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Evaluation of barrier coatings by cycling testing
Progress in Organic Coatings 45 (2002) 405–413
Evaluation of barrier coatings by cycling testing L. Valentinelli a,b , J. Vogelsang c , H. Ochs c , L. Fedrizzi a,∗ a b
Department of ICMMPM, University of Rome “La Sapienza”, via Eudossiana 18, 00184 Rome, Italy Department of Material Engineering, University of Trento, via Mesiano 77, 38050 Povo, Trento, Italy c Sika GmbH, Kornwestheimer Str. 103-107, 70439 Stuttgart, Germany Received 14 March 2002; received in revised form 1 August 2002; accepted 9 August 2002
Abstract A variety of methodology which are supposed to accelerate and/or simulate the effects of time and environment, weathering, on organic coatings degradation are under development since long time. Taking into account that no test can duplicate all of the variables associated with a coatings environment, two modern accelerated tests were carried out and their investigation capabilities were compared: the Norwegian Norsok M 501 and the thermal cycling in electrolyte immersion. The first test highlighted the adhesion performances of coatings and the importance of the zinc-rich primers on the scratch protection. The thermal cycling test has lead to a very rapid loss of film properties. Electrochemical impedance measurements associated to this test have shown the relative importance of electrical resistance and capacitance in predicting corrosion protective performance in presence of very thick coatings. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical impedance spectroscopy; Thermal cycling; Weathering test; Organic coating; Adhesion
1. Introduction The organic coatings have been evaluated using a variety of outdoor and laboratory equipment and procedures for decades. Among these, “use” testing and outdoor exposures are considered the most reliable; they are, respectively, either to apply the coat and then observing coatings conditions over years of actual use or exposing laboratory samples in the form of panels on test racks and then looking at their variations. The data collected correlating the performance of different systems over many years provide a base for the selection of current coatings for specific applications and they also provide an insight into how new coatings could be formulated. Despite the amount of information which they can provide and the trust which they relish, it has not to be forgiven that the economy of a company requires to rapidly develop new, better performing products, while increasing quantity and product field reliability. Different tests and techniques have been studied and performed in the recent years to cut down the time demand and, even so, to improve the capabilities to investigate deeper the characteristics of organic coatings. Salt-spray ∗ Corresponding author. Fax: +39-0461881977. E-mail addresses: [email protected], lorenzo. [email protected] (L. Fedrizzi).
testing following the ISO 7253 standard remains the most used method by which to assess paints for corrosion control purposes, even though severe criticisms of this method have been noted in the literature for many years, mainly regarding the lack of correlation with service experiences [1]. In this context more fashionable approaches are under development; taking into account that the practically found environmental conditions are in continuous variation, in terms of sort, severity and length of time, the idea to introduce in a unique protocol more types of interactions between coating and environment seems to be a more powerful tool for selecting materials. The target is to reproduce what actually happens in reality and strive for duplicate as closely as possible the type of corrosion encountered in the desired use of the products, but also with simultaneously decreasing the time to show failure into the coating. In this paper we performed two different types of test protocols, one dedicated to the evaluation of coating/metal adhesion and one with thermal stress to investigate the barrier properties of the coatings. The first procedure is the Norsok M 501 test, which is a standardised Norwegian weathering test designed for proving the materials supposed to work at very hard environmental conditions. In this cyclic test the scribed coated panel undergoes sessions of UV exposure with humidity, salt fog and a conditioning time at room temperature, as dry part of the cycle. Indeed, it is often observed that alternating wet and
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dry corrosion condition causes faster blistering than continuous exposure to high humidity, because of the increase in internal stress: paint films delaminated from scribe line regions as a result of oxide lifting action caused while anodic corrosion products accumulate under the coating. Moreover, the blisters that are formed are rust filled and apparently also generated by the relative volume of the precipitated, solid corrosion products. The cyclic test should more closely simulate observations made after natural outdoor exposures than those obtained after only a salt spray testing [1,2]. Additionally, with regard to UV-induced degradation of the binders, it has been found that the additional presence of UV-condensation cycles in the corrosion test fundamentally altered the nature of the occurring corrosion/degradation processes. The results produced by this combination of weathering factors appear, qualitatively at least, to be closer representative for the corrosion and degradation observed in natural atmospheric service environments. It may appear that the test results neither of the salt fog nor of the wet/dry cycle give particularly realistic deterioration effects. It is reported that UV-condensation cycles combined with the wet/dry gives a more realistic simulation of the deterioration process for coating systems under atmospheric weathering conditions [3]. The visual evaluation of the degree of sample damages is analysed with respect to rusting and blistering around the scribe and even the remote area within the UV mask (Fig. 1). Even though the mentioned method is defined accelerated, it needs anyway a “long time” to rank the material corrosion protection: the Norsok test is basically planned to last 25 weeks, depending on kind of product, substrate, etc. The modern way to face this bad data is to take advantage of an aging test where temperature plays a big role into the aging of the material, permitting to gather information faster for coating corrosion resistance evaluation.
As Bierwagen et al. [4,5] recently described the electrochemical studies of the organic coats done at room temperature exposure give reproducible informations on film performances, but they do not permit the screening of very high impedance material. On the contrary, an experimental set-up in which the temperature of the sample is varied cyclically above and below the glass transition temperature [6] will establish a synergic degrading effect into the material. This is due to electrolyte transport through the film and its eventual accumulation, loss of adhesion, chemical and physical aging as a consequence of thermal fatigue effects, dielectric characteristic variations due to a more open polymer structure. The cumulative effects of such a thermal cycling in the coat controlled by means of the electrochemical parameters changes should permit the ranking of a variety of materials, for constituents, characteristics and apply purposes, in just a week of time, while remaining objective and reliable. Following this, we carried out our Thermal Cycling test, which consisted of a daily series of electrochemical measurements on a metallic coated sample, performed by means of EIS at different temperatures. The objective of this scientific research is therefore to compare the material selection chances given by different weathering and aging test, precisely the Norsok M 501 and the thermal cycling test, to evaluate their merits and limits and to establish a possible correlation within the results.
2. Experimental 2.1. Materials Various sorts of materials for constructional steelwork have been investigated: nature and composition are reported in Table 1. All of them were applied by air spray to properly sandblasted steel. The final systems curing took then 2 weeks, storing the samples at ambient temperature (standard climate class 2: 23 + −2 ◦ C and 50% relative humidity + −5%). System B was left outdoor during the 2 weeks of drying, following the precautions suggested by the supplier: previous test have effectively shown that outside weathering ages better these materials by producing a better adhesion between steel and primer, and between primer and topcoat, than the normal storage into room, due to UV-induced cross-linking and oxidative curing. 2.2. Norsok test set-up
Fig. 1. Norsok test sample layout: the visual evaluation is made taking into account the whole area inside the UV mask print.
