The Society for Protective Coatings (SSPC) and NACE International issued joint standards which apply to the usage of blast cleaning abrasives to achieve varied degrees of steel surface cleaning before a lining system or protective coating is applied. Inspectors, applicators, coating and lining specifiers, and others employ these standards in determining the correct grade of surface cleanliness for a project.
SSPC-SP 6 (SSI-Sa2) / NACE No. 3
Standard SSPC-SP 6 (SSI-Sa2) / NACE NO. 3 delineates the minimum requirements for what is known as “commercial blast cleaning”, which employs a greater degree of cleaning than industrial blast cleaning. Commercial blast cleaning eradicates all visible corrosion products, oxides, coating, rust, mill scale, dirt, dust, grease, oil, and other foreign matter from all surfaces which can sometimes include pits. SP 6 does allow, on one-third of each square inch of surface area, for minimal streaks, discolorations, or shadows caused by mill scale oxides, stains caused by rust, or minor resides of remaining coating or paint. The remaining two-thirds of each square inch of surface area must be free of all visible residues.
SSPC-SP10 (SSI-Sa2 ½) / NACE No. 2
A different standard, Near-White Blast SSPC-SP10 (SSI-Sa2 ½) / NACE No. 2, requires more thorough cleaning than commercial blast cleaning. The objective of near-white blast cleaning is to remove all mill scale, rust, and coating, but when the added effort to remove 100 percent of all stains (required by white metal blasting) is found not to be necessary. The primary difference between SP 10 and SP 6 is that with SP 10, the same very minor shadows, very minor streaks or minimal discolorations caused by oxides, mill scale, stains caused by rust, or minute residues of coating or paint, can only remain on 5 percent of each square inch of surface area. The remaining 95 percent of the surface area shall be free of all visible residues. Per Blastal Coating Services Inc., “this is probably the best quality surface preparation that can be expected to today for existing plant facility maintenance work.”
SP 6 and SP 10
It can be useful in understanding these standards and the practice of blast cleaning itself to note some of the features the standards share. These features can be grouped into the categories of blast cleaning methods, blast cleaning abrasives, and procedures following blast cleaning.
Blast Cleaning Methods and Operations
Different means of surface preparation may be employed to attain a commercial or near-white blast cleaned surface.
- Using blast nozzles, compressed air, and abrasive in dry abrasive blasting.
- Adding a recirculating closed-cycle abrasive system with compressed air, blast nozzle and abrasive with dry abrasive blasting.
- Employing a recirculating closed-cycle system with centrifugal wheels and abrasive with dry abrasive blasting.
Blast Cleaning Abrasives
The choice of abrasive size and type shall be based on type of blast system used, the grade, type, and surface condition of the steel to be cleaned, and the finished surface to be produced.
Procedures Following Blast Cleaning
- Visible deposits of grease, oil, and other contaminants shall be removed to according to SP1 or another agreed upon method.
- Remaining surface imperfections shall be eliminated according to the requirements of the procurement documents.
The primary difference between SP 6 and SP 10 is the degree of cleanliness required from the blast cleaning. Again, these standards are minimums and should not be construed as a restriction on the usage of superior materials or procedures. It should also be noted that the standards do not necessarily address all possible safety and health problems.