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E.I.S. characterization of protective coatings on aluminium alloys
Electrochimica Acta 44 (1999) 4259±4267
E.I.S. characterization of protective coatings on aluminium alloys K. Bonnel, C. Le Pen, N. PeÂbeÁre* Laboratoire de Cristallochimie, ReÂactivite et Protection des MateÂriaux, UPRESA CNRS 5071, E.N.S.C.T, 118 Route de Narbonne, 31077, Toulouse, cedex 4, France Received 7 August 1998; received in revised form 28 January 1999
Abstract A traditional organic solvent coating and a water-borne coating associated or not with a chromate conversion treatment on aluminium alloys, were characterized by Electrochemical Impedance Spectroscopy in order to study their anti-corrosive properties. It was shown that the impedance diagrams are representative both of the coating and of the chromate treatment. From the results obtained for the organic coating alone, it was concluded that the surface treatment masks the response of the paint. Comparison of the results obtained for the two coatings revealed that the water-borne system confers better barrier properties than the traditional system. Whatever the coating thickness tested, the capacitive behaviour observed for all the systems in the low-frequency domain of the impedance diagrams was attributed to the action of the chromate pigments at the metal interface, which impedes the corrosion of the aluminium alloys. This result was corroborated by measurements carried out for a system in which the chromate pigments were replaced by neutral ®llers. # 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Aluminium alloys; EIS; Organic coatings; Chromate pigments
1. Introduction The organic coatings currently used in the aerospace industry present very high corrosion protection. However, regulations for environment and human health protection have led to the development of lowtoxicity paints and nowadays, water-borne paints have gained notable importance. Painting planes requires surface treatments to improve the adherence of the coating. The conversion treatment used on aluminium alloys consists of anodization in a chromic bath. The oxide layers obtained
* Corresponding author.
have a thickness of between 2 and 3 mm. The oxide ®lms are generally considered to consist of a thin nonporous layer of oxide (barrier layer) covered with a thick porous layer. Sealing the pores signi®cantly improves the corrosion resistance of the system. But, due to a high hydration level, the sealing leads to the decohesion of the anodization layer, as shown by adherence tests. It should be pointed out that these surface treatments will soon be subject to modi®cations for safety and economic reasons. When the anodized layer is not sealed, corrosion protection is essentially aorded by the organic coating which contains inhibitive pigments, generally chromates. The adherence of primers to aluminium anodized in chromic baths is excellent.
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Electrochemical impedance spectroscopy (EIS) has been used for a long time to characterize the behaviour of coated aluminium alloys immersed in aggressive solutions [1±3] and also for the characterization of barrier and porous anodic ®lms on aluminium alloys [4±9] and the sealing of the porous ®lm [10±12]. This paper describes an impedance study carried out for two coated aluminium alloys (aluminium 1050 and aluminium 2024). The surface treatment (anodization in chromic bath) was studied to separate the contribution of the paint and that of the oxide layer to the corrosion resistance of the samples. A water-borne primer was tested by comparison to the traditional organic solvent coating. The aim of this study is to obtain information about the organic coating properties and, more particularly, barrier properties for the transport of aggressive species (water, oxygen, salts). The reactivity of the interface was investigated by taking into account the action of the chromates included in the ®lm. From EIS measurements carried out for the two coatings, the ®nal purpose will be to characterize the anti-corrosive behaviour of the traditional paint and to predict if the water-borne paint will be able to replace it. 2. Experimental Aluminium 2024 T3 and 2024 T3 P (1050 plated on 2024) were anodised in a chromic bath by an industrial process. When the conversion treatment was not applied, the surface was only degreased by methyl ethyl ketone (MEK). Samples of 10 15 cm were covered with two dierent coatings: a traditional organic solvent-based coating (a polyurethane) and a water-borne coating (an epoxy). The solvents used in the polyurethane coatings were a mixture of ketones and aromatic esters and in the case of the epoxy coating, it was only water. The polyurethane coatings were 20 2 2 mm and 30 2 3 mm thick. The water-borne coatings were 24 2 2 mm and 42 2 4 mm thick. The coatings contained chromate pigments (20% weight). The chromate particles occurred as needleshaped crystals about 2 mm in length. For the water-borne coating, a system without chromates was tested. The chromates were replaced by ®llers (20% weight) that are neutral with respect to corrosion (barium sulfate) with a particle size of around 2.2 mm. The paint was 42 2 4 mm thick. The liquid paints were applied by air spraying. A classical three-electrode cell was used: the working electrode with an exposed area of 24 cm2, the saturated calomel reference electrode (SCE) and a platinum auxiliary electrode. The test solution was a 0.5 M NaCl solution. The
Fig. 1. Bode plots for Al 2024 with a chromate conversion treatment for various immersion times in a 0.5 M NaCl solution. A = 24 cm2.
electrochemical impedance measurements were performed using a Solartron 1250 Frequency Response Analyser over a frequency range of 65 kHz to several mHz and a Solartron 1287 Electrochemical interface. For each system, three samples were tested in parallel.
3. Results and discussion The conversion treatment alone was characterized for Al 2024 and Al 1050 (2024 P). Figs 1 and 2 give the impedance spectra in Bode coordinates for dierent immersion times for Al 2024 and Al 1050 respectively. The results obtained for the two samples are similar to the spectra reported by dierent authors on the same type of alloys [4,11]. For short immersion times capacitive behaviour is observed. Due to the high conductivity of the electrolyte solution inside the pores, the porous layer is not detected. For Al 2024 (Fig. 1), a second time constant progressively appears as the immersion time increases and the polarization resistance decreases. As proposed by Mansfeld [4], these
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Fig. 2. As Fig. 1 for Al 1050.
