Surface Preparation Grade Dispute After Abrasive Blasting
A blast crew has prepared a steel substrate to what they claim is SSPC-SP 10 / NACE No. 2 (Near-White Metal Blast Cleaning). The steel originally had tight mill scale and some pitting. After blasting, the inspector observes: uniform gray-white metal on most of the surface, but roughly 5–10% of the area still shows faint, tightly adherent dark staining in the bottom of pits and in a few patches. The blast profile reads 2.5 mils (63 µm), within the 2–3 mil spec. The contractor argues that 'SP 10 allows 5% staining' and wants to proceed. Ambient conditions are acceptable.
1. Evaluate the contractor's claim about SP 10 and staining. What does the standard actually require?
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The contractor is misapplying a remembered figure. Under SSPC-SP 10 / NACE No. 2 (Near-White), the surface shall be free of all visible oil, grease, dirt, dust, mill scale, rust, paint, oxide, corrosion products, and other foreign matter except for random staining limited to no more than 5% of the surface area - and that staining must be tightly adherent and is typically described as light shadows, streaks, or discoloration. It does not permit residual rust or mill scale in pits.
The key judgment: staining ≠ corrosion product in pits. Dark material lodged in pit bottoms is residual corrosion/scale, not acceptable 'random staining.' The 5% figure is a limit on staining, not a license to leave contamination. The inspector should not accept the surface as SP 10 based on the contractor's paraphrase.
2. What should the inspector do, and how should the residual material in pits be handled?
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- Reject the surface as not meeting SP 10. Document with photos and area estimates.
- Re-blast the affected areas, paying attention to pit bottoms where the nozzle angle and dwell time were insufficient. Tightly adherent material in pits often requires a steeper angle and more dwell.
- Re-inspect. If pitting is deep, confirm the specified grade is achievable; if not, the spec/owner may need to address pitting separately (e.g., stripe/penetrating primer), but that is a specification decision, not the inspector's to waive.
- Verify profile remains within 2–3 mils after re-blast (re-blasting can increase profile).
- Coat only after the surface meets the grade and dew-point margin.
The judgment: don't accept a grade based on a remembered percentage; inspect against the actual standard and distinguish staining from contamination.
3. The profile is in spec at 2.5 mils. Does that support accepting the surface? Explain.
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No. Profile and cleanliness are independent requirements. A correct anchor profile (2–3 mils) says the surface is roughened adequately for adhesion; it says nothing about whether rust, scale, or salts remain. A surface can have a perfect profile and still fail the cleanliness grade - and coating over residual corrosion products in pits leads to early corrosion, blistering, and disbondment.
The inspector must satisfy both: the specified surface preparation grade (SP 10) and the specified profile. Passing one does not excuse failing the other.