The test includes a fraction of time of salt spray fog, a conditioning time at room temperature, like dry part of the cycle, and a session of UV exposure with condensation. Every single part of the test is standardised by ASTM or ISO, therefore in Table 2 are presented only the specifications of the used parameters, while for details of the procedure reference may be made to the relative specified standard. The
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Table 1 Type of material System
Layer
Binder and pigments
Thickness (m)
A
1 2 and 3 4
2-Packs highly pigmented zinc epoxy primer 2-Packs epoxy-based intermediate MIO pigmented 2-Packs polyurethane based top coat
387
B
1 2
1-Pack urethane-alkyd primer + zinc phosphate 1-Pack urethane-alkyd resin top coat
179
C
1 2 and 3
1-Pack talc pigmented PVC-acryl resin primer 1-Pack PVC-acryl-based top coat
290
F
1 2 and 3
2-Packs highly pigmented zinc epoxy primer 2-Packs epoxy-based top coat
418
H
1 2 and 3
2-Packs epoxy-based primer 2-Packs low solvent epoxy-based top coat
349
test was performed on scratched panels as visible in Fig. 1, with a 1 mm wide scribe. The test is nominally planned to last 25 cycles for a total amount of 4200 h. 2.3. Thermal cycling test parameters All measurements were performed in a metallic oven that provides through its structure a very good Faraday effect against external electro smog and additionally permits the samples temperature control during the thermal cycling. The measuring cells were realised by gluing a 25 mm wide PMMA tube on the coat surface; due to the high temperature a medium viscosity, fast curing, single component cyanoacrylate adhesive, with a glass transition temperature of 150 ◦ C was used. In particular, cells were filled with the electrolyte solution 3 days before to start the first measurement and then the samples were left at room temperature, so that the coating can be considered in saturation conditions even by the first data acquisition. The Harrison electrolyte (0.05 wt.% NaCl + 0.35 wt.% (NH4 )2 SO4 ) was used, because it has been observed that the presence of ammonium and sulphate species in the electrolyte is more reliable for accelerating of atmospheric corrosion in industrial atmospheres [7], moreover, low concentration electrolyte may allow electrochemically active components of paint system to function during testing more as they would be expected to do in practice [1,8]. Impedance spectra are then daily registered at three different temperatures, 23, 55 and 85 ◦ C for a total amount of 5 days, after leaving the coating at the new reached temperature a couple
of hours to stabilise. The temperature has been varied by leaving the samples into the thermostatic oven, so that the whole system constituted by metal substrate and organic coatings can be considered in a homogeneous state, without any gradient of temperature through its structure. To check the temperature course in time of both oven and coating a pre-experiment was carried out and the result is presented in Fig. 2: the result is a minimum of 2 h of real waiting time for the coating at the fixed temperature before to measure its impedance and to change temperature again. 2.4. EIS measuring set-up The advanced spectrometer from Zahner Elektrik was used for EIS with a two-electrode configuration (a platinum wire as counter); it is characterised by an input impedance >1 T and <5 pF in the frequency range below 100 KHz while it is used in combination with the buffer amplifier connected to the reference and working electrodes. Any other detail of the IM6 Zahner equipments can be found in the User Manual [9].
Table 2 Norsok cyclic test M 501 parameters Test
Duration (h)
Standard
Salt spray Drying in air UV-A 340 nm weatherometer Total duration of one cycle
72 16 80 168
ISO 7253 ASTM D4587, method B
Fig. 2. Variation of samples and oven temperature during the temperature cycling test.
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Fig. 3. The two equivalent circuit models used for analysing EIS data: the values of the R are in and those of the C in Faraday.
2.4.1. Amplitude Amplitude of 20 mV was used; with these small amplitudes there is no doubt that the system behaves linearly. 2.4.2. Frequencies All the measurements were performed between 100 mHz and 100 kHz, with five points per decade and five samples per point. 2.5. Evaluation of the measured data 2.5.1. Qualitative The measurements are first analysed visually. To perform the visual evaluation the data were plotted in a Bode-plot (log(IZI) and phase angle as a function of log(frequency)), which gives a good overall impression without much details [10]. 2.5.2. Quantitative evaluation As was already mentioned the quantitative interpretation of the measurements is performed with an equivalent circuit. These circuits are fitted on the data with the SIM software from Zahner [9]. For all measures and coatings (water uptake and dielectric studies) the two simple circuits in Fig. 3 are used. Both equivalent circuits have merits in the description of EIS results from barrier coatings, even if they are likely the simplest approximation. Model B is suitable to describe parts of EIS spectra over few decades; mainly at higher frequencies the loss capacity has a high accuracy and at low frequencies the coating resistance can be obtained properly. If the fitting of spectra from an intact coating has to cover a wider frequency range, experimental practice showed that model A is better applicable. As most of the barrier coatings show a non-constant phase angle, it is simply necessary to use a CPE and a capacitor in parallel for a proper simulation. (For some cases a Young–Göhr-impedance element might be used with some justification [11].) But also from some theoretical considerations it is quite easy to understand this modification, as water absorption will not only increase the dielectric constant, it also changes the processes of charge flux in the coating. These modified processes are related to hydrogen bonds between water molecules and polymeric
functional groups, yielding in an influence on the relaxation of charges and polarisation on the coating surface [12]. All the experimental data were analysed between 1 and 100 kHz with the model A to assess the coating capacity (CPE element C2 ) and with the model B all over the measured frequency range to evaluate the coating resistance (R6 ). The normal starting values for the fit are presented near each element. The SIM software permit to normalise the value of constant phase element (CPE) according to the following equation: Z=
1 ω0 V (j (ω/ω0 ))α
where ω is the angular frequency, j the imaginary number, V the loss capacitance [F] at ω0 , ω0 the normalising frequency and n the CPE power. Such definition permits, throughout the value of the parameter ω0 , to better model the capacitive behaviour of the evaluated element [11]. CPE will be here used to study small deviations from ideal capacitive behaviour; in our case α was always bigger than 0.82 in all cases. To evaluate and compare the results V is treated as a capacitance (C). It has been shown that a CPE (more precisely a loss capacity) has to be normalised at a certain frequency ω0 and its value should be in that frequency range, where the capacity shows its highest significance, normally in the 100 Hz–10 kHz range. If this normalising frequency is not adjusted and assumed to be ω = 1, then an error in capacity can be estimated to be up to 100% [11,12].