Fig. 4. As Fig. 3 for Al 1050 (*, w, X: results of three independent trials).
Fig. 3. Complex plane plots for Al 2024 with chromate conversion treatment and polyurethane coating (20 mm thick) for two exposure times. A = 24 cm2 (*, w, X: results of three independent trials).
changes are attributed to an attack of the porous layer as well as the barrier layer. For Al 1050 (Fig. 2), no signi®cant change was observed in the impedance diagrams during the two months of immersion. It can be noted that for this system, the capacitive behaviour is more marked and the phase angle measured on the diagram is 808 instead of 868 for the Al 2024. This dierence can be related to the barrier properties of the anodic ®lm introduced by the microstructure of these two materials, based on the presence of copper-rich precipitates in the 2024 alloy. In relation with these results, Brown et al. [13] have shown by transmission electron microscopy that chromate conversion coatings on very high purity aluminium are relatively uniform compared to those observed on lower purity specimens. So they con®rmed that conversion coating growth on very high purity aluminium is considerably dierent due to the signi®cant reduction, or absence, of preferential cathodic sites associated with impurity segregation within the metal substrate.
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The results show the good protection achieved by the anodic layer for Al 1050 (2024 P). In Fig. 3, the impedance diagrams are presented for two immersion times for Al 2024 samples with chromate conversion treatment and polyurethane coating (for each system three measurements are reported). Good reproducibility was noted. Initially, given a short immersion time, a depressed semi-circle can be observed. From this type of diagram, it is possible to determine a resistance value which usually characterizes the barrier properties of the coating. For longer immersion times, the size of this loop decreases and a second loop starts to appear at low frequencies (a few mHz). The aggressive solution reaches the metal substrate and, as can be seen in Fig. 3, the diagrams are modi®ed after 30 days of immersion in comparison with the initial diagram: the high-frequency (HF) part is assumed to represent the electrolytic resistance inside the pores of the coating and the low-frequency (LF) part to represent the reactions occurring at the bottom of the pores [14]. The decrease of the size of the capacitive loop results from the formation of conductive paths due to the penetration of corrosion species and water into the coating [14]. For this coating, new pores are created due to the lixiviation of chromate. This is evidenced by a yellow-green coloration of the NaCl solution during the ®rst month of immersion. The LF part re¯ects the presence of the conversion coating under the paint at the bottom of the pores. After 30 days of immersion and for 5 months, the size of the capacitive loop stays relatively constant as well as the low-frequency part of the diagram (the diagrams are not reported in the paper). These observations indicate high protection of the system with exposure time. In Fig. 4, the impedance diagrams are given for Al 1050 with the same conversion treatment and the same coating. Compared to Fig. 3, the diagrams are dierent: the ®rst loop is less well de®ned and a capacitive behaviour is observed in the LF range after 30 days of immersion. The diagrams are slowly modi®ed with the immersion time. A slight shift of the frequencies is observed when the immersion time increases. The diagrams obtained for the two alloys present similar patterns and are superimposable only over a given frequency range: 65 kHz to a few Hz. The medium and the low-frequency parts are modi®ed depending on the alloy. These results clearly evidence the role of the metal support. The impedance diagrams are representative of both the coating and the chromate conversion treatment. The dierences observed are attributed to the anodization treatment on Al 2024 and Al 1050. As a consequence, the depressed semi-circle observed for the 2024 alloy is not representative of the coating alone. Discrimination between the coating and the surface treatment on the diagrams is not so simple because, as
Fig. 5. Complex plane plots for Al 2024 with chromate conversion treatment and a water-borne coating (24 mm) for two exposure times. A = 24 cm2.
shown in Fig. 2, the surface treatment alone is characterized by a purely capacitive behaviour. In the presence of the coating, the surface treatment in¯uenced the medium frequency part more speci®cally. Thus, it appears critical to propose an equivalent electrical circuit which can model these impedance results. In Fig. 5, we report the impedance diagrams for Al 2024 with the conversion treatment and the waterborne coating. Whatever the immersion time, a capacitive behaviour is obtained (even after two months of immersion). For this paint system, the NaCl solution did not become coloured during the immersion. This indicates that the lixiviation of chromates was not signi®cant and explains how the diagrams did not change with increasing immersion times unlike with those obtained with the polyurethane coating. The results presented in Fig. 5 illustrate that this paint confers a good corrosion resistance to the alloy. As for the polyurethane paint, a signi®cant part of the protection was aorded by the conversion treatment. As a consequence, the remainder of the study focussed on the characterization of the coatings applied to Al 2024 only degreased by MEK. Fig. 6 presents the impedance diagrams, in Bode coordinates, for Al 2024 with the polyurethane coating without surface treatment. Two thicknesses of the coating were tested: 20 mm (Fig. 6a) and 30 mm (Fig. 6b). In both cases, it can be seen that the resistance values were lower than those obtained with the surface treatment plus the coating (Fig. 3). This corroborates the fact that the chromate conversion treatment signi®cantly improves the corrosion resistance of the aluminium alloys and, in addition, shows that the contribution of the organic coating to the impedance
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Fig. 6. Bode plots for Al 2024 with the polyurethane coating without surface treatment for various immersion times. A = 24 cm2; (a) thickness: 20 mm; (b) thickness: 30 mm.
