TOPCOAT FLAKING - A MECHANISM STUDY. PART 1: LABORATORY TESTING -INTERNAL STRESS, MECHANICAL PROPERTIES, ADHESION AND AGEING

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1 Paper No TOPCOAT FLAKING - A MECHANISM STUDY. PART 1: LABORATORY TESTING -INTERNAL STRESS, MECHANICAL PROPERTIES, ADHESION AND AGEING Sten B. Axelsen and Torstein R0ssland StatoilHydro ASA 4035 Stavanger, Norway sbaxe@statoilhydro.com Roy Johnsen Norwegian University of Science and Technology, NTNU 7491 Trondheim, Norway Ole 0ystein Knudsen SINTEF Materials and Chemistry 7465 Trondheim, Norway ABSTRACT Topcoat flaking has been, and still is, considered to be a significant problem on both onshore and offshore structures in the Norwegian sector. Polysiloxanes, the predominant topcoat in the Norwegian market during the last few years, may seem to be particularly vulnerable to this degradation mechanism, but also other generic type topcoats exhibit a similar behavior. Qualification testing according to established standards, e.g. Norsok M-501, does not seem to be able to reveal the potential problem and there is a need for a better understanding of the failure mechanism(s). Such knowledge is a necessity in order both to implement corrective measures to prevent this problem in the future, and to improve the qualification test methodology. A finite element method (FEM) approach has been suggested in order to determine the importance of different coating properties, e.g. adhesion, internal stress, stress-strain behavior, elongation before rupture etc. This is a three-step process: 1) Determination of internal stress and mechanical properties of single coatings, 2) Determination of interfacial factors and any inter-coat effects (e.g. adhesion), and 3) Building and testing the model. Both steps 1) and 2) include an evaluation of the quality of the established data. This paper summarizes the first part of the study on topcoat flaking mechanisms. Keywords: Organic coatings, internal stress, mechanical properties, ageing, adhesion Copyright 2008 by NACE International. Requests for permission to publish this manuscript in any form, in part or in whole must be in writing to NACE International, Copyright Division, 1440 South creek Drive, Houston, Texas The material presented and the views expressed in this paper are solely those of the author(s) and are not necessarily endorsed by the Association. Printed in the U.S.A. 1

2 INTRODUCTION Due to health aspects, polyurethane paints containing isocyanates were banned from the Norwegian offshore industry in the late go's, and polysiloxane (PSG) paints took over as the predominant topcoat for atmospheric conditions. Together with the switch in chemistry, the number of coat layers was also reduced from three to two in order to lower the application cost and increase productivity (faster turnaround times). Failures of the two-coat PSG systems, due to cracking and flaking were, however, reported after relatively short time in service and in the 2004 revision of Norsok M-501' three-coat systems became a requirement. Whether this is an adequate solution to the crackinglflaking problem is still subject for discussion. According to a Norsok task force established to investigate the reported problems, the extent of flaking/delamination of PSG in three-coat systems is reported as less extensive than for twocoat systems 2. Internal stress, subject for a number of studies within the last few decades3-1o, has together with flexibility of the paint films and inter-coat adhesion or adhesion to the substrate, been sug~ested as the main parameters for cracking and flaking. Within a joint industry project at SINTEF,7, extensive studies have been performed to quantify both the formation of internal stress and the mechanical properties of relevant coating systems. This present work is a continuation of the investigations performed within the project mentioned above, where the aim is to establish a better understanding about the relative importance of internal stress, mechanical properties and adhesion. MATERIALS AND EXPERIMENTAL PROCEDURES Internal Stress Internal stress was determined by use of the cantilever (beam) method according to ASTM D "- Aluminum strips (EN AW-3004) with the following dimensions: width, 20 mm; length 220 mm; and thickness 0.23 mm, were used as cantilever substrate. Before coating application, the surface on the side to be coated, was ground with mesh 320 silicon carbide paper and degreased. The organic coatings were applied by spraying. The individual coats from two different commercially available three-coat systems with polysiloxane topcoat were investigated. That is, two zinc-rich primers (ZE1 and ZE2), two epoxy mastics (EM1 and EM2), and two polysiloxane topcoats (PS1 and PS2) were tested. See details in Table 1. The coated strips were allowed to cure in a climate cabinet at 10 C and 80% RH for at least three weeks before exposed to different climatic conditions. Curing conditions were selected based on weather data from installations in the North Sea and Norwegian Sea (see Figure 1). After curing the samples were exposed to variations in both humidity and temperature (only one parameter changed at one time), before they were immersed in distilled water at 40 C, and finally exposed to ultraviolet (UV) radiation. In between each change in the climatic parameters, the samples were allowed to "normalize" at 10 C and 80% RH. Beam deflection was measured before (and after) each change in the climatic exposure. All details on the cyclic exposure are summarized in Table 2. 2

