blast cleaning technology - download.e-bookshelf.de€¦ · (1939), in his book impact cleaning,...

Blast Cleaning Technology

A. Momber

Blast Cleaning Technology

With 385 Figures and 169 Tables

Andreas MomberPrivatdozent Dr.-Ing. habil.HamburgGermany

ISBN: 978-3-540-73644-8 e-ISBN: 978-3-540-73645-5

Library of Congress Control Number: 2007931617

c© 2008 Springer-Verlag Berlin Heidelberg

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Contents

1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.2 Introductory Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11.3 Blast Cleaning Methods and Applications . . . . . . . . . . . . . . . . . . . . . . . . 3

2 Abrasive Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72.1 Classification and Properties of Abrasive Materials . . . . . . . . . . . . . . . . 72.2 Abrasive Material Structure and Hardness . . . . . . . . . . . . . . . . . . . . . . . . 7

2.2.1 Structural Aspects of Abrasive Materials . . . . . . . . . . . . . . . . . . 72.2.2 Hardness of Abrasive Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 13

2.3 Abrasive Particle Shape Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.1 Basic Shape Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.2 Relative Proportions of Particles . . . . . . . . . . . . . . . . . . . . . . . . . 172.3.3 Geometrical Forms of Particles . . . . . . . . . . . . . . . . . . . . . . . . . . 18

2.4 Abrasive Particle Size Distribution and Abrasive Particle Diameter . . . 242.4.1 Particle Size Distribution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

2.4.1.1 General Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.4.1.2 Sieve Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242.4.1.3 Particle Size Distribution Models . . . . . . . . . . . . . . . . 25

2.4.2 Particle Diameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272.4.3 Alternative Abrasive Particle Size Assessment Methods . . . . . . 27

2.5 Density of Abrasive Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302.6 Number and Kinetic Energy of Abrasive Particles . . . . . . . . . . . . . . . . . 30

2.6.1 Abrasive Particle Number and Frequency . . . . . . . . . . . . . . . . . . 302.6.2 Kinetic Energy of Abrasive Particles . . . . . . . . . . . . . . . . . . . . . . 312.6.3 Power Availability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32

2.7 Impurities . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 342.8 Global Abrasive Evaluation Parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . 362.9 Process Behaviour of Abrasive Particles . . . . . . . . . . . . . . . . . . . . . . . . . 37

2.9.1 Fracture Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 372.9.2 Fracture Probability of Abrasive Particles . . . . . . . . . . . . . . . . . . 412.9.3 Effects of Abrasive Material Structure . . . . . . . . . . . . . . . . . . . . 452.9.4 Debris Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

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2.9.5 Disintegration Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 472.9.6 Particle Shape Modification During Abrasive Fragmentation . . 492.9.7 Energy Absorption During Abrasive Fragmentation . . . . . . . . . 512.9.8 Chemical Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53

3 Air and Abrasive Acceleration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.1 Properties of Compressed Air . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 553.2 Air Flow in Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

3.2.1 Air Mass Flow Rate Through Nozzles . . . . . . . . . . . . . . . . . . . . 593.2.2 Volumetric Air Flow Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 613.2.3 Air Exit Flow Velocity in Nozzles . . . . . . . . . . . . . . . . . . . . . . . . 663.2.4 Air Flow in Laval Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 683.2.5 Power, Impulse Flow and Temperature . . . . . . . . . . . . . . . . . . . . 70

3.3 Abrasive Particle Acceleration in Nozzles . . . . . . . . . . . . . . . . . . . . . . . . 723.3.1 General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 723.3.2 Simplified Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753.3.3 Abrasive Flux Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 753.3.4 Abrasive Particle Spacing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76

3.4 Jet Structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 773.4.1 Structure of High-speed Air Jets . . . . . . . . . . . . . . . . . . . . . . . . . 773.4.2 Structure of Air-particle Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 783.4.3 Design Nozzle Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80

3.5 Composition of Particle Jets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 853.5.1 Radial Abrasive Particle Distribution . . . . . . . . . . . . . . . . . . . . . 853.5.2 Particle Velocity Distribution Function . . . . . . . . . . . . . . . . . . . . 853.5.3 Radial Abrasive Particle Velocity Distribution . . . . . . . . . . . . . . 893.5.4 Area Coverage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 903.5.5 Stream Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90

3.6 Parameter Effects on Abrasive Particle Velocity . . . . . . . . . . . . . . . . . . . 943.6.1 Effects of Air Pressure on Particle Velocity . . . . . . . . . . . . . . . . 943.6.2 Effects of Abrasive Mass Flow Rate on Particle Velocity . . . . . 943.6.3 Effects of Abrasive Particle Size on Particle Velocity . . . . . . . . 973.6.4 Effects of Abrasive Particle Shape on Particle Velocity . . . . . . . 1003.6.5 Effects of Abrasive Material Density on Particle Velocity . . . . 1003.6.6 Effects of Stand-off Distance on Particle Velocity . . . . . . . . . . . 1013.6.7 Effects of Nozzle Length and Nozzle Diameter

on Particle Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033.6.8 Effects of Nozzle Design on Particle Velocity . . . . . . . . . . . . . . 1033.6.9 Effects of Nozzle Wall Roughness on Particle Velocity . . . . . . 1043.6.10 Scaling Laws for Abrasive Particle Velocity . . . . . . . . . . . . . . . . 105

