What the standard requires
SSPC-SP 5/NACE No. 1, now maintained under AMPP, sets a visual criterion with no tolerance. After blasting, the entire surface must show bare metal, with no visible residue of mill scale, rust, coating or contaminants, and no stains or shadows. It is the only SSPC blast grade with no allowance for staining. The ISO 8501-1 grade usually compared with it is Sa 3, but the two systems are written differently, so the specification should state which one governs.
How it compares with other blast grades
| Grade | SSPC/NACE | ISO 8501-1 (approximate) | Staining allowed |
|---|---|---|---|
| White metal | SSPC-SP 5/NACE No. 1 | Sa 3 | None |
| Near-white metal | SSPC-SP 10/NACE No. 2 | Sa 2½ | Limited, per the standard |
| Commercial | SSPC-SP 6/NACE No. 3 | Sa 2 | Larger allowance, per the standard |
| Brush-off | SSPC-SP 7/NACE No. 4 | Sa 1 | Tightly adherent material may remain |
When it is specified
- linings in severe immersion or chemical service, where the coating manufacturer requires it;
- some metallizing and thermal spray specifications;
- high-performance systems whose data sheet calls for SSPC-SP 5;
- cases where the owner's specification requires the highest cleanliness.
For many atmospheric systems, near-white metal blast or commercial blast is the grade the data sheet requires, and going further adds time and abrasive without a matching benefit.
Practical points
White metal is demanding to reach and to keep. It requires sharp, clean abrasive, adequate air supply and enough time on pitted or heavily scaled steel. Steel can begin to rerust quickly, so the grade must be present at the moment of coating, not only after blasting. Controlling humidity and steel temperature against the dew point, removing dust, testing for soluble salts and checking the surface profile per ASTM D4417 are part of the same inspection. The photographic guide SSPC-VIS 1 helps with visual comparison.
Where white metal is specified over a large area, the schedule and abrasive consumption should reflect it, since reaching and holding this grade takes longer than lower grades.
White metal and other blast grades are part of our abrasive blasting service.