3 Beam deflection was measured as described in ASTM D , by fixture of one end of the cantilever. Measurements were made from both ends of the samples, and the average deflections were used for stress calculation. Six parallel samples of each coating product were tested. Stress was calculated based on the simplified version of Corcoran's equation", without the term expressing stress relief, as recommended in the standard. Calculations made with the actual coating thicknesses and the coatings' moduli of elasticity proved stress relief to affect the results only marginally. In addition to the effects of climatic parameters on internal stress forming in paint films, three more parameters were included in the internal stress tests: 1. Dry film thickness of the zinc rich primer. Unintentionally, one of the zinc rich primers was applied with a very low film thickness, and as will be discussed in a later section, this coating developed high levels of internal stress. A second test series was conducted with increased film thickness. 2. Time dependency (stress vs. time during curing). 3. Effect of residual stress in the aluminum strips. The cantilever samples are made from coiled aluminum sheets, and show a certain curvature due to residual stresses. In order to make sure that these stresses, or any transverse residual stresses, do not influence on the measurements, some of the coatings were tested on samples from both "as received" and annealed material. Annealing was performed at 440 C for 2 hours, which effectively removed all residual stress from the aluminum strips. Mechanical Properties Young's modulus, tensile strength and elongation at break were determined for all six coating (see Table 1) by tensile tests in a 500 kg universal testing machine at a strain rate of 1 mm/min. Free films were molded in a die of polytetrafluoroethylene (PTFE), and dog bone shaped specimens were punched out from these films. The specimens were 80 mm long, 5 mm wide in the narrow centre area, and 15 mm wide in the ends. Film thicknesses are given in Table 1. The test samples were cured at 10 C and 80% RH for at least three weeks. Six samples were tested after curing, while twelve samples were weathered either by immersion in distilled water at 40 C for three days (6 samples) or by UV radiation for 40 hours at 60 C (6 samples), before testing. Adhesion/Cohesion One of the objectives of this work has been to be able to measure, in the laboratory, adhesive or cohesive strength under both tensile and shear stress load, both wet and dry conditions. Normal pull-off tests, e.g. ISO , have the disadvantage of including an adhesive which complicates the measurement of wet adhesion. Lap shear testing similar to what is being used for testing of adhesives (ASTM D3165'4) was considered. The idea was simply to bond the two specimens together by use of paint instead of an adhesive. This test method does, however, raise an issue with respect to trapping of solvents as the distance from the center of the painted 3

