Testing of Coating Materials in Industrial Practice - American Chemical

Chapter 19

Testing of Coating Materials in Industrial Practice

Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 26, 2018 | https://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch019

Otto Vogt

Sika Chemie GmbH, D 70432 Stuttgart, Germany

Accelerated test procedures for assessing the performance of coating materials are very common in the coating industry. Compared with exposure to service conditions, accelerated tests yield results within a much shorter period of time. But they suffer the drawback that laboratory and field conditions frequently disagree. However, at present there is no alternative to the use of accelerated tests. Examples will demonstrate their application in industrial practice. It will be shown that, despite reservations, accelerated testing can still be a good tool.

Accelerated corrosion testing has been used for many years as a means for evaluating the performance of coatings, when exposure to actual conditions is too time consuming. However, due to frequent differences between test conditions and service environment, the validity of results obtained by laboratory tests is not beyond any doubt. This point has been delt with in detail by numerous scientists. They all agree that exposure to service conditions would be the definitive test for assessment of a material's performance. Concerning accelerated tests the opinions are rather controversial. In particular, the widely used salt spray test has been seriously questioned (1 -2), because the majority of materials are not exposed to the conditions of this test in their working environment. In a literature survey the results of accelerated tests have been compared with natural weathering (3). Some correlations but also a number of inconsistencies have been encountered. For coatings designed for long-term immersion service it has been reported (4), that immersion tests of several months revealed tendencies which standard laboratory tests failed to show. On the other hand it has been pointed out that a combination of thoroughly selected accelerated tests can provide quite satisfactory results (5).

©1998 American Chemical Society

Bierwagen; Organic Coatings for Corrosion Control ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

249

250 This discussion is not very encouraging to the tester, who depends on accelerated laboratory tests, but experience has proved that in most cases allowance for such discrepancies is possible. The following presents a few examples of laboratory testing in industrial practice. It should be emphasized that these investigations were conducted in order to solve relevant problems rather than to compare testing methods.


Examples Influence of surfactants adhering to the steel substrate. Dry blast cleaning is generally accepted to be the most effective means of surface preparation for subsequent coating. But the heavy dust evolution is a serious disadvantage of this procedure, rendering its use increasingly difficult. One should bear in mind that blast cleaning is most frequently used in the course of maintenance work in order to remove old paint layers, which are likely to contain hazardous components, e.g. red lead or asbestos. Efforts are made to minimize dust formation by addition of water in different amounts to the blast-cleaning abrasive, which can be very effective. For example, in case of the moisture injected blast cleaning (6 - 7), which uses only a relatively small quantity of injection water, dust formation is dramatically reduced, though not completely prevented. In cooperation with the BauBerufsgenossenschaft, Wuppertal, Bayer AG, Leverkusen, Màrkische Fachhochschule, Labor fur Korrosionsschutztechnik, Iserlohn, and Peiniger GmbH, Leverkusen, a new approach was made to further improve this method. Preliminary experiments revealed that dispersions as well as surfactants, added to the injection water, can result in an additional dust reduction. Our task was to find out whether the surfactants will be harm full to subsequent coating or not. Since the blast equipment needed adaptions for handling the different admixtures, and also because the dust measuring turned out demanding and expensive, a screening by means of accelerated tests was decided in order to avoid unnecessary cost and work by excluding inappropriate admixtures during an early stage of the investigation. Five surfactants, supplied by Henkel KGaA, were examined. Their chemical constitution is given in Table I. The test procedure was as follows: Steel panels, prepared by using modified moisture injected blast cleaning, were coated. Two coating systems essentially according to the technical terms of delivery 918 300 page 677 (PVC combination) and 687 (EP/PUR) of the Deutsche Bundesbahn were choosen. As reference samples standard blast cleaned panels (Sa 2V2 better) were contemporarily prepared and tested. Since surfactants adhering to the steel substrate were expected to increase the susceptibility to water and, consequently, to affect the adhesion, tests comprising the influence of liquid water or high relative humidity were selected: o

-

r

Condensation water test DIN 50 017. Salt spray test DIN 50 021. A cyclic climatic test according to own specifications (8).



251

In addition, both test and control samples were exposed to natural weathering, which, of course, was unable to make any contributions to the screening. However, if during a later stage of the investigation some surfactants had turned out to be appropriate, then results of long-term testing would have been very helpful. After three weeks of testing the specimens were removed from the cabinets and evaluated. The rating criteria were: (a) rust spots, (b) blistering, (c) undercutting at the scribe, (d) cross-cut test, and (e) pull-off test for adhesion. The evaluation was simply done by comparing each individual test specimen with the corresponding control sample. From this, relative rating numbers ranging from 0 (= breakdown of the coating system) to 10 (= no damage) were derived. Figure 1 shows the results. It can be derived from Figure 1, that surfactant (4) exhibited the best performance, nearly equalled by (2). Consequently all but these two were excluded from further consideration. The behaviour of the other surfactants ranged from indifferent to bad. This result will be quite understandable if the chemical constitution of the surfactants is taken into account. The best performing surfactants are nonionic, whereas the other ones are ionic. And, not too surprising, the worst behaving surfactant (3) is a quaternary ammonium chloride. Thirty three months of natural weathering confirmed the results of accelerated testing. This leads to the following conclusions: - Despite of reservations, laboratory and long-term testing can agree. - Conclusions drawn from accelerated tests are not necessarily misleading. Performance of coatings on aluminium substrates. In connection with the filiform corrosion tests, (see below), the performance of a variety of coating systems on aluminium substrate, in general, and the adhesion in particular, were investigated. Along with outdoor exposure four accelerated tests were conducted: -

