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76 Corrosion Control Through Organic Coatings TABLE 4.4 Chloride Levels after Various Pretreatments Pretreatment Method (see Chapter 5) prior to discharge. Similarly, testing before discharge is needed when using wet blasting or hydrojetting to remove cadmium- or chromium-pigmented coatings. If small quantities of water are used, it may be acceptable to pond the water until the testing can be conducted [13]. 4.4 UNCONVENTIONAL BLASTING METHODS Dry abrasive blasting will not disappear in the foreseeable future. However, other blasting techniques are currently of interest. Some are briefly described in this section: dry blasting with solid carbon dioxide, dry blasting with an ice abrasive, and wet blasting with soda as an abrasive. 4.4.1 CARBON DIOXIDE Average Chloride Level (mg/m 2 ) % Chloride Removal No pretreatment 349 -- Wirebrush to grade SB2 214 39 Needlegun to grade SB2 263 25 UHP hydrojet, 2500 bar, no inhibitor 10 97 Wet blasting with aluminium silicate abrasive, 300 bar, no inhibitor 16 95 Dry grit-blasting to Sa 2 1/2 (copper slag) 56 84 Source: Forsgren, A. and Appelgren, C., Comparison of Chloride Levels Remaining on the Steel Surface after Various Pretreatments, Proc. Pro. Coat. Eur. 2000, Technology Publishing Company, 2000, 271. Rice-sized pellets of carbon dioxide (dry ice) are flung with compressed air against a surface to be cleaned. The abrasive sublimes from solid to gas phase, leaving only paint debris for disposal. This method reportedly produces lower amounts of dust, and thus containment requirements are reduced. Workers are still exposed to any heavy metals that exist in the paint and must be protected against them. Disadvantages of this method are its high equipment costs and slow removal of paint. In addition, large amounts of liquid carbon dioxide (i.e., a tanker truck) are needed. Special equipment is needed both for production of the solid carbon dioxide grains and for blasting. Although carbon dioxide is a greenhouse gas, the total amount of carbon dioxide emissions need not increase if a proper source is used. For example, if carbon dioxide produced by a fossil-fuel-burning power station is used, the amount of carbon dioxide emitted to the atmosphere does not increase. This method can be used to remove paint but is ineffective on mill scale and heavy rust. If the original surface was blast cleaned, the profile is often restored © 2006 by Taylor & Francis Group, LLC
Blast Cleaning and Other Heavy Surface Pretreatments 77 after dry-ice blasting. As Trimber [7] sums up, ‘‘Carbon dioxide blast cleaning is an excellent concept and may represent trends in removal methods of the future.” 4.4.2 ICE PARTICLES Ice is used for cleaning delicate or fragile substrates, for example, painted plastic composites used in aircrafts. Ice particles are nonabrasive; the paint is removed when the ice causes fractures in the coating upon impact. The ice particles’ kinetic energy is transferred to the coating layer and causes conical cracks, more or less perpendicular to the substrate; then lateral and radial cracks develop. When the crack network has developed sufficiently, a bit of coating flakes off. The ice particles then begin cracking the newly exposed paint that was underneath the paint that flaked off. Water from the melted ice rinses the surface free from paint flakes. Foster and Visaisouk [15] have reported that this technique is good for removing contaminants from crevices in the blasted surfaces. Other advantages are [15]: • Ice is nonabrasive and masking of delicate surfaces is frequently unnecessary. • No dust results from breakdown of the blasting media. • Ice melts to water, which is easily separated from paint debris. • Ice can be produced on-site if water and electricity are available. • Escaping ice particles cause much less damage to nearby equipment than abrasive media. Ice-particle blasting has been tested for cleaning of painted compressor and turbine blades on an aircraft motor. The technique successfully removed combustion and corrosion products. The method has also been tested on removal of hydraulic fluid from aircraft paint (polyurethane topcoat) and removal of polyurethane topcoat and epoxy primer from an epoxy graphite composite. 4.4.3 SODA Compressed air or high-pressure water is used to propel abrasive particles of sodium bicarbonate against a surface to be cleaned. Sodium bicarbonate is water-soluble; paint chips and lead can be separated from the water and dissolved sodium bicarbonate, thereby reducing the volume of hazardous waste. The water used with sodium bicarbonate significantly reduces dust. The debris is comprised of paint chips, although it may also be necessary to dispose of the water and dissolved sodium bicarbonate as a hazardous waste unless the lead can be completely removed. The need to capture water can create some difficulties for containment design. This technique is effective at removing paint but cannot remove mill scale and heavy corrosion. In addition, the quality of the cleaning may not be suitable for some paint systems, unless the surface had been previously blast-cleaned. If bare steel is exposed, inhibitors may be necessary to prevent flash rusting. © 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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Page 81 and 82: 4 Blast Cleaning and Other Heavy Su
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