3.7 Abrasive Stream Energy Flow and Nozzle Efficiency . . . . . . . . . . . . . . 107

4 Blast Cleaning Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.1 General Structure of Blast Cleaning Systems . . . . . . . . . . . . . . . . . . . . . 1094.2 Air Compressors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

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4.2.1 General Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1094.2.2 Working Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1114.2.3 Power Rating . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1124.2.4 Economic Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1144.2.5 Aspects of Air Quality . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116

4.3 Blast Machine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.3.1 Basic Parts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1184.3.2 Abrasive Metering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

4.3.2.1 Effects of Process Parameters . . . . . . . . . . . . . . . . . . . 1194.3.2.2 Metering Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1244.3.2.3 Abrasive Mass Flow Adjustment . . . . . . . . . . . . . . . . 127

4.4 Pressure Air Hose Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1284.4.1 Materials and Technical Parameters . . . . . . . . . . . . . . . . . . . . . . . 1284.4.2 Air Hose Diameter Selection . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1294.4.3 Pressure Drop in Air Hose Lines . . . . . . . . . . . . . . . . . . . . . . . . . 130

4.4.3.1 General Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1304.4.3.2 Friction Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1324.4.3.3 Hose Diameter Effects . . . . . . . . . . . . . . . . . . . . . . . . . 1364.4.3.4 Pressure Drop in Fittings and Armatures . . . . . . . . . . 137

4.5 Abrasive Hose Lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1384.5.1 Conveying Modes in Abrasive Hoses . . . . . . . . . . . . . . . . . . . . . 1384.5.2 Critical Conveying Flow Velocities in Abrasive Hoses . . . . . . . 1404.5.3 Optimum Flow Velocities in Abrasive Hoses . . . . . . . . . . . . . . . 1454.5.4 Pressure Drop in Abrasive Hoses . . . . . . . . . . . . . . . . . . . . . . . . . 147

4.6 Nozzles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1544.6.1 Nozzle Types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1544.6.2 Nozzle Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155

4.6.2.1 Fundamentals of Nozzle Wear . . . . . . . . . . . . . . . . . . . 1554.6.2.2 Parameter Effects on Nozzle Wear . . . . . . . . . . . . . . . 1614.6.2.3 Wear Performance of Laminated Ceramic Nozzles . . 164

5 Substrate and Coating Erosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1675.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1675.2 Mechanical Properties of Oxides and Organic Coatings . . . . . . . . . . . . 167

5.2.1 Relevant Mechanical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 1675.2.2 Mechanical Properties of Oxides . . . . . . . . . . . . . . . . . . . . . . . . . 174

5.2.2.1 Deformation Parameters . . . . . . . . . . . . . . . . . . . . . . . . 1745.2.2.2 Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1745.2.2.3 Adhesion Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 174

5.2.3 Mechanical Properties of Organic Coatings . . . . . . . . . . . . . . . . 1755.2.3.1 Deformation Parameters . . . . . . . . . . . . . . . . . . . . . . . . 1755.2.3.2 Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1775.2.3.3 Fracture Mechanics Parameters . . . . . . . . . . . . . . . . . . 181

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5.3 Impact Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825.3.1 Impulse and Energy Considerations . . . . . . . . . . . . . . . . . . . . . . 1825.3.2 Coefficient of Restitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1825.3.3 Energy Absorption . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1855.3.4 Damage Number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1895.3.5 Friction Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191

5.4 Material Loading Due to Solid Particle Impingement . . . . . . . . . . . . . . 1925.4.1 Loading Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1925.4.2 Material Response to Particle Impingement . . . . . . . . . . . . . . . . 1945.4.3 Formation of Radial and Lateral Cracks . . . . . . . . . . . . . . . . . . . 196

5.5 Material Removal Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2005.5.1 General Aspects of Modelling . . . . . . . . . . . . . . . . . . . . . . . . . . . 2005.5.2 Erosion of Plastically Responding Materials . . . . . . . . . . . . . . . 2015.5.3 Erosion of Elastically Responding Materials . . . . . . . . . . . . . . . 205

5.6 Erosion of Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2065.6.1 Brittle Erosion Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2065.6.2 Removal Mechanisms and Modelling . . . . . . . . . . . . . . . . . . . . . 2075.6.3 Removableness of Mill Scale . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209

5.7 Erosion of Bulk Polymers and Elastomers . . . . . . . . . . . . . . . . . . . . . . . . 2105.7.1 Material Removal Mechanisms for Bulk Polymers . . . . . . . . . . 2105.7.2 Material Removal Mechanisms for Elastomers . . . . . . . . . . . . . 2135.7.3 Erosion Resistance of Bulk Polymers . . . . . . . . . . . . . . . . . . . . . 2145.7.4 Erosion Resistance of Elastomers . . . . . . . . . . . . . . . . . . . . . . . . 217

5.8 Erosion of Organic Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2215.8.1 Material Removal Mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . 2215.8.2 Erosion Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2235.8.3 Erosion Durability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228

5.9 Debonding of Organic Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2295.9.1 Indentation Debonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2295.9.2 Impact Debonding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 233

