What Are Soluble Salts?
Soluble salts in the context of steel surface preparation are ionic chemical compounds — primarily chlorides, sulphates and nitrates — that dissolve in water and remain on the steel surface after surface preparation. They are invisible to the naked eye. No visual surface preparation standard — not SP10, not SP5, not WJ-1 — can detect or control soluble salt contamination. Visual inspection alone tells you nothing about salt levels.
The primary sources of soluble salt contamination on steel structures are:
- Marine environment: Airborne sea salt deposits on offshore platforms, ships, coastal structures and pipelines. Chloride ion concentration in marine atmospheres can be orders of magnitude higher than inland industrial environments.
- Industrial atmosphere: Sulphur dioxide from combustion processes converts to sulphate deposits on steel surfaces over time. Power generation, refining and heavy industrial environments are the main sources.
- Contaminated blast media: Recycled abrasive media can accumulate chloride and sulphate contamination from previous use. Using contaminated media introduces salts into the surface profile during blasting.
- Existing coating contamination: Old coatings, particularly on marine structures, may contain soluble salt trapped beneath the film from previous coating cycles. Blasting through a contaminated coating layer can redistribute salts into the new surface profile.
- Wash water: If high-pressure water washing is used for surface pre-treatment and the water source contains dissolved chlorides (such as saline groundwater or untreated seawater), salts can be deposited on the steel surface.
Why Soluble Salts Cause Coating Failure
The mechanism is osmotic blistering — one of the most destructive coating failure modes in service.
When a coating is applied over steel carrying soluble salt contamination, the salts become trapped at the coating–steel interface. Salt solutions have a lower vapour pressure than pure water — they are hygroscopic and attract water molecules through the semi-permeable coating film by osmosis. Over time, water migrates through the coating, accumulates at the salt deposits, and generates localised pressure under the film. This pressure causes the coating to blister, delaminate and ultimately fail — from the inside out, beginning at the steel–coating interface.
The practical consequences: a coating system that might otherwise last 15–20 years in service can fail by blistering within 12–24 months if applied over a salt-contaminated surface. The higher the salt concentration, the faster and more severe the blistering. In immersion service, where water access through the film is continuous, failure can occur even faster.
Critically, this failure mode cannot be detected or predicted by visual inspection at the time of coating application. The surface looks acceptable. The coating looks uniform. The failure only becomes apparent months later, often in the worst possible location — an offshore structure mid-cycle, an internal tank lining already in service, or a pipeline that cannot be shut down for remediation.
How to Test for Soluble Salt Contamination
The standard field method for measuring soluble salt contamination on steel surfaces is the Bresle patch method, standardised in ISO 8502-6 (extraction) and ISO 8502-9 (conductivity measurement). The equivalent ASTM standard is ASTM D4940 for bulk conductivity testing of abrasive blast media.
The Bresle patch test — step by step
- Attach an adhesive Bresle patch (a small rubber cell with a defined area, typically 1250 mm²) to the prepared steel surface.
- Inject a measured volume of deionised water (typically 2–3 mL) into the patch using a syringe.
- Allow the water to remain in contact with the surface for a minimum of 10 minutes (or the time specified in the project specification).
- Extract the water from the patch using the syringe.
- Measure the conductivity of the extracted solution using a calibrated conductivity meter.
- Convert the conductivity reading to equivalent chloride concentration (µg/cm²) using the appropriate conversion factor or calibration curve.
Record the location, date and result for each test point. A minimum number of test points per unit area should be specified in the project documentation — testing density should increase in areas of known or suspected high contamination.
Specific ion test kits
For projects requiring differentiation between chloride, sulphate and nitrate ions — rather than total soluble salt as measured by conductivity — specific ion test strips and colorimetric kits are available. These can help identify the contamination source (marine chlorides vs. industrial sulphates) and guide remediation decisions.
