- Email: [email protected]
AC electrochemical method
Progress in Organic Coatings 138 (2020) 105365
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Accelerated aging of anticorrosive coatings: Two-stage approach to the AC/ DC/AC electrochemical method
T
T. da Silva Lopesa, T. Lopesa, D. Martinsb, C. Carneirob, J. Machadob, A. Mendesa,⁎ a LEPABE – Laboratory for Process Engineering, Environment, Biotechnology and Energy, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b CIN – Corporação Industrial do Norte, S.A., Avenida Dom Mendo, nº 831, 4471-909, Maia, Portugal
A R T I C LE I N FO
A B S T R A C T
Keywords: Neutral salt spray EIS AC/DC/AC Organic primer Metallic corrosion
Organic coatings are nowadays the most common and cost-effective approach to extend the service life of metallic structures subjected to weathering conditions. The development of new and more effective protective coatings is closely related to the rapid assessment of the anticorrosive performance before industrialization. Currently, the coating/metallic substrate system evaluation is made using accelerated standardized test methods. However, the time needed to perform such tests can be long, and in some cases can hold up for thousands of hours. The AC/DC/AC method is an accelerated cyclic test, consisting of a combination of electrochemical impedance spectroscopy (EIS/AC) characterization and cathodic polarisations (DC). The latter technique has been widely used to assess the adhesion of coatings to the metallic substrate but, to the best knowledge of the authors, no clear evidences have been provided that the aging promoted by AC/DC/AC is comparable to the one promoted by standardized accelerated aging tests, such as the neutral salt spray test. Said comparison is necessary to be able to support the use of the AC/DC/AC method to assess the anticorrosive performance of coatings. This work addresses this gap and goes further, proposing an innovative two-stage AC/DC/AC procedure that allows a better control of the physical delamination inherent to this technique, while promoting a chemical and electrochemical attack of coating and metallic substrate. Two water-based organic primers applied on cold rolled steel were aged by neutral salt spray (aging characterized by EIS) and by the proposed AC/DC/AC procedure. The obtained results were critically compared based on the EIS analysis. It was observed that the twostage AC/DC/AC procedure mimicked the degradation mechanisms occurring during the neutral salt spray test, but producing results in less than 25% of the aging time.
1. Introduction The corrosion of metallic structures has a very large impact in the global economy. In fact, the World Corrosion Organisation (WCO) estimates that the worldwide direct cost due to corrosion is currently higher than 1.3 × 1012 € which is equivalent to 3.8% of the global Gross Domestic Product (GDP) [1]. Within all possible causes that can lead to the corrosion of metals, more than half are related to the atmospheric action, i.e. triggered when metals are exposed to the weather conditions and greatly influenced by some climatic factors such as relative humidity, temperature, acidity, salinity, time of wetness and the presence of other pollutants [1–3]. Organic coatings are nowadays the most common and cost-effective approach to prevent and extend the service life of many metallic structures subjected to weathering conditions [3–5]. Protective organic
⁎
coatings are complex formulations comprising a mixture of different pigments and binders [5]. The protection of metal structures using organic coatings is often obtained via multi-layer systems consisting of at least a primer and a topcoat [6]. The primer, assisted by the inhibitive pigments, prevents the primary corrosion and provides adhesion whereas the topcoat is responsible for environmental (e.g. UV, water, mechanical, microbial) resistance and different functional and aesthetic requirements [6]. Nevertheless, it is well established that the barrier properties against corrosive ions and moisture are closely connected to the integrity of the coating which is frequently affected by environmental and mechanical attacks [6]. Given the economic impact of this protective mechanism, there is a continuous and growing research interest in developing high-performance coatings, that simultaneously meet the environmental, health and safety requirements [7]. Accordingly, success among the R&D of
Corresponding author. E-mail address: [email protected] (A. Mendes).
https://doi.org/10.1016/j.porgcoat.2019.105365 Received 18 January 2019; Received in revised form 22 September 2019; Accepted 24 September 2019 Available online 19 October 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
consequently physical, structure of the coating [20]. This modification is the result of the deterioration of the binder’s molecular cohesion, due to a chemical/photochemical attack to the molecular bonds by aggressive agents from the environment [21]. On the other hand, and as previously mentioned, the deterioration promoted by the cathodic polarization is mostly a physical process (local delamination due to the formation of hydrogen bubbles), which is not totally comparable to what happens when the coating is in contact with air or with the environment within the neutral salt spray chamber. Given the physical nature of the deterioration promoted by the AC/ DC/AC method, several studies highlight the importance of this technique to assess the adhesion of the coating to the substrate. Some authors even refer this electrochemical method as an effective technique to assess the anticorrosive performance of a coating/metallic substrate system [9,13]. However, to the best knowledge of the authors, no clear evidences have been provided that the aging promoted by said technique is comparable to the observed using standardized accelerated aging tests, such as the neutral salt spray – NSS – test. Said comparison is necessary to support the use of the AC/DC/AC technique to assess the anticorrosive performance of coatings. In this work, coated cold rolled steel panels were aged by cathodic polarization and by neutral salt spray, both periodically characterized by EIS analysis. To establish a better correlation between both aging techniques, an innovative AC/ DC/AC procedure is proposed comprehending two stages that allow a fast aging, while preventing the coating delamination inherent to the traditional AC/DC/AC procedure. Two water-based primers were studied and the results obtained using the proposed AC/DC/AC procedure and the neutral salt spray test were critically compared based on the EIS analysis.
new and more effective protective coatings is closely connected to the development of characterization methods that are capable of quickly providing reliable information concerning their protective performance [8]. Currently, the stability to corrosion of coating/metallic substrate systems is obtained by applying accelerated weathering tests [9]. Various testing methods have been developed to simulate environmental conditions, such as the neutral salt-spray test (ISO 9227), which involves continuous exposure to salt-fog, and cyclic corrosion tests, such as Prohesion®, QUV®, or the combination of both - Prohesion®/QUV® (ASTM D 5894), which simulates weathering conditions and UV radiation [3,10,11]. Even though these methods promote a faster coating failure, for some specific products the associated testing time is still quite long (sometimes in the range of months or even years [12]).Since they do not give information regarding the corrosion mechanisms, additional information should be obtained from complementary characterization techniques [12]. In recent years, electrochemical characterization techniques, primarily the electrochemical impedance spectroscopy (EIS), proved to be a successful tool for studying the anticorrosive mechanisms of organic coatings [9,12]. EIS is a non-destructive technique that, when combined with accelerated aging techniques, allows monitoring the changes that take place in the coating/ metallic substrate system [12,13]. Given the explicit interest in quickly assessing the performance of an anticorrosive coating, Hollaender et al. developed an unconventional method for testing coated metals. This technique, named AC/DC/ AC, consists of two steps repeated cyclically: first an EIS characterization – short named AC – followed by a DC step where a cathodic potential is applied to the sample [14,15]. The cathodic polarization is used for speeding up the coating failure and it displays two main effects on the coating/metallic substrate system: i) coating mechanical deterioration and consequently pore formation due to the forced passage of ions and water molecules through the coating; and ii) coating delamination at coating/metal interface due to hydrogen production via the enhancement of the cathodic reaction of water hydrolysis – Eq. (1) [8,9,15–17].
