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EIS studies of coated metals in accelerated exposure
Progress in Organic Coatings 46 (2003) 148–157
EIS studies of coated metals in accelerated exposure Gordon Bierwagen a,∗ , Dennis Tallman b , Junping Li a , Lingyun He a , Carol Jeffcoate c a
Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105, USA b Department of Chemistry, North Dakota State University, Fargo, ND 58105, USA c Honeywell, 55 Federal Road, Danbury, CT 06810, USA
Abstract One of the most popular uses of electrochemical impedance spectroscopy (EIS) is the characterization of the protective properties of coatings on corrodible metals. From early studies up to the present time, many EIS studies have been devoted to the study of the changes in the impedance of coated metals as they undergo either natural or artificial exposure to conditions that cause corrosive failure of such systems. With the current improvements in instrumentation and software for EIS studies of coated metals, one no longer needs to be an expert electrochemist to utilize EIS in one’s studies of protection by coatings. In this paper, the use of EIS from the point of view of the coatings scientist will be presented, with an emphasis on its application simultaneous with accelerated exposure. EIS is used by coating scientists for several purposes, among them the detection of changes due to exposure, prediction of the lifetime of corrosion protection, identification of the corrosion processes that lead to failure, ranking of coatings systems, measurement of water uptake by coatings, and the development of models for coating/metal system performance. This paper will discuss several specific examples of the use of EIS in the study of coatings in accelerated exposure and the analysis of EIS data from such studies. The importance of cyclic vs. steady state exposure of samples will be shown by EIS results, and some of the problems in the use of standard continuous salt fog exposure as exemplified by ASTM B117 for a coating specification will be discussed. Considering Tg effects on EIS data will show the importance of considering thermal effects in the testing of coatings. The extremely important role of water uptake in coatings during exposure will also be discussed using EIS results to analyze changes in both the coating resistance (low frequency |Z| data) and capacitance (higher frequency Z data). During exposure to cyclic changes in temperature and electrolyte solution concentration, a coating over a metal substrate appears to undergo both “physical aging” and chemical degradation. The coating appears to have a memory of past exposure events such that each subsequent exposure to water and temperature creates and enlarges transport pathways within the coating for water and electrolyte. As cyclic exposure continues, damage to the bulk-coating layer above the coating/metal interface accumulates until there begins to be a permanent accumulation of electrolyte at this interface and local small-scale corrosion begins. This is the initiation of corrosion failure of the system, but it only occurs following the decrease of bulk-coating layer barrier properties caused by cyclic temperature and humidity processes characteristic of exterior exposure. This whole process can be accelerated by immersion in a flowing electrolyte, emphasizing the role of transport processes in coating degradation processes. If there is simultaneous UV exposure, as Skerry has so well described, one must also account for photodegradation of the outermost layer of the coating system. The role of the coating scientist is now to assimilate the data that EIS now provides us during the exposure process and develop meaningful models for the molecular level changes that occur in the coating film in order to enable use of the EIS results for true coating performance ranking and lifetime prediction. © 2003 Published by Elsevier Science B.V. Keywords: EIS; Photodegradation; Electrolyte; Accelerated testing; Lifetime prediction
1. Introduction 1.1. Brief history of EIS use in coatings Review of the use of electrochemical impedance spectroscopy (EIS) in the study of corrosion protection by coatings has been considered by several authors [1–5]. Early measurements of EIS values on coated metals were very difficult due to the lack of computer controlled data acqui∗
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0300-9440/03/$ – see front matter © 2003 Published by Elsevier Science B.V. doi:10.1016/S0300-9440(02)00222-9
sition systems. Much of this has been reviewed before, and will not be repeated here. From the point of view of coating scientists, when relatively easy to use, control and standardized EIS equipment became available, the domination of this area of research by pure measurement electrochemists ceased. At this time, coating users began to see EIS as a valuable tool to characterize coating systems. The use of equivalent circuit methods to model the physical behavior of coatings as they aged and failed in immersion was introduced, with software that allowed the easy fitting of coating data [1].
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1.2. EIS usage by electrochemists and metallurgists in coatings The use of EIS by electrochemists to study the corrosion protective properties of organic coatings over metals has been dominated by an emphasis on the methodologies and instrumentation of EIS. The primary consideration is the corrosion occurring on the metal itself, and on the metal/coating interface and what happened when this occurred [6]. Very often the period of time during which a coating completely protected a metal was given minimal consideration, for the interest was in what happened to a metal surface as it corroded, and how the coating changed the mechanisms of corrosion [3]. Attention was not directly paid to the coating and the measurement of the properties of the system before failure by corrosion began at the metal/coating interface. Much attention was paid to the metal substrate and its composition. Classes of coatings were compared by EIS, and the EIS modeling of failing systems was very popular. Equivalent circuits with many elements were used to provide physical models for failed systems, but not much modeling of the changes occurring in coatings systems leading to failure was performed. 1.3. EIS measurements by coatings scientists The coatings scientist views EIS as a tool to examine coatings and the way they protect metals. The metal substrate was viewed by the coatings scientist as a feature of the system controlled by the user of the coating, and as something beyond their control [7]. They view the pre-treatment of the substrate as very important to the protective performance of the coating, much like it is for all other coating/substrate systems. They tend to be very concerned with the variability seen in EIS characterization of coated metals [4,8]. Wet adhesion, the ability to maintain adhesion to a metal substrate in the presence of water or electrolyte, was identified as being a very important coating parameter in the performance of coated metals [9]. 2. Accelerated life testing in coatings 2.1. Goals of accelerated life testing Accelerated life testing is used in many areas of science and technology to determine the effective performance lifetime of various types of systems. In general, one seeks a physical or chemical acceleration of the failure of a specific system by placing the system under stress larger than it would receive in its normal lifetime, and monitor its performance to failure from that stress. Ideally the stress only causes the system to fail faster than it normally would, hence the term accelerated, but the mechanism of failure remains the same as in the non-accelerated conditions. A good introduction to accelerated testing along with some introductory
