm. Both the size and the evolution of the pore families are quite different than those obtained from the normal impedance measurements. In the parallel impedance, big difference is observed in the pore size between both families, at least one order of magnitude. Besides, the pore size
beginning. The electrical equivalent circuit employed to fit the experimental measurements is shown in Fig. 14, together with an example of the fitting quality. Although this circuit is the same as that used for fitting the normal impedance measurements, the physical meaning of C1 element is different. Fig. 2B can help to illustrate this point. The parallel current flows along the resin but also through the inner cylinder, and both capacitances are in parallel configuration. Therefore, the total capacitance must be the sum of the two contributions: Ccylinder + Cresin. To compare both magnitudes it is necessary taking into account the geometrical characteristics that it is to say, the geometrical factor, GF. For a cylindrical capacitor GF is given by Eq. (6):
GF =
2πL ln(rm ri )
(6)
Where L is the thickness, and rm and ri are the external (1.15 cm) and the internal (0.9 cm) radius, respectively. Although the radiuses are the same for both elements, cylinder wall and resin, the respective thicknesses are quite different. In the case of the wall, L corresponds to the electrolyte height inside of the cell, 2.2 cm, while for the resin L is the film thickness (9.8 × 10−2 cm). From these values, a GF of 56 cm is obtained for the wall and a GF of 2 cm for the resin. Assuming a dielectric constant around 4 for both materials (PMMA and epoxy resin), the capacitances result to be around 2 × 10−11 F for cylinder wall and around 7 × 10−13 F for the film, i.e. two orders of magnitude lower. Therefore, C1 corresponds here essentially to the capacitance of the cell wall. However, R1 it is related to the ionic conduction through the film (percolating pores), since the wall resistance is infinite. R2 and C2 make reference again to the non-percolating pores, as in the case of the normal impedance. After 20 days of immersion an additional R3C3 combination is included, associated with a new pore family. From that time on the EEC depicted in Fig. 8 was employed to fit the data. No interfacial phenomenon is observed, even at the end of the immersion period as 9
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Fig. 16. A) Resistances and B) capacitances evolution with immersion time, obtained from the parallel impedance plots.
The employed electrical equivalent circuit model took into account the porous nature of the film, both in the normal and in the parallel current distributions. The presence of pores with different sizes were corroborated by scanning electron microscopy. Although in both situations two pore families were distinguished, the size and the evolution were different. Thus, the pores were larger along the film in a parallel direction, which suggested an anisotropic character of the coating. On the other hand, the evolution of both pore families is not clear, although it should be related to the viscoelastic character of the polymer and the plasticizing effect induced by adsorbed water. The lack of interfacial phenomena at low frequency (lower than 1 Hz) in the four electrode configuration indicates good adherence of the film to the metallic substrate.
decreasing observed for both families is more pronounced than that in the normal direction. These results suggest that the changes in the film morphology are bigger in longitudinal direction than in normal one. The large differences may be the result of several factors, among which the polymer's response to the mechanical pressure or the plasticizing effect triggered by the absorbed water would be outstanding. An ongoing research aims at quantifying these aspects. 4. Conclusions The proposed double-cylinder electrolyte cell allows the discrimination between normal and parallel conduction in a thick film applied on carbon steel. Using a three electrode arrangement, impedance measurements of the inner (Zin) and outer (Zext) compartments were performed. It is possible to get an idea about the heterogeneity of the film by comparing the obtained results in both compartments. The impedance spectra measured were quite similar, suggesting a homogenous coating. The total impedance was measured using a four-electrode arrangement. The obtained spectra were, in the low frequency range, more than five orders of magnitude lower than the internal and the external impedances. This evidenced a parallel contribution of a fraction of current flowing along the film, parallel to the metallic substrate. This contribution followed the same pattern that the total impedance.
References [1] G. Grundmeier, W. Schmidt, M. Stratmann, Corrosion protection by organic coatings: electrochemical mechanism and novel methods of investigation, Electrochim. Acta 45 (2000) 2515–2533, http://dx.doi.org/10.1016/S0013-4686(00)00348-0. [2] F. Bellucci, L. Nicodemo, Water transport in organic coatings, Corrosion 49 (1993) 235–247, http://dx.doi.org/10.5006/1.3316044. [3] V.B. Miskovic-Stankovic, D.M. Dražić, Z. Kačarević-Popović, The sorption characteristics of epoxy coatings electrodeposited on steel during exposure to different corrosive agents, Corros. Sci. 38 (1996) 1513–1523, http://dx.doi.org/10.1016/ 0010-938X(96)00042-X.
