Abrasive blasting for surface preparation is an important step in providing a foundation for protective coatings. For years, tests have concluded that high-performance coatings provide excellent corrosion protection and durability when applied over a blast-cleaned surface with a consistent anchor profile.
In today’s business climate, where so much importance is placed on quality assurance and environmental concerns, it’s critical that plant personnel familiarize themselves with the characteristics of abrasives that are available. While no single abrasive can work in every application, there are many types and grades of abrasives available to fulfill most applications. Understanding the characteristics of each abrasive will help you pick the right one for the job and, in the process, help you run your plant more efficiently.
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Silica sand is an inexpensive abrasive that is readily available in many areas of the country. It ranges from 6 to 7 on the Mohs hardness scale and provides an etch or anchor pattern on a steel substrate. Available in many sizes, silica sand is 95 to 99 percent silicon dioxide. However, special precautions must be taken when blasting with silica sand because of the silicosis hazard it presents.* Coal slag is a by-product of the utility industry. This low-cost abrasive is blocky in shape and contains less than 1 percent free silica. Coal slag has a Mohs hardness of 6 and provides an etch or anchor pattern on a steel substrate. In general, it is a one-time-use abrasive. A by-product of the copper ore industry, copper slag is blocky in shape and contains less than 1 percent free silica. It has a Mohs hardness of 6 and provides an etch or anchor pattern on a steel substrate. Because of its high specific gravity, copper slag has a tendency to embed in a steel surface. Available in many sizes, it is considered a low-cost abrasive. A naturally occurring mineral, staurolite is angular in shape and contains less than 1 percent free silica. It has a Mohs hardness of 7 and provides an etch or anchor pattern on a steel substrate. Staurolitemay be recycled several times. It is available in medium to fine sizes. A manufactured abrasive, ceramic is angular in shape and contains less than 1 percent free silica. It has a Mohs hardness of 7. Ceramic is available in various sizes and will provide an etch or anchor pattern on a steel surface. It may be reclaimed several times. Aluminum oxide is a fast cutting, low dusting, higher cost abrasive used in blast cabinets and rooms with reclaim systems. This manufactured abrasive is angular in shape, contains less than 1 percent free silica, and has a Mohs hardness of 8. It will provide an etch or anchor pattern on a steel surface. Aluminum oxide may be reclaimed many times. Glass beads are a fast-cleaning, higher cost abrasive used in blast cabinets and rooms with reclaim systems. Glass beads can be reclaimed many times. They are also used for surface hardening and deburring. This manufactured abrasive has a Mohs hardness of 6. Glass beads are available in many sizes and will peen a steel surface. Glass bead blasting leaves a matte finish. A man-made abrasive, steel shot is spherical and contains less than 1 percent free silica. Its hardness, measured in Rockwell C, is generally 42 to 50. Steel shot may be recycled more than 100 times, which reduces its application costs. It is primarily used in rotary wheel blast machines and for enhancing surface hardness on various metals. Steel grit is a man-made abrasive that is angular in shape and contains less than 1 percent free silica. It can be manufactured to various hardness levels ranging from 42 to 62. Steel grit may be recycled more than 100 times. It 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. * National Institute for Occupational Safety and Health Recommended Standard PB-246-697 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 percent free silica, should be prohibited as an abrasive substance in abrasive blast cleaning operations...The most certain and direct method of eliminating the silicosis hazard is to substitute less toxic material for free silica. In abrasive blasting, where silica sand can be replaced with an abrasive containing less than 1 percent free silica, such substitution should be considered.”
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To determine the right one for a specific job, you must understand the basic definition of an abrasive: a substance used for abrading, smoothing or polishing. Abrasives can be naturally occurring minerals, man-made materials or by-products of another process. (A partial list of common abrasives is featured in the accompanying sidebar.) When choosing from this list, one must consider specific characteristics of the abrasive. They include hardness, shape, size, color, weight, chemical composition, availability, cost and environmental concerns.
Hardness
Hardness determines whether an abrasive particle can etch or provide an anchor pattern on a particular substrate. One way to determine 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 effectively achieve an anchor pattern on a surface have a Mohs hardness of at least 6.0.