5.10 Coating Removal Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2365.10.1 Ploughing/Delamination Model . . . . . . . . . . . . . . . . . . . . . . . . . . 2365.10.2 Debonding Model . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2375.10.3 Effects of a Second Phase in the Coating . . . . . . . . . . . . . . . . . . 239

6 Surface Preparation Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2416.1 Definition of Process and Target Parameters . . . . . . . . . . . . . . . . . . . . . . 241

6.1.1 Process Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2416.1.2 Target Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 242

6.2 Effects of Pneumatic Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2436.2.1 Effects of Air Pressure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2436.2.2 Effects of Nozzle Diameter and Nozzle Length . . . . . . . . . . . . . 2496.2.3 Effects of Nozzle Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 251

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6.3 Effects of Performance Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2566.3.1 Effects of Stand-off Distance . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2566.3.2 Effects of Relative Particle Distance . . . . . . . . . . . . . . . . . . . . . . 2596.3.3 Effects of Impact Angle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2596.3.4 Effects of Exposure Time . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2616.3.5 Effects of Number of Passes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 263

6.4 Effects of Abrasive Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2646.4.1 Effects of Abrasive Mass Flow Rate . . . . . . . . . . . . . . . . . . . . . . 2646.4.2 Effects of Abrasive Flux Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2686.4.3 Effects of Abrasive Particle Diameter . . . . . . . . . . . . . . . . . . . . . 2706.4.4 Effects of Abrasive Particle Shape . . . . . . . . . . . . . . . . . . . . . . . . 2776.4.5 Effects of Abrasive Material Hardness . . . . . . . . . . . . . . . . . . . . 279

6.5 Removal Models . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2816.6 Efficiency of Blast Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 283

6.6.1 Erosion Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2836.6.2 General Aspects of Process Efficiency . . . . . . . . . . . . . . . . . . . . 2846.6.3 Aspects of Site Management . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2866.6.4 Aspects of Operators’ Fatigue . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

6.7 Weld Seam Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2896.8 Underwater Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2906.9 Cost Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 292

7 Health, Safety and Environment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2957.1 Safety Features of Blast Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 295

7.1.1 General Safety Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2957.1.2 Risk of Explosion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296

7.2 Emission of Air Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2977.3 Emission of Body Sound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3017.4 Emission of Dust . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3027.5 Emission of Airborne Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 310

7.5.1 Airborne Lead . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3107.5.2 Other Airborne Metals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 314

7.6 Emission of Minerals and Organic Compounds . . . . . . . . . . . . . . . . . . . 3167.6.1 Asbestos Fibres . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3167.6.2 Organic Compounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317

7.7 Vibrations to the Operator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3187.8 Personal Protective Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3207.9 Confined Spaces . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3217.10 Soil Contamination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3247.11 Waste Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325

7.11.1 General Disposal Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3257.11.2 Abrasive Material Disposal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3287.11.3 Contamination of Abrasive Material and Leachable Metals . . . 3317.11.4 Paint Waste . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 332

x Contents

7.12 Recycling of Abrasive Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3337.12.1 Contamination with Residue . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3337.12.2 Use for Construction Materials . . . . . . . . . . . . . . . . . . . . . . . . . . 334

8 Surface Quality Aspects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3378.1 Surface Quality Features . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3378.2 Visual Cleanliness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 338

8.2.1 Visual Standards . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3388.2.2 Initial Conditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3398.2.3 Preparation Grades . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3418.2.4 Special Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346

8.3 Dissolved Substances . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3498.3.1 Definitions and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3498.3.2 Effects of Dissolved Substances on Coating Performance . . . . 3518.3.3 Substrate Cleanliness After Blast Cleaning . . . . . . . . . . . . . . . . . 357

8.4 Organic Films . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3628.4.1 Definitions and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3628.4.2 Effects of Oil and Grease on Coating Performance . . . . . . . . . . 3638.4.3 Substrate Cleanliness After Blast Cleaning . . . . . . . . . . . . . . . . . 366

8.5 Dust and Embedded Abrasive Particles . . . . . . . . . . . . . . . . . . . . . . . . . . 3678.5.1 Definitions and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3678.5.2 Effects of Dust and Particle Embedment on Coating

Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3708.5.3 Substrate Cleanliness After Blast Cleaning . . . . . . . . . . . . . . . . . 3748.5.4 Fine Cleaning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 379

8.6 Roughness and Profile of Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3808.6.1 Definitions and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3808.6.2 Effects of Roughness on Coating Performance . . . . . . . . . . . . . 3848.6.3 Profile Parameters of Blast Cleaned Metal Substrates . . . . . . . . 394

8.6.3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3948.6.3.2 Effects of Blasting Angle . . . . . . . . . . . . . . . . . . . . . . . 3958.6.3.3 Effects of Stand-off Distance . . . . . . . . . . . . . . . . . . . . 3968.6.3.4 Effects of Air Pressure and Abrasive Particle

Velocity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3978.6.3.5 Effects of Abrasive Particle Size and Shape . . . . . . . . 3998.6.3.6 Effects of Abrasive Material Hardness . . . . . . . . . . . . 4048.6.3.7 Effects of Specific Abrasive Rate . . . . . . . . . . . . . . . . 4048.6.3.8 Effects of Blasting Time. . . . . . . . . . . . . . . . . . . . . . . . 4058.6.3.9 Effects of Substrate Material . . . . . . . . . . . . . . . . . . . . 4068.6.3.10 Effects of Surface Preparation Grade . . . . . . . . . . . . . 4088.6.3.11 Effects of Accessibility . . . . . . . . . . . . . . . . . . . . . . . . 4088.6.3.12 Statistical Assessment Models . . . . . . . . . . . . . . . . . . 408