3. Results 3.1. Thermal cycling test As already shown in the literature [6,13] the barrier properties of the organic coatings are remarkably affected by testing temperature increase. Coatings become more permeable and less protective due to the opening of the polymer structure, particularly when the glass transition temperature is overtaken. In Fig. 4 such a behaviour is quite clear; the tested sample shows an impedance decrease, in the low frequency range, going from spectra acquired at 23 ◦ C to those
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Fig. 4. EIS spectra of system A: the impedance course shows an high grade of reversibility day by day at every temperature.
at 55◦ and 85◦ . Moreover, as observed in Fig. 5 the phase angle values for all measurements at 23 ◦ C undergo a small reduction while those evaluated at higher temperature show a significant loss at lower frequencies. Such behaviour was general and all the tested systems showed this dependence on the testing temperature. However, when aging cycles where introduced the various systems behaved in different ways and taking advantage of the experience of Bierwagen et al. [5] the systems were classified in two categories: reversible and irreversible. Fig. 4 shows the impedance behaviour and the extend of reversibility of system A: all curves at each single temperature seems to overlap themselves in the whole frequency range; this could mean that there is no cumulative damage introduced by this thermal cycling. On the contrary, system B shows an irreversible behaviour. Fig. 6 shows the electric characteristics of this system after
thermal cycling: it shows a variable spectra cycle by cycle, notifying a progressive deterioration, which is not longer recovered. The just defined irreversibility can be correlated with the deterioration which natural thermal cycling may introduce in the organic coatings during their service life, as electrolyte accumulation in the film, loss of adhesion, chemical and physical aging. By fitting of the impedance data it has been found that even for the so called reversible systems (like system A) the capacity trend shows a certain degree of irreversibility: in every case capacity increases with temperature and time. The typical serrated course of the capacity behaviour is reported in Fig. 7. Systems like coating B show a high tendency of increasing capacity cycle by cycle, while system C (which has a reversible behaviour similar to system A) remains to be lower but anyway slowly increasing values. For every cycle the highest capacity can be fitted at
Fig. 5. Reversible change of the phase angle values for system A due to thermal cycling.
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Fig. 6. Irreversible behaviour of the system B due to thermal cycling.
the highest temperature; by decreasing temperature capacity returns to a lower values range. This can be read like a different tendency of adsorbing and desorbing water with temperature, leading to a cumulative structural damage of the polymeric material. From the coating capacity it is then possible to obtain the value of the dielectric constant by the following equation: ε=
Cd ε0 A
where C is the coating capacity, d the coating thickness, ε0 the vacuum dielectric constant and A the sample area. It is well known that dielectric characteristics depends on the measuring temperatures and that generally the dielectric constant increases with temperature, with a law that depends also on the used frequency to measure it [14]. In literature it is possible to find that the dielectric constant changes are due to several causes which are all function of the temper-
ature: the polymer structure and chemical composition, the amount of water present in the material and its interaction with the polymer as well as the pigment–binder interface. The variations of such properties which occur at high temperature cannot be quantitatively allocated, therefore, in our work only the values obtained at room temperature are taken in consideration. Table 3 lists the dielectric constants evaluated at 23 ◦ C at the beginning of the test and after five cycles, and the cell surface rust degree according to ASTM D610-95. The rust degree is defined from values going from 1 up to 10, where 10 is the best one representing no rust spots. Following Table 3, the systems A and H have got a small ε increase and they maintain an outstanding resistance to rust formation. For what concern system B, it has got a very high variation of dielectric constant from the first to the final cycle and it has shown a poor ability to protect from rusting. Such high water up take was in a certain manner expected because of its higher hydrophilic nature compared to the other binders. Additionally, the low thickness (179 m), the slow oxidative cross-linking and the low number of coats (only 2 instead of 3–4) have not allowed then this system to prevent corrosion on set. This resulted in the formation of Table 3 TC results: dielectric constant and cell rusting
Fig. 7. EIS data: capacity course of systems B and C during thermal cycling show different increase in the time.
System
ε1 a
ε5 b
Cell rust degreec
A B C F H
15.94 10.63 10.41 13.56 11.08
18.78 29.65 11.87 39.19 15.01
10 7 Totally rusted interface 10 10
a Note that, this value is referred to the first measurement performed at 23 ◦ C made after 2 days of immersion in electrolyte. b Dielectric constant obtained at 23 ◦ C after five cycles. c ASTM D610-95.
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Fig. 8. Rusted interface of system C after cycling.
little rust spots as remarked in Table 3. Besides system B, even system F shows a great tendency of water adsorption, but in this case the substrate is anyway protected. System F has got the same zinc dust pigmented epoxy primer as system A; this very good primer, which develops outstanding adhesion with the substrate and very good cathodic protection, may have protected the metal independently on the top coat performances with respect to this test. System C seems to adsorb very few water with respect to other cases and at the same time it seems to present a completely corroded interface, as it was possible to observe by peeling after cycling the very weak coating from the substrate (Fig. 8). Probably its chemical nature (being a PVC-based system with only physical drying, not reactive curing) and the total absence of anticorrosive pigments have brought it first to a quick and progressive water permeation right after the first cycle and to its accumulation at the coating–substrate interface, and afterwards this water has contributed to the metal oxidation. In order to explain the very small ε increase during cycling we have to introduce
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a new hypothesis. A polymer capacitance increase is correlated with water penetration into voids, pigment/polymer interfaces as well as bonding of water to polymer chains. But if water penetrates the organic coating to form a continuous water layer at metal/polymer interface the electric circuit to model the physical system can be changed in a series of capacitance (one related to the organic coating, the other one related to the water film). In such a case no large capacitance increase can be measured [15]. As far as the barrier properties are concerned, all tested systems have got excellent performances: they always show resistance higher than 4.3 × 105 m2 (Fig. 9). Such very high range of values is on the other hand very hard to assess, because it ranges at the sensibility limit of the equipment (due to the really measured area of 4.9 cm2 ). Within the 2-pack epoxy coating, system A seems to improve its performance after the second cycle while F and H show a loss of resistance after the first cycle. Comparing this behaviour with the rust degree evaluation of the cell surface, system A is in agreement because it has got an overall good performance, but both F and H showed no rust formation even with lower final resistance. The 1-pack system B resistance scatters at very high values but it shows a few rust spots on the cell surface. The resistance values of system C remain nearly unchanged, what might lead to the misinterpretation of the best performance in this context, whereas it has been found that its interface was completely corroded. Resistance values measured by EIS seem therefore not being very reliable for the corrosion monitoring of coatings with very high thickness [16]. 3.2. Norsok M 501 test Norsok test results provide information either about the adhesion characteristics and the bare metal protection while looking at the scratch, or about the system barrier properties,
Fig. 9. EIS data: resistance calculated at 23 ◦ C for sandblasted surface panels.