Soluble Salt Contamination Limits
There is no single universal limit for soluble salt contamination that applies to all projects and service environments. The applicable limit depends on the service environment and the coating system. The following are the most commonly referenced thresholds:
| Application / Environment | Typical Maximum Limit | Reference |
|---|---|---|
| General industrial / atmospheric | 50–80 µg/cm² (as NaCl equivalent) | Coating TDS / project specification |
| Offshore atmospheric | 30–50 µg/cm² | NORSOK M-501 / coating TDS |
| Marine immersion / ballast tanks | 20–30 µg/cm² | IMO PSPC / coating TDS |
| Internal tank linings (potable water) | ≤10 µg/cm² | Coating TDS / water authority specification |
| Thermal spray (TSA/TSZ) | ≤30 µg/cm² | ISO 2063 / project specification |
These are indicative values. Always verify the applicable limit in the coating manufacturer’s TDS and the project specification. Where the TDS and the project specification conflict, the more stringent limit applies.
What Surface Preparation Does and Does Not Remove
This is the most important practical point: abrasive blasting alone does not reliably remove soluble salt contamination. Blasting removes mill scale, rust and coatings — but chlorides and sulphates embedded in pit corrosion or pressed into the steel surface profile by previous coating operations remain in place after blasting. In fact, aggressive blasting can redistribute salt contamination deeper into the anchor profile, making it harder to remove and harder to detect.
Methods that effectively remove soluble salt contamination:
- UHP water jetting: Water dissolves and flushes chlorides and sulphates from the surface. Highly effective, but introduces flash rust management requirements.
- Fresh water high-pressure washing: Effective for pre-treatment of heavily contaminated surfaces before blast cleaning. Not a substitute for blasting, but reduces salt load significantly.
- Wet abrasive blasting: The water component dissolves and removes salts during the blasting operation. More effective than dry blasting for salt removal.
- Chemical treatment: Proprietary salt-neutralising products applied to the prepared surface can convert chlorides into less harmful compounds. Effectiveness varies and must be verified by Bresle patch testing after treatment.
Dry mechanical tools do not dissolve ionic contamination the way water-based methods do. However, the Bristle Blaster® has been shown in MontiPower-commissioned MTEST laboratory testing (Dr. Prepper No. 4) to physically dislodge and remove rust-bound surface salts, reducing total surface salt levels significantly — from 115 µg/cm² (baseline) down to 5–22 µg/cm² by bristle blasting alone. When combined with a 3 ml surfactant pre-treatment, residual total salts were reduced to 1 µg/cm². Note: this study used only sodium chloride on rusted panels; results may vary for other salt types, steel conditions, or heavily pitted surfaces. Where soluble salt levels exceed the coating manufacturer’s limit on a surface to be mechanically prepared, pre-treatment with water washing, a surfactant wipe, or waterjetting is recommended to bring salt levels within specification, followed by mechanical preparation to restore the anchor profile.
Incorporating Salt Testing Into Your QC Programme
Soluble salt testing should be a mandatory hold point in the quality control plan for any coating project in a marine or industrial atmosphere. Best practice is:
- Test before surface preparation to establish baseline contamination level and identify high-risk areas.
- Test after surface preparation but before priming to confirm contamination is within specification.
- Re-test after any washing, waterjetting or chemical treatment to confirm effectiveness.
- Do not allow priming to proceed in an area that has not passed the salt contamination test.
Document all test results with location, date, time, temperature, RH and the instrument and calibration certificate used. This documentation is essential for warranty claims and for root cause analysis if coating failure occurs.
Key Takeaways
- Soluble salt contamination — primarily chlorides and sulphates — is invisible and cannot be detected by visual inspection or anchor profile measurement.
- Even a small amount of ionic contamination trapped under a coating film causes osmotic blistering that can destroy a coating system within months of application.
- The standard test method is the Bresle patch method (ISO 8502-6/9), measuring conductivity and converting to equivalent chloride concentration in µg/cm².
- Contamination limits vary by service environment — from 80 µg/cm² for general atmospheric to below 10 µg/cm² for potable water tank linings.
- Dry abrasive blasting does not reliably remove soluble salts. The Bristle Blaster® reduces surface salt levels through physical dislodgement of rust-bound contamination (as shown in MTEST testing, Dr. Prepper No. 4), but cannot dissolve and flush ionic contamination the way water-based methods can. Where salt levels exceed specification, water washing, surfactant pre-treatment or UHP jetting is required before mechanical preparation.
- Salt testing should be a mandatory hold point in every coating project QC plan on marine or industrial assets.