2H2 O(l) + 2e− → H2 (g) + 2OH− (aq)
2. Experimental 2.1. Materials Two commercial water-based primers, supplied by CIN- Corporação Industrial do Norte, S.A., were studied: i) a high-quality anticorrosive primer with a zinc phosphate-based anticorrosive pigment and epoxy and amine resins - primer P1; and ii) an oven-cured anticorrosive primer, composed of alkyd and melamine resins and zinc phosphatebased anticorrosive pigment – primer P2. Both primers were applied on cold rolled steel panels by using a conventional spray gun and cured according to the recommendations of the manufacturer. Prior to the painting process, the steel panels’ surface (100 × 150 × 0.5 mm3) was subjected to a treatment consisting of mechanical cleaning (sanding) and degreasing. Three samples of each primer were prepared for both tests considered - the neutral salt spray test and the AC/DC/AC method. Dry film thickness was determined by using a portable thickness gauge (Elcometer®). The identification of the studied samples was done based on the type of primer and the corresponding aging test to which each sample was subjected, as summarized in Table 1. The PVC (pigment
(1)
The cathodic reaction occurs whenever the electrolyte is able to permeate through the coating and reach the substrate, and if the applied cathodic potential is negative enough to trigger said reaction, e.g. for steel immersed in an NaCl aqueous solution, this potential usually has to be more negative than -1.0 VSCE [18]. The overall coating deterioration resulting from the cathodic polarizations is primarily caused by the film-delamination process at the metallic interface [8]. When an organic coating is exposed to air or even to the environment inside the neutral salt spray chamber, most authors agree that the failure of a coating protective system can be the product of many degradation mechanisms (i.e. pore resistance, coating removal, blister formation) [19]. Despite the variety of models, in many cases the protective system fails due to a change in the chemical, and
Table 1 Identification of the coated samples studied (type of primer, PVC, aging test, dry film thickness) and curing procedure. Primer identification (PVC)
Curing procedure
Sample identification
Aging test
Primer 1 - P1 (34.4%)
15 days at room temperature and humidity
Primer 2 - P2 (22.9%)
30 min at 130 °C (oven)
P1NSS1 P1NSS2 P1NSS3 P1DC1 P1DC2 P1DC3 P2NSS1 P2NSS2 P2NSS3 P2DC1 P2DC2 P2DC3
Neutral salt Neutral salt Neutral salt AC/DC/AC AC/DC/AC AC/DC/AC Neutral salt Neutral salt Neutral salt AC/DC/AC AC/DC/AC AC/DC/AC
2
Dry Film thickness / μm spray spray spray
spray spray spray
53 63 58 52 62 56 43 42 42 43 44 44
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
interpret the experimental EIS data of metallic substrates coated with an organic paint obtained in a three-electrode electrochemical cell [16,23–25]. The electrolyte resistance, Rs, measures the resistance between the reference electrode and working electrode. Rp is the pore resistance, usually associated with the number and diameter of pores or capillary channels through which the electrolyte reaches the interface. The pores resistance usually decreases with the coating exposure to aggressive environments; it can, however, increase as a result of pore blockage by corrosion products, a common phenomenon in zinc-rich paints [23,24]. The coating capacitance, Cc, accounts for the water permeation into the coating, and it is expected to increase as more electrolyte is absorbed by the paint coating [8]. The charge transfer resistance, Rct, and the electrical double layer capacitance, Cdl, are interfacial parameters related to the coating delamination or disbanding. Specifically, Rct is the resistance to electron transfer between the electrode and the ionic species present in the electrolyte, and with the onset of reactive phenomena at the surface of the metallic substrate, this parameter is expected to decrease. The ionic species that do not participate in the redox reactions are attracted to the metal interface, electrically screening the charge of the metallic substrate. This charge separation behaves like a capacitor [26]. The strength of the capacitive behavior is reflected by the value of Cdl, which is expected to change with the variation of the electroactive area of the metallic substrate. Although initially Cdl rises due to an increase of the delaminated area, it can later decrease thanks to the accumulation of corrosion products at the interface, that reduces the electroactive area [23]. As soon as the reactive phenomena starts at the surface of the metal substrate, the resulting products may diffuse through the pores at the paint coating and, as consequence, a Warburg element, Zw, needs to be added to the equivalent circuit model to fit the diffusion phenomena [22]. For fitting the equivalent circuit model to the experimental data, the capacitances were treated as constant phase elements (CPE). Ideally, a capacitor is formed by two conducting plates separated by a non-conducting media (dielectric region), in where there is a uniform distribution of charges. In the coating/metallic substrate system, it is assumed that the micro-roughness and heterogeneity of the substrate’s surface generate a non-uniform distribution of charge; the capacitance of this interface must then be fitted to a CPE. The impedance of a CPE, ZQ, is given by:
volume concentration), curing procedure of each primer and the dry film thicknesses measured for each sample are also presented in Table 1. 2.2. Testing methods 2.2.1. Accelerated test The neutral salt spray test (hereby referenced as NSS test) was performed in accordance with ISO 9227:2014. In this test, the samples were placed into a salt spray chamber with constant temperature of 35 °C and were uniformly exposed to a fog of NaCl aqueous solution of (50 ± 5) g L−1 and a pH between 6.5 and 7.2. Samples were collected at specific periods of time and their condition was analysed by electrochemical impedance spectroscopy (EIS), until a maximum exposure time of about 500 h and 300 h for primer P1 and for primer P2, respectively. The maximum exposure time of each sample was established based on the lifetime expectancy (for the NSS test) of each primer indicated by the manufacturer. 2.2.2. Electrochemical impedance spectroscopy (EIS) EIS tests were carried out in a three-electrode electrochemical cell as shown in Fig. 1. This cell consists of an acrylic tube with an O-ring at the bottom edge; in each measurement the tube is placed over the coated metal sample and compressed between two acrylic plates, as indicated in Fig. 1. The acrylic tube was partially filled (200 mL) with the electrolyte, a NaCl aqueous solution of concentration (50 ± 5) g L−1, being the resulting exposed surface area of 36.3 cm2. The coated metal samples behave as a working electrode and an Ag/AgCl sat. KCl. electrode was used as the reference electrode. A platinized niobium mesh was used as counter electrode and positioned ca. 10 mm above the sample. The electrochemical cell was placed inside a Faraday cage to minimize external interference on the system during the EIS measurements. The impedance spectra were recorded at open circuit potential (OCP) by using an AUTOLAB PGSTAT302 N potentiostat with a frequency response analyser (Metrohm Autolab). All EIS tests were carried out over a frequency range from 0.1 Hz to 100 kHz, using a sinusoidal potential perturbation with amplitude of 10 mV. The experimental EIS data were fitted to electrical equivalent circuit models by using ZView® software (version 3.5d, Scribner Associates, Inc.). 2.2.3. Equivalent circuit interpretation One way of interpreting the impedance spectra is using equivalent circuit models, where it is assumed that the various phenomena that occur in an electrochemical system can be modelled using combinations of electrical resistances, condensers and other electrical elements [22]. The equivalent circuit model presented in Fig. 2 is generally used to
ZQ =
1 CCPE,T (jω)CCPE,P
(2)
where CCPE,T and CCPE,P are parameters of the CPE element. CCPE,P has no physical meaning but indicates the deviation from an ideal dielectric behaviour; a CPE element converges to an ideal capacitor as CCPE,P
Fig. 1. Schematic diagram of the three-electrode configuration used to conduct the electrochemical experiments. 3
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 2. (a) Electrical equivalent circuit used to model the impedance data of coated metals. Rs- electrolyte resistance; Rp- pore resistance; Cc- coating capacitance; Rctcharge transfer resistance; Cdl- double layer capacitance; Zw- Warburg impedance; (b) Relationship between the equivalent circuit model and the coating/metallic substrate system.
measurements was automated by using the AUTOLAB PGSTAT302 N potentiostat with a frequency response analyzer (Metrohm Autolab).
approaches 1 [22,27]. In the electrical equivalent circuit of Fig. 2, each individual CPE is parallel to a resistor, so it is possible to calculate its capacitance by using Eq. (3) [28] :
C=
(CCPE,T R)1/ CCPE,P R
3. Results and discussion (3) 3.1. Neutral salt spray test
where R is the resistance of the resistor parallel to the CPE. All capacitance values presented in the next chapter were determined according to Eq. (3).
To investigate the feasibility of using the AC/DC/AC method to simulate the aging promoted by the neutral salt spray test, three samples of the two primers under study were subjected to this standardized test (samples P1NSS1, P1NSS2, P1NSS3, P2NSS1, P2NSS2 and P2NSS3 –Table 1), and the aging was monitored by EIS. The Nyquist plots obtained for one sample of each primer are shown in Fig. 3. From Fig. 3, three stages of degradation were identified for both products. Before the exposure to NSS (0 h), the coatings show a highly capacitive behavior since no water penetration has yet taken place. However, the system quickly evolves into a combination of capacitive and resistive behaviors, signaled by a semicircular plot, indicating the beginning of the water penetration into the coatings’ film, eventually reaching the substrate. In the third stage, a diffusion tail is visible in the low frequency range of the Nyquist plot; although initially the angle of the tail plot is not clearly 45°, suggesting the overlap of phenomena, it signals the onset of the electrocorrosion of the samples’ substrates [24]. To interpret the impedance spectra, it is necessary to fit an adequate equivalent circuit to the experimental EIS results; the model shown in
2.2.4. AC/DC/AC test The AC/DC/AC test is an electrochemical accelerated aging technique that combines DC (direct current) and AC (alternated current) measurements. In the first step, an AC measurement is performed under the conditions described above. This read allows obtaining the initial impedance status of the system. Following, a cathodic constant potential is applied to the sample for a short time; then, the system is allowed to relax until the open circuit potential reaches a new steady state. Finally, a new EIS read is performed in order to evaluate the new condition of the system after the DC step. This sequence is repeated until the loss of the protective capacity of the coating is achieved. The duration and the intensity of the potential applied in the DC step, as well as the relaxation time, may vary according to the characteristics of the system studied. In the next chapter, a new AC/DC/AC methodology comprising two different stages is proposed. The sequence of AC and DC 4
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 3. Nyquist plot evolution for samples P1NSS1 (a) and P2NSS1 (b) subjected to the NSS test. Inset magnification of the plot for measurements carried after 68–500 h of exposure (a) and 132–288 h of exposure (b).
aged by the NSS test, Fig. 5, exhibits a sharp decrease in the first hours of exposure followed by a slower decrease during the intermediate and final stages of the aging test, suggesting that the degradation process follows two different kinetics. During the first hours of exposure, the major decrease of Rt was assigned to a fast increase of the number and size of pores, causing the decrease of the pore resistance and a major degradation of the coating [19,24,31]. With the continuous exposure to salt spray, it is expected that the pore resistance becomes much lower than the charge transfer resistance [29,30], so the gradual decrease of Rt observed during the intermediate and final stages of the test was assigned to a decrease of Rct. Moreover, the charge transfer resistance, Rct, is expected to decrease with the onset of the electrochemical corrosion [19,31]. In the early stages of the NSS test, it is expected that a well-defined electrolyte/metallic substrate interface has not been formed, so the value of Ct (Ct = Cc + Cdl) is mainly influenced by the coating capacitance, Cc. As previously stated, Cc is considered to be a measure of water permeation into the coating, so an increase of its value, and consequently of the total capacitance value, would be expected as the coating ages and incorporates more water/electrolyte. However, in the first hours of exposure to NSS, Ct maintains a constant value for both primers (Fig. 6). The observed stability of this parameter was then assigned to an almost instantaneous water-saturation of the coating film that translates into a stable value of Cc; the studied primers are porous, so a substantial water uptake was expected right after the contact with the electrolyte. In the later stages of the NSS test, with the eventual coating failure, the stability of the Ct values was assigned mainly to a constant Cdl behavior, an indication of a stable electrolyte/metallic substrate interface; the increase of the degraded coating area should balance with the accumulation of corrosion products, resulting in an overall stable electroactive surface area [16].
Fig. 4. Simplified equivalent circuit used to model impedance data. Rs- electrolyte resistance; Rt- total resistance; Ct- total capacitance; Zw- Warburg impedance.