10
Progress in Organic Coatings xxx (xxxx) xxx–xxx
B. Bíaz et al.
[4] B. Liu, Y. Li, H.C. Lin, C.N. Cao, Effect of PVC on the diffusing behavior of water through alkyd coatings, Acta Phys. Chim. Sin. 17 (2001) 244, http://dx.doi.org/10. 1016/S0010-938X(02)00061-6. [5] J.T. Zhang, J.M. Hu, J.Q. Zhang, C.N. Cao, Studies of water transport behavior and impedance models of epoxy-coated metals in NaCl solution by EIS, Prog. Org. Coat. 51 (2004) 145–151, http://dx.doi.org/10.1016/j.porgcoat.2004.08.001. [6] E. Legghe, A. Emmanuel, L. Bélec, A. Margaillan, D. Melot, Correlation between water diffusion and adhesion loss: study of an epoxy primer on steel, Prog. Org. Coat. 66 (2009) 276–280, http://dx.doi.org/10.1016/j.porgcoat.2009.08.001. [7] J. Kittel, N. Celati, M. Keddam, H. Takenouti, New methods for the study of organic coatings by EIS: New insights into attached and free films, Prog. Org. Coat. 41 (2001) 93–98, http://dx.doi.org/10.1016/S0300-9440(00)00155-7. [8] J. Kittel, N. Celati, M. Keddam, H. Takenouti, Influence of the coating-substrate interactions on the corrosion protection: characterisation by impedance spectroscopy of the inner and outer parts of a coating, Prog. Org. Coat. 46 (2003) 135–147, http://dx.doi.org/10.1016/S0300-9440(02)00221-7. [9] R.L. De Rosa, J.T. Grant, L. Kasten, M. Donley, G.P. Bierwagen, "Surface analysis of various methods of preparing Al 2024-T3 surfaces for painting (Vol. 56, No. 4, pp. 395–400)", Corrosion 56 (2000) 669, http://dx.doi.org/10.5006/1.3280569. [10] P. Álvarez, A. Collazo, A. Covelo, X.R. Nóvoa, C. Pérez, The electrochemical behaviour of sol-gel hybrid coatings applied on AA2024-T3 alloy: effect of the metallic surface treatment, Prog. Org. Coat. 69 (2010) 175–183, http://dx.doi.org/10.1016/ j.porgcoat.2010.04.005. [11] J. Luo, C. Lin, Q. Yang, S. Guan, Cathodic disbonding of a thick polyurethane coating from steel in sodium chloride solution, Prog. Org. Coat. 31 (1997) 289–295, http://dx.doi.org/10.1016/S0300-9440(97)00086-6. [12] T. Nguyen, E. Byrd, D. Bentz, C. Lin, In situ measurement of water at the organic coating/substrate interface, Prog. Org. Coat. 27 (1996) 181–193, http://dx.doi.org/ 10.1016/0300-9440(95)00535-8. [13] K.M. Takahashi, T.M. Sullivan, ac impedance measurement of environmental water in adhesive interfaces, J. Appl. Phys. 66 (1989) 3192–3201, http://dx.doi.org/10. 1063/1.344158. [14] K.M. Takahashi, ac impedance measurements of moisture in interfaces between epoxy and oxidized silicon, J. Appl. Phys. 67 (1990) 3419–3429, http://dx.doi.org/ 10.1063/1.345354. [15] E.P. O’Brien, P.F. Reboa, M. Field, D. Pullen, D. Markel, T.C. Ward, A novel impedance sensor design for measuring the distribution and transport of fluids at the interface, Int. J. Adhes. Adhes. 23 (2003) 335–338, http://dx.doi.org/10.1016/ S0143-7496(03)00062-9. [16] A. Miszczyk, T. Schauer, Electrochemical approach to evaluate the interlayer adhesion of organic coatings, Prog. Org. Coat. 52 (2005) 298–305, http://dx.doi.org/ 10.1016/j.porgcoat.2004.09.006. [17] A. Nogueira, X.R. Nóvoa, C. Pérez, On the possibility of using embedded electrodes for the measurement of dielectric properties in organic coatings, Prog. Org. Coat. 59 (2007) 186–191, http://dx.doi.org/10.1016/j.porgcoat.2006.09.035. [18] Q. Su, K.N. Allahar, G.P. Bierwagen, Application of embedded sensors in the thermal cycling of organic coatings, Corros. Sci. 50 (2008) 2381–2389, http://dx. doi.org/10.1016/j.corsci.2008.06.010. [19] E. López-Quiroga, X.R. Nóvoa, C. Pérez, V. Vivier, Current distribution in doublecylinder electrolyte cells: application to the study of corrosion properties of organic coatings, Prog. Org. Coat. 74 (2012) 400–404, http://dx.doi.org/10.1016/j. porgcoat.2011.11.003. [20] Q. Le Thu, H. Takenouti, S. Touzain, EIS characterization of thick flawed organic
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
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coatings aged under cathodic protection in seawater, Electrochim. Acta 51 (2006) 2491–2502, http://dx.doi.org/10.1016/j.electacta.2005.07.049. M. Keddam, X.R. Nóvoa, V. Vivier, The concept of floating electrode for contact-less electrochemical measurements: application to reinforcing steel-bar corrosion in concrete, Corros. Sci. 51 (2009) 1795–1801, http://dx.doi.org/10.1016/j.corsci. 2009.05.006. Keysight-Technologies, Impedance Measurement Handbook. A Guide to Measurement Technology and Techniques, 6th edition– application note, (2016), pp. 1–140. C. Pérez, A. Collazo, M. Izquierdo, P. Merino, X. Nóvoa, Electrochemical impedance spectroscopy study of the corrosion process on coated galvanized steel in a salt spray fog chamber, Corrosion 56 (2000) 1220–1232, http://dx.doi.org/10.5006/1. 3280510. L. Diguet, M. Keddam, H. Takenouti, H. Mazille, Y. Cètre, Corrosion protection by fluorinated polymers: an electrochemical approach from free-standing foils to coatings, Mater. Sci. Forum 289–292 (1998) 397–409 https://www.scientific.net/ MSF.289-292.397. S. Amand, M. Musiani, M.E. Orazem, N. Pébère, B. Tribollet, V. Vivier, Constantphase-element behavior caused by inhomogeneous water uptake in anti-corrosion coatings, Electrochim. Acta 87 (2013) 693–700, http://dx.doi.org/10.1016/j. electacta.2012.09.061. M. Musiani, M.E. Orazem, N. Pébère, B. Tribollet, V. Vivier, Determination of resistivity profiles in anti-corrosion coatings from constant-phase-element parameters, Prog. Org. Coat. 77 (2014) 2076–2083, http://dx.doi.org/10.1016/j. porgcoat.2013.12.013. B. Díaz, L. Freire, P. Merino, X.R. Nóvoa, M.C. Pérez, Impedance spectroscopy study of saturated mortar samples, Electrochim. Acta 53 (2008) 7549–7555, http://dx. doi.org/10.1016/j.electacta.2007.10.042. B. Díaz, X.R. Nóvoa, B. Puga, V. Vivier, Chloride transport through cementitious membranes using pulsed current, Cem. Concr. Compos. 39 (2013) 18–22, http://dx. doi.org/10.1016/j.cemconcomp.2013.03.018. C. Perez, A. Collazo, M. Izquierdo, P. Merino, X.R. Novoa, Characterisation of the barrier properties of different paint systems. Part I. Experimental set-up and ideal Fickian diffusion, Prog. Org. Coat. 36 (1999) 102–108, http://dx.doi.org/10.1016/ S0300-9440(99)00030-2. A. Amirudin, D. Thierry, Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals, Prog. Org. Coat. 26 (1995) 1–28, http://dx.doi.org/10.1016/0300-9440(95)00581-1. B. Fastrup, A. Saarnak, AC impedance of painted and scribed steel panels under atmospheric exposure, Prog. Org. Coat. 16 (1988) 277–290, http://dx.doi.org/10. 1016/0033-0655(88)80021-9. C. Pérez, A. Collazo, M. Izquierdo, P. Merino, X.R. Nóvoa, Characterisation of the barrier properties of different paint systemsPart II. Non-ideal diffusion and water uptake kinetics, Prog. Org. Coat. 37 (1999) 169–177, http://dx.doi.org/10.1016/ S0300-9440(99)00073-9. B. Díaz, X.R. Nóvoa, B. Puga, V. Vivier, Macro and micro aspects of the transport of chlorides in cementitious membranes, Electrochim. Acta 124 (2014) 61–68, http:// dx.doi.org/10.1016/j.electacta.2013.08.105. S. Duval, M. Keddam, M. Sfaira, A. Srhiri, H. Takenouti, Electrochemical impedance spectroscopy of epoxy-vinyl coating in aqueous medium analyzed by dipolar relaxation of polymer, J. Electrochem. Soc. 149 (2002) B520, http://dx.doi.org/10. 1149/1.1512667.
