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BRESLE METHOD – DIRECT SAMPLE PROCEDURE (DSP)

Bresle Patches Method – Direct Sample Procedure

Measuring blind with bresle patches

If you encountered a visually impaired person attempting to undertake a visual inspection of a coating system’s condition then you’d probably be a little surprised…right?

But this slightly clumsy simile is not too far away from the way that many companies carry out their bresle method testing with bresle patches.

In this blog we take a closer look at the process of soluble salt measurement using bresle patches as there can be much confusion when it comes to interpreting the test results. You also find that organizations make errors in the process they follow and the equipment they use – all of which can lead to impaired results and quality problems.

What problems do Bresle patches solve?

It’s common knowledge in the coatings industry that any salt contamination underneath a coating can lead to serious problems in future years.

This is due to the hygroscopic nature of salt: the tendency to attract water – in combination with the permeability of a coating – creates an accumulation of water molecules between the substrate and coating.

The presence of these water molecules, plus the entrapment and migration of oxidation agents, creates the ideal environment for an electrochemical shift. In conjunction with the salt molecules present, this causes corrosion.

Blasting or mechanical cleaning will not remove these salt molecules completely. It can actually cause chloride inclusion on the substrate, worsening the situation. Washing the surface with deionized water is a common solution.

A substrate free of soluble salts is critical in today’s protective coating world and is considered in each professional paint specification. Regulations under IMO PSPC have established that the maximum concentration of soluble salts (measured as Sodium Chloride) on a given surface should be limited to 20 mg·m-2.

The principle of a Bresle patch test

When performing the soluble salt test, water is injected in a bresle patch that’s placed on the surface. This injected water dissolves the salt that’s present at the surface. The solubility in the water depends on the type of salt.

Common salt – also called Sodium Chloride – can be dissolved in cold water to a concentration of 357 g∙l-1. Not only does the solubility differs between the salts, but also the conductivity. When taking a measurement not only the common salt is dissolved, but also all other salts present on the surface. This mixture of salts is eventually measured with a conductivity meter or by other means.

Misunderstanding of what’s measured with bresle patches

As it’s impossible to predict which salts are present at the surface, an assumption is made with the bresle patch method. The term ‘measured as Sodium Chloride’ indicates that this mixture of salts is interpreted as Sodium Chloride.

Clearly indicating how the conductivity is interpreted is essential when creating a report. At present there are several interpretations in use. Some speak about Sodium Chloride, others mention mixed salts or just chlorides, each having a different calculation factor.

Solubility and bresle patch testing

The nominal volume in the test chamber of our TQC Bresle patch is 2.5 cm3. Considering the volume and solubility of salt, it’s possible to dissolve 892,5 mg of common salt in the bresle patch. This correlates to 7.29·105 mg·m-2sodium chloride. Comparing this to the IMO regulation of 20 mg·m-2, there’s a factor of approximately 36,000 between these concentrations.

The solubility of salt is not an issue when conducting the bresle patch test. A level of 20 mg·m-2 sodium chloride results in just 0.025 mg sodium chloride in the patch. Even salts that are harder to dissolve will be present in such concentrations, but this should not lead to solubility problems.

Potential concentrations, where the solubility of these salts will cause a problem, corresponds to contaminations on the surface that won’t pass any guideline by a factor of 100 times.

Dilution and bresle patch testing

The dilution is another key cause of potential errors when testing with bresle patches. In order to make it possible to measure the soluble salts with an electronic conductivity meter, usually a volume of 15 ml. sample liquid is required to fully submerge the instrument’s probe.

Since the actual volume of sample liquid in the bresle patch is only 2.5 ml, it means that the final result has to be multiplied by a factor of 6. Which in turn suggests that any errors made during certain stages of the test could well be multiplied by a factor of 6 as well!

Effect of dilution on the Bresle test results

The average residue of 0.15 ml testing liquid remaining in the patch – plus the inaccuracy and improper use of syringes – leads to some errors. However, the majority of the problems caused with bresle patch testing emerge when diluting the sample liquid, as it’s often done in a 15ml cup.

Good analytical bresle patch practice shows us that the number of steps required to obtain a final test result has to be limited as much as possible. Dilution to 15ml was required in the past. This was to create a sufficient quantity of sample solution to submerge a conductivity probe and to prevent extreme static disturbance from the plastic measuring beaker.

All conductivity gauges on the market are influenced by this static disturbance and this can lead up to a difference of 5 µS·cm- 1 per measurement. Diluting the sample liquid by a factor of 6 automatically implies that the test result has to be multiplied by a factor of 6 as well. Once again, in practice, this means that each deviation or error will be multiplied by 6. So if you take the 5 µS·cm- 1 mentioned above, you could end-up with a 30 µS·cm- 1 error!

New techniques using bresle patches make it possible to measure in smaller samples using the ‘direct sample procedure’ (DSP).

