ABRASIVE BLASTING

Abrasive blasting is generally the preferred method of surface preparation for larger jobs where power or hand tool cleaning cannot be accomplished in a reasonable period of time. When performed properly, abrasive blasting provides one of the highest degrees of surface cleanliness. During abrasive blasting operation, pressurized air is used to accelerate abrasive material against a surface to remove contaminants and oxide films. In addition to cleaning the surface, abrasive blasting imparts a surface profile which greatly enhances coating adhesion. There are a wide variety of abrasive materials. Steel/metal shot and aluminum oxide/refractory slag comprise the bulk of abrasive materials used today. Some points to keep in mind when specifying abrasive materials for surface preparation on a US Navy job include:

  • Acquisition of abrasive shall be in accordance with MIL-A-22262
  • Particle size should be sufficient for recommended profile depth
  • Depth of anchor tooth profile depends on abrasive particle size, hardness, and pressure supplied by the blasting machine

Table 1 provides a list of the generic types of abrasives with examples, while Table 2 provides an abrasive profile comparison chart. The information in this table should be used only for approximating the abrasive size required to obtain a specified anchor pattern. The standard metal used to obtain these results was hot rolled steel with tightly adhering mill scale. The resulting depth of the anchor pattern will vary with the method used for measuring depths as well as any one of numerous other variables, such as type and hardness of steel, thickness of mill scale, and degree of cleaning specified. This information can be used for centrifugal wheel as well as pressure blasting. Pressure blasting should be done using 90-100 psi nozzle pressure. The depth of anchor pattern used in this chart is an average and not a minimum or maximum depth obtainable. Consult local abrasive suppliers for specific technical data.

Table 1. Types of Abrasive
Mineral Slags Manufactured Natural Organic Composite Media
Copper slag
Coal slag
Nickel slag
Silicon carbide
Aluminum oxide
Plastic media
Ferrous metallic
Glass beads
Garnet
Quartz
Silica
Coconut shells
Black walnuts
Pecan shells
Peach pits
Filbert shells
Cherry pits
Sponge


Table 2. Abrasive Profile Comparison Chart
1.0 mil Profile 1.5 mil Profile 2.0 mil Profile
30/60 mesh silica sand
G-80 steel grit
S-110 steel shot
80 mesh garnet
100 aluminum oxide
Clemtex #4
Black Beauty 3060
Silver 120 Sponge Media
16/35 mesh silica sand
G-50 steel grit
S-170 steel shot
36 mesh garnet
50 grit aluminum oxide
Clemtex #3
Black Beauty 3060
16/35 mesh silica sand
G-40 steel grit
S-230 steel shot
36 mesh garnet
36 grit aluminum oxide
Clemtex #3
Black Beauty 2040
Silver 80 Sponge Media
2.5 mil Profile 3-4 mil Profile 5.0 mil Profile
8/35 mesh silica sand
G-40 steel grit
S-280 steel shot
16 mesh garnet
Clemtex #2
Black Beauty 2040
8/20 mesh silica sand
G-25 steel grit
S-330 or 390 steel shot
16 mesh garnet
Clemtex #2
Black Beauty 1240
Silver 30DG Sponge Media
Silver 30 Sponge Media
Silver 16 Sponge Media
S-440, S-550, or S-660
steel shot
Silver Aero-Alox 320
Silver Aero-Alox 320DG


Abrasive Blasting Types

There are three general types of abrasive blasting commonly used today:

Industrial Blasting: No real effort is made in this method to alleviate the dust hazard or to recycle the blasting abrasive. General containment of the surface preparation area is used to collect dust and contain abrasive materials from the environment. Extensive use of PPE is required.

Vacuum Blasting: Minimizes the dust hazard and reclaims the blast abrasive; better on cleaning repetitive, small scale surfaces.

Closed Cycle Blasting: Enclosed cabinet using abrasive that is continuously recycled. Most effective and economical method for removing mill scale. Generally used for small scale shop coating operations.

Abrasive Blast Standards

The standards for abrasive blast cleaning include:

White Metal Blast Cleaning (SSPC-SP 5/NACE No. 1)
A white metal blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxides, corrosion products, and other foreign matter.

Near-White Metal Blast Cleaning (SSPC-SP 10/NACE No. 2)
A near-white metal blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxides, corrosion products, and other foreign matter except for staining. Staining shall be limited to no more than 5% of each 3” x 3” square of surface area and may consist of light shadows, slight streaks, or minor discolorations caused by stains of rust, stains of mill scale or stains of previously applied paint.

Commercial Blast Cleaning (SSPC-SP 6/NACE No. 3)
A commercial blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxides, corrosion products, and other foreign matter, except for staining. Staining shall be limited to no more than 33% of each 3” x 3” square of surface area and may consist of light shadows, slight streaks, or minor discolorations caused by stains of rust, stains of mill scale or stains of previously applied paint. Slight residues of rust and paint may also be left in the bottoms of pits if the original surface is pitted.

Industrial Blast (SSPC-SP 14/NACE No. 8)
An industrial blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dust, and dirt. Traces of tightly adherent mill scale, rust, and coating residues are permitted to remain on 10% of each unit area of the surface (see Section 2.6) if they are evenly distributed. The traces of mill scale, rust, and coating shall be considered tightly adherent if they cannot be lifted with a dull putty knife. Shadows, streaks, and discolorations caused by stains of rust, stains of mill scale, and stains of previously applied coating may be present on the remainder of the surface.

Brush-Off Blasting (SSPC-SP 7/NACE No. 4)
A brush-off blast cleaned surface, when viewed without magnification, shall be free of all visible oil, grease, dirt, dust, loose mill scale, loose rust, and loose paint. Tightly adherent mill scale, rust, and paint may remain on the surface. Mill scale, rust and paint are considered tightly adherent if they cannot be removed by lifting with a dull putty knife.