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68 Corrosion Control Through Organic Coatings 4.1 INTRODUCTION TO BLAST CLEANING By far, the most common pretreatment for steel constructions prior to painting is blast cleaning, in which the work surface is bombarded repeatedly with small solid particles. If the individual abrasive particle transfers sufficient kinetic energy to the surface of the steel, it can remove mill scale, rust, clean steel, or old paint. The kinetic energy (E) of the abrasive particle before impact is defined by its mass (M) and velocity (V), as given in the familiar equation: E = (MV 2 )/2 Upon impact, this kinetic energy can be used to shatter or deform the abrasive particle, crack or deform old paint, or chip away rust. The behavior of the abrasive, as that of the old coating, depends in part on whether it favors plastic or elastic deformation. In general, the amount of kinetic energy transferred, and whether it will suffice to remove rust, old paint, and so forth, depends on a combination of: • Velocity and mass of the propelled abrasive particle • Impact area • Strength and hardness of the substrate being cleaned • Strength and hardness of the abrasive particle In the most-commonly used blasting technique — dry abrasive blasting — velocity of the blasting particles is controlled by the pressure of compressed air. It is more or less a constant for any given dry blasting equipment; the mass of the abrasive particle therefore determines its impact on the steel surface. In wet abrasive blasting, in which water replaces compressed air as the propellant of the solid blasting media, velocity of the particles is governed by water pressure. In hydrojet blasting, the water itself is both the propellant and the abrasive (no solid abrasive is used). Both forms of wet blasting offer the possibility to vary the velocity by changing water pressure. It should be noted however that wet abrasive blasting is necessarily performed at much lower pressures and, therefore, velocities, than hydrojet blasting. 4.2 DRY ABRASIVE BLASTING Only heavy abrasives can be used in preparing steel surfaces for painting. Lighter abrasive media, such as apricot kernels, plastic particles, glass beads or particles, and walnut shells, are unsuitable for heavy steel constructions. Because of their low densities, they cannot provide the amounts of kinetic energy that must be expended upon the steel’s surface to perform useful work. In order to be commercially feasible, an abrasive should be: • Heavy, so that it can bring significant amounts of kinetic energy to the substrate • Hard, so that it doesn’t shatter into dust or deform plastically (thus wasting the kinetic energy) upon impact © 2006 by Taylor & Francis Group, LLC
Blast Cleaning and Other Heavy Surface Pretreatments 69 • Inexpensive • Available in large quantities • Nontoxic 4.2.1 METALLIC ABRASIVES Steel is used as abrasive in two forms: • Cast as round beads, or shot • Crushed and tempered to the desired hardness to form angular steel grit Scrap or low-quality steel is usually used, often with various additives to ensure consistent quality. Both shot and grit have good efficiency and low breakdown rates. Steel shot and grit are used for the removal of mill scale, rust, and old paint. This abrasive can be manufactured to specification and offers uniform particle size and hardness. Steel grit and shot can be recycled 100 to 200 times. Because they generate very little dust, visibility during blasting is superior to that of most other abrasives. Chilled iron shot or grit can be used for the removal of rust, mill scale, heat treatment scale, and old paint from forged, cast, and rolled steel. This abrasive breaks down gradually against steel substrates, so continual sieving to retain only the large particle sizes may be needed if a rough surface profile is desired in the cleaned surface. 4.2.2 NATURALLY OCCURRING ABRASIVES Several naturally occurring nonmetallic abrasives are commercially available, including garnet, zircon, novaculite, flint, and the heavy mineral sands magnetite, staurolite, and olivine. However, not all of these abrasives can be used to prepare steel for maintenance coatings. For example, novaculite and flint contain high amounts of free silica, which makes them unsuitable for most blasting applications. Garnet is a tough, angular blasting medium. It is found in rock deposits in Eastern Europe, Australia, and North America. With a hardness of 7 to 8 Mohr, it is the hardest of the naturally occurring abrasives and, with a specific gravity of 4.1, it is denser than all others in this class except zircon. It has very low particle breakdown on impact, thereby enabling the abrasive to be recycled several times. Among other advantages this confers, the amount of spent abrasive is minimized — an important consideration when blasting old lead- or cadmium-containing paints. The relatively high cost of garnet limits its use to applications where abrasive can be gathered for recycling. However, for applications where spent abrasive must be treated as hazardous waste, the initial higher cost of garnet is more than paid for by the savings in disposal of spent abrasive. Nonsilica mineral sands, such as magnetite, staurolite, and olivine, are tough (5 to 7 Mohr) and fairly dense (2.0 to 3.0 specific gravity) but are generally of finer particle size than silica sand. These heavy mineral sands — as opposed to silica sand — do not contain free silicates, the cause of the disease silicosis. In general, © 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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