4 area to the surface would by far exceed normal coating thickness. To reduce the painted area to dimensions similar to normal coating thickness was considered unpractical. A new test set-up has therefore been suggested. Two thin-walled stainless steel tubes (outer diameter = 10 mm, wall thickness = 1 mm) are bonded together by use of paint. The tubes could be either pulled apart to determine tensile stress resistance, or they could be separated in a torsion set-up to determine the shear strength (see Figure 2). As the samples could be placed directly into the universal testing machine after immersion in water, the test set-up allows determination of wet adhesion properties. Distance between the two tubes (equals coating thickness) is kept constant at 200 ~m. Adhesion/cohesion properties will be studied closely during phase two of the project, and will be published later. Some initial results are, however, reported in this paper. RESULTS AND DISCUSSION Internal Stress Changes in internal stress, for the six tested coatings, as a response to changes in humidity and temperature, water immersion and UV radiation, are summarized in Figure 3 and Table 3. Before looking into the details an important source of error should, however, be brought to attention. Measurement of deflection, i.e. stress measurement, was not performed at the various test conditions, but at room temperature and ambient humidity (typically 60-70% RH). Figure 4 shows that the time between when the samples are taken frorn a given test environment, in this case 1Q C and 80% RH, until the deflection is measured could affect the results significantly. Particularly the zinc rich primers showed a rapid response to the climatic changes. Due to practical reasons samples were taken out in batches, and measurements were made in random order. Time between removal from the climate chamber until measurement would normally be a few minutes (less than ten). This could, however, be sufficient to mask minor or moderate changes in internal stress in the zinc rich primers. The results from the variations in climatic exposure (summarized in Figure 3 and Table 3) showed that: Only one of the zinc rich primers, ZE1, developed a significant internal stress during curing. ZE1 had an internal stress of close to 20 MPa after curing. All the other coatings had an internal stress level below 2 MPa after curing. The ZE1 primer was unintentionally applied with a very low film thickness, average of 17 ~m, and a second series of samples were prepared with an average dry film thickness of 78 ~m. As shown in Figure 5, these samples developed significantly lower levels of internal stress, only about 4-5 MPa. Due to the brittle nature of the ZE1 film and the internal stress (free films showed such degree of deformation that mounting into the test machine was impossible), it was not possible to determine the tensile strength for the cured samples. Mechanical testing after exposure to water or UV does however indicate that 20 MPa could be close to the tensile strength of the ZE1 primer, and hence cracking could be a potential problem for thin film thicknesses. Low film thickness is normally not expected to be a problem with respect to internal stress' 1O. A very rapid film formation was, however, observed on these samples. 4

5 In addition, large zinc particles or clusters of zinc particles, dominates the cross sectional area (see Figure 6), and may hinder any lateral movement during film formation. This may explain why low film thickness seems to be a problem for the ZE1 zinc rich primer. Lowering of the humidity from 80% RH to 38% RH caused only minor changes in the stress level in all coatings. Heating from 10 0 e to 60 0 e increased the stress in all coatings. The second zinc rich primer, ZE2, developed an internal stress of 14 MPa, which is in the same order of magnitude as the tensile strength of these coatings. One of the epoxy mastics developed an internal stress of 7 MPa, the highest stress level measured for these coatings throughout the entire test cycle. Alone, this should not be sufficient to cause any cracking, but together with strain from for instance under film corrosion this could cause problems. The highest reading for the polysiloxanes was just above 3 MPa. This was the highest level of internal stress measured for polysiloxanes in the test program. Water immersion caused only minor changes in the stress level for all coatings except zinc primer ZE1, which showed a significant decrease in internal stress. This is most likely due to swelling from the water uptake and perhaps also a certain softening effect from the water. After the ZE1 samples had been dried for two days at 10 0 e and 80% RH there was a build up of stress again, but to a lower level than before water immersion. This strengthens the theory of a beneficial softening effect from the water. The stress build up during drying was also observed for zinc primer ZE2 and the two epoxy mastics, indicating that the water immersion may have caused a slight wash out effect. However, no attempt was made to confirm such a theory by analysis of the water. The polysiloxanes showed no change in stress level during water immersion, but a build up of slightly compressive stresses during the consecutive drying. With exception of a certain reduction in the stress level of ZE2, UV exposure did not seem to alter the stress levels significantly. At the end of the test cycle all coatings had developed low levels (less than 3 MPa) of compressive stress. Figure 4 does not only show the profound effect of increased film thickness for coating ZE1, it also illustrates the effect of two more parameters: Time dependency, i.e. stress versus curing time. Based on recommendations from the paint producers the coated samples were allowed to cure for at least three weeks at 10 0 e and 80% RH before any changes in the test parameters. Figure 4 shows a tendency for the internal stress to level out after about three weeks. Hence, at least based on the ZE1 measurements, the advice from the suppliers seems reasonable. Residual stresses in the aluminum strips. The difference between as received samples and annealed samples were well within the standard deviations for these measurements. These results have been confirmed by measurements on coatings EM2 and PS2. While there for ZE1 seems to be a tendency for slightly higher stress levels on annealed samples, PS2 showed slightly higher stress levels on the as received samples. The results allow further use of strips made from coiled material. This is considered beneficial as annealing of the aluminum make the strips very sensitive to any mechanical strain, and hence complicates any handling of the samples (e.g. removal of tape used for masking). 5