8.6.4 Height Distribution Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . 4098.6.5 Profiles of “Overblasted” Steel Substrates . . . . . . . . . . . . . . . . . 409

Contents xi

8.7 Surface Integrity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4138.7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4138.7.2 Substrate Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4148.7.3 Residual Stresses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4178.7.4 Substrate Fatigue Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4208.7.5 Sustrate Deformation Behaviour . . . . . . . . . . . . . . . . . . . . . . . . . 4238.7.6 Substrate Deflection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4238.7.7 Tribological Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4238.7.8 Weld Seam Morphology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4258.7.9 Near-surface Layer Chemical Composition . . . . . . . . . . . . . . . . 4258.7.10 Corrosion Resistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 429

8.8 Surface Energy and Work of Adhesion . . . . . . . . . . . . . . . . . . . . . . . . . . 4348.8.1 Definitions and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4348.8.2 Effects of Substrate Surface Energy on Corrosion and

Coating Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4378.8.3 Surface Energies of Blast Cleaned Substrates . . . . . . . . . . . . . . . 438

8.9 Wettability of Metal Substrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4418.9.1 Definitions and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4418.9.2 Effects of Wettability on Coating Performance . . . . . . . . . . . . . 4458.9.3 Wettability of Blast Cleaned Metal Substrates . . . . . . . . . . . . . . 447

8.10 Electron Transport Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 451

9 Coating Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4539.1 Corrosion Protection Performance of Organic Coatings . . . . . . . . . . . . 453

9.1.1 Definitions and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4539.1.2 Coating Performance After Blast Cleaning . . . . . . . . . . . . . . . . . 456

9.1.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4569.1.2.2 Coating Delamination . . . . . . . . . . . . . . . . . . . . . . . . . 4589.1.2.3 Degree of Rusting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4629.1.2.4 Degree of Blistering . . . . . . . . . . . . . . . . . . . . . . . . . . . 463

9.2 Adhesion and Adhesion Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4649.2.1 Definitions and Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . 464

9.2.1.1 Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4649.2.1.2 Adhesion Measurement . . . . . . . . . . . . . . . . . . . . . . . . 465

9.2.2 Adhesion of Coatings and Adhesives to Metal Substrates . . . . . 4689.2.3 Blast Cleaning Parameters Effects on Adhesion . . . . . . . . . . . . . 483

9.2.3.1 Effects of Blasting Angle . . . . . . . . . . . . . . . . . . . . . . . 4839.2.3.2 Effects of Abrasive Type, Size and Shape . . . . . . . . . 4839.2.3.3 Effects of Air Pressure . . . . . . . . . . . . . . . . . . . . . . . . . 4889.2.3.4 Effects of Stand-off Distance . . . . . . . . . . . . . . . . . . . . 4889.2.3.5 Statistical Assessment Models . . . . . . . . . . . . . . . . . . 488

9.3 Mechanical Behaviour of Coatings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4899.4 Corrosion Protection Performance of Coatings . . . . . . . . . . . . . . . . . . . . 4929.5 Deposition and Transport Phenomena . . . . . . . . . . . . . . . . . . . . . . . . . . . 4949.6 Wire Embedment in Polymer Matrices . . . . . . . . . . . . . . . . . . . . . . . . . . 495

xii Contents

9.7 Coating Formation Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4969.7.1 Spreading and Splashing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4969.7.2 Powder Solidification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4989.7.3 Nucleation Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 499

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 501

List of Symbols . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 529

Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 537

Chapter 1Introduction

1.1 Motivation

Reams (1939), in his book Modern Blast Cleaning and Ventilation, and Rosenberger(1939), in his book Impact Cleaning, probably delivered the first serious state-of-the-art reviews about the industrial fundamentals of blast cleaning. They werefollowed by Plaster (1972) with his two-volume compendium on ‘Blast Cleaningand Allied Processes’. In Germany, Horowitz’ (1982) book about Oberflachenbe-handlung mittles Strahlmitteln (Surface Treatment with Blasting Media) becamevery popular and is still a widely used reference. Since then, 25 years of intenseprogress in both industrial applications and scientific research have passed. The aimof this book is to provide an extensive up-to-date engineering-based review aboutthe fundamental principles of blast cleaning.

This book is concerned with the blast cleaning of metallic substrates prior to theapplication of protective coatings or adhesives.

1.2 Introductory Remarks

From the point of view of the material removal mechanism, blast cleaning can beconsidered to be an erosion process. “Erosion”, as a tribological term, is the removalof materials due to the action of impinging solid particles. Erosion is a naturalphenomenon [the correct designation in terms of geology is corrasion (Bates andJackson, 1980)] and there exist a number of impressive examples about the materialremoval capability of natural erosion. One example, the erosion of rock columns, isillustrated in Fig. 1.1.