Test in condensation water without S 0 DIN 50 017. Test in condensation water with S O 2 (Kesternich test) DIN 50 018. Salt spray test DIN 50 021. Cyclic climatic test. 2

Table II shows the coating systems. It has been already mentioned, that the main objective of this investigation concerned the adhesion on aluminium substrate. The results have been recorded elsewhere (8). Another point should be scrutinized here. Standard laboratory tests are not commonly adapted to the specific problems. Because of this it can be risky to rely on one single test which, in individual cases, can yield misleading results. Performing several different tests and combining their results is more promising, because the combined result is likely to be balanced out. In Figure 2 the combined result of all accelerated tests is compared with the result of outdoor exposure (for the rating numbers


252

Table I. Chemical constitution of surfactants


No.

Chemical constitution

1

Triethanolaminolauryl sulfate

2

Alkylaminopolyglycol ether

3

Lauiyldimethylbenzylammonium chloride

4

Fatty alcohol ethoxylate

5

Turkey red oil

Figure 1. Influence of surfactants on the performance of coatings.

Table II. Materials for adhesion testing on aluminium substrate No. 1 2 3

Primer

Top coat

Modified epoxy resin (2 layers) Epoxy resin

Polyurethane

Epoxy coal-tar (2 layers)

4

Epoxy resin ester (zinc rich)

Polyurethane

5

Epoxy resin (zinc rich)

Polyurethane

6 7 8

Bituminous coat (2 layers) Tar pitch coat (2 layers) Zinc silicate

Polyurethane

SOURCE: Adapted from ref. 8.



253 see example 1). On the basis of accelerated testing the systems (1) and (2) are the best. The performance of (3) - (5) is quite acceptable, whereas (6) - (8) are unusable for aluminium substrate. This corresponds with natural weathering as well as with practical experience. If exclusively the salt spray test is regarded, the result would be different (Figure 3). For systems (1) - (3) the assessment remains valid. The other systems generally perform worse, but there are some inconsistencies. In contrast to the outcome of accelerated testing and natural weathering, (7) outperforms (4) and (5), which for their part exhibit a remarkably poor performance. Hence, relying exclusively on the salt spray test would have led to an unjustified rejection of (4) and (5) as well as to an equally unjustified acceptance of (7). From this, the following can be reached: - Individual laboratory tests not adapted to the specific conditions can yield questionable results. Therefore, running several different tests and combining their results is advisable. Filiform corrosion. Filiform corrosion is generally accepted to be a type of differential oxygen concentration cell (see (9) and therein cited literature), in which the head is anodic, whereas the tail acts as cathode. Therefore, the liquid in the head should be acidic and in the tail it should be alkaline. In attempting to make this visible, aluminium panels were pretreated with a pH indicating dye prior to coating with a colourless acrylic varnish. Finally filiform corrosion was initiated as reported below, with two exceptions: only one single immersion procedure was done, and the specimens were stored at ambient temperature instead of 40°C. Photo 1 shows a small thread, which clearly exhibits the anodic and cathodic sites. The colour film, used to photograph the panel, does not truly reproduce the vivid colours observed under the microscope, but it gives a fairly good representation of the pH changes involved. There were filaments too, which showed only a red coloured head or even remained without colour changes. In some cases it may have been that these rather short-lived events were simply not noticed. No cathodic (i.e. blue coloured) areas surrounding the head (10 -11) could be detected. In the past, filiform corrosion, though well known to the car and aeroplane manufacturers, did not play any role in the sector of corrosion protection of constructions. Today the situation is different. For several years powder coated aluminium facades in industrial environment near the coast have been attacked by filiform corrosion. During the search for repair materials coating systems with zinc rich primers turned out to be remarkably resistant to this type of corrosion (12). In order to verify this finding, a lot of different materials were examined within several test series. Only one of this series shall be delt with here. Table III shows the coating materials. The topcoats of systems (1) - (4) are identical. The binders of systems (1) - (3) and the varnishes (5) - (7), respectively, were of the same type but not exactly identical. It should be mentioned, that systems (1) and (4) are the same as systems (5) and (4) of Table II.



254

Figure 2. Averaged accelerated tests and outdoor exposure.