diagram in the presence of an anodization layer is dicult to analyze quantitatively. Whatever the coating thickness, Fig. 6 shows that the modi®cations occurred rapidly, accounting for the degradation of the coating. With increasing immersion times, the high-frequency part decreases whereas the low-frequency limit of the diagrams remains unchanged. For the 20 mm thick coating, the decrease of the high-frequency part was so signi®cant that after 23 days of immersion the HF part disappeared and the value measured corresponds to the electrolytic resistance. Unlike the result obtained with the surface treatment, the HF part of the diagram can be attributed to the properties of the paint so, the decrease of the HF part is related to water and ions penetration into the coating [14,15] and to an increase in the number or/ and in the size of pores, particularly due to chromate lixiviation. The greater the attenuation of the HF part, the higher the porosity of the paint. For the thinner coating, and long immersion times, the porosity was so high that the coating was no longer detected. As previously mentioned, the low-frequency part of
the diagram accounts for the processes occurring at the bottom of the pores. In our case, the LF part was not modi®ed when the immersion times or the thickness increased. Fig. 7 presents the impedance diagrams for two thicknesses of the water-borne coating: 24 mm (Fig. 7a) and 42 mm (Fig. 7b). In a general manner, the diagrams were hardly modi®ed by immersion time or by the thickness of the coating. This appears anomalous because, the total impedance (and in particular the pore resistance) usually shows a clear increase with the coating thickness. As for the polyurethane coating, the low-frequency part of the diagrams is not modi®ed. From the impedance diagrams obtained for the two coatings and for the dierent thicknesses, it is possible to measure the resistance associated to the high-frequency part (RHF) as a function of the immersion time. Whatever the coating, this time constant is very well de®ned. In Fig. 8, the values of RHF are reported. For the polyurethane coating (Fig. 8a), the value of RHF decreases quickly during the ®rst 10 days of immersion to reach a constant value for increasing immersion times. The ®gure clearly evidences the role
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Fig. 7. As Fig. 6 for the water-borne coating; (a) thickness: 24 mm; (b) thickness: 42 mm.
of the thickness on the value of RHF. Initially, the value of RHF is ten times higher for a 30 mm thick layer than for a layer of 20 mm. After 30 days of immersion in 0.5 M NaCl solution, the dierence is more marked and the RHF value is a hundred times higher for the larger thickness. This result is related to the increase of the number of pores in the coating. In a previous work [16], we showed that pigments create heterogeneties in the coating compared to a clear-coat system and facilitate water uptake. Here, an additional process occurred due to chromate lixiviation which creates new pores in the coating. Thus a paint deposited with a low thickness will be more sensitive to this phenomenon because with a thin layer, the chromate pigments will leave the binder easily. The results observed after 30 days of immersion account well for the dierence of chromate lixiviation as a function of the thickness of the coating. For the water-borne coating (Fig. 8b), an original behaviour of RHF was observed. During the ®rst ®ve days, the value increases and then it remains relatively constant as the immersion time continues to increase. In addition, an increase of the coating thickness does not signi®cantly modify the value of RHF. An evalu-
ation of the capacitance of the coatings from the HF loop by the expression C=(2pfcRHF)ÿ1 (RHF is the diameter of the loop and fc the frequency at the top of the semi-circle) shows that, for both systems, the value of the capacitance is similar (around 40±60 nF). This allows us to conclude that the value of RHF for the water-borne paint, like the polyurethane paint, is related to the barrier properties of the coating. The observed increase of RHF for the water-borne system for short immersion times is not clear. Recently, Spengler et al. [17] tested four low-toxicity paints (two of them were water-borne coatings). The measurements carried out for the water-borne paint exposed in the ®eld for six months present an increase of impedance by comparison with the initial diagram. The authors proposed that: `the increase in impedance can be due to a complementary process in the ®lm formation favored by an external exposition, that improves the coalescence among particles'. This behaviour appears to be linked to water-borne systems and needs further investigation. From the values and the evolution of RHF with immersion time it is shown that the water-borne system presents better barrier properties than the poly-
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Fig. 8. Evolution of RHF with immersion times for: (a) polyurethane coating; (b) water-borne coating.
urethane system. These measurements, carried out for the various coating thicknesses, suggest that the value of RHF is not directly proportional to the coating thickness and that the more the system is porous, the more the total impedance (and in particular the RHF) will be sensitive to an increase of the coating thickness. This observation is in agreement with the results presented by Bonora et al. [15] for ¯uoropolymer-coated mild steel where the total impedance increases strongly when the thickness is 60 mm compared to 30 mm. Now, it is interesting to consider the LF part of the diagrams, which characterizes the behaviour of the interface in terms of anticorrosive properties, in relation to the presence of inhibitive pigments in the coating. It is known [18±20] that corrosion inhibition by chromates is due to the reduction of CrOÿ 4 species by aluminium to form an oxide ®lm. This oxide is not very soluble in aqueous solution and thus provides a barrier layer. Skerry et al. [21] have shown that chromates incorporated in a coating confer good protection to the system. These authors have studied the interface between a 2014 aluminium alloy and an organic ®lm (epoxy-amine system) containing chromate or not by Transmission Electron Microscopy. They showed the
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Fig. 9. Bode plots for Al 2024 with the water-borne coating with neutral pigments for various immersion times (42 mm thick). A = 24 cm2.
