On the application of electrochemical impedance spectroscopy to study the self-healing properties of protective coatings

Electrochemistry Communications 9 (2007) 2622 2628 www.elsevier.com/locate/elecom On the application of electrochemical impedance spectroscopy to study the self-healing properties of protective coatings M.L. Zheludkevich a, *, K.A. Yasakau a, A.C. Bastos a, O.V. Karavai a,b, M.G.S. Ferreira a a Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal b Chemistry Department, Belarusian State University, 220050 Minsk, Belarus Received 27 July 2007; received in revised form 7 August 2007; accepted 15 August 2007 Available online 19 August 2007 Abstract Active corrosion protection based on self-healing of defects in coatings is a vital issue for development of new advanced corrosion protection systems. However, there is a significant lack of experimental protocols, which can be routinely used to reveal the self-healing ability and to study the active corrosion protection properties of organic and hybrid coatings. The present work demonstrates the possibility to use EIS (electrochemical impedance spectroscopy) for investigation of the self-healing properties of protective coatings applied on a metal surface. The model EIS experiments supported by SVET (scanning vibrating electrode technique) measurements show that an increase of low frequency impedance during immersion in the corrosive medium is related to the suppression of active corrosion processes and healing of the corroded areas. Thus, EIS can effectively be employed as a routine method to study the self-repair properties of different protective systems. The 2024 aluminium alloy coated with hybrid sol gel film was used as a model system to study the healing of artificial defects by an organic inhibitor (8-hydroxyquinoline). Ó 2007 Elsevier B.V. All rights reserved. Keywords: EIS; SVET; Corrosion; Self-healing; Inhibitor 1. Introduction * Corresponding author. Tel.: +351 234 370255; fax: +351 234 378146. E-mail address: mzheludkevich@ua.pt (M.L. Zheludkevich). The necessity to eliminate hexavalent chromium from corrosion protection systems offered a powerful incentive to the development of new protective coatings, which combine good barrier properties with active corrosion protection ability originated from self-healing of corroded areas. Different strategies have recently been used to impart an active protection component to different coatings. In some cases inhibitors can be directly added to the film [1 3]. However, addition of active inhibiting species to the coating formulation very often leads to degradation of barrier properties of the coating and deactivation of the corrosion inhibitors [4,5]. Different encapsulation strategies using nanoporous layers [6], ion-exchangers [7] and nanocontainers [5,8] are suggested to avoid these negative effects and to obtain a corrosion protection system with good barrier properties and effective self-healing mechanism. Various experimental techniques are used to study corrosion protection performance of protective coatings. Accelerated corrosion tests are very effective to compare the corrosion protection performance of different coatings. However, they do not give any information on the mechanisms of protection. Improvement of protective properties due to a better adhesion or due to a self-healing ability, for example, cannot be differentiated. A conventional DC polarization method is usually not applicable for coated substrates due to important limitations arising from a high IR drop. This method cannot give any useful information on the active corrosion protection and the self-healing ability of protective coatings. Electrochemical impedance spectroscopy (EIS) is widely used to characterize the corrosion protection performance of coating systems. This powerful technique allows not only a comparison between 1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.08.012

M.L. Zheludkevich et al. / Electrochemistry Communications 9 (2007) 2622 2628 2623 performances of different systems but also can give important information on kinetics of evolution of the coating degradation and the corrosion activity during immersion in the corrosive media. The high frequency part of the spectrum is usually related to the barrier properties of the coating, while the low frequency segment reflects the corrosion activity on the metal surface [9]. Several authors, for different corrosion protection systems containing active anticorrosive components, found that the low frequency impedance can increase during immersion [10 13]. Sometimes this increase follows a drop of the impedance caused by a breakdown of the coating with subsequent increased corrosion activity [5,6,14,15]. However, there is no direct confirmation that the increase of the low frequency impedance