Figure 3. Salt spray test and outdoor exposure.



Photo 1. Thread of filiform corrosion on aluminium substrate pretreated with a pH indicating dye. Length approximately 6 mm.

Table III. Materials for filiform corrosion testing No.

Primer

Top coat

1

Epoxy resin (zinc rich)

Polyurethane

2

Epoxy resin (zinc phosphate)

Polyurethane

3

Epoxy resin (micaceous iron oxide)

Polyurethane

4

Epoxy ester resin (zinc rich)

Polyurethane

5

Binder of primer 1 (single layer varnish)

6


7


8




256 In this case, we had to rely exclusively on a laboratory test, because outdoor exposure causes filiform corrosion only at particular sites. It seems that both conditions, industrial and coastal environment, have to meet. This does not apply to our test racks, neither in Stuttgart nor in Gelsenkirchen. In order to be as close as possible to reality, filiform corrosion was not initiated by means of hydrochloric acid fumes, which have been reported to be very effective (12). Instead of this the following simple test procedure was set up: Once a week the X-scribed aluminium panels were immersed in artifical sea water for 5 minutes at ambient temperature. Otherwhise the specimens were stored in a cabinet with 80% relative humidity at 40°C. The pH value of the artifical sea water was adjusted to 4-5 by means of sulfuric acid. During the immersion procedure the panels were visually inspected for signs of filiform corrosion. The intention of this test series was not only to confirm the already known good performance of (1) and (4) but also to examine possible influences of pigmentation and type of binders. Already after 2 weeks the first signs of filiform corrosion became visible. Later, all systems apart from (1) and (4) were gradually attacked by filiform corrosion. After 26 weeks the test was finished. Only systems (1) and (4) and, for comparison, (5) and (8) remained under examination. Photo 2 shows the appearance of systems (4) and (8) after 33 weeks of testing. After such a long time some undercutting and flaking at the scribe of (4) is not significant. However, it is worth noting that only (8) suffered filiform corrosion, though both, (4) as well as (8) are based on the same vehicle. The comparable pair of systems (1) and (5) performed equally. Even after 91 weeks of testing systems (1) and (4) exhibited no signs of filiform corrosion attack. Hence, zinc rich primers, as far as tested, turned out to be indeed resistant to filiform corrosion. This property is strongly related to the zinc dust pigmentation. This finding is in accordance with outdoor exposure recorded in (12) as well as with practical experience gained in the meantime with system (4). From this the following can be deduced: - Even a single laboratory test can yield reliable results, provided it is well adapted to the specific situation.

Conclusions The validity of results achieved by means of accelerated tests is not beyond any doubt, because the correlation with service conditions can be questionable. Despite of this, appropriate testing strategies make accelerated tests a valuable tool.



Photo 2. Appearence of systems (4) and (8) after 33 weeks of filiform corrosion testing. Left: system (4), right: system (8).


258 Acknowledgement The cooperate project referred to in the first example was financially supported by the German Bundesminister fur Forschung und Technologie, DLR Projekttràgerschaft Arbeit und Technik, Kennzeichen 01 HK 199/2.


Literature Cited 1. Cremer, N. D. Pitture Vernic. Eur., Dec. 1992, 68, (12), pp. 27-32, 35-36. 2. Montle, J. F.; Korobov, Y. American Paint & Coatings Journal 76, No. 29, Dec. 23, 1991, pp. 36-44. 3. Boelen, B.; Schmitz, B.; Defourny, J.; Blekkenhorst, F. Corros. Sci., Nov. 1993, 34, (11), pp. 1923-1931. 4. Pawel, S. J. Materials Performance 34, 10 (1955), pp. 37-42. 5. Padinha, Ε. Α.; Ferreira, M . G. S.; Maia, M . A. 8th European Congress of Corrosion, Vol. 2, (Proc. Conf.), Nice, France, 1985, pp. 40.1-40.10. 6. Gieler, R. P. Stahlbau 3/1984, pp. 79-82. 7. Vogt, O. XIXth Patipec Congress, Aachen, Germany, 1988, Vol II, pp. 319-327. 8. Vogt, O. UK Corrosion & Eurocorr 94, Vol I (Proc. Conf.), Bournemouth, England, 1994, pp 20-29. 9. Kobayashi, K.; Shimizu, K.-i.; Tanabe, H.; Masuda, K., Steps into the 90 's, Vol 1 (Proc. Conf.), Queensland, Australia, 1989, pp. 243-249. 10. Leidheiser, H., Jr., Corrosion Nace,Vol.38, No. 7, 1982, pp. 374-383. 11. Hoch, G. M.; Tobias, R. F. Corrosion/71, paper no. 19 (Houston, TX: Nace 1971). 12. Heinrich, M.; Haagen, H.; Schuler, T. farbe + lack 100 (1994), No. 4, pp. 249-252.


Testing of Coating Materials in Industrial Practice - American Chemical

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