presence of a thin ®lm (about 30 nm) which develops when chromate is present in the coating. The thickness of the ®lm remained constant during the exposure time (between 250 and 550 h). By energy dispersive analysis of X-rays, they showed that chromium is incorporated into the surface ®lm. In the absence of chromate, voluminous hydrated alumina was seen to be formed during early stages of immersion. By taking into account these mechanisms of inhibition by chromate, the capacitive behaviour, particularly visible for the polyurethane system (Fig. 6), can be attributed to the presence of the thin oxide layer (acting like a passive layer) at the metal interface which impedes the corrosion of the aluminium alloy. Measurements carried out for increasing immersion times have shown that the behaviour in the LF range remains unchanged. This can be associated to the stability of the oxide layer with time as observed by Skerry et al. [21]. Comparison of the polyurethane and the water-borne systems shows that, for a given coating thickness (Figs. 6a and 7a), the corrosion resistance is obviously the same. But, if we taken into account that the barrier eect is greater for the water-borne system, it can be
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concluded that this system could favourably replace the traditional organic solvent coating. Industrial standard corrosion tests have also evidenced that the water-borne system is highly protective. The system also presents better adherence properties than the conventional one. In order to con®rm the action of chromates, impedance measurements were eected with a water-borne paint containing neutral ®llers (barium sulfate). The diagrams are reported in Fig. 9 for dierent immersion times. This ®gure clearly reveals the role of the ®llers on the barrier properties of the coating (HF part of the diagram) and on the corrosion resistance of the system (LF part of the diagram). As for the system with chromate pigments, it is observed that the RHF values increase with increasing immersion time. Thus, the increase of RHF values for short immersion times is not due to the nature of the ®llers but to the binder. In the presence of chromates, the values are a hundred times higher than in the presence of neutral ®llers. This result is in agreement with previous ®ndings [16]. We have studied the eect of ®llers on the barrier properties of a polyurethane-based ®lm deposited on galvanised steel by coupling electrochemical impedance spectroscopy and a thermostimulated current method. It was shown that the pigments interact with the binder resulting in hardening of the polymer. The pigmented systems (with or without chromates) are subject to more interactions than the clear-coat (from a microstructural viewpoint); these interactions lead to an increase of the proportion of the rigid phase in the coating. Chromates enhance the hardening of the organic coating by comparison with neutral ®llers. This has been explained by the high polarity of chromates that enhances strong interactions with the binder. This electrostatic structuration might be responsible for a global decrease of the diusion of water molecules and ions into the ®lm. In this study, the same behaviour is observed: chromates improve the structure of the coating. In the low-frequency part, the capacitive behaviour is no longer observed. Here, a second capacitive loop is observed with a low resistance value. This corroborates the attribution of the capacitive behaviour to the inhibitive action of chromates at the metal surface. 4. Conclusion The evaluation of the corrosion performance of two paints (a traditional organic solvent coating and a water-borne coating) deposited on aluminium alloys was investigated by EIS. The measurements carried out on the chromate conversion treatment alone and with the coating deposited on this treatment showed ®rstly the role of the metal
support: the results diered between anodization on Al 2024 and Al 1050, and secondly, the response of the systems does not allow the contribution of the oxide layer to the corrosion resistance of the systems to be separated from that of the paint. Nevertheless, very high protection was achieved by both paints deposited on the anodic layer. The impedance diagrams obtained for the two coatings applied to a simply degreased aluminium surface showed that for the traditional organic coating, the resistance of the ®lm decreased as the exposure time increases in agreement with an increase of the number of pores. Conversely, for the water-borne coating, this parameter increased for short immersion times without any explanation actually being found. For both systems investigated, the results underline the signi®cant role of chromates at the metal interface. The challenge is now to replace chromates which are toxic and subject to legislative restrictions.
Acknowledgements This work was carried out in the framework of the `Laboratoire ReÂgional pour l'AmeÂlioration des MateÂriaux Structuraux pour l'AeÂronautique' with the ®nancial support of the ReÂgion Midi-PyreÂneÂes. The authors gratefully acknowledge the Society MAPAERO (Pamiers, France) for the preparation of the coated samples.
References [1] L. Beaunier, I. Epelboin, J.C. Lestrade, H. Takenouti, Surf. Technol. 4 (1976) 237. [2] F. Mansfeld, M.W. Kending, S. Tsai, Corrosion 38 (1982) 478. [3] S. Lin, H. Shih, F. Mansfeld, Corr. Sci. 33 (1992) 1331. [4] F. Mansfeld, M.W. Kendig, J. Electrochem. Soc. 135 (1988) 828. [5] J.B. Bessone, D.R. Salinas, C.E. Mayer, M. Ebert, W.J. Lorenz, Electrochim. Acta 37 (1992) 2283. [6] J. De Laet, J. Scheers, H. Terryn, J. Vereecken, Electrochim. Acta 38 (1993) 2103. [7] J. Bessone, C.E. Mayer, K. Juttner, W.J. Lorenz, Electrochim. Acta 28 (1983) 171. [8] B. Van der Linden, H. Terryn, J. Vereecken, J. Appl. Electrochem. 20 (1990) 798. [9] A. Trichet, P. Gimenez, M. Keddam, Electrochim. Acta 38 (1993) 1957. [10] N. Celati, M.C. Sainte Catherine, M. Keddam, H. Takenouti, Materials Science Forum 192 (194) (1995) 335. [11] O.E. Barcia, J.L. Camara, O.R. Mattos, J. Appl. Electrochem. 17 (1987) 641. [12] M. Koda, H. Takalashi, M. Nagayama, in: 34th
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[13] [14] [15] [16] [17]
Meeting of the International Society of Electrochemists. Erlangen, West Germany, Paper 803, September, 1983. G.M. Brown, K. Shimizu, K. Kobayashi, G.E. Thompson, G.C. Wood, Corr. Sci. 34 (1993) 1045. F. Mansfeld, J. Appl. Electrochem. 25 (1995) 187. P.L. Bonora, F. De¯orian, L. Fedrizzi, Electrochim. Acta 7/8 (1996) 1073. C. Cor®as, N. PeÂbeÁre, C. Lacabanne, Corr. Sci. (in press) (1999). E. Spengler, F.L. Fragata, I.C.P. Margarit, O.R. Mattos, Prog. Org. Coatings 30 (1997) 51.