Fig. 2 is the one usually used to interpret such data. This model includes two parallel resistance and capacitance combinations, considered to contain information regarding coating film and metallic substrate degradation, and the Warburg element that represents the diffusion phenomena [25,29]. A Nyquist plot containing two semicircles and a diffusion tail would be expected. However, the Nyquist plots of P1 and P2 samples aged by NSS, hardly display two semicircles –Fig. 3. This outcome has been reported in the literature and it is common when the paint film is porous [25,29,30], as it is the case of most anticorrosive primers. The observed single-loop in the Nyquist plots was assigned to phenomena that respond in similar frequencies, giving way to a merge between the semicircle representing the coating film degradation and the charge transfer loop [28,31]. Hence, the EIS spectra obtained in this work were considered to be described by a simplified equivalent circuit, as shown in Fig. 4. where the total capacitance (Ct = Cc + Cdl) represents the coating capacitance and the double layer capacitance, and the total resistance (Rt = Rp + Rct) represents the pores resistance and the charge transfer resistance; with a higher exposure time in the salt spray chamber or higher number of AC/DC/AC cycles it can be considered that, due to considerable film deterioration, the pores resistance is much smaller than the charge transfer resistance [29,30]. The simplified equivalent circuit model displayed in Fig. 4 fits quite well the experimental results obtained, as it can be observed in Figure S1 (supplementary information). Figs. 5 and 6 show the history of the average of parameters Rt and Ct obtained for three samples of each primer aged by the NSS test. The standard deviation of the mean is also displayed, and the individual results obtained for each studied sample are presented in Figures S3 and S4 and Table S1. The total resistance (Rt = Rp + Rct) obtained for both primers when
3.2. AC/DC/AC test 3.2.1. Preliminary assessment Previous reports regarding the application of the AC/DC/AC technique to study anticorrosion properties of organic coatings describe experimental procedures that include long cathodic polarizations steps (around 20 min), with potentials ranging from −1 V to −4 V, followed by relaxation times that can exceed 3 h. Table 2, summarizes some of the different AC/DC/AC procedures that can be found in literature. As a first attempt to degrade the two different samples under study, it was decided to apply procedure no. 1 since this is the one most 5
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 5. Total resistance history in NSS test for samples of primer P1 (a) and of Primer P2 (b).
Fig. 6. Total capacitance history in NSS test for samples of primer P1 (a) and of primer P2 (b). Table 2 Various AC/DC/AC procedures found in literature. Procedure number
Cathodic Potential / V
Polarization time / min
Relaxation time / h
Reference
1 2 3 4 5
−4 −2 −3 −1 to −4 −4
20 20 120 20 20
3 3 3 2 to 4 1 to 4
[8,9,17] [13,16] [31] [3] [32]
Fig. 8. Total resistance history in NSS test and as a function of the number of AC/DC/AC cycles for samples of primer P1.
referred in the literature. However, as it was quickly observed, in less than 10 AC/DC/AC cycles (around 210 min per cycle) the samples showed severe film delamination due to hydrogen accumulation between the metallic surface and the coating film. Since the duration of each cycle was considerably long, it was impossible to pinpoint the moment when the water hydrolysis started, even analyzing the impedance spectra obtained after each aging cycle; when the EIS spectrum
Fig. 7. P1 sample after 8 cycles of the AC/DC/AC procedure no.1 (Table 2).
6
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Table 3 Experimental conditions of the proposed AC/DC/AC test. Sample
P1 P2
Stage-I Cathodic Potential / V
Polarization time / s
Relaxation time / s
Number of cycles
Stage-II Cathodic Potential / V
Polarization time / s
Relaxation time / s
−4 −4
150 150
1000 1000
170 13
−1.85 −1.85
150 150
800 800
Total number of cycles / Analysis time
322 / 116 h 63 / 22 h
Fig. 9. Schematic representation of the proposed AC/DC/AC procedure (n1: number of cycles of Stage-I, n2: number of cycles of Stage-II, tp1: polarization time for a Stage-I cycle, tp2: polarization time for a Stage-II cycle, t1: duration of a Stage-I cycle, t2: duration of a Stage-II cycle).
Fig. 10. Nyquist plot evolution for samples P1DC1 (a) and P2DC1 (b) subjected to the proposed AC/DC/AC test. Inset magnification of the plot for cycles 171, 230 and 322 (a) and for cycles 14, 37, 44 and 63 (b).
cathodic potentials of −2 V and −4 V. The EIS data were fitted to the electrical equivalent circuit presented in Fig. 4 and the evolution of Rt was critically compared with the results obtained in NSS – Fig. 5. Taking as example the data obtained for P1 samples (Fig. 8), a cathodic potential of −4 V seemed to mimic the initial fast deterioration reported in NSS but would lead to severe local delamination. Applying −2 V in the DC stage, local delamination would develop more slowly and film degradation was more gradual, very similar to that observed in latter stages of NSS. However, the overall aging time was still considerably long. In the light of these results, an innovative approach was envisioned
showed the first indication of cathodic reaction (diffusion tail in the Nyquist plot), the sample displayed already significant coating delamination – Fig. 7: Given these results, it was consensual that a different approach to the AC/DC/AC technique could be essential to promote an electrochemical aging similar to what is observed in the neutral salt spray chamber. Accordingly, an experimental procedure had to be optimized to better identify the onset of the local delamination and prevent its occurrence in the DC step. In the early stages of the optimization procedure, P1 and P2 samples were aged by AC/DC/AC using shorter polarization and relaxation times (150 s and 1000 s, respectively) and 7
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 11. Total resistance history in NSS test and as a function of the number/aging time of AC/DC/AC cycles for samples of primer P1 (a), and of Primer P2 (b).
Fig. 12. Total capacitance history in NSS test and as a function of the number/aging time of AC/DC/AC cycles for samples of primer P1 (a), and of primer P2 (b).
to take advantage of the fast deterioration promoted by a −4 V cathodic potential and the slower degradation observed with lower cathodic potentials. Accordingly, the proposed AC/DC/AC procedure comprises two different stages as follow:
[33]. Thus, it’s appearance indicates that there is electrolyte-metallic substrate contact, which leads to the redox reactions associated with corrosion phenomena and, if the cathodic potential is high enough, to HER [3]. During the AC/DC/AC method optimization, it was observed that the appearance of the diffusion tail was followed by local delamination. However, decreasing the cathodic potential value during the DC step just after the diffusion tail onset, mitigated said delamination. This latter conclusion was crucial in choosing the diffusion tail onset as the switching-criterion. Table 3 schematizes the optimized experimental parameters used in the AC/DC/AC electrochemical aging tests. This experimental procedure allowed a faster but controlled initial film degradation. By applying short DC steps (150 s), it was possible to identify the cycle when local delamination could start, and thus when Stage-II should be initiated. The cathodic potential chosen for Stage-II was -1.85 V, balancing two requirements: i) hydrogen evolution reaction should not occur; and ii) the degradation of the paint coating film should occur as fast as possible. A schematic representation of the proposed AC/DC/AC procedure is presented in Fig. 9.
1) Stage-I: high cathodic polarization potential is applied to the sample for a short period. This initial stage forces a fast permeation of ions and water molecules into the coating, favoring the formation of pores; 2) Stage-II: lower cathodic polarization potential is applied to the samples to avoid the hydrogen evolution reaction (HER), though allowing continuing the film degradation and subsequent corrosion of the substrate. To determine the number of AC/DC/AC cycles that should be applied in Stage-I before moving to Stage-II, the appearance of a diffusion tail in the Nyquist plot was used as switching-criterion. Although a Warburg-type diffusion process is not directly related to H2 evolution, in this type of systems the diffusion tail is usually associated with the formation of corrosion products that diffuse out the porous coating 8
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 13. Total resistance history in NSS test (a) and as a function of the number of AC/DC/AC cycles (b), for P1 and P2 samples.
To compare the anticorrosive performance of both primers, Rt values obtained for both products were plotted as a function of time for the NSS aging test (Fig. 13a), and as function of the number of cycles (Fig. 13b) for the AC/DC/AC aging test. The results obtained for both primers show a similar pattern for the total resistance, though it is clear that the anticorrosive performance of primer P1 is substantially better than primer P2. The rapid decrease of Rt at the early stages of the aging tests, mostly related to the drop of Rp, is much more accentuated for P2 than for P1. This result suggests that P2 loses more quickly its sealing ability, allowing for an earlier electrolyte/metallic substrate contact. In the latter stages of the tests, the continuous drop of the total resistance is also more evident in P2, which may be the result of a more rapid development of reactive phenomena at the metal surface, leading to a considerable coating and metal substrate deterioration in a shorter aging period.
3.2.2. Accelerated aging results To assess the proposed AC/DC/AC aging method for characterizing anticorrosive coatings, three samples of each primer were subjected to the proposed aging test (samples P1DC1, P1DC2, P1DC3, P2DC1, P2DC2 and P2DC3).The Nyquist plots obtained for one sample of each primer aged by the new procedure (P1DC1 and P2DC1) is shown in Fig. 10. P1 samples required 170 cycles in Stage-I until a diffusion tail was evident in the Nyquist plot (Fig. 10a). Afterwards, the experimental conditions of Stage-II were applied and 152 cycles were performed until achieving a coating degradation similar to that observed after 500 h of exposure to neutral salt spray. P2 samples were submitted to Stage-I operating conditions for 13 cycles and to Stage-II operating conditions for 50 cycles (Fig. 10b). At the end, these samples displayed a similar degradation to the one observed after 300 h of exposure to NSS conditions. The comparison between the degradation promoted by both aging methods was based on the corresponding Nyquist plots. When the diameter of the high-frequency semi-circle obtained during the new procedure matched the one observed in the last EIS measurement of the NSS test, within a relative difference smaller than 20%, the samples aged by the two methods were considered equivalently degraded. The simplified equivalent circuit model displayed in Fig. 4 was fitted to the experimental results (see an example fit in Figure S2). Figs. 11 and 12 show the history of the average of parameters Rt and Ct obtained for three samples of each primer aged by the AC/DC/AC test (individual results can be found in Figures S5 and S6 and Table S2) and compare it with the parameters previously obtained for the NSS test. Comparing the results obtained for both aging tests, it is possible to observe a similar evolution of parameters Rt and Ct (Figs. 11 and 12, respectively) in the evaluation of both primers. During Stage-I of the AC/DC/AC method, the total resistance values show an accentuated decrease very similar to that observed for the NSS test; this result suggests that the high cathodic potential applied during this stage promoted an accelerated coating deterioration. Additionally, it is important to mention that no major local delamination was visually observed, thus the observed Rt decrease can be assigned mainly to the forced permeation of ions and water molecules through the coating, leading to a fast initial degradation. Afterwards, with the onset of StageII (lower cathodic potential), the decrease of Rt is more gradual, as it was observed for the NSS test. Said behavior was assigned to the intensification of the electrochemical corrosion. The obtained results suggest that the proposed AC/DC/AC procedure simulates the two degradation steps previously mentioned for the NSS and further accelerate them, reducing the aging time to less than 25% (Figure S7).
