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114 Corrosion Control Through Organic Coatings The difference may seem unimportant; however, there are advantages to separating the two questions. Testing a coating for initial corrosion protection is relatively inexpensive and straightforward. The stresses — water, heat, electrolyte — that cause corrosion of the underlying metal are exaggerated and then the metal under the coating is observed for corrosion. However, trying to replicate the aging process of a coating is expensive and difficult for several reasons: 1. Coatings of differing type cannot be expected to have a similar response to an accentuated stress. 2. Scaling down wet-dry cycles changes mass transport phenomena. 3. Climate variability means that the balance of stresses, and subsequent aging, is different from site to site. 7.2 WHAT FACTORS SHOULD BE ACCELERATED? The major weathering stresses that cause degradation of organic coatings are: • UV radiation • Water and moisture • Temperature • Ions (salts such as sodium chloride and calcium chloride) and chemicals The first of these weathering factors is unique to organic coatings; the latter three are also major causes of corrosion of bare metals. Most testing tries to reproduce natural weathering and accelerate it by accentuating these stresses. However, it is critically important to not overaccentuate them. To accelerate corrosion, we scale up temperature, salt loads, and frequency of wet-dry transitions; therefore, we must scale down the duration of each temperature–humidity step. The balances of mass transport phenomena, electrochemical processes, and the like necessarily change with every accentuation of a stress. The more we scale, the more we change the balances of transport and chemical processes from that seen in the field and the farther we step from real service performance. The more we force corrosion in the laboratory, the less able are we to accurately predict field performance. For example, a common method of increasing the rate of corrosion testing is to increase the temperature. For certain coatings, the transport of water and oxygen increase markedly at elevated temperature. Even a relatively small increase in temperature above the service range results in large changes in these coating properties. Such coatings are especially sensitive to artificially elevated temperatures in accelerated testing, which may never be seen in service. Other coatings, however, do not see strongly increased oxygen and water transport at the same elevated temperature. An accelerated test at elevated temperatures of these two coatings may falsely show that one was inferior to the other, when in reality both give excellent service for the intended application. And, of course, interactions between stresses are to be expected. Some major interactions that the coatings tester should be aware of include: • Frequency of temperature/humidity cycling. Because the corrosion reaction depends on supplies of oxygen and water, the accelerated test must © 2006 by Taylor & Francis Group, LLC
Corrosion Testing — Background and Theoretical Considerations 115 correctly mimic the mass transport phenomena that occur in the field. There is a limit to how much we can scale down the duration of a temperature–humidity cycle in order to fit more cycles in a 24-hour period. Beyond that limit, the mass transport occurring in the test no longer mirrors that seen in the field. • Temperature/salt load/relative humidity (RH). The balance of these factors helps to determine the size of the active corrosion cell. If that is not to scale in the accelerated test, the results can diverge greatly from that seen in actual field service. Ström and Ström [1] have described instances of this imbalance in which high salt loads combined with low temperatures led to an off-scale cell. • Type of pollutant/RH. Salts such as sodium chloride (NaCl) and calcium chloride (CaCl 2) are hygroscopic but liquefy at different RHs. NaCl liquefies at 76% RH and CaCl 2 at 35% to 40% RH (depending on temperature). At an intermediate RH, for example 50% RH, the type of salt used can determine whether or not a thin film of moisture forms on the sample surface due to hygroscopic salts. Various polymers, and therefore coating types, react differently to a change in one or more of these weathering stresses. Therefore, in order to predict the service life of a coating in a particular application, it is necessary to know not only the environment — average time of wetness, amounts of airborne contaminants, UV exposure, and so on — but also how these weathering stresses affect the particular polymer [2]. 7.2.1 UV EXPOSURE UV exposure is extremely important in the aging and degradation of organic coatings. As the polymeric backbone of a coating is slowly broken down by UV light, the coating’s barrier properties can be expected to worsen. However, UV exposure’s importance in anticorrosion paints is strictly limited. This is because a coating can be protected from UV exposure simply by painting over it with another paint that does not transmit light. The role of UV exposure in testing anticorrosion paints may be said to be “pass/fail.” Knowing if the anticorrosion paint is sensitive to UV light is important. If it is, then it will be necessary to cover the paint with another coating to protect it from the UV light. This additional coating is routinely done in practice because the most important class of anticorrosion paints, epoxies, are notoriously sensitive to UV stress. It does not prevent epoxies from providing excellent service; rather, it merely protects them from the UV light. Because UV light itself plays no role in the corrosion process, the need for UV stress in an accelerated corrosion test is questionable. 7.2.2 MOISTURE There are as many opinions about the proper amount of moisture to use in accelerated corrosion testing of paints as there are scientists in this field. The reason is almost © 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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