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The evaluation of anticorrosive automotive epoxy coatings by means of electrochemical impedance spectroscopy
Progress in Organic Coatings 46 (2003) 121–129
The evaluation of anticorrosive automotive epoxy coatings by means of electrochemical impedance spectroscopy J.J. Suay∗ , M.T. Rodr´ıguez, K.A. Razzaq, J.J. Carpio, J.J. Saura Technology Department, Materials Science and Matallurgical Engineering, Jaume I University, Campus Riu Sec s/n, 12071 Castellón, Spain
Abstract Different epoxy primer coatings for metal substrates were investigated by electrochemical impedance spectroscopy (EIS). The variables were the substrate (cold rolled steel, phosphatised cold rolled steel and phosphatised electrozincated cold rolled steel), the epoxy primer (containing a Pb anticorrosive pigment and non-toxic paint containing Al and Fe oxides) and curing temperature (low temperature 160 ◦ C and high temperature 180 ◦ C). The primers showed excellent behaviour when applied on phosphatised steel and very poor electrochemical characteristics on phosphatised electrozincated steel. The non-toxic primer performed better than the Pb coating on phosphatised steel and electrozincated steel while it performed in a similar way to Pb primer on steel. The curing temperature had a big effect on the primer behaviour. The Pb coating performed better when it was cured at a low temperature. The coat adhesion to the substrate was tested by cathodic disbonding. It was found that the primers have very good adhesion to phosphatised steel and poor adhesion to phosphatised electrozincated steel. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Epoxy primer; Electrochemical impedance spectroscopy; Phosphatised steel
1. Introduction The corrosion protection of automobiles is a problem of great economic importance, both from a national point of view and for the private car owner. Rusting of steel sheet that starts at an interior surface of a body panel and penetrates the sheet and eventually shows through as rust at the exterior exposed surface is known as perforation corrosion. The term cosmetic corrosion is applied to attack which starts at the exterior surface, usually at regions where the paint film is damaged [1]. The concern of automobile manufacturers for corrosion protection may be summed up by the General Motors’ 10-5-2 target, which stands for 10 years protection for perforation, 5 years from cosmetic corrosion and 2 years for engine compartment corrosion. Such targets are achieved mainly by the use of cationic epoxy coatings applied to phosphatised steel or a phosphatised zinc layer applied on steel. Foreign objects damage caused by stone impact is a major concern to the automobile industry. Manifestation of impact damage can either be physical loss of paint which is a cosmetic issue or can result in delamination at the metal–polymer boundary. Delamination at the metal– ∗ Corresponding author. E-mail address: [email protected] (J.J. Suay).
polymer boundary coupled to ingress of water, oxygen and ions leads to corrosion beneath the coating layers (impact induced corrosion). This type of corrosion may potentially lead to perforation which is a concern to the automobile manufacturer from a extended warranty point of view. Various strategies to minimise stone impact damage are currently being pursued by both the automobile manufacturer and coating suppliers. One of this strategies is to use pre-coated sheet steels such as electrogalvanised, hot dipped galvanised and galvannealed substrates. The use of pre-coated sheet steels is a major step undertaken to eliminate the problem of corrosion and perforation [2,3]. According to a recent report on the European automotive industry [4], on average, 40% of the body of an automobile is zinc-coated, of which a third is hot-dipped and the remaining two-thirds are electrogalvanised. Plastics now constitute about 10–12% of the body, 25% of the manufacturing cost of the body shell is attributed to corrosion protection. A body weighing 300 kg includes 22 kg of corrosion protection materials in addition to the weight of the zinc which amounts to 2.5 kg on a 50% galvanised body. The mechanism by which an organic coating protects a metallic material from the environment is a complex process, which is not well understood. Many properties of polymers (processability, curing conditions, application conditions on
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substrate, electric, chemical, thermal, mechanical properties and environmental stability) affect their suitability and reliability as protective organic coatings. A survey of the literature reveals that the protection offered by an organic coating can be ascribed to one of the following mechanisms: (I) depression of the anodic and/or cathodic reaction; (II) introduction of a high electrical resistance into the circuit of the corrosion cell; and (III) as a barrier to aggressive species (oxygen, water and ions) [1,5–9]. At present, for the diagnosis and evaluation of the coating films, several techniques can be used: (I) an external visual observation test; (II) colour and gloss tests of coating films; (III) the adhesion test; (IV) mechanical tests of abrasion-resistance and by stress–strain curve; (V) the accelerated corrosion tests; (VI) the weather resistance test; and (VII) the electrochemical tests [6]. Various electrochemical techniques have been used for evaluating the performance of organic coating/metal systems. The application of electrochemical impedance spectroscopy (EIS) to coated metals has resulted in new information concerning their degradation in corrosive environments. EIS has been shown to be a useful technique in the study of the corrosion performance of polymer-coated metals in recent years [1,5–15]. However, much less research has been done and published on the use of EIS to study the effect of pre-treatment on the entire system of metal/pre-treatment/coating. [8]. The aim of this work is to investigate the corrosive protective properties of two kinds of epoxy coatings (cured at different temperatures and with same curing time), electrodeposited on three types of substrate (steel, phosphatised steel and phosphatised electrozincated steel). One of the coatings contained Pb as an active anticorrosive pigment while the other was toxic pigment free, containing Fe and Al oxides as anticorrosive pigments. An equivalent electrical circuit (Fig. 1) [16–18] was fitted for experimental EIS results and its parameters used in order to discuss anticorrosive characteristics while coat-substrate adhesion was studied by the cathodic disbonding method [19]. The corrosive degradation of the coated metal was evaluated in two main ways: (I) the barrier behaviour is characterised by the impedance measurements; and (II) the corrosion induced disbonding is simulated by the cathodic disbonding tests.
Fig. 1. Schematic figure of the equivalent circuit used for the coated samples.
