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72 Corrosion Control Through Organic Coatings TABLE 4.2 Levels of Selected Compounds/Elements Found in By-Product Abrasives Blasting media Pb Co Cu Cr Ni Zn Copper slag [Eggen and Steinsmo] 0.24% 0.07% 0.14% 0.05% 71 ppm 5.50% Copper slag [Bjorgum] 203 ppm 249 ppm 5.6 ppm 1.4 ppm 129 ppm 10 ppm Nickel slag [Eggen and Steinsmo] 73 ppm 0.43% 0.28% 0.14% 0.24% 0.38% Nickel slag [Bjorgum] 1.2 ppm 2.3 ppm 4.5 ppm 755 ppm 1.1 ppm 15.6 ppm Sources: Bjorgum, A., Behandling av avfall fra bläserensing, del 3. Oppsummering av utredninger vedrorende behandling av avfall fra blåserensing, Report No. STF24 A95326, Foundation for Scientific and Industrial Research at the Norwegian Institute of Technology (SINTEF), Trondheim, 1995 (in Norwegian); Eggen, T. and Steinsmo, U., Karakterisering av flater blast med ulike blåsemidler, Report No. STF24 A94628, SINTEF, Trondheim, 1994 (in Norwegian). and stainless steel, and to be recycled many times before significant particle breakdown occurs. Manufactured abrasives are more costly than by-product slags, usually by an order of magnitude. However, the good mechanical properties of most manufactured abrasives make them particularly adaptable for recycling as many as 20 times. In closed-blasting applications where recycling is designed into the system, these abrasives are economically attractive. Another important use for them is in removing old paints containing lead, cadmium, or chromium. When spent abrasive is contaminated with these hazardous substances, the abrasive might need to be treated and disposed of as a hazardous material. If disposal costs are high, an abrasive that generates a low volume of waste — due to repeated recycling — gains in interest. Silicon carbide, or carborundum, is a dense and extremely hard angular abrasive (specific gravity 3.2, 9 Mohr). It cleans extremely fast and generates a rough surface profile. This abrasive is used for cleaning very hard surfaces. Despite its name, it does not contain free silica. Aluminium oxide is a very dense and extremely hard angular abrasive (specific gravity 4.0, 8.5 to 9 Mohr). It provides fast cutting and a good surface profile so that paint can anchor onto steel. This abrasive generates low amounts of dust and can be recycled, which is necessary because it is quite expensive. Aluminium oxide does not contain free silica. 4.3 WET ABRASIVE BLASTING AND HYDROJETTING In dry abrasive blasting, a solid abrasive is entrained in a stream of compressed air. In wet abrasive blasting, water is added to the solid abrasive medium. Another approach is to keep the water but remove the abrasive; this is called hydrojetting, or water jetting. This pretreatment method depends entirely on water impacting a steel surface at a high enough speed to remove old coatings, rust, and impurities. © 2006 by Taylor & Francis Group, LLC
Blast Cleaning and Other Heavy Surface Pretreatments 73 The presence of an abrasive medium in the dry or wet pretreatment methods results in a surface with a desirable profile. Hydrojetting, on the other hand, does not increase the surface roughness of the steel. This means that hydrojetting is not suitable for new constructions because the steel will never receive the surface roughness necessary to provide good anchoring of the paint. For repainting or maintenance painting, however, hydrojetting may be used to strip away paint, rust, and so forth and restore the original surface profile of the steel. Paul [6] mentions that because dust generation is greatly reduced in wet blasting, this method makes feasible the use of some abrasives that would otherwise be health hazards. This should not be taken as an argument to use health-hazardous abrasives, however, because more user-friendly abrasives are available in the market. 4.3.1 TERMINOLOGY The terminology of wet blasting is confusing, to say the least. The following useful definitions are found in the Industrial Lead Paint Removal Handbook [7]: • Wet abrasive blast cleaning: Compressed air propels abrasive against the surface. Water is injected into the abrasive stream either before or after the abrasive exits the nozzle. The abrasive, paint debris, and water are collected for disposal. • High-pressure water jetting: Pressurized water (up to 20,000 psi) is directed against the surface to remove the paint. Abrasives are not used. • High-pressure water jetting with abrasive injection: Pressurized water (up to 20,000 psi) is directed against the surface to be cleaned. Abrasive is metered into the water stream to facilitate the removal of rust and mill scale and to improve the efficiency of paint removal. Disposable abrasives are used. • Ultra-high-pressure water jetting: Pressurized water (20,000–40,000 psi; can be higher) is directed against the surface to remove the paint. Abrasives are not used. • Ultra-high-pressure water jetting with abrasive injection: Pressurized water (20,000–40,000; can be greater) is directed against the surface to be cleaned. Abrasive is metered into the water stream to facilitate the removal of rust and mill scale and to improve the efficiency of paint removal. Disposable abrasives are used. 4.3.2 INHIBITORS An important question in the area of wet blasting is does the flash rust, which can appear on wet-blasted surfaces, have any long-term consequences for the service life of the subsequent painting? A possible preventative for flash rust is adding a corrosion inhibitor to the water. The literature on rust inhibitors is mixed. Some sources view them as quite effective against corrosion, although they also have some undesirable effects when properly used. Others, however, view rust inhibitors as a definite disadvantage. Which chemicals are suitable inhibitors is also an area of much discussion. © 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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