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references to this area of science is given in the recent book chapter by Meeker et al. [10]. The goal of accelerated life testing of corrosion protection of coatings is to impose a repeatable, measurable stress, in excess of that it normally undergoes, to a coating/metal system. One then determines the time to failure under this stress as well as the changes in system variables under the stress conditions chosen. Ideally, as mentioned above, these imposed stress conditions will not cause the mechanism of corrosion protection failure to change from that seen in normal use of the coating. The goal of accelerated testing is most often to predict the lifetime of the system in question under normal use conditions. Accelerated testing when properly performed enables the user of a system to obtain good estimates of when to replace that system as well as allowing the developer of such systems to study, rank and predict performance lifetimes of new systems with no prior field use history without complete field use testing. The latter use is especially important for “good” systems because the increasing lifetimes of performance make the acquiring of true field performance lifetime data so time-consuming that it is too impractical and expensive to acquire. A proper accelerated test also requires a very clear definition of what constitutes “failure” so that proper measurement methods can be chosen and utilized to characterize the properties involved in “failure” [11]. 2.2. Accelerated testing of the lifetimes of coatings corrosion protection The goal of accelerated testing for coatings corrosion protective lifetimes is twofold: first, to screen and test newly developed coatings systems and second, to qualify new coatings for field use. This type of testing usually involves two components: the imposed stress environment that drives the system to failure and the measurement of system quality during stress imposition. Examples of the stress environments used for accelerating coatings system failure are immersion in electrolyte, continuous salt fog at 35 ◦ C, SO2 salt fog, and cyclic salt fog [12]. These methods were developed around the idea that electrolyte and oxygen are needed for corrosion at a metal surface while increasing temperature increases the transport of oxygen and electrolyte through paint films, and also increases corrosion reaction rates. Skerry and co-workers [13–15] provided evidence that true emulation of the effects of exterior exposure on corrosion protective coatings systems requires inclusion of UV effects. The proper measurement of coating quality during the imposed stress environment is not totally resolved. The ASTM B117 continuous salt fog test in its most used form involves only qualitative examination of panels after exposure by a trained observer, with some guidance given on blister density, appearance judgments, etc. This makes this protocol very weak and almost unusable due to the almost entirely objective nature of the characterization performed, irregardless of the fact that the stress used in the test is unrealistic and takes many coatings above their Tg ’s,
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rendering them non-protective and poor in film properties. A discussion of quantitative coatings measurements which may be used in a corrosion protection lifetime test protocol is given in a recent paper from this laboratory [16]. 2.3. Requirements and recommendations for accelerated test protocol for corrosion protection by coatings A proper test protocol has a stress environment specified, that accelerates the failure of a system by the same mechanism that is observed in field use of the system without altering the failure mechanism. It also provides quantitative measurement(s) which can be performed on the system which will allow clear identification of the failure time as well as the changes in the system with time under the imposed stress. For coatings systems that must provide corrosion protection under exterior exposure conditions, the test protocol must provide the primary stresses that coatings systems endure in exterior exposure. Most generally, these are UV exposure as from the sun along with (cyclic temperature + dilute electrolyte) exposure. This has been most carefully examined, prior to our studies on aircraft coatings, in industrial maintenance coatings and automotive coatings. A more complete discussion of these issues is given in Refs. [12–15]. Based on these references, for our work on exterior coatings, the ASTM D 5894-96 exposure protocol supplemented by various physical measurements was chosen. The details of this are given in Fig. 1. This protocol has been successfully used in our laboratories for many coatings systems, especially aircraft coatings, and it has proven quite useful in examining and ranking coatings [17]. Other test cycles including thermal cycling have also been examined in our laboratory [18], but the one that is generally most useful for ranking and comparing systems in exterior exposure is the protocol shown in Fig. 1.
2.4. Problems in present use of accelerated testing of coatings Many problems exist in the present use of accelerated testing of the corrosion protective properties and lifetimes of coatings. One problem that has arisen constantly is that users are unwilling to stop use of a specification test protocol that has been in place for any extended amount of time. Such a specification test protocol is the ASTM B117 test protocol and the accompanying qualitative observations are used for the measurements for this protocol. The measurements are all qualitative and subjective, and the test method does not emulate the conditions of use and failure in use conditions. The continuous high temperature (35 ◦ C) and continuous high salt concentration (5% NaCl fog) do not fit any common use condition. One must make sure that the temperature does not exceed the Tg of the coating under test otherwise false failures will result [18,19].
3. Lifetime prediction from EIS data 3.1. Previous use of |Z(t)| data Impedance data on coatings have been in use extensively in a semi-quantitative way to measure and predict the corrosion protective lifetimes of organic coatings on metals. As Mansfeld et al. [20] have observed, the most efficient way to analyze EIS data when examining coatings is in Bode plot format. The EIS data from newly applied coatings on metals is often purely capacitive in nature with only one time constant, and only levels off to a low frequency limit at quite a high values of |Z|. As the coating performance decays, the signal begins to show non-linear behavior at intermediate frequencies and displays more than one time constant. Many former workers have examined time changes in EIS data
Fig. 1. Test protocol used with exterior coatings.
