WHAT IS THE RIGHT ABRASIVE FOR THE JOB? Burke Bennett Sales and Service Representative. Clemtex, Inc. Houston, Texas, USA

SSPC 99: The Proceedings of the Seminars WHAT IS THE “RIGHT” ABRASIVE FOR THE JOB? Burke Bennett Sales and Service Representative Clemtex, Inc. Houst...
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SSPC 99: The Proceedings of the Seminars

WHAT IS THE “RIGHT” ABRASIVE FOR THE JOB? Burke Bennett Sales and Service Representative Clemtex, Inc. Houston, Texas, USA An abrasive is a substance used for abrading, smoothing or polishing. Abrasives can be naturally occurring minerals, man made, or byproducts of another process. A partial list common abrasives available is:

ABSTRACT With today's productivity and environmental concerns facing abrasive blasting applications, the importance of becoming familiar with the characteristics of blasting abrasives is a definite requirement to remain viable and competitive in today's business climate.

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INTRODUCTION Abrasive blasting for surface preparation is an important step in providing a foundation for protective coatings. For years, many tests have been conducted with results concluding that high performance coatings provide excellent corrosion protection and durability when applied over a blast cleaned surface with a consistent anchor profile. To achieve the appropriate anchor profile, one must choose an aggressive abrasive for the job. In some cases, the item that requires cleaning might be damaged by residue from the blasting abrasive. In these cases, one must consider a nonaggressive abrasive for the job. In today's business climate, when so much importance is placed on quality assurance and environmental concerns, it has become critical to familiarize yourself with the characteristics of the many abrasives that are available. Unfortunately, one abrasive cannot provide the capability to work in all applications. Fortunately, there are many different types and grades of abrasives available to fulfill most applications. Learning the characteristics of abrasives will allow you pick the right abrasive for the job and be more efficient and profitable.

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coal slag nickel slag staurolite zircon garnet glass beads corn cob granules steel shot

When choosing from a list of abrasives one must consider their specific characteristics. These characteristics include an abrasive's hardness, shape, size, color, weight, chemical composition, availability, cost, and environmental concerns. These characteristics determine which application each abrasive is best suited. Hardness The hardness of an abrasive determines whether an abrasive particle can etch or provide an anchor pattern on a particular substrate. One method of determining the hardness of an abrasive is to use the Mohs hardness scale. The scale ranges from 1 to 10, with 1 being the softest (talc) and 10 being the hardest (diamond). Most abrasives that are used to achieve an anchor pattern on a surface are 6.0 Mohs hardness and higher. There are some applications that do require the use of softer abrasives ranging in the 3.0 to 4.5 scale. These abrasives will not etch steel or glass but generally remove foreign debris and provide a clean

To determine what is the right abrasive for the job one must know the definition of an abrasive. Published by Nanjing EACA Coatings Eng.& Tech.Co.

silica sand copper slag ceramic olivine aluminum oxide silicon carbide walnut shells plastic media steel grit

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What is the “Right” Abrasive for the Job?

surface. These abrasives are used in areas where you do not want to remove any of the substrate's surface or where the abrasive particle or residue could be harmful to that part or surrounding items. Soft abrasives are commonly used around bearings and other easily damaged machinery. Steel shot and grit are considered hard abrasives. Steel abrasives are measured in Rockwell C hardness. These abrasives are produced in hardness ranging from 42 to 65. Steel grit with provide an etch on a steel substrate, where steel shot will provide a peened surface on a steel substrate.

mesh is very coarse and is used for hard to clean surfaces or surfaces that require a deep anchor pattern. A material graded 20 to 40 mesh is considered a general purpose material. A material graded 80 to 120 mesh is considered a fine material and is used for polishing surfaces. Round or spherical abrasives are graded by particle diameter. These abrasives are available in sizes ranging from 0.070 inches to 0.660 inches. All abrasives should be graded to a uniform size to allow for precise flow rates through a metering valve on a blast machine. Hard abrasives must be uniformly graded to provide a consistent anchor profile on the substrate.