Blast cleaning is one of the most frequently utilised treatment methods in modernindustry. The starting point of the utilisation of blast cleaning for industrial purposeswas Tilghman’s patent on “Improvement in cutting and engraving stone, metal,glass, etc.” (Tilghman, 1870). Benjamin Chew Tilghman (1821–1901), an Americanscientist, invented the “cutting, boring, grinding, dressing pulverizing, and engrav-ing stone, metal, glass, wood, and other hard or solid substances, by means of astream of sand or grains of quartz, or of other suitable materials, artificially driven

A. Momber, Blast Cleaning Technology 1C© Springer 2008

2 1 Introduction

Fig. 1.1 The natural erosion (corrasion) of rock columns in Palmyra (Photograph: University ofTokyo)

Fig. 1.2 Benjamin C. Tilghman (Copyright: ATT-Net)

1.3 Blast Cleaning Methods and Applications 3

as projectiles rapidly against them by any suitable method of propulsion” (PatentNo. 108,408, October 18, 1870). It is not only the general idea of what we today callblast cleaning, or grit blasting, covered by this invention, Tilghman also mentioned anumber of methods how to propel the solid particles against the material surface. Hewrote: “The means of propelling the sand . . . is by a rapid jet or current of steam,air, water, or other suitable gaseous or liquid medium; but any direct propellingforce may be used, as, for example, the blows of the blades of a rapidly-revolvingfan, or the centrifugal force of a revolving drum or tube, or any other suitable ma-chine” (Patent No. 108, 408, October 18, 1870). Benjamin Tilghman is portrayed inFig. 1.2.

The industrial applications mentioned by Tilghman included the following: “Ar-ticles of cast or wrought metal may have their surfaces smoothed and cleaned fromslag, scale, or other incrustions.” Reviews about the early developments in industrialblast cleaning were provided by Plaster (1972, 1993). Early applications includedapplications in the foundry industry, steel industry and corrosion protection industry.Today’s applications include the use for micro-machining, polishing, maintenanceand surface preparation for coating applications. A recent advanced application inthe machining industry is grit blast assisted laser milling (Li et al., 2005).

1.3 Blast Cleaning Methods and Applications

Blast cleaning methods, according to corrosion protection applications, can be sub-divided as listed in Table 1.1. Blast cleaning is by definition a method “whereblasting media (as tools) are accelerated in blasting devices of different blastingsystems, and where they are forced to impinge the surface of a target (substrate) tobe treated” (ISO 12944-4, 1998). To define a blast cleaning method completely, thefollowing information is required:

Table 1.1 Blast cleaning methods according to ISO 12944-4 (1998)

Blast cleaning methods Dry abrasive blast cleaning – Centrifugal abrasive blastcleaning

– Compressed-air abrasive blastcleaning

– Vacuum or suction-head abrasiveblast cleaning

Moisture-injection abrasiveblast cleaning

(No further subdivision)

Wet abrasive blast cleaning – Compressed-air wet abrasiveblast cleaning

– Slurry blast cleaning– Pressurised-liquid blast cleaning

Particular applications ofblast cleaning

– Sweep blast cleaning– Spot blast cleaning

4 1 Introduction

� purpose of blasting;� blasting system;� blasting medium type.

A blast cleaning system is designated according to the method, and respectivelythe medium, that accelerates the abrasive particles up to the required velocity. Fromthat point of view, blast cleaning systems can be subdivided into compressed-airblast cleaning, centrifugal blast cleaning and vacuum or suction-head blast cleaning.

The book deals with the application of compressed-air blast cleaning for thetreatment of metallic substrates. This includes the two following applications:

� removal of mill scales, slags and coatings;� substrate preparation for the subsequent application of coating systems or adhe-

sive systems.

An application example is shown in Fig. 1.3. Coating systems to be applied toblast cleaned substrates include basically corrosion protective coatings and wearresistant coatings. The examples provided in Table 1.2 and in Figs. 1.4 and 1.5very well illustrate the effects of surface preparation methods on the performance ofcoating systems.

Figure 1.4 shows the effects of different surface preparation methods on the de-gree of rusting for a variety of coating materials. It can clearly be seen that blastcleaning to a high surface preparation standard (Sa 21/2) could notably improve

Fig. 1.3 Application of blast cleaning for the surface preparation of steel (Photograph:Muehlhan AG, Hamburg)

1.3 Blast Cleaning Methods and Applications 5

Table 1.2 Effect of pipe surface cleanliness on cathodic disbonding; conditions: 30 days/38◦C(Neal, 1999)

Coating Cathodic disbonding in mm

Surface cleanliness

Blast cleaning(white metal)a

Blast cleaning(near-white blast)a

Power wire brush

Epoxy polymer concrete – 5.0 17.1Fusion bonded epoxy (FBE) 6.1 8.9 >40Heat shrink sleeve 12.7 18.0 27.3Tape 14.9 31.8 28.5Coal tar urethane 13.6 16.5 26.9aSee Sect. 8.2.3 for surface preparation grades

the resistance of the coatings against rusting compared to the coatings appliedover untreated substrate (mill scale) and over power tool cleaned (St 2 and St 3)substrates. The situation in Fig. 1.5 is more complex. It can be seen that a lightblast cleaning (sweep blasting) did not succeed for all coating materials. If a PVC-based coating system was applied to the substrate, washing was more effective thanblast cleaning. For the alky-based coatings, however, a preparation performed byblast cleaning substantially improved the resistance of the coating material againstflaking.

