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Corrosion Testing — Practice 131 which provides requirements for materials selection, surface preparation, paint application, inspection, and so on for coatings used on offshore platforms. 8.1.2 CONDENSATION OR HUMIDITY Many tests are based on constant condensation or humidity. Incidentally, constant condensation is not the same as humidity testing. Condensation rates are higher in the former than the latter because, in constant condensation chambers, the back sides of the panels are at room temperature and the painted side faces water vapor at 40°C. This slight temperature differential leads to higher water condensation on the panel. If no such temperature differential exists, the conditions provide humidity testing in what is known as a ‘‘tropical chamber.” The Cleveland chamber is one example of condensation testing; a salt spray chamber with the salt fog turned off, the heater turned on, and water in the bottom (to generate vapor) is a humidity test. Constant condensation or humidity testing can be useful as a test for barrier properties of coatings on less-than-ideal substrates — for example, rusted steel. Any hygroscopic contaminants, such as salts entrapped in the rust, attract water. On new construction, or in the repainting of old construction, where it is possible to blast the steel to Sa2 1 / 2, these contaminants are not be found. However, for many applications, dry abrasive or wet blasting is not possible, and only handheld tools such as wire brushes can be used. These tools remove loose rust but leave tightly adhering rust in place. And, because corrosion-causing ions, such as chloride (Cl − ), are always at the bottom of corrosion pits, the matrix of tightly adhering rust necessarily contains these hygroscopic contaminants. In such cases, the coating must prevent water from reaching the intact steel. The speed with which blisters develop under the coating in condensation conditions can be an indication of the coating’s ability to provide a water barrier and thus protect the steel. Various standard test methods using constant condensation or humidity testing include the International Organization for Standardizaton (ISO) 6270, ISO 11503, the British BS 3900, the North American ASTM D2247, ASTM D4585, and the German DIN 50017. 8.1.3 WEATHERING In UV weathering tests, condensation is alternated with UV exposure to study the effect of UV light on organic coatings. The temperature, amount of UV radiation, length (time) of UV radiation, and length (time) of condensation in the chamber are programmable. Examples of UV weathering tests include QUV-A, QUV-B (® Q-Panel Co.), and Xenon tests. Recommended practices for UV weathering are described in the very useful standard ASTM G154 (which replaces the better-known ASTM G53). 8.1.4 CORROSION TESTS FROM THE AUTOMOTIVE INDUSTRY The automotive industry places great demands on its anticorrosion coatings system and has therefore invested a good deal of effort in developing accelerated tests to help predict the performance of paints in harsh conditions. It should be noted that © 2006 by Taylor & Francis Group, LLC
132 Corrosion Control Through Organic Coatings most automotive tests, including the cyclic corrosion tests, have been developed using coatings relevant to automotive application. These are designed to act quite different from protective coatings. Automotive-derived test methods commonly overlook factors critical to protective coatings, such as weathering and UV factors. In addition, automotive coatings have much lower dry film thickness than do many protective coatings; this is important for mass-transport phenomena. This section is not intended as an overview of automotive industry tests. Some tests that have good correlation to actual field service for cars and trucks, such as the Ford APGE, Nissan CCT-IV, and GM 9540P [4], are not described here. The three tests described here are those believed to be adaptable to heavy maintenance coatings VDA 621-415, the Volvo Indoor Corrosion Test (VICT), and the Society of Automotive Engineers (SAE) J2334. 8.1.4.1 VDA 621-415 For many years, the automotive industry in Germany has used an accelerated test method for organic coatings called the VDA 621-415 [5]; this test has begun to be used as a test for heavy infrastructure paints also. The test consists of 6 to 12 cycles of neutral salt spray (as per DIN 50021) and 4 cycles in an alternating condensation water climate (as per DIN 50017). The time-of-wetness of the test is very high, which implies poor correlation to actual service for zinc pigments or galvanized steel. It is expected that zinc will undergo a completely different corrosion mechanism in the nearly constant wetness of the test than the mechanism that takes place in actual field service. The ability of the test to predict the actual performance of zinc-coated substrates and zinc-containing paints must be carefully examined because these materials are commonly used in the corrosion engineering field. Also, the start of the test (24 hours of 40°C salt spray) has been criticized as unrealistically harsh for latex coatings. 8.1.4.2 Volvo Indoor Corrosion Test or Volvo-cycle The VICT [6] was developed — despite its name — to simulate the outdoor corrosion environment of a typical automobile. Unlike many accelerated corrosion tests, in which the test procedure is developed empirically, the VICT test is the result of a statistical factorial design [7, 8]. In modern automotive painting, all of the corrosion protection is provided by the inorganic layers and the thin (circa 25 µm) electrocoat paint layer. Protection against UV light and mechanical damage is provided by the subsequent paint layers (of which there are usually three). Testing of the anticorrosion or electrocoat paint layer can be restricted to a few parameters, such as corrosion-initiating ions (usually chlorides), time-of-wetness, and temperature. The Volvo test accordingly uses no UV exposure or mechanical stresses; the stresses used are temperature, humidity, and salt solution (sprayed or dipped). The automotive industry has a huge amount of data for corrosion in various service environments. The VICT has a promising correlation to field data; one criticism that is sometimes brought against this test is that it may tend to produce filiform corrosion at a scribe. © 2006 by Taylor & Francis Group, LLC
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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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