Surface Preparation / Testing
Soluble Salt Testing: Why Testing Before Coating Matters
Water-soluble salts can remain on steel even after apparently thorough surface preparation. If they are not identified and adequately controlled before coating application, they can contribute to corrosion, osmotic blistering and premature coating failure.
Why soluble salt contamination matters
Successful protective coating application depends on more than achieving the required visual standard of surface preparation.
A steel surface can appear clean while still containing invisible water-soluble contamination.
Chlorides, sulphates and other ionic contaminants may remain on the substrate following exposure to the environment, previous service conditions or the surface preparation process itself.
If contamination remains beneath a subsequently applied coating system, it can contribute to corrosion activity and premature deterioration of the coating.
This makes soluble salt testing an important part of surface preparation quality control, particularly on existing steelwork and structures exposed to aggressive environments.
Where do soluble salts come from?
Soluble contamination can originate from a number of sources.
These can include:
Marine and coastal environments
Road salts and de-icing materials
Industrial atmospheric contamination
Previous service conditions
Contaminated water
Contaminated abrasive
Existing corrosion products
Contamination retained within corrosion pits
The history and exposure environment of the structure therefore matter when assessing the risk.
Structures exposed to marine conditions or de-icing salts can present a particularly obvious risk, but salt contamination should not automatically be dismissed simply because an asset is inland.
Why blasting alone may not solve the problem
Abrasive blast cleaning is extremely effective at removing coatings, corrosion products and producing the surface profile required by many protective coating systems.
However, achieving a visually clean blast standard does not itself demonstrate that the surface is free from soluble contamination.
Salts can remain within corrosion pits and other surface irregularities. This is particularly relevant when dealing with heavily corroded or previously exposed steelwork.
It is therefore possible to have steel that visually appears extremely clean while soluble contamination remains present.
This is one of the reasons why visual assessment and contamination testing should be considered as different aspects of surface-preparation inspection.
What can happen if salts are coated over?
Residual soluble salts beneath a coating can contribute to electrochemical corrosion processes when moisture becomes available.
They can also contribute to osmotic effects beneath the coating film.
This can manifest itself as coating defects including blistering, loss of adhesion, underfilm corrosion and premature coating breakdown.
The consequences may not become visible immediately after application. A coating can initially appear satisfactory and deterioration may develop later in service.
Identifying contamination before coating application is therefore considerably preferable to investigating its consequences after a coating system has failed.
How are soluble salts tested?
One widely used field approach is commonly known as the Bresle method.
ISO 8502-6 describes the extraction of water-soluble contaminants from a surface for subsequent analysis.
A controlled area of the surface is exposed to a known quantity of suitable water within a sealed cell or patch. Water-soluble material present on the test area is dissolved into the solution.
Conductivity measurement can then be used to assess the extracted ionic contamination.
ISO 8502-9 describes the field method for conductometric determination of water-soluble salts on steel surfaces.
The result provides an assessment of the overall water-soluble ionic contamination represented by conductivity. It should not automatically be interpreted as identifying the concentration of an individual salt such as chloride.
Where identification or measurement of a particular ionic species is required, an appropriate ion-specific method may be necessary.
When should testing take place?
The appropriate testing regime should be established by the project specification, Inspection and Test Plan and the requirements of the coating system.
Depending upon the project, testing may be appropriate before surface preparation, following cleaning operations and before coating application.
Testing before preparation can help establish the existing level of contamination and inform the preparation strategy.
Testing after preparation provides evidence of the condition of the surface at a critical stage before the coating system is applied.
Where cleaning is undertaken because an initial result is unacceptable, further testing may be required to demonstrate the condition of the surface after remedial cleaning.
Test location matters
A single test result should not automatically be assumed to represent an entire structure.
Contamination can vary significantly across an asset.
Areas particularly worth considering can include:
Heavily corroded steel
Deeply pitted areas
Horizontal surfaces
Drainage areas
Locations exposed to spray or standing water
Areas subjected to road salts
Difficult-to-clean details
Areas with different exposure histories
The inspection strategy should therefore consider both the number of tests and where those tests are undertaken.
Testing only the easiest or cleanest areas of a structure may provide little meaningful information about the areas presenting the greatest coating risk.
What is an acceptable salt level?
There is no single universal soluble-salt acceptance limit appropriate to every protective coating project.
Acceptance criteria should come from the applicable project specification, coating-system requirements and other relevant contract documentation.
The coating inspector’s role is to establish that the specified test method has been followed correctly, the required locations and frequency have been addressed, results have been properly recorded and the specified acceptance criteria have been satisfied before coating application proceeds.
Arbitrary acceptance limits should not be substituted for the requirements of the project.
What happens if contamination is too high?
Where results exceed the specified acceptance criteria, the surface normally requires appropriate remedial treatment before coating application.
The appropriate cleaning method depends upon the substrate, contamination, coating system and project requirements.
Because the contaminants of concern are water soluble, suitable washing or other specified cleaning processes may form part of the remedial procedure.
Critically, cleaning should be followed by appropriate verification.
Simply carrying out a cleaning operation does not demonstrate that the required level of surface cleanliness has been achieved.
Soluble salt testing as part of independent inspection
Soluble salt testing should not be treated as an isolated number on an inspection report.
It forms part of the wider assessment of whether a substrate is suitable to receive the specified protective coating system.
That assessment can include:
Visual surface preparation standard
Surface profile
Dust contamination
Soluble contamination
Environmental conditions
Condition of prepared steel
Coating manufacturer’s requirements
Project specification requirements
Inspection and Test Plan requirements
The objective is not simply to perform a test. It is to establish evidence that the surface condition is appropriate before a stage of the work becomes permanently concealed beneath the coating system.
PRACTICAL INSPECTION NOTE
A visually clean surface is not necessarily a chemically clean surface. Particular attention should be given to heavily corroded and pitted steel, where contamination may remain in areas that are difficult to clean effectively. Test locations should be selected to represent the actual condition and exposure of the structure rather than simply the easiest locations to access. Where remedial cleaning is required, verification testing provides the evidence that the cleaning operation has achieved the specified requirement before coating application proceeds.
RELATED STANDARDS
ISO 8502-6:2020 — Extraction of water-soluble contaminants for analysis (Bresle method) • ISO 8502-9:2020 — Field method for the conductometric determination of water-soluble salts • Do not add additional standards without technical verification.
RELATED EXPERTISE
This technical content is provided as general guidance and does not replace the requirements of the applicable project specification, coating manufacturer or current edition of the referenced standard. Official standards should always be obtained from the relevant standards organisation.
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