Shape Color The shape of the abrasive may be angular, blocky, semi-round, or spherical. An angular abrasive has sharp edges which produces the quickest cleaning rate when removing tightly adhered material or contamination from the substrate. The sharp edges of a hard angular abrasive produces steep peeks and valleys in the anchor profile thus increasing the surface area and providing an excellent surface for the mechanical bonding of the coating to the surface. A blocky abrasive has more flat edges than a sharp abrasive. Blocky abrasives provide good cleaning rates on most applications except those where extremely difficult removal of contaminants is present on the substrate. Semi-round and spherical abrasives produce a peened or dimpled surface. These abrasives are generally used for surface hardening or stress relieving of the substrate.

The color may not seem very important when considering the type of abrasive to be selected, however, in certain instances it is a critical factor. Abrasives can leave residue on the surface that could affect the appearance after it is blasted. Darker abrasives tend to be less dusty than lighter colored abrasives because they do not reflect light as easily. This is especially important when blasting inside areas that require lighting. If a blasting job is performed in a manicured or tidy area where the spent material is unable to be completely removed, the color of the abrasive may be important for aesthetic reasons as well. Weight or Specific Gravity The weight or specific gravity of an abrasive will affect the cleaning rate and the anchor profile achieved on the surface. The weight is determined in pounds per cubic foot and the specific gravity is relative density of the particle. An abrasive with a high specific gravity indicates that it will be heavier. When blasted at the same pressure, a heavier abrasive will achieve a deeper anchor pattern than that of a lighter abrasive. Abrasives with a higher specific gravity will tend to be less dusty. Abrasives with a low specific gravity impact the substrate with less force and are used for light cleaning, polishing and deburring.

Size The size of an abrasive affects the speed of the cleaning rate and anchor pattern produced. U.S. sieve analysis or diameter of the particle determines particle size of the abrasive. Abrasives are graded and packaged according to these methods. A U.S. sieve analysis is performed to distribute the abrasive particles in mesh sizes. Abrasives are generally available from 4 mesh to 325 mesh. The smaller the mesh size, the larger the abrasive particle. A material graded 8 to 16 Published by Nanjing EACA Coatings Eng.& Tech.Co.

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SSPC 99: The Proceedings of the Seminars

be provided to minimize the workers exposure limits. The effects of the spent abrasive on the soil must also be considered. In confined areas, the toxicity of the abrasive may require the use of additional safety and engineering controls.

Chemical Composition Chemical composition of abrasives must be considered to assure compatibility of the substrate with the abrasive. When blasting a substrate, abrasive particles may embed or leave residue on the substrate that could be detrimental to the protective coating. In cases where iron abrasive is used to blast on a stainless steel substrate, the embedded particles will form corrosion cells on the substrate.

ABRASIVES There is a long list of abrasives from which to choose. This list includes common abrasives but does not contain all the abrasives that are available. Its intention is to inform the reader and raise his curiosity to look for other abrasives available in his region. Learning the characteristics and capabilities of each abrasive provides an excellent start to completing a successful job.

Availability It is important to insure that the abrasive selected for any particular project is easily obtained in sufficient quantities to finish the job. In order to obtain optimum results, it is best not to have to change abrasives in the middle of a project. Many abrasives are naturally occurring minerals or byproducts from other industries. Therefore, they may not be available for immediate delivery in all geographical regions.

Silica Sand Silica sand is an inexpensive abrasive that is generally readily available in many areas of the country. Mined sand is sharp, where sand dredged from a river is more rounded. It ranges from 6 to 7 on the Mohs hardness scale and will provide an etch or anchor pattern on a steel substrate. Sand is available in many sizes. Sand has a tan to brown tint. Sand is a one-use abrasive that should reclaimed for reuse. Sand is 95% to 99% silicon dioxide (SiO2). Special controls must be taken when blasting with sand because of its silicosis hazard. U.S. Department of Commerce National Institute for Occupational Safety and Health (NIOSH) Recommended Standard PB-246-697 Section 6, a, 2 states “uncontrolled abrasive blasting with silica sand is such a severe silicosis hazard that special attention must be given to this problem. Silica sand or other materials containing more than 1% free silica, should be prohibited as an abrasive substance in abrasive blasting cleaning operations.” It continues “the most certain and direct method of eliminating the silicosis hazard is to substitute other less toxic material for free silica. In abrasive blasting, where silica sand can be replaced with an abrasive containing less than 1% free silica, such substitution should be considered.”