4. Conclusions A new approach to the original AC/DC/AC method is proposed for characterizing the anticorrosive performance of coating/metallic substrate systems by simulating the degradation observed for the neutral salt spray (NSS) tests in shorter testing times. The new test includes two different DC stages that allow preventing the early coating delamination induced by the hydrogen evolution reaction during the cathodic polarization step, while promoting the chemical and electrochemical attack of the coating and substrate. Two water-based organic primers – P1 formulated with epoxy and amine resins and P2 formulated with alkyd and melamine resins – were applied on cold rolled steel and aged by NSS test (combined with EIS analysis) and by the proposed AC/DC/AC procedure. To quantitatively compare the two aging methods, the EIS data were fitted to a simplified equivalent circuit and the respective parameters, total resistance (Rt) and total capacitance (Ct), were determined and critically compared. The results obtained for both aging methods showed that primer P1 displayed a substantially better anticorrosion performance than primer P2. Additionally, for both P1 and P2, the evolution of Rt and Ct, obtained with both techniques were quite similar. This outcome strongly suggests that the proposed AC/DC/AC procedure allowed to obtain essentially the same information as the NSS test, but entailed much shorter testing times, since both primers were characterized in less than 25% of the time required for the NSS tests.
9
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Declaration of Competing Interest
[8] S.J. Garcia, J. Suay, A comparative study between the results of different electrochemical techniques (EIS and AC/DC/AC): application to the optimisation of the cataphoretic and curing parameters of a primer for the automotive industry, Prog. Org. Coat. 59 (2007) 251–258. [9] S.J. Garcia, J. Suay, Optimization of deposition voltage of cataphoretic automotive primers assessed by EIS and AC/DC/AC, Prog. Org. Coat. 66 (2009) 306–313. [10] F. Deflorian, S. Rossi, M. Fedel, Organic coatings degradation: comparison between natural and artificial weathering, Corros. Sci. 50 (2008) 2360–2366. [11] F. Sutter, A. Fernandez-García, J. Wette, P. Heller, Comparison and evaluation of accelerated aging tests for reflectors, Energy Procedia 49 (2014) 1718–1727. [12] G.P. Bierwagen, D. Tallman, J.P. Li, L.Y. He, C. Jeffcoate, EIS studies of coated metals in accelerated exposure, Prog. Org. Coat. 46 (2003) 148–157. [13] M. Bethencourt, F.J. Botana, M.J. Cano, R.M. Osuna, M. Marcos, Lifetime prediction of waterborne acrylic paints with the AC–DC–AC method, Prog. Org. Coat. 49 (2004) 275–281. [14] J. Hollaender, E. Ludwig, S. Hillebrand, Assessing protective layers on metal packaging material by electrochemical impedance spectroscopy, Proceedings of the Fifth International Tinplate Conference, London, 1992, pp. 300–315. [15] J. Hollaender, Rapid assessment of food/package interactions by electrochemical impedance spectroscopy (EIS), Food Addit. Contam. 14 (1997) 617–626. [16] S.J. Garcia, J. Suay, Application of electrochemical techniques to study the effect on the anticorrosive properties of the addition of ytterbiurn and erbium triflates as catalysts on a powder epoxy network, Prog. Org. Coat. 57 (2006) 273–281. [17] S. Lajevardi Esfahani, Z. Ranjbar, S. Rastegar, Evaluation of anticorrosion behavior of automotive electrocoating primers by the AC-DC-AC accelerated test method, Prog. Color Color. Coat. 7 (2013) 187–199. [18] H. Leidheiser, W. Wang, L. Igetoft, The mechanism for the cathodic delamination of organic coatings from a metal-surface, Prog. Org. Coat. 11 (1983) 19–40. [19] A.C.A. de Vooys, B. Boelen, D.H. van der Weijde, Screening of coated metal packaging cans using EIS, Prog. Org. Coat. 73 (2012) 202–210. [20] P.A. Sørensen, S. Kiil, K. Dam-Johansen, C.E. Weinell, Anticorrosive coatings: a review, J. Coat. Technol. Res. 6 (2009) 135–176. [21] D. Kotnarowska, Influence of ultraviolet radiation and aggressive media on epoxy coating degradation, Prog. Org. Coat. 37 (1999) 149–159. [22] V.F. Lvovich, Impedance Spectroscopy: Applications to Electrochemical and Dielectric Phenomena, Wiley, Hoboken, N.J, 2012. [23] F. Mansfeld, Models for the impedance behavior of protective coatings and cases of localized corrosion, Electrochim. Acta 38 (1993) 1891–1897. [24] R. Duraisamy, K. Pownsamy, G. Asgedom, Chemical degradation of epoxy-polyamide primer by electrochemical impedance spectroscopy, ISRN Corros. 2012 (2012) 10. [25] G.W. Walter, A review of impedance plot methods used for corrosion performance analysis of painted metals, Corros. Sci. 26 (1986) 681–703. [26] R.P. O’Hayre, S.-W. Cha, W. Colella, F.B. Prinz, Fuel Cell Fundamentals, John Wiley & Sons, Hoboken, NJ, 2006. [27] P. Zoltowski, On the electrical capacitance of interfaces exhibiting constant phase element behaviour, J. Electroanal. Chem. 443 (1998) 149–154. [28] M.E. Orazem, P. Shukla, M.A. Membrino, Extension of the measurement model approach for deconvolution of underlying distributions for impedance measurements, Electrochim. Acta 47 (2002) 2027–2034. [29] T. Hong, Y.H. Sun, W.P. Jepson, Study on corrosion inhibitor in large pipelines under multiphase flow using EIS, Corros. Sci. 44 (2002) 101–112. [30] Y. Chen, T. Hong, M. Gopal, W.P. Jepson, EIS studies of a corrosion inhibitor behavior under multiphase flow conditions, Corros. Sci. 42 (2000) 979–990. [31] M. Poelman, M.G. Olivier, N. Gayarre, J.P. Petitjean, Electrochemical study of different ageing tests for the evaluation of a cataphoretic epoxy primer on aluminium, Prog. Org. Coat. 54 (2005) 55–62. [32] S.L. Esfahani, Z. Ranjbar, S. Rastegar, An electrochemical and mechanical approach to the corrosion resistance of cathodic electrocoatings under combined cyclic and DC polarization conditions, Prog. Org. Coat. 77 (2014) 1264–1270. [33] A.G. Marques, A.M. Simões, EIS and SVET assessment of corrosion resistance of thin Zn-55% Al-rich primers: effect of immersion and of controlled deformation, Electrochim. Acta 148 (2014) 153–163.
The authors declare that they have no conflict of interest. Acknowledgements The authors would like to thank CIN- Corporação Industrial do Norte for supplying the water-based primers and Ms. Joana Fonseca for her valuable help and collaboration in the development of this project. T. Lopes acknowledges the Portuguese Foundation for Science and Technology (FCT) for her Postdoctoral Grant with Ref. SFRH/BPD/ 102408/2014. The research leading to these results has received funding from: (i) Project POCI-01-0145-FEDER-016387 "SunStorage – Harvesting and storage of solar energy", (ii) Project POCI-01-0145FEDER-030760 “HopeH2 - Efficient, stable and scalable tandem PEC-PV device for solar hydrogen generation”, both funded by the European Regional Development Fund (ERDF), through COMPETE2020 Operational Programme for Competitiveness and Internationalisation (OPCI) and by national funds, through FCT; (iii) Project UID/EQU/ 00511/2019 - Laboratory for Process Engineering, Environment, Biotechnology and Energy – LEPABE funded by national funds through FCT/MCTES (PIDDAC); and (iv) NORTE-01-0145-FEDER-000005 – LEPABE-2-ECO-INNOVATION, supported by North Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the ERDF. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019. 105365. References [1] B. Chico, D. de la Fuente, I. Díaz, J. Simancas, M. Morcillo, Annual atmospheric corrosion of carbon steel worldwide. An integration of ISOCORRAG, ICP/UNECE and MICAT databases, Materials (Basel, Switzerland) 10 (2017) 601. [2] A.A.M.T. Adikari, R.G.N.De S. Munasinghe, S. Jayathilake, Prediction of atmospheric corrosion –a review, Eng. J. Inst. Eng. Sri Lanka 47 (2014) 75–83. [3] K.N. Allahar, G.P. Bierwagen, V.J. Gelling, Understanding ac–dc–ac accelerated test results, Corros. Sci. 52 (2010) 1106–1114. [4] M.L. Zheludkevich, J. Tedim, M.G.S. Ferreira, “Smart” coatings for active corrosion protection based on multi-functional micro and nanocontainers, Electrochim. Acta 82 (2012) 314–323. [5] S.B. Lyon, R. Bingham, D.J. Mills, Advances in corrosion protection by organic coatings: what we know and what we would like to know, Prog. Org. Coat. 102 (2017) 2–7. [6] F. Zhang, P. Ju, M. Pan, D. Zhang, Y. Huang, G. Li, X. Li, Self-healing mechanisms in smart protective coatings: a review, Corros. Sci. 144 (2018) 74–88. [7] H.R. Asemani, P. Ahmadi, A.A. Sarabi, H. Eivaz Mohammadloo, Effect of zirconium conversion coating: adhesion and anti-corrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild steel, Prog. Org. Coat. 94 (2016) 18–27.