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from coated metals in exposure [6,7,21,22]. However, very few authors have considered examining the low frequency |Z| values as a function of exposure time for extended exposure times. This type of data analysis has been done in our laboratories for several types of coatings, and the initial results have already been published [17]. 3.2. Observed trends in EIS vs. exposure time As mentioned above, there have been an extensive number of EIS measurements of coating performance in actual or accelerated exposure. However, much of the focus of these studies has been on the period of time after the metal surface begins corroding, and not on the time over which the coating performance is degrading but the metal surface is essentially intact. However, work in our laboratory indicates that for accurate evaluation of the protective lifetimes of organic coatings over metals, the period of time one should consider in detail is that, before the onset of significant corrosion of the metal substrate during which the coating somehow degrades and loses its corrosion protective properties. When the coating is still largely intact (no physical damage like scratches or stone dings) the metal is not undergoing significant local corrosion. The most important property of the coating is its ability to impede the flow of current between anodic and cathodic areas of the metal substrate. This property is the resistance of the coating and is best characterized from EIS measurements by examining the low frequency limit of |Z(ω)|. In a practical measurement sense, this is the value of |Z(ω)| at the lowest value of ω for which there is still no significant noise as ω → 0, and which does not require an unreasonable measurement time. If one considers the EIS literature on coatings, this value of ω is somewhere between 10−3 and 5×10−2 Hz. In data from our laboratory for |Z| at the lowest frequency that can be accurately measured as a function of exposure time for coatings that are physically intact and provides cor-
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rosion protection, and only for systems which are in steady state over the measurement time necessary to acquire the data, an exponential decay of the low frequency modulus with exposure time has been observed. This has been more completely described in Ref. [17]. Other authors have seen this type of data also and commented likewise on this trend in data of |Z|low freq when plotted vs. exposure time [23]. Examples of these types of data are shown in Figs. 2 and 3. These data are from aircraft coatings as identified in the figure over Al alloy 2024 T-3 panels exposed in the exposure cycle described in Fig. 1. The topcoats are DoD Specification MIL-C -85285, the extended lifetime topcoat (ELT), and primer MIL P23377 materials from Deft Coatings Inc. In Fig. 4 are shown photographs of the panels from which the data in Fig. 3 was measured after the indicated exposure time. The data of Figs. 2 and 3 can be analyzed from the following equations: −t |Z|(t) = |Z|m + (|Z|0 − |Z|m ) exp (1) θ |Z|(t) − |Z|m t ln =− (2) |Z|0 − |Z|m θ where |Z|m is the limiting bare metal value |Z|, |Z|0 is the film resistance values at t = 0, and the constants, θ, has the dimension of time and can be considered the characteristic decay times for the coating under consideration. The decay constant θ (the inverse of the slope of the exponential fit line in the graph of ln(|Z|(t) − |Z|m ) vs. exposure time) is presented for different samples. If one takes Eq. (2), and solves for the time it takes to decay to a specific failure value, |Z|fail , one has an expression for the failure time of the coating as a function of the chosen failure modulus: |Z|0 − |Z|m tfail = θ ln (3) |Z|fail − |Z|m
Fig. 2. |Z|0.012 Hz vs. exposure time for various aircraft coatings—Set A.
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Fig. 3. |Z|0.012 Hz vs. exposure time for various aircraft coatings—Set B.
This analysis holds true for other types of coatings as well, such as alkyd marine coatings and epoxy marine coatings [17], and should be considered for other types of coatings as well. Eq. (3) implies that the failure time is a function of the chosen “failure” modulus value. This is illustrated in Fig. 5, and the corresponding values of the failure rate constant θ are given in Fig. 6. The failure rate constant θ is most useful for comparative ranking of substrate lifetimes when a complete prediction of failure time is not necessary, such as in rapid screening testing. All coating systems examined under the exposure protocol of Fig. 1 have shown the first-order decay behavior described above that allow the use of this lifetime prediction protocol. This is not always true for other exposure protocols, or for shorter time periods of exposure, and this will be discussed in a later publication.
4. Thermal effects in coatings examined electrochemically 4.1. General comments The general effects of temperature are often used to accelerate exposure tests for coating lifetime such as the high temperature periods of the Prohesion test cycle. The reasoning behind the use of increased temperature to “accelerate” failure is that there is an increase in rates of chemical reactions, transport properties, molecular mobility, etc. with increasing temperature. An Arrhenius-type of effect is often invoked in the use of increasing temperature to accelerate the failure of materials [10]. As discussed in Ref. [10], caution must be exercised in extrapolation of results from thermally accelerating failure. The ideal acce-
Fig. 4. Photographs of panels used in testing of Fig. 3.
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Fig. 5. Failure time dependence on failure modulus.
lerated test increases the rate of failure without changing the mechanism of failure. We have examined the effects of temperature on coatings performance by EIS as an attempt to identify the validity of thermal acceleration of coatings failure and to provide numerical evaluation of coatings properties vs. exposure at elevated temperatures [19]. 4.2. Early studies: examination of pipe-line coatings Our earliest studies of thermal effects were performed in conjunction with our examination of pipe-line coatings [24]. We showed that thermal acceleration study had to
consider the effects of the glass transition temperature, Tg , in the coatings under study as well as the effects of the water plasticization of the coating during the measurements. This was more fully documented in Ref. [19], it gives a full discussion of how examination of EIS data as a function of temperature can be used to characterize the Tg in coatings in immersion, as well as the plasticization effect of water on many coating polymers. The barrier and electrical resistance properties of coatings are significantly lowered above their Tg ’s, and care must be taken not to exceed the Tg of the coating under study during accelerated testing if one wants an accurate evaluation of the coating in question. Similarly,
Fig. 6. |Z|(t) decay constant (θ) for different aircraft coating systems.
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use of coatings above their Tg will drastically shorten their lifetime of protection.
Table 1 DSC data on powder coating and ionomer coating Estimated Tg (◦ C)
4.3. Thermal cycling effects What all of these studies have shown is that EIS measurements track very effectively the effects of temperature, water ingress and coating plasticization, and Tg in coatings. Ref. [18] discusses the effect of reversible and irreversible changes due temperature cycling on Tg and other coating properties, and suggests that irreversibility in coatings properties is due to permanent degradation of the coatings properties. Observation of irreversible degradation of coating properties in thermal cycling is an early indicator of coating failures. 4.4. Tg effects in dry and wet systems Below the Tg , the activation energies for diffusion and conductivity are both quite high and the magnitudes of the diffusion coefficient and the conductivity are quite small (resistance is large). Above the Tg , or any other order–disorder transition, the energies of activation for transport processes undergo threshold behavior, and diffusion sharply increases while electrical resistance drops drastically. Plotting the properties of polymer coating films above and below the Tg has shown a distinct change in slope around the Tg . The extent of irreversibility in cycling above and below the Tg is dependent on the coating composition. For coatings in immersion, the plasticization effect of water must be considered, and the reversibility of this plasticization is very important [18,19]. For ionomer types of coating polymers, above the Tg , there is considerable water ingress due to solubility and plasticization effects [25,26]. 4.5. Thermal effects on water ingress and uptake as assessed by EIS We have earlier examined water ingress and uptake by EIS for an epoxy powder coating over steel and a ionomer coating over steel [25,27]. It was noted that the ionomer coating, once taken over its Tg in immersion, had irreversible changes in its low frequency |Z| values and its capacitance as estimated from EIS. Measurement of the electrochemical properties of the film by impedance spectroscopy enables us to calculate the capacitance which incorporates the dielectric constant of the film. Both these properties change with increased water content and plasticization of the polymer film. Caution is required over the choice of frequency since one needs to choose a frequency at which the dielectric properties rather than the electrochemical are measured. Too low a frequency, (below the break point frequency) and the electrochemical properties of the film and any corrosion processes occurring will be measured. The dielectric constants for most polymers are in the range of ε = 3–4 [28].
