FAQ when using of Bresle Salt Test Patches to control salt contamination level before coating:

Even if the steel looks clean and dry, you can still have invisible soluble salts (mostly chlorides, sometimes sulfates) on the surface. These are a big problem because:
• They stay active under the coating – unlike dust or loose rust that might just sit there.
• They attract moisture from the air, even at relatively low humidity.
• They form an electrolyte solution (a thin film of salty water) when they get moisture.

If you don’t check for and remove salts before coating, you risk:
• Much faster corrosion under the paint
• Blistering and delamination of the coating (osmotic blisters)
• Shortened coating lifetime and early repainting/repairs
• Potential warranty issues and failed inspections (many specs limit salt levels)

That’s why standards (like ISO 8502-series) require you to measure and document the salt contamination level, not just “assume” the surface is clean.

Short version:

Salts on steel surfaces are dangerous because they are invisible, attract moisture, and form a conductive electrolyte that accelerates electrochemical corrosion. Chloride salts in particular reduce the critical humidity needed for corrosion, increase the corrosion current, and can draw water through coatings, causing blistering and underfilm rust. Therefore, verifying that the surface is free from soluble salts before coating is essential to ensure long coating life and slow down the corrosion of the steel.

Full version:

Corrosion of steel in air + moisture is an electrochemical reaction. You can think of it as a tiny battery on the steel surface:
• Some areas act as anodes (iron dissolves → Fe²⁺).
• Other areas act as cathodes (oxygen is reduced).
• Ions in water carry charge between these areas, allowing the corrosion current to flow.

Salt speeds this up in several ways:

a) Salt increases the conductivity of the water layer

Pure water doesn’t conduct electricity very well.
Add salt (for example NaCl), and it dissociates into ions (Na⁺, Cl⁻), which makes the solution:
• Much more conductive
• Able to carry more corrosion current

More current = faster corrosion rate.

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b) Salt attracts and holds moisture (hygroscopic effect)

Many salts, especially chlorides:
• Attract water from the air
• Can keep a thin, damp film on the steel even when the relative humidity is not very high

That means:
• The steel spends more time in a “wet” condition, which is when corrosion occurs.
• So instead of corroding only during obvious wet periods (rain, condensation), it can corrode during “normal” humidity too.

More time wet = more hours per day for corrosion to run.

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c) Salt penetrates and breaks down the coating

When steel is already coated:
• Salts under the coating create a concentrated solution when moisture gets in.
• This causes an osmotic pressure difference:
water is drawn through the coating towards the salt-rich area.
• Result: osmotic blistering – paint blisters filled with salty water.

Inside those blisters, the environment is perfect for rapid corrosion:
• High conductivity (salty water)
• Often low oxygen in spots → local anodes
• The coating is lifted, giving corrosion more space to spread

So even a small amount of salt can seriously reduce the life of a coating system.

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d) Chloride ions are chemically aggressive

Chloride ions (Cl⁻):
• Destabilize protective oxide layers on steel.
• Promote pitting and localized attack, which is faster and more dangerous than uniform rusting.

Local pits become strong anodes → very high local corrosion rate.

Soluble salts such as chlorides and sulfates are one of the most common hidden causes of premature coating failure on steel. Even when a blasted surface looks clean, microscopic salt contamination can remain in pits and surface irregularities. These salts are hygroscopic – they attract and hold moisture from the air – and when they dissolve, they create a thin film of highly conductive electrolyte on the steel. This electrolyte dramatically increases the corrosion current and speeds up the rusting process, both on bare steel and under coatings.

If the steel is coated while salt is still present, the risk of osmotic blistering, underfilm corrosion and early delamination increases significantly. Moisture is drawn through the coating towards the salt, forming blisters filled with corrosive salt solution. As a result, the coating system fails long before its expected lifetime, leading to costly repairs, downtime and warranty issues.

Using a Bresle method salt test in accordance with ISO 8502 (for example with the Expertus BresleSampler™ and Salt Test Kit) allows you to measure and document the level of soluble salt contamination on the steel surface before painting. Verifying that the surface is within the specified salt limits is a simple step that greatly reduces the risk of premature coating breakdown and ensures a longer, more reliable service life for your protective coating system.

Definition:
Pitting corrosion means that small, deep pits form on the metal surface. These pits can penetrate deeply into the material, often without any significant visible surface damage.

How does it occur?
It often starts where the protective oxide layer on the metal is weak or damaged. Chloride ions can penetrate and break down this protection, causing corrosion to concentrate in small areas.

Process:
Inside a pit, oxygen is quickly consumed, making the environment more acidic (pH drops) and attracting more chloride ions. This causes the corrosion to accelerate right there, while the rest of the surface remains relatively unaffected.

Consequences:
Pitting is dangerous because it can cause through-holes and severe weakening of the material, even if the rest of the surface appears undamaged.

Summary
Pitting corrosion is thus a self-reinforcing, localized form of corrosion that can quickly destroy steel and other metals in chloride-containing environments, even though the total amount of rust may be small.

SingleTab™ BresleSampler™ has a more "agressive" adhesive than TwinTab™ and can best be used when the surface is judged as difficult to get good adhesion to.
On the other hand the SingleTab™ BresleSampler™ can leave foam remnants when removed. In many cases this is no problem when the surface is blasted after performing the test.
In most other cases TwinTab™ gives a excellent adhesion and well defined area and has the advantage of being easy to remove with no foam rests on the tested surface.

Please see the full description at Instruction for surface sampling with Salt Test Patch TwinTab™ BresleSampler™

Please send us a mail at info@expertus.se and we will inform you about our nearest reseller or distributor!
Thank you!