is related to the active corrosion protection provided by the inhibiting species released from the coating. The term self-healing can be interpreted differently. The classical understanding of self-healing is based on complete recovery of the functionalities of the coating due to real healing of the defect. However, the hindering of the corrosion activity in the defect by the coating itself employing any mechanisms can be also considered as self-healing. Since, the corrosion protective system recovers its main function, namely the corrosion protection, after being damaged. In this work, the second definition of self-healing based on the active corrosion protection originated from inhibiting species is used. In this work, we tried to design a series of model experiments to correlate the increase of the low frequency impedance during immersion with the self-healing conferred by a corrosion inhibitor. A corrosion inhibitor was added, at different stages of the corrosion process, to the electrolyte where aluminium alloy coated with a hybrid sol gel film was immersed. SVET (scanning vibrating electrode technique) was also employed to confirm the EIS results on micro-level demonstrating the effect of organic inhibitors on the local corrosion activity in artificial defects. 2. Experimental 2.1. Model coating system A hybrid protective film on AA2024 was prepared using controllable sol gel route as reported elsewhere [6]. The only difference in this study was that titanium propoxide was used instead of zirconium propoxide. The hybrid sol gel formulation was applied to the substrate by dipcoating with following curing for 1 h at 120 C. The 8-hydroxyquinoline was employed here as an effective corrosion inhibitor for AA2024 [16]. 2.2. Experimental techniques The EIS measurements were carried out at room temperature in a Faraday cage using Gamry FAS2 Femtostat with PCI4 Controller during immersion in 0.5 and 0.05 M NaCl solutions. A three-electrode arrangement, consisting of a saturated calomel reference electrode, a platinum foil as a counter electrode and the exposed sample as a working electrode with a surface area of 3.4 cm 2, was used. The impedance measurements were performed at an open circuit potential with applied 10 mv sinusoidal perturbations in the frequency range of 10 2 10 5 Hz with 10 steps per decade. The SVET measurements were carried out in different NaCl solutions using an apparatus manufactured by Applicable Electronics Inc. (USA), controlled by the ASET software developed by ScienceWares Inc. (USA) and employed micro-electrodes of MicroProbes Inc. (USA). SVET measures potential differences in the solution due to ionic fluxes that arise from electrochemical reactions occurring in the corroding metal surface. The measured potential differences, DV, can be related to the ionic currents, I, that originate them by proper calibration. The micro-electrode had a spherical black platinum tip of 10 lm in diameter and vibrated in two directions (normal and parallel to the surface) with 20 lm amplitude and average distance of 200 lm above the sample. At each point the system waited 0.2 s before the measurement and the sampling time was 0.4 s. All data were recorded by ASET on PC. Experimental results are presented in the form of 2D maps of ionic currents. The maps were generated by QuikGrid (freeware, www.perspectiveedge.com) using data from the electric field normal to the surface. Positive (red) values correspond to the anodic activity and negative values (blue) to the cathodic activity. The artificial scratches were created by a sharp metallic needle (tip diameter about 50 lm) under constant load. 3. Results and discussion 3.1. EIS measurements Recent results obtained on AA2024 coated with doublelayer hybrid sol gel system containing an organic corrosion inhibitor (benzotriazole) demonstrated very interesting behavior of the impedance spectra during immersion [6]. An evolution of the impedance spectra for one of the samples with benzotriazole-containing interlayer is presented in Fig. 1. The Bode plot for the coated alloy before defect formation has three distinguishable time-dependent processes. The time constant at high frequencies is related to the capacitive response of the sol gel film. The resistive plateau at 10 2 10 3 Hz represents the pore resistance of this sol gel film. The relaxation process at about 1 10 Hz is attributed to the capacitance of the intermediate oxide layer present at the metal/coating interface. The resistance of such interlayer, which is situated at lower frequencies, is very important from the corrosion protection standpoint, since it is the last barrier for the corrosive species before reaching the metal surface. The third time constant is weakly defined at lowest frequencies and is related to the corrosion activity.

