Surface Preparation for Immersion Service
A carbon steel tank interior is being prepared for lining with a 100% solids epoxy. The specified surface preparation is NACE No. 2/SSPC-SP 10 (near-white metal blast cleaning). The contractor proposes using a centrifugal blasting machine with steel shot. Ambient temperature is 10°C, relative humidity 85%, and steel temperature is 12°C. The blast media is contaminated with oil from a previous job. The contractor argues that the oil will burn off during blasting. The inspector must decide whether to approve the surface preparation.
1. Should the inspector approve the use of oil-contaminated steel shot? Explain.
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No, the inspector should not approve oil-contaminated steel shot. Oil contamination will be embedded into the steel surface during blasting, creating a barrier that prevents proper adhesion of the epoxy lining. Even if some oil burns off, residues will remain and cause premature failure. The blast media must be clean and free of oil, grease, and other contaminants. The inspector should require the contractor to replace the media with clean, approved abrasive. Additionally, the ambient conditions (10°C, 85% RH, steel temp 12°C) are borderline: the steel temperature is only 2°C above dew point, which is too close; coating application should be postponed or the environment controlled to maintain steel temperature at least 3°C above dew point and within the coating manufacturer's recommendations.
2. What alternative surface preparation method could be considered if blasting is not feasible, and what are the limitations?
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If blasting is not feasible, power tool cleaning to SSPC-SP 11 (power tool cleaning to bare metal) or SSPC-SP 3 (power tool cleaning) could be considered, but these methods are not equivalent to near-white metal blast cleaning and may not provide the required anchor profile for a high-performance epoxy lining. The specified NACE No. 2/SSPC-SP 10 is necessary for immersion service to ensure optimal adhesion and to remove all mill scale, rust, and contaminants. The inspector should not accept a lower standard without approval from the coating manufacturer and the owner. If power tool cleaning is used, the surface must be assessed for profile and cleanliness, and the coating system may need to be upgraded to one with greater tolerance for surface imperfections.