10
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Accelerated aging of anticorrosive coatings: Two-stage approach to the AC/ DC/AC electrochemical method
T
T. da Silva Lopesa, T. Lopesa, D. Martinsb, C. Carneirob, J. Machadob, A. Mendesa,⁎ a LEPABE – Laboratory for Process Engineering, Environment, Biotechnology and Energy, Faculty of Engineering, University of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal b CIN – Corporação Industrial do Norte, S.A., Avenida Dom Mendo, nº 831, 4471-909, Maia, Portugal
A R T I C LE I N FO
A B S T R A C T
Keywords: Neutral salt spray EIS AC/DC/AC Organic primer Metallic corrosion
Organic coatings are nowadays the most common and cost-effective approach to extend the service life of metallic structures subjected to weathering conditions. The development of new and more effective protective coatings is closely related to the rapid assessment of the anticorrosive performance before industrialization. Currently, the coating/metallic substrate system evaluation is made using accelerated standardized test methods. However, the time needed to perform such tests can be long, and in some cases can hold up for thousands of hours. The AC/DC/AC method is an accelerated cyclic test, consisting of a combination of electrochemical impedance spectroscopy (EIS/AC) characterization and cathodic polarisations (DC). The latter technique has been widely used to assess the adhesion of coatings to the metallic substrate but, to the best knowledge of the authors, no clear evidences have been provided that the aging promoted by AC/DC/AC is comparable to the one promoted by standardized accelerated aging tests, such as the neutral salt spray test. Said comparison is necessary to be able to support the use of the AC/DC/AC method to assess the anticorrosive performance of coatings. This work addresses this gap and goes further, proposing an innovative two-stage AC/DC/AC procedure that allows a better control of the physical delamination inherent to this technique, while promoting a chemical and electrochemical attack of coating and metallic substrate. Two water-based organic primers applied on cold rolled steel were aged by neutral salt spray (aging characterized by EIS) and by the proposed AC/DC/AC procedure. The obtained results were critically compared based on the EIS analysis. It was observed that the twostage AC/DC/AC procedure mimicked the degradation mechanisms occurring during the neutral salt spray test, but producing results in less than 25% of the aging time.
1. Introduction The corrosion of metallic structures has a very large impact in the global economy. In fact, the World Corrosion Organisation (WCO) estimates that the worldwide direct cost due to corrosion is currently higher than 1.3 × 1012 € which is equivalent to 3.8% of the global Gross Domestic Product (GDP) [1]. Within all possible causes that can lead to the corrosion of metals, more than half are related to the atmospheric action, i.e. triggered when metals are exposed to the weather conditions and greatly influenced by some climatic factors such as relative humidity, temperature, acidity, salinity, time of wetness and the presence of other pollutants [1–3]. Organic coatings are nowadays the most common and cost-effective approach to prevent and extend the service life of many metallic structures subjected to weathering conditions [3–5]. Protective organic
⁎
coatings are complex formulations comprising a mixture of different pigments and binders [5]. The protection of metal structures using organic coatings is often obtained via multi-layer systems consisting of at least a primer and a topcoat [6]. The primer, assisted by the inhibitive pigments, prevents the primary corrosion and provides adhesion whereas the topcoat is responsible for environmental (e.g. UV, water, mechanical, microbial) resistance and different functional and aesthetic requirements [6]. Nevertheless, it is well established that the barrier properties against corrosive ions and moisture are closely connected to the integrity of the coating which is frequently affected by environmental and mechanical attacks [6]. Given the economic impact of this protective mechanism, there is a continuous and growing research interest in developing high-performance coatings, that simultaneously meet the environmental, health and safety requirements [7]. Accordingly, success among the R&D of
Corresponding author. E-mail address: [email protected] (A. Mendes).
https://doi.org/10.1016/j.porgcoat.2019.105365 Received 18 January 2019; Received in revised form 22 September 2019; Accepted 24 September 2019 Available online 19 October 2019 0300-9440/ © 2019 Elsevier B.V. All rights reserved.
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
consequently physical, structure of the coating [20]. This modification is the result of the deterioration of the binder’s molecular cohesion, due to a chemical/photochemical attack to the molecular bonds by aggressive agents from the environment [21]. On the other hand, and as previously mentioned, the deterioration promoted by the cathodic polarization is mostly a physical process (local delamination due to the formation of hydrogen bubbles), which is not totally comparable to what happens when the coating is in contact with air or with the environment within the neutral salt spray chamber. Given the physical nature of the deterioration promoted by the AC/ DC/AC method, several studies highlight the importance of this technique to assess the adhesion of the coating to the substrate. Some authors even refer this electrochemical method as an effective technique to assess the anticorrosive performance of a coating/metallic substrate system [9,13]. However, to the best knowledge of the authors, no clear evidences have been provided that the aging promoted by said technique is comparable to the observed using standardized accelerated aging tests, such as the neutral salt spray – NSS – test. Said comparison is necessary to support the use of the AC/DC/AC technique to assess the anticorrosive performance of coatings. In this work, coated cold rolled steel panels were aged by cathodic polarization and by neutral salt spray, both periodically characterized by EIS analysis. To establish a better correlation between both aging techniques, an innovative AC/ DC/AC procedure is proposed comprehending two stages that allow a fast aging, while preventing the coating delamination inherent to the traditional AC/DC/AC procedure. Two water-based primers were studied and the results obtained using the proposed AC/DC/AC procedure and the neutral salt spray test were critically compared based on the EIS analysis.
new and more effective protective coatings is closely connected to the development of characterization methods that are capable of quickly providing reliable information concerning their protective performance [8]. Currently, the stability to corrosion of coating/metallic substrate systems is obtained by applying accelerated weathering tests [9]. Various testing methods have been developed to simulate environmental conditions, such as the neutral salt-spray test (ISO 9227), which involves continuous exposure to salt-fog, and cyclic corrosion tests, such as Prohesion®, QUV®, or the combination of both - Prohesion®/QUV® (ASTM D 5894), which simulates weathering conditions and UV radiation [3,10,11]. Even though these methods promote a faster coating failure, for some specific products the associated testing time is still quite long (sometimes in the range of months or even years [12]).Since they do not give information regarding the corrosion mechanisms, additional information should be obtained from complementary characterization techniques [12]. In recent years, electrochemical characterization techniques, primarily the electrochemical impedance spectroscopy (EIS), proved to be a successful tool for studying the anticorrosive mechanisms of organic coatings [9,12]. EIS is a non-destructive technique that, when combined with accelerated aging techniques, allows monitoring the changes that take place in the coating/ metallic substrate system [12,13]. Given the explicit interest in quickly assessing the performance of an anticorrosive coating, Hollaender et al. developed an unconventional method for testing coated metals. This technique, named AC/DC/ AC, consists of two steps repeated cyclically: first an EIS characterization – short named AC – followed by a DC step where a cathodic potential is applied to the sample [14,15]. The cathodic polarization is used for speeding up the coating failure and it displays two main effects on the coating/metallic substrate system: i) coating mechanical deterioration and consequently pore formation due to the forced passage of ions and water molecules through the coating; and ii) coating delamination at coating/metal interface due to hydrogen production via the enhancement of the cathodic reaction of water hydrolysis – Eq. (1) [8,9,15–17].