6 Mechanical Properties Mechanical properties, i.e. Young's modulus, tensile strength and elongation to break, have been characterized for all coatings after curing, UV radiation and water immersion. All readings are summarized in Table 4. Some findings should however be pointed out: The zinc rich primers are in general relatively stiff (high Young's modulus) and have low flexibility (elongation at break typically 0.5% or less), see typical stress-strain curve in Figure 7. There are some differences in the tensile strength of the two tested products. After curing the epoxy mastics were able to withstand elongations of 3-6% before rupture (see Figure 7). Exposure to water, and particularly UV, did, however, reduce the flexibility of these coatings. It further ted to an increase in the Young's modulus. As epoxy mastics normally are not exposed to UV radiation these data may seem irrelevant. However, in areas with topcoat flaking mechanical properties of the intermediate coating are of great interest. Figure 8 summarizes some of the most interesting results from the mechanical testing. The figure shows how water immersion and UV radiation alter the mechanical properties of the tested polysiloxanes. These results are assumed to be a result of increased cross linking. Before the adhesion properties of these coatings are determined, it is not possible to conclude on how important these changes are with respect to topcoat flaking. It is, however, obvious that loss of flexibility is not beneficial with respect to the likelihood for cracking and flaking. Figure 1 shows the climatic variations for a period of one year at an offshore installation located in the Norwegian Sea. In addition to these variations offshore paint systems will experience changes in temperature and humidity due to operational variations. Given the responses to climatic changes summarized in Figure 3, it is clear that the typical load/stress situation is not static, but cyclic. It has therefore been performed two initial tests with cyclic loading. The results from one of these tests are shown in Figure 9. The figure also shows a representative stressstrain curve for this coating (PS2). The initial high load -low cycle trials indicate a fatigue behavior where rupture (which in service means cracking) occurs at stresses below the measured tensile strength. More parallels and different load cycles are, however, needed to map the fatigue behavior of the coatings. This will be a part of the next phase of this project, and published later. Adhesion/Cohesion As mentioned earlier only some initial adhesion tests are performed up to now. The narrow zone with coating (see Figure 2) is the only part of the set-up allowing some flexibility. Hence, the test samples have proven to be very sensitive to any misalignment and more than half of the tests have failed to give valid results. Further adjustments to the test set-up are therefore necessary, and the number of parallels should be increased in future testing (only three parallels have been used in the initial trials) to ensure statistically significant results. Some tests have, however, given two readings, and even though this is not sufficient to make any conclusions a couple of interesting observations are worth mentioning: For coating EM2, purely cohesive fractures were observed. The measured cohesive strength was in the same order of magnitude as the tensile strengths measured in the mechanical testing. 6