2. Experimental details 2.1. Materials The commercial anticorrosive epoxy coatings were cationically deposited on three types of substrate: steel, phosphatised steel and phosphatised electrozincated steel. The conditions of the cationic process, sample identification, coating and substrate type are summarised in Table 1. The test panels (20 mm × 20 mm × 0.25 mm) were pre-treated by mechanical cleaning (polishing), degreasing in an acetone solution and rinsing with distilled water. Coatings were deposited galvanostatically at different current densities and a fixed time (as referred in Table 1) in order to obtain the thickness of 20 m. Anodes were placed parallel to the working electrode (steel panel) at a distance of 1.5 cm. The temperature was under control during the whole cationic process. Other cationic conditions applied were: an anode/cathode difference of 1:4, a bath temperature of 28 ◦ C and an application time of 2 15 . When the painted samples were removed from the cationic bath, they were cured for 10 min at two different temperatures (a low temperature 160 ◦ C and a high temperature 180 ◦ C) as Table 1 shows. One of the coatings used Pb as an anticorrosive pigment while the other was toxic pigment free. 2.2. Testing methods and equipment EIS measurements were carried out on coated samples exposed to 3.5% NaCl in distilled water, for periods up to
Table 1 Substrate, coating and painting conditions of the different samples tested Sample A Substrate Steel
B
C
D
E
F
G
H
I
Steel
Steel
Phosphatised steel
Phosphatised steel
Phosphatised steel
Phosphatised electrozincated steel Pb paint 90 180
Phosphatised electrozincated steel Non-toxic paint 70 180
Phosphatised electrozincated steel Pb paint 70 160
Coating Pb paint Non-toxic paint Pb paint Pb paint Voltage 140 120 170 190 Ta cured 180 180 160 180
Non-toxic paint Pb paint 155 180 180 160
J.J. Suay et al. / Progress in Organic Coatings 46 (2003) 121–129
6 months. The three electrode electrochemical cells were obtained by sticking a glass cylinder on the sample sheet and filling it with the test solution. The exposed surface area was 16.6 cm2 . A carbon sheet counter electrode and an Ag/AgCl saturated calomel as a reference electrode were used. The a.c. impedance data was obtained at the free corrosion potential using an AUTOLAB PSTAT 30 potentiostat and frequency response analyser. The impedance measurements in all cases were carried out over a frequency range of 100 kHz down to 1 mHz using 10 mV amplitude of sinusoidal voltage in a Faraday cage in order to minimise the external interference on the system. The impedance spectra were analysed using Zplot software. The equivalent circuit model, shown in Fig. 1, was employed to analyse the EIS spectra. The circuit consists of a working electrode (metal substrate), a reference electrode, electrolyte resistance Rs , pore resistance Rpo , coating capacitance Cc , polarisation resistance Rp and double layer capacitance Cdl . Fitting the EIS data to the circuit allowed to determine the values of the elements in the equivalent circuit. The tolerance of the evaluated data was usually below 0.1%. The cathodic disbonding test involved scribing the sample through the paint down to the bare metal. The scribe marks were made at right angles to form a 2 cm × 2 cm cross. An electrochemical cell, filled with 3.5% NaCl, aqueous solution, was then placed over the cross and a voltage of −0.59 V (cathodic polarisation) vs. Ag/AgCl reference electrode was applied for typically 24 h in order to accelerate the reduction of water-dissolved oxygen at cathodic sites to form hydroxide ion [14]. After the cathodic treatment, adhesive tape was applied over the coating in the vicinity of the cross and pulled to reveal the extent that the cathodic polarisation produces disbondment. Determination of the extent of disbondment for five samples usually shows good reproducibility. Measurements were made of the average dimension of the disbond away from the scribe in units of mm. The value dx/dt is calculated by dividing this dimension by the time
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of the test in hours, usually 24 h. Note that no observed disbonding defines a default value of 10−4 mm/h.
3. Results 3.1. Electrochemical impedance spectroscopy Direct experimental data are presented in Figs. 2–4 for samples A, D and G (Pb paint cured at 180 ◦ C at different substrates). At the beginning of the test, the behaviour was capacitive for the three substrates. As the exposure time to the electrolyte increases, a new time constant (represented by a second semicircle in a Nyquist plot) appeared in a few days for the electrozincated sample, in 100 days for the steel substrate and up to 264 for phosphatised steel. The values of the resistances and capacitances obtained from the equivalent circuit for samples A, B and C (steel substrate with different primers), are presented in Figs. 5–7. The following trend is observed in the results: • Sample B (non-toxic primer) always shows Rpo and Rp resistance values which are higher than Pb pigment primers (A, C). • Samples A and C give similar results for the resistance parameters for periods up to 100 days. • Cdl values increase with immersion time for all samples. • Samples A and C have very similar Cdl results. They show Cdl stable values for the first 100 days and both have a significant increase in this parameter almost at the same time (100 days), although the sample cured at low temperature (C) have smaller values for this parameter with time. • The Cdl evolution for the non-toxic primer (B) is absolutely different from the others. The Cdl value starts increasing from the first day and remains almost stable up to 60 days.
Fig. 2. Evolution of the impedance module and phase of sample A, at different immersion times: 7 (—), 34 (- - -) and 100 (· · · ) days.
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Fig. 3. Evolution of the impedance module and phase of sample D, at different immersion times: 7 (—), 104 (- - -) and 266 (· · · ) days.
Fig. 4. Evolution of the impedance module and phase of sample G, at different immersion times: 1 (—), 3 (- - -) and 15 (· · · ) days.
Fig. 5. Time dependence of pore resistance (Rpo ) for steel substrate with different primers (sample A (䊐), sample B () and sample C (䊊)), in 3.5% NaCl solution.
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Fig. 6. Time dependence of charge-transfer resistance (Rp ) for steel substrate with different primers (sample A (䊐), sample B () and sample C (䊊)), in 3.5% NaCl solution.
Fig. 7. Time dependence of double layer capacitance (Cdl ) for steel substrate with different primers (sample A (䊐), sample B () and sample C (䊊)), in 3.5% NaCl solution.
