Загрязнение растворимыми солями: почему оно приводит к разрушению покрытия и как его проверить

Soluble salt contamination is one of the most under-specified variables in industrial surface preparation, and a common root cause of premature coating failure. A surface can pass visual inspection for cleanliness, show an adequate anchor profile, and still carry enough ionic contamination to cause osmotic blistering after the coating is applied. This article explains what soluble salts are, why they cause failure, how to test for them, which limits are used, and which preparation methods actually reduce them.

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 SP 10, not SP 5, 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 deposition in marine atmospheres can be orders of magnitude higher than in inland environments.
• Industrial atmosphere: sulphur dioxide from combustion processes forms sulphate deposits on steel surfaces over time. Power generation, refining and heavy industrial environments are the main sources.
• Contaminated blast media: recycled abrasive can accumulate chloride and sulphate contamination from previous use. Using contaminated media introduces salts into the surface profile during blasting. Abrasives are tested for this by conductivity per ASTM D4940.
• Existing coating contamination: old coatings, particularly on marine structures, may have 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 pressure washing is used for pre-treatment and the water contains dissolved chlorides (such as saline groundwater or untreated seawater), salts are deposited on the steel surface instead of removed.

Why do 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: the salts are hygroscopic and draw water through the semi-permeable coating film by osmosis. Over time, water accumulates at the salt deposits and generates localised pressure under the film. The coating blisters, corrosion starts under the film, and the system fails from the inside out, beginning at the steel–coating interface.

The practical consequence is that a coating system designed for a long service life can fail by blistering within the first months or years if it is applied over a salt-contaminated surface. The higher the salt concentration, the faster and more severe the blistering. In immersion service, where water reaches the film continuously, failure can occur even faster.

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 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 do you 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 (conductometric determination). ISO 8502-9 measures all ionic contaminants together and reports a surface density of water-soluble salts; it does not identify individual salts such as chlorides or sulphates. SSPC Guide 15 describes further field extraction and analysis methods. ASTM D4940 is a different test: it applies to blast cleaning abrasives, not to the steel surface.

The Bresle patch test, step by step

1. Attach an adhesive Bresle patch (a flexible cell with a defined test area, typically 1,250 mm²) to the prepared steel surface.
2. Inject a measured volume of deionised water (a few millilitres, as specified for the patch size) into the patch using a syringe.
3. Leave the water in contact with the surface for the time given in the procedure or project specification, cycling it in and out of the syringe to dissolve the salts.
4. Extract the water from the patch using the syringe.
5. Measure the conductivity of the extracted solution (µS/cm) using a calibrated conductivity meter, correcting for the conductivity of the blank water.
6. Convert the conductivity reading to a surface density of soluble salts (mg/m² or µg/cm², usually as NaCl equivalent; 10 mg/m² = 1 µg/cm²) using the formula in ISO 8502-9 or the instrument's conversion.

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, and testing density should increase in areas of known or suspected high contamination.

Ion-specific test kits

Where the specification sets limits for individual ions (chloride, sulphate, nitrate or ferrous iron) rather than total conductivity, ion-specific test strips, titration kits and colorimetric methods are available. They also help identify the contamination source (marine chlorides versus industrial sulphates) and guide remediation decisions.

What are typical soluble salt limits?

There is no single universal limit for soluble salt contamination. The applicable limit depends on the service environment, the coating system and the owner's specification. Note the units: published limits are given either as total salts (NaCl equivalent) or as a specific ion (usually chloride), and in mg/m² or µg/cm². Mixing them up produces ten-fold errors. Published examples:

These are examples, not recommendations for a given project. Always verify the applicable limit in the coating manufacturer's TDS and the project specification. Where the two conflict, the more stringent limit normally applies. How to write the limit into a contract is covered in how to write a surface preparation specification.

What surface preparation does and does not remove salts?

This is the most important practical point: dry abrasive blasting alone does not reliably remove soluble salt contamination. Blasting removes mill scale, rust and coatings, but chlorides and sulphates held at the bottom of corrosion pits or in the profile can remain after blasting. KTA-Tator notes that distinct rust spots forming overnight on a freshly blast-cleaned surface are a strong indication that salts are present. Blasting can also redistribute contamination across the surface, which is why salts concentrated in pitting corrosion deserve particular attention.