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[18] H. Leidheiser Jr, J. Coat. Technol. 53 (1981) 29. [19] M.W. Kendig, A.J. Davenport, H.S. Isaacs, Corr. Sci. 34 (1993) 41. [20] M. Koudelkova, J. Augustynski, H. Bertou, J. Electrochem. Soc. 124 (1977) 1165. [21] B.S. Skerry, G.E. Thompson, G.C. Wood, V. Ashworth, J.B. Johnson, J.D. Scantlebury, V.C.R. McLoughlin, Proceedings of the 6th European Symposium on Corrosion Inhibitors (6SEIC) Annali dell'universita' di Ferrara, 1985, Suppl. N. 8, 521.
E.I.S. characterization of protective coatings on aluminium alloys K. Bonnel, C. Le Pen, N. PeÂbeÁre* Laboratoire de Cristallochimie, ReÂactivite et Protection des MateÂriaux, UPRESA CNRS 5071, E.N.S.C.T, 118 Route de Narbonne, 31077, Toulouse, cedex 4, France Received 7 August 1998; received in revised form 28 January 1999
Abstract A traditional organic solvent coating and a water-borne coating associated or not with a chromate conversion treatment on aluminium alloys, were characterized by Electrochemical Impedance Spectroscopy in order to study their anti-corrosive properties. It was shown that the impedance diagrams are representative both of the coating and of the chromate treatment. From the results obtained for the organic coating alone, it was concluded that the surface treatment masks the response of the paint. Comparison of the results obtained for the two coatings revealed that the water-borne system confers better barrier properties than the traditional system. Whatever the coating thickness tested, the capacitive behaviour observed for all the systems in the low-frequency domain of the impedance diagrams was attributed to the action of the chromate pigments at the metal interface, which impedes the corrosion of the aluminium alloys. This result was corroborated by measurements carried out for a system in which the chromate pigments were replaced by neutral ®llers. # 1999 Published by Elsevier Science Ltd. All rights reserved. Keywords: Aluminium alloys; EIS; Organic coatings; Chromate pigments
1. Introduction The organic coatings currently used in the aerospace industry present very high corrosion protection. However, regulations for environment and human health protection have led to the development of lowtoxicity paints and nowadays, water-borne paints have gained notable importance. Painting planes requires surface treatments to improve the adherence of the coating. The conversion treatment used on aluminium alloys consists of anodization in a chromic bath. The oxide layers obtained
* Corresponding author.
have a thickness of between 2 and 3 mm. The oxide ®lms are generally considered to consist of a thin nonporous layer of oxide (barrier layer) covered with a thick porous layer. Sealing the pores signi®cantly improves the corrosion resistance of the system. But, due to a high hydration level, the sealing leads to the decohesion of the anodization layer, as shown by adherence tests. It should be pointed out that these surface treatments will soon be subject to modi®cations for safety and economic reasons. When the anodized layer is not sealed, corrosion protection is essentially aorded by the organic coating which contains inhibitive pigments, generally chromates. The adherence of primers to aluminium anodized in chromic baths is excellent.
0013-4686/99/$ - see front matter # 1999 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 1 4 1 - 3
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K. Bonnel et al. / Electrochimica Acta 44 (1999) 4259±4267
Electrochemical impedance spectroscopy (EIS) has been used for a long time to characterize the behaviour of coated aluminium alloys immersed in aggressive solutions [1±3] and also for the characterization of barrier and porous anodic ®lms on aluminium alloys [4±9] and the sealing of the porous ®lm [10±12]. This paper describes an impedance study carried out for two coated aluminium alloys (aluminium 1050 and aluminium 2024). The surface treatment (anodization in chromic bath) was studied to separate the contribution of the paint and that of the oxide layer to the corrosion resistance of the samples. A water-borne primer was tested by comparison to the traditional organic solvent coating. The aim of this study is to obtain information about the organic coating properties and, more particularly, barrier properties for the transport of aggressive species (water, oxygen, salts). The reactivity of the interface was investigated by taking into account the action of the chromates included in the ®lm. From EIS measurements carried out for the two coatings, the ®nal purpose will be to characterize the anti-corrosive behaviour of the traditional paint and to predict if the water-borne paint will be able to replace it. 2. Experimental Aluminium 2024 T3 and 2024 T3 P (1050 plated on 2024) were anodised in a chromic bath by an industrial process. When the conversion treatment was not applied, the surface was only degreased by methyl ethyl ketone (MEK). Samples of 10 15 cm were covered with two dierent coatings: a traditional organic solvent-based coating (a polyurethane) and a water-borne coating (an epoxy). The solvents used in the polyurethane coatings were a mixture of ketones and aromatic esters and in the case of the epoxy coating, it was only water. The polyurethane coatings were 20 2 2 mm and 30 2 3 mm thick. The water-borne coatings were 24 2 2 mm and 42 2 4 mm thick. The coatings contained chromate pigments (20% weight). The chromate particles occurred as needleshaped crystals about 2 mm in length. For the water-borne coating, a system without chromates was tested. The chromates were replaced by ®llers (20% weight) that are neutral with respect to corrosion (barium sulfate) with a particle size of around 2.2 mm. The paint was 42 2 4 mm thick. The liquid paints were applied by air spraying. A classical three-electrode cell was used: the working electrode with an exposed area of 24 cm2, the saturated calomel reference electrode (SCE) and a platinum auxiliary electrode. The test solution was a 0.5 M NaCl solution. The
Fig. 1. Bode plots for Al 2024 with a chromate conversion treatment for various immersion times in a 0.5 M NaCl solution. A = 24 cm2.