Figs. 8–10 show the corresponding EIS results of samples D, E and F (phosphatised steel substrate with different primers). The pore resistance of coating (Rpo ) of the three samples was quite similar for 100 days. The Rpo of sample E (no toxic paint) remained almost constant over all immersion time while the Rpo of sample F started decreasing after 120 days and had a big increase for sample D. In the case of Rp , the three samples showed identical behaviour until 100 days (an initially big decrease and an increase at 80 days). From this time sample D showed a big increase, sample E gave medium Rp values and sample F lower values and behaved in a non-stable manner (showing lower Rp values than other samples up until 200 days when it reached similar Rp values to those of sample D). The Cdl of sample E remained almost stable with time while both the Cdl of Pb paints (D and F) increased with time
(especially sample D, that always showed bigger values than sample F). Figs. 11–13 are, respectively, referred to as Rpo , Rp and Cdl values of coats applied to phosphatised electrozincated steel substrate. The values corresponding to sample G (Pb paint cured at high temperature) are not shown because this system failed in the first few days. Sample I (non-toxic paint) had lower Rpo and Rp values than sample H. Rpo and Rp of both samples decreased very quickly with immersion time. The Cdl values corresponding to sample I (Pb paint cured at low temperature) showed a continuos increase with immersion time while the Cdl values corresponding to samples H (non-toxic paint) remained unstable until 80 days of immersion time. Finally, it has to be pointed out that for the immersion times considered, there were not found any significant changes in Cc values for any system.
Fig. 8. Time dependence of pore resistance (Rpo ) for phosphatised steel substrate with different primers (sample D (䊐), sample E () and sample F (䊊)), in 3.5% NaCl solution.
Fig. 9. Time dependence of charge-transfer resistance (Rp ) for phosphatised steel substrate with different primers (sample D (䊐), sample E () and sample F (䊊)), in 3.5% NaCl solution.
Fig. 10. Time dependence of double layer capacitance (Cdl ) for phosphatised steel substrate with different primers (sample D (䊐), sample E () and sample F (䊊)), in 3.5% NaCl solution.
Fig. 11. Time dependence of pore resistance (Rpo ) for electrozincated phosphatised steel substrate with different primers (sample H () and sample I (䊊)), in 3.5% NaCl solution.
Fig. 12. Time dependence of charge-transfer resistance (Rp ) for electrozincated phosphatised steel substrate with different primers (sample H () and sample I (䊊)), in 3.5% NaCl solution.
Fig. 13. Time dependence of double layer capacitance (Cdl ) for electrozincated phosphatised steel substrate with different primers (sample H () and sample I (䊊)), in 3.5% NaCl solution.
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Table 2 Summary of cathodic disbonding test results Steel
Phosphatised steel
Phosphatised electrozincated steel
Sample
−log(dx/dt) (mm/h)
Sample
−log(dx/dt) (mm/h)
Sample
−log(dx/dt) (mm/h)
A B C
2.5541 2.4772 2.6042
D E F
3.7828 3.1494 3.5064
G H I
0.2324 0.2764 0.2447
3.2. Cathodic disbonding The cathodic disbonding results for all tested and recorded samples are shown in Table 2. Very similar adhesion values were observed for the three coatings applied to the same substrate. It was observed that a higher adherence was given when applying the coating to the phosphatised steel and the lower values were those that obtained with phosphatised electrozincated steel substrate.
4. Discussion 4.1. Electrochemical impedance spectroscopy It is generally assumed that the elements of the equivalent circuits are correlated to the corrosion properties of the system [12]. The pore resistance Rpo , is a measure of the porosity and degradation of the coating. Usually Rpo values has been related to the number of pores or capillary channels perpendicular to the substrate surface through which the electrolyte reaches the interface [13]. Though the Rpo can also increase with immersion time, probably as a result of pore or defect blockage by corrosion products, it usually decreases. Some authors have found three regions in the time dependent decreases of Rpo . The pore resistance initially decreases rapidly, then slowly (exhibiting a plateau) and then again rapidly coinciding with the appearance of the second semi-circle. They explain the plateau by making the assumption that the number of pathways formed is approximately constant with time. Cc is the capacitance of the coating, it should be a measure of the water permeation into the coating and is given by Cc =
εε0 A d
(1)
where ε is the dielectric constant of the coating, ε0 is the permitivity of vacuum, A is the area of the coating and d is the thickness. The coating capacitance will usually change during electrolyte absorption because the dielectric constant of water is approximately 20 times greater than that of a typical coating. Cc usually increases at the initial stage of exposure, and seems to be a measure of water absorption and when the coating has been exposed for long times Cc can be correlated to coating disbonding and degradation.