7 For coating PS 1, dry samples showed 100% cohesive fractures, while for the samples immersed in distilled water the fractures were 100% adhesive. Further, the force needed for separating the two tubes was clearly reduced for the water immersed samples. Again, it should be emphasized that the observations above are only indications of that the test set-up may provide useful information on wet adhesion. Further testing is necessary to get statistically significant data. CONCLUSIONS Of the tested coatings, the zinc rich primers developed the highest levels of internal stress. In some instances, stresses in the same order of magnitude as the tensile strength of these coatings were measured. At high temperatures (60 C) the epoxy mastics also developed significant levels of internal stress. In case of additional external strain, cracking could occur. The polysiloxane coatings showed low levels of stress (less than 3 MPa) under all test conditions, and it is considered unlikely that flaking will occur as a result of internal stresses within the topcoat at such low stress levels. Of the tested climatic parameters (temperature, relative humidity, water immersion and UV radiation), the temperature increase from 10 to 60 C caused the most profound change in internal stress. Variations in relative humidity, water immersion and UV radiation gave only minor to moderate changes in the internal stress levels. The zinc rich primers are in general relatively stiff (high Young's modulus: MPa) and have low flexibility (elongation at break typically 0.5% or less). After curing the epoxy mastics were able to withstand elongations of 3-6% before rupture/cracking. Exposure to water or UV reduced the flexibility of these coatings. When exposed to UV radiation both tested polysiloxane coatings lost some of their fiexibility. One of the polysiloxane coatings showed a similar behavior also when immersed in water. The importance of these results with respect to flaking susceptibility is expected to depend on differences in adhesion, which is to be tested in the next phase of the project. ACKNOWLEDGEMENTS Thanks to Egil Aune (StatoiIHydro), Nils-Inge Nilsen (SINTEF) and Ann-Karin Kvernbraten (SINTEF) for their support in the experimental work. 7

8 REFERENCES 1. Norsok M-501, rev. 5 June 2004: Surface preparation and protective coating. Standard Norway, Norway. 2. Norsok Task Force - Polysiloxanes Norway. 3. D.Y. Perera, D. Vanden Eynde, "Moisture and Temperature Induced Stresses (Hygrothermal Stresses) in Organic Coatings", Journal of Coatings Technology, Vol. 59, No. 748 (1987), p Y. Korobov, D.P. Moore, "Performance Testing Methods for Offshore Coatings: Cyclic, EIS and Stress". CORROSION/04, Paper No (NACE, Houston TX, 2007). 5. D.Y. Perera, M. Oosterbroek, "Hygrothermal Stress Evolution During Weathering in Organic Coatings", Journal of Coatings Technology, Vol. 66, No. 833 (1994), p Knudsen, A. Bj0rgum, T. Frydenberg, R. Johnsen, "Development of internal stress in organic coatin9s during curing and exposure". CORROSION/06, Paper No (NACE, Houston TX, 2006) Knudsen, A. Bj0rgum, M. Polanco-Loria, A. 0yen, R. Johnsen, "Internal stress and mechanical properties of paint films". CORROSION/2007, Paper No (NACE, Houston TX, 2007). 8. D.Y. Perera, D. Vanden Eynde, "Considerations on a Cantilever (Beam) Method For Measuring the Internal Stress In Organic Coatin9s", Journal of Coatin9s Technology, Vol. 53, No. 677 (1981), p L.F. Francis, A.V. McCormick, D. M. Vaessen, J.A. Payne, "Development and measurement of stress in polymer coatings", Journal of Materials Science, Vol. 37 (2002), p D.Y. Perera, "On adhesion and stress in organic coatings", Progress in Organic Coatings, Vol. 28 (1996), p ASTM D : Standard Test Method for Measurements of Internal Stresses in Organic Coatings by Cantilever (Beam) Method. ASTM International, United States. 12.E.M. Corcoran, "Determinin9 Stresses in Organic Coatings using Plate Beam Defiection", Journal of Paint Technology, Vol. 41, No. 538 (1969). 13. EN ISO 4624:2003: Paint and varnishes. Pull-off test for adhesion. European Committee for Standardization, Brussels, Belgium. 14.ASTM D : Standard Test Method for Strength Properties of Adhesives in Shear by Tension Loading of Single-Lap-Joint Laminated Assemblies. ASTM International, United States. 8