2H2 O(l) + 2e− → H2 (g) + 2OH− (aq)
2. Experimental 2.1. Materials Two commercial water-based primers, supplied by CIN- Corporação Industrial do Norte, S.A., were studied: i) a high-quality anticorrosive primer with a zinc phosphate-based anticorrosive pigment and epoxy and amine resins - primer P1; and ii) an oven-cured anticorrosive primer, composed of alkyd and melamine resins and zinc phosphatebased anticorrosive pigment – primer P2. Both primers were applied on cold rolled steel panels by using a conventional spray gun and cured according to the recommendations of the manufacturer. Prior to the painting process, the steel panels’ surface (100 × 150 × 0.5 mm3) was subjected to a treatment consisting of mechanical cleaning (sanding) and degreasing. Three samples of each primer were prepared for both tests considered - the neutral salt spray test and the AC/DC/AC method. Dry film thickness was determined by using a portable thickness gauge (Elcometer®). The identification of the studied samples was done based on the type of primer and the corresponding aging test to which each sample was subjected, as summarized in Table 1. The PVC (pigment
(1)
The cathodic reaction occurs whenever the electrolyte is able to permeate through the coating and reach the substrate, and if the applied cathodic potential is negative enough to trigger said reaction, e.g. for steel immersed in an NaCl aqueous solution, this potential usually has to be more negative than -1.0 VSCE [18]. The overall coating deterioration resulting from the cathodic polarizations is primarily caused by the film-delamination process at the metallic interface [8]. When an organic coating is exposed to air or even to the environment inside the neutral salt spray chamber, most authors agree that the failure of a coating protective system can be the product of many degradation mechanisms (i.e. pore resistance, coating removal, blister formation) [19]. Despite the variety of models, in many cases the protective system fails due to a change in the chemical, and
Table 1 Identification of the coated samples studied (type of primer, PVC, aging test, dry film thickness) and curing procedure. Primer identification (PVC)
Curing procedure
Sample identification
Aging test
Primer 1 - P1 (34.4%)
15 days at room temperature and humidity
Primer 2 - P2 (22.9%)
30 min at 130 °C (oven)
P1NSS1 P1NSS2 P1NSS3 P1DC1 P1DC2 P1DC3 P2NSS1 P2NSS2 P2NSS3 P2DC1 P2DC2 P2DC3
Neutral salt Neutral salt Neutral salt AC/DC/AC AC/DC/AC AC/DC/AC Neutral salt Neutral salt Neutral salt AC/DC/AC AC/DC/AC AC/DC/AC
2
Dry Film thickness / μm spray spray spray
spray spray spray
53 63 58 52 62 56 43 42 42 43 44 44
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
interpret the experimental EIS data of metallic substrates coated with an organic paint obtained in a three-electrode electrochemical cell [16,23–25]. The electrolyte resistance, Rs, measures the resistance between the reference electrode and working electrode. Rp is the pore resistance, usually associated with the number and diameter of pores or capillary channels through which the electrolyte reaches the interface. The pores resistance usually decreases with the coating exposure to aggressive environments; it can, however, increase as a result of pore blockage by corrosion products, a common phenomenon in zinc-rich paints [23,24]. The coating capacitance, Cc, accounts for the water permeation into the coating, and it is expected to increase as more electrolyte is absorbed by the paint coating [8]. The charge transfer resistance, Rct, and the electrical double layer capacitance, Cdl, are interfacial parameters related to the coating delamination or disbanding. Specifically, Rct is the resistance to electron transfer between the electrode and the ionic species present in the electrolyte, and with the onset of reactive phenomena at the surface of the metallic substrate, this parameter is expected to decrease. The ionic species that do not participate in the redox reactions are attracted to the metal interface, electrically screening the charge of the metallic substrate. This charge separation behaves like a capacitor [26]. The strength of the capacitive behavior is reflected by the value of Cdl, which is expected to change with the variation of the electroactive area of the metallic substrate. Although initially Cdl rises due to an increase of the delaminated area, it can later decrease thanks to the accumulation of corrosion products at the interface, that reduces the electroactive area [23]. As soon as the reactive phenomena starts at the surface of the metal substrate, the resulting products may diffuse through the pores at the paint coating and, as consequence, a Warburg element, Zw, needs to be added to the equivalent circuit model to fit the diffusion phenomena [22]. For fitting the equivalent circuit model to the experimental data, the capacitances were treated as constant phase elements (CPE). Ideally, a capacitor is formed by two conducting plates separated by a non-conducting media (dielectric region), in where there is a uniform distribution of charges. In the coating/metallic substrate system, it is assumed that the micro-roughness and heterogeneity of the substrate’s surface generate a non-uniform distribution of charge; the capacitance of this interface must then be fitted to a CPE. The impedance of a CPE, ZQ, is given by:
volume concentration), curing procedure of each primer and the dry film thicknesses measured for each sample are also presented in Table 1. 2.2. Testing methods 2.2.1. Accelerated test The neutral salt spray test (hereby referenced as NSS test) was performed in accordance with ISO 9227:2014. In this test, the samples were placed into a salt spray chamber with constant temperature of 35 °C and were uniformly exposed to a fog of NaCl aqueous solution of (50 ± 5) g L−1 and a pH between 6.5 and 7.2. Samples were collected at specific periods of time and their condition was analysed by electrochemical impedance spectroscopy (EIS), until a maximum exposure time of about 500 h and 300 h for primer P1 and for primer P2, respectively. The maximum exposure time of each sample was established based on the lifetime expectancy (for the NSS test) of each primer indicated by the manufacturer. 2.2.2. Electrochemical impedance spectroscopy (EIS) EIS tests were carried out in a three-electrode electrochemical cell as shown in Fig. 1. This cell consists of an acrylic tube with an O-ring at the bottom edge; in each measurement the tube is placed over the coated metal sample and compressed between two acrylic plates, as indicated in Fig. 1. The acrylic tube was partially filled (200 mL) with the electrolyte, a NaCl aqueous solution of concentration (50 ± 5) g L−1, being the resulting exposed surface area of 36.3 cm2. The coated metal samples behave as a working electrode and an Ag/AgCl sat. KCl. electrode was used as the reference electrode. A platinized niobium mesh was used as counter electrode and positioned ca. 10 mm above the sample. The electrochemical cell was placed inside a Faraday cage to minimize external interference on the system during the EIS measurements. The impedance spectra were recorded at open circuit potential (OCP) by using an AUTOLAB PGSTAT302 N potentiostat with a frequency response analyser (Metrohm Autolab). All EIS tests were carried out over a frequency range from 0.1 Hz to 100 kHz, using a sinusoidal potential perturbation with amplitude of 10 mV. The experimental EIS data were fitted to electrical equivalent circuit models by using ZView® software (version 3.5d, Scribner Associates, Inc.). 2.2.3. Equivalent circuit interpretation One way of interpreting the impedance spectra is using equivalent circuit models, where it is assumed that the various phenomena that occur in an electrochemical system can be modelled using combinations of electrical resistances, condensers and other electrical elements [22]. The equivalent circuit model presented in Fig. 2 is generally used to
ZQ =
1 CCPE,T (jω)CCPE,P
(2)
where CCPE,T and CCPE,P are parameters of the CPE element. CCPE,P has no physical meaning but indicates the deviation from an ideal dielectric behaviour; a CPE element converges to an ideal capacitor as CCPE,P
Fig. 1. Schematic diagram of the three-electrode configuration used to conduct the electrochemical experiments. 3
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 2. (a) Electrical equivalent circuit used to model the impedance data of coated metals. Rs- electrolyte resistance; Rp- pore resistance; Cc- coating capacitance; Rctcharge transfer resistance; Cdl- double layer capacitance; Zw- Warburg impedance; (b) Relationship between the equivalent circuit model and the coating/metallic substrate system.
measurements was automated by using the AUTOLAB PGSTAT302 N potentiostat with a frequency response analyzer (Metrohm Autolab).
approaches 1 [22,27]. In the electrical equivalent circuit of Fig. 2, each individual CPE is parallel to a resistor, so it is possible to calculate its capacitance by using Eq. (3) [28] :
C=
(CCPE,T R)1/ CCPE,P R
3. Results and discussion (3) 3.1. Neutral salt spray test
where R is the resistance of the resistor parallel to the CPE. All capacitance values presented in the next chapter were determined according to Eq. (3).
To investigate the feasibility of using the AC/DC/AC method to simulate the aging promoted by the neutral salt spray test, three samples of the two primers under study were subjected to this standardized test (samples P1NSS1, P1NSS2, P1NSS3, P2NSS1, P2NSS2 and P2NSS3 –Table 1), and the aging was monitored by EIS. The Nyquist plots obtained for one sample of each primer are shown in Fig. 3. From Fig. 3, three stages of degradation were identified for both products. Before the exposure to NSS (0 h), the coatings show a highly capacitive behavior since no water penetration has yet taken place. However, the system quickly evolves into a combination of capacitive and resistive behaviors, signaled by a semicircular plot, indicating the beginning of the water penetration into the coatings’ film, eventually reaching the substrate. In the third stage, a diffusion tail is visible in the low frequency range of the Nyquist plot; although initially the angle of the tail plot is not clearly 45°, suggesting the overlap of phenomena, it signals the onset of the electrocorrosion of the samples’ substrates [24]. To interpret the impedance spectra, it is necessary to fit an adequate equivalent circuit to the experimental EIS results; the model shown in
2.2.4. AC/DC/AC test The AC/DC/AC test is an electrochemical accelerated aging technique that combines DC (direct current) and AC (alternated current) measurements. In the first step, an AC measurement is performed under the conditions described above. This read allows obtaining the initial impedance status of the system. Following, a cathodic constant potential is applied to the sample for a short time; then, the system is allowed to relax until the open circuit potential reaches a new steady state. Finally, a new EIS read is performed in order to evaluate the new condition of the system after the DC step. This sequence is repeated until the loss of the protective capacity of the coating is achieved. The duration and the intensity of the potential applied in the DC step, as well as the relaxation time, may vary according to the characteristics of the system studied. In the next chapter, a new AC/DC/AC methodology comprising two different stages is proposed. The sequence of AC and DC 4
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 3. Nyquist plot evolution for samples P1NSS1 (a) and P2NSS1 (b) subjected to the NSS test. Inset magnification of the plot for measurements carried after 68–500 h of exposure (a) and 132–288 h of exposure (b).
aged by the NSS test, Fig. 5, exhibits a sharp decrease in the first hours of exposure followed by a slower decrease during the intermediate and final stages of the aging test, suggesting that the degradation process follows two different kinetics. During the first hours of exposure, the major decrease of Rt was assigned to a fast increase of the number and size of pores, causing the decrease of the pore resistance and a major degradation of the coating [19,24,31]. With the continuous exposure to salt spray, it is expected that the pore resistance becomes much lower than the charge transfer resistance [29,30], so the gradual decrease of Rt observed during the intermediate and final stages of the test was assigned to a decrease of Rct. Moreover, the charge transfer resistance, Rct, is expected to decrease with the onset of the electrochemical corrosion [19,31]. In the early stages of the NSS test, it is expected that a well-defined electrolyte/metallic substrate interface has not been formed, so the value of Ct (Ct = Cc + Cdl) is mainly influenced by the coating capacitance, Cc. As previously stated, Cc is considered to be a measure of water permeation into the coating, so an increase of its value, and consequently of the total capacitance value, would be expected as the coating ages and incorporates more water/electrolyte. However, in the first hours of exposure to NSS, Ct maintains a constant value for both primers (Fig. 6). The observed stability of this parameter was then assigned to an almost instantaneous water-saturation of the coating film that translates into a stable value of Cc; the studied primers are porous, so a substantial water uptake was expected right after the contact with the electrolyte. In the later stages of the NSS test, with the eventual coating failure, the stability of the Ct values was assigned mainly to a constant Cdl behavior, an indication of a stable electrolyte/metallic substrate interface; the increase of the degraded coating area should balance with the accumulation of corrosion products, resulting in an overall stable electroactive surface area [16].
Fig. 4. Simplified equivalent circuit used to model impedance data. Rs- electrolyte resistance; Rt- total resistance; Ct- total capacitance; Zw- Warburg impedance.