Epoxy powder coating detached film sample Dry 100 In 3% NaCl 8 months at 82 25 ◦ C + 1 h at 90 ◦ C Ionomer powder coating detached Dry In 3% NaCl 24 h at25 ◦ C In 3% NaCl 1 h at100 ◦ C
film sample 81 75 74
Wf (%) Nor detected 1.4
Not detected 0.3 1.6
Table 2 Weight measurements on coating films Estimated Tg (◦ C) Epoxy powder coating detached film sample 73 In 3% NaCl for 8 months at 25 ◦ C + 1 h at 90 ◦ C Ionomer powder coating detached film sample In 3% NaCl for 1 h at100 ◦ C 74
Weight (%) 1.35
1.6
Entry of water which has a much higher dielectric constant (εw = 80 is the dielectric constant of water at room temperature) into the coating will increase the coating capacitance (Cc ). From the Brasher and Kingsbury (BK) [29] empirical formula, measuring Cc should be a measurement of water permeation into the coating as given by φ=
log(Cc /C0 ) log εw
(4)
where φ is the volume fraction, Cc the coating capacitance, C0 the coating capacitance at the beginning of exposure, and εw is the dielectric constant of water. There is good agreement between the thermal and weight measurements for water uptake. A slight modification of the BK empirical equation to φ=
(Cc /C0 ) 1 − εw εw
(5)
gives better agreement of these impedance calculations to the thermal and weight measurements for these samples. This was not true for all of our impedance data. As expected, good agreement is possible from weight and thermal measurements of the water uptake by a film. DSC, weight measurement, TGA data are shown in Tables 1–3. The water uptake data for our samples as measured by TGA was the most reliable and the most reproducible results of the methods applied. This method provided us with a very Table 3 TGA measurements of relative water uptake Wet film boiled in 3% NaCl
FBE Sample 1
FBE Sample 2
Ionomer
Water uptake (%)
1.15
1.14
3.27
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Fig. 7. Water uptake estimated by EIS using the BK equation: ionomer powder coating.
accurate, almost absolute method for determining water content when the EIS data based on the BK estimating procedure for water content gives very misleadingly high results. The data calculated by the BK and modified BK equations are given in Figs. 7 and 8 for the ionomer and the epoxy coatings, respectively. As can be seen from these figures, the estimates of water content when using the BK equation are quite large as compared to the weight measurements. The estimate for water content calculated with the modified BK are much closer to the experimentally determined values. The disparities between the TGA and DSC results and the
BK estimates seem to be significant when some of the assumptions in the BK estimate, especially the assumption that there is a separate water phase with no solvency interactions between water and coating, are no longer true. The ionomer coating, as one might expect, shows the greatest disparities between the electrochemical and weight-based estimates of water content. In our measurements, the failures of the assumptions seem to occur especially when water enters the coating above the Tg and/or the water is a strong plasticizer for the coating, or enters it as a neutralizing electrolyte as it does with the ionomer.
Fig. 8. Water uptake estimated by EIS using the modified BK equation: epoxy powder coating.
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This is documented in a recent study in our laboratory. The techniques that we developed in our thermal cycling studies [18] have shown that when panels are exposed above their Tg to aqueous electrolyte and then cooled, the apparent water content as measured by EIS via a BK type of relationship does not equal the amount of water as determined gravimetrically or by TGA. This implies that the water imbibed above the Tg is not phase-separated, as required by the BK relationship, but probably acts as a partial solvent for the polymer phase which apparently alters the dielectric constant significantly yielding a higher apparent water content than predicted by BK. Others have seen related results [30].
5. Summary and conclusions In summary, EIS techniques provide an excellent method to examine the corrosion protective properties of organic coatings as well as other properties of the coatings such as thermal properties, water solubility, and perhaps even physical aging processes. In conjunction with accelerated weathering protocols developed by corrosion scientists to qualitatively estimate the corrosion protective lifetimes of coatings, the numerical evaluation of metal substrate/organic coatings systems by EIS measurements provides an objective assessment of system performance that tracks performance changes in such systems quite well. For relatively undamaged systems, a simple parallel Randles circuit (RC) is sufficient to model systems performance, but as systems become significantly damaged due to exposure and corrosion at the metal/coating interface, more complex circuits are required. These may offer insight into final failure mechanisms. From the work we have presented and the work of others cited in this work, it can be definitely concluded that EIS and other electrochemical measurements of metal/organic coating systems offer significant advantages to those evaluating such systems, especially when used with accelerated exposure protocols. The numerical, objective nature of the results from EIS enable quantitative assessment of changes in metal/coating systems as they are artificially or naturally weathered, and allow objective evaluation of these systems. EIS is an important measurement tool that should be used routinely by all those examining corrosion control by organic coatings.
Acknowledgements This work was performed with the support of the following: (a) Office of Naval Research under grant no. N00014-95-10507, Dr. A.J. Sedriks, program manager; (b) Air Force Office of Scientific Research under grant F49620-96-1-0284, Lt. Col. P. Trulove and Maj. H. DeLong, program managers; (c) a sub-contract from Boeing, Dr. J. Osborne, PI, the prime contractor for DARPA under
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[24] G.P. Bierwagen, C.S. Jeffcoate, D.J. Mills, J. Li, S. Balbyshev, D.E. Tallman, The use of electrochemical noise methods to study thick, high impedance coatings, Prog. Org. Coat. 29 (1996) 21–30. [25] C.S. Jeffcoate, T.L. Wocken, G.P. Bierwagen, Electrochemical assessment of spray-applied thermoplastic coating barrier properties, J. Mater. Eng. Perform. 6 (1997) 417–420. [26] C.S. Jeffcoate, J. Li, G.P. Bierwagen, in: S.R. Taylor, H. Isaacs, E. Brooman (Eds.), Electrochemical Testing of Thick Thermoplastic Powder Coatings, Proceedings of the Electrochemical Society Symposium, vol. 95-16, 1995, p. 60. [27] C.S. Jeffcoate, J. Li, G.P. Bierwagen, Measurement of water ingress into corrosion protective coating films, in: Proceedings of the Symposium on Research in Progress, Corrosion’96, NACE Annual Meeting, Denver, CO, March 1996. [28] C.C. Ku, R.L. Liepens, Electrical Properties of Polymers, Hanser (New York) 1987. [29] D.M. Brasher, A.H. Kingsbury, J. Appl. Chem. 4 (1954) 62. [30] A.M. Simoes, Instituto Superior Técnico, Lisbon, Portugal, Private communication; A.S. Castella, A.M. Simoes, Assessment of water uptake in coil coatings by capacitance measurements, Prog. Org. Coat. 46 (2003) 55–61.