9 TABLE 1 - ORGANIC COATINGS INCLUDED IN THE TEST PROGRAM Code' Generic type Dn Film Thickness (DFTl, urn Specified Actual' Actual ' (internal stress (free film testing) measurements\ ZE1 Zinc epoxy primer ' ZE2 Zinc epoxy primer EM1 Epoxy mastic EM2 Epoxy mastic PS1 Polvsiloxane PS2 Polysiloxane coat system no. 1 ZE1 + EM1 + PS1, and system no. 2 ZE2 + EM2 + PS2. 'OJ Average DFTs measured on all six samples. A total of ten measurements were made on each sample. '''j Also tested with a DFT of 78 1Jm....j Average DFT measured on each sample. Six measurements were made on each sample. - CLIMATIC EXPOSURE, TEST DETAILS TABLE 2 Day(s) Duration Exposure conditions (number of davsl (in some cases 24) 10 C 180% RH (curing) C 138% RH C 180% RH C 180% RH C 180% RH C 1water immersion (distilled water) C 180% RH UV radiation C 180% RH TABLE 3 - TEST RESULTS', STRESS MEASUREMENTS Test Stress (MPal duration ZE1 ZE2 EM1 EM2 PS1 PS2 (davs) Av... 5t.d - Av.- St.d- Av.** St.d- Av. St.d U Av.- 5t.d- Av." St.d*** , ,... For test conditions see Table 2. Av. Average (of SIX samples). St.d. Standard deviation. 9

10 TABLE 4 - MECHANICAL PROPERTIES Coating Young's modulus*j Tensile strength Elongation-to-break (MPa) (MPa) (%) Cured UV Water Cured UV Water Cured UV Water ZE1 No No No data**) data**) data**) ZE No No No data**) 2700 data**) data**) EM EM PS PS *1 0 Determined In the Yo strain area. ** Less than 3 valid readings uo ai... ::J 10 ' 5 Q) a. E Q) I- o 70 cf? 60.~ '0 'E 50 ::J..c:; Q) :p > mq) 0::: -5 - temperature ' humidity J -10 -l---- IU-'--'-- -,- -"- -J.,- --l Jan-98 Feb-98 Apr-98 Jun-98 Jul-98 Sep-98 Nov-98 Dec-98 Feb-99 Mar-99 Figure 1 - Temperature and relative humidity measured on an offshore installation in the Norwegian Sea

11 Shear/torsion: Figure 2 - Test set-up for adhesion/cohesion test. Tubes (outer diameter = 10 mm, wall thickness =1 mm) are bonded together by use of the paint to be tested. ]L-----_ Test duration, days Figure 3 - Stress development as a function of coatings and climatic changes. 11

12 3,5 3 2,5 2 co 0.. ::;;: eli 1,5 '" f!? Ci) 0,5 o -0,5 -L.- ~ o Time, minutes Figure 4 - Sensitivity to time between the samples are taken out of the climate chamber (10 C/80% RH) until the measurement of deflection/stress level. First measurement made after about 30 seconds defines the zero level annealed as received 4 co 0.. ::;;: eli 3 '" ~ U Days Figure 5 - Stress as a function of time during curing at 10 C/80% RH. Coating ZE1. Average OFT 78 fjm. (Tendency is indicated by the solid-drawn line). 12

13 .. - I 20um I Figure 6 - Cross sections of zinc rich primer ZE1 with two different thicknesses. 16, , 14 Epoxy mastic (EM2) Polysiloxane (PS2) 4 2 o r-, , ' o Strain, % Figure 7 - Typical stress - strain behavior of the tested coatings after 3 weeks of curing at 10 C/80% RH

14 25-, , 20 PS2 - UV ro 0.. ::;;: vi f/j Ql... U _::::::::::::;;_-=: PS2 Water PS1 - UV PS1 - Cured ::::::=::::::::::::=::::::=:::::~;;,-::: PS2 - Cured PS1 - Water OL , ~ o Strain, % Figure 8 - Change in mechanical properties of polysiloxane as a function of climatic exposure/weathering (representative curves) ẓ ro a 10.-l 5 ~ / Regular stress-strain curve Cyclic loading 0 " 5 10 Strain (%) Figure 9 - Results from initial trials with cyclic load testing. A regular stress-strain curve is included for comparison. (Cross sectional area is about 1.2 mm 2, i.e. 12 N equals 10 MPa). 14