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Table 4 Norsok M 501 results: cut blistering Cycle
System A
System B
System C
System F
System H
1 2 3 4 5 6 11
10 10 10 10 10 10 10
10 10 10 10 10 4 (few) 4 (medium dense)
10 10 4 4 4 4 4
10 10 10 10 10 6 (few) 6 (few)
10 10 10 4 4 4 4
when looking far away from the scratch. Only the results of the first 11 weeks of test among the 25 originally planned are presented, because even though the samples have not gone through the whole prove period, the external changes they have shown are anyway enough to propose a first fairly good systems ranking. As a first observation, none of the studied systems has shown blistering or rusting in the remote area within the UV mask: from this kind of test it can be concluded that every system behave good in terms of barrier properties. The cut blistering results, according to the ASTM D714-87, are presented in Table 4; they rank from 10 to 0, where number 10 represents no blistering, to illustrate the size blistering and from few to dense blisters to indicate the frequency. The system A is the only one which does not start to delaminate in the cut surrounding zone within 11 week-cycles of test. This confirms the good barrier properties evaluated even by means of the thermal cycling test: low water up-take and very good surface protection. System H, which was identified in the previous section like good performing, behaves in this case roughly bad, showing an early failure and an increase in blister dimensions in the following cycles. The inferior adhesion properties to the substrate of this coating system, compared to the other, as measured by paint supplier, has permitted water to penetrate fast from the scratch section and to easily produce blisters. Therefore, it has to be noticed that, systems with good barrier properties in undamaged conditions can quickly loose their performances in presence of little cuts or defects, if their adhesion is not particularly good. The systems F and B behave better than system H: both show the first blister formation two cycles later. System F produces a more limited number of blisters and of smaller sizes than system B; this may be due to better adhesion of the primer of system F, the same used also in system A. The system C is the first which collapses, right after the second cycle, and which presents a totally swelled scratch at the end of the 11th week. The high permeability and the poor adhesion evaluated in the previous test is confirmed even by this kind of test. The cut rusting degree is reported in Table 5 in four different grades of rust amount from none to completely rusted. The test permits a good assessment of the behaviours of the different systems, showing how their performance
(few) (few) (few) (medium dense) (dense)
(few) (medium dense) (medium dense) (medium dense)
Table 5 Norsok M 501 results: cut rusting Cycle
A
1 2 3 4 5 6 11
0 Small Large Large Large Large Large
B spots spots spots spots spots spots
All All All All All All All
C rusted rusted rusted rusted rusted rusted rusted
All All All All All All All
rusted rusted rusted rusted rusted rusted rusted
F
H
Small spots Small spots Small spots Small spots Large spots All rusted All rusted
All All All All All All All
rusted rusted rusted rusted rusted rusted rusted
degenerates with time; the cut rusting results give clearly evidence for the presence or absence of active anticorrosion pigments in the chemical formulation of the primer. Both the systems C and H have got a totally rusted scratch right after the first week of testing and they are actually those systems without any anticorrosive pigmentation of the primer. Same course is shown even by system B which is based on a zinc phosphate pigmented primer: its protective characteristics seems not to be very appropriated for a timely extended protection of the scratch. The systems A and F show a delayed onset of corrosion in the cut and, in particular for system A, the scratch is still not totally rusted after 11 cycles. A great amount of zinc dust in the primer of systems A and F has contributed to develop a better protection of the bare metal compared to those systems which have not any kind of cathodic protective pigment. This was better visible in the system A, which is characterised by high barrier effect intermediate and top coats, while system F has shown a faster reduction in the galvanic action. The higher permeability or the greater tendency to take water up may produce a faster oxidation of the zinc particles, causing an interruption of the electrical flow between adjacent zinc grains and between the steel substrate. Specific studies are present also in literature which confirm this hypothesis [17].
4. Conclusions Steel samples were protected by different high thickness industrial organic coatings. Two different testing methods were used to assess and to rank the paint behaviour: • the Norsok test by visual observation;
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• the temperature cycles after immersion in Harrison solution by EIS measurement. Norsok test showed to be quite effective in the evaluation of coating adhesion properties and to assess the effectiveness of active primers in protecting metal substrate at scratches. This test was also very effective in providing useful information about barrier protection properties of intact films when exposed to UV radiation and humidity cycles [18]. Nevertheless, such a test is only based on a visual observation making the test itself quite subjective, moreover, time to highlight coating properties is quite long. Thermal cycling of immersed samples showed to be a really drastic test enhancing coating degradation by water up-take. Then samples behaviour can be differentiated very rapidly. However, not always electrical parameters obtained by EIS were able to assess the protection behaviour. The resistance of the coating failed in giving reliable data, always showing very high resistance, even when coating protection failed. Such a parameter has to be carefully used in order to predict coating behaviour, specially in the case if coating thickness is very high and adhesion to the substrate becomes the critical parameter for corrosion protection. On the contrary, CPE confirmed to be very sensitive to coating degradation due to the water up-take. This parameter changed greatly when measured at different temperatures depending on the variation occurring in the barrier properties of the film with temperature. Moreover, this parameter allowed to see that thermal cycling can introduce irreversible degradation in the paint depending on the chemical nature of binder and of the whole coating system. However, also this parameter was sometimes not able to discriminate corrosion behaviour when barrier properties were high due to high coating thickness, and the critical factor became adhesion. Then, some unusual situations were observed: • high CPE variation during thermal cycling maintaining a good protection behaviour (good adhesion properties); • small CPE variation during thermal cycling, with a rapid coating detachment (poor adhesion properties).
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Acknowledgements Authors are grateful to Prof. P.L. Bonora head of the Industrial Corrosion Control Laboratory of the University of Trento, who encouraged the work development, and all Sika collaborators for the technical support. References [1] B.S. Skerry, C.H. Simpson, NACE—Corrosion 91, Paper No. 412. [2] D.A. Jones, Principles and Prevention of Corrosion, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ, 1996. [3] Encyclopedia of Polymer Science and Engineering, Weathering, vol. 17, Wiley, New York, p. 796. [4] G. Bierwagen, J. Li, L. He, D. Tallman, Fundamentals of the measurement of corrosion protection and the prediction of its lifetime in organic coatings, Proceedings of the Second International Symposium on Service Life Prediction Methodology and Metrologies, Monterey, CA, November 14–17, 1999, J. Martin, D. Bauer (Ed.), ACS Books, Washington, DC, 2000. [5] G.P. Bierwagen, J. Li, L. Ellingson, D.E. Tallman, Prog. Org. Coat. 39 (2000) 67–78. [6] J. Li, C.S. Jeffcoate, G.P. Bierwagen, D.J. Mills, D.E. Tallman, Corrosion 54 (1998) 763–771. [7] B.S. Skerry, A. Alavi, K.L. Lindgren, J. Coat. Technol. 60 (765) (1988) 97–106. [8] W. Funke, J. Oil Colour Chem. Assoc. 67 (1984) 71. [9] Thales/IM6 Manual, Zahner, Kronach, Germany. [10] D.H. Van Der Weijde, Proefscchrift, 1996. ISBN 90-9010105-5. [11] C.A. Schiller, W. Strunz, The Evaluation of Experimental Dielectric Data of Barrier Coatings by Means of Different Models, ZahnerelektrikGmbH & Co. KG, El. Acta 46 (24–25) August 15, 2002, pp. 3619–3625. [12] W. Strunz, Private communication. [13] R. Granata, K. Kovaleski, in: Proceedings of the 13th International Congress on Corrosion, vol. 1, Houston, TX, September 1993, p. 24. [14] C.A. Harper, Handbook of Plastics and Elastomers, McGraw-Hill, New York, 1989, pp. 2–27. [15] F. Deflorian, L. Fedrizzi, S. Rossi, P.L. Bonora, Electrochim. Acta 44 (1999) 4243. [16] S. Duval, Y. Camberlin, M. Glotin, M. Keddam, F. Ropital, H. Takenouti, Prog. Org. Coat. 39 (2000) 15–22. [17] S. Feliu, in: D.C. Silvermann, M.W., Kending (Eds.), ASTM STP1188, 1993. [18] O.O. Knudsen, U. Steinsmo, M. Bjordal, S. Nijjer, Protective Coat. (Europe), vol. 9, No. 12, December 2001, pp. 52–56.
