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78 Corrosion Control Through Organic Coatings Most painting contractors are not familiar with this method but, because of similarities to wet abrasive blasting and hydrojetting, they can easily adjust. Because the water mitigates the dust, exposure to airborne lead emissions is significantly reduced but not eliminated; ingestion hazards still exist [15]. 4.5 TESTING FOR CONTAMINANTS AFTER BLASTING Whichever pretreatment method is used, it is necessary before painting to check that the metal surface is free from salts, oils, and dirt. 4.5.1 SOLUBLE SALTS No matter how good a new coating is, applying it over a chloride-contaminated surface is begging for trouble. Chloride contamination can occur from a remarkable number of sources, including road salts if the construction is anywhere near a road or driveway that is salted in the winter. Another major source for constructions in coastal areas is the wind; the tangy, refreshing feel of a sea breeze means repainting often if the construction is not sheltered from the wind. Even the hands of workers preparing the steel for painting contain enough salt to cause blistering after the coating is applied. Rust in old steel can also be a major source of chlorides. The chlorides that originally caused the rust are caught up in the rust matrix; by their very nature, in fact, chlorides exist at the bottom of corrosion pits — the hardest place to reach when cleaning [16,17]. The ideal test of soluble salts is an apparatus that could be used for nondestructing sampling: • On-site rather than in the lab • On all sorts of surfaces (rough, smooth; curved, flat) • Quickly, because time is money • Easily, with results that are not open to misinterpretation • Reliably • Inexpensively Such an instrument does not exist. Although no single method combines all of these attributes, some do make a very good attempt. All rely upon wetting the surface to leach out chlorides and other salts and then measuring the conductivity of the liquid, or its chloride content, afterward. Perhaps the two most-commonly used methods are the Bresle patch and the wetted-filter-paper approach from Elcometer. The Bresle method is described in the international standard ISO 8502-6. A patch with adhesive around the edges is glued onto the test surface. This patch has a known contact area, usually 1250 mm 2 . A known volume of deionized water is injected into the cell. After the water has been in contact with the steel for 10 minutes, it is withdrawn and analyzed for chlorides. There are several choices for analyzing chloride content: titrating on-site with a known test solution; using a conductivity meter; or where facilities permit, using a more sophisticated chloride analyzer. Conductivity meters cannot distinguish between chemical species. If used on heavily © 2006 by Taylor & Francis Group, LLC
Blast Cleaning and Other Heavy Surface Pretreatments 79 rusted steel, the meter cannot distinguish how much of the conductivity is due to chlorides and how much is due simply to ferrous ions in the test water. The Bresle method is robust; it can be used on very uneven or curved surfaces. The technique is easy to perform, and the equipment inexpensive. Its major drawback is the time it requires; 10 minutes for a test is commonly believed to be too long, and there is a strong desire for something as robust and reliable — but faster. The filter-paper technique is much faster. A piece of filter paper is placed on the surface to be tested, and deionized water is squirted on it until it is saturated. The wet paper is then placed on an instrument (such as the SCM-400 from Elcometer) that measures its resistivity. As in the conductivity measurements discussed above, when this is used for repainting applications, it is not certain how much of the resistivity of the paper is due to chlorides and how much is due simply to rust in the test water. In all, the technique is reliable and simple to implement, although initial equipment costs are rather high. Neither technique measures all the chlorides present in steel. The Bresle technique is estimated to have around 50% leaching efficiency; the filter paper technique is somewhat higher. One could argue, however, that absolute values are of very limited use; if chlorides are present in any quantity, they will cause problems for the paint. It does not perhaps matter at all that a measurement technique reports 200 mg/m 2 , when the correct number was 300 mg/m 2 . Both are far too high. This, indeed, is a weakness in the field of pretreatment quality control; it is not known how much chlorides is too much. Although there is some consensus that the acceptable amount is very low, there is no consensus on what the cut-off value is [18–20]. 4.5.2 HYDROCARBONS Like salts, hydrocarbons in the form of oils and grease also come from a variety of sources: diesel fumes, either from passing traffic or stationary equipment motors; lubricating oils from compressors and power tools; grease or oil in contaminated blasting abrasive; oil on operators’ hands; and so on. As mentioned above, the presence of oils and grease on the surface to be painted prevents good adhesion. Testing for hydrocarbons is more complex than is testing for salts for two reasons. First, hydrocarbons are organic, and organic chemistry in general is much more complex than the inorganic chemistry of salts. A simple indicator kit of reagents is quite tricky to develop when organic chemistry is involved. Second, a vast range of hydrocarbons can contaminate a surface, and a test that checks for just a few of them would be fairly useless. What is needed, then, is a test simple enough to be done in the field and powerful enough to detect a broad range of hydrocarbons. Ever game, scientists have developed a number of approaches for testing for hydrocarbons. One approach is ultraviolet (UV) light, or black lights. Most hydrocarbons show up as an unappetizing yellow or green under a UV lamp. This only works, of course, in the dark and, therefore, testing is done under a black hood, rather like turn-of-the-century photography. Drawbacks are that lint and possibly dust show up as hydrocarbon contaminations. In addition, some oils are not detected by black lights [21]. In general, however, this method is easier to use than other methods. © 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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