The polarisation resistance Rp and double layer capacitance Cdl , are two parameters used to specify the disbonding of the top coat and the onset of corrosion at the interface. The specific polarisation resistance is associated with the charge-transfer behaviour of the metal substrate. Rp (like Cdl ) can only be calculated well when at least two time constants are evident in the spectrum. Cdl , the double layer capacitance, is a measure of the area over which coating has disbonded. It can be well measured only at advanced stages of coating deterioration. The trend of Cdl is complex. A change in the Cdl value can be associated with the competition between disbonding and corrosion product accumulation at the interface. The Cdl value increases as water spreads at the interface and the delaminated area extends. On the other hand, the accumulation of the corrosion product at the interface reduces the area of the double layer capacitor, which will lead to a decrease of the Cdl value. Therefore, the change of Cdl may depend on which factor, either the disbonding or the corrosion product accumulation that was more dominant during the corrosion process. However, it should be pointed out that both the increase and the decrease in Cdl are the results of corrosion development at the metal surface and a constant Cdl is an indication of a stable interface [6]. In samples where coating had been applied to steel substrate (A, B and C), it can be seen from the electrochemical parameters calculated that non-toxic paint (B) had fewer pores and was less degraded with the immersion time than samples A and C (Pb coat). The initial increase of Cdl in sample B, indicated that the interface metal-coat is very active initially and probably that a disbonding process takes place there. After 80 days of exposure the Cdl trend is almost constant and it can reflect that the interface has become more and more stable with time which can be interpreted as better corrosion protection. On the other hand, the Pb coatings (A and C) which are only different in the curing temperature that have been applied to them, show quite similar behaviour in all parameters. Nevertheless the sample cured at low temperature (C) showed higher Rp and Rpo values and lower Cdl values than the sample cured at a higher temperature (A), which can be representative of a better performance of the coating (a better corrosion protection of the substrate). A possible explanation can be that a lower curing temperature makes the epoxy coating less bright and makes the interface more
J.J. Suay et al. / Progress in Organic Coatings 46 (2003) 121–129
stable because it has less residual stress and better adhesion to the substrate. Pb coatings showed an increasing Cdl trend which is related to an increase in the disbonding area of the interface, and a big change in this parameters at 80 days indicating a big difference in the protection properties before and after this time. Anticorrosive coat performance was much higher when the epoxy primers were applied to a phosphatised steel substrate than when they were applied to other substrates. It can be clearly seen that the phosphatised-coated interfaces were much more stable than the others (the trend of Rp and Cdl with time were less variable with this substrate than with the others). In the same way the adhesion values to this substrate were the highest too. This effect can be due to a good clamp process between primers and the irregular surface caused by the phosphate process over the steel. The pore resistance for sample D registered an increase with the immersion time. This effect can be a result of a pore or defect blockage by corrosion products. The Cdl value increases for samples D and F can be interpreted by an increase of the delaminated area in the interface (especially in sample D). It has to be pointed out that non-toxic paint (sample E) performed better than Pb paint again. Its Rp and Cdl values remained constant with the immersion times. Furthermore this sample had almost no delamination in the period of time considered. On the other hand, curing temperature had a significant effect on the paints performance. The higher curing temperature made the Pb paint have an unstable interface and gave rise to more delamination than paint cured at low temperature. Perhaps (as it was pointed out before) lower curing temperatures make the epoxy coating less bright and makes the interface more stable because it has less residual stress and better adhesion to the substrate. The paints performed worst when applied to phosphatised electrozincated steel substrate. This can be explained by low adherence (of the coatings of the substrate) and the presence of Cl− in the electrolyte that will make the zinc-phosphatised interface very active. The paint adhesion values to substrate were very low as can be seen in Table 2. The primer degradation process can be clearly seen in the trend of Rp and a Rpo with time, while the increase of Cdl with time can indicate an increase in the delaminate areas. Pb paint cured at high temperature performed very badly. A curing temperature decrease resulted in a better Pb paint performance. The reasons are similar to those pointed out before. Non-toxic paint performance best. It had the highest Rpo and Rp values and the lowest Cdl values. It should also be pointed out that sample I had the lowest delamination area with time. Nevertheless it must be clearly said that in coated phosphatised electrogalvanised steels that have been tested in this study, we have only investigated anticorrosion primers performance and not the system (primer + zinc coated) performance.
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5. Conclusions EIS is an effective tool for studying of the effect of metal pre-treatment on corrosion protection of automobiles. The best properties of paints studied are obtained when applied on phosphatised steel. In this case, it produces a more stable interface. The worst behaviour of paints is obtained in the application to phosphatised electrozincated steel. As regards the coatings, the best behaviour corresponded to the non-toxic paint, followed by the Pb paint cured at low temperature (160 ◦ C) and lastly the Pb paint cured at high temperature (180 ◦ C). An increase in the curing temperature decreases the delamination resistance of the paint. The rate of delamination mainly depends on type of substrate. The best adhesion (paint/substrate) was observed when phosphatised steel substrate was used, while the poorest adhesion corresponded to phosphatised electrozincated steel.
Acknowledgements We thank Ms. S. Gracia Edo, Mr. J.L. Godes Ort´ı, Ms. E. Romero Sales and Mr. J. Ortega Herreros for their help and collaboration. The authors are grateful for the support in this work of Project CICYT 1FD97-0333-C03-03. References [1] A. Amirudin, D. Thierry, Prog. Org. Coat. 28 (1996) 59. [2] A.C. Ramamurthy, W.I. Lorenzen, S.J. Bless, Prog. Org. Coat. 25 (1994) 43. [3] D. Crotty, Met. Finishing (1996) 54. [4] R. Dietz, Proceedings of the Fifth Automotive Corrosion Prevention Conference, SAE, Dearborn, MI, 1991, p. 1. [5] T. Monetta, F. Belluci, L. Nicodemo, L. Nicolais, Prog. Org. Coat. 21 (1993) 353. [6] I. Sekine, Prog. Org. Coat. 31 (1997) 73. [7] V.B. Miiskovic, M.R. Stanic, D.M. Drazic, Prog. Org. Coat. 36 (1999) 53. [8] N. Tang, W.J. Ooij, G. Górecki, Prog. Org. Coat. 30 (1997) 255. [9] V.B. Miskovic, M.D. Maksimovic, Z. Kacarevic, J.B. Zotovic, Prog. Org. Coat. 33 (1998) 68. [10] F. Deflorian, L. Fedrizzi, P.L. Bonora, Electrochem. Acta 44 (1999) 4243. [11] F. Deflorian, V.B. Miskovic, P.L. Bonora, L. Fedrizzi, Corros. Sci. 50 (1994) 439. [12] A. Sabata, W.J. Ooij, R.J. Koch, J. Adhesion Sci. Technol. 7 (1993) 1153. [13] A. Amirudin, D. Thierry, Prog. Org. Coat. 26 (1995) 1. [14] M. Kending, J. Scully, Corrosion 46 (1990) 22. [15] F. Mansfeld, C.H. Tsai, Corrosion 47 (1991) 958. [16] F. Mansfeld, M.W. Kending, S. Tsai, Corrosion 38 (1982) 479. [17] G.W. Walter, Corros. Sci. 26 (1986) 681. [18] F. Mansfeld, C.H. Tsai, H. Shih, in: D. Scantlebury, M. Kendig (Eds.), The Electrochemical Society Softbond Proceedings on Advance in Corrosion Protection by Organic Coatings, PV 89-13, Pennington, NJ, 1989, p. 228. [19] M. Kending, S. Jeanjaquet, R. Brown, F. Thomas, J. Coat. Technol. 68 (1996) 39.