Evaluation of barrier coatings by cycling testing L. Valentinelli a,b , J. Vogelsang c , H. Ochs c , L. Fedrizzi a,∗ a b
Department of ICMMPM, University of Rome “La Sapienza”, via Eudossiana 18, 00184 Rome, Italy Department of Material Engineering, University of Trento, via Mesiano 77, 38050 Povo, Trento, Italy c Sika GmbH, Kornwestheimer Str. 103-107, 70439 Stuttgart, Germany Received 14 March 2002; received in revised form 1 August 2002; accepted 9 August 2002
Abstract A variety of methodology which are supposed to accelerate and/or simulate the effects of time and environment, weathering, on organic coatings degradation are under development since long time. Taking into account that no test can duplicate all of the variables associated with a coatings environment, two modern accelerated tests were carried out and their investigation capabilities were compared: the Norwegian Norsok M 501 and the thermal cycling in electrolyte immersion. The first test highlighted the adhesion performances of coatings and the importance of the zinc-rich primers on the scratch protection. The thermal cycling test has lead to a very rapid loss of film properties. Electrochemical impedance measurements associated to this test have shown the relative importance of electrical resistance and capacitance in predicting corrosion protective performance in presence of very thick coatings. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Electrochemical impedance spectroscopy; Thermal cycling; Weathering test; Organic coating; Adhesion
1. Introduction The organic coatings have been evaluated using a variety of outdoor and laboratory equipment and procedures for decades. Among these, “use” testing and outdoor exposures are considered the most reliable; they are, respectively, either to apply the coat and then observing coatings conditions over years of actual use or exposing laboratory samples in the form of panels on test racks and then looking at their variations. The data collected correlating the performance of different systems over many years provide a base for the selection of current coatings for specific applications and they also provide an insight into how new coatings could be formulated. Despite the amount of information which they can provide and the trust which they relish, it has not to be forgiven that the economy of a company requires to rapidly develop new, better performing products, while increasing quantity and product field reliability. Different tests and techniques have been studied and performed in the recent years to cut down the time demand and, even so, to improve the capabilities to investigate deeper the characteristics of organic coatings. Salt-spray ∗ Corresponding author. Fax: +39-0461881977. E-mail addresses: [email protected], lorenzo. [email protected] (L. Fedrizzi).
testing following the ISO 7253 standard remains the most used method by which to assess paints for corrosion control purposes, even though severe criticisms of this method have been noted in the literature for many years, mainly regarding the lack of correlation with service experiences [1]. In this context more fashionable approaches are under development; taking into account that the practically found environmental conditions are in continuous variation, in terms of sort, severity and length of time, the idea to introduce in a unique protocol more types of interactions between coating and environment seems to be a more powerful tool for selecting materials. The target is to reproduce what actually happens in reality and strive for duplicate as closely as possible the type of corrosion encountered in the desired use of the products, but also with simultaneously decreasing the time to show failure into the coating. In this paper we performed two different types of test protocols, one dedicated to the evaluation of coating/metal adhesion and one with thermal stress to investigate the barrier properties of the coatings. The first procedure is the Norsok M 501 test, which is a standardised Norwegian weathering test designed for proving the materials supposed to work at very hard environmental conditions. In this cyclic test the scribed coated panel undergoes sessions of UV exposure with humidity, salt fog and a conditioning time at room temperature, as dry part of the cycle. Indeed, it is often observed that alternating wet and
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dry corrosion condition causes faster blistering than continuous exposure to high humidity, because of the increase in internal stress: paint films delaminated from scribe line regions as a result of oxide lifting action caused while anodic corrosion products accumulate under the coating. Moreover, the blisters that are formed are rust filled and apparently also generated by the relative volume of the precipitated, solid corrosion products. The cyclic test should more closely simulate observations made after natural outdoor exposures than those obtained after only a salt spray testing [1,2]. Additionally, with regard to UV-induced degradation of the binders, it has been found that the additional presence of UV-condensation cycles in the corrosion test fundamentally altered the nature of the occurring corrosion/degradation processes. The results produced by this combination of weathering factors appear, qualitatively at least, to be closer representative for the corrosion and degradation observed in natural atmospheric service environments. It may appear that the test results neither of the salt fog nor of the wet/dry cycle give particularly realistic deterioration effects. It is reported that UV-condensation cycles combined with the wet/dry gives a more realistic simulation of the deterioration process for coating systems under atmospheric weathering conditions [3]. The visual evaluation of the degree of sample damages is analysed with respect to rusting and blistering around the scribe and even the remote area within the UV mask (Fig. 1). Even though the mentioned method is defined accelerated, it needs anyway a “long time” to rank the material corrosion protection: the Norsok test is basically planned to last 25 weeks, depending on kind of product, substrate, etc. The modern way to face this bad data is to take advantage of an aging test where temperature plays a big role into the aging of the material, permitting to gather information faster for coating corrosion resistance evaluation.
As Bierwagen et al. [4,5] recently described the electrochemical studies of the organic coats done at room temperature exposure give reproducible informations on film performances, but they do not permit the screening of very high impedance material. On the contrary, an experimental set-up in which the temperature of the sample is varied cyclically above and below the glass transition temperature [6] will establish a synergic degrading effect into the material. This is due to electrolyte transport through the film and its eventual accumulation, loss of adhesion, chemical and physical aging as a consequence of thermal fatigue effects, dielectric characteristic variations due to a more open polymer structure. The cumulative effects of such a thermal cycling in the coat controlled by means of the electrochemical parameters changes should permit the ranking of a variety of materials, for constituents, characteristics and apply purposes, in just a week of time, while remaining objective and reliable. Following this, we carried out our Thermal Cycling test, which consisted of a daily series of electrochemical measurements on a metallic coated sample, performed by means of EIS at different temperatures. The objective of this scientific research is therefore to compare the material selection chances given by different weathering and aging test, precisely the Norsok M 501 and the thermal cycling test, to evaluate their merits and limits and to establish a possible correlation within the results.
2. Experimental 2.1. Materials Various sorts of materials for constructional steelwork have been investigated: nature and composition are reported in Table 1. All of them were applied by air spray to properly sandblasted steel. The final systems curing took then 2 weeks, storing the samples at ambient temperature (standard climate class 2: 23 + −2 ◦ C and 50% relative humidity + −5%). System B was left outdoor during the 2 weeks of drying, following the precautions suggested by the supplier: previous test have effectively shown that outside weathering ages better these materials by producing a better adhesion between steel and primer, and between primer and topcoat, than the normal storage into room, due to UV-induced cross-linking and oxidative curing. 2.2. Norsok test set-up
Fig. 1. Norsok test sample layout: the visual evaluation is made taking into account the whole area inside the UV mask print.
