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70 Corrosion Control Through Organic Coatings the heavy mineral sands are effective for blast cleaning new steel but are not the best choice for maintenance applications [1]. Olivine ([Mg,Fe] 2[SiO 4]) has a somewhat lower efficiency than silica sand [2] and occasionally leaves white, chalk-like spots on the blasted surface. It leaves a profile of 2.5 mil or finer, which makes it less suitable for applications where profiling the steel surface is important. Staurolite is a heavy mineral sand that has low dust levels and, in many cases, can be recycled three or four times. It has been reported to have good feathering and does not embed in the steel surface. Zircon has higher specific gravity (4.5) than any other abrasive in this class and is very hard (7.5 Mohr). Other good attributes of zircon are its low degree of dusting and its lack of free silica. Its fine size, however, limits its use to specialty applications because it leaves little or no surface profile. Novaculite is a siliceous rock that can be ground up to make an abrasive. It is the softest abrasive discussed in this class (4 Mohr) and is suitable only for specialty work because it leaves a smooth surface. Novaculite is composed mostly of free silica, so this abrasive is not recommended unless adequate precautions to protect the worker from silicosis can be taken. For the same reason, flint, which consists of 90% free silica, is not recommended for maintenance painting. 4.2.3 BY-PRODUCT ABRASIVES By-product abrasives can be used to remove millscale on new constructions or rust and old paint in maintenance jobs. These abrasives are made from the residue, or slag, leftover from smelting metals or burning coal in power plants. Certain melting and boiler slags are glassy, homogeneous mixtures of various oxides with physical properties that make them good abrasives. However, not all industrial slags have the physical properties and nontoxicity needed for abrasives. Boiler (coal), copper, and nickel slags are suitable and dominate this class of abrasives. All three are angular in shape and have a hardness of 7 to 8 Mohr and a specific gravity of 2.7 to 3.3; this combination makes for efficient blast cleaning. In addition, none contain significant (1%) amounts of free silica. Copper slag is a mixture of calcium ferrisilicate and iron orthosilicate. A byproduct of the smelting and quenching processes in copper refining, the low material cost and good cutting ability of copper slag make it one of the most economical, expendable abrasives available. It is used in many industries, including major shipyards, oil and gas companies, steel fabricators, tank builders, pressure vessel fabricators, chemical process industries, and offshore yards. Copper slag is suitable for removing mill scale, rust, and old paint. Its efficiency is comparable to that of silica sand [2]. It has a slight tendency to imbed in mild steel [3]. Boiler slag — also called coal slag — is aluminum silicate. It has a high cutting efficiency and creates a rough surface profile. It too has a slight tendency to imbed in mild steel. Nickel slag, like copper and boiler slag, is hard, sharp, efficient at cutting, and possesses a slight tendency to imbed in mild steel. Nickel slag is sometimes used in wet blasting (see Section 4.3). © 2006 by Taylor & Francis Group, LLC
Blast Cleaning and Other Heavy Surface Pretreatments 71 4.2.3.1 Variations in Composition and Physical Properties It should be noted that, because these abrasives are by-products of other industrial processes, their chemical composition and physical properties can vary widely. As a result, technical data reported can also vary widely for this class of abrasives. For example, Bjorgum has reported that copper slag created more blasting debris than nickel slag in trials done in conjunction with repainting of the Älvsborg bridge in Gothenburg, Sweden [4]. This does not agree with the information reported by Keane [1], which is shown in Table 4.1. This contradiction in results almost certainly depends on differences in the chemical composition, hardness, and particle size of different sources of the same generic type of by-product abrasive. Because of the very wide variations possible in chemical composition of these slags, a cautionary note should perhaps be introduced when labeling these abrasives as nontoxic. Depending on the source, the abrasive could contain small amounts of toxic metals. Chemical analyses of copper slag and nickel slag used for the Älvsborg bridge work have been reported by Bjorgum [4]. Eggen and Steinsmo have also analyzed the composition of various blasting media [5]. The results of both studies are compared in Table 4.2. Comparison of the lead levels in the nickel slags or of the zinc levels in the copper slags clearly indicates that the amounts of an element or compound can vary dramatically between batches and sources. By-product abrasives are usually considered one-time abrasives, although there are indications that at least some of them may be recyclable. In the repainting of the Älvsborg bridge, Bjorgum found that, after one use, 80% of the particles were still larger than 250 µm; and concluded that the abrasive could be used between three and five times [4]. 4.2.4 MANUFACTURED ABRASIVES TABLE 4.1 Physical Data for By-Product Abrasives Abrasive Degree of dusting Reuse Boiler slag High Poor Copper slag Low Good Nickel slag High Poor Modified from: Good Painting Practice, Vol. 1, J.D. Keane, Ed.. Steel Structures Painting Council, Pittsburgh, PA, 1982. The iron and steel abrasives discussed in Section 4.2.1 are of course man-made. In this section, however, we use the term “manufactured abrasives” to mean those produced for specific physical properties, such as toughness, hardness, and shape. The two abrasives discussed here are very heavy, extremely tough, and quite expensive. Their physical properties allow them to cut very hard metals, such as titanium © 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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