EIS studies of coated metals in accelerated exposure Gordon Bierwagen a,∗ , Dennis Tallman b , Junping Li a , Lingyun He a , Carol Jeffcoate c a
Department of Polymers and Coatings, North Dakota State University, Fargo, ND 58105, USA b Department of Chemistry, North Dakota State University, Fargo, ND 58105, USA c Honeywell, 55 Federal Road, Danbury, CT 06810, USA
Abstract One of the most popular uses of electrochemical impedance spectroscopy (EIS) is the characterization of the protective properties of coatings on corrodible metals. From early studies up to the present time, many EIS studies have been devoted to the study of the changes in the impedance of coated metals as they undergo either natural or artificial exposure to conditions that cause corrosive failure of such systems. With the current improvements in instrumentation and software for EIS studies of coated metals, one no longer needs to be an expert electrochemist to utilize EIS in one’s studies of protection by coatings. In this paper, the use of EIS from the point of view of the coatings scientist will be presented, with an emphasis on its application simultaneous with accelerated exposure. EIS is used by coating scientists for several purposes, among them the detection of changes due to exposure, prediction of the lifetime of corrosion protection, identification of the corrosion processes that lead to failure, ranking of coatings systems, measurement of water uptake by coatings, and the development of models for coating/metal system performance. This paper will discuss several specific examples of the use of EIS in the study of coatings in accelerated exposure and the analysis of EIS data from such studies. The importance of cyclic vs. steady state exposure of samples will be shown by EIS results, and some of the problems in the use of standard continuous salt fog exposure as exemplified by ASTM B117 for a coating specification will be discussed. Considering Tg effects on EIS data will show the importance of considering thermal effects in the testing of coatings. The extremely important role of water uptake in coatings during exposure will also be discussed using EIS results to analyze changes in both the coating resistance (low frequency |Z| data) and capacitance (higher frequency Z data). During exposure to cyclic changes in temperature and electrolyte solution concentration, a coating over a metal substrate appears to undergo both “physical aging” and chemical degradation. The coating appears to have a memory of past exposure events such that each subsequent exposure to water and temperature creates and enlarges transport pathways within the coating for water and electrolyte. As cyclic exposure continues, damage to the bulk-coating layer above the coating/metal interface accumulates until there begins to be a permanent accumulation of electrolyte at this interface and local small-scale corrosion begins. This is the initiation of corrosion failure of the system, but it only occurs following the decrease of bulk-coating layer barrier properties caused by cyclic temperature and humidity processes characteristic of exterior exposure. This whole process can be accelerated by immersion in a flowing electrolyte, emphasizing the role of transport processes in coating degradation processes. If there is simultaneous UV exposure, as Skerry has so well described, one must also account for photodegradation of the outermost layer of the coating system. The role of the coating scientist is now to assimilate the data that EIS now provides us during the exposure process and develop meaningful models for the molecular level changes that occur in the coating film in order to enable use of the EIS results for true coating performance ranking and lifetime prediction. © 2003 Published by Elsevier Science B.V. Keywords: EIS; Photodegradation; Electrolyte; Accelerated testing; Lifetime prediction
1. Introduction 1.1. Brief history of EIS use in coatings Review of the use of electrochemical impedance spectroscopy (EIS) in the study of corrosion protection by coatings has been considered by several authors [1–5]. Early measurements of EIS values on coated metals were very difficult due to the lack of computer controlled data acqui∗
Corresponding author. Tel.: +1-701-231-8294; fax: +1-701-231-8439.
0300-9440/03/$ – see front matter © 2003 Published by Elsevier Science B.V. doi:10.1016/S0300-9440(02)00222-9
sition systems. Much of this has been reviewed before, and will not be repeated here. From the point of view of coating scientists, when relatively easy to use, control and standardized EIS equipment became available, the domination of this area of research by pure measurement electrochemists ceased. At this time, coating users began to see EIS as a valuable tool to characterize coating systems. The use of equivalent circuit methods to model the physical behavior of coatings as they aged and failed in immersion was introduced, with software that allowed the easy fitting of coating data [1].
