A new coating on steel will only last as long as the underlying surface allows. For offshore structures, ships and port facilities, the greatest invisible enemy is salt: chlorides and sulphates that lodge in the rust scars and continue to corrode beneath the fresh paint. This article explains how salts are measured, what the standard limit values are, and how to prepare the surface reliably before coating.
Why salt beneath the coating becomes a problem
Corroded steel is not smooth. Chloride and sulphate crystals accumulate in the depressions of the rust scars; these are not visible to the naked eye. If a coating is applied over them, these salts draw moisture through the coating. This leads to blistering (osmotic blistering), and corrosion resumes beneath the coating. The result is that the coating often fails long before its intended service life, and rectification work at sea or in dock is particularly expensive.
How is salinity measured?
In practice, the Bresle test implemented. A self-adhesive measuring cell (Bresle patch) is affixed to the steel surface and filled with a small, precisely measured quantity of distilled water. The water dissolves the salts from the surface. The method is described in ISO 8502-6 described.
The conductivity of the solution is then measured (ISO 8502-9) and converted into an area density, usually expressed in mg/m² as sodium chloride. The conductivity measurement takes account of all water-soluble salts collectively; it does not distinguish between chlorides and sulphates.
What are the limit values?
There is no standard statutory limit. The decisive factor is the Project Specification, i.e. the specifications set by the coating manufacturer, the client or the regulations to which the contract refers. Two frequently cited examples:
- NORSOK M-501 (Offshore installations, Norway) specifies 20 mg/m² NaCl as the permissible residual salt content.
- IMO PSPC (Ship ballast tanks) permit a maximum of 50 mg/m² of NaCl.
You should therefore check, before you begin, which standard applies to your project and who will carry out the assessment – usually a corrosion protection assessor or inspector.
Why water alone is often not enough
Rinsing with high-pressure fresh water removes loose salts from the surface. However, water alone often fails to reach the crystals deep within the rust scars. Even blasting does not always solve the problem: whilst it roughens the surface and removes rust, it can also work salt residues into the new surface. For this reason, both methods are often combined: blasting for mechanical preparation and a Salt remover for the remaining salts.
This is how salt removal works in practice
- Measuring salinity: Determine the baseline value using the Bresle test.
- Pre-cleaning: Remove grease, oil and loose rust.
- Containment and collection: Enclose the component in such a way that the rinse water is completely collected. At sea, this is usually a requirement.
- Apply the salt remover: using a spray lance, a low-pressure sprayer or a foam lance.
- Leave to soak in: 15 to 30 minutes, depending on the product and the level of soiling.
- Rinsing: using distilled or fully demineralised water and high pressure.
- Post-measurement: Repeat the Bresle test; carry out a second round if necessary.
- Coating: As soon as possible after release and drying, before new salt films form.
From real-life experience
When coating monopiles for an offshore wind farm, the permissible residual salt content was set at 20 mg/m² NaCl, measured in accordance with ISO 8502-6 and ISO 8502-9. Although the steel was new and the piles had been washed with distilled water under high pressure as usual, the required value was not achieved. The on-site inspector was familiar with the Salt Remover from a previous project and recommended it – the feedback from the production team was positive. This example shows that even in the absence of visible corrosion, it is worth carrying out the measurement, and water alone is not always sufficient.
What matters
- Use only distilled or fully demineralised water. Tap water or domestic water contains chlorides itself and distorts the result.
- Check material compatibility. Many de-icing agents are acidic. You should therefore protect aluminium, galvanised surfaces and seals, or test the product on a small area first.
- Collect the rinse water and dispose of it in accordance with the regulations. At sea, the operator’s and the port’s rules also apply.
- Record the result. Measurements taken before and after the test must be included in the test report.
Salt removers for offshore, shipyards and ports
The Power Eco Metal Salt Remover Eco-Nova’s product is a water-based salt remover designed specifically for this stage of the process. Depending on the project, we supply it ready for use or as a concentrate to be diluted on site. On request, we can provide a test sample for a test area – this allows you to see, based on the Bresle value, how well the Salt Remover performs on your component. Find out more on our website Offshore & Shipyard.