Where the salts come from
Marine and de-icing atmospheres deposit chlorides; combustion products and industrial atmospheres deposit sulfates; process fluids, contaminated abrasive and wash water add their own. Salts concentrate at the bottom of pits and within corrosion products, where dry abrasive blasting does not remove them and can even drive them deeper into the profile. A surface can meet a visual cleanliness grade and still carry enough salt to fail the coating.
Why they matter
Coating films are slightly permeable to water. Salt trapped at the steel-coating interface draws water through the film by osmosis, forming blisters and restarting corrosion under the film. Immersion service, such as tank linings and water boxes, is the most sensitive case. Salts also accelerate flash rust on freshly blasted steel.
How they are measured
| Method | What it does |
|---|---|
| ISO 8502-6 (Bresle method) | An adhesive patch cell is applied to the steel, a known volume of deionized water is injected and extracted, dissolving the salts from a defined area |
| ISO 8502-9 | Conductometric determination of the extracted solution, reported as surface density of soluble salts |
| ISO 8502-5 | Chloride determination with ion detection tubes |
| SSPC-Guide 15 | Field methods for retrieving and analyzing soluble salts |
Conductivity responds to all dissolved ionic species, while chloride-specific methods measure chloride only, so the results are not interchangeable. The specification should state the extraction method, the analysis method and the unit of the limit. The abrasive itself can be checked for soluble contaminants by conductivity under ASTM D4940.
Setting and meeting the limit
The acceptance threshold is set by the project specification or the coating manufacturer, often stricter for immersion than for atmospheric service. Surface preparation grades such as SSPC-SP 10/NACE No. 2 do not set it. Tests are taken after final blasting, at representative locations including pitted areas. If a result exceeds the limit, the usual sequence is washing with clean water, often at high pressure, then re-blasting and re-testing until the surface passes. The NACE No. 2 surface preparation guide places the salt test within the full inspection checklist.