Contaminated Abrasive, Low Profile and Salt on a Tank Shell
The specification requires SSPC-SP 10/NACE No. 2, 2.0–3.5 mil surface profile, and soluble salt of 3 µg/cm² or less. The contractor's garnet abrasive has been stored outdoors uncovered for two months; when the inspector opens a bag, the abrasive is damp, caked, and leaves a dark stain on a white rag. After blasting a 400 ft² shell section, the steel shows light shadows and dark streaks that follow the blast nozzle path. Profile tape readings are 1.7, 1.9, 2.1, 2.3 and 2.6 mils. A Bresle patch pulled 25 minutes after blasting reads 9 µg/cm² chloride. The blast pot operator says the streaks 'are just the steel' and offers to re-blast with the same abrasive. The primer is zinc-rich with a 4-hour overcoat window, and the shift ends in 5 hours.
1. Sort the findings into acceptable and nonconforming, and give the basis for each classification.
Read the worked answer
- Light shadows: near-white cleaning permits slight shadowing and discoloration, so shadowing alone is not the issue.
- Dark streaks following the nozzle path: this pattern points to the abrasive, not the steel. Contaminated or damp media smears residue and cuts unevenly. It is a nonconformance in practice because the surface is not clean steel.
- Profile: the specified floor is 2.0 mil. Readings of 1.7 and 1.9 mil are below the floor, so the section fails on profile regardless of the average (2.12 mil).
- Salt: 9 µg/cm² against a 3 µg/cm² limit is a clear failure. The test timing (25 minutes after blasting) is acceptable if the surface was dry and undisturbed.
- Abrasive condition: damp, caked media with a stain on a white rag indicates moisture and probable oil contamination; both will carry into the surface and the profile. Disposition: reject the section and do not prime. The operator's offer to re-blast with the same abrasive would simply redistribute the contamination and drive salts into the pits. The judgement is to separate appearance from the measurable acceptance criteria and then trace the cause.
2. Design the verification and rework sequence before any primer is applied.
Read the worked answer
- Quarantine the abrasive. Sample several bags across the stockpile and test: an aqueous slurry extraction for conductive soluble contamination (ASTM D4940 approach), a solvent wipe on a white cloth for oil, and a moisture check. Compare to the abrasive specification and to the coating's requirements.
- Source conforming media or dry and re-verify the existing stock; do not accept 'it looks dry' as evidence.
- Re-blast the section to SSPC-SP 10 with verified abrasive and verified air supply (clean, dry, oil-free). Check the compressor's separators and traps while you are at it.
- Re-verify profile at multiple points across the section using tape or replica, at the low and high ends of the range, recognising that profile varies with angle, distance and nozzle condition.
- Re-test salt by Bresle after re-blasting, not before. Blasting can both remove and embed chlorides; the number that matters is the one on the finished surface.
- Then apply within the coating window and record the hold points, times and results. If salt persists after a second blast, escalate: pitted steel may need a different removal method or an approved surface-tolerant system, and that is an owner decision, not the contractor's.
3. The contractor argues that re-blasting will consume the 4-hour overcoat window and asks the inspector to allow priming of the current surface. How do you respond and document it?
Read the worked answer
Refuse, and explain the sequence plainly: the overcoat window starts from a conforming primer application. It cannot be consumed by work that should never have produced a coatable surface.
- Record the rejection time, the specific failures (profile 1.7/1.9 mil below the 2.0 mil floor; 9 µg/cm² chloride against 3; contaminated abrasive), and the fact that the contractor was notified in writing.
- Note the schedule consequence as a documented delay caused by nonconforming preparation, so it is not later attributed to inspection.
- If the contractor wants to recover time, the practical route is verified clean abrasive, a defined area, and a primer application straight after acceptance, potentially with extended shift coverage agreed with the owner. The judgement here is independence under schedule pressure: priming a salty, low-profile surface produces an adhesion failure that surfaces months later as blistering or disbonding, and the inspector's signature would be on it.