Results of cathodic disbonding tests on coating materials for pipelines are listedin Table 1.2. It can be recognised that a thorough blast cleaning could notably reducethe delamination widths for all applied coating systems.

Fig. 1.4 Effects of steel substrate quality on the performance of corrosion protective coatings(Kogler et al., 1995)

6 1 Introduction

Fig. 1.5 Effects of surfacepreparation methods on theperformance of a duplexcoating system(Foghelin, 1990)

Table 1.3 Effects of surface preparation methods on the performance of a zinc-dust-basedprotective coating system (Brauns et al., 1964)

Surface preparation Time to complete rusting in months

Splash zone Transition zone Underwater zone

Mill scale 5 16 10Acid pickling 8 8 >24Flame cleaning 8 8 >24Blast cleaninga 24 >24 >24aAbrasive: steel cut wire

Experimental results plotted in Table 1.3 illustrate the effects of different sur-face preparation methods on the rusting of steel samples coated with a zinc-dust-containing paint. The time of rusting was estimated in three corrosive maritimezones, which are typical for the corrosive loading of offshore constructions. Thesezones included, in particular, splash zone, water exchange zone and permanentunderwater exposure. It can be seen that blast cleaning with steel cut wire notablyimproved the performance of the corrosion protection system. The time till completerusting of the paint film occurred could notably be extended if the substrates havebeen blast cleaned irrespectively of the loading zone.

Chapter 2Abrasive Materials

2.1 Classification and Properties of Abrasive Materials

A large number of different types of abrasive materials is available for blast clean-ing applications. Most frequently applied abrasive materials are listed in Table 2.1.Table 2.2 lists numerous physical, chemical and technical properties of commercialabrasive materials. Basically, there can be distinguished between metallic abrasivematerials and non-metallic abrasive materials.

The evaluation of an abrasive material for blast cleaning applications includesthe following important parameters:

� material structure;� material hardness;� material density;� mechanical behaviour;� particle shape;� particle size distribution;� average grain size.

2.2 Abrasive Material Structure and Hardness

2.2.1 Structural Aspects of Abrasive Materials

Structural aspects of abrasive materials include the following features:

� lattice parameters;� crystallographical group and symmetry;� chemical composition;� crystallochemical formula;� cleavage;� inclusions (water–gas inclusion and mineral inclusion).

A. Momber, Blast Cleaning Technology 7C© Springer 2008

8 2 Abrasive Materials

Table 2.1 Annual abrasive consumption in the USA for blast cleaning processes (Hansink, 2000)

Abrasive type Consumption in Mio. of tonnes

Coal boiler slag 0.65Copper slag 0.1–0.12Garnet 0.06Hematite 0.03Iron slag 0.005Nickel slag 0.05Olivine 0.03Silica sand 1.6Staurolite/zirconium 0.08–0.09Steel grit and steel shot 0.35

Table 2.3 lists typical values for some abrasive materials. Table 2.4 displaysa commercial technical data and physical characteristics sheet for a typical blastcleaning abrasive material.

Abrasive particles contain structural defects, such as microcracks, interfaces,inclusions or voids. Very often, these defects are the result of the manufacturingprocess. Strength and fracture parameters of materials can be characterised throughcertain distribution types. A widely applied distribution is the Weibull distribution,and it was shown by Huang et al. (1995) that this distribution type can be applied toabrasive materials. The authors derived the following relationship between fractureprobability, particle strength and particle volume:

F(σF) = 1 − exp

[−VP ·

(σF

σ∗)mW

](2.1)

The strength parameter σ* is a constant, which is related to the defects distri-bution. The power exponent mW is the so-called Weibull modulus; it can be readfrom a graphical representation of (2.1). Low values for m indicate a large intrinsicvariability in particle strength. A Weibull plot for aluminium oxide abrasive par-ticles, based on the results of compressive crushing tests, is displayed in Fig. 2.1.Values for the Weibull modulus estimated for different abrasive materials are listedin Table 2.5. There is a notable trend in the values that both fracture strength andWeibull modulus drop with increasing particle size. Therefore, scatter in strengthof abrasive particles can be assumed to be wider for larger particles. The relation-ship between abrasive particle size and fracture strength of the particles is shown inFig. 2.2. This phenomenon can be explained through the higher absolute number ofdefects in larger particles. The probability that a defect with a critical dimension (forexample, a critical crack length in a fracture mechanics approach) exists increaseswith an increasing number of defects.

This effect was also observed by Larssen-Basse (1993). This author found alsothat the Weibull modulus of abrasive particles depended on the atmospheric humid-ity. Larssen-Basse (1993) performed crushing tests with SiC-particles, and he foundthat, if humidity increased, the Weibull modulus and the number of fragments both

2.2 Abrasive Material Structure and Hardness 9

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Table 2.3 Structural properties of abrasive materials (Vasek et al., 1993)

Material Damaged grains (%) Lattice constant (A) Cell volume (A3)

Almandine 5–60 11.522 (0.006) 1,529.62Spessartine – 11.613 (0.005) 1,566.15Pyrope – 11.457 (0.005) 1,503.88Grossular 30 11.867 (0.005) 1,671.18Andradite 80–90 12.091 (0.009) 1,767.61

increased. This feature can be attributed to moisture-assisted sharpening of the tipsof surface defects present in the particles.