The test includes a fraction of time of salt spray fog, a conditioning time at room temperature, like dry part of the cycle, and a session of UV exposure with condensation. Every single part of the test is standardised by ASTM or ISO, therefore in Table 2 are presented only the specifications of the used parameters, while for details of the procedure reference may be made to the relative specified standard. The
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Table 1 Type of material System
Layer
Binder and pigments
Thickness (m)
A
1 2 and 3 4
2-Packs highly pigmented zinc epoxy primer 2-Packs epoxy-based intermediate MIO pigmented 2-Packs polyurethane based top coat
387
B
1 2
1-Pack urethane-alkyd primer + zinc phosphate 1-Pack urethane-alkyd resin top coat
179
C
1 2 and 3
1-Pack talc pigmented PVC-acryl resin primer 1-Pack PVC-acryl-based top coat
290
F
1 2 and 3
2-Packs highly pigmented zinc epoxy primer 2-Packs epoxy-based top coat
418
H
1 2 and 3
2-Packs epoxy-based primer 2-Packs low solvent epoxy-based top coat
349
test was performed on scratched panels as visible in Fig. 1, with a 1 mm wide scribe. The test is nominally planned to last 25 cycles for a total amount of 4200 h. 2.3. Thermal cycling test parameters All measurements were performed in a metallic oven that provides through its structure a very good Faraday effect against external electro smog and additionally permits the samples temperature control during the thermal cycling. The measuring cells were realised by gluing a 25 mm wide PMMA tube on the coat surface; due to the high temperature a medium viscosity, fast curing, single component cyanoacrylate adhesive, with a glass transition temperature of 150 ◦ C was used. In particular, cells were filled with the electrolyte solution 3 days before to start the first measurement and then the samples were left at room temperature, so that the coating can be considered in saturation conditions even by the first data acquisition. The Harrison electrolyte (0.05 wt.% NaCl + 0.35 wt.% (NH4 )2 SO4 ) was used, because it has been observed that the presence of ammonium and sulphate species in the electrolyte is more reliable for accelerating of atmospheric corrosion in industrial atmospheres [7], moreover, low concentration electrolyte may allow electrochemically active components of paint system to function during testing more as they would be expected to do in practice [1,8]. Impedance spectra are then daily registered at three different temperatures, 23, 55 and 85 ◦ C for a total amount of 5 days, after leaving the coating at the new reached temperature a couple
of hours to stabilise. The temperature has been varied by leaving the samples into the thermostatic oven, so that the whole system constituted by metal substrate and organic coatings can be considered in a homogeneous state, without any gradient of temperature through its structure. To check the temperature course in time of both oven and coating a pre-experiment was carried out and the result is presented in Fig. 2: the result is a minimum of 2 h of real waiting time for the coating at the fixed temperature before to measure its impedance and to change temperature again. 2.4. EIS measuring set-up The advanced spectrometer from Zahner Elektrik was used for EIS with a two-electrode configuration (a platinum wire as counter); it is characterised by an input impedance >1 T and <5 pF in the frequency range below 100 KHz while it is used in combination with the buffer amplifier connected to the reference and working electrodes. Any other detail of the IM6 Zahner equipments can be found in the User Manual [9].
Table 2 Norsok cyclic test M 501 parameters Test
Duration (h)
Standard
Salt spray Drying in air UV-A 340 nm weatherometer Total duration of one cycle
72 16 80 168
ISO 7253 ASTM D4587, method B
Fig. 2. Variation of samples and oven temperature during the temperature cycling test.
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Fig. 3. The two equivalent circuit models used for analysing EIS data: the values of the R are in and those of the C in Faraday.
2.4.1. Amplitude Amplitude of 20 mV was used; with these small amplitudes there is no doubt that the system behaves linearly. 2.4.2. Frequencies All the measurements were performed between 100 mHz and 100 kHz, with five points per decade and five samples per point. 2.5. Evaluation of the measured data 2.5.1. Qualitative The measurements are first analysed visually. To perform the visual evaluation the data were plotted in a Bode-plot (log(IZI) and phase angle as a function of log(frequency)), which gives a good overall impression without much details [10]. 2.5.2. Quantitative evaluation As was already mentioned the quantitative interpretation of the measurements is performed with an equivalent circuit. These circuits are fitted on the data with the SIM software from Zahner [9]. For all measures and coatings (water uptake and dielectric studies) the two simple circuits in Fig. 3 are used. Both equivalent circuits have merits in the description of EIS results from barrier coatings, even if they are likely the simplest approximation. Model B is suitable to describe parts of EIS spectra over few decades; mainly at higher frequencies the loss capacity has a high accuracy and at low frequencies the coating resistance can be obtained properly. If the fitting of spectra from an intact coating has to cover a wider frequency range, experimental practice showed that model A is better applicable. As most of the barrier coatings show a non-constant phase angle, it is simply necessary to use a CPE and a capacitor in parallel for a proper simulation. (For some cases a Young–Göhr-impedance element might be used with some justification [11].) But also from some theoretical considerations it is quite easy to understand this modification, as water absorption will not only increase the dielectric constant, it also changes the processes of charge flux in the coating. These modified processes are related to hydrogen bonds between water molecules and polymeric
functional groups, yielding in an influence on the relaxation of charges and polarisation on the coating surface [12]. All the experimental data were analysed between 1 and 100 kHz with the model A to assess the coating capacity (CPE element C2 ) and with the model B all over the measured frequency range to evaluate the coating resistance (R6 ). The normal starting values for the fit are presented near each element. The SIM software permit to normalise the value of constant phase element (CPE) according to the following equation: Z=
1 ω0 V (j (ω/ω0 ))α
where ω is the angular frequency, j the imaginary number, V the loss capacitance [F] at ω0 , ω0 the normalising frequency and n the CPE power. Such definition permits, throughout the value of the parameter ω0 , to better model the capacitive behaviour of the evaluated element [11]. CPE will be here used to study small deviations from ideal capacitive behaviour; in our case α was always bigger than 0.82 in all cases. To evaluate and compare the results V is treated as a capacitance (C). It has been shown that a CPE (more precisely a loss capacity) has to be normalised at a certain frequency ω0 and its value should be in that frequency range, where the capacity shows its highest significance, normally in the 100 Hz–10 kHz range. If this normalising frequency is not adjusted and assumed to be ω = 1, then an error in capacity can be estimated to be up to 100% [11,12].
3. Results 3.1. Thermal cycling test As already shown in the literature [6,13] the barrier properties of the organic coatings are remarkably affected by testing temperature increase. Coatings become more permeable and less protective due to the opening of the polymer structure, particularly when the glass transition temperature is overtaken. In Fig. 4 such a behaviour is quite clear; the tested sample shows an impedance decrease, in the low frequency range, going from spectra acquired at 23 ◦ C to those
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Fig. 4. EIS spectra of system A: the impedance course shows an high grade of reversibility day by day at every temperature.
at 55◦ and 85◦ . Moreover, as observed in Fig. 5 the phase angle values for all measurements at 23 ◦ C undergo a small reduction while those evaluated at higher temperature show a significant loss at lower frequencies. Such behaviour was general and all the tested systems showed this dependence on the testing temperature. However, when aging cycles where introduced the various systems behaved in different ways and taking advantage of the experience of Bierwagen et al. [5] the systems were classified in two categories: reversible and irreversible. Fig. 4 shows the impedance behaviour and the extend of reversibility of system A: all curves at each single temperature seems to overlap themselves in the whole frequency range; this could mean that there is no cumulative damage introduced by this thermal cycling. On the contrary, system B shows an irreversible behaviour. Fig. 6 shows the electric characteristics of this system after
thermal cycling: it shows a variable spectra cycle by cycle, notifying a progressive deterioration, which is not longer recovered. The just defined irreversibility can be correlated with the deterioration which natural thermal cycling may introduce in the organic coatings during their service life, as electrolyte accumulation in the film, loss of adhesion, chemical and physical aging. By fitting of the impedance data it has been found that even for the so called reversible systems (like system A) the capacity trend shows a certain degree of irreversibility: in every case capacity increases with temperature and time. The typical serrated course of the capacity behaviour is reported in Fig. 7. Systems like coating B show a high tendency of increasing capacity cycle by cycle, while system C (which has a reversible behaviour similar to system A) remains to be lower but anyway slowly increasing values. For every cycle the highest capacity can be fitted at
Fig. 5. Reversible change of the phase angle values for system A due to thermal cycling.