Costs The cost of the abrasive is obviously a critical part of any job. Generally, the cost of freight to get the abrasive from the manufacturer to the job site determines the choice of abrasive. Also important is the friability or breakdown rate of the abrasive. The cost of an abrasive that has a high recycle rate can be lower substantially by reusing it in a blast cabinet or room with a reclaim system. The blasting pressure, hardness, malleability, and size of the abrasive determine its recycle rate. Environmental Concerns The final consideration in choosing an abrasive is the environmental concern. Environmental concerns include respiratory effects of the abrasive on the blaster and the workers in the area. Workers must be provided with approved respiratory equipment and monitored throughout the job. When blasting areas with poor ventilation, proper dust collection equipment must Published by Nanjing EACA Coatings Eng.& Tech.Co.

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What is the “Right” Abrasive for the Job?

Staurolite may be recycled several times. Staurolite is used for an outdoor one time use abrasive as well as in blast cabinets and rooms with reclaim systems. Staurolite is available in medium to fine sizes.

Coal Slag Coal slag is a by-product of the utility industry. It is blocky in shape and contains less than 1% free silica. Coal slag has a Mohs hardness of 6 and will provide an etch or anchor pattern on a steel substrate. Coal slag is black in color and may have a green tint. Coal slag is available in most sizes. Coal slag is mainly comprised of silicon dioxide, aluminum oxide, and iron oxide. Coal slag may be reclaimed a couple of times, however is generally a one time use abrasive. Coal slag is considered a low cost abrasive.

Ceramic Ceramic abrasive is a manufactured abrasive. It is angular in shape and contains less than 1% free silica. Ceramic abrasive is dark brown/gray in color and has a Mohs hardness of 7. Ceramic abrasive is available in various sizes and will provide an etch or anchor pattern on a steel surface. Ceramic abrasive may be reclaimed several times.

Copper Slag Copper Slag is a by-product of the copper ore industry. It is blocky in shape and contains less than 1% free silica. Copper slag has a Mohs hardness of 6 and will provide an etch or anchor pattern on a steel substrate. Copper slag is less than sand or coal slag, however because of its high specific gravity it will have a tendency to embed in a steel surface. Copper slag is black in color and may have a gray tint. Copper slag is available in most sizes. Copper slag is considered a low cost abrasive.

Olivine Olivine is a naturally occurring mineral. It is angular in shape and contains less than 1% free silica. Olivine has a Mohs hardness of 6.5 and will provide an etch or anchor pattern on a steel substrate. Olivine is light green in color. Olivine is available in most sizes. Olivine is mainly comprised of silicon dioxide, magnesium oxide and iron oxide. Olivine may be reclaimed a several times. Olivine is used for an outdoor onetime-use abrasive as well as in blast cabinets and rooms with reclaim systems.

Nickel Slag Nickel slag is a by-product material. It is blocky is shape and contains less than 1% free silica. Nickel slag has a Mohs hardness of 6 and will provide an etch or anchor pattern on a steel substrate. Nickel slag is available in most sizes. Nickel slag may be reclaimed several times, however is generally a one time use abrasive. Nickel slag is considered a low cost abrasive.

Zircon Zircon is a naturally occurring mineral. It is angular in shape and contains less than 1% free silica. Zircon has a Mohs hardness of 6.5 and will provide an etch or anchor pattern on a steel substrate. Zircon is a zirconium silicate that is brown in color. Zircon is a fast cutting abrasive that is used in blast cabinets and rooms with reclaim systems.

Staurolite Staurolite is a naturally occurring mineral. It is angular in shape and contains less than 1% free silica. Staurolite mostly consists of aluminum oxide and iron oxide. Staurolite is reddish-brown and has a Mohs hardness of 7. It will provide an etch or anchor pattern on a steel substrate. Published by Nanjing EACA Coatings Eng.& Tech.Co.

