Various standards and guidelines are used to assess a surface before electroplating. They specify what the surface must look like in order to ensure good adhesion and quality of the coating. The so-called Bresle test is a method that is very simple to use, but nevertheless informative.
Background and origin of the Bresle test
The quality and durability of electroplated coatings depend to a large extent on the cleanliness of the substrate surface. One of the most decisive test methods for assessing surface cleanliness is the Bresle test, which measures in particular the concentration of water-soluble salts on the surface. These salts, if trapped in the coating, can lead to corrosion damage and premature failure of the coatings. In this context, the Bresle test is not only a preventive measure, but also a quality feature in modern electroplating technology.
The Bresle test was originally developed to quickly measure the concentration of water-soluble salts on steel surfaces before coating (painting, electroplating). The sources of salt contamination are manifold and include the abrasive used, environmental influences or inadequate cleaning of residues from previous processing steps. Salt contamination under a coating, for example a paint finish on steel, can lead to adhesion and corrosion problems due to the hygroscopic properties of salt. Salt draws water through a (micro-)porous coating and leads to an accumulation of water molecules between the substrate and the coating.
These molecules, together with salt and other oxidizing agents that are trapped or migrate through the coating during coating, form an electrolytic cell that causes corrosion. Surfaces are often cleaned by blasting before coating. However, in the case of salt contamination, blasting can exacerbate the problem as salt penetrates the base material.
The Bresle test was developed by Åke Bresle, founder of the Swedish company Expertus Kemiteknik AB, which specializes in chemical analysis and the recovery of residual values. The Bresle method itself was first included in the international standard ISO 8502-6 in 1995. Organizations such as the US Navy and the International Maritime Organization (IMO) rely on this test method, which in turn underlines the importance and acceptance of the method among experts. Although a key aspect is the assessment of surface quality prior to painting, it is also increasingly being used in electroplating.
How the Bresle test works
Modern compact devices allow the simple conversion of the measured conductivity into the surface concentration (μg/cm2)The Bresle method uses the different conductivity of salts in water. Each salt has a characteristic conductivity-concentration relationship. The relationship between concentration and conductivity is described in the "CRC Handbook of Chemistry and Physics" (published in regular editions since 1914 by CRC Press LLC New York). However, this relationship is only useful if the dissolved salt is known. Sodium chloride, the main salt in seawater, leads to a sharp increase in conductivity with increasing concentration.
According to the above-mentioned standard DIN EN ISO 8502-6, a special test piece (test patch) is applied to the surface to be tested and a certain amount of deionized water is sprayed under the test piece. All soluble salts present on the surface dissolve in the water. The liquid is drawn off after a precisely defined time and its conductivity is measured. The conductivity of the collected salt solution depends on the amount of water used and its initial conductivity, while the amount of salt in the solution depends on the surface area of the test specimen. The measurement of conductivity is also temperature-dependent and should therefore be carried out at a constant 20 °C. The calculation of the salt load per area is based exclusively on this measurement of the increased conductivity due to the dissolution of water-soluble residues from the metal surface under test.
Carrying out the Bresle test does not require any significant specialist knowledge. After preparing the surface, the Bresle patch is applied according to the manufacturer's instructions. After injecting the water, there is a short waiting time, followed by the measurement of conductivity. The results are compared with the specified limit values in order to evaluate the surface cleanliness. However, the evaluation itself can lead to misinterpretations without a deeper understanding of the matter. It is important to note that the Bresle test does not identify specific ions (salts) and so the conductivity reflects the sum of the dissolved ions. The measured value therefore includes chloride, bromide and fluoride on the surface, for example. Separation using chromatographic methods could lead to a distinction between the individual ion types, but is many times more complex. In most cases, the presence of individual species (e.g. bromides) can also be excluded. As with the IMO-PSPC method, the salt is therefore calculated as a "sodium chloride equivalent".
Significance in electroplating
Electroplating in particular requires good pre-cleaning of the parts, as even the smallest impurities can impair the adhesive strength and quality of the coating. The Bresle test offers a number of advantages in this respect. It enables the early detection of contamination on the surface. This means that appropriate cleaning measures can be taken before coating. As a clean surface ensures good coating adhesion, ensuring adhesion strength increases the service life of the coating and therefore also of the galvanized parts. By being anchored in valid international standards, the method also serves as a meaningful confirmation of the pre-treatment process in the event of a dispute.