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Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
EIS study of epoxy resin applied on carbon steel using double-cylinder electrolyte cell ⁎
B. Bíaz, X.R. Nóvoa, C. Pérez , A. Pintos ENCOMAT Group, University of Vigo, E.E.I. Campus Universitario, 36310 Vigo, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Double-cylinder electrolyte cell Parallel and normal conduction Porous film Interfacial properties
The assessment of the protection properties of an organic film applied on metallic substrate is still a challenge. Barrier features are currently studied by electrochemical impedance spectroscopy (EIS) technique, using a classical three electrode arrangement, where current flow normal to the metallic substrate is measured, nevertheless, any information about parallel ionic conductivity is not acquired. The present paper proposes the use of a double cylinder electrochemical cell, in which the combination of three and four electrode arrangements allows the measurement not only the current flow normal to the metallic substrate but also parallel to the metalcoating interface. The EIS experiments are performed in a thick epoxy resin film applied on carbon steel, during 60 days of immersion in a 0.1 M Na2SO4 + NaOH solution. The impedance values measured using the three electrode configuration are much higher than those obtained by the four electrode arrangement. This result can be explained considering that a current fraction flows parallel to the metal-coating interface. The impedance evolution is explained considering the presence of diverse pore families, which evolve in different ways depending on the ionic motion direction, normal or parallel. Changings are more remarkable in the parallel direction, reflecting the anisotropic character of the film.
1. Introduction Organic coatings represent the most common way to protect metallic structures against corrosion. It is generally accepted that the protective action is based on two main properties: barrier (for oxygen, water and aggressive species), and blocking of the ionic paths between anodic and cathodic areas along the metal/polymer interface [1]. The barrier properties deal with the transport of ions and aggressives through the film, in the direction normal to the metallic substrate. The Electrochemical Impedance Spectroscopy (EIS) technique has been used widespread for this study, mainly using the three electrode arrangement, where the reference electrode is place in the test solution. In such setup, the measurements are related to the sorption characteristics of the whole film in this direction [2–6]. This approach does not discriminate between the characteristics of the film at bulk film and at the metal-coating interface region, where chemical and physical interactions are expected. Kittel et al. employed an additional electrode embedded in the coating, which allows the discrimination between the impedance of the inner part in contact with the metal surface and that of the outer part in contact with the electrolyte [7,8]. However, only information about ionic conductivity in normal direction is acquired.
⁎
On the other hand, parallel conductivity has strong influence on the initiation and propagation of corrosion. It is well-known that, for a given coating, the protection effectiveness against corrosion depends on the surface treatment that could lead to distinct adhesive forces between film and metal [9,10]. Luo et al. stated that in the cathodic disbonding process the ionic transport along the coating/metal interface is more important than through the film [11]. Thus, the interest of this aspect is undoubted, nevertheless, the studies found in the literature are scarce, maybe due to the more complex experimental configuration needed. Some researchers use the Fourier Transform InfraredMultiple Internal Reflection (FTIR-MIR) technique for in situ quantifying the water layer at the coating/substrate interface [12]. The main restriction is that it is limited to substrates that produce total internal reflection. Iron is a poorly-reflective metal, which constrains the use of this technology in the corrosion study. An interesting approach is the measurement of impedance between electrodes located at coating/ substrate interface [13–15] or embedded into the polymer [16–18]. However, placing the electrodes is not a simple task, mainly at the interface level, even though the main constrain is that current flow that crosses the coating in a parallel way is minimum in most of the situations, thus much of the information in this direction is lost [17].
Corresponding author. E-mail address: [email protected] (C. Pérez).
https://doi.org/10.1016/j.porgcoat.2018.02.002 Received 28 August 2017; Received in revised form 13 December 2017; Accepted 6 February 2018 0300-9440/ © 2018 Elsevier B.V. All rights reserved.
