Anchor Profile and Coating Selection on a New Shop-Fabricated Tank
A shop fabricator is blasting and priming new carbon steel tank shell plates for an internal lining. The lining is a 100% solids epoxy, 20-25 mils DFT, for immersion service in a mildly acidic (pH 4-5) process water. The fabricator's blasting program uses a steel grit abrasive that produces a dense, angular profile; the inspector measures 4.0-4.5 mils profile on the shell and 2.0 mils in the weld caps and heat-affected zones. The fabricator's QC tech says 'profile is profile' and points to an SSPC-SP 10 finish that looks uniform. The lining manufacturer's data sheet specifies 2.5-3.5 mils anchor profile and SSPC-SP 10/NACE No. 2 with a chloride limit of 3 µg/cm². The fabricator wants to prime all plates tonight and ship in three days; cure time at shop temperature (70°F) is 24 hours to handle.
1. Evaluate the profile readings against the lining requirement. What is the risk of over-profile on the shell and under-profile on the welds, and what should the inspector require?
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Both readings are out of spec, in opposite directions, and both matter for a 20-25 mil immersion epoxy.
Over-profile on the shell (4.0-4.5 mils vs. 2.5-3.5 mils required): an excessively deep, dense grit profile creates peaks that can protrude through a thin primer or cause 'picture-framing' and shadowing where the coating thins over peaks. For a thick epoxy the greater risk is entrapped air and incomplete wetting at the base of deep valleys, leading to pinholes and osmotic blistering in immersion service - precisely the failure mode a pH 4-5 environment will exploit. Over-profile also increases the surface area and the actual coating consumption, so the specified DFT may not be achieved at valleys.
Under-profile on welds and HAZ (2.0 mils vs. 2.5 minimum): welds and HAZ are the highest-stress, most corrosion-prone regions and the most common lining failure origin. Insufficient profile there means inadequate mechanical adhesion, and the dense, smooth weld cap may also carry slag or oxide that blasting did not fully remove.
The inspector should reject the lot, require re-blasting of the shell with a finer abrasive or adjusted parameters to bring profile into 2.5-3.5 mils, and require additional/angled blasting of welds to reach the minimum. Profile must be verified by replica tape or comparable method at multiple points, including welds, not by visual 'uniformity.'
2. The fabricator wants to prime tonight. What pre-prime verifications and hold points should the inspector enforce before releasing the plates?
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Release requires more than a visual blast check. The inspector should enforce these hold points:
- Surface cleanliness: SSPC-SP 10/NACE No. 2 verified by visual comparator, including welds, with attention to slag, weld spatter, and laminations.
- Profile: 2.5-3.5 mils confirmed by replica tape at a documented sampling plan covering shell, welds, and HAZ.
- Soluble salts: Bresle or equivalent at ≤3 µg/cm² chloride, tested after blasting (blasting can redistribute contamination) and at representative locations including welds.
- Ambient/dew point: steel at least 5°F above dew point, RH and temperature within the product data sheet window; record readings at the time of prime.
- Surface condition at prime time: no visible moisture, no rust bloom, no dust (verify with tape or visual per the spec). If any rust bloom appears between blast and prime, re-blast that area.
- Product verification: correct lining material, batch, mix ratio, induction time, and pot life per data sheet.
Only after all six are documented should the inspector sign the hold point. The three-day ship date is the fabricator's commercial risk, not a basis for waiving hold points; if the schedule cannot accommodate cure, the correct action is to flag the conflict to the owner's representative in writing.