Gauge accuracy and bresle patches

During the evaluation of the bresle patch study results the need for a higher accuracy proved to be a hot issue. Fortunately, the accuracy of the bresle patch test can be increased in two ways.

Firstly, by taking a closer look at the gauge. Previously available handheld or mobile conductivity gauges have a resolution of 1 µS·cm-1, with an accuracy of 1 µS·cm-1. Calculation according to ISO 8502-6 means that the final result has a resolution of 6 mg·m-2, with also an inaccuracy of 6 mg·m-1. When a measurement result is 18 mg·m-1 soluble salts – measured as sodium chloride – the actual value fluctuates between 12 and 24 mg·m-2. This leaves a 33% chance that the actual soluble salt concentration is above the limit of 20 mg·m-2. Increasing the gauge’s resolution to 0.1 µS·cm-1 contributes to a higher accuracy when determining the soluble salt concentration while testing with bresle patches.

However, this is only one part of the analysis…

Besides gauge resolution, dilution also influences the measurement. The earlier mentioned 0.15 ml of residue remaining in the bresle patch causes an error up to 5% in the 15ml diluted solution. When this dilution is not applied, and the measurement is made directly on the pure solution from the bresle patch, the 0.15ml residue will not affect the final result. New gauges can already measure in 2 ml solution with a resolution of 0.1 µS·cm-1.

When measuring in a volume of 2.5ml, the same as the nominal volume of the bresle patch, there’s a significant change in the calculation factor. Using a 2.5ml sample leads to the elimination of the normal calculation factor 6. The concentration of soluble salts measured as sodium chloride is equal to the conductivity in µS·cm-1. This not only makes the determination easier, but more reliable too. Results can now be given with an 1 mg·m-2 uncertainty and resolution of 0.1 mg·m-1 – increasing the accuracy 60 fold!

TQC bresle patch DSP

The new direct sample procedure (DSP) eliminates the use of the 15ml measurement solution. Measurements can now directly be made in the solution that’s extracted from the bresle patch, eliminating the dilution step. This not only increases efficiency, but also eliminates the most error-sensitive part of the old procedure.

To achieve this, inject just 2.5 ml of deionized water into the bresle patch. This also reduces the calculation factor to 1. The reading from the gauge doesn’t have to be multiplied to get the soluble salt measured as sodium chloride concentration in mg·m-1. Due to the measurement in the gauge’s own measuring cell – all static disturbance is also eliminated, further increasing the reliability of the bresle patch analyses.

Quality materials for bresle patch testing

There’s a great difference between the variable soluble salt tests kits on the market. Not only in the gauge, but also the bresle patches differ in quality. A test patch should be as clean as possible; any salts that remain on the bresle patch during its production process will influence the test significantly.

Some of the round bresle patches on the market contribute significantly to the final measurement. During tests these inferior bresle patches contribute on average 0.7 mg·m-2 of soluble salts measured as sodium chloride per patch. High quality bresle patches such as the latex membrane square patches, don’t contain any salt residue. These bresle patches pass multiple wash cycles in a clean room quality production plant to ensure that no contaminants are present.

In Annex A of the ISO 8502-6 standard it states that only certified bresle patches may be used. This annex describes a stress test to ensure bresle patch adhesion and washability.

In relation to the nominal volume of the bresle patch, it has to be injected with an excess of water. Time to leakage has to be determined and eight out of twelve bresle patches must pass in order for the type of patch to be approved. This test must be carried out by an accredited laboratory and the producer must be able to provide a certificate of the test.

High quality bresle patches have passed these tests. Most inferior round bresle patches fail this test by 100% and only one third of the required volume can be injected in the patch, before leakage starts.

When measurements are taken during arbitration using non certified bresle patches, the results will likely be useless. Only certified bresle patches may be used. Some patches also face problems with poor and irreproducible adhesion, making the test surface irregular. Often some 20% additional surface area is exposed due to the fact that the water passes under the edges of the bresle patch. This value isn’t corrected and this causes even bigger errors in the final results. All errors caused by using inferior bresle patches lead to higher results which add up to each other – usually generating a significantly higher, erroneous result.

Climate and bresle patch testing

Each soluble salts report you generate should include climate conditions and substrate temperature.

The ISO 8502-6 standard demands that the test has to be done at 23°C and a relative humidity of 50%. Any deviation from the defined parameters has to be reported and agreed upon by both inspector and customer.

The surface temperature also influences the test and this is another parameter that needs to be recorded. During arbitration the lack of these recorded values also will render the results invalid. Although the descriptions above demonstrate that there’s a lot of ‘science’ behind the proper testing for salt contamination, inspectors can benefit from readily made bresle patch method test-kits. These are readily available and they simplify this type of test considerably.

The recently updated TQC bresle patch kit was the first to support this new technique and enables inspectors to work faster and produce more accurate results. By combining this new technique with high quality gauges and bresle patches you can build the ultimate inspection set for arbitration.

Nico Frankhuizen