Please cite this article as: Bíaz, B., Progress in Organic Coatings (2018), https://doi.org/10.1016/j.porgcoat.2018.02.002
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arrangement, where two platinum wires acted as WE and CE, one in each compartment, respectively. Two Ag/AgCl wires, placed each one in each compartment, were the RE electrodes. It is worthy to highlight that in this arrangement the coated steel is electrically floating. The impedance measured in such configuration, ZTotal, involves the internal and external contributions, as well as the contribution of a fraction of current flowing parallel to the metal-coating interface, as stated elsewhere [19]. The Electrochemical Impedance Spectroscopy, EIS, experiments were performed using an Autolab 30 potentiostat from Ecochemie® equipped with FRA and multiplexer modules. Most of the measurement were done at a frequency scan from 1 MHz down to 10 mHz, even though at longer immersion periods the lowest scanned frequency was 1 mHz, with five points per decade, and amplitude of 100 mVrms at null DC current. The multiplexer module (MUX® control), which is a multichannel device that allows the sequential measurements of the electrode arrangements described above: Zext, Zint and ZTotal, to be completed automatically without manipulation of the set-up. 100 mV amplitude was employed to improve the signal-to-noise ratio of the measurement because of the high coating impedance. The impedance measurements were performed on three replicas. The sample characterization was performed by Optical Microscopy, OM, with an Olympus® GX71/GX51 Inverted Metallurgical Microscope, and by Scanning Electron Microscopy, SEM, and Energy Dispersive Xray, EDX, techniques, employing an Electroscan JSM-54® model JEOL 5410 equipped with an energy dispersive X-ray detector Link ISIS® 300.
In an attempt to overcome these limitations, our research group developed a new experimental design to study the parallel conductivity in a dielectric medium [19]. The experimental setup is based on a double cell used occasionally to study the cathodic protection [11,20]. The design takes advantage of the behaviour of a non-connected electronic conductor (floating electrode) embedded in an ion-conducting medium [21] or contacting it, in the studied case, a polymer film. The general idea is that the current flowing through an ionic-conducting medium that contains a floating electric conductor tends to split in two paths: ionic and electronic, depending on the associated resistances, dielectric and charge transfer resistances, respectively [19,21]. Though the obtained results shown that it was possible the discrimination between parallel and normal current fluxes, a detailed analysis of the parallel impedance evolution at long immersion periods is necessary. Thus, the present study aims on the behaviour of a thick organic film in immersion conditions. For that, a double cylinder electrolyte cell was employed, which allows the tracking of both normal and parallel contributions with immersion time. Two different electrical equivalent circuits were proposed to model the experimental results. 2. Experimental design An adhesive layer based on epoxy resin (Resoltech 3350 HP®), cured at 70 °C during 5 h, was applied on mild steel. This is a structural adhesive used to provide durable bonding of materials such as composites or polymeric layers and aluminium or steel. Thus, the durability of such union depends, among other factors, on the barrier properties of this adhesive. The average thickness was 980 μm. In order to improve the resinmetal adhesion, the metallic substrate was previously grinded with silicon carbide paper 180 grit. Besides, to obtain a uniform coating thickness, the resin surface was also grinded with similar abrasive paper. Fig. 1A depicts a cross-section of the studied samples. The electrochemical measurements were performed using a double cylinder cell, made of Poly-methyl-methacrylate, PMMA. The double cylinders defined inner and outer compartments with the same contact surfaces (S = 2.8 cm2) between the coating and the electrolyte. Details of the geometry are given in Fig. 1B. The electrolyte was a 0.1 M Na2SO4 + NaOH solution in both compartments. NaOH was employed to adjust the pH to 8. The different arrangements of the electrodes employed are depicted in Fig. 2. The three electrode arrangement is illustrated in Fig. 2A, the working electrode (WE) was the metallic substrate coated with the resin, the counter electrode (CE) was a platinum wire, and an Ag/AgCl wire was the reference electrode (RE).This configuration was used in both compartments. The current flows normal to the metallic surface, from CE to WE. The external impedance (Zext) was obtained by placing the RE and CE electrodes in the outer compartment, whereas the internal impedance (Zint) was acquired by placing the RE and CE electrodes in the inner compartment. Fig. 2B shows the four electrode
3. Results and discussion Fig. 3 displays an example of the impedance measurements obtained. It seems striking the value of the total impedance, more than five orders of magnitude lower than the external and internal ones. This behaviour can only be explained assuming that the total impedance is the combination of two contributions: normal impedance, which is the sum of Zext + Zint, and parallel impedance (Zpar) due to a fraction of current that flows along the film, parallel to the metal-coating interface [19]. Thus, the total impedance must be described by a parallel combination of both contributions, as Eq. (1) shows:
ZTotal =
Zpar (Zint + Zext ) Zpar + Zint + Zext
(1)
Taking into account that ZTotal, Zint and Zext are the experimental EIS measurements, the parallel contribution can be obtained from Eq. (2):
Zpar =
1
( ) ( 1
ZTot
−
1 Zext + Zint
)
(2)
This expression indicates that if the normal impedance (Zext + Zint) is high, the total impedance is close to the parallel one, ZTot ≈ Zpar. Fig. 4 corroborates this feature: the total and parallel impedances
Fig. 1. A) Optical picture showing a cross-section of the studied system (50x). B) Sketch showing the dimensions of the double cylinder cell.
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Fig. 2. Electrode arrangements designed to perform the impedance measurements. A) Three electrode configuration to measure the impedance in the inner and the outer compartments with the coated steel as working electrode. B) Four electrode arrangement for measuring the total impedance, where the metallic substrate is electrically floating. For more details see the text.
follow the same pattern. 3.1. Evolution of normal impedances (Zext, Zint): three electrode arrangement The impedance measurements acquired in the inner and outer compartments are quite similar, following the same tendency not only at the beginning (see Fig. 3) but also at the end of the immersion time, as Fig. 5 illustrates. As it can be seen, impedance values overlap in almost all the frequency scanned, which suggests that the coating is a homogeneous film. The small differences observed at frequencies higher than 1 kHz could be attributed to stray current at high frequencies [22], as discussed later. Therefore, in this section the impedance study will be focused on one compartment (the external one, Zext), assuming that the same interpretation can be extrapolated to the internal one. The impedance evolution with immersion time for the external compartment is depicted in Fig. 6. Huge impedance values are measured even at the end of the immersion period, which is not a surprise taking into account the high film thickness. Nonetheless, a clear impedance decrease is observed as the immersion time elapses. On the other hand, it is noticeable, mainly in phase angle, the two contributions located at frequencies higher than 100 Hz.