Fig. 2 is the one usually used to interpret such data. This model includes two parallel resistance and capacitance combinations, considered to contain information regarding coating film and metallic substrate degradation, and the Warburg element that represents the diffusion phenomena [25,29]. A Nyquist plot containing two semicircles and a diffusion tail would be expected. However, the Nyquist plots of P1 and P2 samples aged by NSS, hardly display two semicircles –Fig. 3. This outcome has been reported in the literature and it is common when the paint film is porous [25,29,30], as it is the case of most anticorrosive primers. The observed single-loop in the Nyquist plots was assigned to phenomena that respond in similar frequencies, giving way to a merge between the semicircle representing the coating film degradation and the charge transfer loop [28,31]. Hence, the EIS spectra obtained in this work were considered to be described by a simplified equivalent circuit, as shown in Fig. 4. where the total capacitance (Ct = Cc + Cdl) represents the coating capacitance and the double layer capacitance, and the total resistance (Rt = Rp + Rct) represents the pores resistance and the charge transfer resistance; with a higher exposure time in the salt spray chamber or higher number of AC/DC/AC cycles it can be considered that, due to considerable film deterioration, the pores resistance is much smaller than the charge transfer resistance [29,30]. The simplified equivalent circuit model displayed in Fig. 4 fits quite well the experimental results obtained, as it can be observed in Figure S1 (supplementary information). Figs. 5 and 6 show the history of the average of parameters Rt and Ct obtained for three samples of each primer aged by the NSS test. The standard deviation of the mean is also displayed, and the individual results obtained for each studied sample are presented in Figures S3 and S4 and Table S1. The total resistance (Rt = Rp + Rct) obtained for both primers when
3.2. AC/DC/AC test 3.2.1. Preliminary assessment Previous reports regarding the application of the AC/DC/AC technique to study anticorrosion properties of organic coatings describe experimental procedures that include long cathodic polarizations steps (around 20 min), with potentials ranging from −1 V to −4 V, followed by relaxation times that can exceed 3 h. Table 2, summarizes some of the different AC/DC/AC procedures that can be found in literature. As a first attempt to degrade the two different samples under study, it was decided to apply procedure no. 1 since this is the one most 5
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 5. Total resistance history in NSS test for samples of primer P1 (a) and of Primer P2 (b).
Fig. 6. Total capacitance history in NSS test for samples of primer P1 (a) and of primer P2 (b). Table 2 Various AC/DC/AC procedures found in literature. Procedure number
Cathodic Potential / V
Polarization time / min
Relaxation time / h
Reference
1 2 3 4 5
−4 −2 −3 −1 to −4 −4
20 20 120 20 20
3 3 3 2 to 4 1 to 4
[8,9,17] [13,16] [31] [3] [32]
Fig. 8. Total resistance history in NSS test and as a function of the number of AC/DC/AC cycles for samples of primer P1.
referred in the literature. However, as it was quickly observed, in less than 10 AC/DC/AC cycles (around 210 min per cycle) the samples showed severe film delamination due to hydrogen accumulation between the metallic surface and the coating film. Since the duration of each cycle was considerably long, it was impossible to pinpoint the moment when the water hydrolysis started, even analyzing the impedance spectra obtained after each aging cycle; when the EIS spectrum
Fig. 7. P1 sample after 8 cycles of the AC/DC/AC procedure no.1 (Table 2).
6
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Table 3 Experimental conditions of the proposed AC/DC/AC test. Sample
P1 P2
Stage-I Cathodic Potential / V
Polarization time / s
Relaxation time / s
Number of cycles
Stage-II Cathodic Potential / V
Polarization time / s
Relaxation time / s
−4 −4
150 150
1000 1000
170 13
−1.85 −1.85
150 150
800 800
Total number of cycles / Analysis time
322 / 116 h 63 / 22 h
Fig. 9. Schematic representation of the proposed AC/DC/AC procedure (n1: number of cycles of Stage-I, n2: number of cycles of Stage-II, tp1: polarization time for a Stage-I cycle, tp2: polarization time for a Stage-II cycle, t1: duration of a Stage-I cycle, t2: duration of a Stage-II cycle).
Fig. 10. Nyquist plot evolution for samples P1DC1 (a) and P2DC1 (b) subjected to the proposed AC/DC/AC test. Inset magnification of the plot for cycles 171, 230 and 322 (a) and for cycles 14, 37, 44 and 63 (b).
cathodic potentials of −2 V and −4 V. The EIS data were fitted to the electrical equivalent circuit presented in Fig. 4 and the evolution of Rt was critically compared with the results obtained in NSS – Fig. 5. Taking as example the data obtained for P1 samples (Fig. 8), a cathodic potential of −4 V seemed to mimic the initial fast deterioration reported in NSS but would lead to severe local delamination. Applying −2 V in the DC stage, local delamination would develop more slowly and film degradation was more gradual, very similar to that observed in latter stages of NSS. However, the overall aging time was still considerably long. In the light of these results, an innovative approach was envisioned
showed the first indication of cathodic reaction (diffusion tail in the Nyquist plot), the sample displayed already significant coating delamination – Fig. 7: Given these results, it was consensual that a different approach to the AC/DC/AC technique could be essential to promote an electrochemical aging similar to what is observed in the neutral salt spray chamber. Accordingly, an experimental procedure had to be optimized to better identify the onset of the local delamination and prevent its occurrence in the DC step. In the early stages of the optimization procedure, P1 and P2 samples were aged by AC/DC/AC using shorter polarization and relaxation times (150 s and 1000 s, respectively) and 7
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 11. Total resistance history in NSS test and as a function of the number/aging time of AC/DC/AC cycles for samples of primer P1 (a), and of Primer P2 (b).
Fig. 12. Total capacitance history in NSS test and as a function of the number/aging time of AC/DC/AC cycles for samples of primer P1 (a), and of primer P2 (b).
to take advantage of the fast deterioration promoted by a −4 V cathodic potential and the slower degradation observed with lower cathodic potentials. Accordingly, the proposed AC/DC/AC procedure comprises two different stages as follow:
[33]. Thus, it’s appearance indicates that there is electrolyte-metallic substrate contact, which leads to the redox reactions associated with corrosion phenomena and, if the cathodic potential is high enough, to HER [3]. During the AC/DC/AC method optimization, it was observed that the appearance of the diffusion tail was followed by local delamination. However, decreasing the cathodic potential value during the DC step just after the diffusion tail onset, mitigated said delamination. This latter conclusion was crucial in choosing the diffusion tail onset as the switching-criterion. Table 3 schematizes the optimized experimental parameters used in the AC/DC/AC electrochemical aging tests. This experimental procedure allowed a faster but controlled initial film degradation. By applying short DC steps (150 s), it was possible to identify the cycle when local delamination could start, and thus when Stage-II should be initiated. The cathodic potential chosen for Stage-II was -1.85 V, balancing two requirements: i) hydrogen evolution reaction should not occur; and ii) the degradation of the paint coating film should occur as fast as possible. A schematic representation of the proposed AC/DC/AC procedure is presented in Fig. 9.
1) Stage-I: high cathodic polarization potential is applied to the sample for a short period. This initial stage forces a fast permeation of ions and water molecules into the coating, favoring the formation of pores; 2) Stage-II: lower cathodic polarization potential is applied to the samples to avoid the hydrogen evolution reaction (HER), though allowing continuing the film degradation and subsequent corrosion of the substrate. To determine the number of AC/DC/AC cycles that should be applied in Stage-I before moving to Stage-II, the appearance of a diffusion tail in the Nyquist plot was used as switching-criterion. Although a Warburg-type diffusion process is not directly related to H2 evolution, in this type of systems the diffusion tail is usually associated with the formation of corrosion products that diffuse out the porous coating 8
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Fig. 13. Total resistance history in NSS test (a) and as a function of the number of AC/DC/AC cycles (b), for P1 and P2 samples.
To compare the anticorrosive performance of both primers, Rt values obtained for both products were plotted as a function of time for the NSS aging test (Fig. 13a), and as function of the number of cycles (Fig. 13b) for the AC/DC/AC aging test. The results obtained for both primers show a similar pattern for the total resistance, though it is clear that the anticorrosive performance of primer P1 is substantially better than primer P2. The rapid decrease of Rt at the early stages of the aging tests, mostly related to the drop of Rp, is much more accentuated for P2 than for P1. This result suggests that P2 loses more quickly its sealing ability, allowing for an earlier electrolyte/metallic substrate contact. In the latter stages of the tests, the continuous drop of the total resistance is also more evident in P2, which may be the result of a more rapid development of reactive phenomena at the metal surface, leading to a considerable coating and metal substrate deterioration in a shorter aging period.
3.2.2. Accelerated aging results To assess the proposed AC/DC/AC aging method for characterizing anticorrosive coatings, three samples of each primer were subjected to the proposed aging test (samples P1DC1, P1DC2, P1DC3, P2DC1, P2DC2 and P2DC3).The Nyquist plots obtained for one sample of each primer aged by the new procedure (P1DC1 and P2DC1) is shown in Fig. 10. P1 samples required 170 cycles in Stage-I until a diffusion tail was evident in the Nyquist plot (Fig. 10a). Afterwards, the experimental conditions of Stage-II were applied and 152 cycles were performed until achieving a coating degradation similar to that observed after 500 h of exposure to neutral salt spray. P2 samples were submitted to Stage-I operating conditions for 13 cycles and to Stage-II operating conditions for 50 cycles (Fig. 10b). At the end, these samples displayed a similar degradation to the one observed after 300 h of exposure to NSS conditions. The comparison between the degradation promoted by both aging methods was based on the corresponding Nyquist plots. When the diameter of the high-frequency semi-circle obtained during the new procedure matched the one observed in the last EIS measurement of the NSS test, within a relative difference smaller than 20%, the samples aged by the two methods were considered equivalently degraded. The simplified equivalent circuit model displayed in Fig. 4 was fitted to the experimental results (see an example fit in Figure S2). Figs. 11 and 12 show the history of the average of parameters Rt and Ct obtained for three samples of each primer aged by the AC/DC/AC test (individual results can be found in Figures S5 and S6 and Table S2) and compare it with the parameters previously obtained for the NSS test. Comparing the results obtained for both aging tests, it is possible to observe a similar evolution of parameters Rt and Ct (Figs. 11 and 12, respectively) in the evaluation of both primers. During Stage-I of the AC/DC/AC method, the total resistance values show an accentuated decrease very similar to that observed for the NSS test; this result suggests that the high cathodic potential applied during this stage promoted an accelerated coating deterioration. Additionally, it is important to mention that no major local delamination was visually observed, thus the observed Rt decrease can be assigned mainly to the forced permeation of ions and water molecules through the coating, leading to a fast initial degradation. Afterwards, with the onset of StageII (lower cathodic potential), the decrease of Rt is more gradual, as it was observed for the NSS test. Said behavior was assigned to the intensification of the electrochemical corrosion. The obtained results suggest that the proposed AC/DC/AC procedure simulates the two degradation steps previously mentioned for the NSS and further accelerate them, reducing the aging time to less than 25% (Figure S7).