The evaluation of anticorrosive automotive epoxy coatings by means of electrochemical impedance spectroscopy J.J. Suay∗ , M.T. Rodr´ıguez, K.A. Razzaq, J.J. Carpio, J.J. Saura Technology Department, Materials Science and Matallurgical Engineering, Jaume I University, Campus Riu Sec s/n, 12071 Castellón, Spain
Abstract Different epoxy primer coatings for metal substrates were investigated by electrochemical impedance spectroscopy (EIS). The variables were the substrate (cold rolled steel, phosphatised cold rolled steel and phosphatised electrozincated cold rolled steel), the epoxy primer (containing a Pb anticorrosive pigment and non-toxic paint containing Al and Fe oxides) and curing temperature (low temperature 160 ◦ C and high temperature 180 ◦ C). The primers showed excellent behaviour when applied on phosphatised steel and very poor electrochemical characteristics on phosphatised electrozincated steel. The non-toxic primer performed better than the Pb coating on phosphatised steel and electrozincated steel while it performed in a similar way to Pb primer on steel. The curing temperature had a big effect on the primer behaviour. The Pb coating performed better when it was cured at a low temperature. The coat adhesion to the substrate was tested by cathodic disbonding. It was found that the primers have very good adhesion to phosphatised steel and poor adhesion to phosphatised electrozincated steel. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Epoxy primer; Electrochemical impedance spectroscopy; Phosphatised steel
1. Introduction The corrosion protection of automobiles is a problem of great economic importance, both from a national point of view and for the private car owner. Rusting of steel sheet that starts at an interior surface of a body panel and penetrates the sheet and eventually shows through as rust at the exterior exposed surface is known as perforation corrosion. The term cosmetic corrosion is applied to attack which starts at the exterior surface, usually at regions where the paint film is damaged [1]. The concern of automobile manufacturers for corrosion protection may be summed up by the General Motors’ 10-5-2 target, which stands for 10 years protection for perforation, 5 years from cosmetic corrosion and 2 years for engine compartment corrosion. Such targets are achieved mainly by the use of cationic epoxy coatings applied to phosphatised steel or a phosphatised zinc layer applied on steel. Foreign objects damage caused by stone impact is a major concern to the automobile industry. Manifestation of impact damage can either be physical loss of paint which is a cosmetic issue or can result in delamination at the metal–polymer boundary. Delamination at the metal– ∗ Corresponding author. E-mail address: [email protected] (J.J. Suay).
polymer boundary coupled to ingress of water, oxygen and ions leads to corrosion beneath the coating layers (impact induced corrosion). This type of corrosion may potentially lead to perforation which is a concern to the automobile manufacturer from a extended warranty point of view. Various strategies to minimise stone impact damage are currently being pursued by both the automobile manufacturer and coating suppliers. One of this strategies is to use pre-coated sheet steels such as electrogalvanised, hot dipped galvanised and galvannealed substrates. The use of pre-coated sheet steels is a major step undertaken to eliminate the problem of corrosion and perforation [2,3]. According to a recent report on the European automotive industry [4], on average, 40% of the body of an automobile is zinc-coated, of which a third is hot-dipped and the remaining two-thirds are electrogalvanised. Plastics now constitute about 10–12% of the body, 25% of the manufacturing cost of the body shell is attributed to corrosion protection. A body weighing 300 kg includes 22 kg of corrosion protection materials in addition to the weight of the zinc which amounts to 2.5 kg on a 50% galvanised body. The mechanism by which an organic coating protects a metallic material from the environment is a complex process, which is not well understood. Many properties of polymers (processability, curing conditions, application conditions on
0300-9440/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. doi:10.1016/S0300-9440(02)00219-9
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substrate, electric, chemical, thermal, mechanical properties and environmental stability) affect their suitability and reliability as protective organic coatings. A survey of the literature reveals that the protection offered by an organic coating can be ascribed to one of the following mechanisms: (I) depression of the anodic and/or cathodic reaction; (II) introduction of a high electrical resistance into the circuit of the corrosion cell; and (III) as a barrier to aggressive species (oxygen, water and ions) [1,5–9]. At present, for the diagnosis and evaluation of the coating films, several techniques can be used: (I) an external visual observation test; (II) colour and gloss tests of coating films; (III) the adhesion test; (IV) mechanical tests of abrasion-resistance and by stress–strain curve; (V) the accelerated corrosion tests; (VI) the weather resistance test; and (VII) the electrochemical tests [6]. Various electrochemical techniques have been used for evaluating the performance of organic coating/metal systems. The application of electrochemical impedance spectroscopy (EIS) to coated metals has resulted in new information concerning their degradation in corrosive environments. EIS has been shown to be a useful technique in the study of the corrosion performance of polymer-coated metals in recent years [1,5–15]. However, much less research has been done and published on the use of EIS to study the effect of pre-treatment on the entire system of metal/pre-treatment/coating. [8]. The aim of this work is to investigate the corrosive protective properties of two kinds of epoxy coatings (cured at different temperatures and with same curing time), electrodeposited on three types of substrate (steel, phosphatised steel and phosphatised electrozincated steel). One of the coatings contained Pb as an active anticorrosive pigment while the other was toxic pigment free, containing Fe and Al oxides as anticorrosive pigments. An equivalent electrical circuit (Fig. 1) [16–18] was fitted for experimental EIS results and its parameters used in order to discuss anticorrosive characteristics while coat-substrate adhesion was studied by the cathodic disbonding method [19]. The corrosive degradation of the coated metal was evaluated in two main ways: (I) the barrier behaviour is characterised by the impedance measurements; and (II) the corrosion induced disbonding is simulated by the cathodic disbonding tests.
Fig. 1. Schematic figure of the equivalent circuit used for the coated samples.