Fig. 4. Nyquist plots of total (ZTotal) and parallel (Zparallel) impedances of the studied sample after 9 days immersion in the electrolyte.
The observation of the applied film morphology has been helpful to interpret the impedance spectra in terms of adequate electrical equivalent circuit. Fig. 7A–C correspond to SEM images of the epoxy resin at different magnifications. The main characteristic is the presence of numerous pores or bubbles of varying sizes, from microns down to nanometres. Thus, the model should consider not only their presence but also the
Fig. 3. Impedance modulus of the external (●), internal (★) and total (■) after 9 days of immersion in the electrolyte. The corresponding phase angle is depicted in the insert.
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Fig. 5. Bode plots of the external (●) and internal (★) compartments after 60 days of immersion in the electrolyte.
Fig. 6. Impedance spectra measured in the external compartment at different immersion times.
Fig. 7. SEM images showing a cross-section of the epoxy resin at A) 150x, B) 7.500 x and C) 16.000x. D) Physical interpretation of the electrical equivalent circuit proposed.
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Fig. 8. Experimental and fitted data using the equivalent circuit inserted (see the text for more details) corresponding to Zext after 22 days of immersion.
Fig. 9. Evolution of CfRf parameters with immersion time.
Fig. 10. Nyquist plot measured after 53 days of immersion in the electrolyte. The new EEC is included, as well as the best fitting results obtained.
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Fig. 11. Optical images showing the aspect of the resin at the end of the test at A) 60x and B) 1000x magnifications. C) EDX corresponding to the damaged area displayed in the SEM picture (1700x) inserted.
of the resin, about 1 mm and the huge low frequency impedance values. An example of the fitted quality is depicted in Fig. 8. The interpretation given is not new, Diguet et al. [24] proposed the same model to characterize the delamination of a PTFE coating applied on a 2024 aluminium alloy. Le Thu et al. [20] used equivalent circuits, which include a vacuole branch to account the pre-existing holes in thick coatings. More recently, other authors [25,26] used the same concept to study the water uptake process in organic coatings. In addition, our research group employs this equivalent circuit to model the pore structure of cementitious materials [27,28]. The evolution of the resistance and the capacitance of the resin is depicted in Fig. 9. Rf undergoes a fast decay, more than two orders of magnitude, mainly during the first 42 days of immersion. Surprisingly, only small changes are observed in the capacitance values over the same period. This evolution reflects the electrolyte uptake through preexisting channels, however, little physical-chemical interaction occurs with the resin network [24]. Capacitance values vary from 50 to 70 pF, normalizing by the geometric factor of the flat condenser (d/S) the permittivity ranges from 20 to 27. The initial values seems too high for a dry epoxy resin, although it should be kept in mind that these values are in the order of the potentiostat input capacitance, so the recorded values may be affected by the instrumentation employed [29]. Therefore, although the capacitance values may not be directly related to the coating capacitance, the observed evolution, a regular increase, should correspond to the dielectric behaviour of the resin.
several pore families in terms of size and depth associated. Fig. 7D is a sketch of the proposed electrical equivalent circuit, EEC, and its physical interpretation. Rf represents the resistance of the ionic conduction through the film that reach the metallic substrate (percolating pores) and Cf the dielectric capacitance of the resin. Whereas R2C2 and R3C3 are related to pore families with different morphologies both in size and in depth. Thus, R2 and R3 account the ionic resistances inside of the pores (non-percolating pores), and C2 and C3 represent the double layer capacitance at the pore walls. The impedance is given by Eq. (3):
Z (ω) = R e +
Z1 Z4 Z1 + Z4
(3)
Where
Z1 =
Rf 1 + (jωRf Cf )α1
(
Z2 = R2 1 +
and Z4 =
1 (jωR2 C2)α2
)
Z2 Z3 Z2 + Z3
with
(
and Z3 = R3 1 +
1 (jωR3 C3)α3
)
Re accounts the electrolyte resistance, α1, α2, α3 are the Col-Cole dispersion parameters associated to the corresponding time constants. The other elements have been already explained. A detailed explanation of the fitting procedure has already been provided elsewhere [23]. In this model, the low frequency limit is given by Rf (film resistance), and no interfacial phenomena at the metal-resin interface is considered. This assumption seems reasonable due to the high thickness 6
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Fig. 12. A) Resistances and B) capacitances evolution with immersion time for an epoxy resin applied on carbon steel using a three electrode arrangement. The R2, R3, C2 and C3 meaning is explained in the text.
chloride that suggest the appearance of corrosion products. It is noticeable the presence of a peak corresponding to Si, that probably comes from the silicon carbide abrasive paper used to obtain the flat resin surface. Regarding to the carbon peak, it is compatible with the organic nature of the film. The evolutions of R2, R3, Rct and C2, C3, Cdl parameters are shown in Fig. 12. As stated above, R2C2 and R3C3 refer to the same morphological feature: the presence of different pore families. An estimation of the pore size of both families is possible, based on Eq. (5) [30,31]:
After 42 days of immersion a continuous increase in capacitance is observed, that can be related to the corrosion onset when the electrolyte reaches the metal surface. Fig. 10 displays the Nyquist plot obtained after 53 days of immersion in the tested solution. An additional time constant is clearly distinguished in the low frequency domain, in the range of 10–1 mHz. In this situation, the EEC must include a new time constant related to the corrosion process (RctCdl) that develops at the bottom of the percolating pores. The impedance is also given by Eq. (3), although Z1 now is:
Z1 =
Rf (jωRf Cf )αf +
1 1 + ⎛⎜Z0 Rf ⎞⎟ ⎝ ⎠
with
Z0 =
R ct 1 + (jωR ct Cdl)αdl
τ= (4)
δ2 D
(5)
Where τ (s) is the time constant associated with the ionic motion inside of the non-percolating pores, D (cm2 s−1) corresponds to the diffusion coefficient and δ (cm) the pore length in the current flow direction. Most literature reports diffusion coefficients ranging from 10−12 up to 10−8 cm2 s−1 for ionic species diffusion through organic films
The new EEC and the best fitting values obtained are also shown in Fig. 10. Additionally, visual examination at the end of the experiment reveals signs of corrosion, as Fig. 11A and B illustrate. The corresponding EDX analysis (Fig. 11C) reveals the presence of iron, oxygen and 7
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Fig. 13. Nyquist (A) and Bode (B) plots corresponding to the parallel contribution at different immersion times.