Some applications require softer abrasives ranging in the 3.0 to 4.5 scale range. They will not etch steel or glass, but will generally remove foreign debris and provide a clean surface. They are used in areas where the substrate’s surface need not be removed, or where abrasive particle or residue could damage the substrate or surrounding area. Soft abrasives are often used around bearings and other easily damaged machinery.
Steel shot and steel grit are considered hard abrasives. Steel abrasives are measured in Rockwell C hardness. They are produced in hardness ranging from 42 to 65. Steel grit provides an etch on a steel substrate, whereas steel shot provides a peened surface on a steel substrate.
Shape
The shape of an abrasive may be angular, blocky, semi-round or spherical. An angular abrasive has sharp edges, which ensure the quickest cleaning rate when removing tightly adhering material or contamination from the substrate. The sharp edges of a hard, angular abrasive produce steep peeks and valleys in the anchor profile, thus increasing the surface area and providing an excellent condition for the mechanical bonding of a coating.
A blocky abrasive has mostly flat edges. This abrasive provides good cleaning rates in most applications, except those where hard-to-remove contaminants are present on the substrate. Semi-round and spherical abrasives produce a peened or dimpled surface. Blocky abrasives are generally used for surface hardening or stress relieving of the substrate.
Size
Abrasive particle size affects the cleaning rate and anchor pattern produced. Particle size is determined by U.S. sieve analysis, which distributes the particles in mesh sizes. (Abrasives are graded and packaged according to this method.) Abrasives are generally available from 4 to 325 mesh. The smaller the mesh size, the larger the abrasive particle.
A material graded 8 to 16 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 abrasive. Eighty to 120 mesh is considered fine and is used for polishing surfaces. Round or spherical abrasives are graded by particle diameter. They are available in sizes ranging from 0.070 to 0.660 inches.
Abrasives should be graded to a uniform size. This allows 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.
Color
Color may not seem very important when selecting an abrasive. In certain situations, however, it is a critical factor. Abrasives can leave residue, which can affect the surface appearance of a blasted surface. 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 blasting is performed in a manicured or tidy area where spent material cannot be completely removed, the color of the abrasive may be important for aesthetic reasons as well.
Density or specific gravity
The density or specific gravity of an abrasive affects the cleaning rate and anchor profile achieved on the surface. Density is measured in pounds per cubic foot; specific gravity is measured by the density of the particle relative to the density of water. An abrasive with a high specific gravity is generally heavier. When blasted at the same pressure, a heavy abrasive achieves a deeper anchor pattern than that from a lighter one. Abrasives with a high specific gravity tend to be less dusty. Those with a low specific gravity impact the substrate with less force and are used for light cleaning, polishing and deburring.
Chemical composition
Chemical composition must be considered to assure compatibility of the substrate with the abrasive. When blasting a substrate, particles may be embedded or leave a residue, which could damage the protective coating. In cases where an iron abrasive is used to blast a stainless steel substrate, the embedded particles will form corrosion cells on the substrate.
Availability
It’s important to ensure that the abrasive selected for a project can be easily obtained in sufficient quantities to finish the job. To obtain optimum results, it is best not to change abrasives in the middle of a project. Many abrasives are naturally occurring minerals or by-products from other industries. Therefore, they may not be available for immediate delivery in all geographical regions.
Cost
The cost of the abrasive is a critical part of any job. Often, the cost of freight to transport the abrasive from manufacturer to job site can determine your choice. Also important is its friability or breakdown rate. The cost of an abrasive that has a high recycle rate can be reduced 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 is environmental. Environmental concerns include the abrasive’s respiratory effects on the blaster and other workers in the area. Employees must be provided with approved respiratory equipment and monitored throughout the job.
When blasting in areas with poor ventilation, proper dust collection equipment must be provided to minimize worker exposure limits. The toxicity of junk being removed sometimes makes disposal of the abrasive/junk mix difficult. The effect of 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.
This is an edited version of a paper originally published in “SSPC 99: The Proceedings of the Seminars” by SSPC: The Society for Protective Coatings, 40 24th St., Pittsburgh, PA 15222-4656. Used with permission.