The Bresle method is used to determine the concentration of dissolved salts on metal surfaces before coating applications, such as painting. These salts can cause serious adhesion problems over time.
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Importance
Salt is in the coastal area. Can be tasted on the lips after walking on the beach. The concentration of salt by weight is about 3.5% in seawater. With the spray of waves and by other means, salt enters the air as aerosols, and eventually as particles like dust. This salt dust can be found everywhere near the beach. Salt is hygroscopic, and this trait makes it harmful to the coating.
Salt contamination under the coating, such as paint on steel, can cause adhesion and corrosion problems due to the hygroscopic nature of the salt. Its tendency to draw water through permeable layers creates a buildup of water molecules between substrate and layer. These molecules, together with salts and other oxidizing agents trapped during coating or migrating through layers, create electrolytic cells, causing corrosion. Blast cleaning is often used to clean the surface before coating; However, with salt contamination, blast cleaning can increase the problem by forcing the salt into the base material. Washing the surface with deionized water before coating is a common solution.
IMO PSPC (performance standard for protective coating) regulation regulates permissible maximum concentration of soluble salts on surface to be coated, measured as sodium chloride, from 50 mg -2 . The maximum amount of salt permitted on the surface before coating applications is usually determined by the coating supplier and the user, such as the shipyard. The default value has not been set.
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Origin of the Bresle Method
The Bresle method was launched in 1995 in the ISO 8502-6 and ISO 8502-9 standards. This test was developed to measure the concentration of dissolved salts on steel surfaces before cleaning and blasting coatings. Not only the ISO, but also the US Navy, IMO, NAVSEA, and ASTM adopt this method as their standard. This method remains the main and most flexible test method for soluble salts on metal surfaces.
Principles
The Bresle method uses different salt conductivity in water, each salt having a typical conductivity-versus-concentration relationship. The correlation between concentration and conductivity can be found in the "Handbook of Chemistry and Physics". This relationship is only useful if the dissolved salt is known. Sodium chloride, the main salt in seawater, causes a large increase in conductivity by increasing concentration.
Special fillings are applied to the surface to be tested, and the deionized water volume specified is injected under the patch. Any dissolved salt present on the surface will dissolve in water. The liquid is extracted and the conductivity measured.
The conductivity of the accumulated salt solution depends on the volume of water used and the initial conductivity, and the amount of salt in the solution depends on the patch area. The calculation of salt per area is based on an increase in conductivity but in the PSPC IMO method the salt is calculated as sodium chloride, in the method of ISO 8502-9 it is calculated as a specific mixture of salt, but expressed as Sodium Chloride.
Calculation Factor
Factors are applied to measured conductivity, depending on what is known or assumed about salt contamination and various conditions, to produce meaningful contamination measurements. Some variables are:
- salt type
- the volume of the solution (below the table factor is calculated by the volume of 15 ml solution)
- temperature
- device-specific scale
The common source of error is not knowing the composition of the contamination being measured.
Measurement Tool
There are several test equipment suppliers of Bresle methods.
The principle
Solubility in water depends on the type of salt. Sodium chloride can be dissolved in cold water to a concentration of 357 g? L -1 . Not only is the solubility different between salt but also the conductivity. When performing the Bresle method test, not only dissolved sodium chloride but also all other salts present on the surface. Since it is impossible to predict which salt is on the surface, assumptions are made in the Bresle method. The term "measured as sodium chloride" indicates that this salt mixture is interpreted as sodium chloride. Reporting how conductivity is an important factor when creating a report.
In Practice
All parties involved should be clear about the impact on climate outcomes and the variance of potentially different salt content. Informed agreements must be reached between all parties regarding acceptable reading levels. Depending on the size and nature of the surface to be coated, some readings may need to be taken.
Patch Test
The test patch should be as clean as possible. Contamination of patches can significantly affect results.
ISO 8502-6 standard set out in annex A that certified patches should be used. This appendix describes stress tests to ensure patch adhesion and washing skills. Not provided with a certificate that a patch passing this test will make the results obtained by this patch useless.
Climate
The report of the dissolved salt should include climatic conditions and substrate temperature. ISO 8502-6 requires testing done at 23 ° C and 50% relative humidity, with irregularities reported and approved by inspectors and customers. During arbitration, the absence of these values ​​in the report can result in invalid results.
References
- The Chemical and Physical Handbook
- ISO 8502-6, "Extraction for dissolved contaminants for analysis - Bresle Method"
- ISO 8502-9, "Field method for dissolved salt with conductivity measurement"
Source of the article : Wikipedia
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