Soluble Salt Contamination
Invisible chlorides, sulfates and nitrates on prepared steel can cause blistering and under-film corrosion. How to test for them, set limits and remove them.
Key takeaways
- Soluble salts on prepared steel, mainly chlorides, sulfates and nitrates, are invisible but can cause osmotic blistering and under-film corrosion.
- Dry abrasive blasting often fails to remove salts trapped in pits. Washing, water jetting or wet abrasive blasting is usually needed.
- Salts are extracted from the surface, for example with an ISO 8502-6 patch, then analyzed by conductivity (ISO 8502-9) or ion-specific tests. SSPC Guide 15 reviews field methods.
- There is no universal limit. Specify the ion, the extraction method and the maximum. Immersion service usually gets the tightest limits.
- Watch the units: 1 µg/cm² equals 10 mg/m².
A freshly blasted surface can meet SSPC-SP 10/NACE No. 2 perfectly and still be contaminated. Soluble salts leave no visible trace, yet a few micrograms per square centimetre under a coating can be enough to start blisters or corrosion. This is particularly true for tank linings and other immersion service. Testing for salts and removing them is now routine on high-performance work, and it is required on many marine, bridge and lining projects.
What soluble salts are and where they come from
Soluble salts are ionic compounds that dissolve in water. The ones that matter most for coatings are:
- Chlorides from seawater, salt spray and de-icing salts. They are the most commonly specified and among the most aggressive toward steel.
- Sulfates from industrial and urban atmospheres, combustion products and some process environments.
- Nitrates from agricultural areas, fertilizers and combustion sources.
Salts build up during service, storage and transport, and they can also come from contaminated abrasive or wash water. They concentrate in pits, crevices, at the backs of angles and flanges, and on horizontal surfaces where moisture collects. When steel corrodes in a chloride environment, iron chloride forms at the base of pits, where it stays protected beneath corrosion products.
How salts cause coating failure
Coatings are not perfect barriers; water vapor slowly permeates them. Salt trapped at the interface draws that water in by osmosis and forms a concentrated solution. The solution builds pressure under the film, causing osmotic blistering. It also acts as a conductive electrolyte that drives corrosion at the steel surface (see corrosion science basics). Once corrosion starts under the film, it can spread as undercutting and rust creep.
Salts are also hygroscopic, meaning they attract moisture from the air. That is why blasted steel carrying salts often “rust blooms” within hours, even when conditions look acceptable. Rapid, spotty re-rusting after blasting is a classic field sign of salt contamination.
Testing for soluble salts
Field testing has two steps. First you extract salts from a known area of the surface into a known volume of high-purity water. Then you analyze the solution. SSPC Guide 15 describes the field methods for steel and other non-porous surfaces.
| Step | Method | Reference | Measures / notes |
|---|---|---|---|
| Extraction | Adhesive patch (Bresle) cell | ISO 8502-6 | Standard cell area 1,250 mm²; water injected and withdrawn by syringe |
| Extraction | Sleeve or cell with extraction solution | SSPC Guide 15 | Proprietary kits; results depend on the kit’s extraction efficiency |
| Extraction | Surface swabbing | SSPC Guide 15 | Simple, but extraction efficiency is less consistent |
| Analysis | Conductivity | ISO 8502-9 | All soluble ions, often reported as NaCl equivalent |
| Analysis | Chloride titration strip or detection tube | SSPC Guide 15 | Chloride ion only |
| Analysis | Sulfate (turbidity) and nitrate test strips | SSPC Guide 15 | Ion-specific; used where those ions matter |
Patch test with conductivity
- Choose locations. Test the final prepared surface, favoring pitted areas, crevices, horizontal ledges and previously corroded zones.
- Check the water. Measure the conductivity of the deionized or distilled water as a blank.
- Apply the patch. Press the adhesive cell firmly so no channels leak.
- Extract. Inject the specified volume of water (commonly 3 mL for the standard cell). Cycle it in and out with the syringe several times while keeping the needle in the cell.