The presence of defects, such as cracks and voids, affects the cleaning anddegradation performance of abrasive materials. Number and size of defects are,

Table 2.4 Data sheet for a garnet blast cleaning abrasive material (Reference: GMA Garnet)

Parameter Value

Average chemical compositionSiOa

2 36%Al2O3 20%FeO 30%Fe2O3 2%TiO2 1%MnO 1%CaO 2%MgO 6%

Physical characteristicsBulk density 2,300 kg/m3

Specific gravity 4.1Hardness (Mohs) 7.5–8Melting point 1,250◦CGrain shape Sub-angular

Other characteristicsConductivity 10–15 mS/mMoisture absorption Non-hydroscopicTotal chlorides 10–15 ppmFerrite (free iron) <0.01%Lead <0.002%Copper <0.005%Other heavy metals <0.01%Sulphur <0.01%

Mineral compositionGarnet (Almandine) 97–98%Ilmenite 1–2%Zircon 0.2%Quartz (free silica) <0.5%Others 0.25%

aRefers to SiO2 bound within the lattice of the homo-geneous garnet crystal (no free silica)


Fig. 2.1 Weibull plot for the strength of aluminium oxide particles (Verspui et al., 1997). Abrasiveparticle size: 10–500 μm

therefore, important assessment criteria. Cast steel shot, for example, should notcontain cracked particles, as illustrated in Fig. 2.3, in excess of 15%. Cast steelgrit should not contain cracked particles, as shown in Fig. 2.4, in excess of 40%(SFSA, 1980). Requirements for the defects of particles of metallic abrasive mate-rials are listed in Table 2.6.

Table 2.5 Strength parameters for abrasive materials (Yashima et al., 1987; Huang et al., 1995)

Abrasive material Grain sizein mm

Fracture strengthin MPa

Weibullmodulus

∗a inMPa/mm3

Brown corundum 2.58 67.5 1.98 228.81.85 78.6 2.47 142.81.29 115.4 2.88 135.10.78 200.5 3.47 149.0

Rounded corundum 1.85 96.1 3.41 160.8White corundum 1.29 79.5 2.57 127.3Sintered corundum 1.85 110.8 3.85 174.9Green silicon carbide 1.85 62.2 1.92 155.5Quartz 0.1–2.0 – 21.0 –Glass beads – – 5.90 –

aDefect distribution parameter


Fig. 2.2 Relationship between abrasive particle size and particle fracture strength (values fromHuang et al., 1995)

Fig. 2.3 Cracks in cast steel shot particles; magnification: 10× (SFSA, 1980)


Fig. 2.4 Cracks in cast steel grit particles; magnification: 10× (SFSA, 1980)

2.2.2 Hardness of Abrasive Materials

The hardness of abrasive materials is usually estimated by two types of tests:a scratching test for non-metallic abrasive materials, which delivers the Mohshardness, and indentation tests for metallic materials, which deliver either theKnoop hardness or the Vickers hardness. Respective values for commercial abrasivematerials are listed in Table 2.2.

Mohs hardness is based on a scale of ten minerals, which is provided in Table 2.7.The hardness of a material is measured against the scale by finding the hardest

Table 2.6 Particle defect requirements for metallic abrasive materials (ISO 11124/2-4)

Property Chilled iron grit High-carbon caststeel shot

High-carbon caststeel grit

Low-carboncast steel shot

Particle shape Max. 10% shotor more thanhalf-round

Max. 5%non-round

Max. 10% shot or morethan half-round forgrit up to 700 HV;max. 5% for gritabove 700 HV

Max. 5%non-round

Voids Max. 10% Max. 10% Max. 10% Max. 15%Shrinkagedefects

Max. 10% Max. 10% Max. 10% Max. 5%

Cracks Max. 40% Max. 15% Max. 40% NoneTotal defects Max. 40% Max. 20% Max. 40% Max. 20%


Table 2.7 Mohs scale (Tabor, 1951)

Material Mohs hardness

Talc 1Gypsum 2Calcite 3Fluorite 4Apatite 5Orthoclase (Feldspar) 6Quartz 7Topaz 8Corundum 9Diamond 10

material that the given material can scratch, and/or the softest material that canscratch the given material. For example, if some material is scratched by quartzbut not by feldspar, its hardness on Mohs scale is 6.5. In abrasive standardisation,abrasive particles are being rubbed against a glass plate having a Mohs hardnesscorresponding to 7. If the particles can scratch the plate, their hardness is >Mohs 7.If they do not scratch the plate, their hardness is <Mohs 7. It is because of this pro-cedure that data sheets for mineral abrasive materials often list the Mohs hardnessas >7 only.