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1.2. EIS usage by electrochemists and metallurgists in coatings The use of EIS by electrochemists to study the corrosion protective properties of organic coatings over metals has been dominated by an emphasis on the methodologies and instrumentation of EIS. The primary consideration is the corrosion occurring on the metal itself, and on the metal/coating interface and what happened when this occurred [6]. Very often the period of time during which a coating completely protected a metal was given minimal consideration, for the interest was in what happened to a metal surface as it corroded, and how the coating changed the mechanisms of corrosion [3]. Attention was not directly paid to the coating and the measurement of the properties of the system before failure by corrosion began at the metal/coating interface. Much attention was paid to the metal substrate and its composition. Classes of coatings were compared by EIS, and the EIS modeling of failing systems was very popular. Equivalent circuits with many elements were used to provide physical models for failed systems, but not much modeling of the changes occurring in coatings systems leading to failure was performed. 1.3. EIS measurements by coatings scientists The coatings scientist views EIS as a tool to examine coatings and the way they protect metals. The metal substrate was viewed by the coatings scientist as a feature of the system controlled by the user of the coating, and as something beyond their control [7]. They view the pre-treatment of the substrate as very important to the protective performance of the coating, much like it is for all other coating/substrate systems. They tend to be very concerned with the variability seen in EIS characterization of coated metals [4,8]. Wet adhesion, the ability to maintain adhesion to a metal substrate in the presence of water or electrolyte, was identified as being a very important coating parameter in the performance of coated metals [9]. 2. Accelerated life testing in coatings 2.1. Goals of accelerated life testing Accelerated life testing is used in many areas of science and technology to determine the effective performance lifetime of various types of systems. In general, one seeks a physical or chemical acceleration of the failure of a specific system by placing the system under stress larger than it would receive in its normal lifetime, and monitor its performance to failure from that stress. Ideally the stress only causes the system to fail faster than it normally would, hence the term accelerated, but the mechanism of failure remains the same as in the non-accelerated conditions. A good introduction to accelerated testing along with some introductory
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references to this area of science is given in the recent book chapter by Meeker et al. [10]. The goal of accelerated life testing of corrosion protection of coatings is to impose a repeatable, measurable stress, in excess of that it normally undergoes, to a coating/metal system. One then determines the time to failure under this stress as well as the changes in system variables under the stress conditions chosen. Ideally, as mentioned above, these imposed stress conditions will not cause the mechanism of corrosion protection failure to change from that seen in normal use of the coating. The goal of accelerated testing is most often to predict the lifetime of the system in question under normal use conditions. Accelerated testing when properly performed enables the user of a system to obtain good estimates of when to replace that system as well as allowing the developer of such systems to study, rank and predict performance lifetimes of new systems with no prior field use history without complete field use testing. The latter use is especially important for “good” systems because the increasing lifetimes of performance make the acquiring of true field performance lifetime data so time-consuming that it is too impractical and expensive to acquire. A proper accelerated test also requires a very clear definition of what constitutes “failure” so that proper measurement methods can be chosen and utilized to characterize the properties involved in “failure” [11]. 2.2. Accelerated testing of the lifetimes of coatings corrosion protection The goal of accelerated testing for coatings corrosion protective lifetimes is twofold: first, to screen and test newly developed coatings systems and second, to qualify new coatings for field use. This type of testing usually involves two components: the imposed stress environment that drives the system to failure and the measurement of system quality during stress imposition. Examples of the stress environments used for accelerating coatings system failure are immersion in electrolyte, continuous salt fog at 35 ◦ C, SO2 salt fog, and cyclic salt fog [12]. These methods were developed around the idea that electrolyte and oxygen are needed for corrosion at a metal surface while increasing temperature increases the transport of oxygen and electrolyte through paint films, and also increases corrosion reaction rates. Skerry and co-workers [13–15] provided evidence that true emulation of the effects of exterior exposure on corrosion protective coatings systems requires inclusion of UV effects. The proper measurement of coating quality during the imposed stress environment is not totally resolved. The ASTM B117 continuous salt fog test in its most used form involves only qualitative examination of panels after exposure by a trained observer, with some guidance given on blister density, appearance judgments, etc. This makes this protocol very weak and almost unusable due to the almost entirely objective nature of the characterization performed, irregardless of the fact that the stress used in the test is unrealistic and takes many coatings above their Tg ’s,
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rendering them non-protective and poor in film properties. A discussion of quantitative coatings measurements which may be used in a corrosion protection lifetime test protocol is given in a recent paper from this laboratory [16]. 2.3. Requirements and recommendations for accelerated test protocol for corrosion protection by coatings A proper test protocol has a stress environment specified, that accelerates the failure of a system by the same mechanism that is observed in field use of the system without altering the failure mechanism. It also provides quantitative measurement(s) which can be performed on the system which will allow clear identification of the failure time as well as the changes in the system with time under the imposed stress. For coatings systems that must provide corrosion protection under exterior exposure conditions, the test protocol must provide the primary stresses that coatings systems endure in exterior exposure. Most generally, these are UV exposure as from the sun along with (cyclic temperature + dilute electrolyte) exposure. This has been most carefully examined, prior to our studies on aircraft coatings, in industrial maintenance coatings and automotive coatings. A more complete discussion of these issues is given in Refs. [12–15]. Based on these references, for our work on exterior coatings, the ASTM D 5894-96 exposure protocol supplemented by various physical measurements was chosen. The details of this are given in Fig. 1. This protocol has been successfully used in our laboratories for many coatings systems, especially aircraft coatings, and it has proven quite useful in examining and ranking coatings [17]. Other test cycles including thermal cycling have also been examined in our laboratory [18], but the one that is generally most useful for ranking and comparing systems in exterior exposure is the protocol shown in Fig. 1.
2.4. Problems in present use of accelerated testing of coatings Many problems exist in the present use of accelerated testing of the corrosion protective properties and lifetimes of coatings. One problem that has arisen constantly is that users are unwilling to stop use of a specification test protocol that has been in place for any extended amount of time. Such a specification test protocol is the ASTM B117 test protocol and the accompanying qualitative observations are used for the measurements for this protocol. The measurements are all qualitative and subjective, and the test method does not emulate the conditions of use and failure in use conditions. The continuous high temperature (35 ◦ C) and continuous high salt concentration (5% NaCl fog) do not fit any common use condition. One must make sure that the temperature does not exceed the Tg of the coating under test otherwise false failures will result [18,19].
3. Lifetime prediction from EIS data 3.1. Previous use of |Z(t)| data Impedance data on coatings have been in use extensively in a semi-quantitative way to measure and predict the corrosion protective lifetimes of organic coatings on metals. As Mansfeld et al. [20] have observed, the most efficient way to analyze EIS data when examining coatings is in Bode plot format. The EIS data from newly applied coatings on metals is often purely capacitive in nature with only one time constant, and only levels off to a low frequency limit at quite a high values of |Z|. As the coating performance decays, the signal begins to show non-linear behavior at intermediate frequencies and displays more than one time constant. Many former workers have examined time changes in EIS data
Fig. 1. Test protocol used with exterior coatings.
