Chloride Contamination on a Salt-Exposed Girder: Average Results vs. Failing Locations
A 40-year-old steel girder carries a road over a tidal estuary and receives de-icing runoff. The specification requires abrasive blast to Sa 2½, a 50–85 µm profile, chloride below 5 µg/cm², and coating within 4 hours of blasting. At 09:00 the crew reports blasting complete. Conditions: air 28 °C, RH 78 %, steel 31 °C, dew point 24 °C. Replica tape gives 60–70 µm. Ten Bresle tests return 2, 3, 2, 3, 2, 3, 2, 22, 31 and 3 µg/cm²; the two high results are on the bottom flange beneath a blocked drainage scupper. The abrasive is recycled coal slag from an uncovered pile, last sampled three months ago. Rain is forecast in 18 hours. The contractor proposes spot-blasting only the two high readings and coating the whole girder today.
1. Work through each acceptance criterion and decide whether the girder may be coated today.
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Environmental window - passes, but narrowly. Steel at 31 °C against a 24 °C dew point gives a 7 °C margin; most specifications require at least 3 °C, and 78 % RH is under the usual 85 % product limit. Both hold now, but neither will hold indefinitely on a tidal structure, so the clock matters.
Profile - passes. 60–70 µm sits inside the 50–85 µm band; nothing to challenge.
Soluble salt - fails. Eight of ten readings are 2–3 µg/cm², but acceptance is not a vote. Two locations at 22 and 31 µg/cm² are four to six times the limit, and they coincide with where runoff ponds. A hygroscopic salt film left under a coating pulls water through the film osmotically and produces blistering and disbondment months later, at a repair cost far above the cost of doing it now.
Decision: the girder cannot be coated today. The 4-hour blast-to-coat window will expire while the salt issue is being resolved, so those areas will need re-blasting regardless.
2. The contractor argues that two failing readings over less than 5 % of the area justify coating the remainder and dealing with those spots later. Respond, and state what must happen before any coating is applied.
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Reject the argument, but on mechanism rather than on percentage. Chlorides are mobile. Once dissolved, they wick along the steel surface and into pits and laminations beyond the sampled spot, and blasting can drive them deeper into the profile rather than remove them. The eight passing readings do not prove the steel between them is clean; they prove the sampling grid was too coarse for a known contamination source.
What must happen first:
- Fix the blocked scupper. Coating a surface that is still being wetted from above guarantees a repeat failure and destroys any warranty position.
- Re-clean the bottom flange and at least 300 mm either side by re-blasting or high-pressure fresh-water washing, then re-test on a denser grid, including under laps and at welds.
- Sample the abrasive pile. An uncovered marine-site pile is a credible chloride source; test it by the relevant conductivity method and reject or replace it. Contaminated abrasive would invalidate every reading on the job.
- Check for oil carry-over from the compressor, which the same sampling would reveal.
Only after repeat readings are all under 5 µg/cm², with the source eliminated, is the surface coatable.
3. Rain is forecast in 18 hours. Sequence the actions you would authorise and the documentation you would require, including what happens if the blast-to-coat window is missed.
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Sequence:
- Stop coating; issue a non-conformance record with the ten readings, the grid map, and photographs of the scupper.
- Repair the drainage and re-sample the abrasive before any further blasting, since both affect the whole surface.
- Re-clean and re-test the affected zone in a single mobilisation, so the structure is not left half-blasted over a rain event.
- Coat only within the verified window; if rain arrives first, the blasted steel is left to weather and must be assessed for rust-back before coating.
If the window is missed: do not simply coat over the resulting flash rust. Determine the rust grade against the specification's permitted level; light, uniform staining may be acceptable to some specifications while pitting or heavy rust requires re-blasting. Record the judgement with the evidence that supports it.
Documentation: the sampling grid and instrument calibration, the abrasive test report, the non-conformance and its disposition, environmental readings at the time of coating, and a signed record of who accepted the re-test. The point is that a later failure can be traced to a decision, not to an absence of data.