2. Experimental details 2.1. Materials The commercial anticorrosive epoxy coatings were cationically deposited on three types of substrate: steel, phosphatised steel and phosphatised electrozincated steel. The conditions of the cationic process, sample identification, coating and substrate type are summarised in Table 1. The test panels (20 mm × 20 mm × 0.25 mm) were pre-treated by mechanical cleaning (polishing), degreasing in an acetone solution and rinsing with distilled water. Coatings were deposited galvanostatically at different current densities and a fixed time (as referred in Table 1) in order to obtain the thickness of 20 m. Anodes were placed parallel to the working electrode (steel panel) at a distance of 1.5 cm. The temperature was under control during the whole cationic process. Other cationic conditions applied were: an anode/cathode difference of 1:4, a bath temperature of 28 ◦ C and an application time of 2 15 . When the painted samples were removed from the cationic bath, they were cured for 10 min at two different temperatures (a low temperature 160 ◦ C and a high temperature 180 ◦ C) as Table 1 shows. One of the coatings used Pb as an anticorrosive pigment while the other was toxic pigment free. 2.2. Testing methods and equipment EIS measurements were carried out on coated samples exposed to 3.5% NaCl in distilled water, for periods up to
Table 1 Substrate, coating and painting conditions of the different samples tested Sample A Substrate Steel
B
C
D
E
F
G
H
I
Steel
Steel
Phosphatised steel
Phosphatised steel
Phosphatised steel
Phosphatised electrozincated steel Pb paint 90 180
Phosphatised electrozincated steel Non-toxic paint 70 180
Phosphatised electrozincated steel Pb paint 70 160
Coating Pb paint Non-toxic paint Pb paint Pb paint Voltage 140 120 170 190 Ta cured 180 180 160 180
Non-toxic paint Pb paint 155 180 180 160
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6 months. The three electrode electrochemical cells were obtained by sticking a glass cylinder on the sample sheet and filling it with the test solution. The exposed surface area was 16.6 cm2 . A carbon sheet counter electrode and an Ag/AgCl saturated calomel as a reference electrode were used. The a.c. impedance data was obtained at the free corrosion potential using an AUTOLAB PSTAT 30 potentiostat and frequency response analyser. The impedance measurements in all cases were carried out over a frequency range of 100 kHz down to 1 mHz using 10 mV amplitude of sinusoidal voltage in a Faraday cage in order to minimise the external interference on the system. The impedance spectra were analysed using Zplot software. The equivalent circuit model, shown in Fig. 1, was employed to analyse the EIS spectra. The circuit consists of a working electrode (metal substrate), a reference electrode, electrolyte resistance Rs , pore resistance Rpo , coating capacitance Cc , polarisation resistance Rp and double layer capacitance Cdl . Fitting the EIS data to the circuit allowed to determine the values of the elements in the equivalent circuit. The tolerance of the evaluated data was usually below 0.1%. The cathodic disbonding test involved scribing the sample through the paint down to the bare metal. The scribe marks were made at right angles to form a 2 cm × 2 cm cross. An electrochemical cell, filled with 3.5% NaCl, aqueous solution, was then placed over the cross and a voltage of −0.59 V (cathodic polarisation) vs. Ag/AgCl reference electrode was applied for typically 24 h in order to accelerate the reduction of water-dissolved oxygen at cathodic sites to form hydroxide ion [14]. After the cathodic treatment, adhesive tape was applied over the coating in the vicinity of the cross and pulled to reveal the extent that the cathodic polarisation produces disbondment. Determination of the extent of disbondment for five samples usually shows good reproducibility. Measurements were made of the average dimension of the disbond away from the scribe in units of mm. The value dx/dt is calculated by dividing this dimension by the time
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of the test in hours, usually 24 h. Note that no observed disbonding defines a default value of 10−4 mm/h.
3. Results 3.1. Electrochemical impedance spectroscopy Direct experimental data are presented in Figs. 2–4 for samples A, D and G (Pb paint cured at 180 ◦ C at different substrates). At the beginning of the test, the behaviour was capacitive for the three substrates. As the exposure time to the electrolyte increases, a new time constant (represented by a second semicircle in a Nyquist plot) appeared in a few days for the electrozincated sample, in 100 days for the steel substrate and up to 264 for phosphatised steel. The values of the resistances and capacitances obtained from the equivalent circuit for samples A, B and C (steel substrate with different primers), are presented in Figs. 5–7. The following trend is observed in the results: • Sample B (non-toxic primer) always shows Rpo and Rp resistance values which are higher than Pb pigment primers (A, C). • Samples A and C give similar results for the resistance parameters for periods up to 100 days. • Cdl values increase with immersion time for all samples. • Samples A and C have very similar Cdl results. They show Cdl stable values for the first 100 days and both have a significant increase in this parameter almost at the same time (100 days), although the sample cured at low temperature (C) have smaller values for this parameter with time. • The Cdl evolution for the non-toxic primer (B) is absolutely different from the others. The Cdl value starts increasing from the first day and remains almost stable up to 60 days.
Fig. 2. Evolution of the impedance module and phase of sample A, at different immersion times: 7 (—), 34 (- - -) and 100 (· · · ) days.
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Fig. 3. Evolution of the impedance module and phase of sample D, at different immersion times: 7 (—), 104 (- - -) and 266 (· · · ) days.
Fig. 4. Evolution of the impedance module and phase of sample G, at different immersion times: 1 (—), 3 (- - -) and 15 (· · · ) days.
Fig. 5. Time dependence of pore resistance (Rpo ) for steel substrate with different primers (sample A (䊐), sample B () and sample C (䊊)), in 3.5% NaCl solution.
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Fig. 6. Time dependence of charge-transfer resistance (Rp ) for steel substrate with different primers (sample A (䊐), sample B () and sample C (䊊)), in 3.5% NaCl solution.
Fig. 7. Time dependence of double layer capacitance (Cdl ) for steel substrate with different primers (sample A (䊐), sample B () and sample C (䊊)), in 3.5% NaCl solution.