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from coated metals in exposure [6,7,21,22]. However, very few authors have considered examining the low frequency |Z| values as a function of exposure time for extended exposure times. This type of data analysis has been done in our laboratories for several types of coatings, and the initial results have already been published [17]. 3.2. Observed trends in EIS vs. exposure time As mentioned above, there have been an extensive number of EIS measurements of coating performance in actual or accelerated exposure. However, much of the focus of these studies has been on the period of time after the metal surface begins corroding, and not on the time over which the coating performance is degrading but the metal surface is essentially intact. However, work in our laboratory indicates that for accurate evaluation of the protective lifetimes of organic coatings over metals, the period of time one should consider in detail is that, before the onset of significant corrosion of the metal substrate during which the coating somehow degrades and loses its corrosion protective properties. When the coating is still largely intact (no physical damage like scratches or stone dings) the metal is not undergoing significant local corrosion. The most important property of the coating is its ability to impede the flow of current between anodic and cathodic areas of the metal substrate. This property is the resistance of the coating and is best characterized from EIS measurements by examining the low frequency limit of |Z(ω)|. In a practical measurement sense, this is the value of |Z(ω)| at the lowest value of ω for which there is still no significant noise as ω → 0, and which does not require an unreasonable measurement time. If one considers the EIS literature on coatings, this value of ω is somewhere between 10−3 and 5×10−2 Hz. In data from our laboratory for |Z| at the lowest frequency that can be accurately measured as a function of exposure time for coatings that are physically intact and provides cor-
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rosion protection, and only for systems which are in steady state over the measurement time necessary to acquire the data, an exponential decay of the low frequency modulus with exposure time has been observed. This has been more completely described in Ref. [17]. Other authors have seen this type of data also and commented likewise on this trend in data of |Z|low freq when plotted vs. exposure time [23]. Examples of these types of data are shown in Figs. 2 and 3. These data are from aircraft coatings as identified in the figure over Al alloy 2024 T-3 panels exposed in the exposure cycle described in Fig. 1. The topcoats are DoD Specification MIL-C -85285, the extended lifetime topcoat (ELT), and primer MIL P23377 materials from Deft Coatings Inc. In Fig. 4 are shown photographs of the panels from which the data in Fig. 3 was measured after the indicated exposure time. The data of Figs. 2 and 3 can be analyzed from the following equations: −t |Z|(t) = |Z|m + (|Z|0 − |Z|m ) exp (1) θ |Z|(t) − |Z|m t ln =− (2) |Z|0 − |Z|m θ where |Z|m is the limiting bare metal value |Z|, |Z|0 is the film resistance values at t = 0, and the constants, θ, has the dimension of time and can be considered the characteristic decay times for the coating under consideration. The decay constant θ (the inverse of the slope of the exponential fit line in the graph of ln(|Z|(t) − |Z|m ) vs. exposure time) is presented for different samples. If one takes Eq. (2), and solves for the time it takes to decay to a specific failure value, |Z|fail , one has an expression for the failure time of the coating as a function of the chosen failure modulus: |Z|0 − |Z|m tfail = θ ln (3) |Z|fail − |Z|m
Fig. 2. |Z|0.012 Hz vs. exposure time for various aircraft coatings—Set A.
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Fig. 3. |Z|0.012 Hz vs. exposure time for various aircraft coatings—Set B.
This analysis holds true for other types of coatings as well, such as alkyd marine coatings and epoxy marine coatings [17], and should be considered for other types of coatings as well. Eq. (3) implies that the failure time is a function of the chosen “failure” modulus value. This is illustrated in Fig. 5, and the corresponding values of the failure rate constant θ are given in Fig. 6. The failure rate constant θ is most useful for comparative ranking of substrate lifetimes when a complete prediction of failure time is not necessary, such as in rapid screening testing. All coating systems examined under the exposure protocol of Fig. 1 have shown the first-order decay behavior described above that allow the use of this lifetime prediction protocol. This is not always true for other exposure protocols, or for shorter time periods of exposure, and this will be discussed in a later publication.
4. Thermal effects in coatings examined electrochemically 4.1. General comments The general effects of temperature are often used to accelerate exposure tests for coating lifetime such as the high temperature periods of the Prohesion test cycle. The reasoning behind the use of increased temperature to “accelerate” failure is that there is an increase in rates of chemical reactions, transport properties, molecular mobility, etc. with increasing temperature. An Arrhenius-type of effect is often invoked in the use of increasing temperature to accelerate the failure of materials [10]. As discussed in Ref. [10], caution must be exercised in extrapolation of results from thermally accelerating failure. The ideal acce-
Fig. 4. Photographs of panels used in testing of Fig. 3.
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Fig. 5. Failure time dependence on failure modulus.
lerated test increases the rate of failure without changing the mechanism of failure. We have examined the effects of temperature on coatings performance by EIS as an attempt to identify the validity of thermal acceleration of coatings failure and to provide numerical evaluation of coatings properties vs. exposure at elevated temperatures [19]. 4.2. Early studies: examination of pipe-line coatings Our earliest studies of thermal effects were performed in conjunction with our examination of pipe-line coatings [24]. We showed that thermal acceleration study had to
consider the effects of the glass transition temperature, Tg , in the coatings under study as well as the effects of the water plasticization of the coating during the measurements. This was more fully documented in Ref. [19], it gives a full discussion of how examination of EIS data as a function of temperature can be used to characterize the Tg in coatings in immersion, as well as the plasticization effect of water on many coating polymers. The barrier and electrical resistance properties of coatings are significantly lowered above their Tg ’s, and care must be taken not to exceed the Tg of the coating under study during accelerated testing if one wants an accurate evaluation of the coating in question. Similarly,
Fig. 6. |Z|(t) decay constant (θ) for different aircraft coating systems.
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use of coatings above their Tg will drastically shorten their lifetime of protection.
