Condensation on a Cold Steel Tank Between Blast and Prime
A crew is abrasive-blasting the interior of a carbon steel tank in an open shop in late autumn. Ambient temperature is 12 °C and the steel surface reads 11 °C on a contact thermometer; relative humidity is 68%. Blasting finishes on a 30 m² section and the crew breaks for lunch, leaving the blasted surface exposed for about 90 minutes before the primer is mixed and applied. On return, the surface looks uniformly dull grey with no visible rust, and the applicator wants to prime immediately to stay on schedule. The spec requires an SSPC-SP 10 near-white finish and a zinc-rich primer applied the same shift as blasting. The inspector has a sling psychrometer and a surface thermometer but no dew point meter.
1. Using the available instruments, determine whether the surface was at risk of condensation during the exposure period, and state the decision you would make about priming.
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Estimate dew point from the psychrometer reading. At 12 °C dry bulb and 68% RH, the dew point is roughly 6 °C (water condenses when air is cooled to about 6 °C at that moisture content). The steel is at 11 °C, so it is about 5 °C above dew point - above the common 3 °C margin, so bulk condensation on the steel is not indicated by those numbers.
However, the 90-minute exposure in an open shop is the real risk. A cold tank interior, warm moist air migrating in, or a localized cold spot (nozzle, support, shadowed area) can flash-rust or condense even when the bulk reading looks acceptable. Dull grey alone does not confirm cleanliness; SSPC-SP 10 requires removal of all visible contaminants, and any flash rust would violate the spec.
Decision: do not prime on the assumption of 'looks fine.' Re-verify with a dew point meter or wet-bulb/sling check at multiple locations, inspect for flash rust and moisture, and if any doubt exists re-blast or spot-clean the section before priming. Document the readings and the time gap.
2. The applicator argues that because no rust is visible, the surface still meets SSPC-SP 10. Evaluate this claim and describe what evidence would be needed to accept the surface.
Read the worked answer
Visible absence of rust is necessary but not sufficient for SSPC-SP 10. Near-white blast requires a uniform grey-white appearance with no oil, grease, dirt, mill scale, rust, coating, oxides, or other foreign matter, and a specified surface profile. A lunch break does not change the standard, but exposure can introduce moisture films, flash rust too fine to see at a glance, or airborne contamination.
Evidence to accept: (1) documented blast cleanliness against the SP 10 visual comparator or agreed reference, checked after the break; (2) surface profile measurement (replica tape or comparable) meeting the primer's requirement; (3) confirmation that no moisture or contamination is present; (4) dew point and surface temperature records showing the margin held throughout; (5) a solvent or detergent wash record if the original surface had contaminants.
If any of these cannot be shown, the correct call is to re-clean the affected area rather than accept on appearance alone.