Figs. 8–10 show the corresponding EIS results of samples D, E and F (phosphatised steel substrate with different primers). The pore resistance of coating (Rpo ) of the three samples was quite similar for 100 days. The Rpo of sample E (no toxic paint) remained almost constant over all immersion time while the Rpo of sample F started decreasing after 120 days and had a big increase for sample D. In the case of Rp , the three samples showed identical behaviour until 100 days (an initially big decrease and an increase at 80 days). From this time sample D showed a big increase, sample E gave medium Rp values and sample F lower values and behaved in a non-stable manner (showing lower Rp values than other samples up until 200 days when it reached similar Rp values to those of sample D). The Cdl of sample E remained almost stable with time while both the Cdl of Pb paints (D and F) increased with time
(especially sample D, that always showed bigger values than sample F). Figs. 11–13 are, respectively, referred to as Rpo , Rp and Cdl values of coats applied to phosphatised electrozincated steel substrate. The values corresponding to sample G (Pb paint cured at high temperature) are not shown because this system failed in the first few days. Sample I (non-toxic paint) had lower Rpo and Rp values than sample H. Rpo and Rp of both samples decreased very quickly with immersion time. The Cdl values corresponding to sample I (Pb paint cured at low temperature) showed a continuos increase with immersion time while the Cdl values corresponding to samples H (non-toxic paint) remained unstable until 80 days of immersion time. Finally, it has to be pointed out that for the immersion times considered, there were not found any significant changes in Cc values for any system.
Fig. 8. Time dependence of pore resistance (Rpo ) for phosphatised steel substrate with different primers (sample D (䊐), sample E () and sample F (䊊)), in 3.5% NaCl solution.
Fig. 9. Time dependence of charge-transfer resistance (Rp ) for phosphatised steel substrate with different primers (sample D (䊐), sample E () and sample F (䊊)), in 3.5% NaCl solution.
Fig. 10. Time dependence of double layer capacitance (Cdl ) for phosphatised steel substrate with different primers (sample D (䊐), sample E () and sample F (䊊)), in 3.5% NaCl solution.
Fig. 11. Time dependence of pore resistance (Rpo ) for electrozincated phosphatised steel substrate with different primers (sample H () and sample I (䊊)), in 3.5% NaCl solution.
Fig. 12. Time dependence of charge-transfer resistance (Rp ) for electrozincated phosphatised steel substrate with different primers (sample H () and sample I (䊊)), in 3.5% NaCl solution.
Fig. 13. Time dependence of double layer capacitance (Cdl ) for electrozincated phosphatised steel substrate with different primers (sample H () and sample I (䊊)), in 3.5% NaCl solution.
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Table 2 Summary of cathodic disbonding test results Steel
Phosphatised steel
Phosphatised electrozincated steel
Sample
−log(dx/dt) (mm/h)
Sample
−log(dx/dt) (mm/h)
Sample
−log(dx/dt) (mm/h)
A B C
2.5541 2.4772 2.6042
D E F
3.7828 3.1494 3.5064
G H I
0.2324 0.2764 0.2447
3.2. Cathodic disbonding The cathodic disbonding results for all tested and recorded samples are shown in Table 2. Very similar adhesion values were observed for the three coatings applied to the same substrate. It was observed that a higher adherence was given when applying the coating to the phosphatised steel and the lower values were those that obtained with phosphatised electrozincated steel substrate.
4. Discussion 4.1. Electrochemical impedance spectroscopy It is generally assumed that the elements of the equivalent circuits are correlated to the corrosion properties of the system [12]. The pore resistance Rpo , is a measure of the porosity and degradation of the coating. Usually Rpo values has been related to the number of pores or capillary channels perpendicular to the substrate surface through which the electrolyte reaches the interface [13]. Though the Rpo can also increase with immersion time, probably as a result of pore or defect blockage by corrosion products, it usually decreases. Some authors have found three regions in the time dependent decreases of Rpo . The pore resistance initially decreases rapidly, then slowly (exhibiting a plateau) and then again rapidly coinciding with the appearance of the second semi-circle. They explain the plateau by making the assumption that the number of pathways formed is approximately constant with time. Cc is the capacitance of the coating, it should be a measure of the water permeation into the coating and is given by Cc =
εε0 A d
(1)
where ε is the dielectric constant of the coating, ε0 is the permitivity of vacuum, A is the area of the coating and d is the thickness. The coating capacitance will usually change during electrolyte absorption because the dielectric constant of water is approximately 20 times greater than that of a typical coating. Cc usually increases at the initial stage of exposure, and seems to be a measure of water absorption and when the coating has been exposed for long times Cc can be correlated to coating disbonding and degradation.