2624 M.L. Zheludkevich et al. / Electrochemistry Communications 9 (2007) 2622 2628 Fig. 1. Impedance spectra during immersion in 0.5 NaCl for inhibitor containing double-layer sol gel film (a) and undoped single-layer sol gel film (b) before and after defect formation. A fast drop of impedance at low frequencies occurs when an artificial defect is created in the coating. This drop is related to the breakdown of the oxide film and the corrosion activity started on the naked metal surface. However, after only a relatively short period of time (20 min) the impedance starts rising again towards the initial values (Fig. 1a). In the case of undoped single-layer sol gel film the impedance at low frequencies after defect formation tends to continue the decrease for long immersion without any signs of recovery (Fig. 1b). The decrease of Z mod (impedance modulus) is caused by the permanent development of corrosion processes in the defect zone. Moreover, the third low frequency time constant related to the corrosion activity becomes well defined. After longer immersion time the low frequency part seems to have diffusion control since the phase angle becomes close to 45. The diffusion limitations arise most likely due to formation of layer of the corrosion products on the corroded zone. These products are well visible on the scratches after long immersion (not shown). It is clear that the recovery of the impedance is somehow related to the fact that the corrosion inhibitor is added to the coating since the undoped film does not show this effect. We suggested in our previous works that the increase of low frequency impedance in the case of inhibitor-containing coating is related to the partial recovery of the oxide film and to the suppression of the corrosion activity in the defect. However, no evident confirmation of this suggestion was presented at that time. The following experiments were carried out in order to confirm that the increase of the low frequency impedance during the immersion of the defected coating is caused by the presence of the corrosion inhibitor released from the film. One of the samples was immersed in 0.5 M NaCl solution for one day and then an artificial scratch was created on the surface by a sharp needle. The immersion was continued using the 0.5 M NaCl electrolyte doped with 0.05 mm 8-hydroxyquinoline. The 8-hydroxyquinoline was recently demonstrated as an effective corrosion inhibitor for AA2024 [16]. EIS spectra were obtained during the immersion before and after the defect formation. The Bode plot of the intact sample after one day of immersion in pure chloride electrolyte shows only two time constants without any signs of corrosion activity at low frequencies (Fig. 2a). When the defect is induced in the coating and the electrolyte is doped with the inhibitor no noticeable changes occur in the spectrum at low frequencies. Only the pore resistance of the sol gel film increases continuously. One possible explanation for this is formation of insoluble complex compounds with 8-hydoxyquinoline which act as a poreblocker. These products can sufficiently decrease the mobility of ions in the nano-pores of sol gel film. The low frequency impedance has the same value as before the

M.L. Zheludkevich et al. / Electrochemistry Communications 9 (2007) 2622 2628 2625 Fig. 2. Impedance spectra of undoped sol gel coated alloy during immersion in 0.5 M NaCl + 0.05 mm 8-hydroxyquinoline after defect formation (a) and in undoped 0.05 M NaCl for two days after defect formation and in the following four days in electrolyte containing 0.05 M NaCl + 0.05 mm 8- hydroxyquinoline (b). The plots before the defect formation are also shown. scratch formation even after four days of immersion in concentrated chloride solution. The third low frequency time constant also did not appear on the spectrum demonstrating that the addition of even very low concentrations of 8-hydroxyquinoline passivates the metal surface completely hindering any corrosion activities in the scratch. Thus, this confirms that the presence of an inhibitor in the electrolyte can lead to high values of low frequency impedance due to the effective healing of the defect. The presented experiment models a situation when a corrosion inhibitor is released from the coating in the place where the crack is induced. However, the release of the inhibitor is not instantaneous and its concentration in the defect can reach remarkable values only after a certain induction time [17]. Therefore, the low frequency impedance drops even in the case of an inhibitor-doped coating (Fig. 1a). The next experiment was designed to simulate this situation more