- Measure. Withdraw the solution and read its conductivity with a calibrated meter, then subtract the blank.
- Calculate. Convert to surface density using ISO 8502-9. With a 1,250 mm² cell and 3 mL of water, the conversion works out to roughly 1.2 mg/m² of NaCl equivalent per µS/cm. Recalculate for any other area or volume.
No field method recovers every salt on the surface, and extraction efficiency differs between methods. A conductivity reading includes all ions, while a chloride strip reads chloride alone. Results from different methods are therefore not directly comparable. Always specify the extraction and analysis method along with the limit.
When to test in the work sequence
Salt testing is most useful at three points:
- Before preparation: to judge how contaminated the structure is and whether a wash step or wet method is needed. This is especially useful on maintenance work on marine structures, bridges over salted roads, and chemical plants.
- After washing or water jetting: to confirm the removal method is working before the whole area is processed.
- Immediately before coating: the acceptance test. It confirms the surface meets the limit after all preparation, including any final sweep blast.
Record each test’s location, method, water volume, blank reading and result, so the data can be traced if a failure is investigated later.
Acceptance limits
No single limit is universally accepted. Limits come from the owner’s specification, the coating manufacturer or an industry rule. The ranges below are commonly seen in specifications for chloride and are shown for orientation only.
| Service | Commonly specified chloride maximum |
|---|---|
| Immersion, tank linings | ~3–5 µg/cm² (30–50 mg/m²) |
| Severe atmospheric (marine, splash zone) | ~5–7 µg/cm² (50–70 mg/m²) |
| General atmospheric | ~5–10 µg/cm² (50–100 mg/m²) |
| Ship ballast tanks (IMO PSPC) | 50 mg/m² total soluble salts as NaCl, by conductivity |
Some specifications also set separate limits for sulfates and nitrates, or a total conductivity limit. Use the project’s numbers, units and methods exactly. Mixing up µg/cm², mg/m² and µS/cm is one of the most common reporting errors.
Removing soluble salts
- Pre-wash: pressure-wash heavily contaminated steel with clean, fresh water before blasting, so blasting does not spread salts around.
- Wet abrasive blasting: water helps dissolve and flush salts out of pits while the abrasive cleans.
- Water jetting: high- and ultrahigh-pressure water jetting is effective at removing salts but does not create profile.
- Blast–wash–blast: blast to open the pits, wash to dissolve the exposed salts, then re-blast lightly to remove the flash rust.
- Salt-removal additives: commercial soluble-salt removers can improve washing. The coating manufacturer must accept any residue they leave.
- Clean abrasive: verify abrasive cleanliness. SSPC-AB 1 includes a conductivity requirement for mineral and slag abrasives, tested per ASTM D4940.
Retest after the final preparation step, not after washing. Salts can be re-deposited by dirty wash water, contaminated abrasive or splash from adjacent work. Only the surface you are about to coat counts.
Concrete can also carry soluble salts, which show up as efflorescence or come from de-icing salts and chemical exposure. Remove these during concrete surface preparation, and allow the concrete to dry before coating.
Frequently asked questions
Can I see soluble salts on blasted steel?
No. The salt levels that cause coating failure are invisible. Rapid, spotty rust bloom after blasting is a warning sign, but only a test confirms contamination.
Won’t a thicker coating seal the salts in?
A thicker film slows water permeation but does not stop it. Over time, salts under even a thick film can drive osmotic blistering, especially in immersion service.
How many salt tests should I take?
Follow the specification. If it is silent, agree a frequency based on area and exposure before work starts, and concentrate tests where contamination is most likely: pits, crevices and horizontal surfaces.
How do I convert µg/cm² to mg/m²?
Multiply by 10. For example, 5 µg/cm² equals 50 mg/m².
Educational reference. Coating performance varies by formulation. Always follow the manufacturer’s product data sheet, safety data sheet and your project specification.