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Fig. 6. Irreversible behaviour of the system B due to thermal cycling.
the highest temperature; by decreasing temperature capacity returns to a lower values range. This can be read like a different tendency of adsorbing and desorbing water with temperature, leading to a cumulative structural damage of the polymeric material. From the coating capacity it is then possible to obtain the value of the dielectric constant by the following equation: ε=
Cd ε0 A
where C is the coating capacity, d the coating thickness, ε0 the vacuum dielectric constant and A the sample area. It is well known that dielectric characteristics depends on the measuring temperatures and that generally the dielectric constant increases with temperature, with a law that depends also on the used frequency to measure it [14]. In literature it is possible to find that the dielectric constant changes are due to several causes which are all function of the temper-
ature: the polymer structure and chemical composition, the amount of water present in the material and its interaction with the polymer as well as the pigment–binder interface. The variations of such properties which occur at high temperature cannot be quantitatively allocated, therefore, in our work only the values obtained at room temperature are taken in consideration. Table 3 lists the dielectric constants evaluated at 23 ◦ C at the beginning of the test and after five cycles, and the cell surface rust degree according to ASTM D610-95. The rust degree is defined from values going from 1 up to 10, where 10 is the best one representing no rust spots. Following Table 3, the systems A and H have got a small ε increase and they maintain an outstanding resistance to rust formation. For what concern system B, it has got a very high variation of dielectric constant from the first to the final cycle and it has shown a poor ability to protect from rusting. Such high water up take was in a certain manner expected because of its higher hydrophilic nature compared to the other binders. Additionally, the low thickness (179 m), the slow oxidative cross-linking and the low number of coats (only 2 instead of 3–4) have not allowed then this system to prevent corrosion on set. This resulted in the formation of Table 3 TC results: dielectric constant and cell rusting
Fig. 7. EIS data: capacity course of systems B and C during thermal cycling show different increase in the time.
System
ε1 a
ε5 b
Cell rust degreec
A B C F H
15.94 10.63 10.41 13.56 11.08
18.78 29.65 11.87 39.19 15.01
10 7 Totally rusted interface 10 10
a Note that, this value is referred to the first measurement performed at 23 ◦ C made after 2 days of immersion in electrolyte. b Dielectric constant obtained at 23 ◦ C after five cycles. c ASTM D610-95.
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Fig. 8. Rusted interface of system C after cycling.
little rust spots as remarked in Table 3. Besides system B, even system F shows a great tendency of water adsorption, but in this case the substrate is anyway protected. System F has got the same zinc dust pigmented epoxy primer as system A; this very good primer, which develops outstanding adhesion with the substrate and very good cathodic protection, may have protected the metal independently on the top coat performances with respect to this test. System C seems to adsorb very few water with respect to other cases and at the same time it seems to present a completely corroded interface, as it was possible to observe by peeling after cycling the very weak coating from the substrate (Fig. 8). Probably its chemical nature (being a PVC-based system with only physical drying, not reactive curing) and the total absence of anticorrosive pigments have brought it first to a quick and progressive water permeation right after the first cycle and to its accumulation at the coating–substrate interface, and afterwards this water has contributed to the metal oxidation. In order to explain the very small ε increase during cycling we have to introduce
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a new hypothesis. A polymer capacitance increase is correlated with water penetration into voids, pigment/polymer interfaces as well as bonding of water to polymer chains. But if water penetrates the organic coating to form a continuous water layer at metal/polymer interface the electric circuit to model the physical system can be changed in a series of capacitance (one related to the organic coating, the other one related to the water film). In such a case no large capacitance increase can be measured [15]. As far as the barrier properties are concerned, all tested systems have got excellent performances: they always show resistance higher than 4.3 × 105 m2 (Fig. 9). Such very high range of values is on the other hand very hard to assess, because it ranges at the sensibility limit of the equipment (due to the really measured area of 4.9 cm2 ). Within the 2-pack epoxy coating, system A seems to improve its performance after the second cycle while F and H show a loss of resistance after the first cycle. Comparing this behaviour with the rust degree evaluation of the cell surface, system A is in agreement because it has got an overall good performance, but both F and H showed no rust formation even with lower final resistance. The 1-pack system B resistance scatters at very high values but it shows a few rust spots on the cell surface. The resistance values of system C remain nearly unchanged, what might lead to the misinterpretation of the best performance in this context, whereas it has been found that its interface was completely corroded. Resistance values measured by EIS seem therefore not being very reliable for the corrosion monitoring of coatings with very high thickness [16]. 3.2. Norsok M 501 test Norsok test results provide information either about the adhesion characteristics and the bare metal protection while looking at the scratch, or about the system barrier properties,
Fig. 9. EIS data: resistance calculated at 23 ◦ C for sandblasted surface panels.
