© 2006 by Taylor & Francis Group, LLC

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8 Corrosion Testing — Practice Corrosion tests for organic coatings can be divided into two categories: 1. Test regimes that age the coating. These are the accelerated test methods, including single stress tests, such as the salt spray, or cyclic tests such as the American Society for Testing and Materials (ASTM) D5894. 2. Measurements of coating properties before and after aging. These tests measure such characteristics as adhesion, gloss, and barrier properties (water uptake). The aim of the accelerated test regime is to age the coating in a short time in the same manner as would occur over several years’ field service. These tests can provide direct evidence of coating failure, including creep from scribe, blistering, and rust intensity. They also are a necessary tool for the measurement of coating properties that can show indirect evidence of coating failure. A substantial decrease in adhesion or significantly increased water uptake, even in the absence of rust-through or undercutting, is an indication of imminent coating failure. This chapter provides information about: • Which accelerated tests age coatings • What to look for after an accelerated test regime is completed • How the amount of acceleration in a test is calculated, and how the test is correlated to field data • Why the salt spray test should not be used 8.1 SOME RECOMMENDED ACCELERATED AGING METHODS Hundreds of test methods are used to accelerate the aging of coatings. Several of them are widely used, such as salt spray and ultraviolet (UV) weathering. A review of all the corrosion tests used for paints, or even the major cyclic tests, is beyond the scope of this chapter. It is also unnecessary because this work has been presented elsewhere; the reviews of Goldie [1], Appleman [2], and Skerry and colleagues [3] are particularly helpful. The aim of this section is to provide the reader with an overview of a select group of accelerated aging methods that can be used to meet most needs: • General corrosion tests — all-purpose tests • Condensation or humidity tests • Weathering tests (UV exposure) © 2006 by Taylor & Francis Group, LLC 129
130 Corrosion Control Through Organic Coatings In addition, some of the tests used in the automotive industry are described. These are tests with proven correlation to field service for car and truck paints, which may, with adaptations, prove useful in heavier protective coatings. 8.1.1 GENERAL CORROSION TESTS A general accelerated test useful in predicting performance for all types of coatings, in all types of service applications, is the ‘‘Holy Grail” of coatings testing. No test is there yet, and none probably ever will be (see Chapter 7). However, some general corrosion tests can still be used to derive useful data about coating performance. The two allpurpose tests recommended here are the ASTM D5894 test and the NORSOK test. 8.1.1.1 ASTM D5894 ASTM D5894, “Standard Practice for Cyclic Salt Fog/UV Exposure of Painted Metal (Alternating Exposures in a Fog/Dry Cabinet and a UV/Condensation Cabinet),” is also called “modified Prohesion” or “Prohesion UV.” This test, incidentally, is sometimes mistakenly referred to as ‘‘Prohesion testing.” However, the Prohesion test does not include a UV stress; it is simply a cyclic salt fog (1 hour salt spray, with 0.35% ammonium sulphate and 0.05% sodium chloride [NaCl], at 23°C, alternating with one drying cycle at 35°C). The confusion no doubt arises because the original developers of ASTM D5894 referred to it as ‘‘modified Prohesion.” This test is can be used to investigate both anticorrosion and weathering characteristics. The test’s cycle is 2 weeks long and typically runs for 6 cycles (i.e., 12 weeks total). During the first week of each cycle, samples are in a UV/condensation chamber for 4 hours of UV light at 60°C, alternating with 4 hours of condensation at 50°C. During the second week of the cycle, samples are moved to a salt-spray chamber, where they undergo 1 hour of salt spray (0.05% NaCl + 0.35% ammonium sulphate, pH 5.0 to 5.4) at 24°C, alternating with 1 hour of drying at 35°C. The literature contains warnings about too-rapid corrosion of zinc in this test; therefore, it should not be used for comparing zinc and nonzinc coatings. If zinc and nonzinc coatings must be compared, an alternate (i.e., nonsulphate) electrolyte can be substituted under the guidelines of the standard. This avoids the problems caused by the solubility of zinc sulphate corrosion products. It has also been noted that the ammonium sulphate in the ASTM D5894 electrolyte has a pH of approximately 5; at this pH, zinc reacts at a significantly higher rate than at neutral pH levels. The zinc is unable to form the zinc oxide and carbonates that give it long-term protection. 8.1.1.2 NORSOK NORSOK is suitable for both corrosion and weathering testing. Its cycle is 168 hours long, and it runs for 25 cycles (i.e., 25 weeks total). Each cycle consists of 72 hours of salt spray, followed by 16 hours drying in air, and then 80 hours of UV condensation (ASTM G53). The NORSOK test was developed for the offshore oil industry, particularly the conditions found in the North Sea. The test is part of the NORSOK M-501 standard, © 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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