beginning down to 200 nm at the end of the immersion test. This decreasing can be related to pore blockage (pore splitting) or to full filling with the electrolyte, both processes lead to time constant decreasing because of associated resistance decreasing. On the contrary, size of pore family II (R3C3) has no a clear tendency, ranging between 200 nm to 20 nm. This reflects a dynamic situation probably related to blocking and unblocking processes. It is worth remembering that for the electrolyte accesses to the pores, air must be evacuated via dissolution. As the electrolyte is air saturated, bubbles can easily develop, blocking the pores entry, until further collapse and unblocking. At the low frequency limit, in the order of mHz, the time constant associated with the corrosion process can be distinguished. The high values obtained for charge transfer resistance, about 1 GΩ cm2 indicate the presence of an incipient corrosion process, which is corroborated by the small active surface, estimated from capacitance as 0.02%. Fig. 14. Parallel impedance obtained after 9 days of immersion in the electrolyte together with the fitting result obtained using the EEC depicted in the insert.
3.2. Evolution of parallel impedance (Zpar): four electrode arrangement As above mentioned, the total and parallel impedances follow the same pattern (see Fig. 4). Therefore and in order to study just the parallel contribution, the results presented hereinafter have been obtained from Eq. (2), using the impedance spectra obtained with three (Zext and Zint) and four (Ztot) electrode configurations. Thus, Fig. 13 depicts the parallel impedance evolution with immersion time. As stated before, the obtained values are lower, more than five orders of magnitude, than those measured using a three electrode configuration. Besides, two capacitive loops are clearly distinguished from the
[2,5,32]. These values are obtained by macroscopic experiments, which assume that whole the surface is “active surface”. However, the electrolyte transport only goes through the pores network. In this situation, the diffusion coefficient should be closer to the corresponding in the solution instead of in the polymer matrix bulk [33]. Based on this approach, a diffusion coefficient value of 10−5 cm2 s−1 has been considered to estimate the size of both pore families, named I (R2C2) and II (R3C3). The size of the pore family I (R2C2) evolves from 300 nm at the 8
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Fig. 15. Impedance plots (Zpar) obtained after 60 days of immersion, where resistive behaviour is observed at frequencies lower than 1 Hz.
Fig. 15 corroborates. The impedance spectra show a resistive behaviour at frequencies lower than 1 Hz, only scattered points are recorded, without traces of the presence of double layer like capacitance. The lack of interfacial phenomenon suggests good adherence between the resin and the metallic substrate. The evolution of the resistances and the capacitances obtained from fitting is shown in Fig. 16. As expected, the C1 capacitance, associated with the inner cylinder wall, keeps constant during all the immersion period, indicating no interaction with the electrolyte. It is remarkable the evolution of film resistance, Rf, distinct than that observed in the normal impedance measurements. The values remain quite stable, even though a slight increasing tendency can be observed initially, and decreasing trend at longer immersion periods. The reason for such evolution is not clear, although it could be related to the viscoelastic nature of the polymer. The section considered for the parallel ionic motion is located under the inner cylinder wall, which is submitted to the mechanical pressure necessary to guarantee the electrolyte confinement. Under these conditions, polymer could undergo creep deformation, which tends to compress the film and so, to increase the resistance to ionic motion along the film. The interpretation of R2C2 and R3C3 can be done in the same terms used for normal impedance, even though some differences have to be highlighted. Firstly, the second pore family (R3C3) was not detected initially. Its occurrence can be explained taking into account the plasticizing effect induced by absorbed water that stimulates the local mobility of the polymer [34]. As a result, the initial morphology evolves developing a new pores family. An estimation of pore size for both families can be done using Eq. (5). The pore family I (R2C2) results of bigger size, from 1.7 μm down to 250 nm, whereas family II (R3C3) ranges from 200 nm down to 10 nm. Both the size and the evolution of the pore families are quite different than those obtained from the normal impedance measurements. In the parallel impedance, big difference is observed in the pore size between both families, at least one order of magnitude. Besides, the pore size
beginning. The electrical equivalent circuit employed to fit the experimental measurements is shown in Fig. 14, together with an example of the fitting quality. Although this circuit is the same as that used for fitting the normal impedance measurements, the physical meaning of C1 element is different. Fig. 2B can help to illustrate this point. The parallel current flows along the resin but also through the inner cylinder, and both capacitances are in parallel configuration. Therefore, the total capacitance must be the sum of the two contributions: Ccylinder + Cresin. To compare both magnitudes it is necessary taking into account the geometrical characteristics that it is to say, the geometrical factor, GF. For a cylindrical capacitor GF is given by Eq. (6):
GF =
2πL ln(rm ri )
(6)
Where L is the thickness, and rm and ri are the external (1.15 cm) and the internal (0.9 cm) radius, respectively. Although the radiuses are the same for both elements, cylinder wall and resin, the respective thicknesses are quite different. In the case of the wall, L corresponds to the electrolyte height inside of the cell, 2.2 cm, while for the resin L is the film thickness (9.8 × 10−2 cm). From these values, a GF of 56 cm is obtained for the wall and a GF of 2 cm for the resin. Assuming a dielectric constant around 4 for both materials (PMMA and epoxy resin), the capacitances result to be around 2 × 10−11 F for cylinder wall and around 7 × 10−13 F for the film, i.e. two orders of magnitude lower. Therefore, C1 corresponds here essentially to the capacitance of the cell wall. However, R1 it is related to the ionic conduction through the film (percolating pores), since the wall resistance is infinite. R2 and C2 make reference again to the non-percolating pores, as in the case of the normal impedance. After 20 days of immersion an additional R3C3 combination is included, associated with a new pore family. From that time on the EEC depicted in Fig. 8 was employed to fit the data. No interfacial phenomenon is observed, even at the end of the immersion period as 9
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Fig. 16. A) Resistances and B) capacitances evolution with immersion time, obtained from the parallel impedance plots.
