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120 Corrosion Control Through Organic Coatings It should be noted that the 90 g/m 2 zinc coating in this study is hot-dipped galvanized, and the two thinner coatings are electrogalvanized. It may be that differences other than zinc thickness — for example, structure and morphology of the zinc coating — play a not yet understood role. Further research is needed in this area, to understand the role played by zinc layer structure and morphology in under–cutting. 7.2.3.3 Differences in Absorption and Desorption Rates The rate at which a coating absorbs water is not necessarily the same as the rate at which it dries out. Some coatings have nearly the same absorption and desorption rates, whereas others show slower drying than wetting, or vice versa. In constant stress testing, in which samples are always wet or always dry, this difference does not become a factor. However, as soon as wet-dry cycles are introduced, the implications of a difference between absorption and desorption rates becomes highly important. Two coatings with roughly similar absorption rates can have vastly different desorption rates. The duration of wet and dry periods in modern accelerated tests is measured in hours, not days, and it is quite possible that, for a coating with a slower desorption rate, the drying time in each cycle is shorter than the time needed by the coating for complete desorption. In such cases, the coating that desorbs more slowly than it absorbs can accumulate water. The problem is not academic. Lindqvist [22] has studied absorption and desorption rates for epoxy, chlorinated rubber, linseed oil, and alkyd binders, using a cycle of 6 hours of wet followed by 6 hours of drying. An epoxy coating took up 100% of its possible water content in the wet periods but never dried out in the drying periods. Conversely, a linseed oil coating in this study never reached its full saturation during the 6-hour wet periods but dried out completely during the drying periods. Lindqvist has pointed out that the difference in the absorption and desorption rates of a single paint, or of different types of paint, could go far in explaining why cyclic accelerated tests often do not produce the same ranking of coatings as does field exposure. There is a certain risk to subjecting different types of coatings with unknown absorption and desorption characteristics to a cyclic wet-dry accelerated regime. The risk is that the accelerated test will produce a different ranking from that seen in reality. It could perhaps be reduced by some preliminary measurements of water uptake and desorption; an accelerated test can then be chosen with both wet times and drying times long enough to let all the paints completely absorb and desorb. 7.2.4 TEMPERATURE Temperature is a crucial variable in any accelerated corrosion testing. Higher temperature means more energy available, and thus faster rates, for the chemical processes that cause both corrosion and degradation of cured films. Increasing the temperature — within limits — does not alter the corrosion reaction at the metal surface; it merely speeds it up. A potential problem, however, is what the higher © 2006 by Taylor & Francis Group, LLC
Corrosion Testing — Background and Theoretical Considerations 121 temperature does to the binder. If the chemical processes that cause aging of the binder were simply speeded up without being altered, elevated temperature would pose no problem. But this is not always the case. Every coating is formulated to maintain a stable film over a certain temperature range. If that range is exceeded, the coating can undergo transformations that would not occur under natural conditions [3]. The glass transition temperature (T g) of the polymer naturally limits the amount of acceleration that can be forced by increasing heat stress. Testing in the vicinity of the T g changes the properties of the coatings too much, so that the paint being tested is not very much like the paint that will be used in the field — even if it came from the same can of paint. 7.2.5 CHEMICAL STRESS When the term “chemical stress” is used in accelerated testing, it usually means chloride-containing salts in solution, because airborne contaminants are believed to play a very minor role in paint aging. See Chapter 6 for information about air bourne contaminents. Testers may be tempted to force quicker corrosion testing by increasing the amount of chemical stress. Steel that corrodes in a 0.05% sodium chloride (NaCl) solution will corrode even more quickly in 5% NaCl solution; the same is true for zinc-coated steel. The problem is that the amount of acceleration is different for the two metals. An increase in NaCl content has a much more marked effect for zinccoated substrates than for carbon steel substrates. Ström and Ström [1] have demonstrated this effect in a test of weakly accelerated outdoor exposure of painted zinc-coated and carbon steel samples. In this weakly accelerated test, commonly known as the “Volvo Scab” test, samples are exposed outdoors and sprayed twice a week with a salt solution. Table 7.1 gives the results after 1 year of this test, using different levels of NaCl for the twice-weekly spray. TABLE 7.1 Average Creep from Scribe after 1 Year Weakly Accelerated Field Exposure Material Mean for all electrogalvanized and hot-dipped galvanized painted samples Mean for all cold-rolled steel painted samples Outdoor samples sprayed twice per week with: 0.5% NaCl 1.5% NaCl 5% NaCl 1.3 mm 2.0 mm 3.1 mm 6.2 mm 8.2 mm 9.6 mm Modified from: Ström, M. and Ström, G., SAE Technical Paper Series, 932338, Society of Automotive Engineers, Warrendale, Pennsylvania, 1993. © 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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