Table 1 DSC data on powder coating and ionomer coating Estimated Tg (◦ C)
4.3. Thermal cycling effects What all of these studies have shown is that EIS measurements track very effectively the effects of temperature, water ingress and coating plasticization, and Tg in coatings. Ref. [18] discusses the effect of reversible and irreversible changes due temperature cycling on Tg and other coating properties, and suggests that irreversibility in coatings properties is due to permanent degradation of the coatings properties. Observation of irreversible degradation of coating properties in thermal cycling is an early indicator of coating failures. 4.4. Tg effects in dry and wet systems Below the Tg , the activation energies for diffusion and conductivity are both quite high and the magnitudes of the diffusion coefficient and the conductivity are quite small (resistance is large). Above the Tg , or any other order–disorder transition, the energies of activation for transport processes undergo threshold behavior, and diffusion sharply increases while electrical resistance drops drastically. Plotting the properties of polymer coating films above and below the Tg has shown a distinct change in slope around the Tg . The extent of irreversibility in cycling above and below the Tg is dependent on the coating composition. For coatings in immersion, the plasticization effect of water must be considered, and the reversibility of this plasticization is very important [18,19]. For ionomer types of coating polymers, above the Tg , there is considerable water ingress due to solubility and plasticization effects [25,26]. 4.5. Thermal effects on water ingress and uptake as assessed by EIS We have earlier examined water ingress and uptake by EIS for an epoxy powder coating over steel and a ionomer coating over steel [25,27]. It was noted that the ionomer coating, once taken over its Tg in immersion, had irreversible changes in its low frequency |Z| values and its capacitance as estimated from EIS. Measurement of the electrochemical properties of the film by impedance spectroscopy enables us to calculate the capacitance which incorporates the dielectric constant of the film. Both these properties change with increased water content and plasticization of the polymer film. Caution is required over the choice of frequency since one needs to choose a frequency at which the dielectric properties rather than the electrochemical are measured. Too low a frequency, (below the break point frequency) and the electrochemical properties of the film and any corrosion processes occurring will be measured. The dielectric constants for most polymers are in the range of ε = 3–4 [28].
Epoxy powder coating detached film sample Dry 100 In 3% NaCl 8 months at 82 25 ◦ C + 1 h at 90 ◦ C Ionomer powder coating detached Dry In 3% NaCl 24 h at25 ◦ C In 3% NaCl 1 h at100 ◦ C
film sample 81 75 74
Wf (%) Nor detected 1.4
Not detected 0.3 1.6
Table 2 Weight measurements on coating films Estimated Tg (◦ C) Epoxy powder coating detached film sample 73 In 3% NaCl for 8 months at 25 ◦ C + 1 h at 90 ◦ C Ionomer powder coating detached film sample In 3% NaCl for 1 h at100 ◦ C 74
Weight (%) 1.35
1.6
Entry of water which has a much higher dielectric constant (εw = 80 is the dielectric constant of water at room temperature) into the coating will increase the coating capacitance (Cc ). From the Brasher and Kingsbury (BK) [29] empirical formula, measuring Cc should be a measurement of water permeation into the coating as given by φ=
log(Cc /C0 ) log εw
(4)
where φ is the volume fraction, Cc the coating capacitance, C0 the coating capacitance at the beginning of exposure, and εw is the dielectric constant of water. There is good agreement between the thermal and weight measurements for water uptake. A slight modification of the BK empirical equation to φ=
(Cc /C0 ) 1 − εw εw
(5)
gives better agreement of these impedance calculations to the thermal and weight measurements for these samples. This was not true for all of our impedance data. As expected, good agreement is possible from weight and thermal measurements of the water uptake by a film. DSC, weight measurement, TGA data are shown in Tables 1–3. The water uptake data for our samples as measured by TGA was the most reliable and the most reproducible results of the methods applied. This method provided us with a very Table 3 TGA measurements of relative water uptake Wet film boiled in 3% NaCl
FBE Sample 1
FBE Sample 2
Ionomer
Water uptake (%)
1.15
1.14
3.27
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Fig. 7. Water uptake estimated by EIS using the BK equation: ionomer powder coating.
accurate, almost absolute method for determining water content when the EIS data based on the BK estimating procedure for water content gives very misleadingly high results. The data calculated by the BK and modified BK equations are given in Figs. 7 and 8 for the ionomer and the epoxy coatings, respectively. As can be seen from these figures, the estimates of water content when using the BK equation are quite large as compared to the weight measurements. The estimate for water content calculated with the modified BK are much closer to the experimentally determined values. The disparities between the TGA and DSC results and the
BK estimates seem to be significant when some of the assumptions in the BK estimate, especially the assumption that there is a separate water phase with no solvency interactions between water and coating, are no longer true. The ionomer coating, as one might expect, shows the greatest disparities between the electrochemical and weight-based estimates of water content. In our measurements, the failures of the assumptions seem to occur especially when water enters the coating above the Tg and/or the water is a strong plasticizer for the coating, or enters it as a neutralizing electrolyte as it does with the ionomer.
Fig. 8. Water uptake estimated by EIS using the modified BK equation: epoxy powder coating.
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This is documented in a recent study in our laboratory. The techniques that we developed in our thermal cycling studies [18] have shown that when panels are exposed above their Tg to aqueous electrolyte and then cooled, the apparent water content as measured by EIS via a BK type of relationship does not equal the amount of water as determined gravimetrically or by TGA. This implies that the water imbibed above the Tg is not phase-separated, as required by the BK relationship, but probably acts as a partial solvent for the polymer phase which apparently alters the dielectric constant significantly yielding a higher apparent water content than predicted by BK. Others have seen related results [30].
5. Summary and conclusions In summary, EIS techniques provide an excellent method to examine the corrosion protective properties of organic coatings as well as other properties of the coatings such as thermal properties, water solubility, and perhaps even physical aging processes. In conjunction with accelerated weathering protocols developed by corrosion scientists to qualitatively estimate the corrosion protective lifetimes of coatings, the numerical evaluation of metal substrate/organic coatings systems by EIS measurements provides an objective assessment of system performance that tracks performance changes in such systems quite well. For relatively undamaged systems, a simple parallel Randles circuit (RC) is sufficient to model systems performance, but as systems become significantly damaged due to exposure and corrosion at the metal/coating interface, more complex circuits are required. These may offer insight into final failure mechanisms. From the work we have presented and the work of others cited in this work, it can be definitely concluded that EIS and other electrochemical measurements of metal/organic coating systems offer significant advantages to those evaluating such systems, especially when used with accelerated exposure protocols. The numerical, objective nature of the results from EIS enable quantitative assessment of changes in metal/coating systems as they are artificially or naturally weathered, and allow objective evaluation of these systems. EIS is an important measurement tool that should be used routinely by all those examining corrosion control by organic coatings.
Acknowledgements This work was performed with the support of the following: (a) Office of Naval Research under grant no. N00014-95-10507, Dr. A.J. Sedriks, program manager; (b) Air Force Office of Scientific Research under grant F49620-96-1-0284, Lt. Col. P. Trulove and Maj. H. DeLong, program managers; (c) a sub-contract from Boeing, Dr. J. Osborne, PI, the prime contractor for DARPA under
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