electrochemical impedance measurements were performed using a Solartron 1250 Frequency Response Analyser over a frequency range of 65 kHz to several mHz and a Solartron 1287 Electrochemical interface. For each system, three samples were tested in parallel.
3. Results and discussion The conversion treatment alone was characterized for Al 2024 and Al 1050 (2024 P). Figs 1 and 2 give the impedance spectra in Bode coordinates for dierent immersion times for Al 2024 and Al 1050 respectively. The results obtained for the two samples are similar to the spectra reported by dierent authors on the same type of alloys [4,11]. For short immersion times capacitive behaviour is observed. Due to the high conductivity of the electrolyte solution inside the pores, the porous layer is not detected. For Al 2024 (Fig. 1), a second time constant progressively appears as the immersion time increases and the polarization resistance decreases. As proposed by Mansfeld [4], these
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Fig. 2. As Fig. 1 for Al 1050.
Fig. 4. As Fig. 3 for Al 1050 (*, w, X: results of three independent trials).
Fig. 3. Complex plane plots for Al 2024 with chromate conversion treatment and polyurethane coating (20 mm thick) for two exposure times. A = 24 cm2 (*, w, X: results of three independent trials).
changes are attributed to an attack of the porous layer as well as the barrier layer. For Al 1050 (Fig. 2), no signi®cant change was observed in the impedance diagrams during the two months of immersion. It can be noted that for this system, the capacitive behaviour is more marked and the phase angle measured on the diagram is 808 instead of 868 for the Al 2024. This dierence can be related to the barrier properties of the anodic ®lm introduced by the microstructure of these two materials, based on the presence of copper-rich precipitates in the 2024 alloy. In relation with these results, Brown et al. [13] have shown by transmission electron microscopy that chromate conversion coatings on very high purity aluminium are relatively uniform compared to those observed on lower purity specimens. So they con®rmed that conversion coating growth on very high purity aluminium is considerably dierent due to the signi®cant reduction, or absence, of preferential cathodic sites associated with impurity segregation within the metal substrate.
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The results show the good protection achieved by the anodic layer for Al 1050 (2024 P). In Fig. 3, the impedance diagrams are presented for two immersion times for Al 2024 samples with chromate conversion treatment and polyurethane coating (for each system three measurements are reported). Good reproducibility was noted. Initially, given a short immersion time, a depressed semi-circle can be observed. From this type of diagram, it is possible to determine a resistance value which usually characterizes the barrier properties of the coating. For longer immersion times, the size of this loop decreases and a second loop starts to appear at low frequencies (a few mHz). The aggressive solution reaches the metal substrate and, as can be seen in Fig. 3, the diagrams are modi®ed after 30 days of immersion in comparison with the initial diagram: the high-frequency (HF) part is assumed to represent the electrolytic resistance inside the pores of the coating and the low-frequency (LF) part to represent the reactions occurring at the bottom of the pores [14]. The decrease of the size of the capacitive loop results from the formation of conductive paths due to the penetration of corrosion species and water into the coating [14]. For this coating, new pores are created due to the lixiviation of chromate. This is evidenced by a yellow-green coloration of the NaCl solution during the ®rst month of immersion. The LF part re¯ects the presence of the conversion coating under the paint at the bottom of the pores. After 30 days of immersion and for 5 months, the size of the capacitive loop stays relatively constant as well as the low-frequency part of the diagram (the diagrams are not reported in the paper). These observations indicate high protection of the system with exposure time. In Fig. 4, the impedance diagrams are given for Al 1050 with the same conversion treatment and the same coating. Compared to Fig. 3, the diagrams are dierent: the ®rst loop is less well de®ned and a capacitive behaviour is observed in the LF range after 30 days of immersion. The diagrams are slowly modi®ed with the immersion time. A slight shift of the frequencies is observed when the immersion time increases. The diagrams obtained for the two alloys present similar patterns and are superimposable only over a given frequency range: 65 kHz to a few Hz. The medium and the low-frequency parts are modi®ed depending on the alloy. These results clearly evidence the role of the metal support. The impedance diagrams are representative of both the coating and the chromate conversion treatment. The dierences observed are attributed to the anodization treatment on Al 2024 and Al 1050. As a consequence, the depressed semi-circle observed for the 2024 alloy is not representative of the coating alone. Discrimination between the coating and the surface treatment on the diagrams is not so simple because, as
Fig. 5. Complex plane plots for Al 2024 with chromate conversion treatment and a water-borne coating (24 mm) for two exposure times. A = 24 cm2.