convincingly. The sol gel coated alloy was tested by EIS after one day dipping in a pure 0.05 M NaCl solution (Fig. 2b). After this a defect was created on the surface. The immersion was continued for two days more in the same electrolyte and the impedance spectrum was recorded. Lower concentration of chlorides was used in this test in order to decrease the rate of the corrosion processes since the inhibitor cannot significantly influence a deeply localized attack which can arise after two days immersion in a concentrated NaCl. The impedance values at low frequencies decrease for about 4 times and a well defined third time constant appears on the spectrum. After this the chloride solution was doped by a 0.05 mm 8- hydroxyquinoline. The addition of the inhibitor leads to a slight increase of the impedance and a depression of the third time constant. This effect becomes even more evident when four days after the addition of the inhibitor have passed. This experiment undoubtedly confirms that the increase of the low frequency impedance is related to the healing of the defect by the corrosion inhibiting species. The concentration of inhibitor can also play significant role on the effectiveness of active corrosion protection. Therefore the experiments using different concentration of inhibitor are necessary to deeply simulate the release/ inhibition processes. However it was not a main objective of the present short communication. 3.2. SVET results The scanning vibrating electrode technique was additionally used here to investigate the self-healing processes on the micro-level providing supplemental information for the EIS experiments and reinforcing the conclusions

2626 M.L. Zheludkevich et al. / Electrochemistry Communications 9 (2007) 2622 2628 made above. Three specimens of AA2024 coated by undoped sol gel films and with artificial defects were tested by SVET in the electrolytes described above. Fig. 3 presents the SVET results for the sample immersed in an undoped 0.5 M chloride solution. An artificial scratch was made on the hybrid coating by a sharp metallic needle as shown in Fig. 3a. The measured distribution of the local currents along the surface is demonstrating the starting of corrosion activity after only 1 h of immersion. The anodic process is already well defined in the defect zone (Fig. 3b). Two anodic and two cathodic zones become visible on the scratch after one day showing active Fig. 3. Optical photo shows the artificial scratch (a) and the SVET maps taken in the same zone of AA2024 coated with undoped sol gel film after 1 h (b) and one day (c) of immersion in pure 0.5 M NaCl. Fig. 4. Optical photo shows the artificial scratch (a), the SVET maps taken in the same zone of AA2024 coated with undoped sol gel film after 1 h (b) and two days (c) of immersion in 0.5 M NaCl electrolyte doped with 0.1 mm 8-hydroxyquinoline.

M.L. Zheludkevich et al. / Electrochemistry Communications 9 (2007) 2622 2628 2627 corrosion processes (Fig. 3c). This behavior explains the decrease of the low frequency impedance after the defect formation in the undoped NaCl solution (Fig. 1b). In the case of specimen immersed in the chloride solution doped by the 8-hydroxyquinoline the situation is completely different. After 1 h of immersion there is only a first small sign of corrosion activity (Fig. 4b) which is completely suppressed later, as shown in Fig. 4c. The corrosion activity is not revealed after two days of immersion since the defect is healed by the inhibitor present in the electrolyte. It explains the reason why the impedance spectra did not show any drop of the impedance at low frequencies when the defected sample is immersed in the inhibitor-containing electrolyte starting from the beginning (Fig. 2a). The results presented in Fig. 5 correspond to a sample immersed one day after the scratching in an undoped 0.5 M NaCl and then one day more in the same electrolyte doped with a 0.1 mm 8-hydroxyquinoline. These measurements correspond to the conditions of the EIS experiment presented in Fig. 2b. During the first day of immersion once the scratch is created the anodic corrosion activity is continuously growing from 1.6 la/cm 2 after 1 h to about 20 la/cm 2 after one day of testing (Fig. 5b and c). This corresponds to the remarkable drop of the impedance on Fig. 5. Optical photo shows the artificial scratch (a), the SVET maps taken in the corresponding zone of AA2024 coated with undoped sol gel film after 1 h (b) and one day (c) of immersion in pure 0.5 M NaCl electrolyte. The electrolyte was changed to 0.5 M NaCl + 0.1 mm 8-hydroxyquinoline and the SVET maps were then obtained after 1 h (d) and one day (e).