The principles of two frequently applied indentation hardness tests are illus-trated in Table 2.8. In laboratory practice, an abrasive particle is embedded ina special resin matrix, and it is then being polished in order to obtain an even

Table 2.8 Indentation hardness measurement methods (Images: TWI, Cambridge, UK)

Method Brinell Vickers

Principle

Measurement

Calculationa HB = Fπ2

· D · [D − (D2 − d2)1/2

] HV = 2 · F · sin (136◦/2)

d2

aF= indentation load

d = indentation size = d1 + d2

2D = indenter size


(a)

(b)

Fig. 2.5 Vickers hardness distributions of two cut wire samples (Gesell, 1979). (a) Laboratorysample; (b) Work sample


smooth cross-section where the actual indentation test is being performed. In-dentation hardness values are always dependent on indentation load, and careshould be taken to provide the certain applied indentation load in data sheets.Values from indentation hardness tests and from Mohs hardness tests can be re-lated to each other; exceptions are diamond and corundum (Bowden and Ta-bor, 1964).

The hardness of metallic abrasive particles is a probabilistic parameter, and thehardness values mentioned in data sheets are mainly mean values only. Two typ-ical abrasive hardness distribution diagrams of cut wire samples are provided inFig. 2.5. Figure 2.5a shows the distribution of a laboratory sample, whereas Fig. 2.5billustrates the distribution of a working sample. Although both materials had equalhardness designations of 420 HV, the distributions differed widely. The laboratorysample had a unimodal distribution with a maximum at a Vickers hardness of about430 HV, whereas the working sample featured a multimodal distribution. The hard-ness distribution of the laboratory sample can be expressed through a Normal dis-tribution – this is shown in Fig. 2.6. This result points to a rather homogeneousresponse of the wire material to the indentation with the Vickers pyramid, which isnot always the case (Lange and Schimmoller, 1967). Such a distribution was alsoreported by Flavenot and Lu (1990) for steel wire shot.

Fig. 2.6 Normal distribution function for the laboratory cut wire sample plotted in Fig. 2.5a

2.3 Abrasive Particle Shape Parameters 17

2.3 Abrasive Particle Shape Parameters

2.3.1 Basic Shape Definitions

The following three basic shape definitions are provided for abrasive particles usedfor blast cleaning applications:

� shot;� grit;� cylindrical.

The corresponding designations are listed in Table 2.9. Examples for two shapedefinition are displayed in Fig. 2.7. The term shot characterises grains with a pre-dominantly spherical shape. Their length-to-diameter ratio is <2, and they do notexhibit sharp edges or broken sections. The term grit characterises grains with apredominantly angular shape. These grains exhibit sharp edges and broken sections.The term cylindrical denotes grains that are manufactured by a cutting process. Theirlength-to-diameter ratio is∼1. This shape can only be found with cut steel wire pellets.

2.3.2 Relative Proportions of Particles

Shape parameters characterise the shape of individual particles. Wadell (1933) andHeywood (1933) were probably the first who gave rigorous analyses of shapeparameters. Heywood (1933) considered the shape of a particle to have the followingtwo distinct characteristics:

� the relative proportions of length, breadth and thickness;� the geometrical form.

The relative proportion includes two parameters: (1) the elongation ratio (rE) and(2) the flatness ratio (rF). Both parameters are defined and illustrated in Table 2.10.Bahadur and Badruddin (1990) applied the elongation ratio to investigate the in-fluence of the abrasive particle shape on particle impact erosion processes. Theyfound notable relationships between abrasive type, abrasive particle diameter andabrasive particle shape. Some results of their study are provided in Fig. 2.8. Silicacarbide particles became more elongated and less circular with an increase in theparticle size, while the opposite was the case with aluminium oxide particles. Thegeneral variation of silica oxide was similar to that of silica carbide particles, thoughnot as systematic. The elongation ratios for the silica carbide particles and for the

Table 2.9 Grain shape designations

Designation Grain shape Symbol

Shot Spherical, round SGrit Angular, irregular GCylindrical Sharp-edged C


(a) (b)

Fig. 2.7 Basic shape designations for abrasive particles (Photographs: Kuhmichel GmbH).(a) Grit; (b) Shot

aluminium oxide were very sensitive to the particle size in the range of small parti-cles. For equal grain sizes, silica oxide particles featured much higher elongationratios than silica carbide particles. For a particle diameter of dP = 300 μm, asan example, the elongation ratio was rE = 0.53 for silica carbide, and rE = 0.7for silica oxide. A relationship between particle abrasive size and shape was alsonoted by Djurovic et al. (1999). For starch media, these authors found that smallerparticles were less elongated than larger particles. These results clearly show thatparticle shape may be considered an abrasive material characteristic.

2.3.3 Geometrical Forms of Particles

The geometrical form is a volumetric shape factor, representing the degree to whicha particle approximates an ideal geometric form (cube, sphere or tetrahedron). Thefollowing two parameters can describe the geometrical form of particles: (1) thesphericity (SP) and (2) the roundness (SR).

2.3 Abrasive Particle Shape Parameters 19

Table 2.10 Shape parameters for abrasive particles

Parameter and definition Graphical expression

Shape factor

Fshape = dmin

dmax

Circularity factor

F0 = 4 · π · AP

Perimeter2

Roundness

SR =∑ (

2·rcornerdP

)Ncorner

Sphericity

SP =√

4π

· bP · lP

dcircle

Elongation ratio

rE = lP

bP

Flatness ratio

rF = lP

tP

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