Wet Blasting, Chlorides, and a Marginal Dew Point on a Coastal Viaduct
A coastal viaduct's bottom flanges are being reblasted to SSPC-SP 10 (Near-White) with a 2.5–3.5 mil anchor profile; the specification also caps soluble chloride at 3 µg/cm². The existing system is a zinc-rich primer under epoxy and urethane, with rust bleeding at drain outlets and heavy pitting on the bottom flange. The contractor proposes wet blasting with a corrosion inhibitor to control dust and to reach chloride limits, arguing that the inhibitor prevents flash rust. After a test patch, chloride readings fall from 12 to 2 µg/cm², but the steel is at 8°C with a dew point of 5°C and the ambient RH is 88%; the product data sheet lists a maximum 85% RH. The inspector also measures a 4.2 mil profile in the pitted band and 3.0 mil on adjacent steel.
1. Should the inspector approve wet blasting with the corrosion inhibitor? What must be verified before coating?
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Conditional approval only. Wet blasting with an inhibitor is not automatically acceptable: the project specification may prohibit inhibitors, and many coating manufacturers require that residues be removed because they can cause intercoat adhesion loss. Verify:
- Written confirmation from the coating manufacturer that the specific inhibitor is compatible with the full system.
- Potable water and controlled inhibitor concentration.
- Chloride readings taken after the final rinse on a dry surface, not from the inhibitor-laden slurry. A reading of 2 µg/cm² before rinsing is not proof.
- A coated test patch with adhesion testing before production work.
The drop from 12 to 2 µg/cm² is encouraging but does not by itself establish compliance. If the specification bans inhibitors or the manufacturer will not confirm compatibility, use an alternative method such as dry blasting with dust suppression or a rinse-and-reblast sequence.
2. How should the 4.2 mil profile in the pitted band be evaluated against the 2.5–3.5 mil requirement?
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Anchor profile is measured on the intact steel surface, not inside pits. A 4.2 mil reading in the pitted band largely reflects pit depth and does not mean the surrounding anchor pattern is excessive. The 3.0 mil reading on adjacent steel is within the 2.5–3.5 mil range.
The real concerns are different: deep pits can trap blast debris, salts, and moisture, and high peaks can produce thin spots or pinholes in the primer. The inspector should:
- Confirm pits are clean by visual examination and targeted chloride extraction.
- Require a stripe coat or an additional primer coat to fill pits and cover peaks.
- Check DFT after cure rather than assuming the nominal coat covers the profile.
Do not order more blasting to reduce the 4.2 mil reading; that would only deepen the pits and waste steel.
3. May coating application proceed given the temperature, dew point, and humidity data?
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No. The relative humidity is 88%, above the product data sheet maximum of 85%. That limit is independent of the dew point margin. The steel is 3°C above dew point, which is only the common minimum margin, not a comfortable buffer; at 88% RH, a small temperature drop can put the steel at or below dew point.
Wet blasting also leaves a damp surface. Acceptable options are to dehumidify and heat the steel so that RH is below 85% and the steel is more than 3°C above dew point, then verify with calibrated instruments immediately before coating. If those conditions cannot be held through the induction and cure period, postpone application. Document the hold point and the readings.