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Table 4 Norsok M 501 results: cut blistering Cycle
System A
System B
System C
System F
System H
1 2 3 4 5 6 11
10 10 10 10 10 10 10
10 10 10 10 10 4 (few) 4 (medium dense)
10 10 4 4 4 4 4
10 10 10 10 10 6 (few) 6 (few)
10 10 10 4 4 4 4
when looking far away from the scratch. Only the results of the first 11 weeks of test among the 25 originally planned are presented, because even though the samples have not gone through the whole prove period, the external changes they have shown are anyway enough to propose a first fairly good systems ranking. As a first observation, none of the studied systems has shown blistering or rusting in the remote area within the UV mask: from this kind of test it can be concluded that every system behave good in terms of barrier properties. The cut blistering results, according to the ASTM D714-87, are presented in Table 4; they rank from 10 to 0, where number 10 represents no blistering, to illustrate the size blistering and from few to dense blisters to indicate the frequency. The system A is the only one which does not start to delaminate in the cut surrounding zone within 11 week-cycles of test. This confirms the good barrier properties evaluated even by means of the thermal cycling test: low water up-take and very good surface protection. System H, which was identified in the previous section like good performing, behaves in this case roughly bad, showing an early failure and an increase in blister dimensions in the following cycles. The inferior adhesion properties to the substrate of this coating system, compared to the other, as measured by paint supplier, has permitted water to penetrate fast from the scratch section and to easily produce blisters. Therefore, it has to be noticed that, systems with good barrier properties in undamaged conditions can quickly loose their performances in presence of little cuts or defects, if their adhesion is not particularly good. The systems F and B behave better than system H: both show the first blister formation two cycles later. System F produces a more limited number of blisters and of smaller sizes than system B; this may be due to better adhesion of the primer of system F, the same used also in system A. The system C is the first which collapses, right after the second cycle, and which presents a totally swelled scratch at the end of the 11th week. The high permeability and the poor adhesion evaluated in the previous test is confirmed even by this kind of test. The cut rusting degree is reported in Table 5 in four different grades of rust amount from none to completely rusted. The test permits a good assessment of the behaviours of the different systems, showing how their performance
(few) (few) (few) (medium dense) (dense)
(few) (medium dense) (medium dense) (medium dense)
Table 5 Norsok M 501 results: cut rusting Cycle
A
1 2 3 4 5 6 11
0 Small Large Large Large Large Large
B spots spots spots spots spots spots
All All All All All All All
C rusted rusted rusted rusted rusted rusted rusted
All All All All All All All
rusted rusted rusted rusted rusted rusted rusted
F
H
Small spots Small spots Small spots Small spots Large spots All rusted All rusted
All All All All All All All
rusted rusted rusted rusted rusted rusted rusted
degenerates with time; the cut rusting results give clearly evidence for the presence or absence of active anticorrosion pigments in the chemical formulation of the primer. Both the systems C and H have got a totally rusted scratch right after the first week of testing and they are actually those systems without any anticorrosive pigmentation of the primer. Same course is shown even by system B which is based on a zinc phosphate pigmented primer: its protective characteristics seems not to be very appropriated for a timely extended protection of the scratch. The systems A and F show a delayed onset of corrosion in the cut and, in particular for system A, the scratch is still not totally rusted after 11 cycles. A great amount of zinc dust in the primer of systems A and F has contributed to develop a better protection of the bare metal compared to those systems which have not any kind of cathodic protective pigment. This was better visible in the system A, which is characterised by high barrier effect intermediate and top coats, while system F has shown a faster reduction in the galvanic action. The higher permeability or the greater tendency to take water up may produce a faster oxidation of the zinc particles, causing an interruption of the electrical flow between adjacent zinc grains and between the steel substrate. Specific studies are present also in literature which confirm this hypothesis [17].
4. Conclusions Steel samples were protected by different high thickness industrial organic coatings. Two different testing methods were used to assess and to rank the paint behaviour: • the Norsok test by visual observation;
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• the temperature cycles after immersion in Harrison solution by EIS measurement. Norsok test showed to be quite effective in the evaluation of coating adhesion properties and to assess the effectiveness of active primers in protecting metal substrate at scratches. This test was also very effective in providing useful information about barrier protection properties of intact films when exposed to UV radiation and humidity cycles [18]. Nevertheless, such a test is only based on a visual observation making the test itself quite subjective, moreover, time to highlight coating properties is quite long. Thermal cycling of immersed samples showed to be a really drastic test enhancing coating degradation by water up-take. Then samples behaviour can be differentiated very rapidly. However, not always electrical parameters obtained by EIS were able to assess the protection behaviour. The resistance of the coating failed in giving reliable data, always showing very high resistance, even when coating protection failed. Such a parameter has to be carefully used in order to predict coating behaviour, specially in the case if coating thickness is very high and adhesion to the substrate becomes the critical parameter for corrosion protection. On the contrary, CPE confirmed to be very sensitive to coating degradation due to the water up-take. This parameter changed greatly when measured at different temperatures depending on the variation occurring in the barrier properties of the film with temperature. Moreover, this parameter allowed to see that thermal cycling can introduce irreversible degradation in the paint depending on the chemical nature of binder and of the whole coating system. However, also this parameter was sometimes not able to discriminate corrosion behaviour when barrier properties were high due to high coating thickness, and the critical factor became adhesion. Then, some unusual situations were observed: • high CPE variation during thermal cycling maintaining a good protection behaviour (good adhesion properties); • small CPE variation during thermal cycling, with a rapid coating detachment (poor adhesion properties).
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Acknowledgements Authors are grateful to Prof. P.L. Bonora head of the Industrial Corrosion Control Laboratory of the University of Trento, who encouraged the work development, and all Sika collaborators for the technical support. References [1] B.S. Skerry, C.H. Simpson, NACE—Corrosion 91, Paper No. 412. [2] D.A. Jones, Principles and Prevention of Corrosion, 2nd ed., Prentice-Hall, Englewood Cliffs, NJ, 1996. [3] Encyclopedia of Polymer Science and Engineering, Weathering, vol. 17, Wiley, New York, p. 796. [4] G. Bierwagen, J. Li, L. He, D. Tallman, Fundamentals of the measurement of corrosion protection and the prediction of its lifetime in organic coatings, Proceedings of the Second International Symposium on Service Life Prediction Methodology and Metrologies, Monterey, CA, November 14–17, 1999, J. Martin, D. Bauer (Ed.), ACS Books, Washington, DC, 2000. [5] G.P. Bierwagen, J. Li, L. Ellingson, D.E. Tallman, Prog. Org. Coat. 39 (2000) 67–78. [6] J. Li, C.S. Jeffcoate, G.P. Bierwagen, D.J. Mills, D.E. Tallman, Corrosion 54 (1998) 763–771. [7] B.S. Skerry, A. Alavi, K.L. Lindgren, J. Coat. Technol. 60 (765) (1988) 97–106. [8] W. Funke, J. Oil Colour Chem. Assoc. 67 (1984) 71. [9] Thales/IM6 Manual, Zahner, Kronach, Germany. [10] D.H. Van Der Weijde, Proefscchrift, 1996. ISBN 90-9010105-5. [11] C.A. Schiller, W. Strunz, The Evaluation of Experimental Dielectric Data of Barrier Coatings by Means of Different Models, ZahnerelektrikGmbH & Co. KG, El. Acta 46 (24–25) August 15, 2002, pp. 3619–3625. [12] W. Strunz, Private communication. [13] R. Granata, K. Kovaleski, in: Proceedings of the 13th International Congress on Corrosion, vol. 1, Houston, TX, September 1993, p. 24. [14] C.A. Harper, Handbook of Plastics and Elastomers, McGraw-Hill, New York, 1989, pp. 2–27. [15] F. Deflorian, L. Fedrizzi, S. Rossi, P.L. Bonora, Electrochim. Acta 44 (1999) 4243. [16] S. Duval, Y. Camberlin, M. Glotin, M. Keddam, F. Ropital, H. Takenouti, Prog. Org. Coat. 39 (2000) 15–22. [17] S. Feliu, in: D.C. Silvermann, M.W., Kending (Eds.), ASTM STP1188, 1993. [18] O.O. Knudsen, U. Steinsmo, M. Bjordal, S. Nijjer, Protective Coat. (Europe), vol. 9, No. 12, December 2001, pp. 52–56.