2628 M.L. Zheludkevich et al. / Electrochemistry Communications 9 (2007) 2622 2628 the Bode plot taken after two days (Fig. 2b). The change of the electrolyte to the doped one leads to a slight increase of the corrosion activity especially in the cathodic region (Fig. 5d). This increase can be related to the increased concentration of oxygen in the fresh electrolyte favoring the cathodic reduction of oxygen. However, one day later the corrosion activity lowers at least twice showing decrease of the activity in the scratched area (Fig. 5e). This reduction of the currents related to the active anticorrosion effect of the inhibitor explains the increase of the low frequency impedance in the case of Fig. 2b. However, the corrosion activity is not hindered completely leading to only partial reduction of the impedance without reaching the initial values. The presented SVET results clearly confirm on microlevel the conclusions made from the EIS macro-level tests. It is shown that the decrease of the low frequency impedance after the defect formation is related with the disruption of the coating and the oxide barriers and the following localized corrosion activity in the defected area. The addition of the corrosion inhibitor heals the defect leading to lower corrosion currents and increased impedance. These facts strongly support the suggestion that similar processes occur in the case when the inhibitor is added to the coating. The active substance is released after the defect formation leading to its healing and causing the increase of the low frequency impedance as it is demonstrated in the Fig. 1a and several recent works [5,6,14,15]. The SVET experiments made on the inhibitor-doped twolayer film also demonstrate noticeable delay of the corrosion as reported elsewhere [6]. Thus the experiments reported here provide a clear answer to the question if the increase of the low frequency impedance can be correlated to the self-healing properties of the protective coatings doped with active inhibiting species. 4. Conclusions Electrochemical impedance spectroscopy was demonstrated here as a useful technique for the study of selfhealing processes in protective coatings on metallic substrates. The increase of the low frequency impedance during immersion can be correlated to the active corrosion protection originating the self-repair of defects. The SVET results obtained at micro-level are in good accordance to the EIS measurements. The decrease of the low frequency impedance after the defect formation is related with the disruption of the coating and the oxide barriers and therefore localized corrosion activity in the defect area. The addition of corrosion inhibitor heals the defect leading to lower corrosion currents and increased impedance. References [1] N.N. Voevodin, N.T. Grebasch, W.S. Soto, F.E. Arnold, M.S. Donley, Surf. Coat. Technol. 140 (2001) 24. [2] A.N. Khramov, N.N. Voevodin, V.N. Balbyshev, M.S. Donley, Thin Solid Films 447 (2004) 549. [3] M. Quinet, B. Neveu, V. Moutarlier, P. Audebert, L. Ricq, Prog. Org. Coat. 58 (2007) 46. [4] M. Garcia-Heras, A. Jimenez-Morales, B. Casal, J.C. Galvan, S. Radzki, M.A. Villegas, J. Alloys Compd. 380 (2004) 219. [5] D.G. Shchukin, M.L. Zheludkevich, K.A. Yasakau, S.V. Lamaka, H. Möhwald, M.G.S. Ferreira, Adv. Mater. 18 (2006) 1672. [6] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, R. Serra, S.K. Poznyak, M.G.S. Ferreira, Prog. Org. Coat. 58 (2007) 127. [7] R.G. Buchheit, H. Guan, S. Mahajanam, F. Wong, Prog. Org. Coat. 47 (2003) 174. [8] H. Yang, W.J. van Ooij, Prog. Org. Coat. 50 (2004) 149. [9] M.L. Zheludkevich, R. Serra, M.F. Montemor, K.A. Yasakau, I.M. Miranda Salvado, M.G.S. Ferreira, Electrochim. Acta 51 (2005) 208. [10] A.A.O. Magalhaes, I.C.P. Margarit, O.R. Mattos, Electrochim. Acta 44 (1999) 4281. [11] D. Zhu, W.J. van Ooij, Corros. Sci. 45 (2003) 2177. [12] A. Frignani, F. Zucchi, G. Trabanelli, V. Grassi, Corros. Sci. 48 (2006) 2258. [13] W. Trabelsi, P. Cecilio, M.G.S. Ferreira, M.F. Montemor, Prog. Org. Coat. 54 (2005) 276. [14] M.L. Zheludkevich, D.G. Shchukin, K.A. Yasakau, H. Möhwald, M.G.S. Ferreira, Chem. Mater. 19 (2007) 402. [15] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, M.F. Montemor, P. Cecílio, M.G.S. Ferreira, Electrochem. Commun. 8 (2006) 421. [16] S.V. Lamaka, M.L. Zheludkevich, K.A. Yasakau, M.F. Montemor, M.G.S. Ferreira, Electrochim. Acta 52 (2007) 7231. [17] H. Wang, F. Presuel, R.G. Kelly, Electrochim. Acta 49 (2004) 239.