The employed electrical equivalent circuit model took into account the porous nature of the film, both in the normal and in the parallel current distributions. The presence of pores with different sizes were corroborated by scanning electron microscopy. Although in both situations two pore families were distinguished, the size and the evolution were different. Thus, the pores were larger along the film in a parallel direction, which suggested an anisotropic character of the coating. On the other hand, the evolution of both pore families is not clear, although it should be related to the viscoelastic character of the polymer and the plasticizing effect induced by adsorbed water. The lack of interfacial phenomena at low frequency (lower than 1 Hz) in the four electrode configuration indicates good adherence of the film to the metallic substrate.
decreasing observed for both families is more pronounced than that in the normal direction. These results suggest that the changes in the film morphology are bigger in longitudinal direction than in normal one. The large differences may be the result of several factors, among which the polymer's response to the mechanical pressure or the plasticizing effect triggered by the absorbed water would be outstanding. An ongoing research aims at quantifying these aspects. 4. Conclusions The proposed double-cylinder electrolyte cell allows the discrimination between normal and parallel conduction in a thick film applied on carbon steel. Using a three electrode arrangement, impedance measurements of the inner (Zin) and outer (Zext) compartments were performed. It is possible to get an idea about the heterogeneity of the film by comparing the obtained results in both compartments. The impedance spectra measured were quite similar, suggesting a homogenous coating. The total impedance was measured using a four-electrode arrangement. The obtained spectra were, in the low frequency range, more than five orders of magnitude lower than the internal and the external impedances. This evidenced a parallel contribution of a fraction of current flowing along the film, parallel to the metallic substrate. This contribution followed the same pattern that the total impedance.
References [1] G. Grundmeier, W. Schmidt, M. Stratmann, Corrosion protection by organic coatings: electrochemical mechanism and novel methods of investigation, Electrochim. Acta 45 (2000) 2515–2533, http://dx.doi.org/10.1016/S0013-4686(00)00348-0. [2] F. Bellucci, L. Nicodemo, Water transport in organic coatings, Corrosion 49 (1993) 235–247, http://dx.doi.org/10.5006/1.3316044. [3] V.B. Miskovic-Stankovic, D.M. Dražić, Z. Kačarević-Popović, The sorption characteristics of epoxy coatings electrodeposited on steel during exposure to different corrosive agents, Corros. Sci. 38 (1996) 1513–1523, http://dx.doi.org/10.1016/ 0010-938X(96)00042-X.
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Progress in Organic Coatings xxx (xxxx) xxx–xxx
B. Bíaz et al.
[4] B. Liu, Y. Li, H.C. Lin, C.N. Cao, Effect of PVC on the diffusing behavior of water through alkyd coatings, Acta Phys. Chim. Sin. 17 (2001) 244, http://dx.doi.org/10. 1016/S0010-938X(02)00061-6. [5] J.T. Zhang, J.M. Hu, J.Q. Zhang, C.N. Cao, Studies of water transport behavior and impedance models of epoxy-coated metals in NaCl solution by EIS, Prog. Org. Coat. 51 (2004) 145–151, http://dx.doi.org/10.1016/j.porgcoat.2004.08.001. [6] E. Legghe, A. Emmanuel, L. Bélec, A. Margaillan, D. Melot, Correlation between water diffusion and adhesion loss: study of an epoxy primer on steel, Prog. Org. Coat. 66 (2009) 276–280, http://dx.doi.org/10.1016/j.porgcoat.2009.08.001. [7] J. Kittel, N. Celati, M. Keddam, H. Takenouti, New methods for the study of organic coatings by EIS: New insights into attached and free films, Prog. Org. Coat. 41 (2001) 93–98, http://dx.doi.org/10.1016/S0300-9440(00)00155-7. [8] J. Kittel, N. Celati, M. Keddam, H. Takenouti, Influence of the coating-substrate interactions on the corrosion protection: characterisation by impedance spectroscopy of the inner and outer parts of a coating, Prog. Org. Coat. 46 (2003) 135–147, http://dx.doi.org/10.1016/S0300-9440(02)00221-7. [9] R.L. De Rosa, J.T. Grant, L. Kasten, M. Donley, G.P. Bierwagen, "Surface analysis of various methods of preparing Al 2024-T3 surfaces for painting (Vol. 56, No. 4, pp. 395–400)", Corrosion 56 (2000) 669, http://dx.doi.org/10.5006/1.3280569. [10] P. Álvarez, A. Collazo, A. Covelo, X.R. Nóvoa, C. Pérez, The electrochemical behaviour of sol-gel hybrid coatings applied on AA2024-T3 alloy: effect of the metallic surface treatment, Prog. Org. Coat. 69 (2010) 175–183, http://dx.doi.org/10.1016/ j.porgcoat.2010.04.005. [11] J. Luo, C. Lin, Q. Yang, S. Guan, Cathodic disbonding of a thick polyurethane coating from steel in sodium chloride solution, Prog. Org. Coat. 31 (1997) 289–295, http://dx.doi.org/10.1016/S0300-9440(97)00086-6. [12] T. Nguyen, E. Byrd, D. Bentz, C. Lin, In situ measurement of water at the organic coating/substrate interface, Prog. Org. Coat. 27 (1996) 181–193, http://dx.doi.org/ 10.1016/0300-9440(95)00535-8. [13] K.M. Takahashi, T.M. Sullivan, ac impedance measurement of environmental water in adhesive interfaces, J. Appl. Phys. 66 (1989) 3192–3201, http://dx.doi.org/10. 