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124 Corrosion Control Through Organic Coatings 7.3.1 DIFFERENT SITES INDUCE DIFFERENT AGING MECHANISMS Sites can differ dramatically in weather. Take, for example, a bridge connecting Prince Edward Island to the Canadian mainland and a bridge connecting the island of Öland to the Swedish mainland. At first glance, one could say that these two sites are roughly comparable. Both are bridges standing in the sea, located closer to the North Pole than to the equator. Yet, these two sites induce different stresses in paints. A coating used on the first bridge would undergo much higher mechanical stress, due to heavy floes of sea ice. It would also see much higher salt loads because the Atlantic Ocean has a higher salt concentration than does the Baltic Sea. If these two sites, which at first glance seemed similar, can induce some differences in aging mechanisms, then the difference must be even more drastic between such coastal sites as Sydney, Vladivostok, and Rotterdam or between inland sites such as Aix-en-Provence, Brasilia, and Cincinnati. The point is not academic; it is crucially important for choosing accelerated tests. A mechanically tough coating that is not particularly susceptible to salt would perform well at both sites, but an equally mechanically tough coating that allows some slight chloride permeation may fail at Prince Edward Island and succeed at Öland. A study of coated panels exposed throughout pulp and paper mills in Sweden by Rendahl and Forsgren [31] illustrates the classic problem of using accelerated tests to predict coating performance: the ranking of identical samples can change from site to site. In this study, 23 coating-substrate combinations were exposed at 12 sites in two pulp and paper mills for 5 years. The sites with the most corrosion were the roofs of a digester house and a bleach plant at the sulphate mill. Although these two locations had similar characteristics — same temperature, humidity, and UV exposure — they produced different rankings of coated samples. Both locations agree on the worst sample, but little else. An alkyd paint that gave good results on the bleach plant roof had abysmally poor results on the other roof. Conversely, an acrylic that had significant undercutting on the bleach plant roof performed well on the digester house roof. These results illustrate why there is no “magic bullet”: an accelerated test that correctly predicts the ranking of the 23 samples at the digester house roof may be wildly wrong in predicting the ranking of the same samples at the bleach plant roof of the same mill. Glueckert [32] has reported the same phenomenon based on a study of gloss loss of six coating systems exposed at both Colton, California, and East Chicago, Indiana. The East Chicago location had an inland climate, with a temperature range of −23°C to 38°C. The Colton site had higher temperature, more intense sunlight, and blowing sand. The loss of gloss and ranking of the six coatings is shown in Table 7.2. The two sites identified the same best and worst coating, but ranked the four in between differently. Another study of coatings exposed at various field stations throughout Sweden [2] found no correlations between sites in the corrosion performances of the identical samples, either in the amount of corrosion or in the ranking at each site. In this study, identifying a coating as “always best” or “always worst” was not possible. © 2006 by Taylor & Francis Group, LLC
Corrosion Testing — Background and Theoretical Considerations 125 TABLE 7.2 Exposure Results from Colton, California, and East Chicago, Indiana Coating Gloss loss (%) E. Chicago Gloss loss (%) Colton Even if only one coating and one substrate were to be tested, it would not be possible to design an accelerated test that would perfectly suit all the exposure sites mentioned in this section — much less all the sites in the world. 7.3.2 DIFFERENT COATINGS HAVE DIFFERENT WEAKNESSES Ranking, E. Chicago Ranking, Colton Epoxy-urethane 3 0 1 1 Urethane 38 31 2 3 Waterborne alkyd 56 6 3 2 Epoxy B 65 83 4 5 Acrylic alkyd 68 77 5 4 Epoxy A 98 98 6 6 Data from: Glueckert, A.J., Correlation of accelerated test to outdoor exposure for railcar exterior coatings, in Proc. Corros. 94, NACE, Houston, 1994, Paper 596. Cured coatings are commonly thought of as simple structures: the usual depiction is a layer of binder containing pigment particles. The general view is that of a homogenous, continuous, solid binder film reinforced with pigment particles. In reality, the cured coating is a much more complex structure. For one thing, instead of being a solid, it contains lots of empty space: pinholes, voids after crosslinking, gaps between pigment and binder, and so on. All of these voids are potential routes for water molecules to slip through the cured film. What is important for accelerated testing is that the amount of empty space in the coating is not constant — it can change during weathering, as both the binder and the pigment change. Some pigments, such as passivating pigments, are slowly consumed, causing the empty space between pigment and binder to increase. Other pigments immediately corrode on their surface. The increased volume of the corrosion products can decrease the empty space between particles and binder. Binders also change with time, for many reasons. The stresses in the binder caused by film formation can be increased, or relieved, during aging. The magnitude of the stresses caused by film formation, and what happens to these stresses upon weathering, depends to a large extent on the type of polymer used for the binder. The same could be said for UV degradation, or any stress that ages binders: the binder’s reaction, both in mechanism and in magnitude, depends to a large extent on the specific polymer used. Even if only one exposure site were really to be used, it would not be possible to design an accelerated test that would be suitable for all binders and pigments. © 2006 by Taylor & Francis Group, LLC
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© 2006 by Taylor & Francis Group,
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Corrosion of Ceramic and Composite
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Published in 2006 by CRC Press Tayl
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Preface This book has been written
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Author Amy Forsgren received her ch
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Contents Chapter 1 Introduction ...
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2.4.2 Reactive Reagents ...........
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6.2.4.1 Alkaline Blistering........
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1 Introduction This book is not abo
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Introduction 3 • A dissolution pr
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Introduction 5 1.2.2 ELECTROLYTIC R
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Introduction 7 to the metal is a ne
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Introduction 9 9. Thomas. N.L., Pro
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12 Corrosion Control Through Organi
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4 Blast Cleaning and Other Heavy Su
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© 2006 by Taylor & Francis Group, LLC

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