Aluminum Oxide Aluminum oxide is a manufactured abrasive. It is angular in shape and contains less than 1% free 4

SSPC 99: The Proceedings of the Seminars

silica. Aluminum oxide is dark in color and has a Mohs hardness of 8. Aluminum oxide is available in many sizes and will provide an etch or anchor pattern on a steel surface. Aluminum oxide may be reclaimed many times. Aluminum oxide is a fast cutting, low dusting, higher cost abrasive that is used in blast cabinets and rooms with reclaim systems.

Walnut Shell Walnut shells are a by-product of the farming industry. Walnut shells are blocky in shape and contains less than 1% free silica. Walnut shells have a Mohs hardness of 3 and will not etch a steel substrate. Walnut shells are dark in color and is available in most sizes. Walnut shells are used for deburring, cleaning electrical parts, and cleaning around sensitive threaded or bearing areas.

Glass Beads Glass beads are a manufactured abrasive. They are spherical in shape and contains less than 1% free silica. Glass beads are white in color and have a Mohs hardness of 6. Glass beads are available in many sizes and will peen a steel surface. A surface blasted with Glass beads will have a matted look. Glass beads may be reclaimed many times. Glass beads are a fast cleaning higher cost abrasive that is used in blast cabinets and rooms with reclaim systems. Glass beads are also used for surface hardening and deburring.

Corn Cob Granules Corn cob granules are a by-product of the farming industry. They are blocky in shape and contains less than 1% free silica. Corn cob granules have a Mohs hardness of 4.5 and will not etch a steel substrate. Corn cob granules are dark in color and is available in most sizes. Corn cob granules are used for cleaning electrical parts, aluminum molds, and cleaning around sensitive threaded or bearing areas.

Garnet

Plastic Media

Garnet is a naturally occurring mineral. It is angular in shape and contains less than 1% free silica. Garnet is reddish in color and has a Mohs hardness of 7.5. Garnet is available in many sizes and will provide an etch or anchor pattern on a steel surface. Garnet may be reclaimed many times. Garnet is a fast cutting, low dusting, higher cost abrasive that is used in blast cabinets and rooms with reclaim systems

Plastic media is a man-made abrasive that is blocky in shape and contains less than 1% free silica. Plastic media can have a Mohs hardness of 3 to 4. Plastic media is a soft abrasive that does not etch steel. Plastic media is a high cost abrasive that is primarily used in blast cabinets or room with recovery systems. Plastic media is used to blast molds, threaded areas, and sensitive equipment.

Silicon Carbide

Steel Shot

Silicon carbide is a manufactured abrasive. It is angular in shape and contains less than 1% free silica. Silicon carbide is dark in color and has a Mohs hardness of 8.5. Silicon carbide is available in many sizes and will provide an etch or anchor pattern on a steel surface. Silicon carbide may be reclaimed many times. Silicon carbide is a fast cutting, clean, higher cost abrasive that is used in blast cabinets and rooms with reclaim systems.

Steel shot is a man-made abrasive that is spherical in shape and contains less than 1 % free silica. Steel shot is iron oxide and its hardness is measured in Rockwell C. Its hardness is generally 42 to 50. Steel shot will peen steel. Steel shot is may be recycled more than 100 times which makes its usage costs low. Steel shot is primarily used in rotary wheel blast machines. Steel shot is also used for surface hardness on various metals.

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What is the “Right” Abrasive for the Job?

Steel Grit Steel grit is a man-made abrasive that is angular in shape and contains less than 1% free silica. Steel grit is iron oxide. Steel grit's hardness is measured in Rockwell C. It can be manufactured to various hardnesses ranging from 42 to 62. Steel grit may be recycled more than 100 times which makes its usage costs low. Steel grit is primarily used in blast cabinets and rooms with recovery systems. It is also added to steel shot in many rotary wheel blast machines to help achieve a specified anchor profile. CONCLUSION This paper is written to provide an overview of some the many abrasives that are available. Its intention is to encourage the reader to become aware of other abrasives that may be more competent to complete a job. Each abrasive discussed in this paper has its own benefits as well as detriments. There are many abrasives in today's marketplace which are capable of performing an excellent job. To be competitive is today market, one must become familiar with each abrasive that will economically and successfully complete a job. Costs and productivity of the right abrasive for the job is important, however environmental concerns must be taken into account for your company to remain viable in the marketplace.

Published by Nanjing EACA Coatings Eng.& Tech.Co.

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