1063/1.344158. [14] K.M. Takahashi, ac impedance measurements of moisture in interfaces between epoxy and oxidized silicon, J. Appl. Phys. 67 (1990) 3419–3429, http://dx.doi.org/ 10.1063/1.345354. [15] E.P. O’Brien, P.F. Reboa, M. Field, D. Pullen, D. Markel, T.C. Ward, A novel impedance sensor design for measuring the distribution and transport of fluids at the interface, Int. J. Adhes. Adhes. 23 (2003) 335–338, http://dx.doi.org/10.1016/ S0143-7496(03)00062-9. [16] A. Miszczyk, T. Schauer, Electrochemical approach to evaluate the interlayer adhesion of organic coatings, Prog. Org. Coat. 52 (2005) 298–305, http://dx.doi.org/ 10.1016/j.porgcoat.2004.09.006. [17] A. Nogueira, X.R. Nóvoa, C. Pérez, On the possibility of using embedded electrodes for the measurement of dielectric properties in organic coatings, Prog. Org. Coat. 59 (2007) 186–191, http://dx.doi.org/10.1016/j.porgcoat.2006.09.035. [18] Q. Su, K.N. Allahar, G.P. Bierwagen, Application of embedded sensors in the thermal cycling of organic coatings, Corros. Sci. 50 (2008) 2381–2389, http://dx. doi.org/10.1016/j.corsci.2008.06.010. [19] E. López-Quiroga, X.R. Nóvoa, C. Pérez, V. Vivier, Current distribution in doublecylinder electrolyte cells: application to the study of corrosion properties of organic coatings, Prog. Org. Coat. 74 (2012) 400–404, http://dx.doi.org/10.1016/j. porgcoat.2011.11.003. [20] Q. Le Thu, H. Takenouti, S. Touzain, EIS characterization of thick flawed organic
[21]
[22]
[23]
[24]
[25]
[26]
[27]
[28]
[29]
[30]
[31]
[32]
[33]
[34]
11
coatings aged under cathodic protection in seawater, Electrochim. Acta 51 (2006) 2491–2502, http://dx.doi.org/10.1016/j.electacta.2005.07.049. M. Keddam, X.R. Nóvoa, V. Vivier, The concept of floating electrode for contact-less electrochemical measurements: application to reinforcing steel-bar corrosion in concrete, Corros. Sci. 51 (2009) 1795–1801, http://dx.doi.org/10.1016/j.corsci. 2009.05.006. Keysight-Technologies, Impedance Measurement Handbook. A Guide to Measurement Technology and Techniques, 6th edition– application note, (2016), pp. 1–140. C. Pérez, A. Collazo, M. Izquierdo, P. Merino, X. Nóvoa, Electrochemical impedance spectroscopy study of the corrosion process on coated galvanized steel in a salt spray fog chamber, Corrosion 56 (2000) 1220–1232, http://dx.doi.org/10.5006/1. 3280510. L. Diguet, M. Keddam, H. Takenouti, H. Mazille, Y. Cètre, Corrosion protection by fluorinated polymers: an electrochemical approach from free-standing foils to coatings, Mater. Sci. Forum 289–292 (1998) 397–409 https://www.scientific.net/ MSF.289-292.397. S. Amand, M. Musiani, M.E. Orazem, N. Pébère, B. Tribollet, V. Vivier, Constantphase-element behavior caused by inhomogeneous water uptake in anti-corrosion coatings, Electrochim. Acta 87 (2013) 693–700, http://dx.doi.org/10.1016/j. electacta.2012.09.061. M. Musiani, M.E. Orazem, N. Pébère, B. Tribollet, V. Vivier, Determination of resistivity profiles in anti-corrosion coatings from constant-phase-element parameters, Prog. Org. Coat. 77 (2014) 2076–2083, http://dx.doi.org/10.1016/j. porgcoat.2013.12.013. B. Díaz, L. Freire, P. Merino, X.R. Nóvoa, M.C. Pérez, Impedance spectroscopy study of saturated mortar samples, Electrochim. Acta 53 (2008) 7549–7555, http://dx. doi.org/10.1016/j.electacta.2007.10.042. B. Díaz, X.R. Nóvoa, B. Puga, V. Vivier, Chloride transport through cementitious membranes using pulsed current, Cem. Concr. Compos. 39 (2013) 18–22, http://dx. doi.org/10.1016/j.cemconcomp.2013.03.018. C. Perez, A. Collazo, M. Izquierdo, P. Merino, X.R. Novoa, Characterisation of the barrier properties of different paint systems. Part I. Experimental set-up and ideal Fickian diffusion, Prog. Org. Coat. 36 (1999) 102–108, http://dx.doi.org/10.1016/ S0300-9440(99)00030-2. A. Amirudin, D. Thierry, Application of electrochemical impedance spectroscopy to study the degradation of polymer-coated metals, Prog. Org. Coat. 26 (1995) 1–28, http://dx.doi.org/10.1016/0300-9440(95)00581-1. B. Fastrup, A. Saarnak, AC impedance of painted and scribed steel panels under atmospheric exposure, Prog. Org. Coat. 16 (1988) 277–290, http://dx.doi.org/10. 1016/0033-0655(88)80021-9. C. Pérez, A. Collazo, M. Izquierdo, P. Merino, X.R. Nóvoa, Characterisation of the barrier properties of different paint systemsPart II. Non-ideal diffusion and water uptake kinetics, Prog. Org. Coat. 37 (1999) 169–177, http://dx.doi.org/10.1016/ S0300-9440(99)00073-9. B. Díaz, X.R. Nóvoa, B. Puga, V. Vivier, Macro and micro aspects of the transport of chlorides in cementitious membranes, Electrochim. Acta 124 (2014) 61–68, http:// dx.doi.org/10.1016/j.electacta.2013.08.105. S. Duval, M. Keddam, M. Sfaira, A. Srhiri, H. Takenouti, Electrochemical impedance spectroscopy of epoxy-vinyl coating in aqueous medium analyzed by dipolar relaxation of polymer, J. Electrochem. Soc. 149 (2002) B520, http://dx.doi.org/10. 1149/1.1512667.