Methods that reduce soluble salt contamination:

• Water jetting (HP/UHP): water dissolves and flushes chlorides and sulphates from the surface and from pits. Highly effective, but introduces flash rust management requirements; see water jetting standards WJ-1 to WJ-4 and flash rust prevention.
• Fresh-water pressure washing: effective as pre-treatment of heavily contaminated surfaces before blast cleaning. Not a substitute for blasting, but reduces the salt load significantly.
• Wet abrasive blasting: the water component dissolves and removes salts during blasting. Generally more effective than dry blasting for salt removal.
• Soluble salt removers: proprietary liquid products, used in the wash water or applied directly to the surface, help dissolve and lift salts from the steel. Effectiveness varies and must be confirmed by re-testing after treatment.

Bristle blasting and soluble salts

Dry mechanical tools do not dissolve ionic contamination the way water-based methods do. However, MontiPower-commissioned MTEST laboratory testing (Dr. Prepper No. 4) found that the Bristle Blaster® physically dislodged rust-bound surface salts, reducing total surface salt levels from 115 µg/cm² (baseline) 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. The study used only sodium chloride on rusted panels; results may vary for other salt types, steel conditions and heavily pitted surfaces. In the field, a bristle-blasted field joint for Total E&P in Bolivia recorded a salt result of 1.4 µg/cm² (project report).

Where soluble salt levels exceed the coating manufacturer's limit on a surface to be mechanically prepared, pre-treat with fresh-water washing, a surfactant or salt-remover wash, or water jetting to bring salt levels within specification, then prepare mechanically to restore the cleanliness and anchor profile, and re-test before priming.

How to build 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:

1. Test before surface preparation to establish the baseline contamination level and identify high-risk areas.
2. Test after surface preparation but before priming to confirm contamination is within specification.
3. Re-test after any washing, water jetting or chemical treatment to confirm effectiveness.
4. 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. Profile readings belong in the same record; see the anchor profile measurement field guide.

Key takeaways

• Soluble salt contamination, primarily chlorides and sulphates, is invisible and cannot be detected by visual inspection or anchor profile measurement.
• Ionic contamination trapped under a coating film causes osmotic blistering that can destroy a coating system long before its design life.
• The standard field method is the Bresle patch (ISO 8502-6) with conductometric measurement (ISO 8502-9), reported as a surface density of soluble salts in mg/m² or µg/cm².
• Limits depend on the service environment and specification: for example 50 mg/m² NaCl equivalent for IMO PSPC ballast tanks, and 3–5 µg/cm² chloride in US Navy practice. Watch the units.
• Dry abrasive blasting does not reliably remove soluble salts. The Bristle Blaster® reduced surface salts through physical dislodgement in MTEST laboratory testing (Dr. Prepper No. 4), but it does not dissolve and flush salts like water-based methods. Where salt levels exceed specification, use water washing, salt removers or water jetting before mechanical preparation.
• Salt testing should be a mandatory hold point in every coating project QC plan on marine or industrial assets.

Frequently asked questions

What is an acceptable soluble salt level before coating?

There is no universal value; the coating data sheet and project specification decide. Published examples: IMO PSPC allows 50 mg/m² NaCl equivalent (5 µg/cm²) in ballast tanks; US Navy limits are 3 µg/cm² chloride for immersion and 5 µg/cm² for non-immersion service. Immersion service always needs the lowest limits.

How do you test steel for soluble salts?

Attach a Bresle patch (ISO 8502-6), inject deionised water, extract it after the specified contact time and measure its conductivity (ISO 8502-9). The result is converted to a surface density of soluble salts in mg/m² or µg/cm². Ion-specific kits can separately identify chlorides, sulphates or ferrous ions.

Does abrasive blasting remove soluble salts?

Not reliably. Dry blasting removes rust, scale and coatings, but salts held in pits and at the base of the profile can remain or be redistributed. Fresh-water pressure washing, salt-removing additives, wet abrasive blasting or water jetting are the usual remedies, followed by re-testing before priming.

Can bristle blasting reduce soluble salt levels?

In MontiPower-commissioned MTEST laboratory tests on NaCl-contaminated rusted panels, bristle blasting reduced total surface salts from 115 µg/cm² to 5–22 µg/cm², and to 1 µg/cm² with a surfactant pre-treatment. It does not dissolve salts like water-based methods, so test after preparation and pre-wash where levels exceed the limit.

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