The polarisation resistance Rp and double layer capacitance Cdl , are two parameters used to specify the disbonding of the top coat and the onset of corrosion at the interface. The specific polarisation resistance is associated with the charge-transfer behaviour of the metal substrate. Rp (like Cdl ) can only be calculated well when at least two time constants are evident in the spectrum. Cdl , the double layer capacitance, is a measure of the area over which coating has disbonded. It can be well measured only at advanced stages of coating deterioration. The trend of Cdl is complex. A change in the Cdl value can be associated with the competition between disbonding and corrosion product accumulation at the interface. The Cdl value increases as water spreads at the interface and the delaminated area extends. On the other hand, the accumulation of the corrosion product at the interface reduces the area of the double layer capacitor, which will lead to a decrease of the Cdl value. Therefore, the change of Cdl may depend on which factor, either the disbonding or the corrosion product accumulation that was more dominant during the corrosion process. However, it should be pointed out that both the increase and the decrease in Cdl are the results of corrosion development at the metal surface and a constant Cdl is an indication of a stable interface [6]. In samples where coating had been applied to steel substrate (A, B and C), it can be seen from the electrochemical parameters calculated that non-toxic paint (B) had fewer pores and was less degraded with the immersion time than samples A and C (Pb coat). The initial increase of Cdl in sample B, indicated that the interface metal-coat is very active initially and probably that a disbonding process takes place there. After 80 days of exposure the Cdl trend is almost constant and it can reflect that the interface has become more and more stable with time which can be interpreted as better corrosion protection. On the other hand, the Pb coatings (A and C) which are only different in the curing temperature that have been applied to them, show quite similar behaviour in all parameters. Nevertheless the sample cured at low temperature (C) showed higher Rp and Rpo values and lower Cdl values than the sample cured at a higher temperature (A), which can be representative of a better performance of the coating (a better corrosion protection of the substrate). A possible explanation can be that a lower curing temperature makes the epoxy coating less bright and makes the interface more
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stable because it has less residual stress and better adhesion to the substrate. Pb coatings showed an increasing Cdl trend which is related to an increase in the disbonding area of the interface, and a big change in this parameters at 80 days indicating a big difference in the protection properties before and after this time. Anticorrosive coat performance was much higher when the epoxy primers were applied to a phosphatised steel substrate than when they were applied to other substrates. It can be clearly seen that the phosphatised-coated interfaces were much more stable than the others (the trend of Rp and Cdl with time were less variable with this substrate than with the others). In the same way the adhesion values to this substrate were the highest too. This effect can be due to a good clamp process between primers and the irregular surface caused by the phosphate process over the steel. The pore resistance for sample D registered an increase with the immersion time. This effect can be a result of a pore or defect blockage by corrosion products. The Cdl value increases for samples D and F can be interpreted by an increase of the delaminated area in the interface (especially in sample D). It has to be pointed out that non-toxic paint (sample E) performed better than Pb paint again. Its Rp and Cdl values remained constant with the immersion times. Furthermore this sample had almost no delamination in the period of time considered. On the other hand, curing temperature had a significant effect on the paints performance. The higher curing temperature made the Pb paint have an unstable interface and gave rise to more delamination than paint cured at low temperature. Perhaps (as it was pointed out before) lower curing temperatures make the epoxy coating less bright and makes the interface more stable because it has less residual stress and better adhesion to the substrate. The paints performed worst when applied to phosphatised electrozincated steel substrate. This can be explained by low adherence (of the coatings of the substrate) and the presence of Cl− in the electrolyte that will make the zinc-phosphatised interface very active. The paint adhesion values to substrate were very low as can be seen in Table 2. The primer degradation process can be clearly seen in the trend of Rp and a Rpo with time, while the increase of Cdl with time can indicate an increase in the delaminate areas. Pb paint cured at high temperature performed very badly. A curing temperature decrease resulted in a better Pb paint performance. The reasons are similar to those pointed out before. Non-toxic paint performance best. It had the highest Rpo and Rp values and the lowest Cdl values. It should also be pointed out that sample I had the lowest delamination area with time. Nevertheless it must be clearly said that in coated phosphatised electrogalvanised steels that have been tested in this study, we have only investigated anticorrosion primers performance and not the system (primer + zinc coated) performance.
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5. Conclusions EIS is an effective tool for studying of the effect of metal pre-treatment on corrosion protection of automobiles. The best properties of paints studied are obtained when applied on phosphatised steel. In this case, it produces a more stable interface. The worst behaviour of paints is obtained in the application to phosphatised electrozincated steel. As regards the coatings, the best behaviour corresponded to the non-toxic paint, followed by the Pb paint cured at low temperature (160 ◦ C) and lastly the Pb paint cured at high temperature (180 ◦ C). An increase in the curing temperature decreases the delamination resistance of the paint. The rate of delamination mainly depends on type of substrate. The best adhesion (paint/substrate) was observed when phosphatised steel substrate was used, while the poorest adhesion corresponded to phosphatised electrozincated steel.
Acknowledgements We thank Ms. S. Gracia Edo, Mr. J.L. Godes Ort´ı, Ms. E. Romero Sales and Mr. J. Ortega Herreros for their help and collaboration. The authors are grateful for the support in this work of Project CICYT 1FD97-0333-C03-03. References [1] A. Amirudin, D. Thierry, Prog. Org. Coat. 28 (1996) 59. [2] A.C. Ramamurthy, W.I. Lorenzen, S.J. Bless, Prog. Org. Coat. 25 (1994) 43. [3] D. Crotty, Met. Finishing (1996) 54. [4] R. Dietz, Proceedings of the Fifth Automotive Corrosion Prevention Conference, SAE, Dearborn, MI, 1991, p. 1. [5] T. Monetta, F. Belluci, L. Nicodemo, L. Nicolais, Prog. Org. Coat. 21 (1993) 353. [6] I. Sekine, Prog. Org. Coat. 31 (1997) 73. [7] V.B. Miiskovic, M.R. Stanic, D.M. Drazic, Prog. Org. Coat. 36 (1999) 53. [8] N. Tang, W.J. Ooij, G. Górecki, Prog. Org. Coat. 30 (1997) 255. [9] V.B. Miskovic, M.D. Maksimovic, Z. Kacarevic, J.B. Zotovic, Prog. Org. Coat. 33 (1998) 68. [10] F. Deflorian, L. Fedrizzi, P.L. Bonora, Electrochem. Acta 44 (1999) 4243. [11] F. Deflorian, V.B. Miskovic, P.L. Bonora, L. Fedrizzi, Corros. Sci. 50 (1994) 439. [12] A. Sabata, W.J. Ooij, R.J. Koch, J. Adhesion Sci. Technol. 7 (1993) 1153. [13] A. Amirudin, D. Thierry, Prog. Org. Coat. 26 (1995) 1. [14] M. Kending, J. Scully, Corrosion 46 (1990) 22. [15] F. Mansfeld, C.H. Tsai, Corrosion 47 (1991) 958. [16] F. Mansfeld, M.W. Kending, S. Tsai, Corrosion 38 (1982) 479. [17] G.W. Walter, Corros. Sci. 26 (1986) 681. [18] F. Mansfeld, C.H. Tsai, H. Shih, in: D. Scantlebury, M. Kendig (Eds.), The Electrochemical Society Softbond Proceedings on Advance in Corrosion Protection by Organic Coatings, PV 89-13, Pennington, NJ, 1989, p. 228. [19] M. Kending, S. Jeanjaquet, R. Brown, F. Thomas, J. Coat. Technol. 68 (1996) 39.