4. Conclusions A new approach to the original AC/DC/AC method is proposed for characterizing the anticorrosive performance of coating/metallic substrate systems by simulating the degradation observed for the neutral salt spray (NSS) tests in shorter testing times. The new test includes two different DC stages that allow preventing the early coating delamination induced by the hydrogen evolution reaction during the cathodic polarization step, while promoting the chemical and electrochemical attack of the coating and substrate. Two water-based organic primers – P1 formulated with epoxy and amine resins and P2 formulated with alkyd and melamine resins – were applied on cold rolled steel and aged by NSS test (combined with EIS analysis) and by the proposed AC/DC/AC procedure. To quantitatively compare the two aging methods, the EIS data were fitted to a simplified equivalent circuit and the respective parameters, total resistance (Rt) and total capacitance (Ct), were determined and critically compared. The results obtained for both aging methods showed that primer P1 displayed a substantially better anticorrosion performance than primer P2. Additionally, for both P1 and P2, the evolution of Rt and Ct, obtained with both techniques were quite similar. This outcome strongly suggests that the proposed AC/DC/AC procedure allowed to obtain essentially the same information as the NSS test, but entailed much shorter testing times, since both primers were characterized in less than 25% of the time required for the NSS tests.
9
Progress in Organic Coatings 138 (2020) 105365
T. da Silva Lopes, et al.
Declaration of Competing Interest
[8] S.J. Garcia, J. Suay, A comparative study between the results of different electrochemical techniques (EIS and AC/DC/AC): application to the optimisation of the cataphoretic and curing parameters of a primer for the automotive industry, Prog. Org. Coat. 59 (2007) 251–258. [9] S.J. Garcia, J. Suay, Optimization of deposition voltage of cataphoretic automotive primers assessed by EIS and AC/DC/AC, Prog. Org. Coat. 66 (2009) 306–313. [10] F. Deflorian, S. Rossi, M. Fedel, Organic coatings degradation: comparison between natural and artificial weathering, Corros. Sci. 50 (2008) 2360–2366. [11] F. Sutter, A. Fernandez-García, J. Wette, P. Heller, Comparison and evaluation of accelerated aging tests for reflectors, Energy Procedia 49 (2014) 1718–1727. [12] G.P. Bierwagen, D. Tallman, J.P. Li, L.Y. He, C. Jeffcoate, EIS studies of coated metals in accelerated exposure, Prog. Org. Coat. 46 (2003) 148–157. [13] M. Bethencourt, F.J. Botana, M.J. Cano, R.M. Osuna, M. Marcos, Lifetime prediction of waterborne acrylic paints with the AC–DC–AC method, Prog. Org. Coat. 49 (2004) 275–281. [14] J. Hollaender, E. Ludwig, S. Hillebrand, Assessing protective layers on metal packaging material by electrochemical impedance spectroscopy, Proceedings of the Fifth International Tinplate Conference, London, 1992, pp. 300–315. [15] J. Hollaender, Rapid assessment of food/package interactions by electrochemical impedance spectroscopy (EIS), Food Addit. Contam. 14 (1997) 617–626. [16] S.J. Garcia, J. Suay, Application of electrochemical techniques to study the effect on the anticorrosive properties of the addition of ytterbiurn and erbium triflates as catalysts on a powder epoxy network, Prog. Org. Coat. 57 (2006) 273–281. [17] S. Lajevardi Esfahani, Z. Ranjbar, S. Rastegar, Evaluation of anticorrosion behavior of automotive electrocoating primers by the AC-DC-AC accelerated test method, Prog. Color Color. Coat. 7 (2013) 187–199. [18] H. Leidheiser, W. Wang, L. Igetoft, The mechanism for the cathodic delamination of organic coatings from a metal-surface, Prog. Org. Coat. 11 (1983) 19–40. [19] A.C.A. de Vooys, B. Boelen, D.H. van der Weijde, Screening of coated metal packaging cans using EIS, Prog. Org. Coat. 73 (2012) 202–210. [20] P.A. Sørensen, S. Kiil, K. Dam-Johansen, C.E. Weinell, Anticorrosive coatings: a review, J. Coat. Technol. Res. 6 (2009) 135–176. [21] D. Kotnarowska, Influence of ultraviolet radiation and aggressive media on epoxy coating degradation, Prog. Org. Coat. 37 (1999) 149–159. [22] V.F. Lvovich, Impedance Spectroscopy: Applications to Electrochemical and Dielectric Phenomena, Wiley, Hoboken, N.J, 2012. [23] F. Mansfeld, Models for the impedance behavior of protective coatings and cases of localized corrosion, Electrochim. Acta 38 (1993) 1891–1897. [24] R. Duraisamy, K. Pownsamy, G. Asgedom, Chemical degradation of epoxy-polyamide primer by electrochemical impedance spectroscopy, ISRN Corros. 2012 (2012) 10. [25] G.W. Walter, A review of impedance plot methods used for corrosion performance analysis of painted metals, Corros. Sci. 26 (1986) 681–703. [26] R.P. O’Hayre, S.-W. Cha, W. Colella, F.B. Prinz, Fuel Cell Fundamentals, John Wiley & Sons, Hoboken, NJ, 2006. [27] P. Zoltowski, On the electrical capacitance of interfaces exhibiting constant phase element behaviour, J. Electroanal. Chem. 443 (1998) 149–154. [28] M.E. Orazem, P. Shukla, M.A. Membrino, Extension of the measurement model approach for deconvolution of underlying distributions for impedance measurements, Electrochim. Acta 47 (2002) 2027–2034. [29] T. Hong, Y.H. Sun, W.P. Jepson, Study on corrosion inhibitor in large pipelines under multiphase flow using EIS, Corros. Sci. 44 (2002) 101–112. [30] Y. Chen, T. Hong, M. Gopal, W.P. Jepson, EIS studies of a corrosion inhibitor behavior under multiphase flow conditions, Corros. Sci. 42 (2000) 979–990. [31] M. Poelman, M.G. Olivier, N. Gayarre, J.P. Petitjean, Electrochemical study of different ageing tests for the evaluation of a cataphoretic epoxy primer on aluminium, Prog. Org. Coat. 54 (2005) 55–62. [32] S.L. Esfahani, Z. Ranjbar, S. Rastegar, An electrochemical and mechanical approach to the corrosion resistance of cathodic electrocoatings under combined cyclic and DC polarization conditions, Prog. Org. Coat. 77 (2014) 1264–1270. [33] A.G. Marques, A.M. Simões, EIS and SVET assessment of corrosion resistance of thin Zn-55% Al-rich primers: effect of immersion and of controlled deformation, Electrochim. Acta 148 (2014) 153–163.
The authors declare that they have no conflict of interest. Acknowledgements The authors would like to thank CIN- Corporação Industrial do Norte for supplying the water-based primers and Ms. Joana Fonseca for her valuable help and collaboration in the development of this project. T. Lopes acknowledges the Portuguese Foundation for Science and Technology (FCT) for her Postdoctoral Grant with Ref. SFRH/BPD/ 102408/2014. The research leading to these results has received funding from: (i) Project POCI-01-0145-FEDER-016387 "SunStorage – Harvesting and storage of solar energy", (ii) Project POCI-01-0145FEDER-030760 “HopeH2 - Efficient, stable and scalable tandem PEC-PV device for solar hydrogen generation”, both funded by the European Regional Development Fund (ERDF), through COMPETE2020 Operational Programme for Competitiveness and Internationalisation (OPCI) and by national funds, through FCT; (iii) Project UID/EQU/ 00511/2019 - Laboratory for Process Engineering, Environment, Biotechnology and Energy – LEPABE funded by national funds through FCT/MCTES (PIDDAC); and (iv) NORTE-01-0145-FEDER-000005 – LEPABE-2-ECO-INNOVATION, supported by North Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the ERDF. Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.porgcoat.2019. 105365. References [1] B. Chico, D. de la Fuente, I. Díaz, J. Simancas, M. Morcillo, Annual atmospheric corrosion of carbon steel worldwide. An integration of ISOCORRAG, ICP/UNECE and MICAT databases, Materials (Basel, Switzerland) 10 (2017) 601. [2] A.A.M.T. Adikari, R.G.N.De S. Munasinghe, S. Jayathilake, Prediction of atmospheric corrosion –a review, Eng. J. Inst. Eng. Sri Lanka 47 (2014) 75–83. [3] K.N. Allahar, G.P. Bierwagen, V.J. Gelling, Understanding ac–dc–ac accelerated test results, Corros. Sci. 52 (2010) 1106–1114. [4] M.L. Zheludkevich, J. Tedim, M.G.S. Ferreira, “Smart” coatings for active corrosion protection based on multi-functional micro and nanocontainers, Electrochim. Acta 82 (2012) 314–323. [5] S.B. Lyon, R. Bingham, D.J. Mills, Advances in corrosion protection by organic coatings: what we know and what we would like to know, Prog. Org. Coat. 102 (2017) 2–7. [6] F. Zhang, P. Ju, M. Pan, D. Zhang, Y. Huang, G. Li, X. Li, Self-healing mechanisms in smart protective coatings: a review, Corros. Sci. 144 (2018) 74–88. [7] H.R. Asemani, P. Ahmadi, A.A. Sarabi, H. Eivaz Mohammadloo, Effect of zirconium conversion coating: adhesion and anti-corrosion properties of epoxy organic coating containing zinc aluminum polyphosphate (ZAPP) pigment on carbon mild steel, Prog. Org. Coat. 94 (2016) 18–27.
10