shown in Fig. 2, the surface treatment alone is characterized by a purely capacitive behaviour. In the presence of the coating, the surface treatment in¯uenced the medium frequency part more speci®cally. Thus, it appears critical to propose an equivalent electrical circuit which can model these impedance results. In Fig. 5, we report the impedance diagrams for Al 2024 with the conversion treatment and the waterborne coating. Whatever the immersion time, a capacitive behaviour is obtained (even after two months of immersion). For this paint system, the NaCl solution did not become coloured during the immersion. This indicates that the lixiviation of chromates was not signi®cant and explains how the diagrams did not change with increasing immersion times unlike with those obtained with the polyurethane coating. The results presented in Fig. 5 illustrate that this paint confers a good corrosion resistance to the alloy. As for the polyurethane paint, a signi®cant part of the protection was aorded by the conversion treatment. As a consequence, the remainder of the study focussed on the characterization of the coatings applied to Al 2024 only degreased by MEK. Fig. 6 presents the impedance diagrams, in Bode coordinates, for Al 2024 with the polyurethane coating without surface treatment. Two thicknesses of the coating were tested: 20 mm (Fig. 6a) and 30 mm (Fig. 6b). In both cases, it can be seen that the resistance values were lower than those obtained with the surface treatment plus the coating (Fig. 3). This corroborates the fact that the chromate conversion treatment signi®cantly improves the corrosion resistance of the aluminium alloys and, in addition, shows that the contribution of the organic coating to the impedance
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Fig. 6. Bode plots for Al 2024 with the polyurethane coating without surface treatment for various immersion times. A = 24 cm2; (a) thickness: 20 mm; (b) thickness: 30 mm.
diagram in the presence of an anodization layer is dicult to analyze quantitatively. Whatever the coating thickness, Fig. 6 shows that the modi®cations occurred rapidly, accounting for the degradation of the coating. With increasing immersion times, the high-frequency part decreases whereas the low-frequency limit of the diagrams remains unchanged. For the 20 mm thick coating, the decrease of the high-frequency part was so signi®cant that after 23 days of immersion the HF part disappeared and the value measured corresponds to the electrolytic resistance. Unlike the result obtained with the surface treatment, the HF part of the diagram can be attributed to the properties of the paint so, the decrease of the HF part is related to water and ions penetration into the coating [14,15] and to an increase in the number or/ and in the size of pores, particularly due to chromate lixiviation. The greater the attenuation of the HF part, the higher the porosity of the paint. For the thinner coating, and long immersion times, the porosity was so high that the coating was no longer detected. As previously mentioned, the low-frequency part of
the diagram accounts for the processes occurring at the bottom of the pores. In our case, the LF part was not modi®ed when the immersion times or the thickness increased. Fig. 7 presents the impedance diagrams for two thicknesses of the water-borne coating: 24 mm (Fig. 7a) and 42 mm (Fig. 7b). In a general manner, the diagrams were hardly modi®ed by immersion time or by the thickness of the coating. This appears anomalous because, the total impedance (and in particular the pore resistance) usually shows a clear increase with the coating thickness. As for the polyurethane coating, the low-frequency part of the diagrams is not modi®ed. From the impedance diagrams obtained for the two coatings and for the dierent thicknesses, it is possible to measure the resistance associated to the high-frequency part (RHF) as a function of the immersion time. Whatever the coating, this time constant is very well de®ned. In Fig. 8, the values of RHF are reported. For the polyurethane coating (Fig. 8a), the value of RHF decreases quickly during the ®rst 10 days of immersion to reach a constant value for increasing immersion times. The ®gure clearly evidences the role
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Fig. 7. As Fig. 6 for the water-borne coating; (a) thickness: 24 mm; (b) thickness: 42 mm.
of the thickness on the value of RHF. Initially, the value of RHF is ten times higher for a 30 mm thick layer than for a layer of 20 mm. After 30 days of immersion in 0.5 M NaCl solution, the dierence is more marked and the RHF value is a hundred times higher for the larger thickness. This result is related to the increase of the number of pores in the coating. In a previous work [16], we showed that pigments create heterogeneties in the coating compared to a clear-coat system and facilitate water uptake. Here, an additional process occurred due to chromate lixiviation which creates new pores in the coating. Thus a paint deposited with a low thickness will be more sensitive to this phenomenon because with a thin layer, the chromate pigments will leave the binder easily. The results observed after 30 days of immersion account well for the dierence of chromate lixiviation as a function of the thickness of the coating. For the water-borne coating (Fig. 8b), an original behaviour of RHF was observed. During the ®rst ®ve days, the value increases and then it remains relatively constant as the immersion time continues to increase. In addition, an increase of the coating thickness does not signi®cantly modify the value of RHF. An evalu-
ation of the capacitance of the coatings from the HF loop by the expression C=(2pfcRHF)ÿ1 (RHF is the diameter of the loop and fc the frequency at the top of the semi-circle) shows that, for both systems, the value of the capacitance is similar (around 40±60 nF). This allows us to conclude that the value of RHF for the water-borne paint, like the polyurethane paint, is related to the barrier properties of the coating. The observed increase of RHF for the water-borne system for short immersion times is not clear. Recently, Spengler et al. [17] tested four low-toxicity paints (two of them were water-borne coatings). The measurements carried out for the water-borne paint exposed in the ®eld for six months present an increase of impedance by comparison with the initial diagram. The authors proposed that: `the increase in impedance can be due to a complementary process in the ®lm formation favored by an external exposition, that improves the coalescence among particles'. This behaviour appears to be linked to water-borne systems and needs further investigation. From the values and the evolution of RHF with immersion time it is shown that the water-borne system presents better barrier properties than the poly-
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Fig. 8. Evolution of RHF with immersion times for: (a) polyurethane coating; (b) water-borne coating.
urethane system. These measurements, carried out for the various coating thicknesses, suggest that the value of RHF is not directly proportional to the coating thickness and that the more the system is porous, the more the total impedance (and in particular the RHF) will be sensitive to an increase of the coating thickness. This observation is in agreement with the results presented by Bonora et al. [15] for ¯uoropolymer-coated mild steel where the total impedance increases strongly when the thickness is 60 mm compared to 30 mm. Now, it is interesting to consider the LF part of the diagrams, which characterizes the behaviour of the interface in terms of anticorrosive properties, in relation to the presence of inhibitive pigments in the coating. It is known [18±20] that corrosion inhibition by chromates is due to the reduction of CrOÿ 4 species by aluminium to form an oxide ®lm. This oxide is not very soluble in aqueous solution and thus provides a barrier layer. Skerry et al. [21] have shown that chromates incorporated in a coating confer good protection to the system. These authors have studied the interface between a 2014 aluminium alloy and an organic ®lm (epoxy-amine system) containing chromate or not by Transmission Electron Microscopy. They showed the
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Fig. 9. Bode plots for Al 2024 with the water-borne coating with neutral pigments for various immersion times (42 mm thick). A = 24 cm2.
presence of a thin ®lm (about 30 nm) which develops when chromate is present in the coating. The thickness of the ®lm remained constant during the exposure time (between 250 and 550 h). By energy dispersive analysis of X-rays, they showed that chromium is incorporated into the surface ®lm. In the absence of chromate, voluminous hydrated alumina was seen to be formed during early stages of immersion. By taking into account these mechanisms of inhibition by chromate, the capacitive behaviour, particularly visible for the polyurethane system (Fig. 6), can be attributed to the presence of the thin oxide layer (acting like a passive layer) at the metal interface which impedes the corrosion of the aluminium alloy. Measurements carried out for increasing immersion times have shown that the behaviour in the LF range remains unchanged. This can be associated to the stability of the oxide layer with time as observed by Skerry et al. [21]. Comparison of the polyurethane and the water-borne systems shows that, for a given coating thickness (Figs. 6a and 7a), the corrosion resistance is obviously the same. But, if we taken into account that the barrier eect is greater for the water-borne system, it can be
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concluded that this system could favourably replace the traditional organic solvent coating. Industrial standard corrosion tests have also evidenced that the water-borne system is highly protective. The system also presents better adherence properties than the conventional one. In order to con®rm the action of chromates, impedance measurements were eected with a water-borne paint containing neutral ®llers (barium sulfate). The diagrams are reported in Fig. 9 for dierent immersion times. This ®gure clearly reveals the role of the ®llers on the barrier properties of the coating (HF part of the diagram) and on the corrosion resistance of the system (LF part of the diagram). As for the system with chromate pigments, it is observed that the RHF values increase with increasing immersion time. Thus, the increase of RHF values for short immersion times is not due to the nature of the ®llers but to the binder. In the presence of chromates, the values are a hundred times higher than in the presence of neutral ®llers. This result is in agreement with previous ®ndings [16]. We have studied the eect of ®llers on the barrier properties of a polyurethane-based ®lm deposited on galvanised steel by coupling electrochemical impedance spectroscopy and a thermostimulated current method. It was shown that the pigments interact with the binder resulting in hardening of the polymer. The pigmented systems (with or without chromates) are subject to more interactions than the clear-coat (from a microstructural viewpoint); these interactions lead to an increase of the proportion of the rigid phase in the coating. Chromates enhance the hardening of the organic coating by comparison with neutral ®llers. This has been explained by the high polarity of chromates that enhances strong interactions with the binder. This electrostatic structuration might be responsible for a global decrease of the diusion of water molecules and ions into the ®lm. In this study, the same behaviour is observed: chromates improve the structure of the coating. In the low-frequency part, the capacitive behaviour is no longer observed. Here, a second capacitive loop is observed with a low resistance value. This corroborates the attribution of the capacitive behaviour to the inhibitive action of chromates at the metal surface. 4. Conclusion The evaluation of the corrosion performance of two paints (a traditional organic solvent coating and a water-borne coating) deposited on aluminium alloys was investigated by EIS. The measurements carried out on the chromate conversion treatment alone and with the coating deposited on this treatment showed ®rstly the role of the metal
support: the results diered between anodization on Al 2024 and Al 1050, and secondly, the response of the systems does not allow the contribution of the oxide layer to the corrosion resistance of the systems to be separated from that of the paint. Nevertheless, very high protection was achieved by both paints deposited on the anodic layer. The impedance diagrams obtained for the two coatings applied to a simply degreased aluminium surface showed that for the traditional organic coating, the resistance of the ®lm decreased as the exposure time increases in agreement with an increase of the number of pores. Conversely, for the water-borne coating, this parameter increased for short immersion times without any explanation actually being found. For both systems investigated, the results underline the signi®cant role of chromates at the metal interface. The challenge is now to replace chromates which are toxic and subject to legislative restrictions.
Acknowledgements This work was carried out in the framework of the `Laboratoire ReÂgional pour l'AmeÂlioration des MateÂriaux Structuraux pour l'AeÂronautique' with the ®nancial support of the ReÂgion Midi-PyreÂneÂes. The authors gratefully acknowledge the Society MAPAERO (Pamiers, France) for the preparation of the coated samples.
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